Effect of Hot-pressing Pressure on Some Properties of Polycrystalline Graphite made from Coke Powder with addition of Boron Carbide

Abstract

Polycrystalline graphites were made from a calcined pitch coke powder with boron carbide (B4C) additive in amounts varying from0to16.7wt% by hot-pressing at 2200°C under various pressures from 20 to 400 kg/cm2. The effect of hot-pressing pressure was investigated on some properties such as density, bending strength, electrical resistivity, graphitization degree and oxidation behaviour of the hot-pressed solids.It was found that the density and bending strength of the hot-pressed solids increased remarkably with increasing hot-pressing pressure. The obtained values were, however, depended on the content of B4C additive;that is, in the cases of4.8wt% and16.7wt% B4C additions, the bending strength was about900kg/cm2 and about1400kg/cm2 respectively for solids made at a maximum pressure of400kg/cm2.Electrical resistivity decreased with the increase of hot-pressing pressure and was abou 0.5-0.6×10-3 ohm-cm for all the hot-pressed solids with B4C additive made at400kg/cm2.The d (002) spacings of the hot-pressed solids expanded slightly and three dimensional disordering of graphite structure occurred with the increase of hot-pressing pressure.Their graphitization degree, however, progressed much more than those for the solids without B4C additive. The degradation behaviour of graphite structure is thought to be due to boron diffusion and its interstitial occupation between layers in addition to substitutional positions.All the hot-pressed solids made with B4C additive showed excellent resistance against air oxidation and H2SO4-HNO3acid solution, compared with those of solids without B4C additive and some conventional graphites.