Determination of the standard free energy of formation for niobium and tantalum silicides by means of the three different methods

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The initial objective of this project is determination of the standard free energy of formation for niobium and tantalum silicides by means of EMF measurements, chemical equilibrium and Knudsen effusion method.EMF measurements : applying the silicon concentration cells using molten lithium silicate electrolyte to the niobium-silicon system, the standard free energy of formation for niobium silicides were successfully obtained. In case of the tantalum-silicon system, it was found that tantalum oxide reacts with the constituents of the electrolyte forming intermediate compound. Chemical equilibrium method : silicon distribution between the silver alloy and the mixture of the solid silicides was examined. However, sufficient time to achieve the equilibrium was considered to be unrealistic. Knudsen effusion method : the method was tried, aiming at the tantalum-silicon system, only to obtain unstable values of vapor pressure because of instrumental troubles.Because availability of the EMF method was ensured in the earlier stage of the research project, minor collection to the objectives were decided and the method was expansively applied to the molybdenum or tungsten・silicon system. These metals, molybdenum and tungsten, were known as refractory metals as well as niobium and tantalum. The EMF measurements for these systems were successfully completed. As the results of the experiments, the standard free energies of formation for two niobium silicides, two tungsten silicides and three molybdenum silicides were obtained as functions of temperature. In addition to this, applying the third low method to the results, the values of the standard enthalpy of formation at 298 K for these silicides were also obtained.