High-temperature thermodynamic properties of the vanadium carbides V 2 C and VC 0.73 determined using a galvanic cell te
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Ta2C I BaF2-BaC2 I V,
V2C
(850 t o 1 2 0 0 K)
(D)
VC0.73,
V2C I BaF2-BaC2 I Cr,
Cr23C6
( 8 5 0 to 1100 K)
(E)
VC0.73,
V2C [ BaF2-BaC2 I Mo,
Mo2C
(890to 1247 K)
(F)
Combining the results of this study with previous work llSl and those of Kukarni et aI.,~ following equations for AG~ of V2C and VC0m have been determined:
the
From cell (D), o + AGv~c(-1263) = -152,824(--+9200) + 5 . 4 5 ( - 0 . 2 7 ) T
Joule
for the reaction 2V + C = V2C. From cell (E), o + AGvc0,~(-662) = -96,790.8(---6511.7) + 7 . 0 ( •
J o u l e / g . atom V
From cell (F), o + AGvc0~(-665) = -97,000(•
+ 6.79(•
J o u l e / g . atom V
for the reaction V + 0.73C = VC0.73. I.
INTRODUCTION
V A N A D I U M forms two carbides, f~l VC and V2C. Vanadium carbide exhibits a large degree of nonstoichiometry, and VzC has a narrow homogeneity range. 12lStorms 13J and Storms and McNeal, t41 report that VC exists between VC0.47 and VC0.5 in the temperature ranges of 500 ~ to 2165 ~ and 500 ~ to 2700 ~ respectively. There are few published thermodynamic data on the vanadium carbides, and these are usually measured by calorimetric [4"7] and gas equilibrium techniques, f8,9,1~ This lack of thermochemical data for vanadium carbides VC and V2C is due to the facts that some of the literature reports present data on an uncharacterized sample and due to considerable experimental difficulties. Since their properties are very sensitive to composition, the results of such experiments are meaningless. Unlike many other carbides, the combustion of vanadium carbides may give lower oxides in addition to the highest oxide V205. Furthermore, unlike the highest oxides of most other refractory metals, V2Os melts at the comparatively low temperature vii of about 943 K. Guverich 16! found that when the carbide was burnt in oxygen or air, a liquid film of V205 was formed on the surface of the carbide particles and restricted the access of oxygen to the inner
R.G. COLTTERS, Professor, and Z. NAVA, Assistant Professor, are with the Department of Materials Science, Universidad Simon Bolivar, Caracas 1080, Venezuela. Manuscript submitted December 11, 1989. METALLURGICAL TRANSACTIONS B
layers of the particles. This yielded to an incomplete combustion, and lower oxides were formed in addition to V205. According to Anderson, ~ in the V-O system there are eleven oxides, seven o f them having similar values of free energy of formation (-~ - 167 kJ g. atom -l) and some of them not stoichiometricY 3j Therefore, the heat of combustion of vanadium carbide or the equilibrium pressures over vanadium carbide or the equilibrium pressures over vanadium carbide or the equilibrium pressures over vanadium carbide-vanadium oxide-graphite mixtures cannot always be reliably assigned to a definite reaction. The thermodynamic properties of V2C 04J have been determined indirectly by means of galvanic cell using calcium fluoride as solid electrolyte, but the authors did not state the chemical composition of the sample used. Use of a reversible electromotive force (emf) cell having a carb
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