Standard gibbs energies of formation of the carbides of vanadium by emf measurements
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The measurements were carried out in the temperature range of 816 to 1008 K. The relative partial molar Gibbs energies of carbon have been calculated in the same composition range. The relative integral molar Gibbs energy in the VC single-phase region can be expressed as G M = - 9 8 , 8 5 0 + 72,242Xc + (24.81 - 37.23Xc)T
J/mol
The standard Gibbs energies of formation of V4C3_ x and V2C can be represented as AG ~ = - 6 7 , 2 0 8 + 9.37T
J / m o l ofV060C040
AG ~ = - 6 2 , 5 8 1 + 7.10T
J / m o l of V0.66Co.34
and respectively.
I.
INTRODUCTION
V A N A D I U M is extensively used as an alloying element in steel. It reacts with carbon in steel forming simple vanadium carbides m in contrast to other alloying elements like manganese, which form complex carbides. A knowledge of the stabilities of the vanadium carbides would be valuable in the understanding of their precipitation in steel. Further, in view of the industrial applications of the refractory metal carbides, it is necessary that the thermodynamic properties of these carbides are well understood. The present work, which aims at a study of the standard Gibbs energies of formation of vanadium carbides, is part of a series of thermodynamic measurements pertaining to binary metal-carbon systems in the transition metal series. A study of the thermodynamic properties of the carbides of vanadium is interesting even from a theoretical viewpoint. Vanadium bears a close resemblance to Nb and Ta in group Va in the periodic table in its reaction with carbon. All of these metals form M2C and MC types of carbides. A comparison of the standard Gibbs energies of formation of the carbides of V, Nb, and Ta would give an idea how the M-C bond strengths vary with increasing atomic number in the same group in the periodic table. The recently published vanadium-carbon phase diagram t2] is reproduced in Figure 1. There are three major carbides in this binary system, namely, VC, VaC3_x, and V2C. Among them, both VC and V2C exhibit a range of composition. A number of studies of the standard Gibbs energies of formation of the vanadium carbides can be DU SICHEN, Research Associate, is with the Department of Theoretical Metallurgy, The Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted March 7, 1989. METALLURGICALTRANSACTIONSB
found in literature. Several experimental methods have been employed in these studies. These include (a) the solid-gas equilibration technique involving either CO [3] or CH4-H2 gas mixture, t41 (b) the emf measurements using molten salt electrolyte tS~ or CaF2 solid electrolyte, t6J and (c) the Knudsen effusion method, tT~ While the Knudsen effusion study was carried out between 1600 and 2200 K, the other studies were carried out at relatively low temperatures. The discrepancies among the results of these studies appear to be considerable. The present work aims at obtaining a set of reliable thermodynamic data for the three vanadium carbides using the CaF2 galvanic cell technique. Similar studies have been carried out earlier i
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