The Free Energy of Formation of Tantalum Silicides Using Solid Oxide Electrolytes

Abstract

A method is described for determining the standard free energies of formation of the tantalum silicides from emf measurements on cells with solid thoria‐yttria electrolytes. Measurements were conducted over the temperature range 900°–1100°C in a purified argon atmosphere. Corrections for electronic conduction were established with Ta, Ta2O5 electrodes. Difficulties were encountered because the hard, refractory nature of the compounds prevented fabrication of dense electrodes and because silica films interfered with the measurements. At 1300°K, the measured standard free energies of formation in kcal per gram atom of silicon are: TaSi2 , −8.1 false(+1.8/−3.7false) ; Ta5Si3 , −24.0±3 ; Ta2normalSi , −27.2±3.1 ; Ta4.5normalSi , −37.3±7.2 .

Thermodynamic Assessment of the Si-Ta and Si-W Systems

“…Niessen and Boer [26] [23] were not included in the present optimization due to their significant deviations from those of others. [24][25][26] No thermodynamic data of the liquid phase have been reported for this system.…”

“…Myers and Searcy [24] determined the Gibbs energies of formation (D f G) of four compounds using Knudsen effusion cells. Using solid state EMF cells, Levine and Kolodney [25] measured D f G of aTa 5 Si 3 and Ta 2 Si. The Neumann-Kopp rule to extrapolate the results to the ambient temperature was used in both studies.…”

“…They were first evaluated through the The Si-Ta system Liquidus (0-100 at.% Si) [16] Metallography, TA Used for checking the modeling [21] Metallography, DTA, XRD Used Enthalpy of formation [23] Reaction calorimetry Not used [24] Knudsen effusion cells Used [25] Solid state emf cells Used for checking the modeling [26] Cellular model Used The Si-W system Liquidus (0-100 at.% Si) [27] Metallography, TA, XRD Used for checking the modeling [28] Metallography, TA, XRD Used [29] DTA, XRD Not used Enthalpy of formation [23] Calorimetry Used for checking the modeling [26] Cellular model Not used [31] Knudsen effusion cells Not used [32] Knudsen effusion cells Not used [33] Mass spectroscopy Used [34] Solid state emf cells Used TA, thermal analysis Section I: Basic and Applied Research equilibrium data between the liquid and pure components, as well as the enthalpy of mixing of liquid in similar transition-metal-Si binary systems such as V-Si, [13] Nb-Si, [38] and Cr-Si, [39] and Mo-Si. [40] When a 0 in Eq 5 was preliminarily obtained from the above data, b 0 was added to describe the liquidus lines.…”

“…The parameters of the solid phases were evaluated using the enthalpies of formation of the silicides. [24][25][26][31][32][33][34] The terminal solid solution phases (Ta) and (W) were optimized with the experimental data on the liquidus and solid solubility.…”

“…4 Calculated Si-W phase diagram in comparison with the experimental data [27][28][29] Fig. 3 Calculated enthalpy of formation of the Si-Ta intermetallic compounds at 298 K in comparison with the experimental data [24][25][26] Si-V and Si-Nb systems. [13,38] The minimum values of the three systems are all between À45 and À50 kJ/mol.…”