**3.1 Equilibrium diagram**

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

Reactive Sintering of Aluminum Titanate 505

The composition and mechanical properties of aluminum titanate are strongly influenced by the partial pressure of oxygen from the surrounding atmosphere. It is well known that the valence of the Ti cation in titanium oxides depends on the partial pressure of oxygen (Jürgen et al. 1996). At high oxygen pressures, Ti is tetravalent producing TiO2. Due to entropic reasons, at low oxygen pressures (such as for example in air) there is a small fraction of Ti+3, its amount depending on the temperature. By decreasing the partial pressure of oxygen furthermore, the Ti+3/Ti+4 relationship increases continuously producing TinO2n-1 Magneli phases and other sub-oxides such as Ti4O7, Ti3O5 and Ti2O3. The aluminum titanate phase shows similar behavior (Jürgen et al. 1996). Under high O2 pressure the aluminum titanate is almost a stoichiometric composition phase Al2TiO5. Decreasing the oxygen partial pressure, due to the increase in the Ti+3/Ti+4 ratios, it occurs a gradual interchange of Al+3 by Ti+3 in the aluminum titanate structure. This exchange results in the stoichiometric Al2TiO5 decomposition, to the reduced form of aluminum titanate, Al2O3 and oxygen according to

(3-2z)[(Al+3)2(Ti+4)1(O-2)5] = (Alz+3Ti1-z)2(Ti+4)(O-2)5+3(1-z)Al2O3+½(1-z)O2 (3)

Decreasing the oxygen potential (z→0), the decomposition reaction eventually produces Ti3O5 titanium oxide. The degree of decomposition from Al2TiO5 to Ti3O5 can be related to a continuous change of the aluminum titanate parameter network c, (Asbrink et al., 1967).

Considering the solubility between the Al2TiO5 and the Ti3O5 under various oxygen pressures, the Al2TiO5 was described with a subnet model (Al+3, Ti+3)2(Ti+4)1(O-2)5, this model was derived from the Al2TiO5 orthorhombic structure (Epicer et al. 1991), taking into account the mutual exchange of trivalent cations in one subnet, while in the other subnet

Subsequently, Freudenberg (1987), brings together all the data obtained and proposed a modified diagram (Fig. 2). Where the only stable compound in the Al2O3 – TiO2 system is considered to be, the β- Al2TiO5 phase; this compound decomposes above 1280 ± 1°C (Kato

The Al2TiO5 divides the system in two sub-systems Al2O3 - Al2TiO5 and Al2TiO5 - TiO2 with

It is important to point out the remarkable solubility difference between corundum and titania, the Al2O3 in TiO2 is ≈ one order in magnitude higher than the TiO2 in Al2O3, and the Al2O3 and TiO2 solubility are practically null in Al2TiO5; so it is considered aluminum titanate as a stoichiometric compound. However this claim is only correct for oxidizing

The Al2O3 has a 2.5% molar maximum solubility in TiO2 (1.97 ± 0.18 in weight) at 1726°C, (Slepetys, 1969), whereas the solubility of the latter in the alumina is almost non-existent 0.35% molar between 1300 and 1700°C. While both oxides solubility in the Al2TiO5 is

occupied by Ti+4 and O-2 species there is no influence.

eutectics at titania 38.5 and 80 weight percent respectively (Fig. 2).

**3.1.1 Effect of oxygen partial pressure in aluminum titanate stability** 

the reaction:

et al. 1980).

atmospheres.

completely null ( Golberg, 1968).

transformation between both phases is spontaneous and reversible; it was found that it is almost impossible to obtain α-Al2TiO5 at room temperature, being necessary cooling speeds greater than 800 K/h.)

Fig. 1. Al2O3 -TiO2 Equilibrium Diagram by Lang (1952).

Evidence suggests a congruent transformation of α- Al2TiO5 at 1860ºC, but the possibility of an incongruent fusion or the existence of a solid solution between Al2O3 and Al2TiO5 could not be studied, due to the difficulties of obtaining accurate data, because of the high viscosity of liquid formed.

A second point of importance obtained from this research was the suggestion of an instability region for the β- aluminum titanate between 750°C and 130°C. This phenomenon has been confirmed by subsequent research (Fonseca & Baptista 2003). Lang et al., (1952) concluded that opposing formation and decomposition processes occurred in dynamic equilibrium, promoting decomposition at a certain temperature range, i.e. the β- Al2TiO5 phase is stable above 1300°C, below this temperature aluminum titanate undergoes an eutectoid transformation according to the reaction:

$$\text{Al}\_2\text{TiO}\_5 \Leftrightarrow \text{Al}\_2\text{O}\_3 + \text{TiO}\_2 \tag{2}$$

transformation between both phases is spontaneous and reversible; it was found that it is almost impossible to obtain α-Al2TiO5 at room temperature, being necessary cooling speeds

> 10 30 50 70 90 Al2O3 TiO 2

Evidence suggests a congruent transformation of α- Al2TiO5 at 1860ºC, but the possibility of an incongruent fusion or the existence of a solid solution between Al2O3 and Al2TiO5 could not be studied, due to the difficulties of obtaining accurate data, because of the high

A second point of importance obtained from this research was the suggestion of an instability region for the β- aluminum titanate between 750°C and 130°C. This phenomenon has been confirmed by subsequent research (Fonseca & Baptista 2003). Lang et al., (1952) concluded that opposing formation and decomposition processes occurred in dynamic equilibrium, promoting decomposition at a certain temperature range, i.e. the β- Al2TiO5 phase is stable above 1300°C, below this temperature aluminum titanate undergoes an

Al2TiO5 ⇔ Al2O3 +TiO2 (2)

βAl2O3TiO2 + Liquid

Weigth % Al2O3TiO2

Liquid

1860

α Al2O3TiO2+Liquid

1705

β Al2O3TiO2 + TiO2

1845

TiO2 + Liquid

1860 1820

greater than 800 K/h.)

ºC 2000

1900

1800

Al2O3 +α Al2O3TiO2 1820

Al2O3+β Al2O3TiO2

Fig. 1. Al2O3 -TiO2 Equilibrium Diagram by Lang (1952).

eutectoid transformation according to the reaction:

Al2O3 +

Liquid

1700

viscosity of liquid formed.

#### **3.1.1 Effect of oxygen partial pressure in aluminum titanate stability**

The composition and mechanical properties of aluminum titanate are strongly influenced by the partial pressure of oxygen from the surrounding atmosphere. It is well known that the valence of the Ti cation in titanium oxides depends on the partial pressure of oxygen (Jürgen et al. 1996). At high oxygen pressures, Ti is tetravalent producing TiO2. Due to entropic reasons, at low oxygen pressures (such as for example in air) there is a small fraction of Ti+3, its amount depending on the temperature. By decreasing the partial pressure of oxygen furthermore, the Ti+3/Ti+4 relationship increases continuously producing TinO2n-1 Magneli phases and other sub-oxides such as Ti4O7, Ti3O5 and Ti2O3. The aluminum titanate phase shows similar behavior (Jürgen et al. 1996). Under high O2 pressure the aluminum titanate is almost a stoichiometric composition phase Al2TiO5. Decreasing the oxygen partial pressure, due to the increase in the Ti+3/Ti+4 ratios, it occurs a gradual interchange of Al+3 by Ti+3 in the aluminum titanate structure. This exchange results in the stoichiometric Al2TiO5 decomposition, to the reduced form of aluminum titanate, Al2O3 and oxygen according to the reaction:

$$(\text{3-2z})[(\text{Al}^{\ast 3})\_2(\text{Ti}^{\ast 4})\_1(\text{O}^{\ast 2})\_5] = (\text{Al}\_x \, ^{\ast 3}\text{Ti}\_{1\cdot \text{z}})\_2(\text{Ti}^{\ast 4})(\text{O}^{\ast 2})\_5 + \text{3(1-z)}\text{Al}\_2\text{O}\_3 + \text{4(1-z)}\text{O}\_2 \tag{3}$$

Decreasing the oxygen potential (z→0), the decomposition reaction eventually produces Ti3O5 titanium oxide. The degree of decomposition from Al2TiO5 to Ti3O5 can be related to a continuous change of the aluminum titanate parameter network c, (Asbrink et al., 1967).

Considering the solubility between the Al2TiO5 and the Ti3O5 under various oxygen pressures, the Al2TiO5 was described with a subnet model (Al+3, Ti+3)2(Ti+4)1(O-2)5, this model was derived from the Al2TiO5 orthorhombic structure (Epicer et al. 1991), taking into account the mutual exchange of trivalent cations in one subnet, while in the other subnet occupied by Ti+4 and O-2 species there is no influence.

Subsequently, Freudenberg (1987), brings together all the data obtained and proposed a modified diagram (Fig. 2). Where the only stable compound in the Al2O3 – TiO2 system is considered to be, the β- Al2TiO5 phase; this compound decomposes above 1280 ± 1°C (Kato et al. 1980).

The Al2TiO5 divides the system in two sub-systems Al2O3 - Al2TiO5 and Al2TiO5 - TiO2 with eutectics at titania 38.5 and 80 weight percent respectively (Fig. 2).

It is important to point out the remarkable solubility difference between corundum and titania, the Al2O3 in TiO2 is ≈ one order in magnitude higher than the TiO2 in Al2O3, and the Al2O3 and TiO2 solubility are practically null in Al2TiO5; so it is considered aluminum titanate as a stoichiometric compound. However this claim is only correct for oxidizing atmospheres.

The Al2O3 has a 2.5% molar maximum solubility in TiO2 (1.97 ± 0.18 in weight) at 1726°C, (Slepetys, 1969), whereas the solubility of the latter in the alumina is almost non-existent 0.35% molar between 1300 and 1700°C. While both oxides solubility in the Al2TiO5 is completely null ( Golberg, 1968).

Reactive Sintering of Aluminum Titanate 507

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

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

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

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 -

The raw materials used were reactive grade: Al2O3 (D50=0.60µm), TiO2 (D50=0.88µm), V2O5 (D50=0.60µm), MnO (D50=0.60µm), ferrosilicon (D50=0.69µm), FeTiO3 (D50=0.82µm), and,

Two (2) equimolar mixtures of Al2O3 and TiO2 (56wt%Al2O3 - 44wt%TiO2) were homogeneously mixed with 3, 6 and 9 wt% of each additive using alumina jars and balls, during 6 hours. No binder has been added to the aqueous media powder mixture and it was dried out at 120°C for 24 hours. The material was crushed in an alumina mortar prior to the manufacture of samples by uniaxial die compaction at 300 MPa. Green bodies were reactive sintered at 1450°C, in air for 3 h. Heating was programmed at 5°C/min. whereas cooling at 15°C/min, in order to avoid eutectoid transformation: Al2TiO5 → Al2O3 + TiO2 (Kolomietsev

X-ray diffraction (XRD) analysis has been performed on powders from crushed sintered samples, with grains below 30 µm suitable to obtain rigid specimens. The quantification of Al2TiO5 formed was determined by the internal standard method, through direct determination based on the methodology of Klug and Alexander (1954). In this study, the diffraction signals used were: Al2TiO5 (023), Al2O3 (104) and TiO2 Rutile (110)*,* which are

alumina ball milled 98.5%FeTiO3-1.2%SiO2 purified mineral (D50=0.88µm).

representative of the three components of interest in the studied samples.

temperature of decomposition, i.e., reducing stability (Woermann 1985).

data found in previous research that Fe+3, Mg+2 are the best stabilizers so far.

resistance, (Thomas et al., 1989).

**5. Experimental procedure** 

et al.,1981).

1100°C showed an Al2O3 and TiO2 breakdown.

Fig. 2. Al2O3 -TiO2 Equilibrium Diagram calculated in air, from experimental review by Freudenberg (1987).
