Improved strain-anneal crystal growth technique
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s t a t e growth of l a r g e m e t a l l i c s i n g l e c r y s t a l s b y c o n t r o l l e d r e c r y s t a l l i z a t i o n is f r e q u e n t l y t e r m i n a t e d b y r a n d o m n u c l e a t i o n of u n d e s i r e d c r y s t a l s ( g r a i n s ) a h e a d of the p r o p a g a t i n g s e e d . P r e v i o u s a t t e m p t s to s u p p r e s s n u c l e a t i o n g e n e r a l l y m a d e u s e of a t r a v e l i n g " s h a r p " t e m p e r a t u r e g r a d i e n t and slow growth s p e e d s , e . g . , s p e e d s of 0.25 to 3 c m / h J -7 B e c a u s e c r y s t a l growth o c c u r s m o r e r e a d i l y than n u c l e a t i o n , a moving g r a d i e n t f a v o r s the continued growth of an e x i s t i n g c r y s t a l and i n h i b i t s the f o r m a t i o n of f r e s h n u c l e i in the c o o l e r r e g i o n s , w h e r e n u c l e a t i o n is e n e r g e t i c a l l y l e s s f a v o r a b l e . The use of slow growth s p e e d s is s u p p o r t e d b y the W i l l i a m s o n and S m a l l m a n 8 a n a l y s i s of the s t r a i n anneal technique, which s u g g e s t s that the p r o b a b i l i t y of r a n d o m n u c l e a t i o n is d e c r e a s e d when the g r a d i e n t v e l o c i t y is l e s s than the l i m i t i n g growth v e l o c i t y of the c r y s t a l . N e v e r t h e l e s s , the s u c c e s s f u l growth of a s i n gle c r y s t a l with t h i s m e t h o d is f r e q u e n t l y t e r m i n a t e d b y the r a n d o m n u c l e a t i o n of s e v e r a l g r a i n s m o r e f a v o r a b l y o r i e n t e d for r a p i d growth. Even when c o m p l e t e growth i s a t t a i n e d , l a r g e " p a r a s i t i c " g r a i n s u s u a l l y r e m a i n , l i m i t i n g the amount of useful c r y s t a l . C a r e f u l t h e r m a l m e c h a n i c a l p r o c e s s i n g of the s t a r t i n g m a t e r i a l to m i n i m i z e the n u m b e r of p o t e n t i a l n u c l e a t i o n s i t e s can i m p r o v e r e s u l t s J However, a t t e m p t s to grow c r y s t a l s of o r i e n t a t i o n s that a r e not f a v o r e d b y the cold w o r k e d r e c r y s t a l t i z a t i o n t e x t u r e of the s t a r t i n g m a t e rial are generally unsuccessful. The c l a s s i c a l n u c l e a t i o n and growth c o n c e p t of r e c r y s t a l l i z a t i o n s u g g e s t s an a l t e r n a t i v e a p p r o a c h to s u p p r e s s n u c l e a t i o n . With p u l s e h e a t i n g , r a n d o m n u c l e a tion d u r i n g the g r o w t h of a s e e d c r y s t a l should b e s u p p r e s s e d s i n c e t h e r e is an incubation p e r i o d b e f o r e r e c r y s t a l l i z a t i o n n u c l e i of m i n i m u m s t a b l e s i z e a r e f o r m e d , while t h e r e is no incubation p e r i o d a s s o c i a t e d with the growth of an e s t a b l i s h e d n u c l e u s . Thus, the s e e d can be p r o p a g a t e d b y l i m i t i n g the t i m e at the growth ( r e c r y s t a l l i z a t i o n ) t e m p e r a
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