Aluminum titanate (Al2TiOs) has a potential use in thermal shock applications because of its low thermal expansion coefficient [1]. However, its low mechanical strength, caused by extensive microcracking during cooling, and tendency to decompose into component oxides A1203 and TiO 2 below 1250 °C, limit its application as a structural ceramic [21.
Some theoretical studies on aluminum titanate conclude that AI2TiO 5 would have interesting mechanical properties if it were possible to synthesize a compact free of pores and constituted of ultrafine grains [3,4]. In order to synthesize A12TiO5 ceramics having dense and well-controlled microstructures, many studies have been conducted to prepare A12TiO5 precursors through sol-gel processing using metal alkoxides [3,[5][6][7][8][9]. This is because the alkoxide sol-gel process has been expected to produce very fine crystals from the primary gel particles of 6 -20 nm [3].
In this study Al(OC3H7)3 [aluminum isopropoxide], and Ti(OC2Hs)4 [tetra ethyl orthotitanate: TEOT] were used as starting precursors for AlzTiOs.
TiO2 clear sol was prepared by hydrolysing TEOT-ethyl alcohol solution with distilled water and subsequent peptizing with HC1 [10-13].
Alumina clear sol was prepared by simultaneously hydrolysing and peptizing the aluminum isopropoxide-benzene solution with the distilled water containing HC1 at 85 °C. The two clear sols were mixed at an appropriate pH value determined through zeta-potential measurement according to DLVO theory [14] to obtain a homogeneously mixed gel before drying and calcining to an AlzO3-TiO2 nanocomposite powder.
Zeta potentiometer, X-ray diffractometer (XRD), surface area analyser, TGA/DTA, high-resolution transmission electron microscope (HRTEM), energy dispersive X-ray microanalyser (EDX) and scanning electron microscope (SEM) were used to study the powder processing.
pH values of the obtained TiO2 clear sol (TiO2. nH20) and A1203 clear sol (AlzO3.nHzO) were 1.5 and 3.3 -3.5, respectively, pH value of the mixed clear sol was about 2. The two clear sols remained clear when they were kept separately for 60 days, however, they changed to a semi-transparent gel in 15 min when they were mixed together, resulting in ultra-fine particles. This wet gel is considered to be homogeneously mixed because p H --2 is far from the isoelectric points of hydrous A1203 (boehmite) and hydrous TiO2, as shown in Fig. 1.