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Classification and application of ceramics

Most of the ceramics used in electric light sources belong to special ceramics. According to the difference in structure, as mentioned above, ceramics can basically be divided into two categories, namely ordinary ceramics and special ceramics.

Ceramics used in electric light sources are divided into porous and dense categories.

The microstructure of porous ceramics is usually composed of crystal grains, precipitates outside the crystal grain boundary and pores. Because the refractive index of crystal grains is usually larger than the refractive index of air (n=1), the light incident into the ceramic is scattered at the crystal grain boundary, which seriously affects the light transmission performance. Therefore, ordinary ceramics are opaque and cannot be used as optical materials. They can only be used as device ceramics, such as lamp brackets, insulators, lamp holders and other light source components that require high insulation and heat resistance, as well as installation, fixation, insulation, etc. in electrical appliances. Translucent ceramics in dense ceramics (such as transparent alumina ceramics) were invented due to the need for high-pressure sodium lamp light tubes, which has become the main use.

According to the main components of the porcelain blank, the device porcelain can be divided into barium feldspar porcelain, high-aluminum porcelain, magnesia porcelain, wollastonite porcelain and zircon porcelain, etc. These ceramics have good electrical properties, among which high-aluminum porcelain and magnesia porcelain are the most widely used.

High-aluminum porcelain is a kind of ceramic with A1-O? and SiO. as the main components, of which the content of A1.O, is above 45%. The properties of ceramics are closely related to their composition. In high-aluminum porcelain, with the increase of A1:O3 content, the performance indicators of porcelain are improved. According to the different weight percentage of A1:Os, it is also called 75 porcelain, 92 porcelain, 95 porcelain, 99 porcelain, etc.

A1:O? has three crystal forms; a-ALO?, β-A1?O? and Y-A12Os. They are isomers with different chemical properties. Among them, a-A1O? and Y-A1,Os are of great significance.

a-A1:O: has the most stable chemical properties among isomers. In nature, it exists in the form of corundum (a product of industrial bauxite after chemical cleaning and heat treatment, containing 99.0~99.5% A1:Os). The red transparent one is called ruby (containing chromium), the blue transparent one is called sapphire (containing titanium), and the colorless transparent one is called white jade. Various alumina hydrates can be transformed into a-A12O3 by sintering at high temperature (900~1200℃).

When the content of A1O3 is high, the main crystal phase is a-A1,Os; with the increase of SiO:, mullite (3A1:O3·2SiO2) crystal phase also appears. In 99 porcelain and 95 porcelain, corundum is the only main crystal phase; in 75 porcelain, in addition to corundum, there is also a small amount of mullite.

In electric light sources, ceramic materials used as brackets and insulation are mostly high-aluminum porcelain. Magnesia porcelain is a porcelain with MgO-containing aluminum silicate as the main crystal phase. It is generally used for high-frequency insulation parts. In addition, due to its high expansion coefficient (80~100x10-K-1), which is similar to that of some glasses and alloys, it can be used as vacuum sealing ceramics.

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