• PDF / 1,252,598 Bytes
• 9 Pages / 612 x 792 pts (letter) Page_size
• 83 Downloads / 249 Views

DOWNLOAD

REPORT

---

= 1 - exp[-(t/'c)]

n

where the time constant r and the power dependence n provide useful i n f o r m a t i o n about the r a t e - l i m i t i n g p r o c e s s involved and the s i z e , shape, and d e n s i t y of the p r e c i p i t a t e p a r t i c l e s . F o r the i n i t i a l stage of p r e c i p i t a t i o n of lead in PbSe an a v e r a g e value of n = 1.12 was e x p e r i m e n t a l l y d e t e r m i n e d . This value is i n d i c a t i v e of s p h e r o i d a l l y shaped p r e c i p i t a t e p a r t i c l e s , e s p e c i a l l y if p r e p r e c i p i t a t i o n e x i s t s . A f t e r a p p r o x i m a t e l y 50 pct of the lead p r e c i p i t a t e s , a l o n g e r T p r o c e s s b e c o m e s r a t e - c o n t r o l l i n g and n in the above equation e q u z l s ~-. With r e s p e c t to the s i z e , which was on the o r d e r of 1000A in r a d i u s , and the shape of the lead p r e c i p i t a t e s , the kinetic p r e d i c t i o n s were found to be in good a g r e e m e n t with evidence provided by e x t r a c t i o n r e p l i c a techniques and e l e c t r o n m i c r o s c o p y . F o r the p r e c i p i t a t i o n of s e l e n i u m in PbSe an a v e r a g e value of n = 0.9 was e x p e r i m e n t a l l y d e t e r m i n e d . T h i s value is i n d i c a t i v e of c y l i n d r i c a l l y shaped p r e c i p i t a t e p a r t i c l e s , e s p e c i a l l y if p r e p r e c i p i t a t i o n e x i s t s . Utilizing the exp e r i m e n t a l l y d e t e r m i n e d value for r , an a p p r o x i m a t e size for the p r e c i p i t a t e p a r t i c l e s at e q u i l i b r i u m was c a l c u l a t e d as being about 40.~ in r a d i u s and 3000A in length. Values obtained for the a c t i v a t i o n e n e r g y of p r e c i p i t a t i o n , 0.84 ev for P b , 1.23 ev for Se, a g r e e v e r y c l o s e l y with those for the a c t i v a t i o n e n e r g y of diffusion, 0.83 ev for Pb, 1.20 ev for Se. T h i s i n d i c a t e s that diffusion plays an i m p o r t a n t role in the p r e c i p i t a t i o n of lead and s e l e n i u m in PbSe. I N r e c e n t y e a r s c o n s i d e r a b l e e x p e r i m e n t a l effort has b e e n devoted to the polar s e m i c o n d u c t o r s PbS, PbSe, and P b T e . The m o t i v a t i o n for these s t u d i e s has been the u s e f u l n e s s of these m a t e r i a l s as i n f r a r e d d e t e c t o r s and as t h e r m o e l e c t r i c r e f r i g e r a t i o n and power g e n e r ation e l e m e n t s . In many of these a p p l i c a t i o n s , for opt i m u m p e r f o r m a n c e , it is n e c e s s a r y for these m a t e r i a l s to have specific c a r r i e r c o n c e n t r a t i o n s . T h i s r e q u i r e m e n t can be m e t e i t h e r by doping with a t h i r d e l e m e n t or by s t o i c h i o m e t r i c c o n t r o l . It i s the l a t t e r method of c a r r i e r c o n t r o l with which this p a p e r is c o n c e r n e d . S t o i c h i o m e t r y can be c o n t r o l l e d e i t h e r by vapor dif