POINT DEFECTS AND HIGH TEMPERATURE CREEP OF CoO SINGLE CRYSTALS

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POINT DEFECTS AND HIGH TEMPERATURE CREEP OF CoO SINGLE CRYSTALS

A. Dominguez-Rodriguez, M. Sanchez, R. Marquez, J. Castaing, C. Monty

To cite this version:

A. Dominguez-Rodriguez, M. Sanchez, R. Marquez, J. Castaing, C. Monty. POINT DEFECTS AND

HIGH TEMPERATURE CREEP OF CoO SINGLE CRYSTALS. Journal de Physique Colloques,

1981, 42 (C3), pp.C3-67-C3-72. �10.1051/jphyscol:1981307�. �jpa-00220698�

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JOURNAL DE PHYSIQUE

Colloque C3, supplément au n°6, Tome 42, juin 1381 page C3-67

POINT DEFECTS AND HIGH TEMPERATURE CREEP OF CoO SINGLE CRYSTALS

A. Dominguez-Rodriguez, M. Sanchez, R. Marquez, J . Castaing and

**(*)**

C. Montyv ;

Departamento de Optica, Faaultad de Cienoias, Sevilla, Spain

Résumé.- On a mesuré l'exposant m de la dépendance en pression partielle d'oxygène de la vitesse de fluage en régime stationnaire sur des monocris- taux de CoO. m est positif et varie de 0,5 à 0,1 lorsque la température varie de 1100°C à 1400°C. A température constante m a tendance à augmenter avec la Po^, lorsque celle-ci passe de 1 Pa à 0,21 x ÎO^ Pa.

Ces résultats montrent que le défaut responsable de la diffusion de l'oxygène, qui gouverne la vitesse de fluage, est essentiellement un inter- stitiel d'oxygène.

Ce défaut est à l'état neutre à 1100°C, probablement associé à d'autres défauts, notamment des interstitiels d'oxygène chargés, aux températures plus élevées.

A b s t r a c t . - The oxygen p a r t i a l pressure dependence of the steady s t a t e creep rate was measured in CoO single c r y s t a l s . The exponent m in the r e l a t i o n

£ a Pn-)m l s p o s i t i v e and decreases from 0.5 to 0.1 as the temperature i n c r e a - ses from 1100°C to 1400°C. At constant temperature, m increases s l i g h t l y when PCU increases from I Pa to 0.21 x 10 Pa.

These r e s u l t s i n d i c a t e t h a t the defect responsible for oxygen diffusion, which is assumed to control creep r a t e , i s mainly an oxygen i n t e r s t i t i a l . This defect i s neutral a t 1100°C and probably associated with other defects l i k e charged i n t e r s t i t i a l s , a t higher temperatures.

1 . I n t r o d u c t i o n . - At h i g h t e m p e r a t u r e s , s t e a d y s t a t e c r e e p i s c o n - t r o l l e d by s e l f - d i f f u s i o n . I n b i n a r y c o m p o u n d s , i t i s a s s u m e d t h a t t h e c r e e p r a t e i s p r o p o r t i o n a l t o t h e d i f f u s i o n c o e f f i c i e n t of t h e s l o w e s t moving s p e c i e s a l t h o u g h t h e r e i s a l a c k o f e x p e r i m e n t a l s u p p o r t / 1 , 2 / . Non s t o i c h i o m e t r i c o x i d e s p r o v i d e a good o p p o r t u n i t y t o v e r i f y t h i s a s s u m p t i o n . The c o m p a r i s o n o f a c t i v a t i o n e n e r g i e s f o r c r e e p a n d d i f f u - s i o n a t f i x e d t e m p e r a t u r e , T, a n d v a r i o u s o x y g e n p r e s s u r e s , P Q? ( i . e . v a r i o u s d e p a r t u r e s from s t o i c h i o m e t r y ) a l l o w u s t o v e r i f y t h a t t h e same p o i n t d e f e c t s c o n t r o l t h e two p r o c e s s e s . Such a p r o c e d u r e i s more r e l i a b l e t h a n t h e u s u a l c o m p a r i s o n o f a c t i v a t i o n e n e r g i e s o n l y .

We h a v e u n d e r t a k e n s u c h a work on c o b a l t o u s o x i d e , Co, 0 . I t h a s b e e n shown t h a t o x y g e n d i f f u s e s , a b o u t 10 t i m e s s l o w e r t h a n c o - b a l t / 3 / . An a c t i v a t i o n e n e r g y o f 4 . 1 3 eV h a s b e e n f o u n d f o r o x y g e n d i f f u s i n g v i a s i n g l y c h a r g e d v a c a n c i e s V ' / 4 / . C r e e p a c t i v a t i o n e n e r g y v a l u e s from t h e l i t e r a t u r e a r e r a t h e r s c a t t e r e d / I / . I n c r e a s i n g v a l u e s h a v e b e e n f o u n d f o r i n c r e a s i n g o x y g e n p r e s s u r e s 3 . 8 t o 4 . 4 eV / 5 / , 1.3 t o 2 . 9 eV / 6 / ; a n d 2 . 2 eV i n a i r /!/.

——-

Permanent address: Laboratoire de Physique des Materiaux - C.N.R.S-Bellevue, 1,Place A. Briand, 92190 Meudon, France

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981307

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c3-68 JOURNAL DE PHYSIQUE

Creep r a t e v a r i a t i o n s w i t h PO2 have been measured. Exponents m i n t h e c r e e p law w r i t t e n a s /1/

h a s been found by compression o f s i n g l e c r y s t a l s ( m = 0.45) /5/ and p o l y c r y s t a l s ( m = 0.25) /8/ f o r t e m p e r a t u r e s i n t h e r a n g e 900°C

(0.57 TM)

-

1200°C ( 0 . 7 1 TM) and Po2 from atm. (10 P a ) t o 1 atm.

( l o 5

P a ) . Creep b e n d i n g o f p o l y c r y s t a l s a l s o g i v e s m = 0.25 between 925OC and 1040°C /1/.

The aim o f t h e p r e s e n t work i s t o e x t e n d c r e e p t e s t s on s i n g l e c r y s t a l s i n t h e s t e a d y s t a t e , t h e n t o d e t e r m i n e t h e n , m and 3 p a r a - m e t e r s o f e q u a t i o n (1) and deduce t h e p o i n t d e f e c t s i n v o l v e d i n t h e r a t e c o n t r o l l i n g mechanism o f c r e e p .

2. E x p e r i m e n t a l t e c h n i q u e s . - The c r y s t a l s used i n t h i s s t u d y were s i - m i l a r t o t h o s e used i n /9/. They were c u t i n t o compression specimens o f 3 x 3 x 5 mm3 dimensions w i t h { l o o } f a c e s . They were t h e n a n n e a l e d a t 1200°C f o r two days o r more, i n o r d e r t o a c h i e v e a s t a n d a r d r e f e - r e n c e s t a t e f o r d i s l o c a t i o n s t r u c t u r e and n o n - s t o i c h i o m e t r y .

Compression t e s t s were performed i n a c r e e p machine d e s c r i b e d i n

/ l o / ,

a t t e m p e r a t u r e s betweenllOO°C (0.66 TM) and 1400°C ( 0 . 8 1 T M ) , s t r e s s e s between 5 MPa (p/20,000) and 25 MPa (p/4,000) and Po2 b e t - ween 0.21 atm. ( 0 . 2 1 x 10' Pa) and atm. ( 1 P a ) . B e f o r e c r e e p t e s - t i n g and a f t e r any change of thermodynamic p a r a m e t e r (T o r p o 2 ) , a l o n g enough a n n e a l was performed i n o r d e r t o r e a c h thermodynamic e q u i - l i b r i u m . The a n n e a l i n g t i m e i s d e t e r m i n e d by c o b a l t vacancy d i f f u s i v i - t y . Based on t h e chemical d i f f u s i o n c o e f f i c i e n t

5

/11/, we found t h a t a n n e a l i n g f o r 5 h a t llOO°C and 1 h a t 1400°C a r e more t h a n s u f f i c i e n t . Annealing t i m e s o f 2 h a t 1100°C and 1/2 h a t 1400°C were a l s o t r i e d w i t h no i n f l u e n c e on m and Q v a l u e s .

3. R e s u l t s . - T h i r t e e n COO s i n g l e c r y s t a l s have been s t r a i n e d up t o

E % 0.5 w i t h o p a r a l l e l t o

< l o o >

and t h e l o n g dimension of t h e s p e c i - men. The s t r e s s exponent v a l u e s ( e q u a t i o n ( 1 ) ) n

"

8 . 5 a t 1200°C and n = 6.3 a t 1 4 0 0 " ~ a r e s i m i l a r t o t h o s e f o r N i O s i n g l e c r y s t a l s /12/ and t h e Q v a l u e s measured f ~

Po2

r atm. and a l l t e s t i n g t e m p e r a t u r e s a r e 5.2 eV.

The Po2 e x p o n e n t m h a s b e e n e x t r a c t e d d u r i n g s t e a d y s t a t e c r e e p f o r s t r a i n l o n g e r t h a n 0 . 1 ; 42 oxygen p r e s s u r e changes have been performed.

Average m v a l u e s a r e shown i n t a b l e I. They a r e always p o s i t i v e and tend t o i n c r e a s e from h i g h t e m p e r a t u r e

-

low Po2 c o n d i t i o n s t o low tempera- t u r e

-

h i g h Po2 o n e s .

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Table I.- Values of the m exponent f o r d i f f e r e n t Po2 changes a t v a r i o u s temperatures.

Each v a l u e correspond t o an average of 2 t o 5 measurements.

4. D i s c u s s i o n . - T h e s t e a d y s t a t e creep p r o p e r t i e s of n o n - s t o i c h i o m e t - r i c o x i d e s depends on t h e d i f f u s i v i t y of t h e s l o w e s t species, w h i c h i s c o n t r o l l e d b y m i n o r i t y p o i n t defects / 1 , 2 / . H o w e v e r , it has b e e n s h o w n t h a t f o r c h a r g e d d e f e c t s t h e c o n c e n t r a t i o n of t h e s e m i n o r i t y d e f e c t s d e p e n d s o n t h e b e h a v i o u r of m a j o r i t y p o i n t d e f e c t s / 1 3 / r e s p o n s i b l e f o r t h e d e p a r t u r e f r o m s t o i c h i o m e t r y . I n t h e case of p u r e C O ~ - ~ O , it i s e x p l a i n e d b y t h e p r e s e n c e of a l a r g e a m o u n t of c o b a l t v a c a n c i e s /14,15/:

* 1 0 - ~ a t m . l ~ - ~ a t m . 1 0 - ~ a t m . 0 . 2 1 a t m .

14OO0C

13OO0C

1 2 o o 0 c

l l O O ° C

1 1 I

1 I

+ m = I 0 . 1 6 ^I ³

I I I

-

m = 0 . 1 2 I

1

-

^I

I m- = 0 . 1 2

>

1 I

I m 0 . 1 9 4

1 I _I

I I

I I 1

<

:

^m⁼^{0 . 1 3}

⁷

^I_I

C ¹_I m = 0 . 2 2 ^I₁

>

I + m l = 0 . 1 ~ :

1

I i im = 0 . 2 4

f

>

I 1

I I

I I I

1 1 I

1 1

<

⁸ ^m⁼ ^{0 . 2 8} ¹

>

I ₁

I 1 / m

1

0 . 4 7 4

i 1

I <-m: = 0 . 3 0 >

I I

1 1

1 I 1

< ^I₁ m

= ,

^{0 . 4 6}

+

¹

I

<

^m ⁼ O . 3 6 $

I

1

< m

=:

^{0 . 3 6} ^I

^>

I 1

I ^I

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c3-70 JOURNAL DE PHYSIQUE

w i t h x = [VCO] ⁺ [ ~ k o ] ⁺[.a'Co]

The r a t i o

h a s been d e t e r m i n e d f o r v a r i o u s

Po2 -

^Tu s i n g b i b l i o g r a p h i c d a t a V14, 15/). The r e s u l t s a r e shown on f i g u r e 1 where some disc;epancy

between /14/ and /15/ i s a p p a r e n t . I n most o f o u r t e s t s , t h e d e f e c t V k o i s dominant, e x c e p t a t h i g h T and low Po2 a c c o r d i n g t o /15/.

I f we assume t h a t s t e a d y s t a t e c r e e p i s c o n t r o l l e d by t h e d i f f u - s i o n i n t h e i n t r i n s i c r a n g e o f an oxygen s u b l a t t i c e d e f e c t X I i t c a n b e shown by a n e x t e n s i o n o f t h e formalism o f e q u a t i o n s (1) t o ( 4 ) t h a t / I 3 1 16/

Nhere 1 i s t h e d e g r e e o f i o n i z a t i o n o f m i n o r i t y d e f e c t (1 s 2) Z i s t h e r a t i o

[vcJ/[vb0]

s i g n

+

i s f o r i n t e r s t i t i a l s

and s i g n

-

f o r v a c a n c i e s i n t h e oxygen s u b l a t t i c e .

Creep e x p e r i m e n t s g i v e m v a l u e s t h a t are p o s i t i v e . T h i s i n d i c a t e s t h a t t h e c o r r e s p o n d i n g p o i n t d e f e c t s i n t h e oxygen s u b l a t t i c e a r e i n t e r s t i - t i a l s , a r e s u l t which i s d i f f e r e n t from t h a t f o r BiO /12/.

The change i n t h e m v a l u e s c a n n o t b e e x p l a i n e d by a change i n t h e Z v a l u e b e c a u s e when Z i n c r e a s e s , m s h o u l d i n c r e a s e a c c o r d i n g t o

( 5 ) , i n o p p o s i t i o n t o t h e o b s e r v e d t r e n d . The o p p o s i t e v a r i a t i o n c a n b e o b t a i n e d assuming a change i n t h e i o n i z a t i o n o f i n t e r s t i t i a l oxy- g e n s : f o r Z % 0 , m = 0 f o r 0" and i n c r e a s e s t o m = 0 , 5 f o r 0 . .

i

A more d e t a i l e d a n a l y s i s t a k i n g i n t o a c c o u n t t h e p r e s e n c e o f Vbo,

vco ,

^0:

^,

^{0; and}^{0 ;} ^{i s}u s e l e s s i n view o f t h e l a r g e number of p a r a - m e t e r i n v o l v e d . Moreover, t h e r a t h e r l a r g e d e p a r t u r e from s t o i c h i o m e t r y s u g g e s t ( i ) t h e o c c u r r e n c e n o t o n l y o f i s o l a t e d p o i n t d e f e c t s , b u t a l s o of complex o f p o i n t d e f e c t s

-

Hed /17/ and F r y t /18/ s u g g e s t e d t h a t a t a b o u t 1300°C, 30 %

-

40 % o f t h e d e f e c t s a r e i n c l u d e d i n complexes -

( i i ) t h e m o b i l i t i e s o f t h e d e f e c t s c o u l d depend on Po2.

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I t d o e s n o t seem r e a s o n a b l e t o t r y t o e x p l a i n i n more d e t a i l s s t e a d y s t a t e c r e e p i n view o f such a c o m p l i c a t e d s i t u a t i o n , t o o many unknown p a r a m e t e r s b e i n g invo,lved i n t h e chemical model o f p o i n t de- f e c t s .

Fig. 1.- Stability range for Col-,O in log Po2 (atrn.) versus 1 0 4 1 ~ (K) coordinates showing x values / 9 /

and the ratio CV"col CV"co1 + CV'co1 according to / 1 4 / (solid lines) and /15/ (broken lines). The hatched area shows the range where creep tests were performed.

5 . Conclusion.- High t e m p e r a t u r e s t e a d y s t a t e c r e e p o f COO a p p e a r s t o be c o n t r o l l e d by t h e oxygen d i f f u s i o n t h r o u g h t h e climb of d i s l o c a - t i o n s . T h e P o 2 dependence o f c r e e p r a t e a l l o w s t o deduce t h a t oxygen d i f f u s e s t h r o u g h an i n t e r s t i t i a l mechanism i n t h e Po2, T r a n g e i n v e s - t i g a t e d . T h i s r e s u l t i s remarkable f o r t h r e e r e a s o n s : ( i ) it d i f f e r s from t h a t found f o r ^{N i O ,} a v e r y s i m i l a r o x i d e / 1 2 / ; ( i i ) COO h a s a c l o s e packed oxygen s u b l a t t i c e and it i s d i f f i c u l t t o a c c e p t t h e i d e a t h a t a b i g i o n l i k e oxygen can d i f f u s e more e a s i l y by an i n t e r s t i t i a l mechanism t h a n by a vacancy one i n t h e c r y s t a l ; (iii) t h i s c o n c l u s i o n d i s a g r e e s w i t h t h a t o f Chen e t a l . / 4 / who s u g g e s t e d t h a t oxygen d i f f u - s e s t h r o u g h Vd,

,

b a s e d on d i f f u s i o n measurenlents i n doped specimens.

Recent measurements by J . R o u t b o r t /18/ a r e i n g e n e r a l agreement w i t h o u r s b u t i n d i c a t e a change i n t h e s i g n o f m a t low

Po2,

which c o u l d e x p l a i n t h e whole b e h a v i o u r .

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JOURNAL DE PHYSIQUE

R e f e r e n c e s

/1/ B r e t h e a u , T . , C a s t a i n g , J. R a b i e r , J . , V e y s k i e r e , P . , A d v . P h y s .

28

( 1 9 7 9 ) 8 3 5

/2/ i . l i t c h e l 1 , T . E . , H o b b s , L.W., H e u e r , A . H . , C a s t a i n g , J . , C a d o z ,

J . ,

P h i l i b e r t , J . , A c t a . M e t a l l .

22

( 1 9 7 9 ) 1 6 7 7 /3/ C h e n , W.R. and P e t e r s o n , N . L . , J. P h y s . Chem. S o l i d s

4 1 ( 1 9 8 0 ) 6 4 7

-

/ 4 / C h e n , W . J . , J a c k s o n , R.A., J. P h y s . C h e m . S o l i d s 3 0 ( 1 9 6 9 ) 1 3 0 9

-

/ 5 / C l a u e r , A.K., S e l t z e r , M.S., Wilcox, B . A . , J . M a t . S c i .

6

( 1 9 7 1 ) 1 3 7 9

/ 6 / K r i s h n a m a c h a r i , V . , J o n e s , J . J . , J. Am. C e r a m . S o c . 5 7 ( 1 9 7 4 ) 5 0 6

-

/ 7 / N e h r i n g , V.W., S m y t h , J . R . , Mc G e e , T . D . , J . A m . C e r a m . SOC.

60

( 1 9 7 7 ) 3 2 8

/8/ C l a u e r , A.H., S e l t z e r , M.S., Wilcox, B.A., C e r . i n Sev.

E n v i r . , M a t . S c i . R e s .

5

( 1 9 7 1 )

/ 9 / C a s t a i n g , J . , S p e n d e l , M . , P h i l i b e r t , J . , D o m i n g u e z , A . , M a r q u e z , R . , R e v u e P h y s . A p p l .

15

( 1 9 8 0 ) 2 7 7

/ l o /

G e r v a i s , H , , P e l l i s s i e r , B . , C a s t a i n g , J . , R e v u e I n t . H a u t e s T e m p . R e f r a c t .

15

( 1 9 7 8 )

/11/ F r e e r , R., J . M a t e r . S c i .

15

( 1 9 8 0 ) 8 0 3

/ 1 2 / ~ a b r e r a - ~ a E o , J . , C a s t a i n g , J . , J . P h y s i q u e L e t t . 4 1 ( 1 9 8 0 ) L-119

-

/ 1 3 / M o n t y , C . , D g f a u t s p o n c t u e l s dans l e s s o l i d e s ( L e s E d i - t i o n s de P h y s i q u e , F r a n c e ) C o n f o Y a n t 1 9 7 7 , ch X I 1 / 1 4 / D i e c k m a n n , R . , Z . P h y s . C h e m . N e u e P o l g e , B d .

107

( 1 9 7 7 ) 1 6 9

/ 1 5 / C y r , J . P . , T h P s e d ' & t a t

-

I n s t i t u t N a t i o n a l P o l y t e c h n i q u e de L o r r a i n e ( 1 9 7 6 )

/ 1 6 / D u b o i s , C . , T h 6 s e D o c t e u r - I n g B n i e u r , U n i v e r s . P a r i s X I ( 1 9 7 9 )

/ 1 7 / H e d , A . Z . , J . Chem. P h y s .

50

( 1 9 6 9 ) 2 9 3 5 /18/ F r y t , E . , O x i d . M e t .

10

( 1 9 7 6 ) 311

/ 1 9 / R o u t b o r t , J., T o be p u b l i s h e d