Enthalpies of formation of chromium carbides
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e -t
f,:X//~98 CrTCa
= - 149.2 +
8.5 k J , m o l e -t
fA//~98 C r a C 2
=-
2.9 k J - m o t e-~.
81.1 +
THE chromium-carbon s y s t e m contains the t h r e e c a r b i d e s CrzaC6, C r v C a a n d CraC2,1,2 w h i c h a r e o f considerable technological importance b e c a u s e of t h e i r o c c u r r e n c e in c e r t a i n a l l o y s t e e l s a n d s u p e r a l l o y s a s e i t h e r the p u r e c a r b i d e s o r a s c a r b i d e s i n w h i c h o t h e r t r a n s i t i o n m e t a l s m a y be p a r t i a l l y substituted for chromium. As a preliminary part of a s t u d y of t h e thermodynamic properties of such partially substituted carbides, w h i c h are b e i n g prep a r e d by v a p o r d e p o s i t i o n t e c h n i q u e s , t h e e n t h a l p i e s o f f o r m a t i o n o f t h e pure c a r b i d e s were r e q u i r e d for c o m p a r i s o n p u r p o s e s . Of t h e m a n y p r e v i o u s i n v e s t i gations of the thermodynamic properties of the c h r o m i u m c a r b i d e s a-la t h e r e h a s b e e n only o n e d i r e c t d e t e r m i n a t i o n o f t h e e n t h a l p i e s of f o r m a t i o n of t h e c a r b i d e s .1° R e c e n t i n v e s t i g a t i o n s h a v e u s e d e l e c t r o m o t i v e f o r c e m e a s u r e m e n t s u s i n g s o l i d s t a t e c e l l s n,~2 a n d equilibrium v a p o r p r e s s u r e m e a s u r e m e n t s o f c a r bon monoxide above carbide-oxide-metal m i x t u r e s )a Standard enthalpies of formation obtained f r o m such measurements a r e often inaccurate and thus validate a s e c o n d d i r e c t determination of this data a s a n e c e s s a r y c o n t r i b u t i o n to t h e b a s i c t h e r m o c h e m i c a l p r o p e r t i e s of t h e t r a n s i t i o n m e t a l c a r b i d e s . 1. EXPERIMENTAL 1.1 M a t e r i a l s T h e c h r o m i u m c a r b i d e s a m p l e s u s e d in t h e c o m b u s t i o n e x p e r i m e n t s w e r e p r e p a r e d by t h e r e d u c t i o n of p u r e ( > 9 9 . 6 p c t ) C r 2 0 3 w i t h c a r b o n f o r CrTC3 a n d C r a C z , w h e r e a s Cr2sC6 w a s s y n t h e s i z e d d i r e c t l y f r o m the elements• X - r a y diffraction patterns for each of t h e c a r b i d e s w e r e i n a g r e e m e n t w i t h t h o s e l i s t e d i n t h e A S T M i n d e x . No g r o s s i m p u r i t i e s w e r e d e t e c t e d in t h e X - r a y p a t t e r n s . C h e m i c a l a n a l y s e s s h o w e dt r a c e amounts of impurities with iron, free carbon and o x y g e n at levels that required consideration in the interpretation of the calorimetric measurements (Table I). For subsequent calculations the iron was assumed to exist as iron carbide Fe3C whilst the oxygenwas assumed to be present as Cr2Oa. Theoretical compositions for the carbides are given in Table II. W. M . DAWSON and F. R. SALE are Research Student and Lecturer in Chemical Metallurgy, respectively,Metallurgy Department, Manchester University, Manchester, England. Manuscript submitted May 4 , 1976. METAL
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