**2. Fundamentals of low thermal expansion coefficient aluminum titanate (Al2TiO5) ceramics**

There are two important features to achieve a very low thermal expansion coefficient, in crystalline ceramic structures highly anisotropic. The first aspect involves unit cell crystalline chemistry. The coefficients of thermal expansion of the crystal axes are controlled to develop solid solutions, in an attempt to ensure that the sum of the coefficients of the principal axes gives zero. In the case of polycrystalline ceramic materials, the volumetric

Reactive Sintering of Aluminum Titanate 503

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 -

> **Property Al2TiO5 Reference**  Density (g/cm3) 3.702 Holcombe (1973)

> > -2.9 – -3 10.3 – 11.8 20.1 – 21.8

9.2 – 10.2

1.0 – 1.5 1.5 – 1.7

23 –24.8

25 – 40

Melting Point (ºC) 1860 Lang (1952)

Hardness, Hv (GPa) 5 Wohlfromm (1990)

Thermal shock resistance (Wm-1) 500 Stingl (1986) Thermal Conductivity, k(W/mK) 1.5 –2.5 Stingl (1986)

Lang et al. (1952) studied the Al2O3 -TiO2 equilibrium diagram (Fig. 1), finding the existence of two allotropic forms of aluminum titanate: α- Al2TiO5, a high temperature phase, stable between 1820°C and the melting point at 1860+10ºC and β-Al2TiO5, a low temperature phase stable from room temperature up to ≈ 750ºC and from 1300°C up to inversion temperature 1820°C (at intermediate values, it has instability and decomposes to Al2O3 + TiO2). The

Wohlfromm (1990)

Stingl (1986) Milosevski (1995)

Stingl (1986) Cleveland (1978) Milosevski (1997)

Milosevski (1995)

Milosevski (1997)

Mulita (Morishima and col. 1987), Al2TiO5 - Mulita - ZrO2 (Wohlfrom et al., 1990).

Thermal Expansion Coefficient Average (x10-6 ºC-1) αa20-520 – αa20-1000 αb20-520 – αb20-1000 αc20-520 – αc20-1000

Thermal Expansion Coefficient Average (x10-6 ºC-1) Crystallographic α20-520 – α20-1000 Macroscopic α20-1000 α20-1000 Anisotropy Δα20-520 - Δα20-1000

> Elastic Modulus E(GPa)

Table 1. Aluminum Titanate Physical Properties.

**3.1 Equilibrium diagram** 

Bending Strenght, σ (MPa) 4 – 20

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 of this work, the relationship is:

$$
\beta\_\circ = \alpha\_a + \alpha\_b + \alpha\_c \tag{1}
$$

Where βv = Volumetric thermal expansion coefficient

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

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.
