The thermodynamic properties of condensed phases, i.e. the activities of components, can be determined from partial pressures measured by the Knudsen cell mass spectrometric method. Improvements in accuracy and yield of this method are obtained with the use of twin cells, an idea proposed in the 1960s. The multiple cell method was perfected in 1977 in our laboratory. Changes to molecular beam sampling and furnace assemblies were required to make the multiple Knudsen cell technique work properly. This paper summarizes these prerequisites, and presents a new device and the associated method of measurements, as well as the necessary tests performed with a silver sample in each cell.
A critical analysis of vapor pressure determinations over the ternary Al-C-O system, i.e., mainly the pseudobinary section Al 2 O 3 -Al 4 C 3 , is performed in order to further explain the behavior of alumina in the presence of carbon ͑in contact or not͒ in vacuum furnace environments. The vapor pressure studies allow the determination of standard enthalpies of formation of the oxycarbides, mainly for Al 4 CO 4 :⌬ f H°͑Al 4 CO 4 ,s,298.15 K͒ = −2303 ± 24 kJ mol −1 . Values as retained are compared with those used in phase diagram and thermodynamic data optimizations. The determination of thermodynamic data for the other oxycarbide Al 2 CO is not possible in the context of vapor pressure measurements because nonequilibrium vaporization occurs in concordance with metastable phases.
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