AGGLOMERATE FORMATION, COMPACT CHARACTERISTICS AND SINTERING BEHAVIOUR OF CERAMIC POWDERS

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AGGLOMERATE FORMATION, COMPACT CHARACTERISTICS AND SINTERING BEHAVIOUR OF CERAMIC POWDERS

F. Dogan, A. Roosen, H. Hausner

To cite this version:

F. Dogan, A. Roosen, H. Hausner. AGGLOMERATE FORMATION, COMPACT CHARACTERIS-

TICS AND SINTERING BEHAVIOUR OF CERAMIC POWDERS. Journal de Physique Colloques,

1986, 47 (C1), pp.C1-231-C1-235. �10.1051/jphyscol:1986134�. �jpa-00225563�

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JOURNAL DE PHYSIQUE

Colloque Cl, supplement au n02, Tome 47, fevrier 1986 page cl-231

SGGLOMERATE FORMATION, COMPACT CHARACTERISTICS AND SINTERING BEHAVIOUR O F CERAMIC POWDERS

F. DOGAN, A. ROOSEN and H. HAUSNER

Technische Universitdt Berlin, Institut fiir Nichtmetallische Werkstoffe, Englische Strasse 20, 0-1000 Berlin 1 2 , F.R.G.

!i6sum6

-

La presence a7ag~,lom6rats a une influence importante

--

sur le frictage aes poudres cgramiques. L7interd6pendance entre la structure des poudres compactkes et leur frittage sera discutgedans le cas de li02 avec des caractgristiques diff6rentes.

Abstract

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-m

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The presence of agglomerates has an important influence on the densification behaviour of ceramic powders. The interdependence between the compact structure and its sintering behaviour will be discussed for Ti02 powders with different characteristics.

I

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INTROOUCTION

A significant effort ~ u r i n g the present development of high technology ceramics is directed towards the synthesis and processing of new sinteractive powders. It is well known, that the powder characteristics influence the microstructure of the sintered ceramic material and thereby its properties. It has been recognized, that the presence of agglomerates are very important in this respect.

An agalomerate can be defined as an arrangement of small crystallites:

being held together by electrostatic charges, capillary forces or even small solid state bridges. Sometimes

,

hard, high-strength agglomerates are called aggregates.

I1

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PltOCESSING METHODS AND INFLUENCE ON AGGLOMERATE FORMATION

A~glomerates can be formed in a controlled or an uncontrolled way.

Controlled agglomeration takes place on a routine basis in powder technology, if powders are subjected to a granulation step, in order to facilitate the forming operation. Uncontrolled agglomerate formation can occur during various stages of powder processing. Depend- ing on the conditions, these agglomerates can have a different size, porosity and strength and may be the reason for inhomogeneities in the microstructure of the sintered material.

It is possible to influence the agglomerate formation and characteristics in the various parts of the powder synthesis route. During the wet-chemical preparation of powders the agclomerated state of the precipitates can be altered by the concentration of the solute, the conditions during precipitation (temperature, agitation, aiing) or even by the washing and filtration metho%'~he preparation of monosized powders by the use of controlled nucleation and growth is an example of Lhe different possibilities which do exist here /l/.

The drying conditions for the precipitates are of extreme importance

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986134

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cl-232 JOURNAL DE PHYSIQUE

in respect to the formation of agglomerates in the resulting powder.

The precipitates, especially in the case of gel-type hydroxides, consist of very small particles close to colloidal nature. The capillary forces between the particels are very high and depend besides t%-e particle size on-the surface energy relations within the system. It has been demonstrated, that hard agglomerates, which are being formed during a normal drying step at elevated temperatures, can be avoided by a treatment of the hydroxides with organic solvents.

The organic solvents have a lower surface tension than water and reduce the capillary forces in the small channels within the gel-type structure during drying. Therefore the formation of hard agglomerates cannot take place and loosely bounded structures result after drying /2,3/. The development of strong capillary forces can also be sup- pressed

,

if the precipitates are freeze-dried /4/. Hereby the particles are fixed in their position and their movement is avoided by the absence of a liquid phase and the resulting capillary forces during the drying process. High strength agglomerates cannot form in this way.

The calcination temperature has an important influence on agglomerate strength, since the bridges between single crystallites or particles, which may have been developed already during drying,become stronger with increasing temperature. The characteristics of the agglomerates obtained in the early stages of powder processing are transferred in most cases to the calcined product, which underlines the importance to apply the proper processing steps already in the beginning of the powder synthesis.

If agglomerates of undesired characteristics have been formed during powder preparation, they may be destroyed or at least altered by the milling process, which is used generally in ceramic fabrication.

However, it is difficult or almost impossible to obtain the same powder characteristics in comparison to a powder synthesis route, where from the beginning attention was paid to the preparation of suitable precursors with an appropriate agclomerate characteristic.

Besides milling a sedimentation process can be used to eliminate agglomerates from a powder suspension and an enhanced sinterability of the deagglomerated powder could be demonstrated

/5/.

The characteristics of abtlomerates are reflected by the compact structure, especially by its pore size distribution, which in turn will influence the sintering behaviour. This has been shown already for calcia-stabilized zirconia /6/ and will be discussed here for Yi02 powders.

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EXPE2IMEKTAL METHODS

Three TiO powders of different origin have been used in this in- vestigatign.

Two powders were pr~pared from TiC14 by dissolution of 200 m1 TiC14 in 1880 m1 H 0 at 0 C and the subsequent precipitation of Ti02*aq at 25 ^Cby s6raying the acidic solution into 2000 m1 ammonia

while maintaining a of 10. After removal of the chloride ions by washing the precipit3e with deionized water, one part of the gel was dispersed in 200 m1 water, the other part in a mixture of water and isopropanol (1:l) by milling for one hour with Z r 0 2 pebbles in plastic-lined ball mills. Both suspensions Here freeze-dried and the resulting powders were calcined at

500

C for one hour.

After addition of a PVA-solution a$ a binder the powders were compacted with a pressure of

350

N/mm in an isostatic press into small cylinders of 14 mm in diameter and

5

mm in height with a green density between 51 and

58

% th.d. %he compacts were sintered at temperatures between 800 and 1100 C with a heating rate of 300°c/hr and three hours at peak temperature.

The third powder was of a co~amercial type and was compacted as described above. All powders had the anatase structure.

$'he pore size distributions of the compactswere determined by

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Hg-porosimetry. The d e n s i f i c a t i o n b e h a v i o u r was i n v e s t i g a t e d i n a h i g h - t e m p e r a t u r e d i l a t o m e t e r .

I V

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RESULTS AND DISCUSSION

The g r a i n s i z e ( a g g l o m e r a t e s i z e ) d i s t r i b u t i o n of t h e d i f f e r e n t T i 0 2 powders i s shown i n F i g 1. Yhe commercial powder (K-A) h a s t h e

100 50 l 0 5 1 0.5 0.1

GRAIN SIZE I brn t

F i g . 1 G r a i n s i z e d i s t r i b u t i o n of T i 0 2 powders

s m a l l e s t g r a i n s i z e , f o l l o w e d by t h e powder, which p r e c u r s o r h a d b e e n d i s p e r s e d i n w a t e r

+

a l c o h o l b e f o r e t h e s u b s e q u e n t p r o c e s s i n g s t e p s ( F D

-

^W+P);i n t h e c a s e of t h e c o a r s e s t powder (FD

-

W) t h e p r e c u r s o r was d i s p e r s e d o n l y i n w a t e r . Powder "K-A" had a s p e c i f i c s u r f a c e a r e a of 1 0 . 4 me/g, powder llFD-W1l 55 m 2 / g a n d "FD-W+P"

70.6 m2/g. These v a l u e s do n o t c o r r e s p o n d t o t h e r e s u l t s of t h e g r a i n s i z e measurements and a r e a n i n d i c a t i o n , t h a t a g g l o m e r a t e s of d i f f e r - e n t c h a r a c t e r i s t i c s e x i s t i n t h e t h r e e powders.

The s i n t e r i n g b e h a v i o u r of t h e powders i s shown i n P i g . 2 . A l r e a d y a t 800°C t h e compactL "PD-W+P" h a s a s i n t e r i n g d e n s i t y of 97 th.d.

a n d r e a c h e s

99

% t h . d . a f t e r s i n t e r i n g a t l l O O O C i n comparison w i t h 91.4 '$ t h . d . f o r "FD-W" and 74.4 % t h . d . f o r "K-A" a t t h e same t e m p e r a t u r e .

_ - -

^--.* ^{F D - W}

... ..-. K-A

I l

800 900 1000 1100 1200

TEMPERATURE I "C 1

F i g . 2 S i n t e r i n g d e n s i t y o f Ti02 compacts

C o n s i d e r i n g t h e powder c h a r a c t e r i s t i c s it becomes o b v i o u s , t h a t t h e g r a i n s i z e d i s t r i b u t i o n of t h e d i f f e r e n t powders c a n n o t be u s e d t o

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JOURNAL DE PHYSIQUE

p r e d i c t t h e s i n t e r i . n g b e h a v i o u r . There seems t o be a c o r r e l a t i o n between t h e s p e c l f i c s u r f a c e a r e a and t h e s i n t e r i n g r e s u l t s , b u t i t i s known i n s l n t e r l n g practice a n a h a s been d e m o n s t r a t e d i n a p r e v i o u s i n v e s t i g a t i o n /6/, t h a t a l s o t h e s p e c i f i c s u r f a c e a r e a i s n o t always a c o n c l u s i v e f a c t o r i n d e t e r m i n i n g t h e s i n t e r i n g b e h a v i o u r . A c l e a r c o r r e l a t i o n s e e m s t o e x i s t , however, between t h e p o r e s i z e

d i s t r i b u t i o n i n t h e compact a n d t h e s h r i n k a g e r a t e which i s r e p r e - s e n t a t i v e f o r t h e s i n t e r i n g b e h a v i o u r . I n F i g . 3 t h e p o r e s i z e d i s t r i - b u t i o n i n t h e d i f f e r e n t ; compacts and i n F i g . 4 t h e s h r i n k a g e r a t e s a r e shown. The r e s u l t s of t h e p o r e s i z e d i s t r i b u t i o n a r e r e f l e c t e d i n t h e r e l a t i v e p o s i t i o n of t h e s h r i n k a g e r a t e c u r v e s . The o n s e t of s i n t e r i n g and t h e maximum s h r i n k a g e r a t e i s r e l a t e d t o t h e p o r e s i z e a n d the'maximum of t h e r e l a t i v e p o r e volume. "FD-W+P" h a s a v e r y narrow p o r e s i z e d i s t r i b u t i o n a t a s m a l l p o r e s i z e . A d i s t i n c t i o n between i n t e r a g ~ l o m e r a t e p o r e s and i n t r a a g g l o m e r a t e p o r e s i s n o t p o s s i b l e i n c o n t r a s t t o t h e "FD-W" sample, The compact "K-A" c o n t a i n s t h e l a r g e s t p o r e s i n comparison w i t h t h e two o t h e r specimen and t h e maximum of t h e s h r i n k a g e r a t e i s s h i f t e d t o a h i g h e r t e m p e r a t u r e .

80 o

70 -

-

FD-W+R

--- ^FO-W ... _K-A

-'-,

- . --__

loo 10' 10' 103 lol 10'

PORE RADIUS 1 nm l

F i g . 3 P o r e s i z e d i s t r i b u t i o n i n T i 0 2 compacts

- F D - W + P

--- ^FD-W

; 0.6' ... K.A

F

Z , 0.4-

I

+

(L

200 400 600 800 1000 1200 14W

TEMPERATURE [ ^OC1

F i g . 4 S h r i n k a g e r a t e of T i 0 2 compacts

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Based on t h e r e s u l t s o b t a i n e d i t can be s t a t e d , t h a t t h e c h a r a c t e r - i s t i c s of powder a g g l o m e r a t e s have a n i m p o r t a n t i n f l u e n c e on t h e p o r e s i z e d i s t r i b u t i o n i n t h e compacted b o d i e s and t h e i r s i n t e r i n g b e h a v i o u r . One i m p o r t a n t consequence of t h e a b s e n c e of a & g l o m e r a t e s w i t h a n u n d e s i r a b l e c h a r a c t e r i s t i c i n a powder o r a compacted body i s t h e f a c t , t h a t s i n t e r i n g t o h i g h d e n s i t i e s c a n be a c h i e v e d a t t e m p e r a t u r e s , which a r e s e v e r a l hundred d e g r e e s l o w e r t h a n i n t h e c a s e o f powders, where no a t t e n t i o n h a s been p a i d t o t h e p o s s i b l e o c c u r r e n c e of such a g g l o m e r a t e s . F u r t h e r m o r e a v e r y homogeneous, f i n e g r a i n e d nicrost1.uctur.e c a n be o b t a i n e d . F o r T'iO a n example i s shown i n F i g .

5,

where an a v e r a g e g r a i n s i z e of aboug 0 . 3 m c o u l d be o b t a i n e d i n a sample w i t h 99 P t h . d . a f t e r s i n t e r i n g a!?800°C.

F i g .

5

M i c r o s t r u c t u r e of T i 0 2 V

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CONCLUSION

The p o r e s i z e distribution i n a compacted body g i v e s v a l u a b l e i n - f o r m a t i o n on t h e s i n t e r i n g b e h a v i o u r of c e r a m i c powders. Powder a ~ ~ l o m e r a t e s may i n f l u e n c e t h i s p o r e s i z e d i s t r i b u t i o n v i a t h e i r d i f f e r e n t c h a r a c t e r i s t i c s . I t i s i m p o r t a n t , t h a t even minor p r o c e s s i n g s t e p s d u r i n g t h e p r e p a r a t i o n of powders may d e t e r m i n e t h e c h a r a c t e r i s t i c s of t h e a g g l o m e r a t e s and t h e r e f o r e c a n p l a y a s i g n i f i c a n t r o l e .

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/ 2 / Pampuch, R . and Haberko, K .

,

Keram. Z e i t s c h r . ~ ? 1 9 7 9 )

,

⁴⁷⁸

/3/ van de G r a a f , M .A.C .G., t e r Maat, J .H .H. and B u r g g r a a f , A.J.

i n : Ceramic Powders, 783, P . V i n c e n c i n i (ed.)

,

E l s e v i e r S c i e n t . P u b l . Co., Amsterdam (1983)

/4/ Roosen, A . a n d Hausner, H . i n : Ceramic Powders, 773,

P. V i n c e n c i n i ( e d . ) , E l s e v i e r S c i e n t . P u b l . Co., Amsterdam (1983)

/5/ Rhodes, N.H., J . Am. Ceram. Soc. 64 ( 1 9 8 1 ) , 19

/6/ Roosen, A . a n d Hausner, H., i n : ~ G a n c e s i n Ceramics

g ,

714 The American Ceramic S o c i e t y (1985)