A study of the reaction kinetics for the formation of rare earth-transition metal laves compounds
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AND
P. J. FICALORA
The k i n e t i c s of the r e a c t i o n A t s ~ + 2B(s ~ = AB~ts) was studied by d i f f e r e n t i a l t h e r m a l a n a l y s i s . The A component was P r , Gd, Dy or Er, the B was Fe, Co, or Ni and the r e s u l t i n g A B 2 compound was a MgCu~ Laves i n t e r m e t a l l i c compound. A g e n e r a l i z e d s e a r c h method to d e t e r m i n e the r e a c t i o n m e c h a n i s m was developed and applied to the above r e a c t i o n s . The r e a c t i o n s w e r e found to be diffusion c o n t r o l l e d and the k i n e t i c s b e s t d e s c r i b e d by the V a l e n s i C a r t e r equation. 1I N T E R M E T A L L I C compounds c o n t a i n i n g the r a r e e a r t h m e t a l s a r e b e c o m i n g the s u b j e c t s of i n v e s t i g a tion due to t h e i r i n c r e a s i n g i n d u s t r i a l i m p o r t a n c e . Although the r a r e e a r t h s a r e p r o l i f i c f o r m e r s of i n t e r m e t a l l i c compounds the l i t e r a t u r e 1 i n d i c a t e s that t h e i r i m p o r t a n c e to the s t e e l i n d u s t r y will be as d e s u l f u r i z ing and deoxidizing a g e n t s . Many of the i n t e r m e t a l l i c compounds of the r a r e e a r t h s with cobalt and nickel a r e used as p e r m a n e n t m a g n e t s . F o r example, p e r m a n e n t m a g n e t s b a s e d on SmCo 5 a r e the m o s t p o w e r ful m a g n e t s known. Although a g r e a t deal of i n f o r m a tion on the m a g n e t i c p r o p e r t i e s of the r a r e e a r t h i n t e r m e t a l l i c s is a v a i l a b l e , 2 v e r y little is known r e g a r d ing t h e i r t h e r m o d y n a m i c and k i n e t i c c h a r a c t e r i s t i c s . This i n f o r m a t i o n will be useful in u n d e r s t a n d i n g and p r e d i c t i n g the b e h a v i o r of r a r e e a r t h m e t a l s in alloys and p e r m a n e n t m a g n e t s . The r e a c t i o n s of r a r e e a r t h m e t a l s with iron, cobalt or n i c k e l take place e n t i r e l y in the solid state. Although the k i n e t i c s of solid state r e a c t i o n s is a widely r e s e a r c h e d a r e a , m o s t of the work has b e e n done in the field of c e r a m i c s . D i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) s-8 has b e e n used by m a n y to s t u d y the k i n e t i c s of solid s t a t e r e a c t i o n s . In a t y p i c a l d i f f e r e n t i a l t h e r m a l a n a l y s i s e x p e r i m e n t a r e a c t i v e m i x t u r e and a n o n r e a c t i v e r e f e r e n c e a r e heated at a u n i f o r m r a t e until the r e a c t i v e m i x t u r e has c o m p l e t e l y r e a c t e d . During that t i m e changes in the t e m p e r a t u r e of the r e a c t i v e m i x t u r e r e l a t i v e to the t e m p e r a t u r e of the n o n r e a c t i v e r e f e r e n c e a r e m e a s u r e d . The a r e a under the change in t e m p e r a t u r e u s t i m e c u r v e is a m e a s u r e of the heat of r e a c t i o n . Different m e t h o d s have b e e n s u
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