Investigation of Cobalt (II)-Tungsten (Vl)-Oxide Reduction in Hydrogen: Part II
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p r e s e n t i n v e s t i g a t i o n of CoWO4 m o n o c r y s t a l s r e d u c t i o n s u p p l e m e n t s the p r e v i o u s study about a powdery s a m p l e as it i n c l u d e s a d d i t i o n a l data on both the n a t u r e and d i s t r i b u t i o n of the i n t e r m e d i a t e p h a s e s which o c c u r in the f o r m a t i o n of the f i n a l alloy Co7W6. 1) E X P E R I M E N T A L 1-1) T e c h n i q u e s In addition to the g e n e r a l t e c h n i q u e s and p r o c e d u r e s a l r e a d y d e s c r i b e d i n P a r t I, the r o t a t i o n method was used for o b t a i n i n g d i f f r a c t i o n p a t t e r n s of i n i t i a l or partially reduced crystals. Electron probe m i c r o a n a l y s i s and SEM o b s e r v a t i o n s of both e x t e r n a l f a c e s and p o l i s h e d c r o s s - s e c t i o n s of the s a m e c r y s t a l s w e r e c a r r i e d out u t i l i z i n g a d i s p e r s i v e s p e c t r o m e t e r a s s o c i a t e d with a J.S.M.U 3 ( f r o m JEOLCO) or a $4 (from Kent) s c a n n i n g e l e c t r o n m i c r o s c o p e .
Table I. Initial Material Composition, Structure and Paramagnetic Constants Compared to Bibliographic Data
Lattice Parameters a (A) b (A) c (A) /3 (~
Specimen$2 Powdered Crystal Crystals 4.96 5.62 4.63 90.0
References Ref. 2 Ref. 3
4.975 5.690 4.693 90.19
MagneticProperties: (Powdered Crystals) C X=T_ 0 X(cgsemu) 34 X 10-6 C (cgsemu K) 0.01 O,K 5-+5 Composition:Oxygen,wt pet 20.73
4.9478 5.6827 4.6693 90.0 Ref. 4
4.98 5.69 4.66 90.0 Ref. 5
32.5 X 1 0 -6 34.0X 1 0 -6 0.00976 0.00963 5 9.57 20.86
1-2) M a t e r i a l s P r i s m a t i c m o n o c r y s t a l s , about 1 m m to a few m m long w e r e p r e p a r e d a c c o r d i n g to Van U i t e r t - S o d e n ' s method. 1 The m i x t u r e 1/3 m o l e Co304, 1 m o l e WOs, 1 m o l e Na2WO4, 2H20 was m e l t e d at 1150~ i n a i r , in an a l u m i n a c r u c i b l e , then slowly cooled (5 to 6~ p e r h) down to 800~ and m o r e r a p i d l y down to r o o m t e m p e r a t u r e . The c r y s t a l s w e r e s e p a r a t e d f r o m Na2WO4 i n b o i l i n g w a t e r , and only s m a l l , s u r f a c e c l e a n ones w e r e s e l e c t e d , u n d e r the m i c r o s c o p e , for l a t e r exp e r i m e n t a l u s e . They will s u b s e q u e n t l y be r e f e r r e d to as s p e c i m e n $2. High p u r i t y Co and W oxides and sodium t u n g s t a t e w e r e used a s s t a r t i n g m a t e r i a l s : they a l l c o n t a i n e d l e s s than 0.1 wt pet of total c a t i o n i c i m p u r i t i e s . Thus, in $2, r e s p e c t i v e c o n c e n t r a t i o n s o f N i and F e i m p u r i t i e s are only 1.10 -3 and 1.10 -2 w t p e t . As shown i n T a b l e I, the m e a s u r e d s t r u c t u r a l and m a g n e t i c c o n s t a n t s as well as oxygen c o n c e n t r a t i o n a r e i n good a c c o r d a n c e with the p u b l i s h e d o r c a l c u l a t e d data r e l a tive to CoWO4. 2-5 P. BRACCONI and L. C. DUFOUR are Attach6 de Recherche,
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