Temperature gradient developed across the thickness of resistance-grown pyrolytic graphite

Abstract

Pyrolytic graphite was prepared by resistance heating of graphite substrate at the deposition temperatures of 1550-2270°C, the gas pressures of 5-50 mmHg, and a gas flow rate of 640 cm³/min. To determine the temperature drop established across a layer of pyrolytic graphite, radial holes about 2 mm in diameter were drilled down to the interface between the deposits 0.36-3.05 mm thick and the graphite substrate. The whole was then heated to various temperatures by passing the requisite currents.
Temperature gradient obtained was 140°C/mm for the deposit (type-(a)) prepared at temperatures below about 1800°C, and 350°C/mm for the deposit (type-(b)) prepared above about 2000°C. An estimate of the c-axis thermal conductivity was made, and it was found to increase from 0.015 to 0.04 w cm^-1 deg C^-1 over the temperature range of 1600-2000°C for the type-(a) pyrolytic graphite and to increase from 0.02 to 0.03 w cm^-1 deg C^-1 over the range 2000-2400°C for the type-(b) pyrolytic graphite. Annealing effect was discussed.