**4. Additives**

The thermal instability of aluminum titanate and its low mechanical resistance are the main reasons for the additives use, taking into account these will influence the production process and the final product properties. An important characteristic for all additives is that they do not decrease significantly aluminum titanate thermomechanical properties. Small additions (≤ 5% by weight) are usually added with the aim of forming aluminum titanate solid solutions.

As was mentioned before, the aluminum titanate is formed above and decomposes below the equilibrium temperature 1280°C (Kato et al., 1980), with a free energy of formation given by:

$$\mathsf{\mathsf{\mathsf{\mathsf{\ast}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\ast}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}\mathsf{\mathsf{\mathsf{\mathsf{\prime}}}}}\mathsf{\$$

$$\text{AG}^{\circ}\text{ Al}\_{2}\text{TiO}\_{5} = 17000 \text{ - } 10.95 \text{T} \tag{5}$$

Al2O3 + TiO2

I I I I I I 0 20 40 60 80 100 Al2O3 TiO <sup>2</sup>

Fig. 2. Al2O3 -TiO2 Equilibrium Diagram calculated in air, from experimental review by

The thermal instability of aluminum titanate and its low mechanical resistance are the main reasons for the additives use, taking into account these will influence the production process and the final product properties. An important characteristic for all additives is that they do not decrease significantly aluminum titanate thermomechanical properties. Small additions (≤ 5% by weight) are usually added with the aim of forming aluminum titanate solid

As was mentioned before, the aluminum titanate is formed above and decomposes below the equilibrium temperature 1280°C (Kato et al., 1980), with a free energy of formation given

ΔGº Al2TiO5 = ΔHº - ΔSºT (4)

ΔGº Al2TiO5 = 17000 – 10.95T (5)

mol %

(1553K)

β Al2TiO5

Al2O3 + Al 2TiO5

(2327K) Liquid

(2113K)

+ Liquid

Al 2O3

K 2400 -

2200 -

2400 -

1800 -

1600 -

1400 -

1200 -

Freudenberg (1987).

**4. Additives** 

solutions.

by:

α Al

O2 3

Al 2TiO5 + TiO2

(1978K)

+ L

(2130K)

ºC








Al 2TiO5 + Liq. TiO2

The endothermic reaction is possible due to the entropy (ΔS°) positive contribution. So as other pseudobrookitas, Al2TiO5 can be stabilized entropically (Navrotsky 1975), with certain contributions to cation disorder (Morosin et al., 1972). It is conceivable that the positive effect of entropy can be reinforced with additional entropy in terms of mixing by the formation of aluminum titanate solid solutions. It has been determined empirically that solid solutions containing Fe+3 and Mg+2, provide a lower decomposition temperature, i.e. increasing stability. On the other hand, solid solutions with Cr+3 promote a greater temperature of decomposition, i.e., reducing stability (Woermann 1985).

Jung et al. (1993), studied the replacement of Ti+4 by Ge+4 and Al+3 by Ga+3 and Ge solid solutions combined also with additions of MgO and Fe2O3, finding that the stabilizing effect of the additions decreased in the following order: Fe+3, Mg+2 > Ge+2 > Ga+3, corroborating data found in previous research that Fe+3, Mg+2 are the best stabilizers so far.

Additions such as Fe2O3, MgO or SiO2 were studied, the first two promoting structures of the pseudobrookites type Fe2TiO5 and MgTi2O5 giving complete solid solutions with Al2TiO5 (Brown 1994; Buscaglia et al., 1994; 1995; 1997). The SiO2 has limited solubility (Ishitsuka 1987), however additions up to 3 weight percent produce a slight increase in the mechanical resistance, due to small amounts of liquid phase that densify the material but, larger amounts cause excessive growth of the grain that is detrimental to the mechanical resistance, (Thomas et al., 1989).

Liu et al., (1996), studied the thermal stability of Al2TiO5 with Fe2TiO5 and MgTi2O5 additions finding that material with Fe+3 additions did not show any significant mechanical properties decomposition or degradation and the material with Mg+2 annealed to 1000 - 1100°C showed an Al2O3 and TiO2 breakdown.
