Glass-ceramics with zero expansion

According to Beall and Pinckney (1999), three important criteria must be observed in the development of glass-ceramics with zero expansion. The first two requirements involve low scattering, while the third requirement involves low ionic or atomic absorption. First, the index of refraction of the glass and the crystal phase must be almost identical. Second, the crystallite... 

Tauch, D., Russel, C. Glass-ceramics with zero thermal expansion in the system bao/al2o3/b2o3. J. Non-Cryst. Solids 351(27-29), 2294-2298 (2005)... 

Therefore it would be sheer folly to attempt developing a glass-ceramic with a stable assemblage of cordierite (Mg2Al Si50jg) and P-eucryptite (LiAlSiO ), even though it would seem useful to have sudi a phase mixture to achieve a zero coefficient of thermal expansion in a high-temperature material. 

Thermal Properties. Many commercial glass-ceramics have capitalized on thek superior thermal properties, particularly low or zero thermal expansion coupled with high thermal stabiUty and thermal shock resistance properties that are not readily achievable in glasses or ceramics. Linear thermal expansion coefficients ranging from —60 to 200 x 10 j° C can be obtained. Near-zero expansion materials are used in apphcations such as telescope mirror blanks, cookware, and stove cooktops, while high expansion frits are used for sealing metals. 

Glass-ceramics based on the LijO-AljC -SiC can be tailored, principally through varying the alumina content, to have linear thermal expansivities in the range from close to zero to approximately 18 MK-1. The low expansion materials have excellent resistance to thermal shock whilst those with the higher expansivities can be successfully joined to a range of metals. 

Note that in general the nitrides and carbides of Si, with their lower thermal expansion coefficients, are more resistant to thermal shock than oxides. In theory, a material with zero thermal expansion would not be susceptible to thermal shock. In practice, a number of such materials do actually exist commercially, including some glass-ceramics that have been developed which, as a result of thermal expansion anisotropy, have extremely low a s (see Ch. 4). Another good example is fused silica which also has an extremely low a and thus is not prone to thermal shock. 

Hyde [180] at GEC Engineering Research Centre, Stafford describes how a continuous ceramic fiber preform is impregnated with a sol, which then sohdifies into a gel. Carbon fiber reinforced silica made in this way has non-zero thermal expansion, but much higher strength than a zero-expansion glass ceramic. 

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