FOR MIKE BAYFIELD

12.1 Classes of Materials 491

Insulators and Ceramics Semiconductors have band gap energies that range from ~50 kJjmol to ~300 kJjmol (Table 12.1).Insulators have a band structure similar to that found in semiconductors, except that in insulators the band gap is larger-more than ~350 kJjmol (Table 12.1 and Figure 12.2). Because the energy required to pro- mote an electron from the valence band to the conduction band is about the same as the energy required to break chemical bonds in the material, insulators are not electrically conductive. Many ionic solids and complex network solids and most organic compounds are insulators; elements that are solid insulators include carbon in its diamond form and sulfur.

Ceramics are inorganic ionic solids that are normally hard and brittle and are stable at high temperatures. They are generally electrical insulators. Ceram- ic materials include such familiar objects as pottery, china, cement, roof tiles, and spark-plug insulators.

Ceramic materials (Table 12.2T) come in a variety of chemical forms, in- cluding oxides (oxygen and metals), carbides (carbon and metals), nitrides (nitro- gen and metals), silicates (silica, Si02, mixed with metal oxides), and aluminates (alumina, A1203, mixed with metal oxides).

Ceramics are highly resistant to heat, corrosion, and wear; do not readily deform under stress; and are less dense than the metals used for high- temperature applications. Some ceramics used in aircraft, missiles, and space- craft weigh only 40% as much as the metal components they replace. In spite of these many advantages, the use of ceramics as engineering materials has been limited because they are extremely brittle. Whereas a metal component might suffer a dent when struck, a ceramic part typically shatters because the greater extent of ionic bonding in a ceramic prevents the atoms from sliding over one another.

TABLE 12.2 Properties of Some Ceramic and Nonceramic Materials

Coefficient Melting Density Hardness Modulus of of Thermal

Material Point (0C) (g/cm3) (Mohs)" Elasticity'' Expansion"

Ceramic materials Alumina, AI203 2050 3.8 9 34 8.1 ~ ]-DMODELSilicon carbide, SiC 2800 3.2 9 65 4.3 Silicon Carbide Zirconia, Zr02 2660 5.6 8 24 6.6 Beryllia, BeO 2550 3.0 9 40 10.4

Nonceramic materials Mild steel 1370 7.9 5 17 15 Aluminum 660 2.7 3 7 24

aThe Mohs scale is a logarithmic scale based on the relative ability of a material to scratch another softer material. Diamond, the hardest material. is assigned a value of 10. b A measure of the stiffness of a material when subjected to a load (MPa x 104). The larger the number, the stiffer the material. eIn units of (K-1 x 10-6). The larger the number, the greater the size change upon heating or cooling.

Superconductors Even metals are not infinitely conductive; there is some resistance to electron flow. However, in 1911the Dutch physicist H. Kamerlingh Onnes discovered that when mercury is cooled below 4.2 K, it loses all resistance to the flow of an electrical current. Since that discovery, scientists have found that many sub- stances exhibit this "frictionless" flow of electrons, known as superconductivity. Substances that exhibit superconductivity do so only when cooled below a particular temperature called the superconducting transition temperature, Te.