Influence of the diffusion of oxygen on the aging of zinc oxide-based varistors

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Influence of the diffusion of oxygen on the aging of zinc oxide-based varistors

D. Binesti, Jean-Pierre Bonnet, Marc Onillon, Roger Salmon

To cite this version:

D. Binesti, Jean-Pierre Bonnet, Marc Onillon, Roger Salmon. Influence of the diffusion of oxygen on

the aging of zinc oxide-based varistors. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-919-C1-

923. �10.1051/jphyscol:19861142�. �jpa-00225541�

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INFLUENCE OF THE DIFFUSION OF OXYGEN ON THE AGING OF ZINC OXIDE-BASED VARISTORS

D. BINESTI, J.-P. BONNET, M. ONILLON and R. SALMON

Laboratoire çïe

Chimie

du

Solide du C.N.R.S., Université de Bordeaux I, 351, Cours de la Libération, F-33405 Talence Cedex, France

Résumé - L'étude de la cinétique de réduction de varistan- ces à base d'oxyde de zinc à 650°C et sous atmosphère CO-CO_ a permis de relier la dégradation des caractéristi- ques électriques, observée lors du vieillissement sous champ électrique, à un transfert d'oxygène des grains d'oxyde de zinc vers les phases intergranulaires.

Abstract - The kinetics of reduction of zinc oxide-based varistors has been studied at 650°C in CO-CO, mixtures in order to correlate ageing under electrical stress to diffusion phenomena. The experimental results indicate that the main cause of the ageing is a migration of oxygen from the outer core of the ZnO grains toward the inter- granular phases.

I - INTRODUCTION

The zinc oxide-based varistors exhibit a marked non-linearity in their current vs voltage characteristic, J=f(F). They are in consequence widely used in the manufacturing of devices for the protection of electrical or electronic circuitry against transient overvoltages. For such a purpose, a good stability in time of the J=f(F) characteristic is required /!/.

The non-linear behavior of the material is currently considered as being related to the appearance of Schottky barriers between adjacent ZnO grains, as a result of the trapping of electrons in acceptor levels at the boundary / 2 / . The J=f(F) characteristics are subjected to evolution under an applied electric field / 3 / or again by heating in a low partial pressure of oxygen / 4 / . In either case, the diffusion of species in the vicinity of the grain boundaries would be at the origin of the evolution / 5 / . If such chemical modifica- tions actually occur they are likely to affect the rates of diffusion.

The purpose of the present work is to investigate the diffusion phenomena through comparison of the reduction rates of new and arti- ficially aged materials.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861142

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JOURNAL DE PHYSIQUE

I1

-

EXPERIMENTAL

The ceramics investigated are of the same origin and were designed for surge protection on electric power lines, but :

-

ceramic A is new

-

^ceramicA' has been previously aged for 410 h at 115OC under 76 % of the threshold voltage (A.C.).

Their composition, microstructure and J-F characteristic are repor- ted in Table I, figure 1 and figure 2 respectively.

TABLE I

Concentrations of additives (weight % ) in ZnO based varistors A and A'.

Fig. 1

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Microstructure of varistance A, left, and A', right (back scattered electron images).

Fig. 2

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^3-F characteristics of ceramics A and A'.

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-

c o r i g i n a t i n g from t h e c e n t r a l r e g i o n

-

p o r i g i n a t i n g from t h e o u t e r c o r e .

The r e d u c t i o n wa_s c a r r i e d o u t a t $50°C i n a g a s e o u s m i x t u r e s u c h a s PCO/PC02 = 7.10 a t 1.013 x 1 0 Pa o v e r a l l p r e s s u r e , which c o r r e s p o n d s t o P, 2 atm. The c h a n g e s i n w e i g h t v s t i m e were

"2

measured u s i n g a SETARAM B70 t y p e t h e r m o b a l a n c e w i t h a ~ e l a t i v e s e n s i t i v i t y .

I11 - RESULTS AND DISCUSSION

The c u r v e s , nm/m0 = f ( t ) , o f t h e r e l a t i v e mass c h a n g e s w i t h t i m e a r e p r e s e n t e d on f i g u r e 3. They c l e a r l y show t h a t , f o r t h e new c e r a m i c , t h e r e l a t i v e d e c r e a s e i n mass o c c u r s w i t h a f a s t e r r a t e when t h e sample o r i g i n a t e s from t h e c e n t r a l r e g i o n t h a n from t h e o u t e r c o r e , whereas t h e o p p o s i t e b e h a v i o r i s o b s e r v e d f o r t h e a r t i f i c i a l l y aged c e r a m i c . F u r t h e r m o r e , t h e mass l o s s e s a r e more i m p o r t a n t f o r t h e l a t t e r .

F i g . 3 - R e l a t i v e mass l o s s e s v s t i m e Am/m = f ( t )

---

new m a t e r i a l ( A ) c :sampPe from t h e c e n t r a l r e g i o n a r t i f i c i a l l y a g e d p :sample from t h e o u t e r c o r e m a t e r i a l ( A ' )

The e x t e n s i o n of F i c k ' s s e c o n d law t o a + p o l y c r i s t a l l i n e m a t e r i a l l e a d s t o t h e d e f i n i t i o n of t h e p a r a m e t e r D c a l l e d a p p a r e n t c h e m i c a l d i f f u s i o n c o e f f i c i e n t . I t s e x p e r i m e n t a l d e t e r m i n a t i o n r e q u i r e s t h a t one s i n g l e r e d u c t i o n mechanism i s predominant t h r o u g h o u t t h e r e d u c t i o n t i m e . I t c a n t h e n be a c h i e v e d t h r o u g h t h e u s e of t h e f o l - l o w i n g e q u a t i o n e s t a b l i s h e d by NEWMAN / 6 / f o r a p a r a l l e l e p i p e d i c sample o f d i m e n s i o n s a , b , c :

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JOURNAL DE _PHYSIQUE

w h e r e Am a n d A m , a r e r e s p e c t i v e l y t h e d e c r e a s e s i n mass a t t i m e t a n d t = - a n d R n = n/2

+

^{n n}

.

The v a l u e s o f a n d nm,/m0 c a l c u l a t e d f r o m t h e e x p e r i m e n t a l nm/m0

= f ( t ) c u r v e s . r e c o r d e d f o r r e d u c t i o n t i m e s o f 20 u p t o 60 h o u r s , a r e p r e s e n t e d i n T a b l e 11. The weak w e i g h t l o s s e s o b s e r v e d d u r i n g t h e 20 f i r s t h o u r s o f t h e e x p e r i m e n t s c o r r e s p o n d t o t h e e q u i l i b r a t i o n o f t h e s u r f a c e . The a p p a r e n t c h e m i c a l d i f f u s i o n c o e f f i c i e n t 8 a p p e a r s t o r e m a i n p r a c t i c a l l y . c o n s t a n t , w h e r e a s the Amm/rn r a t i o i n c r e a s e s s i g n i f i c a n t l y when t h e c e r a m i c h a s b e e n a r t i f i c i a l P y a g e d .

[

s a m p l e

i

107D"icm2s-'1

i

¹⁰6 ^Amm/m,

1

TABLE I1

2r

A p p a r e n t d i f f u s i o n c o e f f i c i e n t s D a n d r e l a t i v e m a s s l o s s e s A m , /mo.

The c e r a m i c s s t u d i e d h a v e b e e n p r o c e s s e d u s i n g l i q u i d p h a s e s i n t e - r i n g a t 1300°C. F o r s u c h a t e m p e r a t u r e , t h e e l e c t r o c h e m i c a l p o t e n - t i a l s o f m o s t c o m p o n e n t s a r e e x p e c t e d t o n e a r l y h o m o g e n i z e , a t l e a s t i n t h e g r a i n b o u n d a r i e s r e g i o n . On c o o l i n g

-

w h i c h i s r a t h e r f a s t i n m a n u f a c t u r i n g c o n d i t i o n s - t h e e l e c t r o c h e m i c a l e q u i l i b r i u m c a n n o t b e m a i n t a i n e d s i n c e t h e v a r i a t i o n 3;. /aT

,

f o r a n y i compo- n e n t , i s n o t t h e same i n a l l p h a s e s . ~ l e c t r o & h e m l c a l p o t e n t i a l g r a - d i e n t s a r e t h u s c r e a t e d o n c e t h e c e r a m i c h a s r e a c h e d room t e m p e r a - t u r e , a n d a r e e x p e c t e d t o b e t h e s t e e p e s t i n t h e e x t e r n a l p a r t , w h e r e t h e c o o l i n g i s t h e ' f a s t e s t . I n s u c h a n o n - e q u i l i b r i u m m a t e - r i a l , a n y i n c r e a s e o f i n t e n s i v e t h e r m o d y n a m i c v a r i a b l e s i s l i k e l y t o s t i m u l a t e mass t r a n s f e r i n o r d e r t o b r i n g t h e m a t e r i a l b a c k t o e q u i l i b r i u m . F o r e x a m p l e , t h e a g e i n g t r e a t m e n t a t 115OC a n d u n d e r a n e l e c t r i c f i e l d s h a l l a l l o w o x y g e n t o move f r o m t h e r e g i o n s w h e r e i t s e l e c t r o c h e m i c a l p o t e n t i a l i s h i g h t o w a r d t h o s e w h e r e it i s l o w e r . I t i s w o r t h w h i l e n o t i n g t h a t t h e c h e m i c a l d i f f u s i o n c o e f f i i e n t s r e p o r t e d Lor m o n o c r y s t a l l i n e ZnO a t 650°C / 7 / a r e a t l e a s t 1 0 wea- k e r t h a n D o b t a i n e d a f t e r o u r m e a s u r e m e n t s a t t h e same t e m p e r a t u r e . T h i s means t h a t , i n t h e c a s e s we s t u d i e d , t h e ZnO b u l k was n o t i n - v o l v e d i n t h e f i r s t s t e p o f t h e r e d u c t i o n . The l a t t e r c o u l d a c t u a l l y b e c o n t r o l l e d by i n t e r g r a n u l a r p h a s e s , m a i n l y l o c a t e d a t t h e t r i p l e j o i n t s , w h i c h would p i c k u p o x y g e n d u r i n g t h e a g e i n g t r e a t m e n t , a s i n d i c a t e d b y t h e h i g h v a l u e s of mm/mo f o r t h e s o t r e a t e d m a t e r i a l s .

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r e s u l t o f a t r a n s f e r o f o x y g e n f r o m t h e e x t e r n a l p a r t o f t h e ZnO g r a i n s t o w a r d t h e g r a i n b o u n d a r i e s a n d t h e n t h e i n t e r g r a n u l a r phases, w h i c h would a l t e r t h e S c h o t t k y b a r r i e r s . The m a i n d r i v i n g f o r c e f o r t h i s t r a n s f e r would b e t h e c h a n g e i n t h e e l e c t r i c a l f i e l d n e a r t h e g r a i n b o u n d a r i e s , i n d u c e d by t h e e x t e r n a l e l e c t r i c a l stress.

Such a n i n t e r p r e t a t i o n i s c o n s i s t e n t w i t h t h e r e s t o r a t i o n o f t h e J = f ( F ) c h a r a c t e r i s t i c o b s e r v e d a f t e r h e a t i n g up t h e c e r a m i c a t 400-600°C i n a n a t m o s p h e r e o f p u r e oxygen a n d s u b s e q u e n t c o o l i n g / 8 / .

Acknowledgments

The p r e s e n t r e s e a r c h h a s b e e n p e r f o r m e d w i t h i n a p r o q r a m _- _. s u p p o r t e d by t h e D i r e c t i o n d e s E t u d e s e t - ~ e c h e r c h e s E.D.F.. The a u - t h o r s a r e g r a t e f u l t o t h e S e r v i c e ~ a . t e ' r i e 1 E l e c t r i q u e f o r t h e i r s u p p o r t

.

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