CORRELATION BETWEEN MICROSTRUCTURE OF NiO GRAIN-BOUNDARIES AND INTERGRANULAR DIFFUSION

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CORRELATION BETWEEN MICROSTRUCTURE OF NiO GRAIN-BOUNDARIES AND INTERGRANULAR

DIFFUSION

M. Déchamps, A. Revcolevschi, Franck Barbier

To cite this version:

M. Déchamps, A. Revcolevschi, Franck Barbier. CORRELATION BETWEEN MICROSTRUCTURE

OF NiO GRAIN-BOUNDARIES AND INTERGRANULAR DIFFUSION. Journal de Physique Col-

loques, 1986, 47 (C1), pp.C1-309-C1-313. �10.1051/jphyscol:1986145�. �jpa-00225575�

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JOURNAL DE PHYSIQUE

Colloque C1, suppl4ment au n02, Tome 47, fevrier 1986 page cl-309

CORRELATION BETWEEN MICROSTRUCTURE OF NiO GRAIN-BOUNDARIES AND INTERGRANULAR DIFFUSION

M. DECHAMPS, A. REVCOLEVSCHI and I?. BARBIER

Laboratoire de Chimie Appliquee, Bat. 414, Universit6 Paris-Sud, F-91405 Orsay Cedex, France

R6sum6

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Une d i f f u s i o n i n t e r g r a n u l a i r e p r e f e r e n t i e l l e de Ni e t de Co a & t &

signa7ee dans de 1 'oxyde de n i c k e l p o l y c r i s t a l l i n obtenu p a r oxydation t h e r - mique du n i c k e l . Ces r e s u l t a t s ne sont pas confirm& s u r du NiO obtenu p a r d ' a u t r e s precedes. La s t r u c t u r e des j o i n t s e s t sans doute responsable de l a d i f f e r e n c e de comportement.

A b s t r a c t

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An enhanced i n t e r g r a n u l a r d i f f u s i o n o f N i and Co was observed i n p o l y c r y s t a l l i n e NiO prepared by thermal o x i d a t i o n o f n i c k e l . These r e s u l t s are n o t confirmed i n n i c k e l oxide prepared by o t h e r methods. The s t r u c t u r e o f t h e grain-boundaries i s b e l i e v e d t o be r e s p o n s i b l e f o r t h i s d i f f e r e n t

behaviour.

I

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INTRODUCTION

An enhanced i n t e r g r a n u l a r d i f f u s i o n o f Ni and Co has been observed i n t h e temperature range 500-800°C i n n i c k e l oxide samples prepared by thermal o x i d a t i o n o f n i c k e l a t 1100°C /I/. T h i s r e s u l t , one o f t h e few obtained on n o n - m e t a l l i c m a t e r i a l s , i s n o t s u r p r i s i n g s i n c e i t i s w e l l accepted t h a t d i f f u s i o n proceeds more r a p i d l y along g r a i n - boundaries (GB's) than through t h e l a t t i c e because, i n p a r t i c u l a r , o f t h e atomic jump frequency much g r e a t e r than i n t h e l a t t i c e /2-5/. Numerous experiments, m a i n l y c a r r i e d o u t on metals, support t h i s o p i n i o n and one f r e q u e n t l y f i n d s t h a t g r a i n - boundary d i f f u s i o n c o e f f i c i e n t s D' are several orders o f magnitude g r e a t e r than t h e corresponding l a t t i c e d i f f u s i o n c o e f f i c i e n t s D /6/. _Iti s a l s o w e l l accepted t h a t t h e d i s l o c a t i o n s t r u c t u r e o f t h e GB's, t h a t i s , t h e type o f d i s l o c a t i o n (edge o r screw), i t s dissociatio'n, and i t s p o s s i b l e spreading, can i n f l u e n c e t h e boundary d i f f u s i v i t y . To i l l u s t r a t e t h i s point,one o f t h e most f r u i t f u l models remains t h e p i p e mechanism o f d i f f u s i o n o f T u r n b u l l and Hoffman based on t h e d i s l o c a t i o n descrip- t i o n o f sub grain-boundaries (SGB's) /7,8/. The model p r e d i c t s an i n f l u e n c e o f t h e m i s o r i e n t a t i o n between adjacent c r y s t a l s as w e l l as o f t h e type o f d i s l o c a t i o n on t h e d i f f u s i o n process ; i t a l s o p r e d i c t s an a n i s o t r o p y o f t h e i n t e r g r a n u l a r d i f f u - s i o n .

W i t h i n t h i s frame we have t r i e d t o r e f i n e t h e s t a t i s t i c a l r e s u l t s p r e v i o u s l y obtained on p o l y c r y s t a l l in e NiO ( c a l l e d h e r e a f t e r " o x i d i z e d N i " ) by c o n s i d e r i n g i n t e r g r a n u l a r d i f f u s i o n experiments on w e l l d e f i n e d GB's. T h i s l e d us t o c a r r y o u t a l a r g e number o f experiments on various <011> o r <001> t i l t a x i s b i c r y s t a l l i n e samples grown from t h e m e l t (flame f u s i o n technique), which i n d i c a t e d no p r e f e r e n t i a l g r a i n boundary d i f f u s i o n w i t h r e s p e c t t o l a t t i c e d i f f u s i o n . I n order t o e x p l a i n these c o n t r a d i c t o r y r e s u l t s a s t a t i s t i c a l study o f t h e g r a i n boundary f e a t u r e s was performed by transmission e l e c t r o n microscopy. The o r i e n t a t i o n r e 1 a t i o n s h i p s , t h e coincidence index, t h e o r i e n t a t i o n t e x t u r e o f t h e GB' planes and t h e i r d e t a i l e d microgeometry as w e l l as t h e o x i d a t i o n t e x t u r e , when necessary,were s t u d i e d i n t h e b i c r y s t a l s grown from t h e m e l t and i n t h e " o x i d i z e d N i " . The r e s u l t i n g dat,a were compared w i t h those determined on w e l l annealed NiO specimen. As a r e s u l t i t appears t h a t t h e n i c k e l oxide obtained by o x i d a t i o n i s f a r from thermodynamic e q u i l i b r i u m and i t i s b e l i e v e d t h a t t h i s may be t h e o r i g i n of t h e d i f f e r e n c e i n d i f f u s i o n a l behaviour.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986145

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C1-310 JOURNAL DE PHYSIQUE

Fig. 1

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I n f l u e n c e o f t h e m i s o r i e n t a t i o n O upon t h e r e l a t i v e energy o f [0011 ( a ) and C O l l l (b) symmetric t i l t g r a i n boundaries a t 1520°C /11/. ~b i s t h e GB energy and '(_s i s t h e s u r f a c e energy. The low energy boundaries correspond t o SGB's and t o t h e (122), (111) and (311) GB planes. The symmetrical t i l t boundaries which were used f o r d i f f u - s i o n experiments a r e i n d i c a t e d by arrows.

I 1

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ORIGIN OF THE SAMPLES UNDER STUDY

The o r i g i n and c h a r a c t e r i s t i c s o f t h e t h r e e types o f m a t e r i a l s which were s t u d i e d a r e described elsewhere /9/. L e t us remind b r i e f l y what they a r e :

-

"llOO°C o x i d i z e d N i " obtained by o x i d i z i n g a 30 p m t h i c k n i c k e l f o i l a t l l O O ° C ( t h i s p o l y c r y s t a l l i n e m a t e r i a l was p r o v i d e d by Atkinson who observed on i t an enhanced i n t e r g r a n u l a r d i f f u s i o n / I / ) . Accordinq t o t h e authors p u b l i c a t i o n s t h e t o t a l amount o f m e t a l l i c i m p u r i t i e s contained i n t o t h e o r i g i n a l n i c k e l i s between 100 and 1200 p.p.m.

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"1300°C o x i d i z e d N i " obtained by o x i d i z i n g a 600

urn

t h i c k f o i l a t 1300°C. This f o i l was then annealed d u r i n g 240 hours a t 1600°C l e a d i n g t o an "annealed NiO". The o r i - g i n a l n i c k e l was a h i g h p u r i t y metal p u r i f i e d by i o n exchange. The amount o f metal- l i c i m p u r i t i e s , except A1 and Si, was l e s s than 5 p.p.m.

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c r y s t a l s and b i c r y s t a l s grown from the m e l t by the flame f u s i o n technique

/ l o / ,

which were p r e v i o u s l y i n v e s t i g a t e d i n o r d e r t o determine t h e i r i n t e r g r a n u l a r r e 1 a- t i v e energy ( f i g . 1) /11/. These b i c r y s t a l s were made from Johnson Matthey, grade 1 powder ( m e t a l l i c i m p u r i t i e s < 20 p.p.m.). A c t i v a t i o n a n a l y s i s r e a l i z e d on NiO c r y s t a l s a f t e r growing have confirmed t h e Johnson Matthey a n a l y s i s (except f o r A1 and Si which a r e d i f f i c u l t t o analyse by a c t i v a t i o n i n a Ni m a t r i x ) .

During t h e numerous TEM observations and d i f f r a c t i o n experiments n o t any p r e c i p i t a t e was observed, e i t h e r i n t h e GB r e g i o n o r i n t h e b u l k . Moreover several b i c r y s t a l s grown from t h e m e l t were broken under h i g h vacuum (P Z

lo-'

Pa) along GB's and t h e l a t t e r s analyzed by Auger spectrometry. The r e s u l t i n g a n a l y s i s showed t h a t t h e GB's i n t h e b i c r y s t a l s were f r e e from segregation.

D i f f u s i o n experiments were c a r r i e d o u t on t h e t h r e e types o f m a t e r i a l s (by Atkinson e t a l . i n t h e f i r s t case) t o g e t h e r w i t h e x t e n s i v e geometrical and morphological s t u d i e s performed by TEM.

I 1 1

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DIFFUSION STUDY RESULTS

AS shown i n f i g . 1, t h e r e l a t i v e energy o f t h e two s e r i e s o f symmetrical GB's i s s t r o n g l y dependent on m i s o r i e n t a t i o n , e s p e c i a l l y i n the.case o f <011> t i l t boundaries f o r which t h r e e cusps were evidenced, corresponding t o 12211, { 1111 and (3111 boundary planes ( t h e r e s p e c t i v e coincidence i n d i c e s C a r e 9, 3 and 11)('). Although t h e abso- l u t e value o f t h e i n t e r g r a n u l a r energy remains unknown, t h e experimental evidence o f the cusps i s w e l l e s t a b l i s h e d and a c a u t i o u s examination o f t h e experimental condi- t i o n s showed t h a t t h e cusps correspond t o GB energy minima. Now, Herbeuval e t a1

.,

i n p a r t i c u l a r , have shown on aluminium <011> t i l t GB's t h a t t h e r e e x i s t s a c l o s e

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r e l a t i o n s h i p between t h e energy o f a GB and t h e corresponding i n t e r g r a n u l a r d i f f u s i o n c o e f f i c i e n t D' /12/. By analogy we thought t h a t t h e <O1l>GB1s o f o u r NiO samples were w e l l s u i t e d f o r a v e r i f i c a t i o n o f a s i m i l a r e n e r g y - d i f f u s i o n c o r r e l a t i o n i n an i o n i c m a t e r i a l . Several m i s o r i e n t a t i o n s , i n d i c a t e d by arrows i n f i g .l and corresponding e i t h e r t o h i g h energy m i s o r i e n t a t i o r s , o r t o low energy m i s o r i e n t a t i o n s , were selected.

I n t h e f i r s t case,it was expected t o observe an i m p o r t a n t i n t e r g r a n u l a r d i f f u s i o n ( t h e i n t e r g r a n u l a r d i f f u s i o n c o e f f i c i e n t of N i determined on "1100° oxidized Ni

"

(Po2=1 atm) i s D'6 = 3.10-' exp- (1.78 (eV)/kT), i .e. D'/D %

lo6,

i f we assume as Atkinson f o r t h e GB w i d t h 6

2

7 /I/. I n t h e second case, corresponding t o CSL mi s o r i e n t a t i o n s o f small m i s f i t between a d j a c e n t grains, no d i f f u s i o n o r very 1 im i t e d p r e f e r e n t i a l GB d i f f u s i o n was expected t o be observed ( 1 ) /13/.

Two types o f d i f f u s i o n experiments were undertaken : S e l f - d i f f u s i o n u s i n g 63Ni (quanti- t a t i v e experiments were c a r r i e d 'out by abrasion and counting o f t h e B a c t i v i t y o f t h e samples ; q u a l i t a t i v e experiments used auto-radiography), and h e t e r o - d i f f u s i o n o f homo-valent c o b a l t ( q u a n t i t a t i v e measurements were made using t h e e l e c t r o n microprobe technique). NO enhanced s e l f - d i f f u s i o n o r h e t e r o - d i f f u s i o n were detected a t t h e GB's.

I n order t o complete our i n f o r m a t i o n , a d d i t i o n a l N i s e l f - d i f f u s i o n experiments were undertaken on p o l y c r y s t a l l i n e "1300' o x i d i z e d N i t ' samples,after p o l i s h i n g t h e as-oxi- d i z e d surface, b u t b e f o r e f u r t h e r h i g h temperature annealing. Again, no p r e f e r e n t i a l GB d i f f u s i o n was evidenced on these p o l y c r y s t a l l i ne samples

.

I V

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GRAIN BOUNDARY MICROSTRUCTURE

I n order t o f i n d a p o s s i b l e e x p l a n a t i o n t o these apparent d i f f u s i o n discrepancies, t h e GB's were subjected t o c r y s t a l l o g r a p h i c c h a r a c t e r i z a t i o n , morphological s t u d i e s and d i s l o c a t i o n s t r u c t u r e determinations.

I V . l . C r y s t a l l o g r a p h i c d e s c r i p t i o n o f t h e GB's : such a d e s c r i p t i o n includes, t h e 5 macrocospic parameters needed t o d e f i n e t h e o r i e n t a t i o n r e l a t i o n s h i p e x i s t i n g between t h e adjacent c r y s t a l s and t h e i n d e x a t i o n o f t h e GB plane i n r e l a t i o n t o both c r y s t a l s /14/. T h i s d e t e r m i n a t i o n was c a r r i e d o u t using t h e c r y s t a l l o g r a p h i c data c o l l e c t e d from TEM micrographs and d i f f r a c t i o n p a t t e r n s , according t o methods d e t a i l e d i n several papers /14-1G/. B r i e f l y t h e r e s u l t s can be summarized as f o l l o w s (see r e f . 9 and 14 f o r d e t a i l s ) :

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O r i e n t a t i o n r e l a t i o n s h i p s : these r e l a t i o n s , described by a r o t a t i o n 8 around an a x i s R, were computed f o r each i n t e r f a c e and t h e r e s u l t s used t o l o o k f o r a p o s s i b l e coincidence index C. I n t h e "flame f u s i o n c r y s t a l s " some specimen a r e c l o s e l y approximatinq low values o f C, as i n d i c a t e d i n f i g . 1. I n t h e "llOO°C o x i d i z e d N i "

and i n t h e "annealed NiO" a coincidence index ( w i t h i n t h e l i m i t o f ^Cs 41) can be a l l o t e d t o about 12 % o f t h e couples o f c r y s t a l s adjacent t o each GB ( 2 ) . However no p r e f e r e n t i a l r e l a t i o n s h i p appears, whatever t h e thermal treatment i s .

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O r i e n t a t i o n o f the GB planes : t h i s c h a r a c t e r i s t i c was determined f o r 113 couples o f g r a i n s f o r t h e "llOO°C o x i d i z e d N i t ' and f o r 23 couples f o r t h e "annealed NiO". The r e s u l t s are r e p o r t e d on standard [001] stereographic t r i a n g l e s ( f i g . 2). These data are q u i t e i n t e r e s t i n g because of t h e GB c l e a r c u t tendency, i n b o t h types o f specimen t o a v o i d t h e C O l l ) o r i e n t a t i o n s d e f i n e d w i t h r e s p e c t t o one o f t h e c r y s t a l s , t h i s tendency being s t r o n g l y enhanced by annealing, s i n c e we do n o t observe GB planes w i t h i n 15' o f (011). I t must be emphasized t h a t t h i s cannot r e s u l t from t h e t e x t u r e which develops e i t h e r d u r i n g t h e o x i d a t i o n , o r t h e annealing processes ( 3 ) . Hence, geometrical considerations, l i n k e d w i t h t h e columnar o x i d a t i o n growth o f t h e g r a i n s cannot be evoked t o e x p l a i n t h e absence o f (0111 planes.

( ' )

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^C i s t h e r a t i o o f t h e volume o f t h e Coincidence S i t e L a t t i c e (CSL) u n i t c e l l t o t h a t o f t h e c r y s t a l l a t t i c e c e l l .

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^TheCSL m i s o r i e n t a t i o n s a r e s i n g u l a r o r i e n t a t i o n r e 1 a t i o n s h i p s corresponding t o a h i g h d e n s i t y of s i t e s common t o b o t h l a t t i c e s , which form t h e CSL l a t t i c e . According t o theory, t h e h i g h e r t h e d e n s i t y o f common s i t e s , t h e lower t h e energy o f an i n t e r - face passing through a low index CSL l a t t i c e plane.

(')

-

12 % i n s t e a d o f t h e p r e v i o u s l y r e p o r t e d 50 % by r e f . 14. The disagreement i s due t o t h e erroneous method which was f i r s t used t o determine t h e d e v i a t i o n o f the b i c r y s t a l s from the CSL coincidence m i s o r i e n t a t i o n /21/.

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~ 1 - 3 1 2 JOURNAL DE PHYSIQUE

F i g . 2

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D i s t r i b u t i o n o f t h e GB plane o r i e n t a t i o n i n " o x i d i z e d NiO" (a) and "annealed NiO" ( b ) . The absence o f { O l l } GB plane, already n o t i c e a b l e i n (a) i s s t r o n g l y enhanced a f t e r anneal i n g ( b )

.

As a conclusion i t seems t h a t an i n t e r f a c e corresponding t o a h i g h angle GBy close t o a { O l l ) plane, indexed w i t h r e s p e c t t o one of t h e adjacent c r y s t a l s , i s p a r t i c u l a r l y unstable, as shown by t h e e v o l u t i o n o f t h e d i s t r i b u t i o n o f t h e GB's d u r i n g annealing.

Although the reason o f t h i s tendency i s n o t y e t understood i t appears c l e a r l y t h a t t h e GB1s, i n t h e " l l C O ° C o x i d i z e d N i " , are f a r from being i n t h e e q u i l i b r i u m s t a t e .

I V . 2. Grain boundary morphology : t h e morphology o f t h e v a r i o u s examined i n t e r f a c e s depends on t h e o r i g i n o f t h e NiO and on t h e thermal treatment t o which i t was sub- j e c t e d . F a c e t t i n g doesnot occur i n t h e h i g h angle m i s o r i e n t a t i o n GB1s, whatever t h e o r i g i n and the thermal treatment o f t h e NiO i s . Conversely, t h e morphology o f t h e SGB's i s s t r o n g l y dependent on t h e " h i s t o r y " o f t h e m a t e r i a l . I n t h e "1100°C o x i d i z e d N i " , SGB's a r e n o t facetted, b u t i n t h e "annealed NiO", as w e l l as i n t h e flame f u s i o n

c r y s t a l s , facets are s y s t e m a t i c a l l y observed. They are m o s t l y made o f l a r g e p o r t i o n s o f (0111 and El121 planes and o f (113) and t122.l ledges. I t can be noted t h a t these l a t t e r planes correspond t o cusps i n f i g . l a b , t h a t i s , t o e n e r g e t i c minima o f some GB's.

These observations can be i n t e r p r e t e d as f o l l o w s . A GB can be e i t h e r plane (on a l a r g e scale), o r facetted. F a c e t t i n g has e i t h e r a k i n e t i c o r i g i n (e.g. when t h e GB i s f a r from thermodynamical e q u i l i b r i u m , d u r i n g a f a s t g r a i n growth) or an e n e r g e t i c o r i g i n (e.g. when i t i s c l o s e l y approximating an e q u i l i b r i u m s t a t e ) /17/. Close t o e q u i l i b r i u m , which occurs f o r example a f t e r a l o n g annealing a t h i g h temperature ("annealed NiO"), a GB can reduce i t s surface energy by f a c e t t i n g along the lowest

energy c r y s t a l l o g r a p h i c planes compatible w i t h i t s average o r i e n t a t i o n . Hence, t h e p o s s i b l e e v o l u t i o n o f t h e morphology i s another c r i t e r i o n which may suggest t h a t GB1s are n o t i n an i n t r i n s i c s t r u c t u r a l e q u i l i b r i u m . However f a c e t t i n g w i l l take place o n l y i f t h e s u r f a c e energy o f t h e new facets i s low enough t o compensate f o r t h e increased surface. This i s the reason why h i g h angle GB's do n o t f a c e t e a s i l y , V

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CONCLUSION

Data r e l a t i v e t o i n t e r g r a n u l a r d i f f u s i o n s t u d i e s i n p o l y c r y s t a l l i n e oxides a r e n o t i n good agreement /1,18/ and t h e mechanism o f GB d i f f u s i o n i s n o t y e t w e l l understood.

Wuench and Wasilos /19/ and Osenbach and Stubican /20/, f o r example, suggest t h a t enhanced d i f f u s i o n o f c a t i o n s along GB's i n MgO may be a t t r i b u t e d t o i m p u r i t y segre- g a t i o n a t the boundary. The d i f f u s i o n discrepancies c o u l d be e x p l a i n e d i n a s i m i l a r way s i n c e t h e " l l O O ° C o x i d i z e d N i " i s l i k e l y t h e l e s s pure o f t h e m a t e r i a l s under

investigation,though A t k i n s o n e t a l . have p o i n t e d o u t t h a t t h e r e was no p r e c i p i t a t e a t t h e GB1s.

--

( 3 )

-

The sample t e x t u r e , determined on a l i m i t e d number o f c r y s t a l s by TEM, was shown

a f t e r an a d d i t i o n a l X-Ray study, t o be r e p r e s e n t a t i v e .

(6)

However, t h e comparison o f t h e c h a r a c t e r i s t i c s o f t h e GB's shows t h a t t h e " l l O O ° C o x i d i z e d N i " i s f a r from e q u i l i b r i u m a n d conversely, t h e "annealed H i O " looks q u i t e s i m i l a r t o the flame-fusion c r y s t a l s , where t h e GB's seem t o be almost a t e q u i l i b r i u m . Since t h e p r o p e r t i e s o f a GB are c l o s e l y r e l a t e d t o i t s s t r u c t u r e , i t i s n o t unreaso- nable t o t h i n k t h a t t h e p r e f e r e n t i a l GB d i f f u s i o n observed i n t h e "llCO°C o x i d i z e d W i " i s l a r g e l y due t o the f a c t t h a t t h e g r a i n boundaries a r e f a r from e q u i l i b r i u m . REFERENCES

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