Grain boundary segregation in ceramics
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c e s s charge will be c o m p e n s a t e d by a space c h a r g e cloud of opposite sign v a c a n c i e s extending 20 to 100,~ f r o m the b o u n d a r y . 2 It is r e a s o n a b l e to expect this space c h a r g e effect to c o n t r i b u t e an a d d i t i o n a l d r i v i n g f o r c e for s e g r e g a t i o n of an i m p u r i t y , the magnitude of which will be d e t e r m i n e d by the s t r e n g t h of the dipole effect, and the s t r e n g t h of coupling between the solute and v a c a n c i e s (depending on r e l a t i v e v a l e n c e , v a c a n c y solute b i n d i n g e n e r g y , and so forth). F o r example, T a n has i n v e s t i g a t e d the effect of the s p a c e c h a r g e on the s e g r e g a t i o n of d i v a l e n t i m p u r i t i e s to the s u r f a c e of AgBr.5-7 His model p r e d i c t s the f o r m a t i o n of a p o s i t i v e space charge to c o m p e n s a t e for a net negative c h a r g e on the s u r f a c e of AgBr, and the s u b s e q u e n t i n t e r a c t i o n of a mobile d i v a l e n t i m p u r i t y such as P b ยง with this space c h a r g e . 5'6 U s i n g ion s c a t t e r i n g s p e c t r o m e t r y (ISS), T a n has d e m o n -
II. REVIEW OF IONIC GRAIN BOUNDARY PROPERTIES S e v e r a l p r e v i o u s r e v i e w s have c o n s i d e r e d the s t r u c t u r e and c h a r a c t e r of ionic g r a i n b o u n d a r i e s in detail; 1-3 however, it is a p p r o p r i a t e h e r e to e m p h a s i s s o m e of the r e l e v a n t p r o p e r t i e s of ionic i n t e r f a c e s and the p o s s i b l e effects on s e g r e g a t i o n . 1. C h a r g e d G r a i n B o u n d a r i e s Ionic i n t e r f a c e s in g e n e r a l , 4 and g r a i n b o u n d a r i e s in p a r t i c u l a r , 1-3 can a s s u m e a net c h a r g e due to the b r e a k in p e r i o d i c o r d e r o c c u r r i n g at the i n t e r f a c e . F o r example, Rice has d e m o n s t r a t e d that a s i m p l e 10 deg tilt b o u n d a r y in an ionic such as MgO can only be a c c o m m o d a t e d by the c r e a t i o n of e x c e s s b o u n d a r y v a c a n c i e s to c o n s e r v e charge, 1 Fig. 1. K i n g e r y has c o n s i d e r e d this p r o b l e m of charge m a t c h i n g f u r t h e r , and has modeled the dipole effect expected at a g r a i n b o u n d a r y in a r e g u l a r l y o r d e r e d , c h a r g e d lattice. 2,3 The b a s i s for the dipole effect, is that at t h e r m o d y n a m i c e q u i l i b r i u m in a n ionic m a t e r i a l , a n e x c e s s charge will e x i s t at i n t e r f a c e s which function as v a cancy s i n k s if t h e r e is a d i f f e r e n c e in the e n e r g y of f o r m a t i o n of cation and a n i o n v a c a n c i e s . 2,4 T h i s exW. C. JOHNSON, formerly with the Ford Motor Co. Research Staff, Dearborn, MI, is now Product Manager, Physmal Electronics Ind., Inc., Eden Prairie, MN 55343. This paper is based on a presentation made at a symposium on "Fundamentals of Grain Boundary Segregation'" held at the Niagara
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