Interpretation of the Low-Temperature Thermal Conductivity of Graphite

Abstract

The experimental data observed by Smith and Rasor in measuring the low-temperature thermal conductivity of several types of graphites are interpreted on the basis of a two-medium theory. The theory assumes that, in the artificial polycrystalline graphites, the nongraphitic regions are isotropic thermal conductors and that these regions can be taken in series with the graphite particles to obtain a total conductivity. In this way, the anomalous temperature dependence of the artificial graphites can be immediately explained. Using this theory as a guide, the crystallite size of a given sample can be found in an unambiguous manner, and for the various graphite types discussed by Smith and Rasor are found to correlate with independent previous estimates. Furthermore, the observed effects of neutron damage and bromination can be readily explained in a phenomenological manner. Although this theory still needs refining in several aspects, it appears to be substantiated by all the available evidence and helps to establish the usefulness of low-temperature thermal conductivity measurements as a probe to detect internal defects and microstructure ln graphite.