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Role of particle size of alumina on the formation of aluminium titanate as well as on sintering and microstructure development in sol–gel alumina–aluminium titanate composites
Materials Chemistry and Physics 124 (2010) 92–96
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Role of particle size of alumina on the formation of aluminium titanate as well as on sintering and microstructure development in sol–gel alumina–aluminium titanate composites M. Jayasankar, K.P. Hima, S. Ananthakumar, P. Mukundan, P. Krishna Pillai, K.G.K. Warrier ∗ Materials and Minerals Division, National Institute for Interdisciplinary Science & Technology (NIIST) (Formerly Regional Research Laboratory (CSIR)), Thiruvananthapuram, India
a r t i c l e
i n f o
Article history: Received 27 May 2009 Received in revised form 24 March 2010 Accepted 26 May 2010 Keywords: Ceramics Sol–gel Surface Thermal expansion
a b s t r a c t With a view to develop low temperature fine grained alumina–aluminium titanate composite, influence of alumina particle size on the temperature of formation of the aluminium titanate, sintering behaviour and microstructure development of alumina–aluminium titanate composite prepared through a sol–gel core shell approach is reported. The alumina matrix composite containing 20 wt% aluminium titanate has been prepared from alumina powders having different average particle size in the range 300–600 nm. The alumina particle size appears to have no significant influence on the formation temperature of in situ formed aluminium titanate. However, the microstructural analysis of the dense ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increase in the alumina particle size. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. Smaller starting alumina particle size led to finer grain size composites. The present study therefore shows that although the starting particle size of alumina has no significant role on the lowering of formation temperature of aluminium titanate, it does influence the microstructure of the composite. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Aluminium titanate (AT) has drawn much attention as a high temperature material due to low thermal expansion coefficient and high thermal shock resistance [1]. Aluminium titanate exhibits two allotropic forms such as ␣ and , where  aluminium titanate is the stable phase [2]. Aluminium titanate is usually formed by the solidstate reaction between alumina and titania above the eutectoid temperature 1280 ◦ C [3]. Aluminium titanate exhibits microcracking during cooling from the sintering temperature especially above critical sintered grain size of 1.5 m bringing out the necessity for obtaining fine-grained microstructure [4]. Alumina is widely employed in various applications because of its high temperature stability, chemical inertness and wear resistance. However, alumina has poor thermal shock resistance. Earlier reports suggest addition of aluminium titanate particles of controlled size for improving the thermo-mechanical response of alumina–aluminium titanate composite [5,6]. Improved flaw tolerance has been observed as a result of addition of AT due to induced residual stresses by virtue of thermal expansion mismatch between alumina and aluminium titanate [7]. The
∗ Corresponding author. Tel.: +91 471 2515289; fax: +91 471 491712. E-mail address: [email protected] (K.G.K. Warrier). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.072
alumina–aluminium titanate composites exhibit functional as well as structural properties for application as thermal barrier coating, insulating components for diesel engines and as high temperature substrates [8]. Further improvement in mechanical properties of alumina–aluminium titanate composite is attributed to the combined effect of internal stresses developed due to AT and large particle size of alumina [9]. Padture et al. reported that the alumina–aluminium titanate composite with a uniform distribution of large alumina particles results in excellent flaw tolerance and steady-state toughness [7]. Thus the particle size of alumina appears to play an important role along with that of aluminium titanate in the development of the fine grained microstructure for obtaining better mechanical properties in alumina–aluminium titanate composite. The control of grain size of alumina by a novel synthesis route where the alumina particle was coated by titania particle followed by formation of aluminium titanate phase in situ getting distributed among alumina grains is reported recently [10]. However, the effect of starting particle size of alumina on the sintered microstructure of the composite has not been well investigated. The possibility of controlling the aluminium titanate grain size below the critical grain size through controlling the alumina particle size has therefore been investigated. In this work we report sintering and the microstructure development of alumina–aluminium titanate composite (Al–AT) starting from three different alumina particle sizes through a sol–gel core-
M. Jayasankar et al. / Materials Chemistry and Physics 124 (2010) 92–96
shell approach in order to obtain information on the influence of size of alumina on the formation temperature of aluminium titanate and the resultant microstructure of the composite. 2. Experimental Titanium tetrachloride (99%purity M/s Kerala Minerals and Metals Ltd., Kollam India) and A16 SG alumina powder (99%purity ACC-ALCOA Chemicals, India, Av. Particle size 0.45 m) were used as precursors for the synthesis of alumina–aluminium titanate composite. In a typical experiment, titanium tetra chloride was dissolved in ice cold distilled water (0.2 M) and was hydrolysed by slow addition of ammonium hydroxide (10% S.D. Fine Chemicals, India), solution under constant stirring at room temperature, until the reaction mixture attained pH = 9.0 [11]. The precipitate was separated by filtration and was washed free of chloride ions (as was confirmed by AgCl test) with hot distilled water. The precipitate (5 g) was further dispersed in 1000 ml water and was peptised by addition of 10% HNO3 (Merck, India) solution. The titania sol of known concentration was used to coat on alumina kept in suspension, which was prepared by dispersing alumina powder (40 g) in 1000 ml of water. Stirring was continued for 30 min and the additional titania required for aluminium titanate was added in the form of titanium hydroxide precipitate (50.04 g) [12]. The whole mixture was then peptised to a stable suspension by further addition of 10% HNO3 (Merck, India). The sol was then flocculated by addition of ammonium hydroxide (10% S.D. Fine Chemicals, India), at a controlled pH = 6.5 which was further dried in an oven at 70 ◦ C. Alumina–aluminium titanate precursor was thus prepared. The experiment was repeated using alumina powders having average particle size of 300 nm, 450 nm and 600 nm, respectively (represented as Al–20%AT0.30, Al–20%AT0.45 and Al–20%AT0.60, where Al – alumina, AT – aluminium titanate). The powders were weighed and ball milled in a polyethylene mill bottle with zirconia balls for 72 h, using distilled water as the mixing medium. The sintering of alumina–aluminium titanate precursor compact was carried out at a temperature of 1350 ◦ C with 3 h soaking time. The typical sintering schedule was as follows: RT (room temperature) to 800 ◦ C at a rate of 3 K min−1 , 800–1200 ◦ C at a rate of 5 K min−1 and then up to maximum sintering temperatures at 10 K min−1 . The relative density of the sintered specimen was calculated by rule of mixtures and presented in Table 2. The theoretical density of alumina and aluminium titanate was taken as 3.90 g cm−3 and 3.70 g cm−3 , respectively. 2.1. Characterization The coating of titania on alumina particle was observed under Transmission Electron Microscope (FEI, TECNAI 30 S-Twin (Netherlands)) operated at 300 kV and equipped with an energy-dispersive X-ray analyzer (EDX). The powder was dispersed in acetone and a drop of the suspension was deposited on a carbon-coated copper grid (TEM) and dried. The dried precursor gels were subjected to DTA analysis (Shimadzu, DTA-50H) at a heating rate of 5 K min−1 up to 1400 ◦ C. The dried gels were individually heated at 1350 ◦ C in electric muffle furnace and then cooled in air to room temperature. The phase identification was done using XRD (Philips PW 1170) in the 2Â range of 20–60 CuK␣. Cylindrical pellet of size 6 mm diameter and 8 mm height made by uniaxial compaction (200 MPa) was used for the dilatometric studies by thermo mechanical analyzer (TMA-60H, Shimadzu, Japan) at a heating rate of 5 K min−1 up to 1400 ◦ C. The precursor was calcined at 900 ◦ C compacted uniaxially at 200 MPa using 2 wt% PVA as a binder and was sintered in the range 1350–1450 ◦ C. The particle size analysis was carried out using Laser Particle Size Analyzer on the powders calcined at 1350 ◦ C (Zeta Sizer, Malvern Instruments UK). The sample for the particle size analysis was made by ultrasonic dispersion of the respective powders (1 g) in aqueous medium (100 ml) at pH <2.0. Average of three measurements within a standard deviation of 3.0% is reported. Morphological characterization of the sintered pellets was performed using scanning electron micrograph (SEM, JEOL, JSM 5600 LV, Japan) on samples after polishing and thermally etching at 1325 ◦ C. The average grain size was calculated by the linear intercept method using the relation G = 1.5L/MN where 1.5 is a geometry dependent proportionality constant, L is the total test line length, M the magnification and N the total number of intercepts. For the grain size determination four representative micrographs with the magnification of 4000× were analyzed. A test line of 9 cm length was marked on the micrograph and the number of intersections of the line with grain boundaries (at least 10 no of grains) was counted [13].
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Table 1 Particle size of alumina. Material
Particle size (nm)
Purity (%)
Source
␣-Alumina ␣-Alumina ␣-Alumina
450 300 600
99.5 99.5 99.5
ACC-Alcoa (ASG-16) Ballmilled for 72 h Calcined at 900 ◦ C/3 h
Fig. 1. Particle size distribution of the as-received, ball milled and calcined alumina powder.
age particle size increases from 450 nm to 600 nm. The particle size measurements of the as received, ball milled and calcined aluminas are shown in Table 1 and the size distribution is presented in Fig. 1. The coating on alumina particle with titania sol was performed by a sol–gel method as reported earlier [10]. The titania coating on alumina particle was observed under Transmission electron microscope (TEM) on composite precursor sample after calcining at 1000 ◦ C (Fig. 2). The alumina particle was uniformly coated with the nanotitania with an average coating thickness of 24 nm. The low temperature in situ formation of aluminium titanate can be attributed to high contact area available due to the novel core–shell approach.
3. Results and discussion 3.1. Powder characterization Initial particle size of alumina which was used for the preparation of the composite, was in the range of 400–450 nm as was measured by laser particle size analyser. On further ball milling for 72 h, the average particle size of alumina decreases from 450 nm to 300 nm. On calcination of as received alumina powder, the aver-
Fig. 2. TEM image of titania coated alumina particle.
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Fig. 3. Particle size distribution of sol–gel coated Alumina–20%AT0.45 at 80 ◦ C and 1000 ◦ C.
The mechanism of coating in the present synthesis proceeds by already reported by the authors [10]. The particle size analysis of alumina–aluminium titanate composite precursor dried at 80 ◦ C shows an average particle size of 380 nm which was increased to 450 nm at 1000 ◦ C (Fig. 3). It is interesting to note that the distribution is monomodal and this indicates the extent of uniformity in coating.
Fig. 4(a–c) shows the differential thermal analysis (DTA) of the alumina–aluminium composite precursors. The endothermic peak at 80 ◦ C corresponds to the decomposition of physically absorbed water. A peak at 300 ◦ C attributes to the evolution of nitrates [14]. An endothermic peak at 1300 ◦ C shows the formation of aluminium titanate in the composite precursor (A–20%AT). From the DTA analysis we can conclude that the alumina particle size has no influence on the formation temperature of aluminium titanate in the composite. The formation of aluminium titanate was confirmed by the XRD analysis. The low temperature of formation aluminium titanate is attributed to the high area of contact between the fine alumina and titania particles under the present condition of coating where a diffusion rate is favoured. The shrinkage behaviour of A–20%AT0.45 composite is also conducted. The shrinkage observed at 1100 ◦ C is due to the sintering which is initiated in presence of rutile which is formed as a result of transformation of anatase phase and this further results in near completion at 1350 ◦ C. The formation rutile phase at 1100 ◦ C is also confirmed by XRD analysis and presented in Fig. 5. There is a small peak at 1300 ◦ C which may be due to the formation of aluminium titanate in situ in the alumina matrix. The densification of the specimen proceeds faster during the aluminium titanate formation but was retarded after the formation of aluminium titanate. 3.2. XRD characterization XRD analysis was used to identify different phases in the composite precursor powders (Fig. 6) calcined at 1300 ◦ C over a period of 3 h. The composite calcined at 1300 ◦ C shows the presence of aluminium titanate and alumina in the XRD while there is no evidence for any unreacted titania. This situation does not happen in the
Fig. 4. (a) DTA analysis of Al–20%AT0.45. (b) DTA analysis of Al–20%AT0.60. (c) DTA analysis of Al–20%AT0.30.
M. Jayasankar et al. / Materials Chemistry and Physics 124 (2010) 92–96
Fig. 5. XRD of alumina–aluminium titanate composite precursor (Al–20%AT) calcined at 1100 ◦ C/3 h. Alumina (䊉) and rutile (). Table 2 Sintered densities of alumina–aluminium titanate composites at 1350 ◦ C. Particle size of alumina (nm)
Material designation
Archimedes density (g cm−3 )
Relative density (% theoretical density)
300 450 600
Al–20%AT0.30 Al–20%AT0.45 Al–20%AT0.60
3.81 3.79 3.78
98.7 ± 0.2% 98.4 ± 0.2% 98.2 ± 0.2%
95
Fig. 6. XRD pattern of alumina and composite precursors calcined at 1300 ◦ C. (a) Alumina, (b) Al–20%AT0.45, (c) Al–20%AT0.60, and (d) Al–20%AT0.30. Aluminium titanate () and alumina (䊉).
contact points among the particles and the higher fractions of fine particles resulting in shorter diffusion paths. This in turn increases the average coordination number of particles, leading to enhanced sintering [16]. 3.4. SEM and particle size analysis
solid-state route of preparing aluminium titanate [15] as revealed from literature. 3.3. Sintering studies The as received alumina sample shows 98% sintered density at 1550 ◦ C but the composite shows >98% at 1350 ◦ C. This was an indication that alumina can be sintered at low temperature in presence of a second phase like aluminium titanate. The pure ␣-alumina can be nearly fully densified at 1550 ◦ C whereas the composite attains the sintered density >98% at 1350 ◦ C. The results are shown in Table 2. This can be attributed to a large number of
Alumina–aluminium titanate fabricated by reaction sintering shows low sintered densities, abnormal grain growth and even unreacted titania particles due to the special characteristics of the alumina–titania reaction process [17]. Bartolome et al. reported the in situ formation of alumina–10% aluminium titanate composite starting from mixture of alumina and titania powders, achieved sintered composite at 1600 ◦ C. The average grain size of the composite following the solid state mixing route and calculated using linear intercept method was 5 m at 1600 ◦ C [15]. The equimolar mixture of alumina and titania blended with different percentage (1–10 wt%) of equal mixture (1:1) FeTiO3 and Fe2 O3 also indicates
Fig. 7. Scanning electron micrographs of facture surface. (a) Alumina at 1550 ◦ C polished and thermally etched sintered surface of the materials, (b) Al–20%AT0.30 at 1350 ◦ C, (c) Al–20%AT0.45 at 1350 ◦ C, and (d) Al–20%AT0.60 at 1350 ◦ C.
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in the precoated alumina particles get converted to rutile at 1200 ◦ C and get distributed at the grain boundaries of alumina which could initiate the sintering process. However, aluminium titanate once formed may preclude further densification of the composite. 4. Conclusion
Fig. 8. Particle size measurement Al–20%AT precursor calcined at 1350 ◦ C.
certain unreacted Al2 O3 [18]. Park et al. reported the effect of starting powder on Al2 TiO5 morphology and grain growth of Al2 O3 was investigated in alumina–aluminum titanate composites using Al2 O3 , TiO2 and calcined Al2 TiO5 as starting powders. Depending on the composition of starting powders, various Al2 TiO5 morphologies, such as rod-like, polyhedron-like, and irregular shape were observed. When Al2 O3 and TiO2 were used as starting powder, the rod-like shape and the irregular shape of Al2 TiO5 were observed. The average grain size of alumina increases with increase in the content of aluminium titanate when it was prepared from Al2 O3 and TiO2 mixture as starting material. All these studies indicate the importance of grain size and morphologies of alumina–aluminium titanate composite [19]. Recently microcrack free alumina–aluminium titanate composite was prepared by colloidal filtration method using alumina and titania, which was sintered at 1450 ◦ C. But the grain size was under 5 m for 10% addition of aluminium titanate [20]. Alumina powder covered by titanium isopropoxide precursor followed by heat treatment at 1300 ◦ C resulted in high density in alumina 20 vol.% of aluminium titanate. However, composites with larger second phase contents (20–40 vol.%) gave lower densities (95% of theoretical) [21]. Microstructural analysis of the present composite shows that the aluminium titanate grains are well distributed between alumina grains. All the samples were sintered at 1350 ◦ C with 3 h soaking time. Fig. 7(a) shows the SEM of fractured alumina sintered at 1550 ◦ C. Fig. 7(b)–(d) shows the SEM of sintered and thermally etched surfaces of Al–20%AT0.30, Al–20%AT0.45 and Al–20%AT0.60, respectively. Al–20%AT0.30 shows fine grain structure here the average grain size was less than 1 m which can be supported by the particle size measurement of the powder sample calcined at 1350 ◦ C (920 nm in Fig. 8). The coating approach followed in the present work minimises grain growth and using fine-grained Al2 O3 , there is an increased effect of grain growth inhibition. Similarly the Al–20%AT0.45 and Al–20%AT0.60 composites show average grain size of greater than 1 m and 1.5 m, respectively. These results are comparable with the particle size measurements (Fig. 8). The titania
Nanosize alumina–aluminium titanate composite was achieved through a titania coated alumina particle precursor. The starting alumina particle size has no significant influence on the formation temperature of aluminium titanate in the composite. XRD analysis confirms the formation of aluminium titanate in the Al–20%AT composite sintered at 1350 ◦ C and having >98% sintered density. SEM photograph of sintered composite shows that the average grain size of the composite varied with respect to the initial particle size of alumina. The sintered grain size of alumina–aluminium titanate composite prepared starting from ball milled alumina powder shows lower grain size. The final microstructure of the ceramic is determined by the starting alumina powder. The present study reveals information on an effective engineering and design of the alumina–aluminium titanate composite with desired microstructure. The proposed method may provide a new avenue for fabricating Al–AT composite with improved thermal properties. Acknowledgements The authors acknowledge the members of Materials and Minerals division for discussion on the results obtained. One of the authors, M. Jayasankar, is grateful to Department of Science and Technology (DST) and CSIR, India, for providing financial support and research fellowship, respectively. References [1] H.A.J. Thomas, R. Stevens, Br. Ceram. Trans. J. 88 (1989) 144. [2] A.V. Prasadarao, U. Selvaraj, S. Komarneni, A.S. Bhalla, R. Roy, J. Am. Ceram. Soc. 75 (1992) 1529. [3] E. Kato, K. Daimon, J. Takahashi, J. Am. Ceram. Soc. 63 (1980) 355. [4] Y. Ohya, Z. Nakagawa, K. Hamano, J. Am. Ceram. Soc. 70 (1987) 184. [5] J.L. Ruryan, S.J. Bennison, J. Eur. Ceram. Soc. 7 (1991) 93. [6] R. Uribe, C. Baudin, J. Am. Ceram. Soc. 86 (2003) 846. [7] N.P. Padture, S.J. Bermison, H.M. Chan, J. Am. Ceram. Soc. 76 (1993) 2312. [8] H. Morishima, Z. Kato, K. Uematsu, K. Saito, T. Yano, N. Ootsuka, J. Mater. Sci. Lett. 6 (1987) 389. [9] S. Pratapa, I.M. Low, J. Mater. Sci. 33 (1998) 3047. [10] M. Jayasankar, S. Ananthakumar, P. Mukundan, W. Wunderlich, K.G.K. Warrier, J. Solid State Chem. 181 (2008) 2748. [11] M. Jayasankar, S. Ananthakumar, P. Mukundan, K.G.K. Warrier, Mater. Lett. 61 (2007) 790. [12] M. Jayasankar, S. Ananthakumar, P. Mukundan, K.G.K. Warrier, J. Am. Ceram. Soc. 90 (2007) 3091. [13] U.S. Hareesh, M. Sternitzke, R. Janssen, N. Claussen, K.G.K. Warrier, J. Am. Ceram. Soc. 84 (2004) 1024. [14] A.K. Vasudevan, T.V. Mani, A.D. Damodaran, K.G.K. Warrier, J. Mater. Sci. 14 (1995) 1317. [15] J. Barolome, J. Requena, J.S. Moya, M. Li, F. Guiu, Acta Mater. 44 (1996) 1361. [16] L.C. Lim, J. Ma, P.M. Wong, Acta Mater. 48 (2000) 2263. [17] J. Barolome, J. Requena, J.S. Moya, M. Li, F. Guiu, Fatigue Fract. Eng. Mater. Struct. 20 (1997) 789. [18] I.B. de Arenas, O. Gil, J. Mater. Process. Technol. 143/144 (2003) 838. [19] S.-Y. Park, S.-W. Jung, Y.-B. Chung, Ceram. Int. 29 (2003) 707. [20] S. Bueno, R. Moreno, C. Baudin, J. Eur. Ceram. Soc. 21 (2003) 71. [21] H.M. Okamura, E.A. Barringer, H.K. Bowen, J. Mater. Sci. 24 (1989) 1867.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Role of particle size of alumina on the formation of aluminium titanate as well as on sintering and microstructure development in sol–gel alumina–aluminium titanate composites M. Jayasankar, K.P. Hima, S. Ananthakumar, P. Mukundan, P. Krishna Pillai, K.G.K. Warrier ∗ Materials and Minerals Division, National Institute for Interdisciplinary Science & Technology (NIIST) (Formerly Regional Research Laboratory (CSIR)), Thiruvananthapuram, India
a r t i c l e
i n f o
Article history: Received 27 May 2009 Received in revised form 24 March 2010 Accepted 26 May 2010 Keywords: Ceramics Sol–gel Surface Thermal expansion
a b s t r a c t With a view to develop low temperature fine grained alumina–aluminium titanate composite, influence of alumina particle size on the temperature of formation of the aluminium titanate, sintering behaviour and microstructure development of alumina–aluminium titanate composite prepared through a sol–gel core shell approach is reported. The alumina matrix composite containing 20 wt% aluminium titanate has been prepared from alumina powders having different average particle size in the range 300–600 nm. The alumina particle size appears to have no significant influence on the formation temperature of in situ formed aluminium titanate. However, the microstructural analysis of the dense ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increase in the alumina particle size. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. Smaller starting alumina particle size led to finer grain size composites. The present study therefore shows that although the starting particle size of alumina has no significant role on the lowering of formation temperature of aluminium titanate, it does influence the microstructure of the composite. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Aluminium titanate (AT) has drawn much attention as a high temperature material due to low thermal expansion coefficient and high thermal shock resistance [1]. Aluminium titanate exhibits two allotropic forms such as ␣ and , where  aluminium titanate is the stable phase [2]. Aluminium titanate is usually formed by the solidstate reaction between alumina and titania above the eutectoid temperature 1280 ◦ C [3]. Aluminium titanate exhibits microcracking during cooling from the sintering temperature especially above critical sintered grain size of 1.5 m bringing out the necessity for obtaining fine-grained microstructure [4]. Alumina is widely employed in various applications because of its high temperature stability, chemical inertness and wear resistance. However, alumina has poor thermal shock resistance. Earlier reports suggest addition of aluminium titanate particles of controlled size for improving the thermo-mechanical response of alumina–aluminium titanate composite [5,6]. Improved flaw tolerance has been observed as a result of addition of AT due to induced residual stresses by virtue of thermal expansion mismatch between alumina and aluminium titanate [7]. The
∗ Corresponding author. Tel.: +91 471 2515289; fax: +91 471 491712. E-mail address: [email protected] (K.G.K. Warrier). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.072
alumina–aluminium titanate composites exhibit functional as well as structural properties for application as thermal barrier coating, insulating components for diesel engines and as high temperature substrates [8]. Further improvement in mechanical properties of alumina–aluminium titanate composite is attributed to the combined effect of internal stresses developed due to AT and large particle size of alumina [9]. Padture et al. reported that the alumina–aluminium titanate composite with a uniform distribution of large alumina particles results in excellent flaw tolerance and steady-state toughness [7]. Thus the particle size of alumina appears to play an important role along with that of aluminium titanate in the development of the fine grained microstructure for obtaining better mechanical properties in alumina–aluminium titanate composite. The control of grain size of alumina by a novel synthesis route where the alumina particle was coated by titania particle followed by formation of aluminium titanate phase in situ getting distributed among alumina grains is reported recently [10]. However, the effect of starting particle size of alumina on the sintered microstructure of the composite has not been well investigated. The possibility of controlling the aluminium titanate grain size below the critical grain size through controlling the alumina particle size has therefore been investigated. In this work we report sintering and the microstructure development of alumina–aluminium titanate composite (Al–AT) starting from three different alumina particle sizes through a sol–gel core-
M. Jayasankar et al. / Materials Chemistry and Physics 124 (2010) 92–96
shell approach in order to obtain information on the influence of size of alumina on the formation temperature of aluminium titanate and the resultant microstructure of the composite. 2. Experimental Titanium tetrachloride (99%purity M/s Kerala Minerals and Metals Ltd., Kollam India) and A16 SG alumina powder (99%purity ACC-ALCOA Chemicals, India, Av. Particle size 0.45 m) were used as precursors for the synthesis of alumina–aluminium titanate composite. In a typical experiment, titanium tetra chloride was dissolved in ice cold distilled water (0.2 M) and was hydrolysed by slow addition of ammonium hydroxide (10% S.D. Fine Chemicals, India), solution under constant stirring at room temperature, until the reaction mixture attained pH = 9.0 [11]. The precipitate was separated by filtration and was washed free of chloride ions (as was confirmed by AgCl test) with hot distilled water. The precipitate (5 g) was further dispersed in 1000 ml water and was peptised by addition of 10% HNO3 (Merck, India) solution. The titania sol of known concentration was used to coat on alumina kept in suspension, which was prepared by dispersing alumina powder (40 g) in 1000 ml of water. Stirring was continued for 30 min and the additional titania required for aluminium titanate was added in the form of titanium hydroxide precipitate (50.04 g) [12]. The whole mixture was then peptised to a stable suspension by further addition of 10% HNO3 (Merck, India). The sol was then flocculated by addition of ammonium hydroxide (10% S.D. Fine Chemicals, India), at a controlled pH = 6.5 which was further dried in an oven at 70 ◦ C. Alumina–aluminium titanate precursor was thus prepared. The experiment was repeated using alumina powders having average particle size of 300 nm, 450 nm and 600 nm, respectively (represented as Al–20%AT0.30, Al–20%AT0.45 and Al–20%AT0.60, where Al – alumina, AT – aluminium titanate). The powders were weighed and ball milled in a polyethylene mill bottle with zirconia balls for 72 h, using distilled water as the mixing medium. The sintering of alumina–aluminium titanate precursor compact was carried out at a temperature of 1350 ◦ C with 3 h soaking time. The typical sintering schedule was as follows: RT (room temperature) to 800 ◦ C at a rate of 3 K min−1 , 800–1200 ◦ C at a rate of 5 K min−1 and then up to maximum sintering temperatures at 10 K min−1 . The relative density of the sintered specimen was calculated by rule of mixtures and presented in Table 2. The theoretical density of alumina and aluminium titanate was taken as 3.90 g cm−3 and 3.70 g cm−3 , respectively. 2.1. Characterization The coating of titania on alumina particle was observed under Transmission Electron Microscope (FEI, TECNAI 30 S-Twin (Netherlands)) operated at 300 kV and equipped with an energy-dispersive X-ray analyzer (EDX). The powder was dispersed in acetone and a drop of the suspension was deposited on a carbon-coated copper grid (TEM) and dried. The dried precursor gels were subjected to DTA analysis (Shimadzu, DTA-50H) at a heating rate of 5 K min−1 up to 1400 ◦ C. The dried gels were individually heated at 1350 ◦ C in electric muffle furnace and then cooled in air to room temperature. The phase identification was done using XRD (Philips PW 1170) in the 2Â range of 20–60 CuK␣. Cylindrical pellet of size 6 mm diameter and 8 mm height made by uniaxial compaction (200 MPa) was used for the dilatometric studies by thermo mechanical analyzer (TMA-60H, Shimadzu, Japan) at a heating rate of 5 K min−1 up to 1400 ◦ C. The precursor was calcined at 900 ◦ C compacted uniaxially at 200 MPa using 2 wt% PVA as a binder and was sintered in the range 1350–1450 ◦ C. The particle size analysis was carried out using Laser Particle Size Analyzer on the powders calcined at 1350 ◦ C (Zeta Sizer, Malvern Instruments UK). The sample for the particle size analysis was made by ultrasonic dispersion of the respective powders (1 g) in aqueous medium (100 ml) at pH <2.0. Average of three measurements within a standard deviation of 3.0% is reported. Morphological characterization of the sintered pellets was performed using scanning electron micrograph (SEM, JEOL, JSM 5600 LV, Japan) on samples after polishing and thermally etching at 1325 ◦ C. The average grain size was calculated by the linear intercept method using the relation G = 1.5L/MN where 1.5 is a geometry dependent proportionality constant, L is the total test line length, M the magnification and N the total number of intercepts. For the grain size determination four representative micrographs with the magnification of 4000× were analyzed. A test line of 9 cm length was marked on the micrograph and the number of intersections of the line with grain boundaries (at least 10 no of grains) was counted [13].
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Table 1 Particle size of alumina. Material
Particle size (nm)
Purity (%)
Source
␣-Alumina ␣-Alumina ␣-Alumina
450 300 600
99.5 99.5 99.5
ACC-Alcoa (ASG-16) Ballmilled for 72 h Calcined at 900 ◦ C/3 h
Fig. 1. Particle size distribution of the as-received, ball milled and calcined alumina powder.
age particle size increases from 450 nm to 600 nm. The particle size measurements of the as received, ball milled and calcined aluminas are shown in Table 1 and the size distribution is presented in Fig. 1. The coating on alumina particle with titania sol was performed by a sol–gel method as reported earlier [10]. The titania coating on alumina particle was observed under Transmission electron microscope (TEM) on composite precursor sample after calcining at 1000 ◦ C (Fig. 2). The alumina particle was uniformly coated with the nanotitania with an average coating thickness of 24 nm. The low temperature in situ formation of aluminium titanate can be attributed to high contact area available due to the novel core–shell approach.
3. Results and discussion 3.1. Powder characterization Initial particle size of alumina which was used for the preparation of the composite, was in the range of 400–450 nm as was measured by laser particle size analyser. On further ball milling for 72 h, the average particle size of alumina decreases from 450 nm to 300 nm. On calcination of as received alumina powder, the aver-
Fig. 2. TEM image of titania coated alumina particle.
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Fig. 3. Particle size distribution of sol–gel coated Alumina–20%AT0.45 at 80 ◦ C and 1000 ◦ C.
The mechanism of coating in the present synthesis proceeds by already reported by the authors [10]. The particle size analysis of alumina–aluminium titanate composite precursor dried at 80 ◦ C shows an average particle size of 380 nm which was increased to 450 nm at 1000 ◦ C (Fig. 3). It is interesting to note that the distribution is monomodal and this indicates the extent of uniformity in coating.
Fig. 4(a–c) shows the differential thermal analysis (DTA) of the alumina–aluminium composite precursors. The endothermic peak at 80 ◦ C corresponds to the decomposition of physically absorbed water. A peak at 300 ◦ C attributes to the evolution of nitrates [14]. An endothermic peak at 1300 ◦ C shows the formation of aluminium titanate in the composite precursor (A–20%AT). From the DTA analysis we can conclude that the alumina particle size has no influence on the formation temperature of aluminium titanate in the composite. The formation of aluminium titanate was confirmed by the XRD analysis. The low temperature of formation aluminium titanate is attributed to the high area of contact between the fine alumina and titania particles under the present condition of coating where a diffusion rate is favoured. The shrinkage behaviour of A–20%AT0.45 composite is also conducted. The shrinkage observed at 1100 ◦ C is due to the sintering which is initiated in presence of rutile which is formed as a result of transformation of anatase phase and this further results in near completion at 1350 ◦ C. The formation rutile phase at 1100 ◦ C is also confirmed by XRD analysis and presented in Fig. 5. There is a small peak at 1300 ◦ C which may be due to the formation of aluminium titanate in situ in the alumina matrix. The densification of the specimen proceeds faster during the aluminium titanate formation but was retarded after the formation of aluminium titanate. 3.2. XRD characterization XRD analysis was used to identify different phases in the composite precursor powders (Fig. 6) calcined at 1300 ◦ C over a period of 3 h. The composite calcined at 1300 ◦ C shows the presence of aluminium titanate and alumina in the XRD while there is no evidence for any unreacted titania. This situation does not happen in the
Fig. 4. (a) DTA analysis of Al–20%AT0.45. (b) DTA analysis of Al–20%AT0.60. (c) DTA analysis of Al–20%AT0.30.
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Fig. 5. XRD of alumina–aluminium titanate composite precursor (Al–20%AT) calcined at 1100 ◦ C/3 h. Alumina (䊉) and rutile (). Table 2 Sintered densities of alumina–aluminium titanate composites at 1350 ◦ C. Particle size of alumina (nm)
Material designation
Archimedes density (g cm−3 )
Relative density (% theoretical density)
300 450 600
Al–20%AT0.30 Al–20%AT0.45 Al–20%AT0.60
3.81 3.79 3.78
98.7 ± 0.2% 98.4 ± 0.2% 98.2 ± 0.2%
95
Fig. 6. XRD pattern of alumina and composite precursors calcined at 1300 ◦ C. (a) Alumina, (b) Al–20%AT0.45, (c) Al–20%AT0.60, and (d) Al–20%AT0.30. Aluminium titanate () and alumina (䊉).
contact points among the particles and the higher fractions of fine particles resulting in shorter diffusion paths. This in turn increases the average coordination number of particles, leading to enhanced sintering [16]. 3.4. SEM and particle size analysis
solid-state route of preparing aluminium titanate [15] as revealed from literature. 3.3. Sintering studies The as received alumina sample shows 98% sintered density at 1550 ◦ C but the composite shows >98% at 1350 ◦ C. This was an indication that alumina can be sintered at low temperature in presence of a second phase like aluminium titanate. The pure ␣-alumina can be nearly fully densified at 1550 ◦ C whereas the composite attains the sintered density >98% at 1350 ◦ C. The results are shown in Table 2. This can be attributed to a large number of
Alumina–aluminium titanate fabricated by reaction sintering shows low sintered densities, abnormal grain growth and even unreacted titania particles due to the special characteristics of the alumina–titania reaction process [17]. Bartolome et al. reported the in situ formation of alumina–10% aluminium titanate composite starting from mixture of alumina and titania powders, achieved sintered composite at 1600 ◦ C. The average grain size of the composite following the solid state mixing route and calculated using linear intercept method was 5 m at 1600 ◦ C [15]. The equimolar mixture of alumina and titania blended with different percentage (1–10 wt%) of equal mixture (1:1) FeTiO3 and Fe2 O3 also indicates
Fig. 7. Scanning electron micrographs of facture surface. (a) Alumina at 1550 ◦ C polished and thermally etched sintered surface of the materials, (b) Al–20%AT0.30 at 1350 ◦ C, (c) Al–20%AT0.45 at 1350 ◦ C, and (d) Al–20%AT0.60 at 1350 ◦ C.
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in the precoated alumina particles get converted to rutile at 1200 ◦ C and get distributed at the grain boundaries of alumina which could initiate the sintering process. However, aluminium titanate once formed may preclude further densification of the composite. 4. Conclusion
Fig. 8. Particle size measurement Al–20%AT precursor calcined at 1350 ◦ C.
certain unreacted Al2 O3 [18]. Park et al. reported the effect of starting powder on Al2 TiO5 morphology and grain growth of Al2 O3 was investigated in alumina–aluminum titanate composites using Al2 O3 , TiO2 and calcined Al2 TiO5 as starting powders. Depending on the composition of starting powders, various Al2 TiO5 morphologies, such as rod-like, polyhedron-like, and irregular shape were observed. When Al2 O3 and TiO2 were used as starting powder, the rod-like shape and the irregular shape of Al2 TiO5 were observed. The average grain size of alumina increases with increase in the content of aluminium titanate when it was prepared from Al2 O3 and TiO2 mixture as starting material. All these studies indicate the importance of grain size and morphologies of alumina–aluminium titanate composite [19]. Recently microcrack free alumina–aluminium titanate composite was prepared by colloidal filtration method using alumina and titania, which was sintered at 1450 ◦ C. But the grain size was under 5 m for 10% addition of aluminium titanate [20]. Alumina powder covered by titanium isopropoxide precursor followed by heat treatment at 1300 ◦ C resulted in high density in alumina 20 vol.% of aluminium titanate. However, composites with larger second phase contents (20–40 vol.%) gave lower densities (95% of theoretical) [21]. Microstructural analysis of the present composite shows that the aluminium titanate grains are well distributed between alumina grains. All the samples were sintered at 1350 ◦ C with 3 h soaking time. Fig. 7(a) shows the SEM of fractured alumina sintered at 1550 ◦ C. Fig. 7(b)–(d) shows the SEM of sintered and thermally etched surfaces of Al–20%AT0.30, Al–20%AT0.45 and Al–20%AT0.60, respectively. Al–20%AT0.30 shows fine grain structure here the average grain size was less than 1 m which can be supported by the particle size measurement of the powder sample calcined at 1350 ◦ C (920 nm in Fig. 8). The coating approach followed in the present work minimises grain growth and using fine-grained Al2 O3 , there is an increased effect of grain growth inhibition. Similarly the Al–20%AT0.45 and Al–20%AT0.60 composites show average grain size of greater than 1 m and 1.5 m, respectively. These results are comparable with the particle size measurements (Fig. 8). The titania
Nanosize alumina–aluminium titanate composite was achieved through a titania coated alumina particle precursor. The starting alumina particle size has no significant influence on the formation temperature of aluminium titanate in the composite. XRD analysis confirms the formation of aluminium titanate in the Al–20%AT composite sintered at 1350 ◦ C and having >98% sintered density. SEM photograph of sintered composite shows that the average grain size of the composite varied with respect to the initial particle size of alumina. The sintered grain size of alumina–aluminium titanate composite prepared starting from ball milled alumina powder shows lower grain size. The final microstructure of the ceramic is determined by the starting alumina powder. The present study reveals information on an effective engineering and design of the alumina–aluminium titanate composite with desired microstructure. The proposed method may provide a new avenue for fabricating Al–AT composite with improved thermal properties. Acknowledgements The authors acknowledge the members of Materials and Minerals division for discussion on the results obtained. One of the authors, M. Jayasankar, is grateful to Department of Science and Technology (DST) and CSIR, India, for providing financial support and research fellowship, respectively. References [1] H.A.J. Thomas, R. Stevens, Br. Ceram. Trans. J. 88 (1989) 144. [2] A.V. Prasadarao, U. Selvaraj, S. Komarneni, A.S. Bhalla, R. Roy, J. Am. Ceram. Soc. 75 (1992) 1529. [3] E. Kato, K. Daimon, J. Takahashi, J. Am. Ceram. Soc. 63 (1980) 355. [4] Y. Ohya, Z. Nakagawa, K. Hamano, J. Am. Ceram. Soc. 70 (1987) 184. [5] J.L. Ruryan, S.J. Bennison, J. Eur. Ceram. Soc. 7 (1991) 93. [6] R. Uribe, C. Baudin, J. Am. Ceram. Soc. 86 (2003) 846. [7] N.P. Padture, S.J. Bermison, H.M. Chan, J. Am. Ceram. Soc. 76 (1993) 2312. [8] H. Morishima, Z. Kato, K. Uematsu, K. Saito, T. Yano, N. Ootsuka, J. Mater. Sci. Lett. 6 (1987) 389. [9] S. Pratapa, I.M. Low, J. Mater. Sci. 33 (1998) 3047. [10] M. Jayasankar, S. Ananthakumar, P. Mukundan, W. Wunderlich, K.G.K. Warrier, J. Solid State Chem. 181 (2008) 2748. [11] M. Jayasankar, S. Ananthakumar, P. Mukundan, K.G.K. Warrier, Mater. Lett. 61 (2007) 790. [12] M. Jayasankar, S. Ananthakumar, P. Mukundan, K.G.K. Warrier, J. Am. Ceram. Soc. 90 (2007) 3091. [13] U.S. Hareesh, M. Sternitzke, R. Janssen, N. Claussen, K.G.K. Warrier, J. Am. Ceram. Soc. 84 (2004) 1024. [14] A.K. Vasudevan, T.V. Mani, A.D. Damodaran, K.G.K. Warrier, J. Mater. Sci. 14 (1995) 1317. [15] J. Barolome, J. Requena, J.S. Moya, M. Li, F. Guiu, Acta Mater. 44 (1996) 1361. [16] L.C. Lim, J. Ma, P.M. Wong, Acta Mater. 48 (2000) 2263. [17] J. Barolome, J. Requena, J.S. Moya, M. Li, F. Guiu, Fatigue Fract. Eng. Mater. Struct. 20 (1997) 789. [18] I.B. de Arenas, O. Gil, J. Mater. Process. Technol. 143/144 (2003) 838. [19] S.-Y. Park, S.-W. Jung, Y.-B. Chung, Ceram. Int. 29 (2003) 707. [20] S. Bueno, R. Moreno, C. Baudin, J. Eur. Ceram. Soc. 21 (2003) 71. [21] H.M. Okamura, E.A. Barringer, H.K. Bowen, J. Mater. Sci. 24 (1989) 1867.