Preparation of AINTiO2 powder compacts using colloidal methods

Ceramics International 15 (1989) 255-270

M e t a l l o r g a n i c C o m p o u n d s as Preceramlc M a t e r i a l s II. Oxide Ceramics G. P o u s k o u l e l i Ceramic Section, Mineral Sciences Laboratory, CANMET, Energy Mines and Resources Canada, Ottawa, Canada K I A O G I (Received 6 September 1988; accepted 2 November 1988)

Abstract: The preparation of oxide ceramics from metallorganic precursors is reviewed and discussed in this paper. Sol-gel techniques through which these precursors are turned into ceramic materials are also addressed. Systems such as silica, alumina, titania, zirconia, as well as their mixtures, are examined and some new developments in the synthesis and processing of ceramic precursors are presented.

1 INTRODUCTION

2 SILICA

Remarkable progress has been made in the last two decades in the science of processing ceramic materials. A whole range of organic and inorganic solution- and vapor-phase-based techniques have emerged that offer extensive opportunities for producing not only existing products with a very tight control on their homogeneity and performance but which, in some cases, also offer the opportunity of producing entirely new products, especially in the field of composites. These developments have been spurred by both a progressive realization that the final performance of a ceramic depends largely on its microstructural characteristics and also on the recognition that in many cases, ceramics can be expected to display metal-based components especially in hightemperature, corrosive or erosive conditions. As a follow-up to a previous paper which dealt with the non-oxide ceramics and metallorganic precursors, 1 the present paper will present an overview of the use of organically modified preceramic compounds for the synthesis of oxide ceramics, glasses and composites.

The use of organic compounds containing Si for the production of SiO2 was initiated in the early 1960s z based on the chemistry of the metal oxides studied much earlier. The sol-gel technique for ceramics also emerged in the same era, and since then they have evolved together.

2. 1 Sol-gel In general terms, the sol-gel process involves three stages: a partial hydrolysis of the metallorganic compound in order to produce reactive monomers, a polycondensation of the reactive monomers which forms oligomers of colloidal size (sol), and finally an additional hydrolysis which initiates and further promotes the polymerization and cross-linking of the precursor. The last step, gelation, will produce polymeric compounds of given morphology which upon pyrolysis yield oxide ceramics. These operations although complicated can be presented in the following simplified manner:

Monomer formation (partial hydrolysis): M(OR)x + H 2 0 255

© 1989 Minister of Supply and Services Canada

•

(RO)~MOH + ROH (1)

B. Grobety, A. Mocellin

272

common methods. 12 These techniques however greatly affect the homogeneity of composite powders. Colloidal or filter pressing has also been gaining popularity in recent years, but will not be considered here. Attention thus will be focused to granulate formation by spray-drying and to their subsequent uniaxial cold-pressings. It is generally observed that parameters such as the degree of granulation, the binder content, etc., affect not so much the chemical but the density distribution in the powder pills and the sintered products. 13'14 The results of our compaction experiments will be illustrated on densified micrographs. They suggest that the nature of the powder is more important to the final microstructure than any pressing cycle parameters.

(a)

2 STARTING MATERIALS AND EXPERIMENTAL METHODS The particle size distributions (Figs 1 and 2) of the three powders used in the following experiments were determined by sedimentation (Sedigraph 5000 D--Micromeritics). Powders and liquids were used as received (Table 1). A polyvinylpyrrolidone was used as the steric stabilizer. For reasons pertaining to the mechanism of reaction between A1N and TiO2,1° most experiments were conducted on nominal 1"STiO 2 + 2A1N molar compositions. The sedimentation volumes however have also been measured for pure A1N, TiO 2 (Fig. 3) as well as for varying A1N/TiO 2 powder mixtures. Such volumes were determined by placing 5 g of powder (or dry mixture) into a glass cylinder, then adding 50 ml of liquid. After shaking the cylinder for 30s, the suspension was treated for another 5rain by ultrasound. The cylinders were closed using a rubber

1 oo.

o

(b)

.......':".'7,.-,

9o.o 80.

o

':..

~

70.0 G0.0

~

50.0

- i

_~ 4 0 . 0 .~

AIN HC Starr.k

30.0 20.0

.... TiO2 Tioxlde

":.. \

1o.o

\\ ~ 20.0

ii 10.0

t i

i

i 5.0

i

i

i

ii t.O

,'T.,"~

,

i

i

0.5

Equivalent spherical dianeter [.J~]

Fig. 1.

Size distributions of the starting powders.

0.1

(e) Fig. 2. SEM aspect of the starting powders: (a) AIN HC Starck Grade C; (b) AIN Tokuyama; (c) TiO2 HP Tioxide.

Preparation of AIN-Ti02 powder compacts using colloidal methods Table 1.

273

Origin and properties of investigated powders and suspending liquids (data from suppliers) Powders

Density (g/cm 3)

dso(/~m)

3.26 3'26 4.25

1.8 1.2 1.4

AIN Tokuyama Soda AIN HC Starck Grade C TiO 2 Tioxide HP Liquids

Products

d(g/cm3) a

HBI b

HaOa

ec

n-Hexane Cyclohexane Xylene Trichlorethylene Dibutylphthalate Benzaldehyde Acetone Methanol Ethanol Isopropanol

Merck no. 4367 9 666 8 681 11 872 800 919 801 756 14 6012 983 995

0.66 0.788 0.862 1.463 1.045 1.044 0.792 0.791 0.789 0.784

0 0 4 -9.5 -8.7 18.7 18.7 18-7

0.01 0.01 0.01 ---0.2 0.01 0.2 0.2

1.87 2.02 2.4 3.42 6.43 17.8 20.7 32.7 24.5 19.9

a Indications of the producer. b Gordy, J. Chem. Phys., 7 (1939) and 9 (1941) 970. c Riddick & Bunger, Organic solvents, Techniques of Chemistry IL 1986.

plug. After 7 days the sedimentation heights were measured. For all the powders over 9 9 w t % are sedimented after this time. The zeta potential of the powders were determined by electrophoretic mobility measurements using the Hfickel equation 15 for liquids with low electrolyte concentrations. The reported zeta potentials are average values of three measurements. The experiments were done with as-received powders and suspension liquids. The influence of the drying step on the mixtures was examined on various isopropanol suspensions (2:1 weight ratio) mixed in a ball-mill for 24 h and then spray dried. The different treatment parameters

Table 2. 2 AIN (Starck) + 1 '5 TiO z mixture number

(1) (2) (3) (4) (5) (6) (7)

are shown in Table 2. The drying temperature and air flow rate were 150°C and --,40m3/h at STP respectively. Ultrasonic treatment and surfactant addition were performed after ball-milling. The granule size distribution of the dried products was determined by automatic image analysis. F r o m 1000 to 1500 granules were sized for each sample. The dried mixtures were compacted on an uniaxial press in a steel die prelubricated with stearic acid. The pressing rate was 0.4MPa/s up to a maximum pressure of 190MPa. The progress of compaction was followed by recording the plunger displacement with time. The microstructures of the compacts were observed in a scanning electron microscope.

Suspension preparation and spray-drying parameters

Surfactant concentration (% weight of solids)

Ultrasound treatment

Flow rate of the suspension through the nozzle (ml/h)

0.0 No 0.0 Yes 1.0 Yes 2.0 Yes 4.0 Yes 2.0 Yes Sample collected in the drying chamber with

Flow rate of the spraying air through the nozzle (I/h; STP)

Dried on a heating plate 800 800 1 500 400 1 500 400 1 500 400 800 800 granule sizes between 60-125/zm

274

B. Grobety, A. Mocellin E

Liquid n - Hexane

1.87

Cyclohexane

2.02

X vlene

2.4

Sediment densities

(~ absolute densities) --

AIN TIO z

--

Trichloretbylene

3.42

Dibutylphtalate

6.43

Benzaldehyde

17.8

Isopropanol

19.9

Acetone

20.7

Ethanol

24.5

Methanol

32.7

1

I

EL3 [

J

F I L_ l

.....

J

L ....

I

I 30

I

I 20

(a) Liquid Cyclohexane

HB I

Sediment densities

I'L.],

0

I

Xy(ene

4

r_Jl

Acetone

8.7

[-

D~butylphtalate Isopropanol

(% absolute densiUes) --AIN TiO2

__

_L_.

g .5

r__j

+87

]

I 20

1~)

i I

I

I I 30

(b)

Fig. 3. Sedimentdensity of pure AIN (HC Starck) and TiO2 (Tioxide HP) in differentorganic liquids: (a) with increasing dielectricconstants; (b) with increasingHBI values.

3 RESULTS

AND

DISCUSSION

3. 1 Suspension characteristics

In a flocculated system, the particles sediment quickly to a loose and thick layer. A well dispersed powder sediments very slowly and forms a thin dense layer. The sedimentation volume therefore decreases with the suspension quality. Figure 3 shows the results obtained with pure A1N and TiO2 respectively, together with the corresponding dielectric constant (Fig. 3(a)) and hydrogen bonding index (HBI, Fig. 3(b)) of the various liquids that were used.

Table 3.

No simple relationship is apparent between sediment densities and those physical properties of the dispersing media. Alcohols, benzaldehyde and dibutylphthalate proved to be the best dispersants. In subsequent experiments however, benzaldehyde was discarded because of its high boiling point which causes difficulties in spray-drying. Dibutylphthalate on the other hand has an HBI value close to 10, which, according to findings of Davies and Karuhn 16 would be an optimum for dispersing metal oxides. Acetone, however, exhibits a poor dispersion ability and the discrepancy is not understood. Amongst the alcohols, methanol is toxic and isopropanol appears more effective than ethanol. All subsequent experiments therefore used isopropanol. It is interesting also to compare experimental sedimentation volumes of 1.STIO2 + 2A1N mixtures with calculated volumes which are obtained by linear interpolation from the measured sediment volumes of the A1N and TiO2 end members in the corresponding liquids. The experimental volumes are always larger than the calculated ones (Table 3). The difference between both values decreases with the increasing dispersion ability of the liquid. The mixed suspension seems to be less stable than the end members, because particles, which are stable in their own dispersion, tend to coagulate with particles of the other powder in the mixed system, thus increasing the sediment volume. With decreasing dispersion ability of the liquid, the number of particles involved in such heterocoagulation processes seems to increase when poor suspending media are used. Also, using Tokuyama A1N, which is closer to being monodisperse, helps lower the sediment volume of the mixture. To further evaluate the stability of the polydispersed-polychemical system A1N-TiO2, we have calculated the repulsive part of the interaction energy between the smallest particles of both

Comparison between measured and calculated sediment volumes

Medium

Measured (ml)

Calculated (ml)

Difference (ml)

1. 2AIN (HC Starck) + 1.5 TiO= (Tioxide HP) Isopropanol Methanol Acetone Cyclohexane

5.8 6.9 1 1.2 16.4

5.2 5.7 7.9 1 2.0

0'6 1.2 3"3 4.4

2. 2AIN (Tokuyarna) + 1.5 7702 (Tioxide HP) Isopropanol

4.9

4.5

0'4

Preparation of AIN-Ti02 powder compacts using colloidal methods powders using the expression given by Lyklema 6 for fluids with low electrolyte concentrations:

275

30

eala2~bOl~bO2

2O

VR - 2(a 1 + a 2 + H) where e is the dielectric constant of the liquid, al, a 2 are particle radii, ~ol, ~bo2zeta potentials, and H the distance separating surfaces of the particles. This approach considers the particles as electrical point charges. O f course such an assumption is questionable. However, this is not too bad an assumption when the Debye scaling length is comparable to the size of the particles. The zeta potential of A1N H C Starck is 30 mV _ 2mV, that for A1N T o k u y a m a is 2 9 m V + 2 m V , values similar to that from A1203 in isopropanol, 17 which is not surprising since the surface of AIN is strongly oxidized by h u m i d air. 18 The TiO 2 potential is 21 m V + 2mV. The Van der Waals attraction between two particles on the other h a n d was estimated from:

vA=

A123ala2 6H(al + a2)

@.2*m 8.1 ~n 6 --5

. . . .

I

0

. . . .

56

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,

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.

~

150

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. . . .

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280

250

300

H (na)

(b)

where A12 3 is the combined H a m a k e r constant, given by

dr~o~l).6¢il~

The indices l, 2, 3 represent the constituents of the suspension: hlN{1} TIO2{2 }, isopropanol{3}. The H a m a k e r constant depends strongly on the density and the chemical nature of the surface of the materials. Based on dispersion data, Vincent 19 has calculated the following values: A l 1 = 2 2 . 6 × 10- :o j, A 33 = 5"9 × 10- 20 j. There are no values for A1N in the literature. But since A1N is strongly oxidized on the surface, we have taken the value for A1203, i.e. A22 = 15.5 × 10-z°J. 1° The interaction energies are given in k T units versus distance between the particles (Fig. 4). The minimal repulsion energy to stabilize a suspension is a disputed value 2 and lies, depending on the author, between 5 and 15 kT. F r o m Figs 1, 4(a) and 4(b), it appears that all TiO 2 particles should be stable, whereas up to 1 0 w t % HC-Starck-A1N particles lie within the instability range. The calculation also suggests (Fig. 4(c)) that mixing A1N + TiO z tends to decrease the suspension stability. The measured sedimentation volumes o f x A1N + y TiO2 mixtures with different x/y weight ratios confirm the existence of an interaction between A1N and TiOz particles. In a suspension with no or only

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and

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........................................................................................................

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50

100

150

200

5¢ltto

dal

: O,

250

,t~la

300

H (nia)

(c) Fig. 4. Calculated interaction energies between particles; (a) identical A1N-AIN of varying radius (HC Starck); (b) idem for TiO 2 (HP Tioxide); (c) TiO2-TiO2 and TiO2-AIN with different radii (HP Tioxide, HC Starck).

h o m o c o a g u l a t i o n the sediment density would be a simple addition of the partial sediment densities measured in the pure system, i.e. dse~ = X dA~N+ Y dTio2., These values would lie on a straight line between the densities for pure AIN (x = 1) and pure TiO 2 (x = 0). Heterocoagulation between A1N and TiO z particles, which in their pure suspensions are stable, lowers the sediment densities (Fig. 5). The difference between calculated and measured values is larger for the TiO2-rich mixtures.

276

B. Grobety, A. Mocellin

30.0 -'~ IC 5 . 8

'i

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g

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[ 48,0

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] GO.O

~ 80.0

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. 100.0

z(NeLght) RIN

3.0 6,0

I

2~0

I

I

I

4.0

1 5,0

I 8,0

~(Neight of solids) POP

Fig. 5. Influence of variable mixture composition on the sediment density in isopropanol (AIN HC Starck, TiO2 HP Tioxide).

Fig. 6. Sediment volume of 2AIN (HC Starck)+ 1-5TiO 2 (Tioxide HP) mixture versus surfactant concentration (% mass of dry powder).

A possible explanation for such an effect may be found from considering the respective particle size distributions (Fig. I). In order that an equal number of AIN and TiO2 particles of appropriate sizes for heterocoagulation be present (i.e. AIN < 0.2/tm, TiO 2 < 0.6/zm), then a significantly higher weight percentage of TiO2 than A1N must be present in the suspension. Other factors, such as particle shapes of course may also be important. A straightforward method to improve the stability of an aqueous suspension is to change its electrolyte concentration by varying the pH or by adding appropriate salts. This is not very easily done in organic liquids, instead a polymeric steric stabilizer is used more frequently. Here we used polyvinylpyrrolidone (PVP). During spray-drying the PVP also acts as binder. The right quantity of stabilizer is of great importance: when the surfaces are not completely covered it is possible that the same chain be attached at two surfaces at the same time thus promoting coagulation. This risk is highest at low polymer concentrations. At too high a polymer content, the unadsorbed molecules can form bridges between two polymer chains already attached at two different surfaces. 11 The sediment volume measurements of A1N-TiO2 suspensions with decreased stability for low concentrations reflect the decreased stability for low surfactant content. The value for 1% PVP is markedly higher than for the suspension without any surfactant (Fig. 6). The optimal stabilizer concentration is between 4% and 6%. At higher contents the sediment volume is slightly increasing again, indicating a lowering of the stability. Difficulties during drying and sintering, but also economic reasons, limit the concentration to around 2% in practice.

3.2 Spray-drying mixed suspensions Spray-drying is a technique of wide industrial use and is thus worth investigating. It must however be borne in mind that results obtained with a laboratory-size spray-drier such as that used in the present work, may not be transferable to an industrial operation. Some interesting trends are nonetheless observed. The reference 1"5 TiO2 + 2 AIN isopropanol suspension with or without PVP stabilizer was dried under varying conditions of flow rate through the nozzle and air temperature respectively. The granule sizes that were found in the collector remained rather small because of the small dimensions of the drying chamber. Whenever a big droplet reaches its wall, being still somewhat wet, it sticks there instead of being reflected into the stream. The maximum granule diameter collected was ~ 30 #m (Fig. 7(b)), whereas it could reach ,-, 100/~m in the drying chamber (Fig. 7(c)). Binderless suspensions would not granulate (Fig. 7(a)). It was also observed (Fig. 8) that for binder containing suspensions the concentration of solids in the suspension to be sprayed does not markedly influence the distribution of granule sizes. The chemical homogeneity of the dried mixture on the other hand was found related to the state of granulation. Ungranulated powders (mixtures 1 and 2 in Table 2) always exhibit poor homogeneity. Figure 9(b) illustrates large heterogeneities in a green compact, which are found to be A1N-rich by electron microprobe analysis. Granulated mixtures (nos. 3 to 6 in Table 2) of the same composition conversely are substantially more homogeneous (Figure 9(a)), presumably because the particle configuration that

Preparation of AIN-TiO 2 powder compacts using colloidal methods

277

g

0.0

~.0

10.0

15.0

20.0

2~.0

~e.o

dianeter (~1

Fig. 8. Granule size distribution of ungranulated (a) and granulated (b) powders taken in the collector.

(a)

prevails in the suspension is maintained during drying thanks to the effect of PVP preventing segregation. Mixtures containing Tokuyama A1N were always found to remain homogeneous regardless of operating conditions. This is thought to be

(b)

(a)

(c)

(b)

Fig. 7. SEM aspects of dried mixtures: (a) ungranulated powder(mixture I); (b) granulated powdertaken in the collector (mixture 3); (c) granulated powder taken from the drying chamber waif (mixture 7).

Fig. 9. Optical micrographs of polished sections through green bodies: (a) body made with a granulated powder (mixture 3) (p = 190 MPa); (b) body made with an ungranulated powder (mixture l) (p = 190 MPa).

278

B. Grobety, A. Mocellin ..................... :............ :........ I. T. T. .

related to the different particle size distribution of this powder. Nearly all particles of mixtures with the latter A1N are above the minimum size required for stability. The density difference between A1N and TiO2 which could favour selective sedimentation, is partly offset by the coarser size of the A1N.

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~.

: :

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3.3 Cold pressing behaviour

::

..................................

0 :

20

The compaction does not influence the chemical homogeneity of the mixtures. The density of the compacts however depends strongly on the compaction procedure and the starting material. The two constituents (A1N Starck, TiO2 Tioxide) of the mixture show markedly different behaviour during pressing (Fig. 10). The rather bad compressibility of TiO 2 is due to its very rough surface (Fig. 2(a)) which increases the friction forces even under low pressing pressures, when particle rearrangement should be dominant. The densification of smoother A1N is easier, in particular at low applied pressures, and that of the A1N/TiO 2 mixture is intermediate as might be anticipated (Fig. 10). Thus, A1N does not have a significant lubricating effect on TiO 2. In granulated powders, the compaction mechanism is presumably more complex. Granule rearrangement followed by deformation may be expected to take place first upon the application of the pressing load. Then, at higher pressures, individual particle rearrangement and perhaps deformation may be occurring. Experiments show that in the lowpressure regime the density of granulated powders remains lower than for the ungranulated ones, but increases more rapidly with increasing load. The pressure at which both densities match each other, sometimes referred to as the joining pressure, is believed to correspond to the onset of individual particle rearrangement. It was found here to be in excess of 10MPa, substantially higher than in i

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Fig. l l . Cold densification of 2AIN (HC Starck)+ 1.5TIO2 (Tioxide) mixtures with different binder contents.

systems containing a more common binder such as PVA. Polyvinylpyrrolidone therefore appears to be a very strong binder. The influence of binder content and granule size upon compaction was also investigated. Figure 11 shows the compaction behaviour of the base 1.5TiO 2 + 2A1N (Starck) mixture with different amounts of PVP added. The lower relative density achieved with the higher binder content simply reflects the lower absolute density of the binder itself: 1% PVP by weight corresponds to ~ 6% by volume. In the first approximation the densification rate and mechanism are not markedly affected by the amount of binder in the range investigated here. Figure 12 shows finally that varying the granule size at constant 2% PVP content also does not influence the later stage of cold compaction or the ultimate green density of the pressed bodies.

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111

iiil/ilil :

288

p L'HPa]

Fig. 10. C o l d d e n s i f i c a t i o n o f p u r e A I N ( H C S t a r c k ) , T i O 2 ( T i o x i d e ) a n d a 2 A l N ( H C S t a r c k ) + 1"5 T i O 2 ( T i o x i d e ) m i x t u r e .

Powder processing is a crucial step in the preparation of composite ceramics, particularly the reactively sintered ones. When markedly different constituents such as AIN and TiO 2 are used, and in the absence of a sufficient scientific base, a rather

Preparation o f A l N - T i O 2 powder compacts using colloidal methods

technological approach is necessary to identify adequate operating conditions. In this work, it was found possible to prepare stable suspensions of AIN and TiO 2 powders in all proportions. The sedimentation volume proved to be a simple but reliable parameter to rank dispersing liquids. The classical DLVO theory, although of questionable applicability here, was nonetheless useful for understanding stability differences observed when A1N powders of different origins are used. The introduction of a steric stabilizing agent proved to be helpful for isopropanol A1N + TiO2 suspensions. Polyvinylpyrrolidone which was selected for that purpose also served as a strong binder in subsequent granulation via spray-drying. Thus the homogeneity of the suspensions could be maintained during drying and subsequent handling. The cold pressing per se and its products do not appear to be markedly affected by variations in the spraydrying parameters, A1N to TiO2 ratio or binder content providing a minimum one to two weight per cent PVP has been added.

ACKNOWLEDGEMENT Thanks are due to J. Castano for technical help and to Professor T. Ring for critically reading the manuscript. This work was supported by the Swiss National Science Foundation under contract 4.842.0.85.19.

REFERENCES 1. VERWEY, E. S. W. & OVERBEEK, J. Th. G. (eds). In Theory of Lyophobic Colloids, Elsevier, Amsterdam, 1948, p. 135. 2. BAROUCH, E., MATIJEVIC, E., RING, T. A. & FINLAN, M., Heterocoagulation, II. Interaction energy of two unequal spheres. J. Colloid Interface Sci., 67 (1978) 1. 3. MATIJEVIC, E., Preparation and characterisation of monodispersed metal hydrous oxide sols. Prog. Colloid. Polymer. Sci., 61 (1976) 222.

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