Observed Dependence of the Low-Temperature Thermal and Electrical Conductivity of Graphite on Temperature, Type, Neutron Irradiation, and Bromination

Abstract

The thermal conductivity of various graphites in the temperature interval 10\ifmmode^\circ\else\textdegree\fi{} to 300\ifmmode^\circ\else\textdegree\fi{}K has been determined and the effect of neutron irradiation and bromination investigated. The thermal conductivity of large crystallite natural graphite and of nonpitch-bonded graphite is found to vary as ${T}^{2}$ at low temperatures, as does the specific heat, in accordance with the simple theory of lattice heat conduction. This is in contrast to the anomalous ${T}^{2.7}$ dependence exhibited by various pitch-bonded graphites. The anomaly is explained in a subsequent paper in terms of the effect of ungraphitized pitch on the total thermal resistivity of pitch-bonded graphites.Neutron irradiation is observed to cause the thermal conductivity of graphite to decrease markedly at a rate which decreases with exposure time. Also, the exponent of the temperature dependence decreases with exposure. The effect of bromination on the thermal conductivity of graphite is determined and compared with the effect of neutron irradiation. The results indicate that the change in the concentration of conduction electrons is not the principal mechanism by which neutron irradiation decreases the thermal conductivity.The electrical resistivity shows a negative temperature coefficient for all graphites, except the large crystallite natural graphite. Neutron irradiation increases the electrical resistivity to a saturation value and decreases the magnitude of the temperature coefficient.