Ceramics and glasses

Ceramics are materials of both the past and the future. They are the most durable of all materials—ceramic pots and ornaments survive from 5000 BC; Roman cement still bonds the walls of villas. It is ceramics ‘ durabil­ity, particularly at high temperatures, that generates interest in them today. They are exceptionally hard (diamond, a ceramic, is the hardest of them all) and can tolerate higher temperatures than any metal. Ceramics are crystalline (or partly crystalline) inorganic compounds. They include high-performance technical ceramics such as alumina (used for elec­tronic substrates, nozzles, and cutting tools), traditional, pottery-based ceramics (including brick and whiteware for baths, sinks, and toilets), and hydrated ceramics (cements and concretes) used for construction. All are hard and brittle and have generally high melting points and low ther­mal expansion coefficients, and most are good electrical insulators. When perfect they are exceedingly strong, but tiny flaws, hard to avoid, propa­gate as cracks when the material is loaded in tension or bending, drasti­cally reducing the strength. The compressive strength, however, remains high (eight to 18 times the strength in tension). Impact resistance is low, and stresses due to thermal shock are not easily alleviated by plastic deformation, so large temperature gradients or thermal shock can cause failure.

Glass. Discovered by the Egyptians and perfected by the Romans, glass is one of the oldest manmade materials. For most of its long history it was a possession for the rich—as glass beads, ornaments, and vessels and as glaze on pottery. Its use in windows started in the 15th century, but it was not widespread until the 17th. Now, of course, it is so universal and cheap that, as bottles, we throw it away.

Glass is a mix of oxides, principally silica, SiO2, that does not crystallize when cooled after melting. Pure glass is crystal-clear. Adding metal oxides produces a wide range of colors. Nickel gives a purple hue, cobalt a blue, chromium a green, uranium a green-yellow, iron a green-blue. The addition of iron gives a material that can absorb wavelengths in the infrared range so that heat radiation can be absorbed. Colorless, nonmetallic particles (fluo­rides or phosphates) are added from 5-15% to produce a translucent or an almost opaque white opalescence in glass and glass coatings. Photochromic glass changes color when exposed to UV Filter glass protects from intense light and UV radiation; it is used in visors for welding.

Profiles for six ceramics follow, in the order in which they appear in Table 12.1.