J. Leitner

The low-temperature heat capacities of mixed oxides in the Bi–Ca–O system have been determined by the relaxation method at temperatures from 15 to about 225K. The high-temperature heat capacities have been measured from 340 to about 1030K using differential scanning calorimetry (DSC) in a stepwise mode. From the results, standard molar entropies and molar heat capacities at 298.15K as well

The already published data of temperatures and enthalpies of decomposition for mercury-based phases were combined with our experimental results from DSC and low-temperature calorimetry and the data for the Ba–Cu–O system taken from the literature. The consistent set of thermodynamic data of all phases in the Hg–Ba–Cu–O system was used for the calculation of the equilibrium phase composition under varying

A technique has been developed for the calculation of chemical equilibrium of a closed multicomponent heterogeneous system for the given values of temperature, pressure and feed composition. The method is based on the consecutive exclusion and inclusion of the phases from and into the calculation process till the system is thermodynamically stable with respect to the phases not included in the system. Thermodynamic stability is tested using the Kuhn-Tucker condition. Proof of convergence of the proposed iterative process is given, allowing a test of the validity of Kuhn-Tucker condition to be made. Convergence of the whole calculation process is also discussed.

Heat capacity and enthalpy increments of ternary strontium bismuth(III) oxides SrBi 2 O 4 and Sr 2 Bi 2 O 5 were measured by the relaxation time method (2–210 K), DSC (253–352 K) and drop calorimetry (570–1072 K). Temperature dependencies of the molar heat capacity in the form C pm = 161.97 + 45.936 × 10 −3 T – 1.7862 × 10 6 /T 2 J K −1 mol −1 and C pm = 197.48 + 87.463 × 10 −3 T – 1.9282 × 10 6 /T 2 J K −1 mol −1 for SrBi 2 O 4 and Sr 2 Bi 2 O 5 , respectively, were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S • m (298.15 K) = 206.1 ± 1.1 J K −1 mol −1 for SrBi 2 O 4 and S • m (298.15 K) = 261.2 ± 1.4 J K −1 mol −1 for Sr 2 Bi 2 O 5 , were evaluated from the low temperature heat capacity measurements.

Some empirical methods for the estimation of standard molar heat capacity (C pm) of solid mixed oxides are reviewed and the reliability of the obtained data in phase equilibria calculations is examined. Following the comparison of predicted values of C pm (298.15 K) with more than 300 experimental data the most widely used Neumann±Kopp rule (NKR) is found to be very universal but in some cases the mean deviation of 3.3% is too high, giving rise to a relatively large error in equilibrium calculation results. On the other hand, the method based on binary oxide contributions proposed by Berman and Brown [Contrib. Mineral. Petrol. 89 (1985) 168] for the estimation of temperature dependencies C pm (T) of silicates and other minerals formed by is less general, but more accurate. In comparison with the NKR, the most pronounced drawback of this method is the necessity to know the experimental values of C pm for a set of mixed oxides, so that the individual contributions of constituent binary oxides can be evaluated.

Palladium dental casting alloys are alternatives to gold alloys. The aim of this study was to determine the electrochemical behaviour and the corrosion mechanism of binary silver–palladium alloys. Seven binary silver–palladium alloys and pure palladium and silver were tested in a model saliva solution. Electrochemical tests included corrosion potential, polarization resistance, and potentiodynamic polarization measurements. The corrosion products, which may be theoretically formed, were determined by thermodynamic calculation. The behaviour of silver and silver-rich alloys was dominated by the preferential formation of a thiocyanate surface layer, which controlled the free corrosion potential. Palladium dissolved in the form of a thiocyanate complex, but the surface became passivated by either palladium oxide or solid palladium thiocyanate layer, the thermodynamic calculations indicating preference for the oxide. Palladium-rich alloys showed evidence of silver depletion of the surface, resulting in behaviour similar to palladium. Examination of binary silver–palladium alloys has made possible determination of the role of the components of the alloys and model saliva in the corrosion behaviour. The findings are applicable to the more complex commercial dental alloys containing silver and palladium as major components.

ABSTRACT Oxidation reactions during plasma spraying of metallic powders give rise to oxide crusts on powder particle surfaces. The first oxidation stage occurs in flight of molten particles. It is usually followed by the second stage after hitting a substrate. To investigate the oxidation products immediately after the first stage, abrupt stopping of in-flight oxidation is possible by trapping and quenching the flying particles in liquid nitrogen. In oxide crusts on plasma sprayed and liquid nitrogen quenched particles of a Fe-12%Cr alloy, two spinel oxides were indicated by Mössbauer spectroscopy and X-ray diffraction. Both are solid solutions of the type Fe3O4 - Cr3O4 (i.e., Fe3−x Cr x O4, 0 ≤x ≤ 3). One of the oxides, tetragonally distorted spinel, is characterized by the mean value ofx ≈ 2.3. It is only stable at very high temperatures. The other spinel oxide is cubic withx slightly lower than 2, i.e. almost stoichiometric chromite FeCr2O4. From thermodynamic considerations it follows that in the Fe3O4 - Cr3O4 system there is no miscibility gap at high temperatures. The simultaneous existence of both oxides is probably due to non-equilibrium conditions during liquid nitrogen quenching of trapped particles.