Geometrical Construction of Auxiliary Axes in an Ellingham Diagram

To facilitate the assessment of the oxide stability in H2-H2O or CO-CO2 atmospheres, auxiliary axes are constructed in the Ellingham diagram. Based on A. Ghosh’s approach, the geometrical interpretation of the diagram is proposed for the reaction 2X + O2 = 2Y, where X and Y could be originated from H2 and H2O or CO and CO2. Two cases are considered when oxygen partial pressures are lower and higher than one bar. By a geometrical method, it is proved that with an appropriate set-up of values relating to the auxiliary axes, the axes representing the ratio between the equilibrium partial pressure of hydrogen and that of water vapour, as well as the ratio between the equilibrium partial pressure of carbon monoxide and that of carbon dioxide, can be constructed. The geometrical method on the construction of axes using thermodynamic derivation is explained in the paper.

Generalized Ellingham diagrams for utilization in solid oxide fuel cells

Generalized Ellingham diagram for the P-O-H and the Ni-P-OH systems have been constructed to investigate thermodynamically the chemical stability of nickel anode against the gaseous impurities containing phosphorous compounds. In the same way as the original Ellingham diagram, the oxygen potential is used as the vertical axis, while the temperature is adopted as horizontal axis. For the P-O-H system which contains many gaseous species, the dominant areas of gaseous species are displayed with a parameter of their partial pressure in an analogous way to the aqueous species in the Pourbaix diagram. The multicomponent Ellingham diagram for the Ni-P-O-H system was constructed in a similar manner to the multicomponent Pourbaix diagram. The obtained diagrams have been discussed to examine the reactivity of nickel anodes with phosphorus compounds in SOFCs in terms of operational variables such as temperature, oxygen potential, overpotential under the anode polarization and so on.

Phase Transformation of Mo and W over Co OR Its Alloy in Contact with Si

ABSTRACTW or Mo directly deposited on Si cold substrate by electron-beam gun at a base pressure of 10-6 torr is not able to form silicide even annealed at 900 °C in either N2 or H2 ambient. We present an easy way that Mo and W silicides can be formed on the same depositing and annealing conditions with the help of an intervened layer of cobalt or its alloy. Investigation was made on various metallizations of Mo (or W)/Co/Si, W/Co-Mo/Si, and Co/Mo/Si in normal flowingnitrogen or in H2 ambient at various temperatures. In the systems of Mo (or W)/Co/Si and W/Co-Mo/Si, the overlying Mo (or W) can be transformed into silicide at 900 °C, while in the Co/Mo/Si system, where stable Co-Mo compounds are formed in advance, no silicide can be formed. Why silicide is formed in preference to metal oxide in N2 environment at higher temperature is based on Ellingham diagram.

The Role of Oxygen in the Hot Isostatic Pressing of High Tc Superconductors

ABSTRACTThe bismuth-based 2212 and 2223 superconductors and the yttrium-based 123 and 124 high Tc superconductors are oxide ceramics, so that oxidizing conditions must be maintained in the HIP to avoid oxygen loss and decomposition. This has been done in an argon gas HIP by using glass encapsulation, but internal gas pressure retards densification. The thermodynamics of the release of oxygen by the superconductive ceramics and the stability of the metal to oxygen can be shown on a free energy diagram (Ellingham diagram). The base metals (as Fe and Cu) are reactive with respect to superconductive ceramics but not silver.

ELECTROLYTIC REDUCTION AND ELLINGHAM DIAGRAMS FOR OXY-ANION SYSTEMS

Standard reversible decomposition voltages, E0, for various decomposition reactions of K2CO3, Na2CO3, Li2CO3, BaCO3, CaCO3, MgCO3, Na2SO4, NaNO3, and KNO3 are calculated from thermal data and the values obtained plotted against temperature. That reaction which has the lowest values of E0 is considered to be the most likely to occur when the particular salt is electrolyzed. The results obtained indicate that in the case of the carbonates metal deposition is possible only for K2CO2 and Na2CO3 above 870 °K. In all other cases metal oxide and carbon (or CO at higher temperatures) are the favored cathode products. For Na2SO4, Na2S and Na2O are the favored cathode products and for the nitrates, nitrite and oxide or nitrogen evolution and oxide formation appear equally feasible thermodynamically.Once the likely electrolytic decomposition products are known, a method is presented whereby the products formed by reduction of the oxy-anions by metals (and in the case of the carbonates, by carbon) may be predicted. This method involves drawing an appropriate redox line on an Ellingham diagram. Finally, a comparison is made between the thermodynamic predictions and the few experimental results available.