Scientific Principles

Introduction:

Ceramics have characteristics that enable them to be used in a wide variety of applications including:

The diversity in their properties stems from their bonding and crystal structures.

Atomic Bonding:

Two types of bonding mechanisms occur in ceramic materials, ionic and covalent. Often these mechanisms co-exist in the same ceramic material. Each type of bond leads to different characteristics.

Ionic bonds most often occur between metallic and nonmetallic elements that have large differences in their electronegativities. Ionically-bonded structures tend to have rather high melting points, since the bonds are strong and non-directional.

The other major bonding mechanism in ceramic structures is the covalent bond. Unlike ionic bonds where electrons are transferred, atoms bonded covalently share electrons. Usually the elements involved are nonmetallic and have small electronegativity differences.

Many ceramic materials contain both ionic and covalent bonding. The overall properties of these materials depend on the dominant bonding mechanism. Compounds that are either mostly ionic or mostly covalent have higher melting points than compounds in which neither kind of bonding predominates.

Table 1: Comparison of % Covalent and Ionic character with several ceramic compound's melting points.
Ceramic CompoundMelting Point % Covalent character% Ionic character
Magnesium Oxide279827%73%
Aluminum Oxide205037%63%
Silicon Dioxide171549%51%
Silicon Nitride190070%30%
Silicon Carbide250089%11%

Classification:

Ceramic materials can be divided into two classes: crystalline and amorphous (noncrystalline). In crystalline materials, a lattice point is occupied either by atoms or ions depending on the bonding mechanism. These atoms (or ions) are arranged in a regularly repeating pattern in three dimensions (i.e., they have long-range order). In contrast, in amorphous materials, the atoms exhibit only short-range order. Some ceramic materials, like silicon dioxide (SiO2), can exist in either form. A crystalline form of SiO2 results when this material is slowly cooled from a temperature (T > TMP @1723 C). Rapid cooling favors noncrystalline formation since time is not allowed for ordered arrangements to form.

Crystalline Silicon dioxide Amorphous Silicon dioxide 
(regular pattern) (random pattern)
Figure 1: Comparison in the physical strucuture of both crystalline and amorphous Silicon dioxide

The type of bonding (ionic or covalent) and the internal structure (crystalline or amorphous) affects the properties of ceramic materials. The mechanical, electrical, thermal, and optical properties of ceramics will be discussed in the following sections.

Thermal Properties:

The most important thermal properties of ceramic materials are heat capacity, thermal expansion coefficient, and thermal conductivity. Many applications of ceramics, such as their use as insulating materials, are related to these properties.

Thermal energy can be either stored or transmitted by a solid. The ability of a material to absorb heat from its surrounding is its heat capacity. In solid materials at T > 0 K, atoms are constantly vibrating. The atomic vibrations are also affected by the vibrations of adjacent atoms through bonding. Hence, vibrations can be transmitted across the solid. The higher the temperature, the higher the frequency of vibration and the shorter the wavelength of the associated elastic deformation.

The potential energy between two bonded atoms can be schematically represented by a diagram:

Figure 2: Graph depicting the potential energy between two bonded atoms

The distance at which there is minimum energy (potential well) represents what is usually described as the bond length. A good analogy is a sphere attached to a spring, with the equilibrium position of the spring corresponding to the atom at the bond length (potential well). When the spring is either compressed or stretched from its equilibrium position, the force pulling it back to the equilibrium position is directly proportional to the displacement (Hooke's law). Once displaced, the frequency of oscillation is greatest when there is a large spring constant and low mass ball. Ceramics generally have strong bonds and light atoms. Thus, they can have high frequency vibrations of the atoms with small disturbances in the crystal lattice. The result is that they typically have both high heat capacities and high melting temperatures.

As temperature increases, the vibrational amplitude of the bonds increases. The asymmetry of the curve shows that the interatomic distance also increases with temperature, and this is observed as thermal expansion. Compared to other materials, ceramics with strong bonds have potential energy curves that are deep and narrow and correspondingly small thermal expansion coefficients.

The conduction of heat through a solid involves the transfer of energy between vibrating atoms. Extending the analogy, consider each sphere (atom) to be connected to its neighbors by a network of springs (bonds). The vibration of each atom affects the motion of neighboring atoms, and the result is elastic waves that propagate through the solid. At low temperatures (up to about 400 ), energy travels through the material predominantly via phonons, elastic waves that travel at the speed of sound. Phonons are the result of particle vibrations which increase in frequency and amplitude as temperature increases. Phonons travel through the material until they are scattered, either through phonon-phonon interactions* or at lattice imperfections. Phonon conductivity generally decreases with increasing temperature in crystalline materials as the amount of scattering increases. Amorphous ceramics which lack the ordered lattice undergo even greater scattering, and therefore are poor conductors. Those ceramic materials that are composed of particles of similar size and mass with simple structures (such as diamond or BeO) undergo the smallest amount of scattering and therefore have the greatest conductivity.

At higher temperatures, photon conductivity (radiation) becomes the predominant mechanism of energy transfer. This is a rapid sequence of absorptions and emissions of photons that travel at the speed of light. This mode of conduction is especially important in glass, transparent crystalline ceramics, and porous ceramics. In these materials, thermal conductivity increases with increased temperature.

Although the thermal conductivity is affected by faults or defects in the crystal structure, the insulating properties of ceramics essentially depend on microscopic imperfections. The transmission of either type of wave (phonon or photon) is interrupted by grain boundaries and pores, so that more porous materials are better insulators. The use of ceramic insulating materials to line kilns and industrial furnaces are one application of the insulating properties of ceramic materials.

The electron mechanism of heat transport is relatively unimportant in ceramics because charge is localized. This mechanism is very important, however, in metals which have large numbers of free (delocalized) electrons.

*Phonon-phonon interactions are another consequence of the asymmetry in the interaction potential between atoms. When different phonons overlap at the location of a particular atom, the vibrational amplitudes superimpose. In the asymmetrical potential well, the curvature varies as a function of the displacement. This means that the spring constant by which the atom is retained also changes. Hence the atom has the tendency to vibrate with a different frequency, which produces a different phonon.

Table 2: Comparison of thermal properties of different ceramic materials.
MaterialMelting Temp ()Heat Capacity
(J/kg K)
Coefficient of Linear
Expansion 1/ Cx10-6
Thermal Conductiv-ity
(W/m K)
Aluminum metal66090023.6247
Copper metal106338616.5398
Alumina20507758.830.1
Fused silica16507400.52.0
Soda-lime glass7008409.01.7
Polyethylene120210060-2200.38
Polystyrene65-75136050-850.13

One of the most interesting high-temperature applications of ceramic materials is their use on the space shuttle. Almost the entire exterior of the shuttle is covered with ceramic tiles made from high purity amorphous silica fibers. Those exposed to the highest temperatures have an added layer of high-emittance glass. These tiles can tolerate temperatures up to 1480 C for a limited amount of time. Some of the high temperatures experienced by the shuttle during entry and ascent are shown in Figure 3.

Figure 3: Diagram of space shuttle's ascent and descent temperatures

The melting point of aluminum is 660 C. The tiles keep the temperature of the aluminum shell of the shuttle at or below 175 C while the exterior temperatures can exceed 1400 C. The tiles cool off rapidly, so that after exposure to such high temperatures they are cool enough to be held in the bare hand in about 10 seconds. Surprisingly, the thickness of these ceramic tiles varies from only 0.5 inches to 3.5 inches.

Figure 4: Graph of inner temperature of tile versus tile thickness.

The shuttle also uses ceramic applications in fabrics for gap fillers and thermal barriers, reinforced carbon-carbon composites for the nose cone and wing leading edges, and high temperature glass windows.

Optical Properties:

An optical property describes the way a material reacts to exposure to light. Visible light is a form of electromagnetic radiation with wavelengths in the range of 400 to 700 nm corresponding to an energy range of 3.1 to 1.8 electron volts (eV) (from E = hc/, where c = 3 x 10i17 nm/s and h = 4.13 x 10-15 eV s).

When light strikes an object it may be transmitted, absorbed, or reflected. Materials vary in their ability to transmit light, and are usually described as transparent, translucent, or opaque. Transparent materials, such as glass, transmit light with little absorption or reflection. Materials that transmit light diffusely, such as frosted glass, are translucent. Opaque materials do not transmit light.

Two important mechanisms for the interaction of light with the particles in a solid are electronic polarizations and transitions of electrons between different energy states. The distortion of the electron cloud of an atom by an electric field, in this case the electric field of the light, is described as polarization. As a result of polarization, some of the energy may be absorbed, i.e., converted into elastic deformations (phonons), and consequently heat. On the other hand, the polarization may propagate as a material-bound electromagnetic wave with a different speed than light. When light is absorbed and reemitted from the surface at the same wavelength, it is called reflection. Metals, for example, are highly reflective, and those with a silvery appearance reflect the whole range of visible light. The energy levels of electrons are quantized, i.e., each electron transition between levels requires a certain specific amount of energy. The absorption of energy results in the shifting of electrons from the ground state to a higher, excited state. The electrons then fall back to the ground state, accompanied by the reemission of electromagnetic radiation.

In nonmetals, the lower energy bonding orbitals make up what is called the valence band, and the higher energy antibonding orbitals form the conduction band. The separation between the two bands is the band gap energy, and is generally large for nonmetals, smaller for semiconductors, and nonexistent in metals.

The energy range for visible light is from 1.8 to 3.1 eV. Materials with band gap energies in this range will absorb those corresponding colors (energies) and transmit the others. They will appear transparent and colored. For example, the band gap energy of cadmium sulfide photocells is about 2.4 eV and so it absorbs the higher energy (blue and violet) components of visible light. It has a yellow-orange color as a result of the transmitted portions of the spectrum. This type of light-induced conductivity is called photoconductivity.

Materials with band gap energies less than 1.8 eV will be opaque because all visible light will be absorbed by electron transitions from the valence to the conduction band. Dissipation of this absorbed energy may be by direct return to the valence band or by more complicated transitions involving impurities. Pure materials with band gap energies greater than 3.1 eV will not absorb light in the visible range and will appear transparent and colorless.

Light that is emitted from electron transitions in solids is called luminescence. If it occurs for a short time it is fluorescence, and if it lasts for a longer time it is phosphorescence.

Light that is transmitted from one medium into another, such as from air into glass, undergoes refraction. This is the apparent bending of light rays that results from the change in speed of the light. The index of refraction (n) of a material is the ratio of the speed of light in a vacuum (c = 3 x 108 m/s) to the speed of light in that material (n = c/v). The change in speed is the result of electronic polarization. Since the effect of polarization increases with the size of the atoms, glasses which contain heavy metal ions (such as lead crystal) have higher indices of refraction than those composed of smaller atoms (such as soda-lime glass).

Figure 5: This figure represents the refraction of light as it passes from a medium with low optical density (such as air) to one of higher optical density (such as water or glass). Light maintains its frequency but its speed is changed in the more dense medium. Therefore, the wavelength must change accordingly. Snell's law (n1 sin q1 = n2 sin q2) can be used to relate the indices of refraction (n), the angles (q) of incidence and refraction, and the speed (v) of light in the two media: n1/n2 = q2/q1 = v1/v2)

Internal scattering of light in an inherently transparent material may render a material translucent or opaque. Such scattering occurs at density fluctuations, grain boundaries, phase boundaries, and pores.

Many applications take advantage of the optical properties of materials. The transparency of glasses make them useful for windows, lenses, filters, cookware, labware, and objects of art. Conversions between light and electricity are the basis for the use of semiconducting materials such as gallium arsenide in lasers and the widespread use of LED's (light-emitting diodes) in electronic devices. Fluorescent and phosphorescent ceramics are used in electric lamps and television screens. Finally, optical fibers transmit telephone conversations, cable television signals, and computer data based on the total internal reflection of the light signal.

Mechanical Properties:

Mechanical properties describe the way that a material responds to forces, loads, and impacts. Ceramics are strong, hard materials that are also resistant to corrosion (durable). These properties, along with their low densities and high melting points, make ceramics attractive structural materials.

Structural applications of advanced ceramics include components of automobile engines, armor for military vehicles, and aircraft structures. For example, titanium carbide has about four times the strength of steel. Thus, a steel rod in an airplane structure can be replaced by a TiC rod that will support the same load at half the diameter and 31% of the weight.

Other applications that take advantage of the mechanical properties of ceramics include the use of clay and cement as structural materials. Both can be formed and molded when wet but produce a harder, stronger object when dry. Very hard materials such as alumina (Al2O3) and silicon carbide (SiC) are used as abrasives for grinding and polishing.

The principal limitation of ceramics is their brittleness, i.e., the tendency to fail suddenly with little plastic deformation. This is of particular concern when the material is used in structural applications. In metals, the delocalized electrons allow the atoms to change neighbors without completely breaking the bond structure. This allows the metal to deform under stress. Work is done as the bonds shift during deformation. But, in ceramics, due to the combined ionic and covalent bonding mechanism, the particles cannot shift easily. The ceramic breaks when too much force is applied, and the work done in breaking the bonds creates new surfaces upon cracking.

Figure 6: Stress-Strain diagrams for typical (a) brittle and (b) ductile materials

Brittle fracture occurs by the formation and rapid propagation of cracks. In crystalline solids, cracks grow through the grains (transgranular) and along cleavage planes in the crystal. The resulting broken surface may have a grainy or rough texture. Amorphous materials do not contain grains and regular crystalline planes, so the broken surface is more likely to be smooth in appearance.

The theoretical strength of a material is the tensile stress that would be needed to break the bonds between atoms in a perfect solid and pull the object apart. But all materials, including ceramics, contain minuscule structural and fabrication flaws that make them significantly weaker than the ideal strength. Any flaw, such as a pore, crack, or inclusion, results in stress concentration, which amplifies the applied stress. Pores also reduce the cross-sectional area over which a load is applied. Thus, denser, less porous materials are generally stronger. Similarly, the smaller the grain size the better the mechanical properties.

In fact, ceramics are the strongest known monolithic materials, and they typically maintain a significant fraction of their strength at elevated temperatures. For example, silicon nitride (Si3N4, = 3.5 g/cm3) turbocharger rotors have a fracture strength of 120 ksi at 70 F and 80 ksi at 2200 F.

Figure 7: Tensile, compressive and bending testing for materials

Compressive (crushing) strength is important in ceramics used in structures such as buildings or refractory bricks. The compressive strength of a ceramic is usually much greater than their tensile strength. To make up for this, ceramics are sometimes prestressed in a compressed state. Thus, when a ceramic object is subjected to a tensile force, the applied load has to overcome the compressive stresses (within the object) before additional tensile stresses can increase and break the object. Safety glass (thermal tempered glass) is one example of such a material. Ceramics are generally quite inelastic and do not bend like metals. Rigidity varies with the composition and structure. The ability to deform reversibly is measured by the elastic modulus. Materials with strong bonding require large forces to increase space between particles and have high values for the modulus of elasticity. In amorphous materials, however, there is more free space for the atoms to shift to under an applied load. As a result, amorphous materials such as glass are more easily flexed than crystalline materials such as alumina or silicon nitride.

The fracture toughness is the ability to resist fracture when a crack is present. It depends on the geometry of both the object and the crack, the applied stress, and the length of the crack. Composites are being developed which retain the desirable properties of the ceramics while reducing their tendency to fracture. For example, the introduction of carbon fiber whiskers inhibits crack propagation through a ceramic and improves toughness.

Glass ceramics such as those that are used to make ovenware are composed of a matrix of glass in which tiny ceramic crystals grow, such that the final matrix is actually composed of fine crystalline grains (average size < 500 nm). Because their grain size is so small, these materials are transparent to light. In addition, since fracture strength is inversely proportional to the square of the grain size, the materials are strong. In other words, the presence of the crystals improves the mechanical and thermal properties of the glass--the glass ceramics are strong, resistant to thermal shock, and good thermal conductors.

Electrical Properties:

The electrical properties of ceramic materials vary greatly, with characteristic measures spanning over many orders of magnitude (see Table 3). Ceramics are probably best known as electrical insulators. Some ceramic insulators (such as BaTiO3) can be polarized and used as capacitors. Other ceramics conduct electrons when a threshold energy is reached, and are thus called semiconductors. In 1986, a new class of ceramics was discovered, the high Tc superconductors. These materials conduct electricity with essentially zero resistance. Finally, ceramics known as piezoelectrics can generate an electrical response to a mechanical force or vice versa.

Table 3: Electrical Resistivity of different materials.

TypeMaterialResistivity (-cm)
Metallic conductors:Copper1.7 x 10-6
CuO23 x 10-5
Semiconductors:SiC10
Germanium40
Insulators:Fire-clay brick108
Si3N4 > 1014
Polystyrene1018
Superconductors:YBa2Cu3O7-x < 10-22 (below Tc)

Anyone who has used a portable cassette player, personal computer, or other electronic device is taking advantage of ceramic dielectric materials. A dielectric material is an insulator that can be polarized at the molecular level. Such materials are widely used in capacitors, devices which are used to store electrical charge. The structure of a capacitor is shown in the diagram.

Figure 8: Diagram of capacitor.

The charge of the capacitor is stored between its two plates. The amount of charge (q) that it can hold depends on its voltage (V) and its capacitance (C).

q = CV

The dielectric is inserted between the plates of a capacitor, raising the capacitance of the system by a factor equal to its dielectric constant, k.

q = (kC)V

Using materials that have large dielectric constants allows large amounts of charge to be stored on extremely small capacitors. This is a significant contribution to the continuing miniaturization of electronics (e.g., lap top computers, portable CD players, cellular phones, even hearing aids!).

The dielectric strength of a material is its ability to continuously hold electrons at a high voltage. When a capacitor is fully charged, there is virtually no current passing through it.

But sometimes very strong electric fields (high voltages) excite large numbers of electrons from the valence band into the conduction band. When this happens current flows through the dielectric and some of the stored charge is lost. This may be accompanied by partial breakdown of the material by melting, burning, and/or vaporization. The magnetic field strength necessary to produce breakdown of a material is its dielectric strength. Some ceramic materials have extremely high dielectric strengths. For example, electrical porcelain can handle up to 300 volts for every .001 inches (mil) of the material!

Table 4: Electrical property constants of different ceramic materials.
MaterialDielectric constant at 1 MHzDielectric strength (kV/cm)
Air1.0005930
Polystyrene2.54 - 2.56240
Glass (Pyrex)5.6142
Alumina4.5 - 8.416 - 63
Porcelain6.0 - 8.016 - 157
Titanium dioxide14 - 11039 - 83

Electrical current in solids is most often the result of the flow of electrons (electronic conduction). In metals, mobile, conducting electrons are scattered by thermal vibrations (phonons), and this scattering is observed as resistance. Thus, in metals, resistivity increases as temperature increases. In contrast, valence electrons in ceramic materials are usually not in the conduction band, thus most ceramics are considered insulators. However, conductivity can be increased by doping the material with impurities. Thermal energy will also promote electrons into the conduction band, so that in ceramics, conductivity increases (and resistivity decreases) as temperature increases.

Although ceramics were historically thought of as insulating materials, ceramic superconductors were discovered in 1986. A superconductor can transmit electrical current with no resistance or power loss. For most materials, resistivity gradually decreases as temperature decreases. Superconductors have a critical temperature, Tc, at which the resistivity drops sharply to virtually zero.

Figure 9: Electrical Resistivity vs. Temperature for superconducting and nonsuperconducting materials.

Pure metals and metal alloys were the first known superconductors. All had critical temperatures at or below 30K and required cooling with liquid helium. The new ceramic superconductors usually contain copper oxide planes such as YBa2Cu3O7 discovered in 1987 with Tc = 93 K. They have critical temperatures above the boiling point of liquid nitrogen (77.4 K), which makes many potential applications of superconductors much more practical. This is due to the lower cost of liquid nitrogen and the easier design of cryogenic devices.

Figure 10: Unit cell for YBCO superconductor.

In addition to their critical temperature, two other parameters define the region where a ceramic material is superconducting: 1) the critical current and 2) the critical magnetic field. As long as the conditions are within the critical parameters of temperature, current, and magnetic field, the material behaves as a superconductor. If any of these values is exceeded, superconductivity is destroyed.

Applications of superconductors which rely on their current carrying ability include electrical power generation, storage and distribution. SQUIDS (Superconducting Quantum Interference Devices) are electronic devices that use superconductors as sensitive detectors of electromagnetic radiation. Possible applications in the field of medicine include the development of advanced MRI (Magnetic Resonance Imaging) units based on magnets made of superconducting coils.

The magnetic applications of superconductors are also of major importance. Superconductors are perfect diamagnets, meaning that they will repel magnetic fields. This exclusion of an applied magnetic field is called the Meissner effect and is the basis for the proposed use of superconductors to magnetically levitate trains.

Some ceramics have the unusual property of piezoelectricity, or pressure electricity. These are part of a class known as "smart" materials which are often used as sensors. In a piezoelectric material, the application of a force or pressure on its surface induces polarization and establishes an electric field, i.e., it changes a mechanical pressure into an electrical impulse. Piezoelectric materials are used to make transducers, which are found in such common devices as phonograph pickups, depth finders, microphones, and various types of sensors. In ceramic materials, electric charge can also be transported by ions. This property can be tailored by means of the chemical composition, and is the basis for many commercial applications. These range from chemical sensors to large scale electric power generators. One of the most prominent technologies is that of fuel cells. It is based on the ability of certain ceramics to permit the passage of oxygen anions, while at the same time being electronic insulators. Zirconia (ZrO2), stabilized with calcia (CaO), is an example of such a solid electrolyte.

Fuel cells were first used in spacecraft such as the Apollo capsules and the space shuttle. At night the fuel cells were used to generate electric power, by combusting hydrogen and oxygen from gas cylinders. During the day, solar cells took over, and the excess power was used to purify and reclaim oxygen from exhaust gas and the atmosphere exhaled by the astronauts. The lambda probe in the exhaust manifold of cars works on the same principle and is used to monitor engine efficiency.

Ceramic Processing:

Processing of ceramic materials describes the way in which ceramic objects (e.g., glass windows, turbocharger rotor blades, optical fibers, capacitors) are produced.

Processing begins with the raw materials needed to produce the finished components, and includes many individual steps that differ significantly depending on the type of ceramic material, crystalline versus glass.

Processing of Crystalline CeramicsGlass Processing
Raw Material SelectionRaw Material Selection
PreparationMelting
ConsolidationPouring
SinteringAnnealing

Raw material selection involves obtaining and preparing the right materials for the final product. Traditional ceramics use various forms of clay. Glass makers start with primarily silica. Advanced ceramics use several different raw materials depending on the applications (i.e., properties needed).

MaterialUses
Al2O3 (aluminum oxide)Spark-plug insulating bodies,
substrates for microelectronic packaging
MgO (magnesium oxide)electrical insulators, refractory brick
SiO2 (Silicon dioxide)cookware, optical fibers
ZrO2 (zirconium oxide)cubic zirconia, oxygen sensors
SiC (silicon carbide)kiln parts, heating elements,
abrasives
Si3N4 (silicon nitride)turbocharger rotors, piston valves

For crystalline ceramics, the characteristics of the raw materials (powders) such as their particle size and purity are very important as they affect the structure (e.g., grain size) and properties (e.g., strength) of the final component. Since strength increases with decreasing grain size, most starting powders are milled (or ground) to produce a fine powder (diameter < 1 m). Since dry powders are difficult to shape, processing additives like water, polymers, etc. are added to improve their plasticity. Consolidation involves forming the ceramic mixture into the specified shape. There are many techniques available for this step:

Figure 11: Ceramic processing aides.

Sintering is the final step in the process. Sintering at high temperatures (800 to 1800 C) causes densification that gives the ceramic product its strength and other properties. During this process, the individual ceramic particles coalesce to form a continuous solid network and pores are eliminated. Typically, the mictrostructure of the sintered product contains dense grains, where an individual grain is composed of many starting particles.

Figure 12: Microstructure of raw, formed, and sintered ceramic products

Glass processing is different from crystalline processing. One of the considerations that must be examined is the solidifying behavior of glass. Glasses are most commonly made by rapidly quenching a melt. This means that the elements making up the glass material are unable to move into positions that allow them to form the crystalline regularity. The result is that the glass structure is disordered or amorphous.

One of the most notable characteristics of glasses is the way they change between solid and liquid states. Unlike crystals, which transform abruptly at a precise temperature (i.e., their melting point) glasses undergo a gradual transition. Between the melting temperature (Tm) of a substance and the so-called glass transition temperature (Tg), the substance is considered a supercooled liquid. When glass is worked between Tg and Tm, one can achieve virtually any shape. The glass blowing technique is a fascinating demonstration of the incredible ability to deform a glass.

Figure 13: Specific Volume vs. Temperature graph for a typical ceramic material

Glass processing does not require an optimum size particle (although smaller pieces melt faster). The selections of glass raw materials and chemical additives (which, for example, can alter the color of the glass) are heated up (700 - 1600 C), melted and finally poured onto or into a quick-cool form or plate. There are four different forming techniques used to fabricate glass.

TechniqueApplication
PressingTable ware
BlowingJars
DrawingWindows
Fiber formingFiber optics

During the glass formation, there may be stresses that have been introduced by rapid cooling or special treatments that the glass needs (such as layering or strengthening). Additional heat treatment is needed to "heal" the glass. Annealing, in which the glass is heated to the annealing point (a temperature just below the softening point where the viscosity is approximately 108 Poise) and then slowly cooled to room temperature, is one such process. Tempering is also a follow-up heat treatment in glass processing in which the glass is reheated and cooled in oil or a jet of air so that the internal and external parts have different properties. The tempering reduces the tendency of glass to fail. Tempered glass can then be used in conditions prone to stresses like car windows.

Summary:

The term "ceramic" once referred only to clay-based materials. However, new generations of ceramic materials have tremendously expanded the scope and number of possible applications. Many of these new materials have a major impact on our daily lives and on our society.

Ceramic materials are inorganic compounds, usually oxides, nitrides, or carbides. The bonding is very strong--either ionic or network covalent. Many adopt crystalline structures, but some form glasses. The properties of the materials are a result of the bonding and structure.

Ceramics can withstand high temperatures, are good thermal insulators, and do not expand greatly when heated. This makes them excellent thermal barriers, for applications that range from lining industrial furnaces to covering the space shuttle to protect it from high reentry temperatures.

Glasses are transparent, amorphous ceramics that are widely used in windows, lenses, and many other familiar applications. Light can induce an electrical response in some ceramics, called photoconductivity. Fiber optic cable is rapidly replacing copper for communications, as optical fibers can carry more information for longer distances with less interference and signal loss than traditional copper wires.

Ceramics are strong, hard, and durable. This makes them attractive structural materials. The one significant drawback is their brittleness, but this problem is being addressed by the development of new materials such as composites.

Ceramics vary in electrical properties from excellent insulators to superconductors. Thus, they are used in a wide range of applications. Some are capacitors, others semiconductors in electronic devices. Piezoelectric materials can convert mechanical pressure into an electrical signal and are especially useful for sensors. There is now a strong research effort to discover new high Tc superconductors and to develop possible applications.

The processing of crystalline ceramics follows the basic steps that have been used for ages to make clay products. The materials are selected, prepared, formed into a desired shape, and sintered at high temperatures. Glasses are processed by pouring in a molten state, working into shape while hot, and then cooling. New methods such as chemical vapor deposition and sol-gel processing are presently being developed. Ceramics has advanced far beyond its beginnings in clay pottery. Ceramic tiles cover the space shuttle as well as our kitchen floors. Ceramic electronic devices make possible high-tech instruments for everything from medicine to entertainment. Clearly, ceramics are our window to the future.

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