Thermal Diffusivity of Heterogeneous Materials

Abstract

The concept of the thermal diffusivity as a characteristic constant of a heterogeneous material is examined. By considering the derivation of the heat conduction equation, and the concept of the thermal wavelength associated with a particular problem, a general criterion for the homogeneity of a material under transient conditions is obtained. This criterion limits the scale of heterogeneity of the material (expressed as a lattice spacing) to be much smaller than the minimum thermal wavelength associated with the problem. The flash method of measuring the thermal diffusivity is considered in detail. The criterion developed limits the particle size of a particulate phase to be much smaller than the sample thickness for the sample to be considered homogeneous, and for the usual relation between the thermal diffusivity and thermal conductivity to hold.