Anisotropy thermal expansion

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. 

Thermal expansion anisotropy in single-phase materials... 

Figure 13.4 Schematic of how thermal expansion anisotropy can lead to the development of thermal stresses upon cooling of a polycrystalline solid, (a) Arrangement of grains prior to cooling shows relationship between thermal expansion coefficients and grain axis. (/>) Unconstrained contraction of grains. Here it was assumed that 0 = 0.

Heating or cooling of ceramics for which the thermal expansion is anisotropic. The magnitude of the stresses will depend on the thermal expansion anisotropy, and can cause polycrystalline bodies to spontaneously microcrack. This damage cannot be avoided by slow cooling, but can be avoided if the grain size is kept small. 

For (random) polycrystals, the thermal expansion coefficient is often estimated by averaging the single-crystal values and these values are included in Table 2.2. In some cases, microcracking caused by thermal expansion anisotropy can influence the overall expansion behavior (see Section 3.7). Although not discussed here, it is also important to note that thermal expansion coefficients often vary with temperature in many ceramic systems. 

One type of pore that is worthy of further consideration is the extreme case of a microcrack. In Section 2.9 it was shown that thermal expansion anisotropy can lead to residual stresses in ceramics and, in some cases, the formation of localized (spontaneous) microcracking. Microcracks may only represent a small fraction of porosity in a body but their ability to concentrate stress can lead to substantial reductions in the elastic constants. For a random array of circular microcracks, radius a, the SC approach shows the elastic constants ix and B of the microcracked material can be approximated by... 

Thermal expansion anisotropy exists in polycrystalline ceramics with crystal structures. 

Figure 7.7 Correlation between thermal expansion anisotropy, as measured by the ratio of the thermal expansion along the a-axis, Oa, to that along the c-axis, and (a) A-group element (b) C13.

The large thermal expansion anisotropy often results in large Internal stresses and structural problems such as delamination between planes as will be seen in Ch. 5. Sec. 3. 

The extreme thermal expansion anisotropy and very low longitudinal expansion of the carbon and aramid fibre systems are clear. All measurements are for room temperature or slightly above. Once the Tg of the polymer matrix has been exceeded the CTE in either direction will increase. 

A. G. Evans, Microfracture from thermal expansion anisotropy— I. Single phase systems, Acta Metcdlurgica, 26(12) 1845-1853,1978. 

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