Thermal Effects in the HTBB-3200pg Furnace on Metal-Carbon Eutectic Point Implementation

The general statement that a temperature fixed-point cell will show better melting and freezing plateaux with better temperature uniformity along the dimensions of the fixed point is understood to be valid for metal-carbon (M-C) eutectics as well as for pure metal fixed points. In this article, it is shown that improved temperature uniformity in the central part of the high-temperature blackbody BB3200pg (HTBB), where the M-C fixed point is implemented, results in flatter and longer plateaux. Pyrolitic graphite rings, clamped together by a spring, form the heated cavity of the HTBB. As a first step, the relative electrical resistivity of each pyrolitic graphite ring was measured using a method advised by the furnace manufacturer. Next, the ring positions were optimized, taking into account their relative resistivities, in order to obtain a more homogeneous temperature distribution. Subsequent measurement of the temperature uniformity at the furnace walls confirmed the improvement. Measuring the melting plateaux of the Pt-C eutectic with different arrangements of the rings, and thereby operating the fixed-point cell in different temperature distributions, confirmed the influence of the temperature distribution on the plateau shape, with the best plateau shape corresponding to the most homogeneous temperature distribution.