Graphite-liquid-vapor triple point pressure and the density of liquid carbon

Abstract

A detailed experimental investigation of the graphite-liquid-vapor triple point using a 400 W Nd:YAG continuous-wave laser as the heat source was completed and the triple point pressure accurately placed at 107 ± 2 atm (10.8 ± 0.2 MPa). Both X-ray diffraction and detailed microstructure of the recrysallized graphite were obtained to confirm melt. Careful experimental procedures were employed to address some of the remaining uncertainties in previous carbon triple point studies. These included accurate pressure measurements combined with apparatus designed to minimize pressure excursions. Also systematic melt experiments were completed in helium and argon as a function of sample size and laser power density to assure that sufficient power was available to produce melt at the triple point pressure and to prove, for the first time, that only carbon vapor was present at the sample surface during melting. Maximum mass loss rates were measured and used to confirm that non-equilibrium pressure excursions were experimentally insignificant. The presence of a bright laser-generated vapor or particle plume interfered with the temperature measurement and prevented the determination of the carbon triple point temperature. Finally, the density of liquid carbon near the triple point was calculated by quantitatively measuring the void fraction and density of the recrystallized melt. This density was found to be 1.37±0.06 g/cm 3.