Thermodynamic studies on chromium carbides by the electromotive force (emf) method

• Published in: Journal of alloys and compounds Vol. 321; no. 1; pp. 138 - 145
• Main Author: Kleykamp, Heiko
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 31.05.2001; Elsevier
• Subjects: Condensed matter: structure, mechanical and thermal properties; Electromotive force; Enthalpy; Entropy and other thermodynamical quantities; Exact sciences and technology; Physics; Thermal properties of condensed matter; Thermal properties of crystalline solids; Thermodynamic properties; Transition metal compounds; Electromotive force; Transition metal compounds; Thermodynamic properties; Enthalpy; Stoichiometry; Gibbs free energy; Inorganic compounds; Chromium carbides; Temperature effects; Transition element compounds; Experimental study; Heat of formation
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/S0925-8388(01)01012-X

The Gibbs energies of formation Δ f G° of the three carbides Cr 3C 2, Cr 7C 3 and Cr 23C 6 existing in the Cr–C system were re-determined by electromotive force (emf) measurements between 930 and 1120 K using the galvanic cells Cr, CrF 2∣CaF 2∣CrF 2, Cr mC n, Cr pC q which give at 1100 K: Δ f G° 〈Cr 3C 2〉=−84.3 kJ/mol, Δ f G° 〈Cr 7C 3〉=−177 kJ/mol and Δ f G° 〈Cr 23C 6〉=−407 kJ/mol. The third-law enthalpies of formation Δ f H° at 298 K were evaluated to be Δ f H° 〈Cr 3C 2〉=−72.3 kJ/mol, Δ f H° 〈Cr 7C 3〉=−144 kJ/mol and Δ f H° 〈Cr 23C 6〉=−344 kJ/mol. Previous investigations by other authors diverge from these results up to ±50%. Obsolete thermodynamic data of Cr 2O 3 used in oxygen combustion calorimetry and in CO equilibrium pressure measurements as well as an oxycarbide formation may be the reasons. The experimental difficulties of emf measurements using a CaF 2 electrolyte are outlined. Recommended Δ f G° values of the chromium carbides are given.