Thermal Properties
All solids expand on heating and contract on cooling. The thermal expansion of solids is because of basic structure in which atoms occupy mean position at a fixed distance from each other. In fact atoms are not stationary however vibrate about mean position which changes with temperature. The distance between mean locations enhance with increasing temperature and decrease with temperature. Such modification in the mean distance between the atoms results in expansion or contraction. Coefficient of linear expansion is explained as the increase in length per unit length per degree rise in temperature. For decrease in temperature same property is regarded as negative. The linear changes in three mutually perpendicular directions will constitute coefficient of volume expansion. If in three orthogonal directions the coefficients of linear expansion are equivalent then the solid is thermally isotropic. The thermal expansion is zero at absolute zero temperature & is generally related to specific heat and melting point of a substance.
It is interesting to note that experiments demonstrate that total volume change on heating between absolute zero and melting point is same for all elements. This means that coefficient of thermal expansion is low for high melting point solids. Most solids that are used for high temperature applications that mean refractory materials have linearly varying thermal coefficients. The exception are silica (SiO2) & zirconia (ZrO2) which because of polymorphic transformation show irregular behaviour .
The coefficient of thermal expansion is significant consideration whereas designing structure to operate at high temperature. The restricted deformations shall cause forces to act upon the part and therefore induce stresses. Further throughout moulding process proper care ought to be exercised for due consideration of volume and linear changes after solidification so that dimensional accuracy and tolerances might be maintained. Apparently this consideration suppose greater importance in case of such materials which are difficult to machine. Ceramics and other refractory materials are instances.
The coefficient of expansion of plastics might be controlled by addition of filler material, generally enhancing filler material would reduce the coefficient. Expansion in case of reinforced plastics tends to occur in the direction of reinforcement. There are various plastics utilized in conjunction with metals of common use and it might be noted that use of fillers enable material engineer to control the coefficient such that metal & plastic expand equally. Unequal expansion shall introduce undesirable deformation and stresses.