Extractive Metallurgy Division - Free Energies of Formation of Gaseous Metal Oxides

The standard free energies of formation of some gaseous metal oxides together with those of their condensed oxides have been plotted against temperature. The heats of formation of the gaseozcs oxides are tabulated. An explanation is given of the use of the data to predict the importance of the gaseous metal oxides at high tenzperatures. RECENT mass spectrometric work1 has resulted in the detection of several hitherto unknown gaseous oxides and the measurement of their free energies of formation. Of especial interest is the fact that several of the refractories in common use either as crucibles or furnace tubes, and metals used in Knudsen cells and in furnace windings, have been found to have one or more stable gaseous oxides. In view of the present emphasis on high-temperature techniques, the development of vacuum metallurgy.z heating by electron bombardment and plasma jets, and the resultant shift to ever higher temperatures, it is no longer possible to plan experiments without taking into consideration the significance of the presence of these gases in the system, the reactions they undergo. and their contribution to the vapor pressure. A good example of what went wrong in earlier work, when the existence of these gaseous molecules was not yet known, is presented by the work of Hoch, Nakata, and Johnston. Hoch and coworkers attempted to measure the vapor pressure of solid ZrO2 by letting the vapor in equilibrium with zir-conia effuse from a tantalum Knudsen cell. The free energies of formation of solid zirconia and tantala are such that a short calculation showed that the reaction