Zero-expansion ceramic

There are many examples of rational synthesis. A good example is Sialon, in which A1 and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N4. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed by them. The zero-expansion ceramic CaQ5Ti2P30j2, possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network [3]. 

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. 

Many aluminosilicate glass ceramics are based on framework structures of AIO4 tetrahedra, which, when crystallized, posseses low thermal expansions. This gives the glass ceramics based on them near zero expansions and thus excellent dimensional stability, thermal shock resistance, and mechanical strength. Aluminosilicate glass ceramics are used commercially as telescope mirrors, thermally stable structures for satellites and space probes, gyroscope components, heat-resistant windows, stove tops, and cookware. 

Zirconyl pyrophosphate, (ZrO)2P207, mp = 2070°C, p = 3.88 g/cc, is stable up to about 1600 C, and calcium orthophosphate, Ca3(P04)2 which melts at 1820°C, can be used for some purposes at temperatures up to 1600°C. The zirconyl compound, which loses P2O5 above 1600°C, has potential use as a low-expansion ceramic, although its strength is rather limited [3]. Up to llOO C the net expansion is <1.7 x 10 C (Table 12.27). Zirconyl pyrophosphate is a good electrical insulator and its dielectric constant is similar to that of alumina [4]. Solid solutions of composition Ca Sr,. Zr4(P04)g and Zr 2-Nx i show almost zero thermal expansion over a considerable temperature range, and are promising materials for future application (see below). 

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. 

In addition to Neoceram N-0, Nippon Electric Glass Co., Ltd., also produces another glass-ceramic that demonstrates zero expansion. Neoceram N-11 is a white, nontransparent glass-ceramic that contains P-spodumene solid solution primary crystals measuring approximately 1 pm. This glass-ceramic is used for induction cooker top plates, Kitchenware, (Section 4.2.1) or for optical components, such as the coupler housings of optical fibers (Neoceram 1992, 1995). 

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... 

Neoceram, Zero-Expansion Glass-Ceramics for Innovative Application," Nippon Electric Glass Co. Ltd., 11 (1992). 

Li20-Si02 (LAS). The trade names of such glass-ceramic matrix materials are Corningware, Zerodur and Ceran. This type of glass-ceramic matrix material has nearly zero thermal expansion and high thermal shock resistance. It is used for the production of optical and telescopic mirrors. 

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. 

Therefore the dielectric constant is changed with temperature and the resonant frequency will change with temperature, and the microwave devices cannot respond at a specific frequency if the dielectric materials in microwave frequencies show a large TCK and thermal expansion coefficient a due to the thermal expansion of dielectric materials and the temperature dependence of polarizability. In general, the a of dielectric ceramics, which is well known as the slope of the Cockbain equation, is about 10 ppm/°C. Therefore control of TCP can be achieved by adequate manipulations of TCK. It is an important requirement for practical applications to control the stable TCP, nearly zero, which is available to temperature-stable microwave devices. 

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