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SOLID SOLUTION FORMATION DURING LIQUID PHASE SINTERING
E. Kostić, S. Kiss, D. Cerović
To cite this version:
E. Kostić, S. Kiss, D. Cerović. SOLID SOLUTION FORMATION DURING LIQUID PHASE SINTERING. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-441-C1-445.
�10.1051/jphyscol:1986166�. �jpa-00225597�
JOURNAL DE PHYSIQUE
Colloque C1, supplgment au n02, Tome 47, f6vrier 1986 page cl-441
SOLID SOLUTION FORMATION DURING LIQUID PHASE SINTERING
E. K O S T I ~ , S.J. KISS* and D. C E R O V I ~
" B o r i s ~ i d r i c " I n s t i t u t e of N u c l e a r S c i e n c e s , P.O.B. 5 2 2 , Lab.
1 7 0 , 11001 B e l g r a d e , Y u g o s l a v i a
Resum6 - Les r 6 s u l t a t s obtenus s u r l e s f r i t t e s de MgAl 0 o n t montr6 que l a formation de s o l u t i o n s s o l i d e s e n t r e c e r t a i n s c o n s t i t u g n t s de l a phase l i q u i d e e t du s p i n e l l e , f a v o r i s a i t l a d e n s i f i c a t i o n . La f o r m a t i o n de s o l u t i o n s s o l i - des n'a pas une i n f l u e n c e importante s u r l e grossissement des grains.
A b s t r a c t - Results obtained d u r i n g l i q u i d phase s i n t e r i n g o f s p i n e l powder showed t h a t s o l i d s o l u t i o n f o r m a t i o n between t h e s p i n e l and some o f l i q u i d components enhanced d e n s i f i c a t i o n . However t h e s o l i d s o l u t i o n f o r m a t i o n d i d n o t i n f l u e n c e g r a i n growth process d u r i n g s i n t e r i n g o f MgAl 204 powder.
'and College of Ceramics, 34300 Arandjelovac, Yugoslavia I - INTRODUCTION
Using knowledge gained by t h e i n v e s t i g a t i o n o f A1 0 powders c o n s o l i d a t i o n /1,2/ we chose several a d d i t i o n s , each o f them c o n t a i n i n g $a8 and Si02 (and a l s o some d i v a l e n t o r t r i v a l e n t c a t i o n s ) , t o enhance the d e n s i f i c a t i o n o f i n a c t i v e s p i n e l powder by t h e l i q u i d phase s i n t e r i n g process. Analysis o f m i c r o s t r u c t u r e s showed t h a t r e a c t i o n s de- veloping d u r i n g s i n t e r i n g d i d n o t e x h i b i t considerable i n f l u e n c e on t h e g r a i n growth process d u r i n g s p i n e l s i n t e r i n g .
I1 - EXPERIMENTAL
The experiments were performed u s i n g a s t o i c h i o m e t r i c MgA120 powder synthesized c t 1 1 0 0 ~ ~ . By X-ray a n a l y s i s o f t h e s t a r t i n g powder o n l y t h e ~ g h ~ 0 phase was detected.
Grinded s p i n e l powder contained agglomerates up t o 30 pm. The i n i i v i d u a l c r y s t a l l i n e s i z e was about 0.8 pm. The ? p e c i f i c surface area o f the s p i n e l powder as determined by the BET method was 2.0 m /g. Compositions o f the used a d d i t i v e s from CaO-MgO-Si02, Ca0-Zn0-Si02 and Ca0-Cr203-Si02 systems are given i n Table 1.
Tab1 e 1 . Compositions and me1 t i n g temperatures o f used a d d i t i v e s a d d i t i v e
D2 D
4
composition (wt.%)
CaO MgO ZnO Cr203 Si O2
20 23.5 - - 56.5
30.5 8.0 - - 61.5
19.6 - 32.4 - 48.0
52.0 - - 7 .O 41 .O
m e l t i n g temperature
(OC) 1390 1320 1170 1430
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986166
~ 1 - 4 4 2 JOURNAL DE PHYSIQUE
The compositions o f these a d d i t i v e s were chosen w i t h t h e i n t e n t i o n t o a v o i d the appearance o f new c r y s t a l 1 i ne phases as r e a c t i o n products. The a d d i t i v e s were prepa- red by mi 11 i n g the corresponding pure oxides i n ethanol, i n a p l a n e t a r y b a l l m i 11.
Each o f t h e mentioned a d d i t i v e s i n an mount from 1 t o 5 wt.% was blended w i t h s p i n e l pow- der i n the p l a n e t a r y b a l l m i l l . The obtained m i x t u r e s were pressed a t 19.6 MPa. Sin- t e r i n g was done a t 15500C i n a i r f o r 2 hours.
I11 - RESULTS
Density and open p o r o s i t y measurements (Table 2), as we1 1 as X-ray, m i c r o s t r u c t u r a l a n a l y s i s and e l e c t r o n microprobe i n v e s t i g a t i o n were performed on t h e s i n t e r e d com- pacts.
Table 2. Density ( d ) and open p o r o s i t y (OP) o f s i n t e r e d s p i n e l samples a t 1 5 5 0 ~ ~
TD - t h e o r e t i c a l d e n s i t y
- a d d i t i v e amount (wt.%)
0 1 2 3 4 5
The X r a y d i f f r a c t i o n p a t t e r n o f the samples c o n t a i n i n g 5 wt.% a d d i t i v e s showed o n l y the existence o f MgA1204.
M i c r o s t r u c t u r a l a n a l y s i s o f the s i n t e r e d s p i n e l samples c o n t a i n i n g a minimum amount o f a d d i t i v e t o o b t a i n o n l y closed p o r o s i t y , i n d i c a t e d t h e i n f l u e n c e of d e n s i f i c a t i o n a i d on the type o f m i c r o s t r u c t u r e . More heterogeneous m i c r o s t r u c t u r e s were found i n m a t e r i a l s w i t h a d d i t i v e s D and D2. The average g r a i n s i z e s o f s p i n e l m a t e r i a l w i t h optimum d e n s i t y f o r each o! t h e used a d d i t i v e s , a r e given i n Table 3.
a d d i t i v e s
D2 D3 D4
Table 3. Average g r a i n s i z e o f s i n t e r e d s p i n e l w i t h 3 wt.% a d d i t i v e a t 1 5 5 0 ~ ~
d(%TD) OP(%)
72.4 25.4
75.4 20.6
78.3 16.8
90.3 0
88.5 0
87.3 0
E l e c t r o n microprobe data obtained f o r s p i n e l w i t h 5 wt.% D3 are given i n Fig.1 and s p i n e l w i t h 5 wt.% Dq i n Fig.2. O p t i c a l micrographs o f t h e r m a l l y etched s i n t e r e d s p i n e l samples are g i v e n i n Fig.3.
d(%TD) OP(%)
72.4 25.4
77.4 20.0
84.0 11.2
90.4 0
87.5 0
86.8 0
average g r a i n s i z e (urn) d e n s i t y (%TD)
I V - DISCUSSION
a d d i t i v e s
O2 O3 O4
3.0 2.9 2.5 4.4
90.3 90.4 92.6 92.7
-
Data presented above i n d i c a t e a very slow s i n t e r i n g r a t e o f the pure s p i n e l powder.
As a consequence, o n l y 73% TD has been reached a f t e r f i r i n g a t 1550'~. I n t h e p r e - sence o f t h e a d d i t i v e s , t h e d e n s i f i c a t i o n process was enhanced as can be seen i n Ta-
d(%TD) OP(%)
72.4 25.4
- -
86.8 6.4
92.6 0
91.9 0
91.4 0
d(%TD) OP(%)
72.4 25.4
- -
89.2 2.1
92.7 0
92.3 0
91.3 0
b l e 2. The g r e a t e s t improvement was obtained w i t h a d d i t i v e s D and D4. The d e n s i t i e s obtained w i t h a d d i t i v e D2 were lower than w i t h a d d i t i v e D3 bu?. the l e a s t e f f e c t i v e a d d i t i v e was Dl .
This i s a l s o made c l e a r from t h e study o f open p o r o s i t i e s . I n samples c o n t a i n i n g 2 wt.% a d d i t i v e the open p o r o s i t y c o n t i n u o u s l y decreases from Dl t o D4.
The closed p o r o s i t y i s h i g h e r w i t h t h e a d d i t i o n o f D and D than w i t h t h e a d d i t i o n o f D~ and D4. The lower closed POrOSIty i s a consequence of b e g t e r p a c k ~ n g o f t h e agglo- merates. The improvement i n t h e s p i n e l agglomerate packing due t o t h e presence o f t h e m e l t caused b y a d d i t i v e s i s more e f f i c i e n t f o r D3 and D4 a d d i t i v e s than f o r D and D2.
Thus, t h e e f f e c t i s n o t o n l y r e l a t e d t o t h e presence o f the m e l t b u t a l s o t o I t s com- p o s i t i o n , perhaps because o f a d i f f e r e n t w e t t i n g angle. Our conclusion t h a t a t each s i n t e r i n g temperature t h e optimum d e n s i t y f o r a g i v e n powder can be obtained o n l y i n t h e presence o f a f i x e d amount o f an a d d i t i v e was proved by data shown i n Table 2. A s i m i l a r e f f e c t has been r e p o r t e d e a r l i e r /3/. I n t h e present work t h e h i g h e s t d e n s i t i e s were reached w i t h 3 wt.% a d d i t i v e . I f t h e amount o f a d d i t i v e i s s m a l l e r than t h e o p t i - mum, t h e open p o r o s i t y i n t h e s i n t e r e d m a t e r i a l s decreases from Dl t o D Higher amounts o f a d d i t i v e s b r i n g about a decrease o f d e n s i t i e s , which i s l e s s w ~ t h a d d i t i v e s D3 and D4 than w i t h Dl and D2.
Analyzing t h e p r o p e r t i e s o f t h e a d d i t i v e s i t was obvious t h a t t h e i r m e l t i n g p o i n t s were n o t a c r u c i a l parameter. I n t h e presence o f D4 t h e l i q u i d phase was i n c o n t a c t w i t h t h e s p i n e l d u r i n g a s h o r t e r time than i n t h e m a t e r i a l w i t h D3 although t h e f i n a l d e n s i t i e s were i d e n t i c a l . The f a c t t h a t t h e needed amount o f a d d i t ~ v e f o r reaching a f i x e d den- s i t y (below t h e optimum one) was t h e l e a s t w i t h D4 seems t o i n d i c a t e t h e m e l t i n g p o i n t does n o t p l a y an important r o l e . Judging from the SiO content, t h e m e l t formed by t h e D4 a d d i t i v e should be l e s s viscous than o t h e r ones /4f.
Since e q u i l i b r i u m polycomponent phase diagrams f o r t h e used system are unknown, the amount and composition o f t h e l i q u i d present d u r i n g t h e s i n t e r i n g are n o t known e i t h e r . The most i m p o r t a n t f a c t i s t h a t t h e s t a r t i n g m e l t s obtained d u r i n g t h e s i n t e r i n g a r e n o t i n thermodynamical e q u i l i b r i u m w i t h the s p i n e l .
From the secondary e l e c t r o n image, Figs. 1 . I and 2.1., one can i d e n t i f y the regions where l i q u i d phases had occurred. The d i s t r i b u t i o n o f Ca and S i i n such an area i s displayed i n Figs. 1.2 and 2.2. Data given i n Figs. 1.3 and 2.3. i n d i c a t e t h e uniform d i s t r i b u t i o n o f ZnO and Cr,03 throughout the l i q u i d and s o l i d phases. Q u a n t i t a t i v e microprobe a n a l y s i s o f t h e ' s ~ n t e r e d samples c o n t a i n i n g a d d i t i v e D3 i n d i c a t e s t h a t b o t h the l i q u i d phase and t h e s p i n e l c o n t a i n about 1 wt.% ZnO. The same a n a l y s i s o f mate- r i a l w i t h a d d i t i v e D4 shows t h a t t h e content o f C r 0 i n t h e l i q u i d phase, as w e l l as i n the s p i n e l i s about 0.3 wt.%. I t i s obvious tha? 80th ZnO and Cr203 from t h e l i q u i d phase have entered t h e s o l i d m a t r i x . We assume t h a t both mentioned o x ~ d e s form a s o l i d s o l u t i o n w i t h s p i n e l . We c o u l d h o t c o n f i r m t h i s assumption by X-ray a n a l y s i s because of the low ZnO and Cr203 content i n t h e i n v e s t i g a t e d systems.
I n a d d i t i o n , these r e s u l t s i n d i c a t e t h a t h i g h e r d e n s i t i e s are obtained i n samples con- t a i n i n g a d d i t i v e s which do n o t i n c l u d e MgO. There i s a d i f f e r e n c e between t h e d e n s i t i e s of m a t e r i a l s c o n t a i n i n g Dl and D , which i n d i c a t e s t h e importance o f t h e MgO amount i n t h e a d d i t i v e . The i n f l u e n c e o f ?.he MgO content i n t h e a d d i t i v e s on t h e d e n s i f i c a t i o n process p o i n t s t o s p i n e l d i s s o l u t i o n i n the m e l t . Analyzing the e f f e c t o f a d d i t i v e com- p o s i t i o n on the f i n a l density, we came t o t h e conclusion t h a t t h e amount o f t h e a v a i l a b - l e l i q u i d f o r s i n t e r i n g depends upon t h e amount o f d i s s o l v e d MgA1204 i n t h e s t a r t i n g melt. Judging by our data (Tables 1 and 2), t h e a b i l i t y o f t h e s t a r t i n g m e l t t o d i s - s o l v e MgA1204 decreases w i t h i n c r e a s i n g MgO content. Less d i s s o l u t i o n o f MgAIZDq i n t h e presence o f D3 than D4 was expected, because o f the q u a n t i t y o f d i v a l e n t ions ne- cessary t o form s p i n e l compound contained i n them. Higher d e n s i t i e s obtained w i t h a l e s s e r amount o f a d d i t i v e p o i n t t o more l i q u i d phase present under t h e given c o n d i t i o n s . S o l i d s o l u t i o n formaticn between d i v a l e n t and t r i v a l e n t s p i n e l ions and corresponding ad- d i t i v e components i s an a d d i t i o n a l f a c t o r t h a t i n f l u e n c e s processes accompanying s i n - t e r i ng.
I n a d d i t i o n t o the amount o f l i q u i d , t h e e f f i c i e n c y o f the p a r t i c l e rearrangement pro- cesses i n the given system d u r i n g s i n t e r i n g depends on v i s c o s i t y and t h e w e t t i n g angle of the l i q u i d . When t h e s i n t e r i n g process was f o l l o w e d by s o l i d s o l u t i o n formation,
C I - 4 4 4 JOURNAL DE PHYSIQUE
1 2 3
Fig.1. E l e c t r o n microprobe a n a l y s i s o f t h e s p i n e l w i t h 5 wt.% D3 1.1. E l e c t r o n image w i t h t h e l i n e L, x600
1.2. D i s t r i b u t i o n o f Ca and S i a l o n g t h e l i n e L 1.3. D i s t r i b u t i o n o f A l , Mg and Zn a l o n g t h e l i n e L
Fig.2. E l e c t r o n microprobe a n a l y s i s o f t h e s p i n e l w i t h 5 w t . % D4 2.1. E l e c t r o n image w i t h t h e l i n e L, x600
2.2. D i s t r i b u t i o n o f Ca and S i a l o n g t h e l i n e L 2.3. D i s t r i b u t i o n o f A l , Mg and Cr a l o n g t i m e l i n e L
a b c
Fig.3. O p t i c a l micrographs o f t h e r m a l l y etched s i n t e r e d s p i n e l samples w i t h 3 wt.% a d d i t i v e s D2 ( a ) , D3 ( b ) and D4 ( c ) .
higher d e n s i t i e s , as well as slower decrease of the sintered d e n s i t i e s , a f t e r the op- timum one, were noticed. Solid solution formation processes improve the wetting of spinel p a r t i c l e s , favouring a more convenient arrangement (Fig.3).
A t the same time the b e t t e r wetting homogenizes d i s t r i b u t i o n of the l i q u i d i n t h e ma- t e r i a l being sintered. In the presence of Dl and D2 the liquid penetrates between the spinel p a r t i c l e s due t o capillary forces. When densification i s a l s o accompanied by s o l i d solution formation, a stronger chemical a f f i n i t y betueen the liquid and solid phases i s expected. In the l a s t case wetting i s b e t t e r and the favourable arrangement changes slowly. I f a l i q u i d i s more viscous containing higher Si02 content, b e t t e r d i s t r i b u t i o n of the liquid phase can be expected i n the materials with Dq and D3 than D and D l .
T'2e influence of used additives on grain growth process during s i n t e r i n g depends on the composition of s i n t e r i n g aid (Table 3). Liquids formed i n the presence of Dl and D2 wet spinel p a r t i c l e s t o a l e s s e r degree which causes p a r t i c l e s i n t e r i n g predomi- nantly within agglomerates. I n the l a s t case the grain growth i s controlled by s o l i d s t a t e process. However, d i f f e r e n t microstructures between materials with D3 and D4 were a l s o noticed. Besides the f a c t t h a t s o l i d solution formation took place with both additives and t h a t spinel was wetted b e t t e r than in the presence of D l and D2, dissolution i n t o the liquid formed with D3 i s retarded. In our opinion the viscosity of t h e present liquid phase i s an important f a c t o r f o r grain growth process. Less viscous liquids enable f a s t e r migration of diffusing species. As a consequence of retarded dissolution and migration processes f i n e r grains were obtained with a d d i t i - ve Dj.
V - CONCLUSION
In the presence of additives which are able t o r e a c t with the spinel b e t t e r densifi- cations of MgAl 0 were achieved. The optimum density was obtained with the fixed amount of each g d t i t i v e . Grain s i z e of sintered spinel depended on additive conposi- t i o n .
REFERENCES
/ I / . Kostid,E. and Kiss, S.J., Sci. of Ceramics, 11 (1901) 309
/2/. Kostif ,E., Ceramica (Florence), 33 (1980) 16- /3/. KostiC,E., Mat.Sci .Res., 10 ( 1 9 7 q 379
/4/. Voskoboynikov, V.G. e t al, Svoistva zhydkih domenih shlakov, Moskva, "Metal 1 urgiya", 1975.
