High-temperature thermodynamic stability of ditantalum carbide (Ta 2 C)
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The CO(g) p r e s s u r e in e q u i l i b r i u m with a Ta2C-Ta2Os-Ta m i x t u r e h a s b e e n m e a s u r e d at t e m p e r a t u r e s b e t w e e n 1740 and 1900 K using the t o r s i o n - e f f u s i o n t e c h n i q u e . F r o m the e q u i l i b r i u m d a t a , the following equation for AG} of Ta2C h a s b e e n o b t a i n e d : AG)~ (=~300) = --47,000 (•
+ 2.1T
F r o m the enthalpy t e r m in the AG~ equation, a value of - 4 7 . 9 (• kcal/mole has been c a l c u l a t e d for AH~98 of Ta2C which i s in good a g r e e m e n t with s e v e r a l c a l o r i m e t r i c r e s u l t s . T H E R E a r e two c a r b i d e s (TaC and Ta2C) in the T a - C s y s t e m . 1 The high t e m p e r a t u r e t h e r m o d y n a m i c s t a b i l i t y of TaC h a s b e e n m e a s u r e d , 2 but s i m i l a r d a t a for Ta2C a r e not a v a i l a b l e . In t h i s s t u d y , the t h e r m o d y n a m i c p r o p e r t i e s of Ta2C have b e e n d e t e r m i n e d by m e a s u r i n g the CO(g) p r e s s u r e in e q u i l i b r i u m with a T a 2 C - T a 2 O s - T a m i x t u r e at t e m p e r a t u r e s b e t w e e n 1470 ~ and 1620~ The torsion-effusion technique was used; this method has b e e n u s e d p r e v i o u s l y to obtain h i g h - t e m p e r a t u r e t h e r m o d y n a m i c d a t a for the c h r o m i u m c a r b i d e s . 3
at any t e m p e r a t u r e , a p r e s s u r e (P) can be c a l c u l a t e d u s i n g the following r e l a t i o n s h i p : 3'4
p= "r_O ad
[1]
w h e r e 9 i s the t o r s i o n c o n s t a n t of the t o r s i o n s t r i p , a i s the a v e r a g e a r e a of the two effusion h o l e s , and d i s the a v e r a g e of the p e r p e n d i c u l a r d i s t a n c e s b e t w e e n the a x e s of the two effusion h o l e s and the s u s p e n s i o n a x i s . B e c a u s e the d i f f e r e n c e b e t w e e n the m e a s u r e d a r e a s of the two effusion h o l e s i s l e s s than 4 p c t , and the p e r p e n d i c u l a r d i s t a n c e s b e t w e e n e a c h a x i s differ by only 2 p c t , EXPERIMENTAL a v e r a g e v a l u e s (a and d) a r e u s e d in Eq. [1] to c a l c u l a t e A Ta-Ta2C mixture was prepared by hot pressing tan- the CO(g) p r e s s u r e . The t o r s i o n constant (z) of the P t talum (99.8 pct purity) and spectroscopic grade graphite 10 p c t Ni t o r s i o n s t r i p w a s d e t e r m i n e d using the t o r at 1850~ under a pressure of 3800 psi in 12 mm diam s i o n - p e n d u l u m m e t h o d , and the m e a s u r e d value w a s in graphite dies for 30 min. The hot-pressed samples were good a g r e e m e n t with that given by the s u p p l i e r . A s in sand blasted and then annealed at 1800~ for 4 h followed o u r p r e v i o u s s t u d y , 3 k n i f e - e d g e effusion h o l e s w e r e by 2 h at 1600~ The annealed Ta-Ta2C mixture was m a d e , and F r e e m a n - S e a r c y c o r r e c t i o n s 5 can be n e crushed to -325 mesh and mixed with Ta205 powder g l e c t e d in Eq. [1]. (99.999 pct p
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