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Resistor. Electric Furnace

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Производство и промышленные технологии

In ordinary electric wiring plastics are commonly used as insulating sheathing for the wire itself. PROPERTIES Ceramics possess chemical mechanical physical thermal electrical and magnetic properties that distinguish them from other materials such as metals and plastics.

Английский

2015-09-16

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Resistor

Resistor, component of an electric circuit that resists the flow of direct or alternating electric current. Resistors can limit or divide the current, reduce the voltage, protect an electric circuit, or provide large amounts of heat or light.

An electric current is the movement of charged particles called electrons from one region to another. The amount of resistance to the flow of current that a resistor causes depends on the material it is made of as well as its size and shape. Resistors are usually placed in electric circuits, which are devices formed when current moves through an electrical conductor (a material that allows the current to flow without much resistance, such as copper wire) and when the conductor makes a complete loop.

When a voltage, or electric potential, is applied to opposite ends of a circuit, it causes current to flow through the circuit. As the current flows, it encounters a certain amount of resistance from the conductor and any resistors in the circuit. Each material has a characteristic resistance. For example, wood is a bad conductor because it offers high resistance to the current; copper is a better conductor because it offers less resistance. In any electric circuit, the current in the entire circuit is equal to the voltage across that circuit divided by the resistance of the circuit. Resistors are often made to have a specific value of resistance so that the characteristics of the circuit can be accurately calculated.

Physicists sometimes explain the flow of current through a material, such as a resistor, by comparing it to water flowing through a pipe. A pressure difference maintained across two ends of the pipe by a pump is like the potential difference, or voltage, across a wire maintained by a battery. The rate of flow of water, analogous to the rate of flow of charge (current), depends on the type of pipe used. A long and thin water pipe offers more resistance than a short and thick one or a pipe that has obstructions. Similarly, the resistance of a conductor is dependent upon several factors, including its length, cross section, temperature, and a property called resistivity.  Resistivity is an intrinsic characteristic of the material itself defined by the voltage divided by the density of current (current per unit cross section area) flowing across the material.

A material of high resistivity will require a higher electrical field to cause a given current density. If the resistivity of a material is known, as well as its dimensions, it can be used to calculate the resistance of a particular piece of material. The resistivity of a material is also dependent upon temperature. When a material resists the flow of current, it converts the electrical energy into other kinds of energy such as heat and light. This energy causes resistors to heat up and glow when enough current flows through them.

Resistors are designed to have a specific value of resistance. Most resistors used in electric circuits are cylindrical items a few millimeters long with wires at both ends to connect them to the circuit. Resistors are often color coded by three or four color bands that indicate the specific value of resistance. Some resistors obey Ohm’s law, which states that the current density is directly proportional to the electrical field when the temperature is constant. The resistance of a material that follows Ohm’s law is constant, or independent of voltage or current, and the relationship between current and voltage is linear. Modern electronic circuits depend on many devices that deviate from Ohm’s law. In devices such as diodes, the current does not increase linearly with voltage and is different for two directions of current.

Resistors can help divide voltages, and when combined with other elements can help convert voltages for a specific electrical design. Resistors can also be used to provide intense light or heat. For example, the heating element in a household cooking range is a resistor, as is the tungsten filament in a common incandescent lamp. Resistors with adjustable resistance are called rheostats or potentiometers. These types of resistors are used in appliances when the current needs to be adjusted or when the resistance needs to be varied, as with lights that dim or adjustable generators.

Electric Furnace

Electric Furnace, electrically heated device used industrially for melting metals or firing ceramics. It is also known as an electrothermic furnace.

The simplest type of electric furnace is the resistance furnace, in which heat is generated by passing a current through a resistance element surrounding the furnace or by utilizing the resistance of the material being heated. The heating element in an externally heated furnace may take the form of a coil of metal wire wound around a tube of refractory material or it may be a tube of metal or other resistive material such as carborundum. Resistance furnaces are particularly useful in applications in which a small furnace, with precisely controlled temperatures, is needed. Small resistance furnaces are widely used in laboratories and in shops for the heat treatment of tools. Larger furnaces are used for firing ceramics and melting brass. The highest temperature at which resistance furnaces are operated, for example, in the manufacture of graphite, is in the neighborhood of 4100° C (7366° F).

The electric-arc furnace is the most widely used type of electric furnace for the production of quality alloy steels and range in capacity from 227 kg (500 lb) to 181 metric tons. In these furnaces the heat is generated by an arc struck between the metal being heated and one or more electrodes suspended above the metal. A typical form of arc furnace has three electrodes, fed by a three-phase power supply, giving three heating arcs. The electrodes are made either of graphite or of carbon.

A more recently developed type of electric furnace is the induction furnace, consisting of a crucible in which a metallic charge is heated by eddy currents induced magnetically. Around the crucible is wound a coil through which high-frequency alternating currents are passed. The magnetic field of this coil sets up eddy currents in the metal in the crucible. Induction furnaces have a number of advantages, chief among them being the speed at which metal can be melted. At comparatively low frequencies the induced eddy currents exert a stirring action on the molten metal. Because the higher frequencies are the most effective for heating, some induction furnaces have two coils, one for high-frequency current and one for low-frequency. The earlier types of induction furnaces operated at frequencies between 60 and 60,000 cycles per second, but some modern furnaces are designed to use frequencies of 1 million cycles or more per second.

A special type of furnace, called an electrolytic furnace, is used in the production of aluminum, magnesium, and sodium. In the electrolytic furnace, a salt is fused by the heat generated by the passage of a large electric current and is at the same time electrolyzed so that the pure metal is deposited at one electrode.

Insulation 

INTRODUCTION

Insulation, any material that is a poor conductor of heat or electricity, and that is used to suppress the flow of heat or electricity.

ELECTRIC INSULATION

The perfect insulator for electrical applications would be a material that is absolutely nonconducting; such a material does not exist. The materials used as insulators, although they do conduct some electricity, have a resistance to the flow of electric current as much as 2.5 × 1024 greater than that of good electrical conductors such as silver and copper. Materials that are good conductors have a large number of free electrons (electrons not tightly bound to atoms) available to carry the current; good insulators have few such electrons. Some materials such as silicon and germanium, which have a limited number of free electrons, are semiconductors and form the basic material of transistors.

In ordinary electric wiring, plastics are commonly used as insulating sheathing for the wire itself. Very fine wire, such as that used for the winding of coils and transformers, may be insulated with a thin coat of enamel. The internal insulation of electric equipment may be made of mica or glass fibers with a plastic binder. Electronic equipment and transformers may also use a special electrical grade of paper. High-voltage power lines are insulated with units made of porcelain or other ceramic, or of glass.

The specific choice of an insulation material is usually determined by its application. Polyethylene and polystyrene are used in high-frequency applications, and mylar is used for electrical capacitors. Insulators must also be selected according to the maximum temperature they will encounter. Teflon is used in the high-temperature range of 175° to 230° C (350° to 450° F). Adverse mechanical or chemical conditions may call for other materials. Nylon has excellent abrasion resistance, and neoprene, silicone rubber, epoxy polyesters, and polyurethanes can provide protection against chemicals and moisture.

THERMAL INSULATION

Thermal insulating materials are used to reduce the flow of heat between hot and cold regions. The sheathing often placed around steam and hot-water pipes, for instance, reduces heat loss to the surroundings, and insulation placed in the walls of a refrigerator reduces heat flow into the unit and permits it to stay cold.

Thermal insulation may have to fulfill one or more of three functions: to reduce thermal conduction in the material where heat is transferred by molecular or electronic action; to reduce thermal convection currents, which can be set up in air or liquid spaces; and to reduce radiation heat transfer where thermal energy is transported by electromagnetic waves. Conduction and convection can be suppressed in a vacuum, where radiation becomes the only method of transferring heat. If the surfaces are made highly reflective, radiation can also be reduced. Thus, thin aluminum foil can be used in building walls, and reflecting metal on roofs minimizes the heating effect of the sun. Thermos bottles or Dewar flasks (see Cryogenics) provide insulation through an evacuated double-wall arrangement in which the walls have reflective silver or aluminum coatings. See also Heat Transfer.

Air offers resistance to heat flow at a rate about 15,000 times higher than that of a good thermal conductor such as silver, and about 30 times higher than that of glass. Typical insulating materials, therefore, are usually made of nonmetallic materials and are filled with small air pockets. They include magnesium carbonate, cork, felt, cotton batting, rock or glass wool, and diatomaceous earth. Asbestos was once widely used for insulation, but it has been found to be a health hazard and has, therefore, been banned in new construction in the U.S.

In building materials, air pockets provide additional insulation in hollow glass bricks, insulating or thermopane glass (two or three sealed glass panes with a thin air space between them), and partially hollow concrete tile. Insulating properties are reduced if the air space becomes large enough to allow thermal convection, or if moisture seeps in and acts as a conductor. The insulating property of dry clothing, for example, is the result of air entrapped between the fibers; this ability to insulate can be significantly reduced by moisture.

Home-heating and air-conditioning costs can be reduced by proper building insulation. In cold climates about 8 cm (about 3 in) of wall insulation and about 15 to 23 cm (about 6 to 9 in) of ceiling insulation are recommended. The effective resistance to heat flow is conventionally expressed by its R-value (resistance value), which should be about 11 for wall and 19 to 31 for ceiling insulation.

Superinsulation has been recently developed, primarily for use in space, where protection is needed against external temperatures near absolute zero. Superinsulation fabric consists of multiple sheets of aluminized mylar, each about 0.005 cm (about 0.002 in) thick, and separated by thin spacers with about 20 to 40 layers per cm (about 50 to 100 layers per in).


Ceramics

INTRODUCTION

Ceramics (Greek keramos, "potter's clay"), originally the art of making pottery, now a general term for the science of manufacturing articles prepared from pliable, earthy materials that are made rigid by exposure to heat. Ceramic materials are nonmetallic, inorganic compounds—primarily compounds of oxygen, but also compounds of carbon, nitrogen, boron, and silicon. Ceramics includes the manufacture of earthenware, porcelain, bricks, and some kinds of tile and stoneware.

Ceramic products are used not only for artistic objects and tableware, but also for industrial and technical items, such as sewer pipe and electrical insulators. Ceramic insulators have a wide range of electrical properties. The electrical properties of a recently discovered family of ceramics based on a copper-oxide mixture allow these ceramics to become superconductive, or to conduct electricity with no resistance, at temperatures much higher than those at which metals do. In space technology, ceramic materials are used to make components for space vehicles.

The rest of this article will deal only with ceramic products that have industrial or technical applications. Such products are known as industrial ceramics. The term industrial ceramics also refers to the science and technology of developing and manufacturing such products.

PROPERTIES

Ceramics possess chemical, mechanical, physical, thermal, electrical, and magnetic properties that distinguish them from other materials, such as metals and plastics. Manufacturers customize the properties of ceramics by controlling the type and amount of the materials used to make them.

Chemical Properties

Industrial ceramics are primarily oxides (compounds of oxygen), but some are carbides (compounds of carbon and heavy metals), nitrides (compounds of nitrogen), borides (compounds of boron), and silicides (compounds of silicon). For example, aluminum oxide can be the main ingredient of a ceramic—the important alumina ceramics contain 85 to 99 percent aluminum oxide. Primary components, such as the oxides, can also be chemically combined to form complex compounds that are the main ingredient of a ceramic. Examples of such complex compounds are barium titanate (BaTiO3) and zinc ferrite (ZnFe2O4). Another material that may be regarded as a ceramic is the element carbon (in the form of diamond or graphite).

Ceramics are more resistant to corrosion than plastics and metals are. Ceramics generally do not react with most liquids, gases, alkalies, and acids. Most ceramics have very high melting points, and certain ceramics can be used up to temperatures approaching their melting points. Ceramics also remain stable over long time periods.

Mechanical Properties

Ceramics are extremely strong, showing considerable stiffness under compression and bending. Bend strength, the amount of pressure required to bend a material, is often used to determine the strength of a ceramic. One of the strongest ceramics, zirconium dioxide, has a bend strength similar to that of steel. Zirconias (ZrO2) retain their strength up to temperatures of 900° C (1652° F), while silicon carbides and silicon nitrides retain their strength up to temperatures of 1400° C (2552° F). These silicon materials are used in high-temperature applications, such as to make parts for gas-turbine engines. Although ceramics are strong, temperature-resistant, and resilient, these materials are brittle and may break when dropped or when quickly heated and cooled.

Physical Properties

Most industrial ceramics are compounds of oxygen, carbon, or nitrogen with lighter metals or semimetals. Thus, ceramics are less dense than most metals. As a result, a light ceramic part may be just as strong as a heavier metal part. Ceramics are also extremely hard, resisting wear and abrasion. The hardest known substance is diamond, followed by boron nitride in cubic-crystal form. Aluminum oxide and silicon carbide are also extremely hard materials and are often used to cut, grind, sand, and polish metals and other hard materials.

Thermal Properties

Most ceramics have high melting points, meaning that even at high temperatures, these materials resist deformation and retain strength under pressure. Silicon carbide and silicon nitride, for example, withstand temperature changes better than most metals do. Large and sudden changes in temperature, however, can weaken ceramics. Materials that undergo less expansion or contraction per degree of temperature change can withstand sudden changes in temperature better than materials that undergo greater deformation. Silicon carbide and silicon nitride expand and contract less during temperature changes than most other ceramics do. These materials are therefore often used to make parts, such as turbine rotors used in jet engines, that can withstand extreme variations in temperature.

Electrical Properties

Certain ceramics conduct electricity. Chromium dioxide, for example, conducts electricity as well as most metals do. Other ceramics, such as silicon carbide, do not conduct electricity as well, but may still act as semiconductors. (A semiconductor is a material with greater electrical conductivity than an insulator has but with less than that of a good conductor.) Other types of ceramics, such as aluminum oxide, do not conduct electricity at all. These ceramics are used as insulators—devices used to separate elements in an electrical circuit to keep the current on the desired pathway. Certain ceramics, such as porcelain, act as insulators at lower temperatures but conduct electricity at higher temperatures.

Magnetic Properties

Ceramics containing iron oxide (Fe2O3) can have magnetic properties similar to those of iron, nickel, and cobalt magnets (see Magnetism). These iron oxide-based ceramics are called ferrites. Other magnetic ceramics include oxides of nickel, manganese, and barium. Ceramic magnets, used in electric motors and electronic circuits, can be manufactured with high resistance to demagnetization. When electrons become highly aligned, as they do in ceramic magnets, they create a powerful magnetic field which is more difficult to disrupt (demagnetize) by breaking the alignment of the electrons.

MANUFACTURE

Industrial ceramics are produced from powders that have been tightly squeezed and then heated to high temperatures. Traditional ceramics, such as porcelain, tiles, and pottery, are formed from powders made from minerals such as clay, talc, silica, and feldspar. Most industrial ceramics, however, are formed from highly pure powders of specialty chemicals such as silicon carbide, alumina, and barium titanate.

The minerals used to make ceramics are dug from the earth and are then crushed and ground into fine powder. Manufacturers often purify this powder by mixing it in solution and allowing a chemical precipitate (a uniform solid that forms within a solution) to form. The precipitate is then separated from the solution, and the powder is heated to drive off impurities, including water. The result is typically a highly pure powder with particle sizes of about 1 micrometer (a micrometer is 0.000001 meter, or 0.00004 in).

Molding

After purification, small amounts of wax are often added to bind the ceramic powder and make it more workable. Plastics may also be added to the powder to give the desired pliability and softness. The powder can then be shaped into different objects by various molding processes. These molding processes include slip casting, pressure casting, injection molding, and extrusion. After the ceramic is molded, it is heated in a process known as densification to make the material stronger and more dense.

Slip Casting

Slip casting is a molding process used to form hollow ceramic objects. The ceramic powder is poured into a mold that has porous walls, and then the mold is filled with water. The capillary action (forces created by surface tension and by wetting the sides of a tube) of the porous walls drains water through the powder and the mold, leaving a solid layer of ceramic inside.

Pressure Casting

In pressure casting, ceramic powder is poured into a mold, and pressure is then applied to the powder. The pressure condenses the powder into a solid layer of ceramic that is shaped to the inside of the mold.

Injection Molding

Injection molding is used to make small, intricate objects. This method uses a piston to force the ceramic powder through a heated tube into a mold, where the powder cools, hardening to the shape of the mold. When the object has solidified, the mold is opened and the ceramic piece is removed.

Extrusion

Extrusion is a continuous process in which ceramic powder is heated in a long barrel. A rotating screw then forces the heated material through an opening of the desired shape. As the continuous form emerges from the die opening, the form cools, solidifies, and is cut to the desired length. Extrusion is used to make products such as ceramic pipe, tiles, and brick.

Densification

The process of densification uses intense heat to condense a ceramic object into a strong, dense product. After being molded, the ceramic object is heated in an electric furnace to temperatures between 1000° and 1700° C (1832° and 3092° F). As the ceramic heats, the powder particles coalesce, much as water droplets join at room temperature. As the ceramic particles merge, the object becomes increasingly dense, shrinking by up to 20 percent of its original size . The goal of this heating process is to maximize the ceramic’s strength by obtaining an internal structure that is compact and extremely dense.

APPLICATIONS

Ceramics are valued for their mechanical properties, including strength, durability, and hardness. Their electrical and magnetic properties make them valuable in electronic applications, where they are used as insulators, semiconductors, conductors, and magnets. Ceramics also have important uses in the aerospace, biomedical, construction, and nuclear industries.

Mechanical Applications

Industrial ceramics are widely used for applications requiring strong, hard, and abrasion-resistant materials. For example, machinists use metal-cutting tools tipped with alumina, as well as tools made from silicon nitrides, to cut, shape, grind, sand, and polish cast iron, nickel-based alloys, and other metals. Silicon nitrides, silicon carbides, and certain types of zirconias are used to make components such as valves and turbocharger rotors for high-temperature diesel and gas-turbine engines. The textile industry uses ceramics for thread guides that can resist the cutting action of fibers traveling through these guides at high speed.

Electrical and Magnetic Applications

Ceramic materials have a wide range of electrical properties. Hence, ceramics are used as insulators (poor conductors of electricity), semiconductors (greater conductivity than insulators but less than good conductors), and conductors (good conductors of electricity).

Ceramics such as aluminum oxide (Al2O3) do not conduct electricity at all and are used to make insulators. Stacks of disks made of this material are used to suspend high-voltage power lines from transmission towers. Similarly, thin plates of aluminum oxide , which remain electrically and chemically stable when exposed to high-frequency currents, are used to hold microchips.

Other ceramics make excellent semiconductors. Small semiconductor chips, often made from barium titanate (BaTiO3) and strontium titanate (SrTiO3), may contain hundreds of thousands of transistors, making possible the miniaturization of electronic devices.

Scientists have discovered a family of copper-oxide-based ceramics that become superconductive at higher temperatures than do metals. Superconductivity refers to the ability of a cooled material to conduct an electric current with no resistance. This phenomenon can occur only at extremely low temperatures, which are difficult to maintain. However, in 1988 researchers discovered a copper oxide ceramic that becomes superconductive at -148° C (-234° F). This temperature is far higher than the temperatures at which metals become superconductors.

Thin insulating films of ceramic material such as barium titanate and strontium titanate are capable of storing large quantities of electricity in extremely small volumes. Devices capable of storing electrical charge are known as capacitors. Engineers form miniature capacitors from ceramics and use them in televisions, stereos, computers, and other electronic products.

Ferrites (ceramics containing iron oxide) are widely used as low-cost magnets in electric motors. These magnets help convert electric energy into mechanical energy. In an electric motor, an electric current is passed through a magnetic field created by a ceramic magnet. As the current passes through the magnetic field, the motor coil turns, creating mechanical energy. Unlike metal magnets, ferrites conduct electric currents at high frequencies (currents that increase and decrease rapidly in voltage). Because ferrites conduct high-frequency currents, they do not lose as much power as metal conductors do. Ferrites are also used in video, radio, and microwave equipment. Manganese zinc ferrites are used in magnetic recording heads, and bits of ferric oxides are the active component in a variety of magnetic recording media, such as recording tape and computer diskettes.

Aerospace

Aerospace engineers use ceramic materials and cermets (durable, highly heat-resistant alloys made by combining powdered metal with an oxide or carbide and then pressing and baking the mixture) to make components for space vehicles. Such components include heat-shield tiles for the space shuttle and nosecones for rocket payloads.

Bioceramics

Certain advanced ceramics are compatible with bone and tissue and are used in the biomedical field to make implants for use within the body. For example, specially prepared, porous alumina will bond with bone and other natural tissue. Medical and dental specialists use this ceramic to make hip joints, dental caps, and dental bridges. Ceramics such as calcium hydroxyl phosphates are compatible with bone and are used to reconstruct fractured or diseased bone.

Nuclear Power

Engineers use uranium ceramic pellets to generate nuclear power. These pellets are produced in fuel fabrication plants from the gas uranium hexafluoride (UF6). The pellets are then packed into hollow tubes called fuel rods and are transported to nuclear power plants.

Building and Construction

Manufacturers use ceramics to make bricks, tiles, piping, and other construction materials. Ceramics for these purposes are made primarily from clay and shale. Household fixtures such as sinks and bathtubs are made from feldspar- and clay-based ceramics.

Coatings

Because ceramic materials are harder and have better corrosion resistance than most metals, manufacturers often coat metal with ceramic enamel. Manufacturers apply ceramic enamel by injecting a compressed gas containing ceramic powder into the flame of a hydrocarbon-oxygen torch burning at about 2500° C (about 4500° F). The semimolten powder particles adhere to the metal, cooling to form a hard enamel. Household appliances, such as refrigerators, stoves, washing machines, and dryers, are often coated with ceramic enamel.

Plastics

INTRODUCTION

Plastics, materials made up of large, organic (carbon-containing) molecules that can be formed into a variety of products. The molecules that compose plastics are long carbon chains that give plastics many of their useful properties. In general, materials that are made up of long, chainlike molecules are called polymers. The word plastic is derived from the words plasticus (Latin for “capable of molding”) and plastikos (Greek “to mold,” or “fit for molding”). Plastics can be made hard as stone, strong as steel, transparent as glass, light as wood, and elastic as rubber. Plastics are also lightweight, waterproof, chemical resistant, and produced in almost any color. More than 50 families of plastics have been produced, and new types are currently under development.

Like metals, plastics come in a variety of grades. For instance, nylons are plastics that are separated by different properties, costs, and the manufacturing processes used to produce them. Also like metals, some plastics can be alloyed, or blended, to combine the advantages possessed by several different plastics. For example, some types of impact-resistant (shatterproof) plastics and heat-resistant plastics are made by blending different plastics together.

Plastics are moldable, synthetic (chemically-fabricated) materials derived mostly from fossil fuels, such as oil, coal, or natural gas. The raw forms of other materials, such as glass, metals, and clay, are also moldable. The key difference between these materials and plastics is that plastics consist of long molecules that give plastics many of their unique properties, while glass, metals, and clay consist of short molecules.

USES OF PLASTICS

Plastics are indispensable to our modern way of life. Many people sleep on pillows and mattresses filled with a type of plastic—either cellular polyurethane or polyester. At night, people sleep under blankets and bedspreads made of acrylic plastics, and in the morning, they step out of bed onto polyester and nylon carpets. The cars we drive, the computers we use, the utensils we cook with, the recreational equipment we play with, and the houses and buildings we live and work in all include important plastic components. The average 1998-model car contains almost 136 kg (almost 300 lb) of plastics—nearly 12 percent of the vehicle’s overall weight. Telephones, textiles, compact discs, paints, plumbing fixtures, boats, and furniture are other domestic products made of plastics. In 1979 the volume of plastics produced in the United States surpassed the volume of domestically produced steel.

Plastics are used extensively by many key industries, including the automobile, aerospace, construction, packaging, and electrical industries. The aerospace industry uses plastics to make strategic military parts for missiles, rockets, and aircraft. Plastics are also used in specialized fields, such as the health industry, to make medical instruments, dental fillings, optical lenses, and biocompatible joints.

GENERAL PROPERTIES OF PLASTICS

Plastics possess a wide variety of useful properties and are relatively inexpensive to produce. They are lighter than many materials of comparable strength, and unlike metals and wood, plastics do not rust or rot. Most plastics can be produced in any color. They can also be manufactured as clear as glass, translucent (transmitting small amounts of light), or opaque (impenetrable to light).

Plastics have a lower density than that of metals, so plastics are lighter. Most plastics vary in density from 0.9 to 2.2 g/cm3 (0.45 to 1.5 oz/cu in), compared to steel’s density of 7.85 g/cm3 (5.29 oz/cu in). Plastic can also be reinforced with glass and other fibers to form incredibly strong materials. For example, nylon reinforced with glass can have a tensile strength (resistance of a material to being elongated or pulled apart) of up to 165 Mega Pascal (24,000 psi).

Plastics have some disadvantages. When burned, some plastics produce poisonous fumes. Although certain plastics are specifically designed to withstand temperatures as high as 288° C (550° F), in general plastics are not used when high heat resistance is needed. Because of their molecular stability, plastics do not easily break down into simpler components. As a result, disposal of plastics creates a solid waste problem.

CHEMISTRY OF PLASTICS

Plastics consist of very long molecules each composed of carbon atoms linked into chains. One type of plastic, known as polyethylene, is composed of extremely long molecules that each contain over 200,000 carbon atoms. These long, chainlike molecules give plastics unique properties and distinguish plastics from materials, such as metals, that have short, crystalline molecular structures.

Although some plastics are made from plant oils, the majority are made from fossil fuels. Fossil fuels contain hydrocarbons (compounds containing hydrogen and carbon), which provide the building blocks for long polymer molecules. These small building blocks, called monomers, link together to form long carbon chains called polymers. The process of forming these long molecules from hydrocarbons is known as polymerization. The molecules typically form viscous, sticky substances known as resins, which are used to make plastic products.

Ethylene, for example, is a gaseous hydrocarbon. When it is subjected to heat, pressure, and certain catalysts (substances used to enable faster chemical reactions), the ethylene molecules join together into long, repeating carbon chains. These joined molecules form a plastic resin known as polyethylene.

Joining identical monomers to make carbon chains is called addition polymerization, because the process is similar to stringing many identical beads on a string. Plastics made by addition polymerization include polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Joining two or more different monomers of varying lengths is known as condensation polymerization, because water or other by-products are eliminated as the polymer forms. Condensation polymers include nylon (polyamide), polyester, and polyurethane.

The properties of a plastic are determined by the length of the plastic’s molecules and the specific monomer present. For example, elastomers are plastics composed of long, tightly twisted molecules. These coiled molecules allow the plastic to stretch and recoil like a spring. Rubber bands and flexible silicone caulking are examples of elastomers.

The carbon backbone of polymer molecules often bonds with smaller side chains consisting of other elements, including chlorine, fluorine, nitrogen, and silicon. These side chains give plastics some distinguishing characteristics. For example, when chlorine atoms substitute for hydrogen atoms along the carbon chain, the result is polyvinyl chloride, one of the most versatile and widely used plastics in the world. The addition of chlorine makes this plastic harder and more heat resistant.

Different plastics have advantages and disadvantages associated with the unique chemistry of each plastic. For example, longer polymer molecules become more entangled (like spaghetti noodles), which gives plastics containing these longer polymers high tensile strength and high impact resistance. However, plastics made from longer molecules are more difficult to mold.

THERMOPLASTICS AND THERMOSETTING PLASTICS

All plastics, whether made by addition or condensation polymerization, can be divided into two groups: thermoplastics and thermosetting plastics. These terms refer to the different ways these types of plastics respond to heat. Thermoplastics can be repeatedly softened by heating and hardened by cooling. Thermosetting plastics, on the other hand, harden permanently after being heated once.

The reason for the difference in response to heat between thermoplastics and thermosetting plastics lies in the chemical structures of the plastics. Thermoplastic molecules, which are linear or slightly branched, do not chemically bond with each other when heated. Instead, thermoplastic chains are held together by weak van der Waal forces (weak attractions between the molecules) that cause the long molecular chains to clump together like piles of entangled spaghetti. Thermoplastics can be heated and cooled, and consequently softened and hardened, repeatedly, like candle wax. For this reason, thermoplastics can be remolded and reused almost indefinitely.

Thermosetting plastics consist of chain molecules that chemically bond, or cross-link, with each other when heated. When thermosetting plastics cross-link, the molecules create a permanent, three-dimensional network that can be considered one giant molecule. Once cured, thermosetting plastics cannot be remelted, in the same way that cured concrete cannot be reset. Consequently, thermosetting plastics are often used to make heat-resistant products, because these plastics can be heated to temperatures of 260° C (500° F) without melting.

The different molecular structures of thermoplastics and thermosetting plastics allow manufacturers to customize the properties of commercial plastics for specific applications. Because thermoplastic materials consist of individual molecules, properties of thermoplastics are largely influenced by molecular weight. For instance, increasing the molecular weight of a thermoplastic material increases its tensile strength, impact strength, and fatigue strength (ability of a material to withstand constant stress). Conversely, because thermosetting plastics consist of a single molecular network, molecular weight does not significantly influence the properties of these plastics. Instead, many properties of thermosetting plastics are determined by adding different types and amounts of fillers and reinforcements, such as glass fibers.

Thermoplastics may be grouped according to the arrangement of their molecules. Highly aligned molecules arrange themselves more compactly, resulting in a stronger plastic. For example, molecules in nylon are highly aligned, making this thermoplastic extremely strong. The degree of alignment of the molecules also determines how transparent a plastic is. Thermoplastics with highly aligned molecules scatter light, which makes these plastics appear opaque. Thermoplastics with semialigned molecules scatter some light, which makes most of these plastics appear translucent. Thermoplastics with random (amorphous) molecular arrangement do not scatter light and are clear. Amorphous thermoplastics are used to make optical lenses, windshields, and other clear products.

MANUFACTURING PLASTIC PRODUCTS

The process of forming plastic resins into plastic products is the basis of the plastics industry. Many different processes are used to make plastic products, and in each process, the plastic resin must be softened or sufficiently liquefied to be shaped.

Forming Thermoplastics

Although some processes are used to manufacture both thermoplastics and thermosetting plastics, certain processes are specific to forming thermoplastics.

Injection Molding

Injection molding uses a piston or screw to force plastic resin through a heated tube into a mold, where the plastic cools and hardens to the shape of the mold. The mold is then opened and the plastic cast removed. Thermoplastic items made by injection molding include toys, combs, car grills, and various containers.

Extrusion

Extrusion is a continuous process, as opposed to all other plastic production processes, which start over at the beginning of the process after each new part is removed from the mold. In the extrusion process, plastic pellets are first heated in a long barrel. In a manner similar to that of a pasta-making or sausage-stuffing machine, a rotating screw then forces the heated plastic through a die (device used for forming material) opening of the desired shape.

As the continuous plastic form emerges from the die opening, it is cooled and solidified, and the continuous plastic form is then cut to the desired length. Plastic products made by extrusion include garden hoses, drinking straws, pipes, and ropes. Melted thermoplastic forced through extremely fine die holes can be cooled and woven into fabrics for clothes, curtains, and carpets.

Blow Molding

Blow molding is used to form bottles and other containers from soft, hollow thermoplastic tubes. First a mold is fitted around the outside of the softened thermoplastic tube, and then the tube is heated. Next, air is blown into the softened tube (similar to inflating a balloon), which forces the outside of the softened tube to conform to the inside walls of the mold. Once the plastic cools, the mold is opened and the newly molded container is removed. Blow molding is used to make many plastic containers, including soft-drink bottles, jars, detergent bottles, and storage drums.

Blow Film Extrusion

Blow film extrusion is the process used to make plastic garbage bags and continuous sheets. This process works by extruding a hollow, sealed-end thermoplastic tube through a die opening. As the flattened plastic tube emerges from the die opening, air is blown inside the hollow tube to stretch and thin the tube (like a balloon being inflated) to the desired size and wall thickness.

The plastic is then air-cooled and pulled away on take-up rollers to a heat-sealing operation. The heat-sealer cuts and seals one end of the thinned, flattened thermoplastic tube, creating various bag lengths for products such as plastic grocery and garbage bags. For sheeting (flat film), the thinned plastic tube is slit along one side and opened to form a continuous sheet.

Calendering

The calendering process forms continuous plastic sheets that are used to make flooring, wall siding, tape, and other products. These plastic sheets are made by forcing hot thermoplastic resin between heated rollers called calenders. A series of secondary calenders further thins the plastic sheets. Paper, cloth, and other plastics may be pressed between layers of calendered plastic to make items such as credit cards, playing cards, and wallpaper.

Thermoforming

Thermoforming is a term used to describe several techniques for making products from plastic sheets. Products made from thermoformed sheets include trays, signs, briefcase shells, refrigerator door liners, and packages. In a vacuum-forming process, hot thermoplastic sheets are draped over a mold. Air is removed from between the mold and the hot plastic, which creates a vacuum that draws the plastic into the cavities of the mold. When the plastic cools, the molded product is removed. In the pressure-forming process, compressed air is used to drive a hot plastic sheet into the cavities and depressions of a concave, or female, mold. Vent holes in the bottom of the mold allow trapped air to escape.

Forming Thermosetting Plastics

Thermosetting plastics are manufactured by several methods that use heat or pressure to induce polymer molecules to bond, or cross-link, into typically hard and durable products.

Compression Molding

Compression molding forms plastics through a technique that is similar to the way a waffle iron forms waffles from batter. First, thermosetting resin is placed into a steel mold. The application of heat and pressure, which accelerate cross-linking of the resin, softens the material and squeezes it into all parts of the mold to form the desired shape. Once the material has cooled and hardened, the newly formed object is removed from the mold. This process creates hard, heat-resistant plastic products, including dinnerware, telephones, television set frames, and electrical parts.

Laminating

The laminating process binds layers of materials, such as textiles and paper, together in a plastic matrix. This process is similar to the process of joining sheets of wood to make plywood. Resin-impregnated layers of textiles or paper are stacked on hot plates, then squeezed and fused together by heat and pressure, which causes the polymer molecules to cross-link. The best-known laminate trade name is Formica, which is a product consisting of resin-impregnated layers of paper with decorative patterns such as wood grain, marble, and colored designs. Formica is often used as a surface finish for furniture, and kitchen and bathroom countertops. Thermosetting resins known as melamine and phenolic resins form the plastic matrix for Formica and other laminates. Electric circuit boards are also laminated from resin-impregnated paper, fabric, and glass fibers.

Reaction Injection Molding (RIM)

Strong, sizable, and durable plastic products such as automobile body panels, skis, and business machine housings are formed by reaction injection molding. In this process, liquid thermosetting resin is combined with a curing agent (a chemical that causes the polymer molecules to cross-link) and injected into a mold. Most products made by reaction injection molding are made from polyurethane.

Forming Both Types of Plastics

Certain plastic fabrication processes can be used to form either thermoplastics or thermosetting plastics.

Casting

The casting process is similar to that of molding plaster or cement. Fluid thermosetting or thermoplastic resin is poured into a mold, and additives cause the resin to solidify. Photographic film is made by pouring a fluid solution of resin onto a highly polished metal belt. A thin plastic film remains as the solution evaporates. The casting process is also used to make furniture parts, tabletops, sinks, and acrylic window sheets.

Expansion Processes

Thermosetting and thermoplastic resins can be expanded by injecting gases (often nitrogen or methyl chloride) into the plastic melt. As the resin cools, tiny bubbles of gas are trapped inside, forming a cellular plastic structure. This process is used to make foam products such as cushions, pillows, sponges, egg cartons, and polystyrene cups.

Foam plastics can be classified according to their bubble, or cell, structure. Sponges and carpet pads are examples of open-celled foam plastics, in which the bubbles are interconnected. Flotation devices are examples of closed-celled foam plastics, in which the bubbles are sealed like tiny balloons. Foam plastics can also be classified by density (ratio of plastic to cells), by the type of plastic resin used, and by flexibility (rigid or flexible foam). For example, rigid, closed-celled polyurethane plastics make excellent insulation for refrigerators and freezers.

IMPORTANT TYPES OF PLASTICS

A wide variety of both thermoplastics and thermosetting plastics are manufactured. These plastics have a spectrum of properties that are derived from their chemical compositions. As a result, manufactured plastics can be used in applications ranging from contact lenses to jet body components.

Thermoplastics

Thermoplastic materials are in high demand because they can be repeatedly softened and remolded. The most commonly manufactured thermoplastics are presented in this section in order of decreasing volume of production.

Polyethylene

Polyethylene (PE) resins are milky white, translucent substances derived from ethylene (CH29CH2). Polyethylene, with the chemical formula [8CH28CH28]n (where n denotes that the chemical formula inside the brackets repeats itself to form the plastic molecule) is made in low- and high-density forms. Low-density polyethylene (LDPE) has a density ranging from 0.91 to 0.93 g/cm3 (0.60 to 0.61 oz/cu in). The molecules of LDPE have a carbon backbone with side groups of four to six carbon atoms attached randomly along the main backbone. LDPE is the most widely used of all plastics, because it is inexpensive, flexible, extremely tough, and chemical-resistant. LDPE is molded into bottles, garment bags, frozen food packages, and plastic toys.

High-density polyethylene (HDPE) has a density that ranges from 0.94 to 0.97 g/cm3 (0.62 to 0.64 oz/cu in). Its molecules have an extremely long carbon backbone with no side groups. As a result, these molecules align into more compact arrangements, accounting for the higher density of HDPE. HDPE is stiffer, stronger, and less translucent than low-density polyethylene. HDPE is formed into grocery bags, car fuel tanks, packaging, and piping.

Polyvinyl Chloride

Polyvinyl chloride (PVC) is prepared from the organic compound vinyl chloride (CH29CHCl). PVC is the most widely used of the amorphous plastics. PVC is lightweight, durable, and waterproof. Chlorine atoms bonded to the carbon backbone of its molecules give PVC its hard and flame-resistant properties.

In its rigid form, PVC is weather-resistant and is extruded into pipe, house siding, and gutters. Rigid PVC is also blow molded into clear bottles and is used to form other consumer products, including compact discs and computer casings.

PVC can be softened with certain chemicals. This softened form of PVC is used to make shrink-wrap, food packaging, rainwear, shoe soles, shampoo containers, floor tile, gloves, upholstery, and other products. Most softened PVC plastic products are manufactured by extrusion, injection molding, or casting.

Polypropylene

Polypropylene is polymerized from the organic compound propylene (CH38CH9CH2) and has a methyl group (8CH3) branching off of every other carbon along the molecular backbone. Because the most common form of polypropylene has the methyl groups all on one side of the carbon backbone, polypropylene molecules tend to be highly aligned and compact, giving this thermoplastic the properties of durability and chemical resistance. Many polypropylene products, such as rope, fiber, luggage, carpet, and packaging film, are formed by injection molding.

Polystyrene

Polystyrene, produced from styrene (C6H5CH9CH2), has phenyl groups (six-member carbon ring) attached in random locations along the carbon backbone of the molecule. The random attachment of benzene prevents the molecules from becoming highly aligned. As a result, polystyrene is an amorphous, transparent, and somewhat brittle plastic. Polystyrene is widely used because of its rigidity and superior insulation properties. Polystyrene can undergo all thermoplastic processes to form products such as toys, utensils, display boxes, model aircraft kits, and ballpoint pen barrels. Polystyrene is also expanded into foam plastics such as packaging materials, egg cartons, flotation devices, and styrofoam.

Polyethylene Terephthalate

Polyethylene terephthalate (PET) is formed from the reaction of terephthalic acid (HOOC8C6H48COOH) and ethylene glycol (HOCH28CH2OH), which produces the PET monomer [8OOC8C6H48COO8CH2CH28]n. PET molecules are highly aligned, creating a strong and abrasion-resistant material that is used to produce films and polyester fibers. PET is injection molded into windshield wiper arms, sunroof frames, gears, pulleys, and food trays. This plastic is used to make the trademarked textiles Dacron, Fibre V, Fortrel, and Kodel. Tough, transparent PET films (marketed under the brand name Mylar) are magnetically coated to make both audio and video recording tape.

Acrylonitrile Butadiene Styrene

Acrylonitrile butadiene styrene (ABS) is made by copolymerizing (combining two or more monomers) the monomers acrylonitrile (CH2CHCN) and styrene (C6H5CH9CH2). Acrylonitrile and styrene are dissolved in polybutadiene rubber [8CH9CH8CH9CH8] n, which allows these monomers to form chains by attaching to the rubber molecules.

The advantage of ABS is that this material combines the strength and rigidity of the acrylonitrile and styrene polymers with the toughness of the polybutadiene rubber. Although the cost of producing ABS is roughly twice the cost of producing polystyrene, ABS is considered superior for its hardness, gloss, toughness, and electrical insulation properties. ABS plastic is injection molded to make telephones, helmets, washing machine agitators, and pipe joints. This plastic is thermoformed to make luggage, golf carts, toys, and car grills. ABS is also extruded to make piping, to which pipe joints are easily solvent-cemented.

Polymethyl Methacrylate

Polymethyl methacrylate (PMMA), more commonly known by the generic name acrylic, is polymerized from the hydrocarbon compound methyl methacrylate (C4O2H8). PMMA is a hard material and is extremely clear because of the amorphous arrangement of its molecules. As a result, this thermoplastic is used to make optical lenses, watch crystals, aircraft windshields, skylights, and outdoor signs. These PMMA products are marketed under familiar trade names, including Plexiglas, Lucite, and Acrylite. Because PMMA can be cast to resemble marble, it is also used to make sinks, countertops, and other fixtures.

Polyamide

Polyamides (PA), known by the trade name Nylon, consist of highly ordered molecules, which give polyamides high tensile strength. Some polyamides are made by reacting dicarboxylic acid with diamines (carbon molecules with the ion –NH2 on each end), as in nylon-6,6 and nylon-6,10. (The two numbers in each type of nylon represent the number of carbon atoms in the diamine and the dicarboxylic acid, respectively.) Other types of nylon are synthesized by the condensation of amino acids.

Polyamides have mechanical properties such as high abrasion resistance, low coefficients of friction (meaning they are slippery), and tensile strengths comparable to the softer of the aluminum alloys. Therefore, nylons are commonly used for mechanical applications, such as gears, bearings, and bushings. Nylons are also extruded into millions of tons of synthetic fibers every year. The most commonly used nylon fibers, nylon-6,6 and nylon-6 (single number because this nylon forms by the self-condensation of an amino acid) are made into textiles, ropes, fishing lines, brushes, and other items.

Thermosetting Materials

Because thermosetting plastics cure, or cross-link, after being heated, these plastics can be made into durable and heat-resistant materials. The most commonly manufactured thermosetting plastics are presented below in order of decreasing volume of production.

Polyurethane

Polyurethane is a polymer consisting of the repeating unit [8R8OOCNH8R’8]n, where R may represent a different alkyl group than R’. Alkyl groups are chemical groups obtained by removing a hydrogen atom from an alkane—a hydrocarbon containing all carbon-carbon single bonds. Most types of polyurethane resin cross-link and become thermosetting plastics. However, some polyurethane resins have a linear molecular arrangement that does not cross-link, resulting in thermoplastics.

Thermosetting polyurethane molecules cross-link into a single giant molecule. Thermosetting polyurethane is widely used in various forms, including soft and hard foams. Soft, open-celled polyurethane foams are used to make seat cushions, mattresses, and packaging. Hard polyurethane foams are used as insulation in refrigerators, freezers, and homes.

Thermoplastic polyurethane molecules have linear, highly crystalline molecular structures that form an abrasion-resistant material. Thermoplastic polyurethanes are molded into shoe soles, car fenders, door panels, and other products.

Phenolics

Phenolic (phenol-formaldehyde) resins, first commercially available in 1910, were some of the first polymers made. Today phenolics are some of the most widely produced thermosetting plastics. They are produced by reacting phenol (C6H5OH) with formaldehyde (HCOH). Phenolic plastics are hard, strong, inexpensive to produce, and they possess excellent electrical resistance. Phenolic resins cure (cross-link) when heat and pressure are applied during the molding process. Phenolic resin-impregnated paper or cloth can be laminated into numerous products, such as electrical circuit boards. Phenolic resins are also compression molded into electrical switches, pan and iron handles, radio and television casings, and toaster knobs and bases.

Melamine-Formaldehyde and Urea-Formaldehyde

Urea-formaldehyde (UF) and melamine-formaldehyde (MF) resins are composed of molecules that cross-link into clear, hard plastics. Properties of UF and MF resins are similar to the properties of phenolic resins. As their names imply, these resins are formed by condensation reactions between urea (H2NCONH2) or melamine (C3H6N6) and formaldehyde (CH2O).

Melamine-formaldehyde resins are easily molded in compression and special injection molding machines. MF plastics are more heat-resistant, scratch-proof, and stain-resistant than urea-formaldehyde plastics are. MF resins are used to manufacture dishware, electrical components, laminated furniture veneers, and to bond wood layers into plywood.

Urea-formaldehyde resins form products such as appliance knobs, knife handles, and plates. UF resins are used to give drip-dry properties to wash-and-wear clothes as well as to bond wood chips and wood sheets into chip board and plywood.

Unsaturated Polyesters

Unsaturated polyesters (UP) belong to the polyester group of plastics. Polyesters are composed of long carbon chains containing [8OOC8C6H48COO8CH28CH2]n. Unsaturated polyesters (an unsaturated compound contains multiple bonds) cross-link when the long molecules are joined (copolymerized) by the aromatic organic compound styrene.

Unsaturated polyester resins are often premixed with glass fibers for additional strength. Two types of premixed resins are bulk molding compounds (BMC) and sheet molding compounds (SMC). Both types of compounds are doughlike in consistency and may contain short fiber reinforcements and other additives. Sheet molding compounds are preformed into large sheets or rolls that can be molded into products such as shower floors, small boat hulls, and roofing materials. Bulk molding compounds are also preformed to be compression molded into car body panels and other automobile components.

Epoxy

Epoxy (EP) resins are named for the epoxide groups (cycl-CH2OCH; cycl or cyclic refers to the triangle formed by this group) that terminate the molecules. The oxygen along epoxy’s carbon chain and the epoxide groups at the ends of the carbon chain give epoxy resins some useful properties. Epoxies are tough, extremely weather-resistant, and do not shrink as they cure (dry).

Epoxies cross-link when a catalyzing agent (hardener) is added, forming a three-dimensional molecular network. Because of their outstanding bonding strength, epoxy resins are used to make coatings, adhesives, and composite laminates. Epoxy has important applications in the aerospace industry. All composite aircraft are made of epoxy. Epoxy is used to make the wing skins for the F-18 and F-22 fighters, as well as the horizontal stabilizer for the F-16 fighter and the B-1 bomber. In addition, almost 20 percent of the Harrier jet’s total weight is composed of reinforcements bound with an epoxy matrix (see Airplane). Because of epoxy’s chemical resistance and excellent electrical insulation properties, electrical parts such as relays, coils, and transformers are insulated with epoxy.

Reinforced Plastics

Reinforced plastics, called composites, are plastics strengthened with fibers, strands, cloth, or other materials. Thermosetting epoxy and polyester resins are commonly used as the polymer matrix (binding material) in reinforced plastics. Due to a combination of strength and affordability, glass fibers, which are woven into the product, are the most common reinforcing material. Organic synthetic fibers such as aramid (an aromatic polyamide with the commercial name Kevlar) offer greater strength and stiffness than glass fibers, but these synthetic fibers are considerably more expensive.

The Boeing 777 aircraft makes extensive use of lightweight reinforced plastics. Other products made from reinforced plastics include boat hulls and automobile body panels, as well as recreation equipment, such as tennis rackets, golf clubs, and jet skis.

HISTORY OF PLASTICS

Humankind has been using natural plastics for thousands of years. For example, the early Egyptians soaked burial wrappings in natural resins to help preserve their dead. People have been using animal horns and turtle shells (which contain natural resins) for centuries to make items such as spoons, combs, and buttons.

During the mid-19th century, shellac (resinous substance secreted by the lac insect) was gathered in southern Asia and transported to the United States to be molded into buttons, small cases, knobs, phonograph records, and hand-mirror frames. During that time period, gutta-percha (rubberlike sap taken from certain trees in Malaya) was used as the first insulating coating for electrical wires.

In order to find more efficient ways to produce plastics and rubbers, scientists began trying to produce these materials in the laboratory. In 1839 American inventor Charles Goodyear vulcanized rubber by accidentally dropping a piece of sulfur-treated rubber onto a hot stove. Goodyear discovered that heating sulfur and rubber together improved the properties of natural rubber so that it would no longer become brittle when cold and soft when hot. In 1862 British chemist Alexander Parkes synthesized a plastic known as pyroxylin, which was used as a coating film on photographic plates. The following year, American inventor John W. Hyatt began working on a substitute for ivory billiard balls. Hyatt added camphor to nitrated cellulose and formed a modified natural plastic called celluloid, which became the basis of the early plastics industry. Celluloid was used to make products such as umbrella handles, dental plates, toys, photographic film, and billiard balls.

These early plastics based on natural products shared numerous drawbacks. For example, many of the necessary natural materials were in short supply, and all proved difficult to mold. Finished products were inconsistent from batch to batch, and most products darkened and cracked with age. Furthermore, celluloid proved to be a very flammable material.

Due to these shortcomings, scientists attempted to find more reliable plastic source materials. In 1909 American chemist Leo Hendrik Baekeland made a breakthrough when he created the first commercially successful thermosetting synthetic resin, which was called Bakelite (known today as phenolic resin). Use of Bakelite quickly grew. It has been used to make products such as telephones and pot handles.

The chemistry of joining small molecules into macromolecules became the foundation of an emerging plastics industry. Between 1920 and 1932, the I.G. Farben Company of Germany synthesized polystyrene and polyvinyl chloride, as well as a synthetic rubber called Buna-S. In 1934 Du Pont made a breakthrough when it introduced nylon—a material finer, stronger, and more elastic than silk. By 1936 acrylics were being produced by German, British, and U.S. companies. That same year, the British company Imperial Chemical Industries developed polyethylene. In 1937 polyurethane was invented by the German company Friedrich Bayer & Co. (see Bayer AG), but this plastic was not available to consumers until it was commercialized by U.S. companies in the 1950s. In 1939 the German company I.G. Farbenindustrie filed a patent for polyepoxide (epoxy), which was not sold commercially until a U.S. firm made epoxy resins available to the consumer market almost four years later.

After World War II (1939-1945), the pace of new polymer discoveries accelerated. In 1941 a small English company developed polyethylene terephthalate (PET). Although Du Pont and Imperial Chemical Industries produced PET fibers (marketed under the names Dacron and Terylene, respectively) during the postwar era, the use of PET as a material for making bottles, films, and coatings did not become widespread until the 1970s. In the postwar era, research by Bayer and by General Electric resulted in production of plastics such as polycarbonates, which are used to make small appliances, aircraft parts, and safety helmets. In 1965 Union Carbide Corporation introduced a linear, heat-resistant thermoplastic known as polysulfone, which is used to make face shields for astronauts and hospital equipment that can be sterilized in an autoclave (a device that uses high pressure steam for sterilization).

Today, scientists can tailor the properties of plastics to numerous design specifications. Modern plastics are used to make products such as artificial joints, contact lenses, space suits, and other specialized materials. As plastics have become more versatile, use of plastics has grown as well. By the year 2005, annual global demand for plastics is projected to exceed 200 million metric tons (441 billion lb).

PLASTICS AND THE ENVIRONMENT

Every year in the United States, consumers throw millions of tons of plastic away—of the estimated 190 million metric tons (420 billion pounds) of municipal waste produced annually in the United States, about 9 percent are plastics. As municipal landfills reach capacity and additional landfill space diminishes across the United States, alternative methods for reducing and disposing of wastes—including plastics—are being explored. Some of these options include reducing consumption of plastics, using biodegradable plastics, and incinerating or recycling plastic waste.

Source Reduction

Source reduction is the practice of using less material to manufacture a product. For example, the wall thickness of many plastic and metal containers has been reduced in recent years, and some European countries have proposed to eliminate packaging that cannot be easily recycled.

Biodegradable Plastics

Due to their molecular stability, plastics do not easily break down into simpler components. Plastics are therefore not considered biodegradable. However, researchers are working to develop biodegradable plastics that will disintegrate due to bacterial action or exposure to sunlight. For example, scientists are incorporating starch molecules into some plastic resins during the manufacturing process. When these plastics are discarded, bacteria eat the starch molecules. This causes the polymer molecules to break apart, allowing the plastic to decompose. Researchers are also investigating ways to make plastics more biodegradable from exposure to sunlight. Prolonged exposure to ultraviolet radiation from the sun causes many plastics molecules to become brittle and slowly break apart. Researchers are working to create plastics that will degrade faster in sunlight, but not so fast that the plastic begins to degrade while still in use.

Incineration

Some wastes, such as paper, plastics, wood, and other flammable materials can be burned in incinerators. The resulting ash requires much less space for disposal than the original waste would. Because incineration of plastics can produce hazardous air emissions and other pollutants, this process is strictly regulated.

Recycling Plastics

All plastics can be recycled. Thermoplastics can be remelted and made into new products. Thermosetting plastics can be ground, commingled (mixed), and then used as filler in moldable thermoplastic materials. Highly filled and reinforced thermosetting plastics can be pulverized and used in new composite formulations.

Chemical recycling is a depolymerization process that uses heat and chemicals to break plastic molecules down into more basic components, which can then be reused. Another process, called pyrolysis, vaporizes and condenses both thermoplastics and thermosetting plastics into hydrocarbon liquids.

Collecting and sorting used plastics is an expensive and time-consuming process. While about 35 percent of aluminum products, 40 percent of paper products, and 25 percent of glass products are recycled in the United States, only about 5 percent of plastics are currently recovered and recycled. Once plastic products are thrown away, they must be collected and then separated by plastic type. Most modern automated plastic sorting systems are not capable of differentiating between many different types of plastics. However, some advances are being made in these sorting systems to separate plastics by color, density, and chemical composition. For example, x-ray sensors can distinguish PET from PVC by sensing the presence of chlorine atoms in the polyvinyl chloride material.

If plastic types are not segregated, the recycled plastic cannot achieve high remolding performance, which results in decreased market value of the recycled plastic. Other factors can adversely affect the quality of recycled plastics. These factors include the possible degradation of the plastic during its original life cycle and the possible addition of foreign materials to the scrap recycled plastic during the recycling process. For health reasons, recycled plastics are rarely made into food containers. Instead, most recycled plastics are typically made into items such as carpet fibers, motor oil bottles, trash carts, soap packages, and textile fibers.

To promote the conservation and recycling of materials, the U.S. federal government passed the Resource Conservation and Recovery Act (RCRA) in 1976. In 1988 the Plastic Bottle Institute of the Society of the Plastics Industry established a system for identifying plastic containers by plastic type. The purpose of the "chasing arrows" symbol that appears on the bottom of many plastic containers is to promote plastics recycling. The chasing arrows enclose a number (such as a 1 indicating PET, a 2 indicating high density polyethylene (HDPE), and a 3 indicating PVC), which aids in the plastics sorting process.

By 1994, 40 states had legislative mandates for litter control and recycling. Today, a growing number of communities have collection centers for recyclable materials, and some larger municipalities have implemented curbside pickup for recyclable materials, including plastics, paper, metal, and glass.


Statistical Mechanics

Statistical Mechanics, in physics, field that seeks to predict the average properties of systems that consist of a very large number of particles. Statistical mechanics employs principles of statistics to predict and describe particle motion.

Statistical mechanics was developed in the 19th century, largely by British physicist James Clerk Maxwell, Austrian physicist Ludwig Boltzmann, and American mathematical physicist J. Willard Gibbs. These scientists believed that matter is composed of many tiny particles (atoms and molecules) in constant motion. These scientists knew that determining the motions of the particles by assuming each particle individually obeys Newtonian mechanics is unworkable, because any sample of matter contains an enormous number of particles. For example, a cubic foot of air contains about a trillion trillion (1 followed by 24 zeroes) particles. Rather than dealing with all of these microscopic particles individually, Maxwell, Boltzmann, and Gibbs developed statistical techniques to average the microscopic dynamics of individual particles and obtain their macroscopic (large-scale) thermodynamic features. Through their calculations they discovered that temperature is a measure of the average kinetic energy of microscopic particles. They also found that entropy is proportional to the logarithm of the number of ways a given macroscopic system can be microscopically arranged.

Statistical mechanics had to be extended in the 1920s to incorporate the new principles of quantum theory. The nature of particles is regarded differently in quantum theory than in classical physics, which is based on Newton's laws of motion. In particular, two classical particles are in principle distinguishable; just as two cue balls can be distinguished by placing an identifying mark on one, so in principle can classical particles. In contrast, two identical quantum particles are indistinguishable, even in principle, requiring new formulations of statistical mechanics. Furthermore, there are two quantum mechanical formulations of statistical mechanics corresponding to the two types of quantum particles—fermions and bosons. The formulation of statistical mechanics designed to describe the behavior of a group of classical particles is called Maxwell-Boltzmann (MB) statistics. The two formulations of statistical mechanics used to describe quantum particles are Fermi-Dirac (FD) statistics, which applies to fermions, and Bose-Einstein (BE) statistics, which applies to bosons.

Two formulations of quantum statistical mechanics are needed because fermions and bosons have significantly different properties. Fermions—particles that have odd half-integer spin—obey the Pauli exclusion principle, which states that two fermions cannot be in the same quantum mechanical state. Some examples of fermions are electrons, protons, and helium-3. On the other hand, bosons—particles that have integer spin—do not obey the Pauli exclusion principle. Some examples of bosons are photons and helium-4. While only one fermion at a time can be in a particular quantum mechanical state, it is possible for multiple bosons to be in a single state.

The phenomenon of superconductivity dramatically illustrates the differences between systems of quantum mechanical particles that obey Bose-Einstein statistics instead of Fermi-Dirac statistics. At room temperature, electrons, which have spin y, are distributed among their possible energy states according to FD statistics. At very low temperatures, the electrons pair up to form spin-0 Cooper electron pairs, named after the American physicist Leon Cooper. Since these electron pairs have zero spin, they behave as bosons, and promptly condense into the same ground state. A large energy gap between this ground state and the first excited state ensures that any current is “frozen in.” This causes the current to flow through without resistance, which is one of the defining properties of superconducting materials.

Friction

INTRODUCTION

Friction, force that opposes the motion of an object when the object is in contact with another object or surface. Friction results from two surfaces rubbing against each other or moving relative to one another. It can hinder the motion of an object or prevent an object from moving at all. The strength of frictional force depends on the nature of the surfaces that are in contact and the force pushing them together. This force is usually related to the weight of the object or objects. In cases involving fluid friction, the force depends upon the shape and speed of an object as it moves through air, water, or other fluid.

Friction occurs to some degree in almost all situations involving physical objects. In many cases, such as in a running automobile engine, it hinders a process. For example, friction between the moving parts of an engine resists the engine’s motion and turns energy into heat, reducing the engine’s efficiency. Friction also makes it difficult to slide a heavy object, such as a refrigerator or bookcase, along the ground. In other cases, friction is helpful. Friction between people’s shoes and the ground allows people to walk by pushing off the ground without slipping. On a slick surface, such as ice, shoes slip and slide instead of gripping because of the lack of friction, making walking difficult. Friction allows car tires to grip and roll along the road without skidding. Friction between nails and beams prevents the nails from sliding out and keeps buildings standing.

When friction affects a moving object, it turns the object’s kinetic energy, or energy of motion, into heat. People welcome the heat caused by friction when rubbing their hands together to stay warm. Frictional heat is not so welcome when it damages machine parts, such as car brakes.

CAUSES OF FRICTION

Friction occurs in part because rough surfaces tend to catch on one another as they slide past each other. Even surfaces that are apparently smooth can be rough at the microscopic level. They have many ridges and grooves. The ridges of each surface can get stuck in the grooves of the other, effectively creating a type of mechanical bond, or glue, between the surfaces.

Two surfaces in contact also tend to attract one another at the molecular level, forming chemical bonds. These bonds can prevent an object from moving, even when it is pushed. If an object is in motion, these bonds form and release. Making and breaking the bonds takes energy away from the motion of the object.

Scientists do not yet fully understand the details of how friction works, but through experiments they have found a way to describe frictional forces in a wide variety of situations. The force of friction between an object and a surface is equal to a constant number times the force the object exerts directly on the surface. The constant number is called the coefficient of friction for the two materials and is abbreviated µ. The force the object exerts directly on the surface is called the normal force and is abbreviated N. Friction depends on this force because increasing the amount of force increases the amount of contact that the object has with the surface at the microscopic level. The force of friction between an object and a surface can be calculated from the following formula:

F = µ × N

In this equation, F is the force of friction, µ is the coefficient of friction between the object and the surface, and N is the normal force.

Scientists have measured the coefficient of friction for many combinations of materials. Coefficients of friction depend on whether the objects are initially moving or stationary and on the types of material involved. The coefficient of friction for rubber sliding on concrete is 0.8 (relatively high), while the coefficient for Teflon sliding on steel is 0.04 (relatively low).

The normal force is the force the object exerts perpendicular to the surface. In the case of a level surface, the normal force is equal to the weight of the object. If the surface is inclined, only a fraction of the object’s weight pushes directly into the surface, so the normal force is less than the object’s weight.

KINDS OF FRICTION

Different kinds of motion give rise to different types of friction between objects. Static friction occurs between stationary objects, while sliding friction occurs between objects as they slide against each other. Other types of friction include rolling friction and fluid friction. The coefficient of friction for two materials may differ depending on the type of friction involved.

Static friction prevents an object from moving against a surface. It is the force that keeps a book from sliding off a desk, even when the desk is slightly tilted, and that allows you to pick up an object without the object slipping through your fingers. In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting. This force depends on the coefficient of static friction (µs) between the object and the surface and the normal force (N) of the object.

A book sliding off a desk or brakes slowing down a wheel are both examples of sliding friction, also called kinetic friction. Sliding friction acts in the direction opposite the direction of motion. It prevents the book or wheel from moving as fast as it would without friction. When sliding friction is acting, another force must be present to keep an object moving. In the case of a book sliding off a desk, this force is gravity. The force of kinetic friction depends on the coefficient of kinetic friction between the object and the surface on which it is moving (µk) and the normal force (N) of the object. For any pair of objects, the coefficient of kinetic friction is usually less than the coefficient of static friction. This means that it takes more force to start a book sliding than it does to keep the book sliding.

Rolling friction hinders the motion of an object rolling along a surface. Rolling friction slows down a ball rolling on a basketball court or softball field, and it slows down the motion of a tire rolling along the ground. Another force must be present to keep an object rolling. For example, a pedaling bicyclist provides the force necessary to the keep a bike in motion. Rolling friction depends on the coefficient of rolling friction between the two materials (µr) and the normal force (N) of the object. The coefficient of rolling friction is usually about t that of sliding friction. Wheels and other round objects will roll along the ground much more easily than they will slide along it.

Objects moving through a fluid experience fluid friction, or drag. Drag acts between the object and the fluid and hinders the motion of the object. The force of drag depends upon the object’s shape, material, and speed, as well as the fluid’s viscosity. Viscosity is a measure of a fluid’s resistance to flow. It results from the friction that occurs between the fluid’s molecules, and it differs depending on the type of fluid. Drag slows down airplanes flying through the air and fish swimming through water. An airplane’s engines help it overcome drag and travel forward, while a fish uses its muscles to overcome drag and swim. Calculating the force of drag is much more complicated than calculating other types of friction.

EFFECTS OF FRICTION

Friction helps people convert one form of motion into another. For example, when people walk, friction allows them to convert a push backward along the ground into forward motion. Similarly, when car or bicycle tires push backward along the ground, friction with the ground makes the tires roll forward. Friction allows us to push and slide objects along the ground without our shoes slipping along the ground in the opposite direction.

While friction allows us to convert one form of motion to another, it also converts some energy into heat, noise, and wear and tear on material. Losing energy to these effects often reduces the efficiency of a machine. For example, a cyclist uses friction between shoes and pedals, the chain and gears, and the bicycle’s tires and the road to make the bicycle move forward. At the same time, friction between the chain and gears, between the tires and the road, and between the cyclist and the air all resist the cyclist’s motion. As the cyclist pedals, friction converts some of the cyclist’s energy into heat, noise, and wear and tear on the bicycle. This energy loss reduces the efficiency of the bicycle. In automobiles and airplanes, friction converts some of the energy in the fuel into heat, noise, and wear and tear on the engine’s parts. Excess frictional heat can damage an engine and braking system. The wearing away of material in engines makes it necessary to periodically replace some parts.

Sometimes the heat that friction produces is useful. When a person strikes a match against a rough surface, friction produces a large amount of heat on the head of the match and triggers the chemical process of burning. Static friction, which prevents motion, does not create heat.

REDUCING FRICTION

Reducing the amount of friction in a machine increases the machine’s efficiency. Less friction means less energy lost to heat, noise, and wearing down of material. People normally use two methods to reduce friction. The first method involves reducing the roughness of the surfaces in contact. For example, sanding two pieces of wood lessens the amount of friction that occurs between them when they slide against one another. Teflon creates very little friction because it is so smooth.

Applying a lubricant to a surface can also reduce friction. Common examples of lubricants are oil and grease. They reduce friction by minimizing the contact between rough surfaces. The lubricant’s particles slide easily against each other and cause far less friction than would occur between the surfaces. Lubricants such as machine oil reduce the amount of energy lost to frictional heating and reduce the wear damage to the machine surfaces caused by friction.


Materials Science and Technology

I

INTRODUCTION

Materials Science and Technology, the study of materials, nonmetallic as well as metallic, and how they can be adapted and fabricated to meet the needs of modern technology. Using the laboratory techniques and research tools of physics, chemistry, and metallurgy, scientists are finding new ways of using plastics, ceramics, and other nonmetals in applications formerly reserved for metals.

II

RECENT DEVELOPMENTS

The rapid development of semiconductors (see Semiconductor) for the electronics industry, beginning in the early 1960s, gave materials science its first major impetus. Having discovered that nonmetallic materials such as silicon could be made to conduct electricity in ways that metals could not, scientists and engineers devised ways of fashioning thousands of tiny integrated circuits (see Integrated Circuit) on a small chip of silicon. This then made it possible to miniaturize the components of electronic devices such as computers.

In the late 1980s, materials science research was given renewed emphasis with the discovery of ceramics that display superconductivity at higher temperatures than metals do. If the temperature at which these new materials become superconductive can be raised high enough, new applications, including levitating trains and superfast computers, are possible.

Although the latest developments in materials science have tended to focus on electrical properties, mechanical properties are also of major, continuing importance. For the aircraft industry, for instance, scientists have been developing, and engineers testing, nonmetallic composite materials that are lighter, stronger, and easier to fabricate than the aluminum and other metals currently used to form the outer skin of aircraft.

III

MECHANICAL PROPERTIES OF MATERIALS

Engineers must know how solid materials respond to external forces, such as tension, compression, torsion, bending, and shear. Solid materials respond to these forces by elastic deformation (that is, the material returns to its original size and form when the external force is lifted), permanent deformation, or fracture. Time-dependent effects of external forces are creep and fatigue, which are defined below.

Tension is a pulling force that acts in one direction; an example is the force in a cable holding a weight. Under tension, a material usually stretches, returning to its original length if the force does not exceed the material's elastic limit (see Elasticity). Under larger tensions, the material does not return completely to its original condition, and under even greater forces the material ruptures.

Compression is the decrease in volume that results from the application of pressure. When a material is subjected to a bending, shearing, or torsional (twisting) force, both tensile and compressive forces are simultaneously at work. When a rod is bent, for example, one side of it is stretched and subjected to a tensional force, and the other side is compressed.

Creep is a slowly progressing, permanent deformation that results from a steady force acting on a material. Materials subjected to high temperatures are especially susceptible to this deformation. The gradual loosening of bolts, the sagging of long-span cables, and the deformation of components of machines and engines are all noticeable examples of creep. In many cases the slow deformation stops because the force causing the creep is eliminated by the deformation itself. Creep extended over a long time eventually leads to the rupture of the material.

Fatigue can be defined as progressive fracture. It occurs when a mechanical part is subjected to a repeated or cyclic stress, such as vibration. Even when the maximum stress never exceeds the elastic limit, failure of the material can occur even after a short time. With some metals, such as titanium alloys, fatigue can be avoided by keeping the cyclic force below a certain level. No deformation is apparent during fatigue, but small localized cracks develop and propagate through the material until the remaining cross-sectional area cannot support the maximum stress of the cyclic force. Knowledge of tensile stress, elastic limits, and the resistance of materials to creep and fatigue are of basic importance in engineering. See also Metals.

Superconductivity

I

INTRODUCTION

Superconductivity, phenomenon displayed by certain conductors that demonstrate no resistance to the flow of an electric current. Superconductors also exhibit strong diamagnetism; that is, they are repelled by magnetic fields. Superconductivity is manifested only below a certain critical temperature Tc and a critical magnetic field Hc, which vary with the material used. Before 1986, the highest Tc was 23.2 K (-249.8° C/-417.6° F) in niobium-germanium compounds. Temperatures this low were achieved by use of liquid helium, an expensive, inefficient coolant. Ultralow-temperature operation places a severe constraint on the overall efficiency of a superconducting machine. Thus, large-scale operation of such machines was not considered practical. But in 1986 discoveries at several universities and research centers began to radically alter this situation. Ceramic metal-oxide compounds containing rare earth elements were found to be superconductive at temperatures high enough to permit using liquid nitrogen as a coolant. Because liquid nitrogen, at 77K (-196° C/-321° F), cools 20 times more effectively than liquid helium and is 10 times less expensive, a host of potential applications suddenly began to hold the promise of economic feasibility. In 1987 the composition of one of these superconducting compounds, with Tc of 94K (-179° C/-290° F), was revealed to be YBa2Cu307 (yttrium-barium-copper-oxide). It has since been shown that rare-earth elements, such as yttrium, are not an essential constituent, for in 1988 a thallium-barium-calcium copper oxide was discovered with a Tc of 125K (-148° C/-234° F).

II

HISTORY

Superconductivity was first discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes, who observed no electrical resistance in mercury below 4.2 K (-268.8° C/-451.8° F). The phenomenon was better understood only after strong diamagnetism was detected in a superconductor by Karl W. Meissner and R. Ochsenfeld of Germany in 1933. The basic physics of superconductivity, however, was not realized until 1957, when the American physicists John Bardeen, Leon N. Cooper, and John R. Schrieffer advanced the now celebrated BCS theory, for which the three were awarded the 1972 Nobel Prize in physics. The theory describes superconductivity as a quantum phenomenon (see Quantum Theory), in which the conduction electrons move in pairs and thus show no electrical resistance. In 1962 the British physicist Brian D. Josephson examined the quantum nature of superconductivity and proposed the existence of oscillations in the electric current flowing through two superconductors separated by a thin insulating layer in a magnetic or electric field. The effect, known as the Josephson effect, subsequently was confirmed by experiments.

III

APPLICATIONS

Because of their lack of resistance, superconductors have been used to make electromagnets that generate large magnetic fields with no energy loss. Superconducting magnets have been used in diagnostic medical equipment, studies of materials, and in the construction of powerful particle accelerators. Using the quantum effects of superconductivity, devices have been developed that measure electric current, voltage, and magnetic field with unprecedented sensitivity.

The discovery of better superconducting compounds is a significant step toward a far wider spectrum of applications, including faster computers with larger storage capacities, nuclear fusion reactors in which ionized gas is confined by magnetic fields, magnetic levitation (lifting or suspension) of high-speed (“Maglev”) trains, and perhaps most important of all, more efficient generation and transmission of electric power. The 1987 Nobel Prize in physics went to West German physicist J. Georg Bednorz and Swiss physicist K. Alex Müller for their discovery of materials that are superconductive at temperatures higher than had been thought possible. See Electricity; Magnetism.


Metals

I

INTRODUCTION

Metals, group of chemical elements that exhibit all or most of the following physical qualities: they are solid at ordinary temperatures; opaque, except in extremely thin films; good electrical and thermal conductors (see Conductor, Electrical); lustrous when polished; and have a crystalline structure when in the solid state. Metals and nonmetals are separated in the periodic table by a diagonal line of elements. Elements to the left of this diagonal are metals, and elements to the right are nonmetals. Elements that make up this diagonal—boron, silicon, germanium, arsenic, antimony, tellurium, polonium, and astatine—have both metallic and nonmetallic properties. The common metallic elements include the following: aluminum, barium, beryllium, bismuth, cadmium, calcium, cerium, chromium, cobalt, copper, gold, iridium, iron, lead, lithium, magnesium, manganese, mercury, molybdenum, nickel, osmium, palladium, platinum, potassium, radium, rhodium, silver, sodium, tantalum, thallium, thorium, tin, titanium, tungsten, uranium, vanadium, and zinc. Metallic elements can combine with one another and with certain other elements, either as compounds, as solutions, or as intimate mixtures. A substance composed of two or more metals, or a substance composed of a metal and certain nonmetals such as carbon are called alloys. Alloys of mercury with other metallic elements are known as amalgams.

Within the general limits of the definition of a metal, the properties of metals vary widely. Most metals are grayish in color, but bismuth is pinkish, copper is red, and gold is yellow. Some metals display more than one color, a phenomenon called pleochroism. The melting points of metals range from about -39° C (about -38° F) for mercury to 3410° C (6170° F) for tungsten. Osmium and iridium (specific gravity 22.6) are the most dense metals, and lithium (specific gravity 0.53) is the least dense. The majority of metals crystallize in the cubic system, but some crystallize in the hexagonal and tetragonal systems (see Crystal). Bismuth has the lowest electrical conductivity of the metallic elements, and silver the highest at ordinary temperatures. (For conductivity at low temperatures, see Cryogenics; Superconductivity.) The conductivity of most metals can be lowered by alloying. All metals expand when heated and contract when cooled, but certain alloys, such as platinum and iridium alloys, have extremely low coefficients of expansion.

II

PHYSICAL PROPERTIES

Metals are generally very strong and resistant to different types of stresses. Though there is considerable variation from one metal to the next, in general metals are marked by such properties as hardness, the resistance to surface deformation or abrasion; tensile strength, the resistance to breakage; elasticity, the ability to return to the original shape after deformation; malleability, the ability to be shaped by hammering; fatigue resistance, the ability to resist repeated stresses; and ductility, the ability to undergo deformation without breaking. See Materials Science and Technology.

III

CHEMICAL PROPERTIES

Reactivity Series

Chemists can list metals according to how quickly they undergo chemical reactions, such as burning or dissolving in acids. The result is called a reactivity series. A metal at the top of the series generally reacts more vigorously than those that are below it in the series, and the more reactive metal can take their place (or displace them) in various compounds or in solution. In some reactions, however, such as reduction reactions, the order of reactivity is reversed.

Metals typically have positive valences in most of their compounds, which means they tend to donate electrons to the atoms to which they bond. Also, metals tend to form basic oxides. Typical nonmetallic elements, such as nitrogen, sulfur, and chlorine, have negative valences in most of their compounds—meaning they tend to accept electrons—and form acidic oxides (see Acids and Bases; Chemical Reaction).

Metals typically have low ionization potentials. This means that metals react easily by loss of electrons to form positive ions, or cations. Thus, metals can form salts (chlorides, sulfides, and carbonates, for example) by serving as reducing agents (electron donors).

IV

ELECTRON STRUCTURE

In early attempts to explain the electronic configurations of the metals, scientists cited the characteristics of high thermal and electrical conductivity in support of a theory that metals consist of ionized atoms in which the free electrons form a homogeneous sea of negative charge. The electrostatic attraction between the positive metal ions and the free-moving and homogeneous sea of electrons was thought to be responsible for the bonds between the metal atoms. Free movement of the electrons was then held to be responsible for the high thermal and electrical conductivities. The principal objection to this theory was that the metals should then have higher specific heats than they do.

Metallic Bonding

Silver, a typical metal, consists of a regular array of silver atoms that have each lost an electron to form a silver ion. The negative electrons distribute themselves throughout the entire piece of metal and form nondirectional bonds between the positive silver ions. This arrangement, known as metallic bonding, accounts for the characteristic properties of metals: they are good electrical conductors because the electrons are free to move from one place to another, and they are malleable (as shown here) because the positive ions are held together by nondirectional forces.

In 1928 the German physicist Arnold Sommerfeld proposed that the electrons in metals exist in a quantized arrangement in which low energy levels available to the electrons are almost fully occupied (see Atom; Quantum Theory). In the same year the Swiss-American physicist Felix Bloch and later the French physicist Louis Brillouin used this idea of quantization in the currently accepted “band” theory of bonding in metallic solids.

According to the band theory, any given metal atom has only a limited number of valence electrons with which to bond to all of its nearest neighbors. Extensive sharing of electrons among individual atoms is therefore required. This sharing of electrons is accomplished through overlap of equivalent-energy atomic orbitals on the metal atoms that are immediately adjacent to one another. This overlap is delocalized throughout the entire metal sample to form extensive orbitals that span the entire solid rather than being part of individual atoms. Each of these orbitals lies at different energies because the atomic orbitals from which they were constructed were at different energies to begin with. The orbitals, equal in number to the number of individual atomic orbitals that have been combined, each hold two electrons, and are filled in order from lowest to highest energy until the number of available electrons has been used up. Groups of electrons are then said to reside in bands, which are collections of orbitals. Each band has a range of energy values that the electrons must possess to be part of that band; in some metals, there are energy gaps between bands, meaning that there are certain energies that the electrons cannot possess. The highest energy band in a metal is not filled with electrons because metals characteristically possess too few electrons to fill it. The high thermal electrical conductivities of metals is then explained by the notion that electrons may be promoted by absorption of thermal energy into these unfilled energy levels of the band.

Temperature

I

INTRODUCTION

Temperature, in physics, property of systems that determines whether they are in thermal equilibrium (see Thermodynamics). The concept of temperature stems from the idea of measuring relative hotness and coldness and from the observation that the addition of heat to a body leads to an increase in temperature as long as no melting or boiling occurs. In the case of two bodies at different temperatures, heat will flow from the hotter to the colder until their temperatures are identical and thermal equilibrium is reached (see Heat Transfer). Thus, temperatures and heat, although interrelated, refer to different concepts, temperature being a property of a body and heat being an energy flow to or from a body by virtue of a temperature difference. See Energy.

Temperature changes have to be measured in terms of other property changes of a substance. Thus, the conventional mercury thermometer measures the expansion of a mercury column in a glass capillary, the change in length of the column being related to the temperature change. If heat is added to an ideal gas contained in a constant-volume vessel, the pressure increases, and the temperature change can be determined from the pressure change by Gay-Lussac's law (see Gases), provided the temperature is expressed on the absolute scale.

II

TEMPERATURE SCALES

One of the earliest temperature scales was that devised by the German physicist Gabriel Daniel Fahrenheit. According to this scale, at standard atmospheric pressure, the freezing point (and melting point of ice) is 32° F, and the boiling point is 212° F. The centigrade, or Celsius scale, invented by the Swedish astronomer Anders Celsius, and used throughout most of the world, assigns a value of 0° C to the freezing point and 100° C to the boiling point. In scientific work, the absolute or Kelvin scale, invented by the British mathematician and physicist William Thomson, 1st Baron Kelvin, is most widely used. In this scale, absolute zero is at -273.16° C, which is zero K, and the degree intervals are identical to those measured on the Celsius scale (see Absolute Zero). The corresponding “absolute Fahrenheit” or Rankine scale, devised by the British engineer and physicist William J. M. Rankine, places absolute zero at -459.69° F, which is 0° R, and the freezing point at 491.69° R. A more consistent scientific temperature scale, based on the Kelvin scale, was adopted in 1933.

III

EFFECTS OF TEMPERATURE

Temperature plays an important part in determining the conditions in which living matter can exist. Thus, birds and mammals demand a very narrow range of body temperatures for survival and must be protected against extreme heat or cold. Aquatic species can exist only within a narrow temperature range of the water, which differs for various species. Thus, for example, the increase in temperature of river water by only a few degrees as a result of heat discharged from power plants may kill most of the native fish.

The properties of all materials are also markedly affected by temperature changes. At arctic temperatures, for example, steel becomes very brittle and breaks easily, and liquids either solidify or become very viscous, offering high frictional resistance to flow. At temperatures near absolute zero, many materials exhibit strikingly different characteristics. At high temperatures, solid materials liquefy or become gaseous; chemical compounds may break up into their constituents.

The temperature of the atmosphere is greatly influenced by both the land and the sea areas. In January, for example, the great landmasses of the northern hemisphere are much colder than the oceans at the same latitude, and in July the situation is reversed. At low elevations the air temperature is also determined largely by the surface temperature of the earth. The periodic temperature changes are due mainly to the sun's radiant heating of the land areas of the earth, which in turn convect heat to the overlying air. As a result of this phenomenon, the temperature decreases with altitude, from a standard reference value of 15.5° C (60° F) at sea level (in temperate latitudes), to about -55° C (about -67° F) at about 11,000 m (about 36,000 ft). Above this altitude, the temperature remains nearly constant up to about 33,500 m (about 110,000 ft).


Machine Tools

I

INTRODUCTION

Machine Tools, stationary power-driven machines used to shape or form solid materials, especially metals. The shaping is accomplished by removing material from a workpiece or by pressing it into the desired shape. Machine tools form the basis of modern industry and are used either directly or indirectly in the manufacture of machine and tool parts.

Machine tools may be classified under three main categories: conventional chip-making machine tools, presses, and unconventional machine tools. Conventional chip-making tools shape the workpiece by cutting away the unwanted portion in the form of chips. Presses employ a number of different shaping processes, including shearing, pressing, or drawing (elongating). Unconventional machine tools employ light, electrical, chemical, and sonic energy; superheated gases; and high-energy particle beams to shape the exotic materials and alloys that have been developed to meet the needs of modern technology.

II

HISTORY

Modern machine tools date from about 1775, when the English inventor John Wilkinson constructed a horizontal boring machine for producing internal cylindrical surfaces. About 1794 Henry Maudslay developed the first engine lathe. Later, Joseph Whitworth speeded the wider use of Wilkinson's and Maudslay's machine tools by developing, in 1830, measuring instruments accurate to a millionth of an inch. His work was of great value because precise methods of measurement were necessary for the subsequent mass production of articles having interchangeable parts.

The earliest attempts to manufacture interchangeable parts occurred almost simultaneously in Europe and the United States. These efforts relied on the use of so-called filing jigs, with which parts could be hand-filed to substantially identical dimensions. The first true mass-production system was created by the American inventor Eli Whitney, who in 1798 obtained a contract with the U.S. government to produce 10,000 army muskets, all with interchangeable parts.

During the 19th century, such standard machine tools as lathes, shapers, planers, grinders, and saws and milling, drilling, and boring machines reached a fairly high degree of precision, and their use became widespread in the industrializing nations. During the early part of the 20th century, machine tools were enlarged and made even more accurate. After 1920 they became more specialized in their applications. From about 1930 to 1950 more powerful and rigid machine tools were built to utilize effectively the greatly improved cutting materials that had become available. These specialized machine tools made it possible to manufacture standardized products very economically, using relatively unskilled labor. The machines lacked flexibility, however, and they were not adaptable to a variety of products or to variations in manufacturing standards. As a result, in the past three decades engineers have developed highly versatile and accurate machine tools that have been adapted to computer control, making possible the economical manufacture of products of complex design. Such tools are now widely used.

III

CONVENTIONAL MACHINE TOOLS

Lathe, Milling Machine, Planer, and Shaper

A selection of basic machine tools shows a variety of functions and methods of crafting a workpiece. The job at hand usually determines which tool will be used. For instance, a person making a rounded handle would use a lathe, while a person making a breadboard would use a planer. In order to use power tools efficiently and safely, either the workpiece or the actual tool must be stationary. A planer is an example of a stationary machine tool because the workpiece is moved, or fed, into it. To use the shaper, the workpiece must be kept stationary while the tool is moved across it.

Among the basic machine tools are the lathe, the shaper, the planer, and the milling machine. Auxiliary to these are drilling and boring machines, grinders, saws, and various metal-forming machines.

A

Lathe

A lathe, the oldest and most common type of turning machine, holds and rotates metal or wood while a cutting tool shapes the material. The tool may be moved parallel to or across the direction of rotation to form parts that have a cylindrical or conical shape or to cut threads. With special attachments, a lathe may also be used to produce flat surfaces, as a milling machine does, or it may drill or bore holes in the workpiece.

B

Shaper

The shaper is used primarily to produce flat surfaces. The tool slides against the stationary workpiece and cuts on one stroke, returns to its starting position, and then cuts on the next stroke after a slight lateral displacement. In general, the shaper can produce almost any surface composed of straight-line elements. It uses a single-point tool and is relatively slow, because it depends on reciprocating (alternating forward and return) strokes. For this reason, the shaper is seldom found on a production line. It is, however, valuable for tool and die rooms and for job shops where flexibility is essential and relative slowness is unimportant because few identical pieces are being made.

C

Planer

The planer is the largest of the reciprocating machine tools. Unlike the shaper, which moves a tool past a fixed workpiece, the planer moves the workpiece past a fixed tool. After each reciprocating cycle, the workpiece is advanced laterally to expose a new section to the tool. Like the shaper, the planer is intended to produce vertical, horizontal, or diagonal cuts. It is also possible to mount several tools at one time in any or all tool holders of a planer to execute multiple simultaneous cuts.

D

Milling Machine

In a milling machine, a workpiece is fed against a circular device with a series of cutting edges on its circumference. The workpiece is held on a table that controls the feed against the cutter. The table conventionally has three possible movements: longitudinal, horizontal, and vertical; in some cases it can also rotate. Milling machines are the most versatile of all machine tools. Flat or contoured surfaces may be machined with excellent finish and accuracy. Angles, slots, gear teeth, and recess cuts can be made by using various cutters.

E

Drilling and Boring Machines

Some Conventional Machine Tools

Conventional machine tools prepare workpieces for further fitting and use. Drills, grinders, punch presses, surface grinders, and boring machines are used extensively in industry. Particularly useful in large-scale production, these power tools produce uniform holes and smooth surfaces far faster and more accurately than they could be produced by hand.

Hole-making machine tools are used to drill a hole where none previously existed; to alter a hole in accordance with some specification (by boring or reaming to enlarge it, or by tapping to cut threads for a screw); or to lap or hone a hole to create an accurate size or a smooth finish.

Drilling machines vary in size and function, ranging from portable drills to radial drilling machines, multispindle units, automatic production machines, and deep-hole-drilling machines. See Drill.

Boring is a process that enlarges holes previously drilled, usually with a rotating single-point cutter held on a boring bar and fed against a stationary workpiece. Boring machines include jig borers and vertical and horizontal boring mills.

F

Grinders

Grinding is the removal of metal by a rotating abrasive wheel; the action is similar to that of a milling cutter. The wheel is composed of many small grains of abrasive, bonded together, with each grain acting as a miniature cutting tool. The process produces extremely smooth and accurate finishes. Because only a small amount of material is removed at each pass of the wheel, grinding machines require fine wheel regulation. The pressure of the wheel against the workpiece can be made very slight, so that grinding can be carried out on fragile materials that cannot be machined by other conventional devices. See Grinding and Polishing.

G

Saws

Commonly used power-driven saws are classified into three general types, according to the kind of motion used in the cutting action: reciprocating, circular, and band-sawing machines. They generally consist of a bed or frame, a vise for clamping the workpiece, a feed mechanism, and the saw blade.

H

Cutting Tools and Fluids

Because cutting processes involve high local stresses, frictions, and considerable heat generation, cutting-tool material must combine strength, toughness, hardness, and wear resistance at elevated temperatures. These requirements are met in varying degrees by such cutting-tool materials as carbon steels (steel containing 1 to 1.2 percent carbon), high-speed steels (iron alloys containing tungsten, chromium, vanadium, and carbon), tungsten carbide, and diamonds and by such recently developed materials as ceramic, carbide ceramic, and aluminum oxide.

In many cutting operations fluids are used to cool and lubricate. Cooling increases tool life and helps to stabilize the size of the finished part. Lubrication reduces friction, thus decreasing the heat generated and the power required for a given cut. Cutting fluids include water-based solutions, chemically inactive oils, and synthetic fluids.

I

Presses

Presses shape workpieces without cutting away material, that is, without making chips. A press consists of a frame supporting a stationary bed, a ram, a power source, and a mechanism that moves the ram in line with or at right angles to the bed. Presses are equipped with dies (see Die) and punches designed for such operations as forming, punching, and shearing. Presses are capable of rapid production because the operation time is that needed for only one stroke of the ram.

IV

UNCONVENTIONAL MACHINE TOOLS

Unconventional machine tools include plasma-arc, laser-beam, electrodischarge, electrochemical, ultrasonic, and electron-beam machines. These machine tools were developed primarily to shape the ultrahard alloys used in heavy industry and in aerospace applications and to shape and etch the ultrathin materials used in such electronic devices as microprocessors.

A

Plasma Arc

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas (see Plasma) to melt and displace material in its path. The materials cut by PAM are generally those that are difficult to cut by any other means, such as stainless steels and aluminum alloys.

B

Laser

Laser-beam machining (LBM) is accomplished by precisely manipulating a beam of coherent light (see Laser) to vaporize unwanted material. LBM is particularly suited to making accurately placed holes. The LBM process can make holes in refractory metals and ceramics and in very thin materials without warping the workpiece. Extremely fine wires can also be welded using LBM equipment.

C

Electrodischarge

Electrodischarge machining (EDM), also known as spark erosion, employs electrical energy to remove metal from the workpiece without touching it. A pulsating high- frequency electric current is applied between the tool point and the workpiece, causing sparks to jump the gap and vaporize small areas of the workpiece. Because no cutting forces are involved, light, delicate operations can be performed on thin workpieces. EDM can produce shapes unobtainable by any conventional machining process.

D

Electrochemical

Electrochemical machining (ECM) also uses electrical energy to remove material. An electrolytic cell is created in an electrolyte medium, with the tool as the cathode and the workpiece as the anode. A high-amperage, low-voltage current is used to dissolve the metal and to remove it from the workpiece, which must be electrically conductive. A wide variety of operations can be performed by ECM; these operations include etching, marking, hole making, and milling.

E

Ultrasonic

Ultrasonic machining (USM) employs high-frequency, low-amplitude vibrations to create holes and other cavities. A relatively soft tool is shaped as desired and vibrated against the workpiece while a mixture of fine abrasive and water flows between them. The friction of the abrasive particles gradually cuts the workpiece. Materials such as hardened steel, carbides, rubies, quartz, diamonds, and glass can easily be machined by USM.

F

Electron Beam

In electron-beam machining (EBM), electrons are accelerated to a velocity nearly three-fourths that of light. The process is performed in a vacuum chamber to reduce the scattering of electrons by gas molecules in the atmosphere. The stream of electrons is directed against a precisely limited area of the workpiece; on impact, the kinetic energy of the electrons is converted into thermal energy that melts and vaporizes the material to be removed, forming holes or cuts. EBM equipment is commonly used by the electronics industry to aid in the etching of circuits in microprocessors. See Microprocessor.

Flame Retardant

I

INTRODUCTION

Flame Retardantmaterial added or applied to a product to increase the resistance of that product to fire. Flame retardants, also called fire retardants, are less flammable than the materials they protect, burn slowly, and do not propagate fire. Some flame retardants prevent the spread of flame; others burn and thereby create a layer of char that inhibits further combustion.

Flame retardants are generally added to wood, paper, plastics, textiles, and composites to meet governmental regulations for buildings, aircraft, automobiles, and ships. Flame retardants can be incorporated into a material either as a reactive component or as an additive component. Reactive-type flame retardants are preferable because they produce stable and more uniform products. Such flame retardants are incorporated into the polymer structure of some plastics. Additive-type flame retardants, on the other hand, are more versatile and economical. They can be applied as a coating to wood, woven fabrics, and composites, or as dispersed additives in bulk materials such as plastics and fibers.

The chemicals in a flame retardant determine how it works. Most flame retardants contain elements from any of three groups in the periodic table of elements: group IIIa (including boron and aluminum); group Va (including nitrogen, phosphorus, arsenic, and antimony); and group VIIa (including fluorine, chlorine, and bromine). Elements of different groups that are combined in a single flame retardant may work more effectively together than they would separately.

II

GROUP IIIA FLAME RETARDANTS

Flame retardants that contain boron or aluminum increase the amount of char, or burnt material, formed in the early stage of a fire. The char forms a protective layer that prevents oxygen from reaching the inner layers of the material and thus sustaining the fire (see Combustion). Chemicals commonly used for this purpose include borax, boric acid, and hydrated aluminum oxide.

III

GROUP VA FLAME RETARDANTS

Phosphorus can function as a flame retardant in both its solid phase and its liquid phase. Phosphorus-containing compounds such as phosphoric acid work by forming a surface layer of protective char. Nitrogen is used mainly in combination with phosphorus; such combinations have proved effective in cellulose, polyester, and polyurethane products. Arsenic, because of its toxicity, is now rarely used in flame retardants. Antimony by itself is ineffective as a flame retardant and is used only in combination with halogens, especially bromine and chlorine.

IV

GROUP VIIA FLAME RETARDANTS

Bromine works as a flame retardant in its gaseous phase. Bromine-containing compounds are incorporated into flammable materials. When these materials are exposed to flame, the bromine dissociates from the material and forms a heavy gas. This dissociation disperses heat, and the bromine gas forms an insulating layer around the material. The layer prevents flames from spreading by inhibiting access to oxygen and by slowing the transfer of heat. Chlorine works in a similar manner in both its liquid and gaseous phases. The most important fluorine-containing flame retardants are the chlorofluorocarbons, which are used as blowing agents in polyurethane and polystyrene foams. The use of bromine and chlorine in fire retardants is somewhat restricted, however, because a high concentration of these elements can diminish the flexibility, mechanical properties, and durability of materials.


Welding

I

INTRODUCTION

Welding, in engineering, any process in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. Most of the processes may be grouped into two main categories: pressure welding, in which the weld is achieved by pressure; and heat welding, in which the weld is achieved by heat. Heat welding is the most common welding process used today. Brazing and soldering (see Solder) are other means of joining metals.

With the development of new techniques during the first half of the 20th century, welding replaced bolting and riveting in the construction of many types of structures, including bridges, buildings, and ships. It is also a basic process in the automotive and aircraft industries and in the manufacture of machinery. Along with soldering and brazing, it is essential in the production of virtually every manufactured product involving metals.

The welding process best suited to joining two pieces of metal depends on the physical properties of the metals, the specific use to which they are applied, and the production facilities available. Welding processes are generally classified according to the sources of heat and pressure used.

The original pressure process was forge welding. Forge welding was practiced for centuries by blacksmiths and other artisans. The metals are brought to a suitable temperature in a furnace, and the weld is achieved by hammering or other mechanical pressure. Forge welding is used rarely in modern manufacturing.

The welding processes most commonly employed today include gas welding, arc welding, and resistance welding. Other joining processes include thermite welding, laser welding, and electron-beam welding.

II

GAS WELDING

Gas welding is a nonpressure process using heat from a gas flame. The flame is applied directly to the metal edges to be joined and simultaneously to a filler metal in wire or rod form, called the welding rod, which is melted to the joint. Gas welding has the advantage of involving equipment that is portable and does not require an electric power source. The surfaces to be welded and the welding rod are coated with flux, a fusible material that shields the material from air, which would result in a defective weld.

III

ARC WELDING

Arc-welding processes, which have become the most important welding processes, particularly for joining steels, require a continuous supply of either direct or alternating electrical current. This current is used to create an electric arc, which generates enough heat to melt metal and create a weld (see Electric Arc).

Arc welding has several advantages over other welding methods. Arc welding is faster because of its high heat concentration, which also tends to reduce distortion in the weld. Also, in certain methods of arc welding, fluxes are not necessary. The most widely used arc-welding processes are shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc.

A

Shielded Metal Arc

In shielded metal-arc welding, a metallic electrode, which conducts electricity, is coated with flux and connected to a source of electric current. The metal to be welded is connected to the other end of the same source of current. By touching the tip of the electrode to the metal and then drawing it away, an electric arc is formed. The intense heat of the arc melts both parts to be welded and the point of the metal electrode, which supplies filler metal for the weld. This process, developed in the early 20th century, is used primarily for welding steels.

B

Gas-Tungsten Arc

In gas-tungsten arc welding, a tungsten electrode is used in place of the metal electrode used in shielded metal-arc welding. A chemically inert gas, such as argon or helium, is used to shield the metal from oxidation. The heat from the arc formed between the electrode and the metal melts the edges of the metal. Metal for the weld may be added by placing a bare wire in the arc or the point of the weld. This process can be used with nearly all metals and produces a high-quality weld. However, the rate of welding is considerably slower than in other processes.

C

Gas-Metal Arc

In gas-metal welding, a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas or by coating the electrode with flux. The electrode is fed into the electric arc, and melts off in droplets to enter the liquid metal that forms the weld. Most common metals can be joined by this process.

D

Submerged Arc

Submerged-arc welding is similar to gas-metal arc welding, but in this process no gas is used to shield the weld. Instead, the arc and tip of the wire are submerged beneath a layer of granular, fusible material formulated to produce a proper weld. This process is very efficient but is generally only used with steels.

IV

RESISTANCE AND THERMITE WELDING

In resistance welding, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to great pressure, and a heavy current is applied briefly. The point where the two metals meet creates resistance to the flow of current. This resistance causes heat, which melts the metals and creates the weld. Resistance welding is extensively employed in many fields of sheet metal or wire manufacturing and is particularly adaptable to repetitive welds made by automatic or semiautomatic machines.

In thermite welding, heat is generated by the chemical reaction that results when a mixture of aluminum powder and iron oxide, known as thermite, is ignited. The aluminum unites with the oxygen and generates heat, releasing liquid steel from the iron. The liquid steel serves as filler metal for the weld. Thermite welding is employed chiefly in welding breaks or seams in heavy iron and steel sections. It is also used in the welding of rail for railroad tracks.

V

NEW PROCESSES

The use of electron beams and lasers for welding has grown during the second half of the 20th century. These methods produce high-quality welded products at a rapid rate. Laser welding and electron-beam welding have valuable applications in the automotive and aerospace industries.


Brazing

Brazing, a method of joining two metal surfaces by using nonferrous filler metal heated to above 430° C (800° F), but below the melting point of the metals to be joined. The kinds of filler metal used include brass, bronze, or a silveralloy; the filler metal distributes itself between the surfaces to be bonded by capillary action. Brazing is different from welding; in welding, partial melting of the surfaces may occur, and the filler metal is not distributed by capillarity. Brazing differs from ordinary soldering only in the temperature of the operation; ordinary, or soft, solder melts at temperatures below 430° C, but brazing alloys, sometimes called hard solder, melt above that temperature.

In general, brazing requires careful cleaning of the surfaces to be joined and the use of flux, such as borax, to reduce any oxide film on the surfaces. In mass production, furnaces are often used to heat the parts to be brazed, or the parts are brazed by dipping in baths of molten filler alloys. For single, nonrepetitive operations, the joint is usually heated with a gas, oxyacetylene, or oxyhydrogen torch.

Electric Arc

Electric Arc, type of continuous electric discharge, giving intense light and heat, formed between two electrodes in a gas at low pressure or in open air. It was first discovered and demonstrated by the British chemist Sir Humphry Davy in 1800.

To start an arc, the ends of two pencil-like electrodes, usually made of carbon, are brought into contact and a large current (about 10 amp) is passed through them. This current causes intense heating at the point of contact, and if the electrodes are then separated, a flamelike arc is formed between them. The discharge is carried largely by electrons traveling from the negative to the positive electrode, but also in part by positive ions traveling in the opposite direction. The impact of the ions produces great heat in the electrodes, but the positive electrode is hotter, because the electrons impinging on it have greater total energy. In an arc in air at normal pressure, the positive electrode reaches a temperature of 3500° C (6332° F).

The intense heat of the electric arc is often utilized in special furnaces to melt refractory materials. Temperatures of about 2800° C (5072° F) can easily be obtained with such a furnace. Arcs are also used as a high-intensity light source. Arc lights have the advantage of being concentrated sources of light, because some 85 percent of the light intensity comes from a small area of the tip of the positive carbon electrode. Such lamps were formerly much used for street lighting, but are now chiefly employed in motion picture projectors. Mercury-vapor lamps and sodium-vapor lamps are enclosed arc lamps in which the arc is maintained in an atmosphere of mercury or sodium vapor at reduced pressure.

Forging

I

INTRODUCTION

Forging, process of shaping iron and other malleable metals by hammering or pressing them after making them plastic by application of heat. Forging techniques are useful in the working of metal because the metal can be given the desired form, and the process improves the structure of the metal, particularly by refining the grain size of the metal. Forged metal is stronger and more ductile than cast metal and exhibits greater resistance to fatigue and impact.

II

HAND FORGING

Sometimes called smithing, or blacksmithing, hand forging is the simplest form of forging and it is one of the methods by which metal was first worked. The metal to be forged is first heated to red heat in the fire of a forge, and then is beaten into shape on a metal anvil with sledges or hammers. The forge consists of an open hearth, made of some durable, refractory substance such as firebrick, which is provided with a number of air openings, or tuyeres, through which air is forced by a bellows or blower fan. Charcoal, coke, and coal are used as fuels in the forge. Hammers and other tools are employed by the blacksmith in the various forging operations.

In general, six basic types of forging exist: upsetting, or decreasing the length and increasing the diameter of the metal; swaging, decreasing the diameter of the metal; bending; welding, joining two pieces of metal together by semifusion; punching, the forming of small openings in the metal; and cutting out, the forming of large holes in the metal.

A piece of metal, called the work, is upset when it is struck along the longest dimension (for example, the end of a rod or bar), which shortens and thickens it. Swaging is accomplished by hammering the metal stock while it is held on the anvil within any one of various concave tools called swages. Bending is accomplished either by hammering the work around a form or by leveraging it against a supporting fulcrum. In forge welding of iron, a flux such as borax is first applied to the heated metal to remove any oxides from the surfaces of the two pieces, and the pieces are then joined by hammering them together at high temperature; a welded joint of this kind, when properly made, is entirely homogeneous and is as strong, that is, uniform, as the parent metal. To punch small holes, the work is supported on a ring-shaped piece of metal atop the anvil, and a punch of the proper shape is driven through the work by hammer blows. Larger holes are cut out, and portions of the work are cut off with heavy, sharp chisels similar to cold chisels used to cut cold metal. Combinations of several of these operations can produce forgings of a wide variety of shapes.

III

MACHINE FORGING

The chief difference between hand forging and machine forging is that in the latter technique various types of machine-powered hammers or presses are used instead of hand sledges. These machines enable the operator to strike heavy blows with great rapidity and thus to produce forgings of large size and high quality as swiftly as required by modern production-line methods. Another advantage of machine forging is that the heavier the blows struck during forging, the greater the improvement in the quality of metallic structure. Fine-grain size in the forging, which is particularly desirable for maximum impact resistance, is obtained by working the entire piece. With large, hand-forged metal, only the surface is deformed, whereas the machine hammer or press will deform the metal throughout the entire piece.

A special type of machine forging is drop forging, also called impact-die forging. Drop forging consists of placing soft, hot metal between two shaping dies (see Die). The upper one of these dies is hammered, or dropped, on the lower die, forcing the heated metal into the shaped die cavities, as in coin-making operations.

For reducing part of a piece of metal stock to a predetermined size, forging rolls are sometimes employed. These consist of a pair of grooved, cam-shaped rollers through which the metal is passed. The rollers touch each other and work on the metal during only part of each rotation and therefore reduce only part of the stock that is fed to them.

Machine-forging operations are frequently accomplished by use of a series of dies mounted on the same press or hammer. The dies are arranged in sequence so as to form the finished forging in a series of steps. After the piece has been partially formed by one stroke, it is moved to the next die for further shaping on the next stroke.

Fatigue (materials)

Fatigue (materials), in metals, progressive deterioration, that ultimately results in the breaking of the metal. Fatigue is caused by repeated application of stress to the metal, and the deformation of a material or object as a result of the stress is known as creep. The fatigue strength of a typical steel alloy is about 50 percent of the ultimate strength and 75 percent of the elastic strength but may be considerably lower, particularly for the strongest heat-treated steels. If the elastic strength of a steel beam is about 45,000 kg (about 100,000 lb), it could withstand a continuous stress of about 41,000 kg (about 90,000 lb) for centuries, with no measurable yielding. A stress of about 36,000 kg (about 80,000 lb) alternately applied and withdrawn, however, would probably cause fatigue failure after a few million applications. Fatigue is not important in civil engineering structures, in which stress is generally continuous, but in an engine turning at 3000 rpm, any stress to which an engine part is subjected will often be applied millions of times within a few hours of operation. Fatigue failures account for an overwhelming majority of all structural failures in cyclic devices such as engines, and design engineers must consider fatigue strength, rather than elastic strength or ultimate strength, in their calculations.

The problem of metal fatigue has gained great importance in the field of air transport since the end of World War II. The increased stresses of high-speed flight with heavy loads at high altitudes have posed difficult problems for structural engineers, especially in wing and engine design. The exact structural changes that occur as a result of fatigue are not known. The failure usually starts at a point of stress concentration and proceeds along the intercrystalline planes of the metal. The break often shows a characteristic coarsely crystalline structure except where the surfaces are worn smooth by rubbing against one another after the break has started. The term fatigue is not an entirely appropriate one, because no amount of rest between stress applications has any measurable effect upon the ultimate failure. See Materials Science and Technology; Metallography; Metals; Tensile Strength.

Flame

Flame, glowing body of mixed gases undergoing the process of combustion. Flames generally consist of a mixture of oxygen (or air) and another gas, usually such combustible substances as hydrogen, carbon monoxide, or hydrocarbon.

A typical flame is that of a burning candle. When the candle is lighted, the heat of the match melts the wax, which is carried up the wick and then vaporized by the heat. The vaporized wax is then broken down by the heat and, finally, combines with the oxygen of the surrounding air, producing a flame and generating heat and light. The candle flame consists of three zones that are easily distinguished. The innermost zone, a nonluminous cone, is composed of a gas-air mixture at a comparatively low temperature. In the second, or luminous, cone, hydrogen and carbon monoxide are produced by decomposition and begin to react with oxygen to form water and carbon dioxide, respectively. In this cone the temperature of the flame—about 590° to 680° C (about 1090° to 1250° F)—is great enough to dissociate the gases in the flame and produce free particles of carbon, which are heated to incandescence and then consumed. The incandescent carbon produces the characteristic yellow light of this portion of the flame. Outside the luminous cone is a third, invisible cone in which the remaining carbon monoxide and hydrogen are finally consumed.

If a cold object is introduced into the outer portions of a flame, the temperature of that part of the flame will be lowered below the point of combustion, and unburned carbon and carbon monoxide will be given off. Thus, if a porcelain dish is passed through a candle flame, it will receive a deposit of carbon in the form of soot. Operation of any kind of flame-producing stove in a room that is unventilated is dangerous because of the production of carbon monoxide, which is poisonous.

All combustible substances require a definite proportion of oxygen for complete burning. (A flame can be sustained in an atmosphere of pure chlorine, although combustion is not complete.) In the burning of a candle, or of solids such as wood or coal, this oxygen is supplied by the surrounding atmosphere. In blowpipes and various types of gas burners, air or pure oxygen is mixed with the gas at the base of the burner so that the carbon is consumed almost instantaneously at the mouth of the burner. For this reason such flames are nonluminous. They also occupy a smaller volume and are proportionately hotter than a simple candle flame. The hottest portion of the flame of a Bunsen burner has a temperature of about 1600° C (about 2910° F). The hottest portion of the oxygen-acetylene flames used for welding metals reaches 3500° C (6330° F); such flames have a bluish-green cone in place of the luminous cone. If the oxygen supply is reduced, such flames have four cones: nonluminous, bluish-green, luminous, and invisible.

The blue-green cone of any flame is often called the reducing cone, because it is insufficiently supplied with oxygen and will take up oxygen from substances placed within it. Similarly, the outermost cone, which has an excess of oxygen, is called the oxidizing cone. Intensive studies of the molecular processes taking place in various regions of flames are now possible through the techniques of laser spectroscopy.