High temperature thermodynamic properties of the tungsten carbide WC determined using a galvanic cell technique
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I.
INTRODUCTION
THERE are two tungsten carbides, WzC and WC, which are stable below 2000 ~ although only the carbide WC is stable at room temperature. In view of the metallurgical importance of WC, there have been some attempts to measure the thermodynamic properties of this compound by calorimetric and gas equilibrium techniques. 2'3'4 There is significant disagreement among the reported data on the stability of WC. The disagreement is due to the difficulty in the application of calorimetric methods and gas equilibrium techniques to refractory metal compounds, which arises from their stability to solvents, incomplete combustion, low rate of reaction, and thermal segregation problems even at high temperatures. The thermodynamic properties of some refractory carbides have been determined indirectly by means of galvanic cells. Several investigators have used a single crystal of calcium fluoride as solid electrolyte in the measurement of the free energy of formation of carbide systems, e.g.; ThC and ThC2; 5-8 Mn23C6, MnTC3, and MnsC2;9 Mn23C6 and MnsC3;l~ MnTC3;t! UFeC2 and UWc; 12 Cr7C3 and Cr3C2; 13 Cr3C2;~4and Fe-Mn-C system. 15 Use of a reversible electromotive-force cell having a carbide electrolyte to measure directly the carbon activity of some refractory carbides offers a convenient experimental method. It also avoids some of the problems inherent in the experimental methods2'3'4 for studies which rely on precise determination of gas compositions. Particularly in the case of carbides, since these compounds can dissolve oxygen, it is necessary when making thermodynamic measurements to take oxygen solubility into account. Besides the dissolution of oxygen, the carbide can have a high percentage of vacancies in the carbon sublattice. If this should occur, the measured gas partial pressure will be lowered. However, despite the advantages that open circuit measurements offer, the emf method has not so far been used to measure the thermodynamic properties of WC. In previous work ~6 a study has been made of the BaF2-BaC2 system for use as solid electrolyte for the determination of the chemical potential of carbon. 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 June 1, 1982. METALLURGICAL TRANSACTIONS A
Consequently, the present study is part of a systematic investigation on the stability of refractory metal carbides by simple galvanic cells using BaF2-BaC2 as a solid electrolyte. In this investigation, two carbon concentration cells were used to measure the thermodynamic properties of WC. (A) Cr, Cr23C6 I BaF2-BaC2 I W, WC
(873to 1132K)
(B) Mo, Mo2C [ BaF2-BaC2 [ W, WC
(889 to 1309 K)
II.
EXPERIMENTAL
All the materials used and their specifications are listed in Table I. In a dry box, the BaC2 was first ground in an agate mortar to - 325 mesh and then weighed proportions of the BaC2 and BaF2 powders were mixed to produ
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