Composition–Oxygen Partial Pressure Diagram of the Cr–B–O Ternary System Based on the Standard Gibbs Energies of Formation of CrB₄, CrB₂, Cr₃B₄, Cr₅B₃ and CrBO₃ Determined by Solid Electrolyte

抄録

The standard Gibbs energies of formation, Δ_fG^o, of CrB₄, CrB₂, Cr₃B₄, Cr₅B₃, and CrBO₃ existing along the oxidation path of Cr–B binary alloy were determined using the electromotive forces of galvanic cell composed of the ZrO₂–Y₂O₃ solid electrolyte. The electromotive force showed plateaus and decreases in the process that CrB₄–CrB₂ two-phase alloy used for the cell materials were oxidized via two- and three-phase regions along its oxidation path. From the plateaus of electromotive forces corresponding to the cell materials in the three-phase regions, the values of Δ_fG^o(CrB₄), Δ_fG^o(CrB₂), Δ_fG^o(Cr₃B₄), Δ_fG^o(Cr₅B₃), and Δ_fG^o(CrBO₃) in the temperature range from 1273 to 1346 K were determined as follows:

Δ_fG^o(CrB₄)/J (mol of compd.)⁻¹ = 167500 − 261.2 T ± 7200

Δ_fG^o(CrB₂)/J (mol of compd.)⁻¹ = −21020 − 79.22 T ± 2100

Δ_fG^o(Cr₃B₄)/J (mol of compd.)⁻¹ = −173400 − 95.47 T ± 2500

Δ_fG^o(Cr₅B₃)/J (mol of compd.)⁻¹ = −318900 + 20.64 T ± 5800

Δ_fG^o(CrBO₃)/J (mol of compd.)⁻¹ = −958800 + 59.24 T ± 4100

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

Fig. 8 Composition-oxygen partial pressure diagram of the Cr–B–O ternary system at 1300 K. The horizontal lines show the following three-phase equilibria: (1) CrB₂–CrB₄–B₂O₃; (2) Cr₃B₄–CrB₂–B₂O₃; (3) CrB–Cr₃B₄–B₂O₃; (4) Cr₅B₃–CrB–B₂O₃; (5) Cr₅B₃–CrBO₃–B₂O₃; (6) Cr₂B–Cr₅B₃–CrBO₃; (7) Cr₂B–Cr₂O₃–CrBO₃; (8) Cr–Cr₂B–Cr₂O₃; (9) CrB₄–B–B₂O₃.