Devitrified Glass

any of the inorganic materials obtained by the volume crystallization of glass and composed of one or more crystalline phases that are uniformly distributed throughout the vitreous phase. By selecting the composition of the glass, in most cases containing additives that accelerate the volume crystallization (catalysts, nucleators), it is possible to plan the corresponding crystalline and vitreous phases. The crystals in the planned phases appear and grow uniformly throughout the bulk of the material as a result of heat treatment. Devitrified glass was first made in the 1950’s. In most devitrified glass the size of the crystals does not exceed 1 micron and the extent of the crystalline phase varies from 20 to 95 percent by volume. In other countries, materials similar to devitrified glass are called Pyroceram, devitroceramics, or vitreous porcelain.

The properties of devitrified glass depend on the properties of both the crystalline and vitreous phases. For example, devitrified glass containing mullite has an excellent high-temperature strength and functions well as an electrical insulator; if the devitrified glass contains β-spodumene as the crystalline phase, it may have a low (zero or even negative) coefficient of thermal expansion. Devitrified glass is characterized by an absence of porosity; it does not absorb water, is impermeable to gases, and has a high heat resistance and low thermal conductivity. The density ranges from 2,400 to 2,700 kg/m³; bending strength is from 100 to 200 meganewtons/m², and compressive strength is between 500 and 1,000 meganewtons/m². Devitrified glass is a good dielectric. Most types of devitrified glass are opaque, but there are some in which the crystal dimensions are small compared with the wavelength in the visible part of the spectrum. Such types are transparent, and their integrated transmission can reach 70 to 80 percent for a thickness of 10 mm. The properties of devitrified glass do not change with lengthy storage.

The production technology for items of devitrified glass differs little from that for glass items. In some cases, items can be formed by the methods of ceramic technology. Sometimes photosensitive additives are introduced into the glass composition to generate the crystals. Certain types of devitrified glass are produced by using slags. Devitrified glass is used in soldering and in hermetically sealing electronic vacuum devices. It is also used in optics and in manufacturing, for example, electrical insulators and tableware.