MICROWAVES SINTERING OF BaTiO3 BASED CERAMICS

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MICROWAVES SINTERING OF BaTiO3 BASED CERAMICS

G. Desgardin, L. Mazo, L. Quemeneur, B. Raveau

To cite this version:

G. Desgardin, L. Mazo, L. Quemeneur, B. Raveau. MICROWAVES SINTERING OF Ba- TiO3 BASED CERAMICS. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-397-C1-402.

�10.1051/jphyscol:1986159�. �jpa-00225590�

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MICROWAVES SINTERING OF BaTiO, BASED CERAMICS

G. DESGARDIN, L. MAZO, L. QUEMENEUR and B. RAVEAU

Laboratoire de Cristallographie, Chimie et Physique des

Solides, U.A. 251, ISMRa, Universite de Caen, F-14032 Caen Cedex, France

Rdsumd - Le f r i t t a g e micro-ondes du t i t a n a t e de baryum en presence de LiF ou BaLiF3 e s t p o s s i b l e , contrairement au c a s de BaTiOj pur. L ' i n f l u e n c e de d i f - f g r e n t s paramstres a d t d d t u d i s e : puissance i n c i d e n t e , p u i s s a n c e absorbde, durde du f r i t t a g e , s t o e c h i o m e t r i e , t e n e u r s en LiF e t BaLiF3. Les p r o p r i S t 6 s d i g l e c t r i q u e s s o n t c o r r g l d e s h l a s t r u c t u r e , 2 l a morphologie e t 2 l a t e n e u r en l i t h i u m de La cgramique. Contrairement au c a s de BaLiP3, l e f r i t t a g e de BaTiO en prdsence de LiF met en jeu une physicochimie d i f f d r e n t e de c e l l e obse&e en f r i t t a g e c l a s s i q u e .

A b s t r a c t - The microwaves s i n t e r i n g of barium t i t a n a t e w i t h LiF and BaLiF3 i s p o s s i b l e c o n t r a r y t o pure BaTi03. The i n f l u e n c e o f t h e d i f f e r e n t parameters on t h e s i n t e r i n g was s t u d i e d : i n c i d e n t and absorbed power, time, stoichiome- t r y , LiF and BaLiF3 c o n t e n t s . The d i e l e c t r i c p r o p e r t i e s a r e c o r r e l a t e d t o t h e s t r u c t u r e , t h e morphology and t h e l i t h i u m c o n t e n t of t h e ceramic. It i s con- cluded t h a t , t h e microwaves s i n t e r i n g o f " ~ a T i 0 3 " with LiF i s c h a r a c t e r i z e d by a chemistry d i f f e r e n t from t h e one observed f o r conventional s i n t e r i n g contra- ry t o t h e case of BaLiF 3'

I - INTRODUCTION

The p l a c e o f barium t i t a n a t e a s a d i e l e c t r i c powder f o r type I1 c a p a c i t o r s i s w e l l known. The p r e v i o u s r e s u l t s o b t a i n e d i n Caen and Lannion / 1 , 2 / have shown t h e possi- b i l i t y of s i n t e r i n g BaTi03 a t low temperature by a d d i t i o n of LiF o r BaLi?). From t h e s e s t u d i e s it a l s o appears t h a t t h e m i c r o s t r u c t u r e and of course t h e d ~ e l e c t r i c p r o p e r t i e s of t h e ceramic a r e c l o s e l y r e l a t e d t o t h e thermal t r e a t m e n t . For t h i s reason, t h e use of microwaves could be of i n t e r e s t s i n c e it could change t h e chemical p r o c e s s involved d u r i n g s i n t e r i n g . So t h e p r e s e n t work d e a l s w i t h t h e microwavessin- t e r i n g o f ceramics i n t h e systems BaTi03-LiF and BaTi03-BaLiF3.

I1 - EXPERIMENTAL Powder s y n t h e s i s

Two s o r t s of products were used, l a b e l l e d A and B r e s p e c t i v e l y . A i s a commercial p r e f i r e d productx, c l o s e t o t h e s t o i c h i o m e t r y "BaTi03". I n c r e a s i n g amounts of BaLiF3 from 1 t o 4 mole p e r c e n t , were added and mixed by wet-milling i n alcohol.

B product corresponds t o a non-stoichiometric "BaTiO? (Ti/Ba = 0.975) : B ~ c o ~ * * and Ti02X* were mixed i n a l c o h o l during 48 h. i n a shaker-mill, d r i e d i n an i n f r a - r e d

% AN2 CHORI XX PROLABO Normapur product.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986159

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r a d i a t o r and then c a l c i n e d a t 1150°C f o r 30 mn. LiF was t h e n added, t h e homogeneiza- t i o n of t h e mixture beeing i d e n t i c a l t o t h a t used f o r A.

For a l l t h e samples, an o r g a n i c b i n d e r was then added, t h e powder was t h e n p r e s s e d i n t h e form of a c y l i n d e r ( 1 = 30 mm, 0 = 7 mm) u s i n g an i s o s t a t i c p r e s s .

S i n t e r i n g c o n d i t i o n s

The c y l i n d r i c a l sample was p l a c e d a t t h e c e n t r e o f a Grooved r e s o n a n t a p p l i c a t o r p r e v i o u s l y d e s c r i b e d 131. A f t e r a r a p i d i n c r e a s e of t h e temperature ( t o r: 30 s e c . ) , t h e d i s t r i b u t i o n o f t h e temperature i n t h e m a t e r i a l was maintened a t a c o n s t a n t va- l u e by modifying t h e coupling of t h e c a v i t y . I n a l l t h e d i f f e r e n t r u n s , t h e tempe- r a t u r e corresponding t o t h i s p l a t e a u was about 1 0 0 0 ° ~ .

C h a r a c t e r i z a t i o n

The f o l l o w i n g a n a l y z e s were made : (1) scanning e l e c t r o n microscope o b s e r v a t i o n s , (2) X-ray d i f f r a c t i o n a n a l y s i s , (3) l i t h i u m a n a l y s i s (A.A.), ( 4 ) d i e l e c t r i c measurements.

I11 - RESULTS

11 A ¹¹m a t e r i a l s : "BaTiOg" + y BaLiF3

-

The p r i n c i p a l r e s u l t s concerning t h e d e n s i f i c a t i o n can be summarized a s f o l l o w s :

- For P i > 200 W, t h e s i n t e r i n g of t h e samples took p l a c e . But one o f t e n observed cracks and a l s o f u s i o n of t h e m a t e r i a l .

- An e x c e l l e n t d e n s i f i c a t i o n w i t h o u t any crack was o b t a i n e d f o r P i = 200 W and Pa -.

50 W and f o r t h e composition with 2.5 X BaLiF3. It i s worth n o t i n g t h a t t h i s com- p o s i t i o n corresponds t o t h e optimum composition o b t a i n e d by normal h e a t i n g .

- The formation of c r a c k s i s very s e n s i t i v e t o t h e BaLiFj c o n t e n t and t o t h e Pa va- l u e even f o r P i c l o s e t o 200 W.

Thus, we p r e s e n t h e r e t h e d i e l e c t r i c p r o p e r t i e s and a n a l y s i s o n l y f o r c e r a m i c s s i n t e - r e d i n t h e optimal c o n d i t i o n s ( i . e . P i = 200 W, Pa = 50 W).

The a n a l y s i s of t h e curves ^E= f (T) and tg6 = f (T) ( F i g . 1) f o r d i f f e r e n t compositions shows t h a t E i n c r e a s e s w i t h t h e BaLiF3 c o n t e n t , whereas t h e d i e l e c t r i c peak i s t r a n s - l a t e d towards t h e low temperature region. The c o n s i d e r a t i o n of t h e c u r v e s ^E= f(T) f o r d i f f e r e n t h e a t i n g times ranging from 1 t o 60 mn (Fig. 2) shows t h a t they e x h i b i t a s i m i l a r shape ; one n o t e s a g a i n t h e i n f l u e n c e of the.BaLiF3 content : E i n c r e a s e s p r o g r e s s i v e l y with t h e time f o r t h e 2 moles % BaLiF3 samples whereas i t i n c r e a s e s up about 2800, and t h e n remains c o n s t a n t f o r t h e 2.5 mole % BaLiF sample. 3

Fig. 1 - D i e l e c t r i c c h a r a c t e r i s t i c s ^E= f(T) and tg6 = f ( T ) v e r s u s BaLiFj c o n t e n t f o r X) mn dwelling time.

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The e v o l u t i o n of t h e t e g r a g o n a l d i s t o r t i o n of t h e p e r o v s k i t e c e l l v e r s u s t h e l i t h i u m c o n t e n t (Table 1) i n t h e s i n t e r e d m a t e r i a l , and t h e s i n t e r i n g time shows t h a t t h e p e r o v s k i t e c e l l appears a s cubic by X-ray d i f f r a c t i o n a n a l y s i s a s soon a s t h e BaLiF?

c o n t e n t i s g r e a t e r t h a n 2.5 mole %. A comparison w i t h t h e d i e l e c t r i c measurements shows t h a t t h e t r u e symmetry i s n o t c u b i c s i n c e t h e r e s u l t i n g p e r o v s k i t e i s i n f a c t f e r r o e l e c t r i c . The "cubic" s y m e t r y i s n o t d i r e c t l y c o r r e l a t e d t o t h e g l o b a l lithium c o n t e n t i n t h e c e r a m i c , c o n t r a r y t o t h e r e s u l t s p r e v i o u s l y observed by conventional h e a t i n g / I / . Nevertheless i t seems t h a t a minimum l i t h i u m c o n t e n t of abo6t 500 ppm i s n e c e s s a r y t o t h e s t a b i l i t y of t h e c u b i c p e r o v s k i t e i n an agreement w i t h t h e pre- v i o u s r e s u l t s /4/.

Table 1

Evolution of t h e s t r u c t u r e and l i t h i u m content f o r d i f f e r e n t nominal compositions BaTi03 + y X BaLiF v e r s u s microwavesheating time 3

The SEM o b s e r v a t i o n s (Fig. 3) show t h a t t h e s i z e of t h e g r a i n s i n c r e a s e s w i t h t h e h e a t i n g time, and i n t h e same time from the c e n t r e t o t h e s u r f a c e of t h e c y l i n d r i c a l b a r . This l a t t e r d i s t r i b u t i o n of t h e g r a i n s i s i n agreement w i t h t h e thermal gra- d i e n t due t o t h e microwavesheating which s t a r t s a t t h e c e n t r e of t h e b a r . A t t h e c e n t e r of t h e b a r , t h e g r a i n s a r e g e n e r a l l y w e l l - f a c e t t e d ( f o r t = 30 mn), whereas t h e i n t e r g r a n u l a r p o r o s i t y i s g r e a t e r c l o s e t o t h e s u r f a c e . A weak i n t r a g r a n u l a r p o r o s i t y i s a l s o observed.

I t B m a t e r i a l s 11 : "BaTiO," + y LiF

<

The i n f l u e n c e of t h e P i and Pa powers and LiF c o n t e n t summarized i n Table 2 shows

0 1 5 15 30 60 75

t e t r a g o n a l Pseudo-cubic

530 530 500 480 420 380 420

...

t e t r a g o n a l Pseudo-cubic Cubic

740 700 700 600 620 520 460

...

t e t r a g o n a l Pseudo-cubic Cubic

855 830 800 715 700 720 680

% BaLiF3

%

2.5 '

t h a t w e l l s i n t e r e d ceramics without any c r a c k s could only be o b t a i n e d f o r i n i t i a l LiF c o n t e n t s of 7.5 and 10 mole X. For each c o n t e n t one observes a minimum Pa v a l u e n e c e s s a r y t o i n i t i a t e t h e s i n t e r i n g , and a maximum Pa v a l u e beyond which c r a c k s a r e observed. The d i f f e r e n c e ?a(max)-Pa(min) depends on t h e LiF c o n t e n t .

The d i e l e c t r i c curves s = f(T) f o r 7.5 and 10 mole % LiF (Fig. 4 ) a r e very d i f f e r e n t from t h o s e o b t a i n e d w i t h a conventional h e a t i n g / 4 / . They a r e c h a r a c t e r i z e d by a maximum c l o s e t o 8o0c,and one beyona OOC. For t h e s e two compositions v a l u e s a r e r a t h e r

Dwelling time S t r u c t u r e (RX) L i content (PPM) S t r u c t u r e (RX) Li c o n t e n t (PPM) S t r u c t u r e (RX) Li c o n t e n t (PPM)

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s i m i l a r and vary i n t h e same way a s a f u n c t i o n of t h e h e a t i n g time. It must be poin- t e d o u t t h a t t h e Curie temperature does n o t vary. The d i e l e c t r i c l o s s e s a r e r a t h e r good a t room temperature b u t i n c r e a s e r a p i d l y beyond 20°C, f o r long s i n t e r i n g times ( t = 2 0 mn).

Fig. 3 - M i c r o s t r u c t u r e s f o r A (BaTi03 + 2.5 % BaLiF3) and B (BaTi03 + 7.5 Z LiF).

Dwelling times : A l e = 10 mn ; B1 = 15 mn ; e2, ^A3⁼^{30 mn} ^;^{B2, 3 3}⁼^{60 mn} ^;^{A4, B4}⁼

1 h. c l a s s i c a l h e a t l n g . A1, A2, B 1 , B2 : m l c r o s t r u c t u r e a t t h e c e n t r e of t h e bar.

A3, B3 : m i c r o s t r u c t u r e a t t h e circumference of t h e bar.

Table 2

S i n t e r i n g of B compositions v e r s u s i n c i d e n t and absorbed microwave powers.

% LiF Pi(W) PR(W) Pa R e s u l t s Z LiF Pi(W) PR(M) Pa R e s u l t s

< 100 not s i n t e r e d < 100 not s i n t e r e d

O ' ⁼¹⁰⁰ ^{~ 6 0}< 4 0 n o t s i n t e r e d 10 X = I 0 0 >75 <25 n o t s i n t e r e d

= l o o <60 >40 p a r t i a l l y (mole) = l o 0 =75 =25 c o r r e c t l y s i n t e r e d

molten = 100 <70 >30 cracks

...

< 100 n o t s i n t e r e d < 100 n o t s i n t e r e d 7.5 % = l o 0 >70 <30 not s i n t e r e d l5 % = l o 0 >80 <20 n o t s i n t e r e d (mole) = l o 0 =70 =30 c o r r e c t l y = l o 0 <80 > 2 0 c r a c k s

s i n t e r e d

= I 0 0 <65 >35 p a r t i a l l y molten

The symmetry of the s t r u c t u r e remains t e t r a g o n a l whatever t h e s i n t e r i n g time may be.

However, t h e t e t r a g o n a l d i s t o r t i o n d e c r e a s e s r a p i d l y d u r i n g t h e f i r s t f i v e minutes of t h e h e a t i n g (Fig. 5 a ) , and then slows down. The l i t h i u m c o n t e n t (Fig. 5b) decrea- s e s r e g u l a r y d u r i n g s i n t e r i n g .

The SEM i n v e s t i g a t i o n (Fig. 3b) shows a behaviour s i m i l a r t o t h a t observed f o r t h e A m a t e r i a l s . However no i n t r a g r a n u l a r p o r o s i t y can be seen. For a same h e a t i n g time t h e g r a i n s a r e b i g g e r t h a n t h o s e of t h e A type.

I V - DISCUSSION

From t h e s e r e s u l t s i t i s c l e a r t h a t i t i s p o s s i b l e t o s i n t e r BaTi03 i n t h e presence of l i t h i u m s a l t s by u s i n g microwaves c o n t r a r y t o pure BaTiOg. The s i n t e r i n g i s q u i t e r e p r o d u c i b l e , b u t a l s o very s e n s i b l e t o t h e experimental c o n d i t i o n s . The important

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E I conventional

Fig. 4 - D i e l e c t r i c c h a r a c t e r i s t i c s v e r s u s temperature f o r d i f f e r e n t dwelling times i n t h e presence of 7.5 % LiF.

Fig. 5 - E v o l u t i o n s : a ) of t h e d i s t o r t i o n of t h e t e t r a g o n a l c e l l ; b) of t h e l i t h i u m c o n t e n t d u r i n g dwelling time, i n t h e presence of 7.5 X and 10 9; LiF.

A d i f f e r e n t behaviour w i t h r e s p e c t t o microwaves i s observed f o r "A" and "B" m a t e r i a l s :

"A" m a t e r i a l s e x h i b i t c h a r a c t e r i s t i c s v e r y s i m i l a r t o t h o s e observed f o r i d e n t i c a l samples s i n t e r e d i n a conventional way ( 1 0 0 0 ' ~ - 1 o r 2 h.). This s i m i l a r i t y can be e x p l a i n e d by t h e e a s y formation of t h e s o l i d s o l u t i o n BaTil-xLix03-3xF3xdue t o t h e presence of t h e p r e c u r s o r s BaTi03 and BaLiF3.

On t h e o p p o s i t e , t h e d i f f e r e n t r e s u l t s o b t a i n e d f o r t h e B m a t e r i a l s f o r conventional and microwave h e a t i n g can be explained by a d i f f e r e n t chemistry. The d i f f i c u l t y t o o b t a i n a d e c r e a s e of t h e t e t r a g o n a l d i s t o r t i o n i n t h e microwavesprocess ( c o n t r a r y t o t h e conventional h e a t i n g where t h e c u b i c c e l l i s o b t a i n e d ) can indeed be i n t e r p r e t e d i n t h e f o l l o w i n g way : d u r i n g t h e microwave h e a t i n g t h e temperature i n c r e a s e s very r a p i d l y i n such a way t h a t most of t h e f l u o r i n e can b e trapped i n t h e p e r o v s k i t e ma- t r i x : t h e small v a r i a t i o n of t h e Curie temperature observed by t h i s method i s i n agreement with a low f l u o r u r e content i n t h e f i n a l ceramic.

The c o n s i d e r a t i o n o f t h e l i t h i u m c o n t e n t s and e s p e c i a l l y t h e i r e v o l u t i o n v e r s u s time confirms t h e d i f f e r e n c e s between t h e two modes of h e a t i n g . I n t h e conventional hea- t i n g t h e formation of t h e "cubic" phase t a k e s p l a c e a t about 850°C, and a f t e r a pla- t e a u a t 1 0 0 0 ' ~ d u r i n g a time g r e a t e r than 1 h. t h e "cubic" form r e t u r n s t o t e t r a g o - n a l , owing t o t h e simultaneous l o s s of f l u o r u r e and l i t h i u m , tending t o form again pure BaTi03 / I / . I n t h e microwaves h e a t i n g one a l s o observes a t e t r a g o n a l o r pseudo- c u b i c symmetry f o r s i m i l a r l i t h i u m c o n t e n t s , b u t i n t h i s c a s e t h e d i s t o r t i o n decrea-

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s e s w i t h t h e l i t h i u m c o n t e n t . Thus i t appears t h a t t h e s e s i m i l a r c r y . s t a l l o g r a p h i c a 1 c h a r a c t e r i s t i c s f o r a same composition of t h e ceramic correspond t o a d i f f e r e n t me- chanism and a t l e a s t t o a d i f f e r e n t s t a g e i n t h e two s i n t e r i n g processus.

V - REFERENCES

/ I / Desgardin, G., Mey, I., Raveau, B. and Haussonne, J.M., Am. Ceram. Soc. B u l l . , 64 (1985) 564.

/ 2 / g s g a r d i n , G., B a j o l e t , Ph., Raveau, B. and Haussonne, J.M., S c i . Ceramics 12, St-Vincent, I t a l i a , 1983, p. 435-440.

131 Thiebaut, J.M., Berteaud, A.J. and Roussy, G . , J. Yicrowave Powder, 14 (1979) 217.

/ 4 / Desgardin, G., Xey, I., Raveau, B. and Haussonne, J.M., S i l i c a t e s I n d u s t r i e l s , 7-8 (1984), 159.

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