Vaporization Coefficient of Graphite and Composition of the Equilibrium Vapor

Abstract

Direct comparisons of the rates of evaporation and of effusion from a Knudsen cell yield a value of about 0.15 for the vaporization coefficient of graphite. The absolute effusion rates, taken together with other data, not only confirm the existence of triatomic carbon in the equilibrium vapor but permit the evaluation of the thermodynamic properties of the triatomic vapor. In the region of 2400°K the vapor pressure of the triatomic form is logp3(atmos)=−40296.0T+9.811which yields 184.4 kcal/mole for the heat of sublimation and 77.41 eu for the entropy of C3. This value for the heat agrees with that obtained in mass spectrometric measurements by Chupka and Inghram. The partial pressures at 2400°K of C1, C2, and C3 are 4.29×10—8, 1.13×10—8, and 1.05×10—7 atmos respectively. A comparison with the mass spectrometric data yields 2.7 for the ratio of the ionization cross section of C3 to C1 and values of 0.37, 0.34, and 0.08 for the individual vaporization coefficients of C1, C2, and C3 respectively. The vapor pressure equations used to determine the amount of monatomic and diatomic carbon in the equilibrium vapor are discussed.