Oxygen permeability of calcia-stabilized zirconia
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ZrO2(CaO)
(P62 = 1 aim)
solid electrolyte
N2 - 02 I (P~)z = 0.39- 10 -3 aim). I
Oxygen permeability of calcia-stabilized zirconia is proportional to (I - p,I14~ 02 i. From permeability measurement, the conduction properties of the electrolyte were log cr~ = 0.28
-
the
5100/T
and log P e = - 0 . 8 7 + 15,400//T w h e r e a~ i s t h e / ) - t y p e e l e c t r o n i c c o n d u c t i v i t y at PO2 = 1 a t m , and PC i s the oxygen a c t i v ity at which the p - t y p e e l e c t r o n i c c o n d u c t i v i t y and the ionic c o n d u c t i v i t y a r e e q u a l .
C A L C I A - s t a b i l i z e d z i r c o n i a is u s e d m a i n l y a s a c o m m e r c i a l s o l i d e l e c t r o l y t e of o x y g e n c o n c e n t r a t i o n c e l l at high t e m p e r a t u r e , and its m o s t i m p o r t a n t a p p l i c a t i o n is an oxygen s e n s o r for m o n i t o r i n g the oxygen a c t i v i t y o r p r e s s u r e in liquid m e t a l s and g a s e s . Oxygen, h o w e v e r , p e r m e a t e s t h r o u g h z i r c o n i a at high t e m p e r a t u r e , c o n s e q u e n t l y o x y g e n p e r m e a b i l i t y m a y be a m a i n s o u r c e of e x p e r i m e n t a l e r r o r in the e l e c t r o c h e m i c a l m e a s u r e m e n t of oxygen in i n e r t g a s e s s u c h a s n i t r o g e n , a r g o n and so f o r t h . F o r e x a m p l e , the m e a s u r e d E M F of the following c e l l d e p e n d s on the flow r a t e of g a s e s ; 1'2 P t / a i r / Z r O 2 ( C a O ) / O ~ (in N2 o r A r ) / P t . S i m i l a r l y , oxygen p e r m e a b i l i t y g i v e s an influence on the E M F of the following c e l l f o r m o n i t o r i n g the oxygen content in liquid iron; 3 M o - M o O J Z r O 2 ( C a O ) / O (in F e ) . In o r d e r to c o n f i r m the path of t r a n s f e r that m a i n l y c o n t r i b u t e s to oxygen p e r m e a b i l i t y t h r o u g h z i r c o n i a , it was d e c i d e d to m e a s u r e the p e r m e a b i l i t y of both oxygen and n i t r o g e n . If the m a i n c a u s e of oxygen t r a n s f e r w e r e not due to the e l e c t r o n i c c o n d u c t i o n of the z i r c o n i a e l e c t r o l y t e , both o x y g e n and n i t r o g e n would p e r m e a t e t h r o u g h z i r c o n i a . O x y g e n p e r m e a t i o n due to K n u d s e n d i f f u s i o n t h r o u g h p o r e s , d i f f u s i o n t h r o u g h g r a i n b o u n d a r i e s and m o l e c u l a r diffusion t h r o u g h m i c r o c r a c k s w i l l be r e d u c e d by i m p r o v i n g the p h y s i c a l p r o p e r t i e s of z i r c o n i a , i.e., b y m a k i n g d e n s e c e r a m i c s . T h e r e f o r e , in the a b s e n c e of t h e s e o t h e r e f f e c t s , oxygen t r a n s f e r which m a y c a u s e an e x p e r i m e n t a l e r r o r d u r i n g the E M F m e a s u r e m e n t of o x y g e n c o n c e n t r a t i o n c e l l would be due to e l e c t r o n i c conduction. T h e oxygen p e r m e a b i l i t y due to e l e c t r o n i c conduction, and the r
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