Methods of Measuring Thermal Expansion

Abstract

When heat is added to or removed from a solid material there is a change in dimensions, △L. If the material is isotropic, then the change in dimensions is the same in all directions and the mean coefficient of linear thermal expansion is defined as $$ {{\alpha }_{m}} = \frac{1}{{{{L}_{0}}}}\frac{{\Delta L}}{{\Delta T}} $$ (1) where Lo is the length at some reference temperature, preferably 293 K. The limiting value of this definition (at constant pressure P) for a differential change in temperature is defined as the coefficient of linear thermal expansion or as the expansivity $$ \\alpha = \frac{1}{{{L_0}}}{\left( {\frac{{\partial L}}{{\partial T}}} \right)_p}\ $$ (2) The relative change in dimensions, or the thermal expansion (△L)/L o, is usually expressed in parts per million (µm/m) or as a percent.