Dissociation Pressure of Aluminum Carbide Using a Rotating Knudsen Cell

An attempt was made to eliminate surface depletion effects in the vaporization of aluminum carbide (Al₄C₃) by carrying out measurements in a Knudsen cell rotating at an angle of 45 deg from the vertical. Results showed that the surface depletion effects encountered with this material are due to formation of layers of graphite over individual particles of aluminum carbide rather than to formation of a macroscopic layer of graphite over the sample. In order to achieve near-equilibrium pressures, vaporization experiments were carried out in which the sample was ground prior to each experiment and the amount of aluminum vaporized during each experiment minimized. Third law treatment of these data yielded ΔH° (298) of 91.6 kcal mol^-1 (383.3 kJ mol^-1) for the decomposition of ¼Al₄C₃ to form Al(g) and ¾C(C), in good agreement with data based on calorimetric studies.