Thermal Diffusivity of Heterogeneous Materials and Non-Fibrous Insulators

Abstract

The flash technique has proven to be a fast and accurate method for determining the thermal diffusivity (and hence conductivity of a wide range of homogeneous materials from cryogenic temperatures into the molten region. The applicability of this technique has been extended to many heterogeneous materials including certain layered, dispersed and fiber-reinforced composites. Extensions of its use for some types of insulators has been limited due to the larger temperature rise which occurs on the front face of highly insulating materials, partial in-depth absorption of the laser energy by porous or translucent samples and problems associated with rear face temperature transient measurements on such materials. There are also difficulties involved in measuring the thermal diffusivity of large-grain heterogeneous materials where the grain size is of the order of the usual sample thicknesses used. Substituting step heating for the laser pulse tended to overcome problems associated with both large-grain heterogeneous materials and many insulating materials.

A step-wise heating apparatus was developed and tested using several materials including solid insulators and large-grain carbon magnesite brick. Results were compared to the standard flash technique where applicable, and procedures were developed which permitted these materials to be measured quickly and accurately. For example, diffusivity measurements were made on refractory brick materials from room temperature to 500C (seven data points) in less than four hours. Typical results are presented.