Hot strip determination of the thermal conductivity tensor and heat capacity of crystals

Abstract

A method of determining the principal thermal conductivities and the specific heat capacity for an anisotropic crystal is presented, tested, and discussed. Two parallel metal strips are deposited onto a plane face of the specimen. One strip serves as a heater, the other as a temperature sensor. A pulse of constant dc power is applied to the heater, and the response at the sensor is measured by means of an ac bridge, monitored by a lock-in amplifier. The experiment is on-line with a PDP-11/34 computer system. The expression for the temperature rise in case of infinitely long strips is fitted to the sensor temperature data. In the isotropic case one experiment yields both thermal conductivity and heat capacity. In an anisotropic case experiments have to be performed with two or three different strip orientations, if all principal thermal conductivities are to be determined. A precision of 0.1% is readily obtained, and the method is thus valuable in cases where small changes in thermal conductivity are to be determined. We estimate that the accuracy generally obtainable is about 2% as regards thermal conductivity and heat capacity. The accuracy in determining thermal diffusivities is mainly limited by geometrical factors and may ultimately be better than 1%. Results are given for isotropic CaF2 and anisotropic SiO2 at 300 K.