Determination of Gibbs Energy of Mixing of Tungsten-Boron Binary System by Electromotive Force Measurement Using Solid Electrolyte

Abstract

The thermodynamic properties for the tungsten-boron binary system were determined by measuring electromotive forces of galvanic cells using an Y2O3-stabilized ZrO2 solid oxide electrolyte. Assuming that W2B and αWB are the stoichiometric compounds, and W2B5−x and W1−xB3 are the nonstoichiometric compounds having solubility widths of 0.670 ≤ XB ≤ 0.690 and 0.805 ≤ XB ≤ 0.822, respectively, they were treated as the intermediate phases of W0.667B0.333, αW0.50B0.50, W0.330B0.670 ~ W0.310B0.690, and W0.195B0.805 ~ W0.178B0.822. The Gibbs energies of mixing, ∆mixG, determined in the present study are listed as follows: $$ \begin{aligned} & \Delta_{\text{mix}} G(\text{W}_{0.667} \text{B}_{0.333} )/{\text{J}}\,{\text{mol}}^{ - 1} = {-}78070 \, + \, 26.01T \pm 70 \, [1305{-}1422{\text{ K}}(1032{-}1149^\circ {\text{C}})], \\ & \Delta_{\text{mix}} G(\alpha \text{W}_{0.50} \text{B}_{0.50} )/{\text{J}}\,{\text{mol}}^{ - 1} = {-}86140 \, + \, 20.19T \pm 200 \, [1310{-}1399{\text{ K }}(1037{-}1126^\circ {\text{C}})], \\ & \Delta_{\text{mix}} G(\text{W}_{0.330} \text{B}_{0.670} )/{\text{J}}\,{\text{mol}}^{ - 1} = {-}78910 \, + \, 18.11T \pm 200 \, [1228{-}1410{\text{ K }}(955{-}1137^\circ {\text{C}})], \\ & \Delta_{\text{mix}} G(\text{W}_{0.310} \text{B}_{0.690} )/{\text{J}}\,{\text{mol}}^{ - 1} = {-}77350 \, + \, 17.52T \pm 500 \, [1228{-}1410{\text{ K }}(955{-}1137^\circ {\text{C}})], \\ & \Delta_{\text{mix}} G(\text{W}_{0.195} \text{B}_{0.805} )/{\text{J}}\,{\text{mol}}^{ - 1} = \, {-}63920 \, + \, 12.08T \pm 500 \, [1170{-}1340{\text{ K }}(897{-}1067^\circ {\text{C}})], \\ & \Delta_{\text{mix}} G(\text{W}_{0.178} \text{B}_{0.822} )/{\text{J}}\,{\text{mol}}^{ - 1} = \, {-}60090 \, + \, 11.15T \pm 200 \, [1170{-}1340{\text{ K }}(897{-}1067^\circ {\text{C}})]. \\ \end{aligned} $$ Using the thermodynamic properties determined in the present study, the composition-oxygen partial pressure diagram of the tungsten-boron-oxygen system was constructed under the conditions at 1273 K (1000 °C) and a total pressure of 1 bar (100 kPa). It is useful to understand the oxidation property of tungsten-boron binary alloys.