The induction heating of pyrolytic graphite

Abstract

It is shown that in the induction heating of a long cylinder of sufficient wall thickness, the temperature difference between the inside and outside wall, for a given power input, is inversely proportional to the square root of the frequency. This can be most easily observed in materials of low thermal conductivity, and is here applied to pyrolytic graphite, which is a good thermal insulator normal to the surface on which it is deposited. At outer temperatures of 1000–1500°C, the temperature difference across a pyrolytic graphite wall is negligible at 500 kc/s, but is remarkably large at frequencies below 10 kc/s. For example, using about 25 kW of power at 3·6 kc/s, a pyrolytic graphite crucible 105 mm dia. and 160 mm long, with about 10 mm total wall thickness, was heated to a surface temperature of 1500°C, while at the same time the interior temperature rose to 2700°C in equilibrium. Using a pyrolytic graphite susceptor in this way, sizable systems can be heated to high temperatures without any additional thermal insulation. The larger the diameter of the susceptor, the lower the frequency which can be used for efficient coupling, and thus the larger the temperature difference which can be established across the susceptor wall. Furthermore, when used as radiation screening, thermal shields of slotted pyrolytic graphite are up to 100 per cent more effective at frequencies below 10 kc/s than at radio frequencies.