Ambient and high temperature experiments on boron-doped polycrystalline graphites

Abstract

Physical properties have been measured as functions of boron content and temperature for a series of extruded, resin bonded graphites. The boron concentration range considered was from 0 to 0.79 weight per cent; up to at least 0.16 per cent the solubility appears to be substitutional. Effects of the boron at room temperature were determined for lattice spacing, elastic modulus, electrical resistivity and thermal conductivity. Temperature effects on electrical resistivity and thermal conductivity are reported to 2500°C and for coefficient of thermal expansion to 645°C. The observed results are discussed in qualitative terms and appear to be self-consistent. The boron exhibited a surprisingly strong effect on the thermal conductivity: an addition of 0.1 per cent B reduced the room temperature λ by ~ 20 per cent although the electrical conductivity was simultaneously increased. Temperature effects on the electrical resistivity vary in a complex manner with boron concentration and it is of interest to note that at 0.081 per cent B the electrical resistivity was found to be stable and constant within about ±3 per cent over the temperature interval 25–2500°C.