Aluminum, crystal structure thermal expansion

For a nonisotropic crystal, such as a hexagonal one, the thermal expansion varies with the axis. For example, in the case of alumina, its average linear thermal expansion coefficient normal to the c-axis is 8.3 x 10" per °C, whereas it is 9.0 x 10 per °C parallel to the same axis. Another interesting phenomenon taking place in these crystals is that both c/a and a a decrease as temperature is increased. That means, as stated earlier, the structure becomes more symmetrical. Some materials, such as aluminum titanate, show contraction with an increase in temperature in a direction normal to the c-axis. These materials exhibit resistance to thermal shock as their volume expansion with temperature becomes low. 

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