SAXS AND TEM STUDIES OF PHASE SEPARATION IN BaO-SiO2 GLASSES

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SAXS AND TEM STUDIES OF PHASE SEPARATION IN BaO-SiO2 GLASSES

E. Zanotto, A. Craievich, P. James

To cite this version:

E. Zanotto, A. Craievich, P. James. SAXS AND TEM STUDIES OF PHASE SEPARA- TION IN BaO-SiO2 GLASSES. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-107-C9-110.

�10.1051/jphyscol:1982920�. �jpa-00222449�

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JOURNAL DE PHYSIQUE

Colloque C9, supplément au n°12, Tome 4Z, décembre 1982 page C9-107

SAXS AND TEM STUDIES OF PHASE SEPARATION IN BaO-Si0

2

GLASSES

E.D. Zanotto, A.F. Craievich and P.F. James

Department of Ceramics, Glasses & Polymers, Sheffield University, U.K.

*Instituto de Fisioa e Quimiaa de Sao Carlos, Brazil

Résumé - La séparation de phases amorphes des verres de BaO-SiC>2 a été étudiée par diffusion de rayons X aux petits angles et microscopie électronique de transmission, dans la région de germination et croissance de la lacune de miscibilité.

Les diamètres des particules sphéroidales précipitées, déterminés avec la loi de Guinier et par microscopie électronique, sont en accord. La cinétique de croissance des particules est bien décrite par la théorie classique de "coarsening".

Les intégrales des courbes de diffusion de rayons X ont permis de déterminer la frontière de la lacune de miscibilité et le temps nécessaire pour que la matrice atteigne sa composition d'équilibre. La germination se produit pendant le re- froidissement du liquide ou dans les tous premiers stades du traitement therm- ique. La processus de "coarsening" commence avant que la matrice atteigne la composition d'équilibre.

Abstract - Amorphous phase separation was studied in the nucleation and growth region of the Ba0-Si(>2 miscibility gap. Nearly spherical droplets were precipi- tated, the diameters obtained from SAXS and TEM being in reasonable agreement.

The growth kinetics of the droplets were well described by the classical theory of coarsening. From the integrated SAXS intensities the times to reach the equilibrium matrix compositions at 743 and 760°C were determined, and the binodal boundary located. Nucleation occurred during quenching from the melt or within a very short time of heat treatment. Coarsening began well before the attainment of the equilibrium matrix composition.

1- Introduction.- The kinetics of amorphous phase separation (APS) in glasses can be studied by small-angle X-ray scattering (SAXS) or by transmission electron micros- copy (TEM). Apart from the intrinsic scientific interest, the determination of the APS kinetics in glasses which crystallize internally may lead to a better under- standing of the influence of APS on the nucleation and growth of crystals in glass ceramics. We report here a study of the kinetics of APS in BaO-SiC>2 glasses with 27.0 and 28.3 m o U BaO (glasses '27.0' and '28.3B').

2. Experimental.- Well mixed batches of silquartz Si02 and Fisons BaC03 (AR grade) were sintered at 1300°C for 24 hours and melted in electric furnaces at 1550°C. The crushing/remelting technique was used for homogenization. The melts were pressed between two steel plates yielding clear glasses about 1 mm thick. Chemical analysis indicated 0.3 wtX SrO as the main impurity, and less than 0.1 wt% total of other impurities. Samples of about 20 x 6 x 1 mm were heated at 743°C or 760°C (Tg ~ 700°C) for various times, air quenched, annealed at 650°C (or below) and ground and pol- ished to 0.03-0.05 mm for the SAXS experiments. Ion beam thinned foils and carbon replicas were studied in a Hitachi HU 11A electron microscope. For the SAXS a Rigaku small angle goniometer was used with slit collimation. A graphite monochromator in the scattered beam isolated the CuK radiation. A scintillation detector and a step scanning device were used to determine the intensities. The cross-section of the beam had negligible width and was taken as 'infinitely high' [1], This condition was satisfied for most specimens. The experimental SAXS curves were corrected for parasitic scattering and normalized to a constant sample thickness and to a constant beam intensity.

°Present Address : LURE, Université Paris-Sud, Orsay, France

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982920

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C9-108 JOURNAL DE PHYSIQUE

3. R e s u l t s . - P a r t o f t h e BaO-Si02 p h a s e a i a g r a m i n c l u d i n g t h e m i s c i b i l i t y g a p i s shown i n F i g . 1 . F o r t h e t e m p e r a t u r e s u s e d ( 7 4 3 , 7 6 0 ° C ) t h e two g l a s s e s s t u d i e d h e r e were s i t u a t e d i n t h e ' n u c l e a t i o n and g r o w t h ' r e g i o n o f t h e gap.

No p h a s e s e p a r a t i o n was d e t e c t e d by TEM i n t h e a s - p r e p a r e d g l a s s 28.3B b u t a f t e r h e a t t r e a t m e n t a t 760°C f o r 8 . 2 5 h n e a r l y s p h e r i c a l d r o p l e t s were c l e a r l y o b s e r v e d ( F i g . 2 ) . G l a s s e s 27.0 and 28.3B, h e a t t r e a t e d a t 743OC f o r t i m e s up t o 1 7 h , g a v e s i m i l a r m i c r o g r a p h s , o n l y t h e number and s i z e o f d r o p l e t s v a r y i n g . C r y s t a l l i z a t i o n o f t h e m a t r i x p h a s e was o b s e r v e d f o r s a m p l e s h e a t e d f o r l o n g e r t i m e s , t h e amorphous p a r t i c l e s r e m a i n i n g u n d i s t u r b e d .

A mean d i a m e t e r D o f t h e d r o p l e t s was d e t e r m i n e d u s i n g t h e G u i n i e r e q u a t i o n [ 2 ] J ( q ) = ~ ( O ) e x p ( - ~ ~ q ~ 1 2 0 )

w h e r e J ( q ) i s t h e SAXS i n t e n s i t y f o r a ' d i l u t e ' s y s t e m o f s p h e r i c a l p a r t i c l e s o f c o n s t a n t e l e c t r o n i c d e n s i t y i n a n homogeneous m a t r i x , q i s t h e modulus o f t h e s c a t - t e r i n g v e c t o r and J ( O ) i s t h e SAXS i n t e n s i t y f o r q = 0. At s m a l l a n g l e s q " 2 n d X w h e r e E i s t h e s c a t t e r i n

3

a n g l e and t h e X-ray w a v e l e n g t h (0.154 nm). T y p i c a l G u i n i e r p l o t s ( l o g J v s E

,

F i g . 3 ) w e r e l i n e a r o v e r a w i d e r a n g e , i n d i c a t i n g n a r r o w s i z e d i s t r i b u t i o n s . The mean d i a m e t e r s o b t a i n e d f r o m t h e s l o p e s o f t h e s e p l o t s a r e shown i n F i g . 4 f o r g l a s s 27.0 h e a t e d a t 743OC and g l a s s 28.3B a t 743 and 760°C. The mean d i a m e t e r s from t h i n f o i l TEM a r e a l s o shown f o r g l a s s 28.3B a t 760°C. The d i a m e t e r s from SAXS a n d TEM a r e i n r e a s o n a b l e a g r e e m e n t . The s m a l l d i s c r e p a n c y i n v a l u e s f r o m SAXS and TEM a t e a r l y t i m e s o f p h a s e s e p a r a t i o n may be e x p l a i n e d by ( a ) e r r o r s i n t h e measurement o f s m a l l d r o p l e t s u s i n g TEM a n d / o r ( b ) e r r o r s i n t h e v a l u e s f r o m t h e G u i n i e r p l o t s d u e , f o r e x a m p l e , t o t h e h i g h p a r t i c l e c o n c e n t r a t i o n 121. The good a g r e e m e n t a t l o n g e r t i m e s s u g g e s t s t h a t t h e a s s u m p t i o n s o f G u i n i e r ' s l a w a r e b e t t e r s a t i s f i e d i n t h e l a t e r s t a g e s o f s e p a r a t i o n .

The d i a m e t e r s m e a s u r e d i n r e p l i c a m i c r o g r a p h s w e r e a b o u t 2.5 t i m e s l a r g e r t h a n t h o s e o b t a i n e d f r o m SAXS o r t h i n f o i l m i c r o g r a p h s , i n d i c a t i n g t h a t t h e r e p l i c a v a l - u e s w e r e s u b j e c t t o l a r g e e r r o r s . T h i s p o i n t i s f u l l y d i s c u s s e d e l s e w h e r e [ 3 ] .

F o r g l a s s 28.3B h e a t e d a t 760°C t h e s l o p e o f t h e s t r a i g h t l i n e p l o t o f l o g D v s . I o g ( t i m e ) i n F i g . 4 i s 0.32 k 0 . 0 2 . A t 743"C, t h e s l o p e i s 0.34 a . 0 3 w h e r e a s f o r s h o r t p e r i o d s o f h e a t t r e a t m e n t t h e s l o p e i s a b o u t 0.5. F o r d i f f u s i o n c o n t r o l l e d g r o w t h , t h e s l o p e s h o u l d be 0.50 i n t h e i n i t i a l s t a g e o f APS ( 4 1 a n d 0.33 i n t h e f i n a l s t a g e 151 (when t h e e q u i l i b r i u m c o m p o s i t i o n o f t h e m a t r i x i s r e a c h e d ) . T h e r e - f o r e , t h e r e s u l t s f o r g l a s s 28.3B a g r e e w e l l w i t h t h e t h e o r i e s [ 4 , 5 ] . F o r g l a s s 27.0 a t 743°C t h e s l o p e i s 0 . 4 3 a . 0 3 . I n t h i s c a s e t h e a g r e e m e n t w i t h t h e t h e o r i e s i s n o t a s good. I t s h o u l d b e s t r e s s e d , h o w e v e r , t h a t t h e s e t h e o r i e s w e r e d e r i v e d f o r a s m a l l volume f r a c t i o n o f t h e s e c o n d p h a s e , a n d t h i s c o n d i t i o n i s more c l o s e l y o b e y e d i n g l a s s 28.3B.

F o r ' i n f i n i t e l y ' h i g h beam c r o s s - s e c t i o n t h e i n t e g r a t e d SAXS i n t e n s i t y i n r e c i - p r o c a l s p a c e i s g i v e n by

Q = c o n s t . J ( q ) q d q F o r a ' t w o e l e c t r o n i c d e n s i t i e s ' s y s t e m

I

Q a

-

) ( ~ p ) ~

w h e r e i s t h e volume f r a c t i o n o f s c a t t e r i n g p a r t i c l e s and A p t h e e l e c t r o n d e n s i t y d i f f e r e n c e b e t w e e n p a r t i c l e s and m a t r i x [ 2 ] . Thus when t h e m a t r i x r e a c h e s i t s e q u i - l i b r i u m c o m p o s i t i o n i n t h e f i n a l s t a g e o f p h a s e s e p a r a t i o n Q i s c o n s t a n t . H e r e Q was d e t e r m i n e d b y a method d e s c r i b e d i n [6]. F o r g l a s s 28.3B a c o n s t a n t v a l u e o f Q i s a p p r o a c h e d ( F i g . 5 ) a f t e r 3-4h a t 760°C and a f t e r 7h a t 743OC. F o r g l a s s 2 7 . 0 a t 743OC, Q i s c o n s t a n t f r o m t

<

2.5h o f h e a t t r e a t m e n t . The i n c r e a s e i n Q f o r g l a s s 28.3B, a f t e r 30.2h a t 743OC, i s p r o b a b l y a s s o c i a t e d w i t h t h e o n s e t o f c r y s t a l - l i z a t i o n i n t h i s s p e c i m e n . A s i m i l a r e f f e c t was o b s e r v e d i n Li20-SiOg g l a s s e s 171.

From t h e SAXS c u r v e s , ( e v e n when i n r e l a t i v e u n i t s ) i t is p o s s i b l e t o o b t a i n t h e r e l a t i v e number o f p a r t i c l e s p e r u n i t volume o f g l a s s , N , t h e i n t e r f a c i a l a r e a p e r u n i t volume S v , and t h e p o s i t i o n o f t h e b i n o d a l b o u n d a r y . F o r a beam o f l i n e a r and ' i n f i n i t e ' c r o s s - s e c t i o n t h e SAXS i n t e n s i t y e x t r a p o l a t e d t o q = 0 i n t h e G u i n i e r p l o t , J ( O ) , i s g i v e n a p p r o x i m a t e l y by [ 8 , 9 ]

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F i g . 1. BaO-Si02 system. C a l c u l a t e d b i n c d a l ( s o l i d curve) and s p i n c d a l (da- shed curve) a r e shown [ 131. Experiment- a l p o i n t s 0 [14]

, P

[15] , - ( t h i s work)

.

Heat t r e a t m e n t s used here ( 0 ) .

I I ' " " ' 1

E ' i d e g r e e s 1 2

F i g . 3 . G u i n i e r p l o t s f o r g l a s s 28.3B h e a t t r e a t e d a t 7 6 0 ' ~ . J i n a r b i t r a r y u n i t s (a.u. )

.

I I I I 1

5 15 2 5

t I h o u r s 1

F i g . 5 . I n t e g r a t e d SAXS i n t e n s i t y f o r g l a s s e s h e a t t r e a t e d a t 7 4 3 O ~ and

7 6 0 ' ~ . Q i n a r b i t r a r y u n i t s ( a . u . )

.

Fig.2. TEM micrographs of g l a s s 28.3B;

a s prepared ( t o p ) , and heated 8.25h a t 7 6 0 ' ~ (bottom)

.

The b a r s d e n o t e 0.2pm.

Fig.4. LogloD v s . Loglo(tirne) p l o t s f o r g l a s s 27 - 0 a t 743 OC and g l a s s 28.3B a t 7 4 3 ' ~ and 7 6 0 ' ~ . V e r t i c a l b a r s i n d i c a t e v a l u e s from TEM f o r g l a s s 28.3 a t 7 6 0 ' ~ .

2 7 0 7 ' 3 ' ~ 0 28 1. 7 4 3 ' ~

/ 28 3 s 760°c

Fig.6. I n v e r s e of t h e number of amorphous d r o p l e t s v e r s u s time of h e a t t r e a t m e n t i n BaO-Si02 g l a s s e s .

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-C9-110 JOURNAL DE PHYSIQUE

~ ( 0 ) = c o n s t . ( A ~ ) ~ N D ~ .

T h e r e f o r e , i f A p i s assumed c o n s t a n t t h e v a r i a t i o n o f N w i t h t i m e o f i s o t h e r m a l t r e a t m e n t may be f o l l o w e d . F i g . 6 shows t h e i n v e r s e of t h e r e l a t i v e number o f drop- l e t s ( f r o m sAxS) v e r s u s time o f h e a t t r e a t m e n t . S t r a i g h t l i n e s f i t t h e e x p e r i m e n t a l d a t a r e a s o n a b l y w e l l f o r g l a s s 27.0 h e a t e d a t 743OC and g l a s s 28.3B a t 760°C. For g l a s s 28.3B a t 743OC, t h e r e i s a change i n t h e s l o p e a f t e r 8h when N b e g i n s t o de- c r e a s e more r a p i d l y . Of p a r t i c u l a r i n t e r e s t i s t h e o b s e r v a t i o n t h a t t h e number o f d r o p l e t s d e c r e a s e s from t h e f i r s t h e a t t r e a t m e n t time g i v e n , s u g g e s t i n g t h a t most o f t h e n u c l e a t i o n of amorphous p a r t i c l e s o c c u r s e i t h e r d u r i n g c o o l i n g from t h e m e l t o r w i t h i n a v e r y s h o r t t i m e o f h e a t t r e a t m e n t . The h i g h e s t r a t e o f d i s s o l u t i o n o f p a r - t i c l e s , however, o c c u r s when t h e m a t r i x c o m p o s i t i o n r e a c h e s e q u i l i b r i u m . T h i s i s c l e a r from F i g s . 5 and 6 f o r g l a s s 28.3B h e a t e d a t 743OC. Also i t s h o u l d be n o t e d t h a t tlhe e a r l y p e r i o d of s l o w e r d e c r e a s e i n N f o r t h i s g l a s s a t 743°C c o i n c i d e s w i t h t h e t z v a r i a t i o n o f d r o p l e t d i a m e t e r ( ~ i g . 4 ) . I t s h o u l d be s t r e s s e d t h a t t h e N - ~ v a l u e s f o r d i f f e r e n t g l a s s e s , o r t h e same g l a s s h e a t e d a t d i f f e r e n t t e m p e r a t u r e s , a r e n o t d i r e c t l y comparable b e c a u s e (Ap)2 i s n o t t h e same i n e a c h c a s e . A l s o , t h e N-I v a l u e s would be s m a l l e r f o r g l a s s e s i n which t h e p h a s e s e p a r a t i o n was i n t h e e a r l y s t a g e s , a s i n t h i s c a s e (AP)2 would be s m a l l e r . T h i s r e i n f o r c e s t h e c o n c l u - s i o n t h a t N b e g i n s t o d e c r e a s e w e l l b e f o r e t h e a t t a i n m e n t o f t h e e q u i l i b r i u m m a t r i x c o m p o s i t i o n . These r e s u l t s c o n t r a d i c t a c l a s s i c a l s t u d y [ 3 1 which showed t h a t c o a r s e n i n g s t a r t e d when t h e m a t r i x phase r e a c h e d e q u i l i b r i u m . However, i n agreement w i t h t h e p r e s e n t r e s u l t s , Kirkwood (101 found t h a t c o a r s e n i n g o f p r e c i p i t a t e s i n m e t a l l i c a l l o y s began w e l l b e f o r e t h e a t t a i n m e n t of t h e e q u i l i b r i u m m a t r i x c o m p o s i t i o n .

According t o P o r o d ' s law [ I l l , t h e a s y m p t o t i c v a l u e o f J ( ~ a t l a r g e a n g l e s ) ~ ~ (q + m) f o r 3 'two e l e c t r o n i c d e n s i t i e s ' s y s t e m and a l i n e a r and ' i n f i n i t e ' X-ray beam c r o s s - s e c t i o n , i s p r o p o r t i o n a l t o t h e s u r f a c e a r e a o f t h e i n t e r f a c e between t h e p a r t i c l e s and t h e m a t r i x . The a b s o l u t e v a l u e s o f t h e s u r f a c e a r e a p e r u n i t volume, Sv, d e t e r m i n e d from

I { J ( ~ ) ~ ~ > ~ * / Q I = s,/8~@~(1-@1)

were 1.2 t o 2.0 t i m e s s m a l l e r t h a n t h o s e o b t a i n e d by t h e method o f Debye e t a 1 [ 1 2 ] . However, t h e v a l u e s from b o t h methods d e c r e a s e d w i t h time o f h e a t t r e a t m e n t f o r b o t h g l a s s e s , a s e x p e c t e d f o r t h e c o a r s e n i n g s t a g e .

From t h e r a t i o o f t h e c o n s t a n t v a l u e s o f Q f o r t h e two g l a s s e s t h e e q u i l i b r i u m c o m p o s i t i o n o f t h e m a t r i x phase was d e t e r m i n e d a t 743°C a s d e s c r i b e d i n [ 7 , 9 1 . Simi- l a r l y , by comparing t h e v a l u e s o f Q a t 743'C and 760°C f o r g l a s s 28.3B, t h e m a t r i x c o m p o s i t i o n was d e t e r m i n e d a t 760°C. The c o m p o s i t i o n s ( i n mole% BaO) were 32.0-33.0 a t 743OC and 31.5-32.0 a t 760°C ( F i g . 1 ) . These v a l u e s a r e 1-2 mole% BaO o u t s i d e t h e b i n o d a l c a l c u l a t e d by H a l l e r e t a l . [ 1 3 ] which was based on d a t a [ I 4 1 a c c u r a t e t o w i t h i n _ + I mole% BaO and k50-10O0C. D i f f e r e n c e s i n t h e l e v e l s o f SrO, t h e main i m p u r i t y , c o u l d a l s o e x p l a i n t h e d i s c r e p a n c y .

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