Melting Line of Graphite.

Abstract

A thin plate of highly oriented pyrolytic graphite (HOPG) with the hexagonal axis (c axis) perpendicular to its surface was sandwiched between two plates of the window material and heated by an electric current pulse. The quasistatic heating process has been affected, in which the graphite sample undergoes thermal expansion only along the c axis and is melted at a pressure of 0.3-2GPa. The set of thermodynamic quantities characterizing completely the thermodynamic states of the sample in such a process (specific volume, enthalpy, temperature, and pressure) as well as the electrical resistivity, were measured with an uncertainty <5%. It has been found that under the above pressures the HOPG melts at the temperatures of 6.3 to 6.7kK, which are substantially higher than the literature values derived from indirect measurements. The jumps in the volume, resistivity and enthalpy of carbon on melting have been determined as well as values of the isochoric heat capacity and the sound velocity of the graphite and liquid carbon. The heat capacities in the vicinity of the melting line turned out to be close to the Dulong-Petit value while the sound velocity of liquid carbon clearly demonstrates an increase with volume indicating a change from the planar sp^{2} to tetrahedral sp^{3} covalent bonding.