A little bit of physics (Несколько текстов для зачёта), страница 2

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