Glass-ceramics chemical properties

The glass-ceramic phase assemblage, ie, the types of crystals and the proportion of crystals to glass, is responsible for many of the physical and chemical properties, such as thermal and electrical characteristics, chemical durabiUty, elastic modulus, and hardness. In many cases these properties are additive for example, a phase assemblage comprising high and low expansion crystals has a bulk thermal expansion proportional to the amounts of each of these crystals. 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Because of their unique combination of physical and chemical properties, manufactured carbons and graphites are widely used in several forms in high temperature processing of metals, ceramics, glass, and fused quartz. A variety of commercial grades is available with properties tailored to best meet the needs of particular appHcations (45). Industrial carbons and graphites are available in a broad range of shapes and sizes. 

Silicon shows a rich variety of chemical properties and it lies at the heart of much modern technology/ Indeed, it ranges from such bulk commodities as concrete, clays and ceramics, through more chemically modified systems such as soluble silicates, glasses and glazes to the recent industries based on silicone polymers and solid-state electronics devices. The refined technology of ultrapure silicon itself is perhaps the most elegant example of the close relation between chemistry and solid-state physics and has led to numerous developments such as the transistor, printed circuits and microelectronics (p. 332). 

Beryllium oxide shows excellent thermal conductivity, resistance to thermal shock, and high electrical resistance. Also, it is unreactive to most chemicals. Because of these properties the compound has several applications. It is used to make refractory crucible materials and precision resistor cores as a reflector in nuclear power reactors in microwave energy windows and as an additive to glass, ceramics and plastics. 

Chemical Properties. The chemical durability is a function of the durability of the crystals and the residual glass. Generally, highly siliceous glass-ceramics with low alkali residual glasses, such as glass-ceramics based on quartz and (3-spodumene, have excellent chemical durability and corrosion resistance similar to that obtained in borosilicate glasses. 

The name fine ceramics is based on the grain size distribution of the hard components in the ceramic mass. This rather differs from the distribution as it is seen in the ceramic branch of industry which produces for instance bricks, the coarse ceramic industry. Another difference is that all fine ceramic products are provided with a protective and in some cases also decorative coating, a so-called glaze. In this section much attention will be paid to glazes because this technique is rather unique for fine ceramics and because it offers the possibility to explore the subject glass and some important physical and chemical properties of materials. 

In many cases, more than one crystalline phase forms. Often, phases that form initially transform into another phase as heat treatment progresses. For example, crystalline phases of /5-quartz solid solutions (e.g., (S-eucryptite), which are not stable at high temperatures, convert to more stable phases (e.g., 6-spodumene, cristobalite, sapphirine, or lithium disilicate). The stable high temperature phase may have different properties— in this case, higher thermal expansion. Therefore, glass ceramics with the same chemical composition may have very different properties, depending on the heat treatment. 

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