Experimental study of the sublimation of graphite at high temperatures

Abstract

The equilibrium vaporization of pyrolytic graphite was studied using rf induction heating and a time‐of‐flight mass spectrometer. rf induction heating was demonstrated for the first time as a compatible and practical heating technique for high temperature mass spectrometry. Relative ion intensities were measured for C₁(g), C₂(g), C₃(g), and C₄(g) in the temperature range 2320 K‐3000 K and partial vapor pressures were obtained. The vapor pressures of C₂ and C₃ in the temperature range 2400 K‐3000 K can be represented by logP(C₂) (atm) = 9.971 − 44 230/T(K) and logP (C₃) (atm) = 9.944 − 40 670/T(K). Heats of vaporization obtained by the second and third law methods are as follows: $$\begin{array}{ccc}\mathrm{Species}& \mathrm{Second\; law}{\mathrm{\Delta H°}}_{298}(\mathrm{kcal}/\mathrm{mole})& \mathrm{Third\; law}{\mathrm{\Delta H°}}_{298}(\mathrm{kcal}/\mathrm{mole})\\ {\mathrm{C}}_1& \text{170.3 ± 1.1}& \\ {\mathrm{C}}_2& \text{206.0 ± 1.3}& \text{202.7 ± 0.6}\\ {\mathrm{C}}_3& \text{194.4 ± 1.2}& \text{202.1 ± 1.5}\\ {\mathrm{C}}_4& & \text{242.7}\end{array}$$ Thermodynamic functions calculated by Strauss and Thiele were used forC₃. JANAF thermodynamic data were used for all other species. The entropy change in the vaporization of C₃ determined from this work is 45.55 eu at 2740 K.