METASTABLE TETRAGONAL CeO2-ZrO2 SOLID SOLUTION

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METASTABLE TETRAGONAL CeO2-ZrO2 SOLID SOLUTION

S. Meriani

To cite this version:

S. Meriani. METASTABLE TETRAGONAL CeO2-ZrO2 SOLID SOLUTION. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-485-C1-489. �10.1051/jphyscol:1986173�. �jpa-00225604�

METASTABLE TETRAGONAL Ce0,-ZrO, SOLID SOLUTION

S. MERIANI

I s t i t u t o d i C h i m i c a A p p l i c a t a , v i a Valerio, 2, 1-34127 T r i e s t e , I t a l y

RGsum6 : En r e f r o i d i s s a n t , en c o n d i t i o n s h o r s G q u i l i b r e , l e s solu- t i o n s s o l i d e s de s t r u c t u r e f l u o r i n e zircone-oxyde de c6- rium, on a obtenu une n o u v e l l e phase, r i c h e en CeO , de s t r u c t u r e t k t r a g o n a l e au l i e u de l a phase cubique, 2 s t a b l e .3 haute temp6rature.

A b s t r a c t : When c e r i a - z i r c o n i a s o l i d s o l u t i o n s with f l u o r i t e t y p e of s t r u c t u r e undergo a non-equilibrium c o o l i n g t r e a t m e n t , a new m e t a s t a b l e , Ce02 r i c h phase, of t e t r a g o n a l syrnmetry is obtained i n s t e a d of t h e h i g h temperature c u b i c phase.

I

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Ceria

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These s o l i d s o l u t i o n s ' encounter a growing i n t e r e s t among t h e s c i e n t i f i c and t e c h n o l o g i c a l commdnities, because of t h e i r p o t e n t i a l use a s a high

s t r e n g t h / 4 / and e l e c t r c o n d u c t i v e ceramics / 5 / ,

Recently, a r e v i s e d phase diagram of t h e system Z r O --CeO . below 1400°C

2 2

was proposed, u s i n g hydrotherml t e c h n i q u e s i n o r d e r t o a c c e l e r a t e r e - a c t i o n r a t e s a t low temperatures / 6 / .

With r e g a r d t o t h e p r e v i o u s l y r e p o r t e d diagrams, t h i s new i n v e s t i g a t i o n d i s c l o s e d an e u t e c t o i d decomposition of t h e s o l i d s o l u t i o n based on t h e t e t r a g o n a l z i r c o n i a t y p e of s t r u c t u r e between 1000 and 1100°C. The two outcoming phases were t h e monoclinic and c u b i c s o l i d s o l u t i o n s . In t h e d i s c u s s i o n it has been p o i n t e d o u t t h a t o t h e r a u t h o r s has found and r e - ported a t e t r a g o n a l s o l i d s o l u t i o n s t a b l e down t o room temperature/l-3/

o r s l i g h t l y above it / 4 / . This was j u s t i f i e d on t h e b a s i s of t h e l a r g e e x c e s s energy necessary f o r t h e e u t e c t o i d r e a c t i o n t o t a k e p l a c e . Where a s t h e ( t ) - ( m ) t r a n s f o r m a t i o n , d i f f b s i o n l e s s , needs only a much smal- l e r anount of energy.

S i n t e r i n g r a t e and d i f f u s i o n c o e f f i c i e n t s of t h e Ce(+4) and Z r ( + 4 ) i o n s have been e v a l u a t e d i n t h e 1 6 mol % c e r i a - z i r c o n i a s o l i d s o l u t i o n with t e t r a g o n a l symmetry preserved throughout t h e whole experimental p e r i o d , which l a s t e d s e v e r a l hours, a t e l e v a t e d temperatures / 7 / .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986173

JOURNAL DE PHYSIQUE

This work was undertaken t o study t h e behaviour of t h e two o x i d e s mix- t u r e s above 1400°C. Although t h i s l a t t e r temperature was n o t exceeded, d u r i n g t h e hydrothermal i n v e s t i g a t i o n s of Tani e t a 1 . , / 6 / i t i s assumed t h a t two phases a r e s t a b l e up t o 1600°C: t h e c u b i c f l u o r i t e t y p e s o l i d

s o l u t i o n , based on t h e c e r i a s t r u c t u r e , and a n o t h e r based on t h e t e t r a - gonal z i r c o n i a .

The temperature of 1600°C was reached d u r i n g t h e d r y - f i r i n g s performed t o p r e p a r e t h e s i n t e r e d samples. The c e r i a c u b i c t y p e s t r u c t u r e was found t o extend from t h e 100 t o t h e 4 0 rnol % c e r i a c o n c e n t r a t i o n / 6 / . The r e s u l t s obtained i n t h e p r e s e n t i n v e s t i g a t i o n i n d i c a t e t h a t a d i f f e - r e n t phase could be o b t a i n e d i f a not-quenching c o o l i n g r a t e was a p p l i - ed t o samples f i r e d a t 1450 - 1600°C and c o n t a i n i n g 60-20 rnol % c e r i a .

I1

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Samples were prepared by wet m i l l i n g Zr02 - ^CeO powders, of commercial o r i g i n , f o r two days i n a g a t e b a l l m i l l s . The 2 d r i e d samples were pres- sed i n t o d i s k s which were f i r e d i n a n e l e c t r i c f u r n a c e up t o t h e s o a k - i n q temperature, i n a i r , a t 10°C/min. The specimens were k e p t 24 h a t 1450°C, 15 h a t 1500°C, 12 h a t 1550°C and 6 hours a t 16'0O0C.

The c o o l i n g s t e p , which was named "non-equilibrium", followed t h e natu- r a l c o o l i n g r a t e of t h e f u r n a c e : on averaqe about I0C/sec between t h e soaking temperatures and 900°C. A t t h a t moment t h e samples were quench- ed down t o room temperature t o be examined by X-ray d i f f r a c t i o n analy- s i s (XRD) and scanning e l e c t r o n microscopy (SEMI.

I n o r d e r t o r e v e a l t h e m i c r o s t r u c t u r e , a f t e r diamond p o l i s h i n g , a t h e r - mal e t c h i n g of about 1 hour a t 1600°C was a p p l i e d t o t h e samples.

I11 - RESULTS AND DISCUSSION

The r e s u l t s a r e summarized i n F i g . 1 (a-b-c-d) which r e p o r t s t h e d(hk1) v a l u e s of t h e c u b i c f l u o r i t e s t r u c t u r e , l a b e l l e d ( 4 2 2 ) , (51 1 - ^333).

The compositians ranged from t h e pure Ce02 t o t h e 30 rnol % Ce02-70 mol%

Z r O mixture. The recorded 'Id" v a l u e s were observed t o d e c r e a s e with de- c r e a s i n g CeO 2 c o n c e n t r a t i o n . However, they became c o n s t a n t f o r Ce02 con- t e n t lower t h a n 60 rnol % value a t 1 450°C.

The c o n s t a n t v a l u e s of t h e ( h k l ) s p a c i n g s a t 1450" C i n d i c a t e d t h a t a two-phase region was encountered, a s expected on t h e b a s i s of t h e phase e q u i l i b r i u m diagram. The o t h e r e q u i l i b r i u m phase t u r n e d o u t t o be t h e s o l u t i o n , based on t h e t e t r a g o n a l z i r c o n i a (TZ") , c o n t a i n i n g about 1 8

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2 0 mole% c e r i a . The (224) and (422-242) peaks appeared a t 1.065 and a t 1.055 A , r e s p e c t i v e l y , with minor d i f f e r e n c e s a t t h e o t h e r temperatures.

f i r e d a t 1600, 1550, 1500 and 1450 OC and cooled under a "not -

quenching" r a t e . CeO i s r e p o r t e d a s mole f r a c t i o n s ( . X I .

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What was unexpected was t h e s p l i t t i n g of t h e c u b i c f l u o r i t e d i f f r a c t i o n peaks i n t o two o r t h r e e o t h e r l i n e s , a s r e p o r t e d i n Fig. 1 . The new li- n e s became c o n s t a n t f o r CeO c o n t e n t lower than 55 mol % a t 1500°C, 45 mol% a t 1550°C, 40 mol% a t 2 1 6 0 0 0 ~ . A n u n c e r t a i n t y of + 1 mol% i s es- t i m a t e d .

A t c o n c e n t r a t i o n s lower t h a n 60 mol% c e r i a , t h e ( h k l ) l i n e s of samples f i r e d above 1 45 0 O C , became more and more splittedshe s p l i t t i n g i n c r e a s e d with i n c r e a s i n g f i r i n q temperature.

The r e p o r t e d "d" spacings a r e somewhat l a r g e r t h a n t h e above mentioned TZO v a l u e s , i n agreement with t h e h i g h e r c e r i a c o n c e n t r a t i o n . They i n d i - c a t e t h a t t h e phase w i t h t h e f l u o r i t e t y p e s t r u c t u r e has undergone a de- formation, outcoming i n t o a new, m e t a s t a b l e phase which h a s been c o n s i - dered of t e t r a c ~ o n a l symmetry and named T Z ' .

Since a "not-quenching" c o o l i n g r a t e of about I 0 C / s e c was a p p l i e d , t h e c u b i c f l u o r i t e s t r u c t u r e could n o t be f r o z e n i n t h e sample a t room tem- p e r a t u r e . On t h e o t h e r hand, t h i s l a t t e r could n o t develop a t r u e phase s e p a r a t i o n , a b l e t o d e l i v e r b o t h t h e t e t r a g o n a l ( T Z O ) and t h e c e r i a - r i c h phase.

JOURNAL DE _PHYSIQUE

I t i s worth mentioning t h a t a s h o r t t r e a t m e n t below 1450°C of t h e T Z ' samples, r e s t o r e d t h e expected t e t r a g o n a l TZO and t h e CCF c u b i c phases.

I n o r d e r t o r e v e a l t h i s new phase, SEM imaqes were o b t a i n e d on samples c o n t a i n i n g 60 and 40 mol % c e r i a , which e x h i b i t e d t h e f l u o r i t e and t h e TZ' phase s t r u c t u r e r e s p e c t i v e l y . Their m i c r o s t r u c t u r e s a r e r e p o r t e d i n Fig. 2 and 3 . The f i n e grooves a r e I u e t o t h e thermal e t c h i n g and a r e made e v i d e n t by t h e t i l t i n g of tire samples under t h e microscope.

Fig. 2 = 60 Ce02 - 40 Zr02 mol % Fig. 3 = 40 CeO

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The white b a r e q u a l s 10 ym

Samples f i r e d a t 1600°C - cooled under " n o t - quenching rate'; p o l i s h e d and t h e r m a l l y e t c h e d - Cubic phase i n Fig. 2 and T Z ' phase i n Fig. 3 . The s u r f a c e of t h e " f l u o r i t e " s t r u c t u r e sample i s homogeneous, a s it i s r e p o r t e d i n Fig. 2 , whereas a q u i t e d i f f e r e n t s u r f a c e t e x t u r e was r e v e - a l e d by t h e T Z ' sample, which i s p r e s e n t e d a l s o i n Fig. 4 under l a r g e r m a o n i f i c a t i o n .

The T Z ' sample e x h i b i t s a m i c r o s t r u c t u r e made of "twinned" c o l o n i e s /8/

t h a t develop most probably according o r a d i f f u ~ s i o n l e s s t y p e of t r a n s - formation. This phenomenon i s " c o o l i n g - r a t e " dependent and r e l a t e d t o t h e c u b i c - t e t r a g o n a l t r a n s f o r m a t i o n which, on t h e o t h e r hand, i s depen-- dent on t h e o v e r a l l composition of t h e sample,

I t could be a m a t t e r of i n v e s t i g a t i o n t o t e s t f o r i t s f u r t h e r t r a n s f o r - m a b i l i t y a c c o r d i n g t o s t r e s s - a s s i s t e d mechanisms t o t h e monoclinic sym- metry. Taking i n t o account a somewhat s i m i l a r phenorfienon, i n v e s t i g a t e d w i t h i n t h e z i r c o n i a - y t t r i a system by Lantery e t a 1 / 8 / , f u r t h e r i n v e s t i - g a t i o n w i l l be undertaken t o d e f i n e t h e p h y s i c a l and mechanical proper- t i e s of t h i s new, m e t a s t a b l e , T Z ' phase.

A few words of comment about t h e unusual m i c r o s t r u c t u r e of t h e g r a i n boundary p r e s e n t e d i n Fig. 2 which r e c a l l s a " z i p " i n t e r f a c e " .

s i o n induced g r a i n boundary m i g r a t i o n o r d i s c o n t i n u o u s p r e c i p i t a t i o n of two p h a s e s s t a r t i n g w i t h i n t h e i n t e r f a c e of a two component p o l y c r y s t a l - l i n e , s i n g l e p h a s e , s o l i d .

I n c e r a m i c systems n e i t h e r phenomenon h a s b e e n , s o f a r , u n e q u i v o c a l l y observed / 9 / .

I n t h i s c a s e t h e s t r i k i n g resemblance of t h e " z i p " i n t e r f a c e of F i g . 2 w i t h t h e e x p e c t e d shape of a g r a i n boundary m i g r a t i o n due t o d i s c o n t i n - uous p r e c i p i t a t i o n , c a n n o t b e overlooked. The t h e r m a l h i s t o r y of t h e s e samples d o e s n o t f i t i n t h e sequence proposed by o t h e r a u t h o r s t o l e t g r a i n boundary phenomena o c c u r ; however, it l o o k s l i k e i n t h i s case a r a t h e r t h i c k i n t e r f a c e m a t e r i a l becane t h e l o c u s of f u r t h e r growth of t h e main phase through n u c l e a t i o n snd syrnr.letrica1 expansion from ad j a - c e n t g r a i n s u r f a c e s . F u r t h e r i n v e s t i q a t i o n i s i n p r o g r e s s t o c l a r i f y it.

F i g . 4 - SEX 2 e t a i l of t h e TZ' phase s u r z a c e I V - REFERENCES

/ I / Duwez, P. and O d e l l , F . , J. Am. Ceram. Soc. 33 ( 1 9 5 0 ) 274.

/ 2 / Longo, V. and R o i t t i , S . , Ceramurgia I n t . 1 ( 1 9 7 1 ) 4.

/ 3 / Yoshimura, M a n d Bowen, E.K., Am. Cera1.t. Soc. B u l l . 56 ( 1 9 7 7 ) 301.

/ 4 / Lange, F.F., Rockwell I n t . Tech. R e p . O N R - N 0 0 0 1 4 - 7 7 - C - 0 4 4 1 - ~ 1 9 8 1 . / 5 / I v e r s T i f f e e , T. ,Kfistner, and Grahning, F. Proc. E l e c t r o c e r a m .

I n t e r n a t i o n a l Conference; B r u x e l l e s , Xay 1984.

/ 6 / Tani,F..,Yoshimura,M. and Somiya,S., J.Am.Ceram.Soc. 66 ( 1 9 8 3 ) 506.

/ 7 / E l Sayed, A.M., Sgjrensen, 0. T. and Meriani, S . , S c i e n c e of Ceramics 1 2 ( 1 9 8 3 ) 355. P . V i n c e n z i n i Ed., Ceramurgica P u b l . , Faenza, I t a l y / 8 / L a n t e r i , V . , Heuer, A.H. and F l i t c h e l l , T.E., Advances i n Ceramics

Vol. 12 pag. 1 1 8 . M. Claussen e t a l . Ed., Am. Ceram. Soc. Pub1.'84 / 9 / Handwerker, C . A . , Coble, R.L. and B l a n d e l l , J . E . , Advances i n Ce- r a m i c s , Vol. 6 pag. 213. M.F. Yan and A.B. Heuer Ed., Am. Ceramic SOC. Pub1.1983.

V -- ACKNOVJLEDGmNT

T h i s work was s u p p o r t e d by t h e I t a l i a n C o n s i g l i o Nazionale d e l l e Ricerche whose f i n a n c i a l a s s i s t a n c e i s g r a t e f u l l y aknowledged.