Morphology relations during surface-transport controlled sintering
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d o m i n a n t m a s s - t r a n s p o r t m e c h a n i s m d u r i n g the s i n t e r i n g of s p h e r i c a l p a r t i c l e s has been c l a s s i c a l l y identified by the t i m e - d e p e n d e n c y of the i s o t h e r m a l neck growth of s p h e r e - t o - s p h e r e and s p h e r e - t o - p l a t e c o n f i g u r a t i o n s .1-~ Added s u p p o r t for the choice of a given m e c h a n i s m is u s u a l l y d e r i v e d f r o m c a l c u l a t i o n s of the activation e n e r g y a s s o c i a t e d with the p r o c e s s and s c a l i n g laws, p r o p o s e d by H e r r i n g , s have been u s e d to s u p p l e m e n t this a p p r o a c h of identifying the s i n t e r ing m e c h a n i s m . However, in spite of the a p p a r e n t s u c c e s s e s of this approach, d i s c r e p a n c i e s in the r e p o r t e d r e s u l t s and c o n t r o v e r s y c o n c e r n i n g their s i g n i f i c a n c e exist. 6 T h i s i s due, i n p a r t , to the d i f f e r e a t a p p r o x i m a t i o n s and a s s u m p t i o n s employed in the v a r i o u s modifications of the neck growth t i m e - d e p e n dency r e l a t i o n . F u r t h e r m o r e , the task of c r i t i c a l l y evaluating the r e s u l t s of s i n t e r i n g e x p e r i m e n t s is often made m o r e difficult by the r e l a t i v e lack of r e l i a b l e diffusion data. .Typically, the t i m e - d e p e n d e n t neck growth equation is e x p r e s s e d as
x) n = Bt.
[1 ]
Schwed, 1~ and Rockland. 1' When the r e s u l t s of s i n t e r ing e x p e r i m e n t s could not be totally explained in t e r m s of a single m e c h a n i s m , the concept of a m u l t i - m e c h a n i s m s i n t e r i a g has b e e n advocated. M u l t i - m e c h a n i s m s i n t e r i n g has quite often been suggested for p r o c e s s e s w h e r e the c a l c u l a t e d t i m e exponent n did not a g r e e with the anticipated value proposed for a given m e c h a n is m)2 The d o m i n a n c e of s u r f a c e diffusion a s the m e c h a n i s m for low t e m p e r a t u r e s i n t e r i n g has long been a c cepted. However, r e c e n t e x p e r i m e n t a l o b s e r v a t i o n s on the s u r f a c e d i f f u s i v i t i e s of m e t a l s and c e r a m i c s have shown that the s i g n i f i c a n c e of this m e c h a n i s m a n d its d o m i n a n c e extend to the r e l a t i v e l y high t e m p e r a t u r e r e g i o n s , at l e a s t in the i n i t i a l s t a g e s of s i n t e r ing. ~'14 The a b s e n c e of d e n s i f i c a t i o n in s u r f a c e - d i f fusion s i n t e r i n g along with the cited u n c e r t a i n t i e s in n e c k - g r o w t h e x p e r i m e n t s b r i n g in focus the need for a n o t h e r approach. Nichols and M u l l i n s ~s and Nichols is have provided a c o m p u t e r modeling of the m o r p h o l o g ical changes o c c u r r i n g d u r i n g s u r f a c e - d i f f u s i o n c o n t r o l l e d s i n t e r i n g . T h e i r a n a l y s i s has shown that th
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