Vapor Pressure and Heat of Sublimation of Tungsten.

The vapor pressure of tungsten was measured by the Langmuir method in the temperature range 2600 to 3100 K using a vacuum microbalance. Four series of data gave concordant results and three of the four series gave second and third law heats of sublimation in excellent agreement. A vapor pressure equation representing the data is log P(atm) = -45385/T + 7.871, based on our mean third law heat and tabulated entropies at 2800 K. The mean third law heat of sublimation at 298.15 K is 205.52 ± 1.1 kcal mol-1 (859.90 ± 4.6 kJ mol-1) where the uncertainty is an overall estimated error. Rates of vaporization are about 1/2 those previously accepted for tungsten.

Vapor Pressure and Heat of Sublimation of Platinum.

The vapor pressure of platinum was measured by the Langmuir method in the temperature range 1700-2000 K using a vacuum microbalance. Eight series of data gave concordant results and an average third-law heat of sublimation of 564.49 kJ mol-1 with an estimated overall uncertainty of 2.1 kJ mol-1 (134.92 ±0.5 kcal mol-1). Three out of eight second-law heats agreed with the third-law heats within one standard error but there was a tendency for second-law heats to be low. This was attributed to small systematic errors in the measurements. A vapor-pressure equation representing the data is log P(atm) = -29020/T+7.502, based on our third-law heat and tabulated entropies evaluated at 1800 K. Our data agree well with several previous Langmuir determinations but significantly decrease the error in the heat previously accepted. Inability to obtain saturation pressures at lower temperatures, previously reported in the literature, was confirmed. It was shown that microgram quantities of carbon are capable of blocking the sublimation reaction.

Vapor Pressure and Heat of Sublimation of Rhenium.

The vapor pressure of rhenium was measured by the Langmuir method in the temperature range 2350-3050 °K using a vacuum microbalance. The least squares line through the four series of data points is 4.5756 log P(atm) = 32.26 - 180700/T. Least squares lines for each of the four series yield heats and entropies of sublimation higher than the corresponding third law values. The vapor pressure equation based on the average heat and entropy is, 4.5756 log P(atm) = 33.36 - 183500/T. The selected third law heat of sublimation, Δ H s ° (298) is 185.9 kcal mol-1. Our recommended equation for the vapor pressure is 4.5756 log P (atm) = 31.86 - 180200/T based on our mean third law heat and tabulated values for the entropies and enthalpies.

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 »Al4C3 to form Al(g) and ¾C(C), in good agreement with data based on calorimetric studies.