**3. Aluminum titanate (Al2TiO5) ceramics**

The Al2TiO5 is a one mole Al2O3 and one mole TiO2 compound. This material is conventionally prepared by reactive sintering of Al2O3 and TiO2 powders. Its interest as polycrystalline ceramic material arose from the work of Bachmann (1948), who found that the thermal expansion of aluminum titanate, in the studied temperature range, could be lower than that of the vitreous silica. This material has interesting features for applications such as thermal insulator and can also withstand strong thermal gradients. This aluminum titanate emerged as a promising ceramics for technological applications; summarizing its most important physical properties, in Table 1.

The material presents two major problems: the thermodynamics instability of the Al2TiO5 below 1280 °C and its poor mechanical resistance related to an extensive microcracking which is, in turn, responsible for the low thermal expansion. Decomposition can be controlled or at least delayed, with oxides additions, such as MgO (Ishitsuka and col., 1987;

thermal expansion coefficient is related to the sum of unit cells coefficients of thermal expansion. In orthorhombic crystal structures as that of the pseudobrookita, material object

As the anisotropic crystalline structures have principal axes with positive and negative expansion coefficients, it is necessary to examine the thermal expansion coefficients of all the members of an isostructural family and chemically design a solid solution whose α*i* addition is close to zero. Bayer (1971; 1973), studied the unit cell, of the pseudobrookita structure. Provided that the sum of the thermal expansion coefficients of the principal axes (α*i*) add zero, it occurs an inevitable combination of positive and negative values. This condition leads to, very high (at GPa levels), micromechanical stresses at grain boundaries, during cooling from the temperatures of ceramic processing. The development of these internal stresses, promotes the breakdown of the grain boundaries, which causes a decrease in the structural integrity of the polycrystalline ceramic body. However, the existence of this microcracking depends on the microstructural grain size. Kuszyk and Bradt (1973) noted that the rigidity of the ceramic body decreased as increasing grain size, determining a critical grain size. Once determined this size, is simply necessary a process production control to achieve a compromise between the microcracking and the required structural mechanical resistance. Another possibility is to produce a material with large grain size and extensive microcracking with low mechanical resistance but where the main interest is the low thermal expansion (Hasselman, 1977; Stingl, 1986; Sheppard 1988; Huber, 1988). However, several researchers (Buessem, 1966; Cleveland, 1977; 1978) have suggested that the presence of the extensive internal microcracking, contributes to an increase in the resistance to fracture of these polycrystalline ceramics highly anisotropic, activating mechanisms such as: shielding, branching or cracks deviation. Experimentally, this hypothesis has not been demonstrated, so it is a concept that must be handled carefully.

The Al2TiO5 is a one mole Al2O3 and one mole TiO2 compound. This material is conventionally prepared by reactive sintering of Al2O3 and TiO2 powders. Its interest as polycrystalline ceramic material arose from the work of Bachmann (1948), who found that the thermal expansion of aluminum titanate, in the studied temperature range, could be lower than that of the vitreous silica. This material has interesting features for applications such as thermal insulator and can also withstand strong thermal gradients. This aluminum titanate emerged as a promising ceramics for technological applications; summarizing its

The material presents two major problems: the thermodynamics instability of the Al2TiO5 below 1280 °C and its poor mechanical resistance related to an extensive microcracking which is, in turn, responsible for the low thermal expansion. Decomposition can be controlled or at least delayed, with oxides additions, such as MgO (Ishitsuka and col., 1987;

α*i* = Thermal expansion coefficients of principal crystal axes

βv= α*a* + α*b* + α*<sup>c</sup>* (1)

of this work, the relationship is:

Where βv = Volumetric thermal expansion coefficient

**3. Aluminum titanate (Al2TiO5) ceramics** 

most important physical properties, in Table 1.

Wohlfrom and col., 1990) and Fe2O3 (Tilloca, G., 1991, Brown et al., 1994), which forms solid solutions between the Al2TiO5 and the isoestructurals MgTi2O5 and Fe2TiO5. The mechanical strength can be increased with good results preparing composite materials such as: Al2TiO5 - Mulita (Morishima and col. 1987), Al2TiO5 - Mulita - ZrO2 (Wohlfrom et al., 1990).


Table 1. Aluminum Titanate Physical Properties.
