The low-temperature specific heat of graphite

Abstract

In a companion paper, a calorimeter especially suitable for the accurate low-temperature specific-heat measurement of polycrystalline materials was described and its use with graphite samples was reported. Using this apparatus, it was noted that there were significant differences in the heat capacity of various types of graphite. The present paper deals with further experiments and theoretical analysis undertaken to identify the cause of these differences. The experimental program was based on a set of graphite samples prepared in such a way as to control electronic and structural factors in nearly independent fashion. The results show that the major difference in heat capacity between various graphites, in the liquid-helium temperature range, is not due to electronic factors. The theoretical analysis leads to the conclusions that, first, both electronic and lattice-vibration theories are in good agreement with experiment, and, second, in lampblack graphite the random stacking of layer planes can account both qualitatively and quantitatively for the specfic heat in excess of that of natural graphite. Estimated values of the elastic constants of graphite are presented.