Use of spodumene for liquid-phase-sintering of aluminium titanate

July 1998

Materials Letters 36 Ž1998. 118–122

Use of spodumene for liquid-phase-sintering of aluminium titanate C.G. Shi, I.M. Low

)

Materials Research Group, Department of Applied Physics, Curtin UniÕersity of Technology, Perth, WA 6845, Australia Received 10 November 1997; revised 4 January 1998; accepted 5 January 1998

Abstract The use of b-spodumene ŽLi 2 O P Al 2 O 3 P 4SiO 2 . has been investigated as a liquid-phase sintering aid for the densification aluminium titanate ŽAT. ceramics. XRD and DTA were used to characterise the effect of spodumene on the phase relations, sintering and densification behaviour of AT. The results show that the presence of spodumene significantly reduced the porosity and enhanced the densification of AT. Sillimanite, glassy, and lithium aluminium silicate ŽLAS. phases formed in samples containing spodumene additions greater than 2.5 wt.%. The presence of up to 15 wt.% spodumene improved the physical and mechanical properties of AT. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Spodumene; Aluminium titanate; Liquid-phase sintering; Reaction-sintered; Sillimanite; Glassy; Lithium aluminium silicate

1. Introduction Aluminium titanate ŽAT., Al 2TiO5 , is formed by a solid-state reaction of equimolar mixtures of alumina and titania. The formation of AT occurs in two stages. In the initial reaction stage, nucleation of AT cells takes place at 13458C w1x which is followed by the elimination of Al 2 O 3 and TiO 2 dispersoids, which are previously trapped. This occurs through a diffusion-controlled mechanism and takes place at 13858C, although values of 13758C w2x and 13808C w3x have also been obtained. AT is a ceramic material which has low thermal expansion coefficient Ž0.5 = 10y6 –1.0 = 10y6 Ky1 ., high melting point, and low thermal conductivity

)

Corresponding author.

w4,5x. These characters make it appropriate for structural applications where thermal insulation and thermal shock resistance are required. Possibilities for utilising this material include use as refractory components of internal combustion engines and casting clean metal alloys without residues. However, AT has two substantial problems withstanding its wider applications w6x. The first problem is associated with its thermal instability at high temperatures where it decomposes into a-Al 2 O 3 and TiO 2 –rutile at 12808C. This thermal instability can be controlled by adding stabilisers such as w7–10x Fe 2 O 3 , MgO, SiO 2 and ZrO 2 . The second problem inherent in AT is related to extensive microcracking which develops during cooling from the sintering temperature. The microcracks are associated with the thermal expansion behaviour or the degree of thermal anisotropy.

00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 1 3 - 5

C.G. Shi, I.M. Low r Materials Letters 36 (1998) 118–122

AT is also difficult to sinter and densify without the presence of additives which serve to lower the sintering temperature, enhance the densification, and improve the mechanical properties. However, the use of additives such as Fe 2 O 3 , MgO, and ZrO 2 can degrade the thermal shock performance of AT. Hence it is essential to use an additive which has a similar or lower thermal expansion coefficient than AT. One such additive is spodumene ŽLi 2 O P Al 2 O 3 P 4SiO 2 .. Spodumene has been widely used in the glass and ceramic industry for decades as a lithia-bearing flux and low-expansion filler in whiteware bodies w11– 13x. The a-polymorph of spodumene is a monoclinic pyroxene which is stable under ambient conditions. This phase undergoes an irreversible phase change at 10808C, forming the more open tetragonal polymorph, b-spodumene, which melts at 14238C. The transformation is accompanied by a 30% volume increase due to a density change from 3.2 to 2.4 g cmy3 . It is also commonly used for making glasses and ceramics which are harder, smoother, chemical and thermal shock resistant. Recently, spodumene has been as a liquid-phase sintering aid for the densification of alumina w14x and mullite w15–17x ceramics. This paper describes the use of spodumene as a flux for cost-effective processing and densification of AT ceramics. The effect of spodumene on the sintering behaviour, phase relations, physical and mechanical properties is discussed.

2. Experimental procedure 2.1. Powder processing Commercial titania ŽSCM Chemicals., alumina ŽA1000SG, Alcoa of Australia., and b-spodumene powder ŽGwalia Consolidated. were used as the starting materials for the synthesis of AT ceramics. The chemical analyses of these starting raw materials are shown in Table 1. A solid-state method was used to synthesise reaction-sintered spodumene-modified AT ceramics. An equimolar powder mixture of alumina and titania, containing 0–30 wt.% spodumene, was thoroughly mixed using mortar and pestle for 20 min, followed by 2 h in a Turbula mixer. The formulations for each

119

Table 1 Chemical analyses Žwt.%. of starting raw materials

Al 2 O 3 SiO 2 TiO 2 SO4 Li 2 O Fe 2 O 3 CaO MgO Na 2 O K 2O P2 O5 LOI

Spodumene

Alumina

Titania

27.0 64.0 0.02

99.7 - 0.01 - 0.01

0.5 0.8 98.0 0.2

8.10 0.16 0.04 0.02 0.19 0.05 0.11 0.3

0.03 0.05 - 0.01 0.06 0.06 - 0.01 0.46

0.5

of the samples are shown in Table 2. The powder mixture was then uniaxially pressed in a metal die at 150 MPa to yield cylindrical pellets of height 5 mm and diameter 12.8 mm. These pellets were sintered in a Ceramic Engineering high temperature furnace in air at 16008C for 3 h and then furnace-cooled to room temperature. 2.2. Materials characterisation Analysis of phases formed and their abundance was performed with a Siemens D500 X-ray diffractometer. The operating conditions used were: Cu-K a radiation Ž l s 0.15418 nm. produced at 40 kV and 0.158 receiving slit, goniometer range s 15–608, step size s 0.06, counting time 1 srstep, and post-diffraction graphite monochromator with NaI detector and PHA. The approximate abundance of crystalline phases present was computed from their relative peak intensity ratios. The content of glassy phase

Table 2 Formulations of various spodumene-modified aluminium titanate ceramics Sample

Alumina Žwt.%. Titania Žwt.%. Spodumene Žwt.%.

ATR0 ATR2.5 ATR5 ATR10 ATR15 ATR30

56 54.5 53 50 47 39

44 43 42 40 38 31

0 2.5 5 10 15 30

120

C.G. Shi, I.M. Low r Materials Letters 36 (1998) 118–122

was estimated from the assumption that the total content of sillimanite and glass should equal the amount of spodumene added. DTA measurements on the powder mixture were carried out on a Netzsch STA-409C instrument in nitrogen atmosphere at a heating rate of 208Crmin with empty reference. The water displacement method was use to measure the apparent porosity and bulk density of the sintered samples which had been boiled in water for 2 h, followed by 24 h soaking. The hardness of sintered and polished samples Ž1 m finish. was measured using a Rockwell Hardness tester ŽScale H, 60 Kg f ..

3. Results and discussion 3.1. Phase relations Aluminium titanate ŽAl 2TiO5 . was the only phase observed for samples ATR0 and ATR2.5, and the major phase for all other samples. For samples containing 5 wt.% or more spodumene, moderate amount of glassy and sillimanite phases formed. Some traces of lithium aluminium silicate ŽLAS. w18x, LiAlŽSiO 3 . 2 were also observed. The X-ray diffractograms of AT containing 2.5–15 wt.% spodumene are shown in Fig. 1. The formation of sillimanite is clearly evident for spodumene content greater than 2.5 wt.%. The

Table 3 Approximate phase abundances of various spodumene-modified AT ceramics Sample

AT Žwt.%. Sillimanite Žwt.%. Glassy phase Žwt.%.

ATR0 100 ATR2.5 100 ATR5 95 ATR10 90 ATR15 85 ATR30 70

0 ;0 4.5 8 10 22

0 0 0.5 2 5 8

approximate percentage phase compositions Žexcluding LAS. of the range of spodumene-modified aluminium titanates obtained are shown in Table 3. The yield of sillimanite and glassy phases was found to increase with the addition of spodumene with an associated decrease in the abundance of AT. The formation of AT in the samples is well established to occur via an endothermic reaction between Al 2 O 3 and TiO 2 at approximately 13858C w3–5x:

a-Al 2 O 3 q TiO 2 ™ b-Al 2TiO5

Ž 1.

The thermogram of sample ATR15, recorded in the temperature range 20–15008C is shown in Fig. 2. The endotherm peak at 13858C is a result of AT formation and the shoulder-endotherm at 13508C is due to the melting of spodumene. The latter represents a shift of 738C from the melting point of pure b-spodumene Ž14238C., suggesting that there is an eutectic formed between spodumene and AT. The exothermic reaction at approximately 9508C is believed to arise from the reaction between alumina and spodumene to form sillimanite: Li 2 O P Al 2 O 3 P 4SiO 2 q 3Al 2 O 3 ™ Li 2 O q 4Al 2 SiO5 Ž 2. The lithia would either form as a glassy phase or form an interstitial solid solution with AT. Similarly, the formation of LAS is thought to derive from excess spodumene and occur via the following reaction:

Fig. 1. XRD plots of sintered aluminium titanate samples containing various amounts of spodumene: Ža. 2.5 wt.%, Žb. 5 wt.%, and Žc. 15 wt.%. Note the formation of sillimanite Žv . as a second phase. The apparent shift in the d-spacing of AT peaks is probably due to a solid solution effect.

Li 2 O P Al 2 O 3 P 4SiO 2 ™ 2LiAl Ž SiO 3 . 2

Ž 3.

The presence of sillimanite, lithia–glass, and LAS will not expect to cause much degradation on the

C.G. Shi, I.M. Low r Materials Letters 36 (1998) 118–122

121

Fig. 2. Thermogram of sample ATR15.

thermomechanical properties of AT because they also have relatively good strength and thermal shock properties w13,19–21x. Indeed, the presence of spodumene has found to reduce the thermal expansion coefficient w16,17,22x and thermal shock resistance w23x of mullite ceramics. 3.2. Physical and mechanical properties The shrinkage, apparent porosity and density results for the samples are shown in Figs. 3–5. The

Fig. 4. Porosity of sintered spodumene-modified aluminium titanate ceramics.

Fig. 3. Shrinkage of sintered spodumene-modified aluminium titanate ceramics.

Fig. 5. Density of sintered spodumene-modified aluminium titanate ceramics.

122

C.G. Shi, I.M. Low r Materials Letters 36 (1998) 118–122

Acknowledgements The authors thank Mr. G. Ferguson of SCM Chemicals ŽAustralia. for supplying the titania powder. Mr. R.D. Skala and Mr. I. Sills provided valuable technical assistance on XRD and DTA, respectively.

References Fig. 6. Rockwell hardness of sintered spodumene-modified aluminium titanate ceramics.

shrinkage increased with increased spodumene addition due to the increased levels of liquid phase. The low porosity obtained for the spodumene-modified AT results from the spodumene liquid phase serving to enhance sintering and densification. The bulk density of the products increased to a maximum value of 3.23 g cmy3 at 2.5 wt.% spodumene and decreased thereafter for higher spodumene contents. This was consistent with the results of spodumenemodified mullites by Low et al. w16x and Maity et al. w21x who found that additions of 20–90 wt.% spodumene resulted in decreased density due to the formation of 5–10 m m pores caused by the excess liquid phase, and the less dense spodumene phase. The hardness results for the samples are shown in Fig. 6. In contrast to spodumene-modified mullites w15–17x, the presence of spodumene caused a considerable improvement in hardness of AT due to enhanced densification, especially for the reactionsintered samples. The hardness increased steadily and reached a maximum value of 112 HRH at 15 wt.% spodumene.

4. Conclusion Spodumene is an excellent additive for liquidphase-sintering of AT ceramics. Sillimanite, glassy and LAS phases also formed in AT ceramics containing greater than 2.5 wt.% spodumene. The presence of up to 15 wt.% spodumene improved the sintering kinetics and thus the densification of AT. It also imparted better physical and mechanical properties.

w1x V. Buscaglia, P. Nanni, G. Battilana, G. Aliprandi, C. Carry, J. Eur. Ceram. Soc. 13 Ž1994. 411. w2x M.S.J. Gani, R. McPherson, Thermochim. Acta 7 Ž1973. 251. w3x B.N. Battacharyya, S. Sen, Glass Ceram. Bull. ŽIndia. 12 Ž1965. 92. w4x A. Feltz, F. Schmidt, J. Eur. Ceram. Soc. 6 Ž1990. 107. w5x H.A.J. Thomas, R. Stevens, Br. Ceram. Trans. 88 Ž1989. 144. w6x H.A.J. Thomas, R. Stevens, E. Gilbart, J. Mater. Sci. 26 Ž1991. 3613. w7x G. Tilloca, J. Mater. Sci. 26 Ž1991. 2809. w8x G. Battilana, V. Buscaglia, P. Nanni, G. Aliprandi, in: P. Vincenzini ŽEd.., High Performance Materials in Engine Technology, Techna, 1995, pp. 147–154. w9x M. Ishitsuka, T. Sato, T. Endo, M. Shimada, J. Am. Ceram. Soc. 70 Ž1987. 69. w10x H. Morishima, Z. Kato, K. Uematsu, J. Am. Ceram. Soc. 69 Ž1986. C226. w11x C.A. Cowan, G.A. Bole, R.L. Stone, J. Am. Ceram. Soc. 33 Ž1950. 193. w12x J. Kusnik, K.W. Terry, Mater. Sci. Forum 34–36 Ž1988. 931. w13x J.H. Fishwick, Applications of Lithium in Ceramics, Cahners Books, Boston, MA, 1974. w14x B.L. Latella, G.R. Burton, B.H. O’Connor, J. Am. Ceram. Soc. 78 Ž1995. 1895. w15x I.M. Low, T.D. Garrod, D. Zhou, D.N. Phillips, Z. Pillai, J. Mater. Sci. 32 Ž1997. 3812. w16x I.M. Low, P.M. Suherman, D.N. Phillips, J. Mater. Sci. Lett. 16 Ž1997. 982. w17x I.M. Low, T. Garrod, D.N. Phillips, Z. Pillai, Trans. Indian Ceram. Soc. 55 Ž1996. 27. w18x C. Li, Acta Crystallogr. 27 Ž1971. 1132. w19x S.B. Roy, S.K. Chakravorty, Trans. Indian Ceram. Soc. 29 Ž1970. 134. w20x S. Maity, B.K. Sarkar, J. Eur. Ceram. Soc. 16 Ž1996. 1083. w21x S. Maity, T.K. Mukhopadhyay, B.K. Sarkar, J. Eur. Ceram. Soc. 17 Ž1997. 749. w22x H. Kobayashi, N. Ishibashi, T. Akiba, T. Mitamura, J. Ceram. Soc. Jpn. 96 Ž1990. 1023. w23x K. Abdullah, Third Year Physics Project 392 Report, Department of Applied Physics, Curtin University of Technology, Perth, Australia, No. UGr296, 1996.