Small area contact methods

Abstract

In measuring the termal conductivity k of materials at high temperatures, error due to heat radiation may be suppressed by making the sample very small (less than 1 mm), or by carrying out the measurement in a small region of a larger sample. To do this, one causes electrical heating in a small region of the sample near a small area contact, observes the temperature rise indirectly by measuring the thermoelectric voltage δV or the change in resistance δR at the contact, and relates these to the applied voltage V, by equations which do not include factors dependent upon the detailed shape of the contact or of the sample. With this approach one can make complete measurements of thermal and electrical properties of very small samples, including the dependence on temperature of the electrical and thermal conductivities, the Seebeck coefficient, and the thermal diffusivity. Small area contact methods are being applied at our laboratories to three domains of measurements. For measurements at moderately high temperatures (up to 600°C), experimental techniques are being developed to exploit the advantages of working on a semi-micro scale. Since small samples are used, it is practical to use a hot plate in a bell jar, and a relatively small heater. A flexible arrangement which has been developed for experimental work permits the use of micromanipulators and allows visual observation of the sample under a microscope as it is being heated. For studying high-temperature thermoelectric materials, techniques have been evolved for making small area contacts in materials which tend to shatter at lower temperatures. In this case, a wire is dipped into the molten material, and the sample is then cooled to the range where measurements are made. For studies of thermal conductivity at very high temperatures (∼2000°C), where high solubility and high rate of diffusion become problems, the small area contact configuration has been modified to a necked-down region between two wider parts in a homogeneous sample. By observing the δR which occurs because of Joule heating, one can determine σ and k. In all the applications, the agreement with conventional measurements that has been obtained shows that the basic equations are correct. Various applications also show that the small area contact method is quite versatile and that it presents possibilities for avoiding or minimizing difficulties which occur in using conventional techniques. Experimental problems encountered in using the technique are being studied and have not, so far, proved to be excessively formidable.