Aluminium titanate-titania ceramics synthesized by sintering and hot pressing

Ceramics International 15 (1989) 155-160

Aluminium Titanate-Titania Ceramics Synthesized by Sintering and Hot Presstng I. S t a m e n k o v i b Institutc "Boris Kidri~'-Vin~a, P.O. Box 522, 11001 Belgrade, Yugoslavia (Rcccivcd 15 January 1988: acccptcd 12 March 1988)

Abstract: Akmlinium titanate was prepared by beating single oxides (mechanically

mixed) with an excess of2wt% MgO at 1450 C. Using the vibro mill, the surface area of tile obtained AI2TiO 5 increased to 32 re-'g- ~. while tbe particle diametcr ranged between 0.6 and 3/ma. The composite ceramics built from aluminium titanate matrix and rutilc as a dispcrscd phase was obtained by sintering at 1500 C and hot pressing at 1350 C. All samples obtained through thc sintcring procedure wcrc charactcrized by high intergranular porosity and cracks and a mean grain diameter of 6"5/ma; TiO_, played thc role of sintering promoter enabling the samples to rcach 90% of relative theoretical density. Dilatomctric measurements revealcd a clockwise hysteresis of expansion contraction curves of samples in which TiO, content ranged up to 32.3 vol%. The hysteresis was connected with the grain boundaries' tendency towards cracking during the cooling phase of sintering. The inflection temperature of dilatometric cooling curves ranged between 600 C and 700 C. Hot pressed AI.,TiO_~-TiO_,ceramics reached a density level of over 96% of the theoretical density, contained low intergranular porosity and a fine-grained structure: the mean grain diameter was 3"8/lm and no size change with TiO_, content was noticed. The presence of rupture grain boundaries was noticed during microstructural investigations of all samples. Hot pressed samples also showed the hysteresis but in a much lesser extent, due to their liner structure. The inflection temperatures of cooling curves ranged between 260 C and 370 C for pure aluminium titanate and AI,TiO s + 32.2 vol% TiO, samples, respectively.

1 INTRODUCTION

p r e p a r a t i o n : c r a c k p r o p a g a t i o n : a n d to the specific industrial applications. 6 T h e role o f TiO2 c o n t e n t d u r i n g the sintering o f A I 2 T i O 5 was investigated as p a r t o f research into which p r o p e r t i e s w o u l d be a p p r o p r i a t e for a host m a t e r i a l in nuclear waste m a n a g e m e n t a n d as a b i o m a t e r i a l , v T o b r o a d e n the existing k n o w l e d g e on the influence o f T i O z on a l u m i n i u m t i t a n a t e densification, e x p e r i m e n t a l w o r k in the field o f sintering a n d hot pressing o f A l z T i O s - T i O 2 mixtures was realized.

D e s p i t e the u n f a v o u r a b l e c o n s e q u e n c e s o f the extremes thermal expansion anisotropy, aluminium t i t a n a t e has a l r e a d y p e n e t r a t e d into the field o f c o m m e r c i a l a p p l i c a t i o n . Its use as a t h e r m a l insulating layer in swirl c h a m b e r a n d e x h a u s t gas outlet c o a t i n g s was r e p o r t e d as a successful a t t e m p t o f overall engine efficiency increase. ~ T o d a y , the i n v e s t i g a t i o n o f a l u m i n i u m t i t a n a t e is directed t o w a r d s the m e c h a n i s m o f its synthesis 2'3 p o w d e r 155

Ceramics International 0272-8842/89/$03"50 i('~ 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

1. Stamenkovi('

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2 EXPERIMENTAL RESULTS

PROCEDURE

AND

2. 1 Starting p o w d e r s

In the preparation of the aluminium titanate, the following procedure was used: an equimolar mixture of titania (anatase) 99.00% pure and alumina 99.65% pure powder, together with 2 wt% excess of MgO in the form of Mg(CH3COO)24H20 (a compound very soluble in alcohol), was homogenized in a laterally rotating plastic bottle for 5 h, using ethanol as a dispersive liquid. After drying, the powder mixture was cold pressed at 50MPa and heated in air at 145@'C for 3 h. In order to increase its sinterability, the obtained aluminium titanate was wet milled in vibro mill (Fritsch GmbH, ldar-Oberstein, FRG); the surface area versus milling time curve being shown in Fig. 1. Electron micrographs of aluminium titanate and titania powders are shown in Fig. 2. Table 1 summarizes the measured properties of starting and produced powders.

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2.2 Densification o f A I 2TiO 5- TiO2 ceramics

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Fig. I.

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Influence of milling time on AI2TiO~ powder surface a rea.

Table 1. Powder

Crystal phases

The aluminium titanate milled for 1.5 h and TiO2-R were used to prepare the samples of the desired composition. The powder mixtures containing TiO 2 up to 30 wt% were homogenized by tumbling in a

Characteristic properties of powders Surface area (m2g -j )

Diameter (Hm) Calculated

Electron microscope

TiO2-A" TiO2-R

Anatase Rutile

7.3 3-6

0'4 0.8

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Corundum Orthorhombic (rutile) c

7.6 3"2

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0.8-1 06-3

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TiO2-A heated at 1100 C for 2 h Milling time 90min

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Aluminium titanate-titania ceramics s.rnthesized by sinterhlg and hot presshlg

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plastic bottle, in ethanol media, for 5 h. After drying, the mixtures were cold pressed. Explorative pressing revealed that pressure had very little effect on green pellet density: increasing the pressure from 75 MPa up to 150MPa the green pellets density only increased to 3% of relative theoretical density. To prepare the green pellets, having a diameter of 12 mm and a height of approximately 12 mm, for sintering experiments, the pressure of 100 MPa was accepted. Sintering experiments in air were realized in a muffle kiln at 1500°C with 2h of soak time. Heating--cooling rates were 300°Ch-~ and 200"C h - 1, respectively. The hot pressing of samples, prepressed at 50 MPa, was performed in a 30 mm graphite die with pyrolitic carbon paper spacers. An atmosphere of pure argon was used in the hot pressing experiments. The pressure of 25 MPa was applied for 30 min at the maximum temperature. The heating rate was 1400°C h - i while the cooling rate ranged between 300 and 400°C h-1 The relationship between the densities of the sintered and hot-pressed samples and the theoretical ones given in Table 1, are reported in Fig. 3. Densified samples were examined using a scanning electron microscope and some of the micrographs are shown in Figs 4 and 5. The measurement of the thermal expansion characteristics was carried out using a dilatometer, with a heating--cooling rate of 10°Cmin - t, in air. Some examples of dilatometric curves are shown in Fig. 6.

3 DISCUSSION The X-ray analysis of the obtained aluminium titanate (Table 1) revealed the presence of TiO 2, confirming the published data dealing with the complexity of single oxides direct interaction. 2 The presence of MgTiO 3 or MgTi205 was not detected

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Fig. 4. Scanningelectron micrographsof fracture surface of IA) AI2TiOs---II.I vol% TiO2, (B) AI2TiOs--22"5vol% TiO2 and (C) AI,TiOs--32"2vol% TiO2 samples sintered at 1500~'C. in the produced AI2TiOs; it was presumed that added MgO existed in the form of solid solution by substituting of 2AI atoms by Mg and Ti ones) Obtained aluminium titanate lumps were loosely composed of relatively coarse particles and no BET surface area was measurable. The intensive comminution by vibro mill in a tungsten carbide container resulted in a relatively high increase in surface area,

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I. Stamenkot,i('

Fig. 5. Scanning electron micrographs of fracture surface of (A) AlzTiO s, (B) Al_,TiOs--11.1 vol% TiO_,, (C) AI_,TiO5 22.5 vol% and (D) AI2TiOs--32.2vol% samples hot pressed at 1350 C.

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Linear thermal expansion of sintered and hot pressed Al2TiOs-TiO _, samples.

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Ahtminium titanate titania ceramics synthesized hy sintering and hot presshlg

as is shown in Fig. 1. The milled aluminium titanate powders (Fig. 2) consisted of larger, sharp-edged particles with diameters ranging between 10 and 20pm and the matrix of particles had diameters between 0"6 and 3 pm. So, the gradual chipping of large particles could be accepted as evidence that the milling mechanism occurred in the experiments. When the bulk of the large particles were reduced in diameter, a significant surface area increase might have been expected and the obvious surface area increase, beginning after approximately 1 h of milling time, is seen in the milling curve (Fig. 1). There is no doubt that the 'two-step' procedure, i.e. single oxide interaction followed by densification, suffers tremendously by milling limits itself, particularly when submicron and low impurity ceramics are synthesized. The milling time of !.5h was chosen in order to prevent further contamination with lining material and to attain the surface area close to that of TiO2-R. The relatively low density level of the sintered samples, as evident from Fig. 3, confirms its poor sinterability and is connected with a high cowdent/ionic binding ratio greater than 30:70, 9 and powder characteristics not highly suitable for densification by pressureless sintering. The linearity of the interdependence between sample densities and TiOz-R content should be ascribed to the existence of a two-phase material, where AlzTiO 5 is acting as a matrix component. The presence of TiO2 evidently enhanced the samples densification to a notable extent. In changing the densification procedure to hot pressing, no abrupt density increase of pure AI,TiO 5 was detected to confirm its poor sinterability. However, the samples containing TiOz were enabled to reach such high densities as 97% of the theoretical density (TD). The promoting effects of TiO2 might be related to the increased contact area between constituent particles enhancing correspondent interdiffusional parameters. No rutile phase was detected in investigation of'as hot pressed' pure AIzTiO 5 samples by X-ray technique due to the completed single oxide interaction and/or TiO2 diffusion into synthesized titanate. Therefore, after sample quenching in air at 900"C for 4 h and cooling at 300°C h - t, the X-ray analysis again revealed traces of free TiOz, evidently as a consequence of a spontaneous Al-titanate eutectoid like decomposition, a If the micrographs of sintered samples in Fig. 4 are analysed (except of the pure aluminium titanate, the

159

consistency of which was not suitable for microscopic investigation) a substantial a m o u n t of intergranular pores and cracks are noticed in all samples. When compared with particles of the starting powders, grain growth has occurred at the investigated sintering temperature; the mean grain diameter of 6.5 pm was measured in the samples of TiO 2 content up to 32.2 vol%. The process of grain spheroidization and neck formation between particles of inhomogeneous array was observed but it was more pronounced in samples with a higher TiO a content. One could speculate about the sintering rate through enhanced AI2TiOs-TiO 2 interfacial and diffusional parameters relevant to the sintering process. The microstructure of hot pressed samples (Fig. 5) is characterized by a fine grained structure and limited intergranular porosity. The mean grain diameter is 3"8 pm: no size change was then noticed with TiO a content ranging up to 32 vol%. Some of the larger grains contained closed pores created during heating procedure, probably as a consequence of huge thermal expansion anisotropy of AIaTiO 5 crystals, t° These granular voids may have acted as obstacles to the mass transport during the sintering process occurring during the hot pressing procedure. The interparticle bonding is apparently loose, due to the presence of microcracks created in the cooling step of hot pressing. According to the dilatometric measurements up to 1200~C (Fig. 6) the total linear expansion did not exceed the value of 1.2%. This is in good agreement with the data obtained in the investigation of AI2TiO.~ crystal expansion using the X-ray technique I~ and those obtained with aluminium titanate containing AI203 + T i O , . tz Obtained heating~zooling dilatometric curves demonstrated the clockwise hysteresis related with internal ruptures or microcracks created during cooling. The levels of hysteresis noticed with sintered samples were much higher than those of hot pressed samples. This effect could be connected with the higher consistency of hot pressed samples in which microcracks played a less obvious role. Negative expansion values during heating, particularly distinct with AIzTiO5-TiO 2 sintered samples, could be ascribed to the process of recombination of grain boundaries, i.e. microcracks, created during previous cooling phases of sintering and/or heat treatment, closing. The recombination process proceeds due to the effect of surface forces at close distances 1° and/or chemical interactions, inducing in both cases restoration of intergranular bonds.13

160

The inflection regions of the cooling curves could be explained by the reopening and/or further formation of microcracks caused by internal stresses. Corresponding inflection temperatures of sintered samples ranged between 600 and 700°C; no relationship with TiO2 content was noticed. The inflection temperatures of hot pressed samples were much lower and ranged from 260 up to 370°C if aluminium titanate contained TiO2 up to 32.2 vol%. This could be connected with the lower sensitivity to microcracking of the relatively fine grained microstructure typical of hot pressed samples.

REFERENCES 1. WALZER, P., HENRICK, H. & LANGER, M., Ceramic Components in Passenger-Car Diesel Engines. Materials aml Design, 7 (1986) 75. 2. FREUDENBERG, B. & MOCELLIN, A., Aluminium Titanate Formation by Solid-State Reaction of Fine AI,O 3 and TiO 2 Powder. J. Am. Ceram. Sot., 70 (1987) 33. 3. TARASOVSKIJ, V. P., LUKIN, E. S. & POPOVA, N. A., Synthcsis of Aluminium Titanate. Ogneupory(1986) 21 (in Russian).

L Stamenkovii' 4. OKIMURA, H., BARR1NGER, E. A. & BOWEN, H. K., Preparation and Sintering of Monosized AI203-TiO 2 Composite Powder. J. Am. Ceram. Soc., 69 (1986) C-22. 5. HAMANO, K., OHYA, Y. & NAKAGAWA, Z., Crack Propagation Resistance of Aluminium Titanate Ceramics. Int. J. High Technology Ceramics, ! (1985) 129. 6. Anon. Alumina Titania Powder by Vapor Phase Reaction, New Materials Japan, 4 (1987) 4. 7. STAM ENKOVIC, I. & ONDRACEK, G., Preparation and Thermal Expansion of AI,TiOs-TiO: Sintered Bodies. Meeting on Yugoslav-German Cooperation in Materials Development, Ljubljana-Brdo (May 1987). 8. PENA, P., DE AZA, S. & MOYA, J. S., Thermal Stability of AI,,TiOs-AI6Si,Ot ~-ZrOz Composites Obtained by Reaction Sintering. Proceedings of the Science of Ceramics 14 (to be published). 9. TARASOVSKIJ, V. P. & LUKIN, E. S., Aluminium Titanate--Methods of Synthesis, Microstructure and Properties. Ogneupory, 6 11985) 24 (in Russian). 10. BUESSEM, W. R. & LANGE, F. F., Residual Stresses in A nisotropic Ceramics. hTterceram, 15 (1966) 229. I 1. TOULOUKIAN, Y., POWELL, R., HO, C. & NICOLAU, M., Thermophysical Properties of Matter, Vol. 13, IF1/Plenum, New York, 1973. 12. PERSON, M., HERMANSSON, L. & CARLSSON, R., Investigation of the Stability of Aluminium Titanate Ceramics. Science of Ceramics, I 1 ( 1981) 479. 13. BYRNE, W. P., MORRELL, R. & LAWSOW, J., Thermal Expansion Characterization and Thermal Stability of Aluminium Titanate. Proceedings of the Science of Ceramics 14 (to be published).