Free energy of formation of barium ferrites
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(+2275) J, - 6 6 , 8 7 5 (+435) J, - 7 4 , 4 3 7 (+250) J
at 1300 K r e s p e c t i v e l y .
THE
f e r r i t e s of b a r i u m have a t t r a c t e d a t t e n t i o n of t e c h n o l o g i s t s b e c a u s e of t h e i r i n t e r e s t i n g m a g n e t i c p r o p e r t i e s . ~ T h e p h a s e d i a g r a m of BaO-Fe2Os s y s t e m s p r o p o s e d by Goto and T a k a d a , z shows the p r e s e n c e of t h r e e s t a b l e c o m p o u n d s : 2 BaO 9Fe2Os, BaO 9Fe203 and BaO 96 Fe2Os. B a r i u m h e x a f e r r i t e BaO 96 Fe2Os i s a l i n e compound with n e g l i g i b l e s o l u b i l i t y of e i t h e r Fe203 o r BaO 9Fe2Os. The m u t u a l s o l u b i l i t y of BaO" Fe2Os and 2 BaO 9Fe2Os i s a l s o v e r y s m a l l . The compound 2 BaO 9Fe2Os, however, has an u n u s u a l l y l a r g e n o n s t o i c h i o m e t r y r a n g e b e c a u s e of e x c e s s oxygen, s-4 P a v l e n i s h v i l i e t a l 4 r e p o r t e d that 2 BaO 9Fe203 is s t a ble only above 1223 K and when cooled slowly i n a i r i t s c o m p o s i t i o n changes to BaFeOz.8. V a n Hook s showed that n o n s t o i c h i o m e t r i c BaFeO2.5 +x d e c o m p o s e s to s t o i c h i o m e t r i c BaFeOzs at 1133 K i n a i r and at 1523 K u n d e r 10 arm p r e s s u r e . A l a r g e n u m b e r of s t a b l e p h a s e s have a l s o b e e n r e p o r t e d s i n the s y s t e m BaOF e 2 0 3 - F e O in w u s t i t e r i c h zone. V e r y few a t t e m p t s have, h o w e v e r , b e e n m a d e to o b t a i n t h e r m o d y n a m i c data for v a r i o u s b a r i u m f e r r i t e s . M a s s a z z a s and L a n d i y a e t al 7 d e t e r m i n e d the heats of f o r m a t i o n of 2 BaO 9Fe2Os, BaO 9Fe2Oa a n d BaO 96 Fe2Os f r o m c a l o r i m e t r i c data. E s t i m a t i o n of t h e r m o d y n a m i c s t a b i l i t y of t h e s e p h a s e s at e l e v a t e d t e m p e r a t u r e s f r o m c a l o r i m e t r i c data o b t a i n e d at low t e m p e r a t u r e s is, however, not a l w a y s j u s t i f i e d u n l e s s heat capacity data i s known a c c u r a t e l y . R e c e n t l y , it has b e e n p o s s i b l e to u s e g a l v a n i c c e l l s i n c o r p o r a t i n g s o l i d e l e c t r o l y t e s to o b t a i n a c c u r a t e and r e l i a b l e t h e r m o d y n a m i c data on v a r i o u s b i n a r y and t e r n a r y oxide s y s t e m s . 8 The a i m of this p a p e r i s to r e p o r t the t h e r m o d y n a m i c data on v a r i o u s s t a b l e p h a s e s i n BaO-Fe2Os s y s t e m u s i n g g a l v a n i c c e l l s with b a r i u m f l u o r i d e (BaF2) as solid e l e c t r o l y t e .
THEORETICAL
CONSIDERATIONS
With b a r i u m f l u o r i d e a s a s o l i d e l e c t r o l y t e and b a r i u m oxide as a r e f e r e n c e e l e c t r o d e , a g a l v a n i c c e l l s i m i l a r to one c o n s t r u c t e d by Benz and W a g n e r 9 can be a s s e m b l e d as follows: BRAHMA DEO is with Research and Development, Hindustan Steel Limited, Ranchi-83400
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