Solubility and thermodynamic properties of oxygen in solid molybdenum
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Using the known v a l u e of the f r e e e n e r g y of f o r m a t i o n of MOO2, the c h e m i c a l p o t e n t i a l of oxygen in the dilute s o l i d s o l u t i o n i s c a l c u l a t e d to be _ i o = AG~ = - 10,760 + (6.92 + R in )~)) T + 1000 c a l / g - a t o m oxygen /1~) ~ pO 2 The h e a t of s o l u t i o n of oxygen in Solid Mo, ~ = - 10,760 9 3000 c a l / g - a t o m oxygen, and the e x c e s s e n t r o p y f o r the i n t e r s t i t i a l s o l i d s o l u t i o n A~o(XS , i) = - 9 . 1 0 ~: 1.5 c a l / d e g r e e , g - a t o m oxygen, a s s u m i n g that the oxygen a t o m s r e s i d e in the o c t a h e d r a l i n t e r s t i c e s of b c c MO.
T H E e l e m e n t s C, O, and N, d i s s o l v e d i n t e r s t i t i a l l y have a s t r o n g effect upon the m e c h a n i c a l p r o p e r t i e s of t h e b c c r e f r a c t o r y m e t a l s , C r , Mo, W, Nb, and Ta. For this reason there has been considerable interest in o b t a i n i n g b a s i c d a t a f o r the i n t e r s t i t i a l e l e m e n t r e f r a c t o r y m e t a l s y s t e m s , Such a s diffusion, p h a s e e q u i l i b r i u m , and t h e r m o d y n a m i c data, which would be of u s e in t h e t e c h n o l o g y of t h e s e m e t a l s and t h e i r a l loys. As p a r t of a g e n e r a l s t u d y of 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 the i n t e r s t i t i a l e l e m e n t s in the r e f r a c t o r y m e t a l s , we have i n v e s t i g a t e d the b e h a v i o r of oxygen in m o l y b d e n u m . 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 a g a s d i s s o l v e d in a m e t a l can often be d e d u c e d f r o m m e a s u r e m e n t s of the s o l u b i l i t y of the g a s a s a function of p r e s s u r e and t e m p e r a t u r e . T h i s m e t h o d h a s not b e e n a p p l i e d to the m o l y b d e n u m - o x y g e n s y s t e m , b e c a u s e the v a p o r p h a s e in e q u i l i b r i u m with s o l u t i o n s of oxygen in m o l y b d e n u m c o n s i s t s p r e d o m i n a n t l y of m o l y b d e n u m o x i d e s , with only a s m a l l f r a c t i o n of m o l e c u l a r oxygen, at t e m p e r a t u r e s of i n t e r e s t . T h e r e f o r e a n o t h e r t e c h nique h a s b e e n c o n s i d e r e d , n a m e l y that of c a l c u l a t i n g 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 oxygen in s o l i d Mo f r o m s o l u b i l i t y d a t a and the f r e e - e n e r g y of f o r m a t i o n of MoO2, the oxide in e q u i l i b r i u m with the t e r m i n a l s o l i d solution. Of the two q u a n t i t i e s r e q u i r e d f o r the c a l c u l a t i o n , the f r e e e n e r g y of f o r m a t i o n of MoO~ i s w e l l known. However, the o n l y laublished d a t a f o r the s o l u b i l i t y of oxygen in Mo, t h o s e of F e w and Manning, ~ exhibit s o m e questionable features. For example, their solubility c u r v e is a l i n e a r , r a t h e r than e x p o n e n t i a l function of t e m p e r a t u r e , and e x
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