Determination of Gibbs Energies of Formation of Cr₃B₄, CrB₂, and CrB₄ by Electromotive Force Measurement Using Solid Electrolyte

Abstract

The standard Gibbs energies of formation, Δ_fG°, of intermediate compounds of Cr₃B₄, CrB₂, and CrB₄ in the boron-rich side of the Cr–B binary system were determined by measuring electromotive forces of the galvanic cells using ZrO₂–Y₂O₃ solid electrolyte. The phase equilibria for the Cr–B binary system were clarified to construct the cells using the solid electrolyte. The measured electromotive forces of the cells were evaluated as linear temperature functions under the conditions that the transport numbers of oxide ion in solid electrolyte were regarded as 1.0. The Δ_fG° functions determined from the electromotive forces via the Nernst equation were as follows:

Δ_fG°(Cr₃B₄)/J (mol of compd)⁻¹ = −264540 − 26.697 T ± 970 (1256–1322 K),

Δ_fG°(CrB₂)/J (mol of compd)⁻¹ = −84572 − 32.442 T ± 790 (1253–1350 K),

Δ_fG°(CrB₄)/J (mol of compd)⁻¹ = −105120 − 57.921 T ± 2200 (1280–1352 K).

The present Δ_fG° values satisfied the phase equilibria in the Cr–B binary system. Using the Δ_fG° values determined in the present study, the composition-oxygen partial pressure diagram of the Cr–B–O ternary system was constructed under the conditions of 1300 K and a total pressure of 1 bar (100 kPa). It is useful to understand the oxidation path of the boron-rich side of Cr–B binary alloys.

Fig. 4 Δ_fG°(Cr₃B₄), Δ_fG°(CrB₂), and Δ_fG°(CrB₄) values expressed for the formulation of one mole of atoms at 1300 K compared with those in the references.^11,12)