The coefficient of area expansion is a factor that relates change in a material’s external area to temperature. Materials generally expand as they are heated, causing their overall surface area to increase. The extent of this area increase for each degree on a temperature scale is the area expansion coefficient.
Stricter definitions of area expansion define a pressure for the expansion coefficient because pressure may slightly alter the degree of expansion of a heated material. Expansion coefficients may also be defined at specific temperature ranges because of the nonlinearity of the expansion of materials in certain cases, such as when they undergo phase changes. For linear expansion, the coefficient is a constant value. Nonlinear expansion over a range of temperatures can be defined using a function of temperature, pressure and, in some cases, volume.
To obtain this expression, the infinitesimal change in area that corresponds to an infinitesimal change in volume is defined for the material. The changes in area are experimentally measured or modeled at each temperature, and the values are curve-fitted or numerically integrated to give the total expression. For the coefficient of area expansion to be valid, the material must be isotropic, having the same physical properties in all directions. Otherwise, different directions will have different expansion coefficients.