High temperature thermodynamic properties of the chromium carbides Cr 7 C 3 and Cr 3 C 2 determined using a galvanic cel
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I.
INTRODUCTION
CHROMIUM forms three stable carbides, Cr23C6, Cr7C3, and Cr3C2. ~ Below 1000 ~ no significant homogeneity range has been detected for either Cr23C6, Cr7C3, or Cr3C:. 2 There have been many investigations of the high temperature thermodynamic properties of the chromium carbides.3-'~ Of the many previous investigations of the thermodynamic properties of the Cr7C3 and Cr3C2 there have been only two determinations of the free energy of formation of Cr7C312 and CrC212'~3using a galvanic cell technique. These galvanic cell studies have used calcium fluoride as solid electrolyte. Many of these earlier studies are unreliable, and there are large uncertainties in their results. Results from the more reliable investigations will be compared with those from this study. High temperature galvanic cells have several advantages over conventional gas equilibrium techniques; 6-1~ the use of a reversible electromotive-force cell having a carbide electrolyte to measure directly the carbon activity offers a convenient experimental method. Accordingly, the purpose of the present investigation has been to measure the free energy of formation of Cr7C3 and Cr3 C2 as part of systematic studies on the stability of refractory metal carbides by simple galvanic cells using BaF2-BaC2 as a solid electrolyte. In this investigation, three carbon concentration cells were used to measure the thermodynamic properties of Cr7C3 and Cr3C2: (A) Cr, Cr23C61BaF2-BaC2[Cr23Cr6,Cr7C3 (920 to 1250 K) (B) Cr23C6, Cr7C3IBaF2-BaC2IW, WC (900 to 1200 K) (C) WC, WIBaF2-BaC2ICr3C2, Cr7C3 II.
(975 to 1173 K).
EXPERIMENTAL
All the materials used and their specifications are listed in Table I. The phases present in the electrolyte and electrodes were verified by X-ray diffraction analysis. R.G. COLTTERS is Professor, Department of Materials Science, Universidad Simon Bolivar, Caracas, Venezuela. G.R. BELTON is Director, Central Research Laboratories, The Broken Hill Proprietary Company Limited, Australia. Manuscript submitted September 22, 1983. METALLURGICALTRANSACTIONSB
The procedure for the preparation of BaF2-BaC2 electrolyte and the electrode pellets, together with the experimental cell, was substantially the same as that described in detail by Coltters and Belton. ~4j5 Briefly, metal-carbide and carbide (A)-carbide (B) electrodes were prepared by dry mixing appropriate weights of high purity powders and then pressing at 15 MPa into pellets of 10 mm diameter and approximately 3 mm thickness. These pellets were sintered at 1100 ~ for seven days under a mixture of 99 pct argon and 1 pct hydrogen. Electrolyte pellets were sintered in boron nitride crucibles at approximately 900 ~ for about 10 hours under a mixture of 99 pct argon and 1 pct hydrogen. The sandwich type cell was pressed between two boron nitride inserts within a loosely fitting alumina tube. Tungsten foil discs and wires were used as electrical contacts and leads of the galvanic cell. Argon containing 1 pet hydrogen was flowing through the cell. The cell assembly was isolated by me
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