**6. Discussions of results**

#### **6.1 Additives selection**

The additives selection is based on the cation radius, which must be related to the aluminum titanate cations i.e., Al+3 and Ti+4, in order to be able to replace them in the solid solution to be formed. The thermal expansion can be related to the degree of crystalline distortion which is known to increase with the difference between the radii of the cations.

Consequently, the octahedral distortion is much greater in Al2TiO5 than in Fe2TiO5, due to the small radius of Al+3 ions, which facilitate a tendency to the tetrahedral coordination (Bayer, G., 1973). To avoid extreme distortion, the replacing selected cations must have a radii at least close to the Al+3=0.54 Å; i.e., V+5=0.59 Å; Mn+4 =0.76 Å; Si+4 =0.41 Å; Fe+3=0.67 and Ti+4=0.76. Hence, all additives used in this work fulfill the requirement. On the other hand, the aluminum titanate is formed by an equimolar reaction between Al2O3 and TiO2. However, due to entropic reasons, in low O2 pressure conditions, as in air for example, there is a small fraction of Ti+3, its quantity being a function of the temperature. Hence, the reaction of transformation can be expressed in terms of the intermediate reaction of the titanium oxide Ti3O5 in the following manner:

$$2\text{ Al}\_2\text{O}\_3 + 1/3\text{ Ti}\_3\text{O}\_5 + 1/6\text{ O}\_2 \Leftrightarrow \text{Al}\_2\text{TiO}\_5\tag{6}$$

The **Ti3O5** can be seen in terms of Ti**+3** Ti**+4** O5**-2** for which there is the possibility of forming *"limited solid solutions"* by cationic substitution as:

$$(\text{1-x})\text{Al}\_2\text{TiO}\_5 + \text{xTi}\_3\text{O}\_5 \Leftrightarrow (\text{Al}^{\ast 3}\text{I}\_{1\cdot \alpha}\text{Ti}^{\ast 3}\text{}\_{\text{x}})\_2\text{Ti}^{\ast 4}\text{O}\_5\text{-}^2\tag{7}$$

Studying the affinity diagram of multivalent oxides of the transition metals with O2, it was observed that below Ti, the order of decreasing affinity with O2 is V, Mn and Fe (Fig.3.).

Sintered sample surfaces were carefully ceramographically prepared to minimize damage and, in some cases it was needed to chemically etch in ambient 15%HF solution for 60 s, to reveal grain boundaries. The microstructure characterization was carried out using compositional back scattered electron images (BSEI) from scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Evaluation of grain size and phases

In order to quantify the stabilization of Al2TiO5, sintered samples previously thermal treated at 1100°C for 100 h. were Si internal standard XRD analyzed, as in the as-sintered condition. To determine the type of Fe ion in solution, it was used Mössbauer Spectroscopy with the isotope iron 57Fe, in the samples with addition of ilmenite and ferrosilicon. The source used was 57Co, the Mösssbauer transition is 14.41 keV, with the excited level of nuclear spin I = 3/2 and fundamental level I = 1/2. The extent of the isomeric shift provides information on the valence of the atom to which belong the core, as the electronic layers and therefore the

Thermal expansion analysis in the temperature range of 25 to 1000°C and 1450°C, at 5°C/min heating and cooling ramps, has been performed on selected samples with good

The additives selection is based on the cation radius, which must be related to the aluminum titanate cations i.e., Al+3 and Ti+4, in order to be able to replace them in the solid solution to be formed. The thermal expansion can be related to the degree of crystalline distortion

Consequently, the octahedral distortion is much greater in Al2TiO5 than in Fe2TiO5, due to the small radius of Al+3 ions, which facilitate a tendency to the tetrahedral coordination (Bayer, G., 1973). To avoid extreme distortion, the replacing selected cations must have a radii at least close to the Al+3=0.54 Å; i.e., V+5=0.59 Å; Mn+4 =0.76 Å; Si+4 =0.41 Å; Fe+3=0.67 and Ti+4=0.76. Hence, all additives used in this work fulfill the requirement. On the other hand, the aluminum titanate is formed by an equimolar reaction between Al2O3 and TiO2. However, due to entropic reasons, in low O2 pressure conditions, as in air for example, there is a small fraction of Ti+3, its quantity being a function of the temperature. Hence, the reaction of transformation can be expressed in terms of the intermediate reaction of the

The **Ti3O5** can be seen in terms of Ti**+3** Ti**+4** O5**-2** for which there is the possibility of forming

 (1-x)Al2TiO5 + x**Ti3O5** ⇔ (Al**+3**1-xTi**+3**x)2Ti**+4**O5**-2** (7) Studying the affinity diagram of multivalent oxides of the transition metals with O2, it was observed that below Ti, the order of decreasing affinity with O2 is V, Mn and Fe (Fig.3.).

α Al2O3 + 1/3 **Ti**3**O5** + 1/6 O2 ⇔ Al2TiO5 (6)

which is known to increase with the difference between the radii of the cations.

density of electrons in the nucleus, are sensitive to chemical bonding.

present has been performed by image analysis.

stabilization behavior.

**6.1 Additives selection** 

**6. Discussions of results** 

titanium oxide Ti3O5 in the following manner:

*"limited solid solutions"* by cationic substitution as:

Fig. 3. Oxygen partial pressure (PO2 ) vs. Oxygen/Metal (x en MOx) for the 3d metals.

#### **6.2 Structure analysis**

The XRD results showed, for aluminum titanate without additive (Fig. 4.), that the selected temperature and time are sufficient for a near 100% Al2TiO5 reaction of formation, as the most important peaks correspond to this compound with a minimum of Al2O3 and TiO2 remnants. It is important to point out that in all XRD, are represented the PDF values for all the constituents expected in each case, although they are not present.

Fig. 4. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 without addition, sintered at 1450°C for 3 hours.

Reactive Sintering of Aluminum Titanate 511

Fig. 7. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with industrial FeSi2.Si: 3, 6

Both pure and concentrated mineral ilmenite (FeTiO3) (Figs.8 and 9),promoted the formation of Al2TiO5 in all compositions studied. Phases such as Fe2O3, TiO2, or Fe2TiO5 product of decomposition and reaction of the FeTiO3, due to the oxidizing atmosphere, were not

Fig. 8. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with pure FeTiO3: 3, 6 y 9

Other remark is that expected phases, product of the reaction of contaminant SiO2 with the parent Al2O3 and TiO2, in the samples containing mineral did not show in the XRD spectra. Nevertheless, the most important reflections correspond to the Al2TiO5 corroborating the

and 9 wt% addition, sintered at 1450°C for 3 hours.

wt% addition, sintered at 1450°C for 3 hours.

beneficial effect of this additive in its formation (Fig. 9).

detected.

3w% FeSi2.Si

6w% FeSi2.Si

9w% FeSi2.Si

In the samples with V2O5, the formation of Al2TiO5 decreases as addition contents increase due to an intergranular liquid phase formed, identified by SEM-EDX, that inhibits the reaction between the main constituents (Fig. 5.)

Fig. 5. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with V2O5: 3, 6 and 9 wt% addition, sintered at 1450°C for 3 hours.

Regarding the MnO addition, it promotes Al2TiO5 formation with contents, as depicted by the aluminum titanate principal signals which increase in intensity whereas those of Al2O3 and TiO2 are suppressed. MnTiO3 appears as product of TiO2 and MnO eutectic reaction. (Fig. 6.).

Fig. 6. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with MnO: 3, 6 y 9 wt% addition, sintered at 1450°C for 3 hours.

In the case of ferrosilicon added compositions (Fig. 7), the Al2TiO5 formation reaction occurs but not complete and, the main signals of Al2O3 and TiO2 are slightly shifted, corresponding to Al2SiO5 and Al4Ti2SiO12 being the latter, a product of a ternary eutectic transformation.

In the samples with V2O5, the formation of Al2TiO5 decreases as addition contents increase due to an intergranular liquid phase formed, identified by SEM-EDX, that inhibits the

Fig. 5. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with V2O5: 3, 6 and 9 wt%

Fig. 6. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with MnO: 3, 6 y 9 wt%

In the case of ferrosilicon added compositions (Fig. 7), the Al2TiO5 formation reaction occurs but not complete and, the main signals of Al2O3 and TiO2 are slightly shifted, corresponding to Al2SiO5 and Al4Ti2SiO12 being the latter, a product of a ternary eutectic transformation.

Regarding the MnO addition, it promotes Al2TiO5 formation with contents, as depicted by the aluminum titanate principal signals which increase in intensity whereas those of Al2O3 and TiO2 are suppressed. MnTiO3 appears as product of TiO2 and MnO eutectic reaction.

reaction between the main constituents (Fig. 5.)

addition, sintered at 1450°C for 3 hours.

addition, sintered at 1450°C for 3 hours.

(Fig. 6.).

Fig. 7. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with industrial FeSi2.Si: 3, 6 and 9 wt% addition, sintered at 1450°C for 3 hours.

Both pure and concentrated mineral ilmenite (FeTiO3) (Figs.8 and 9),promoted the formation of Al2TiO5 in all compositions studied. Phases such as Fe2O3, TiO2, or Fe2TiO5 product of decomposition and reaction of the FeTiO3, due to the oxidizing atmosphere, were not detected.

Fig. 8. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with pure FeTiO3: 3, 6 y 9 wt% addition, sintered at 1450°C for 3 hours.

Other remark is that expected phases, product of the reaction of contaminant SiO2 with the parent Al2O3 and TiO2, in the samples containing mineral did not show in the XRD spectra. Nevertheless, the most important reflections correspond to the Al2TiO5 corroborating the beneficial effect of this additive in its formation (Fig. 9).

Reactive Sintering of Aluminum Titanate 513

It can be seen that the best results are obtained firstly for the pure ilmenite, secondly the mineral ilmenite, then the MnO, vanadium oxide and ferrosilicon additions respectively.

The composition without addition (Fig. 10), shows the characteristic microstructure of the aluminum titanate: a porous and microcracked Al2TiO5 matrix phase and the presence of unreacted Al2O3 and TiO2, due to the formation reaction kinetics, which is a process leaded by nucleation and growth of Al2TiO5 grains and finally the diffusion of the reactants remnants through the matrix, this is controlled for a very slow reacting species diffusion, as

**a)**

**Al**

**AT b)**

Element Wt% At % Ok 44.21 62.44 AlK 31.00 25.96 TiK 21.56 10.17 VK 3.22 1.43 Total 100.00 100.00 **Al**

**a) b)**

**O**

Element Wt% At % Ok 37.10 54.72 AlK 37.44 26.74 TiK 25.45 17.54 Total 100.00 100.00

**0.90 1.80 2.70 3.60 4.50 5.40** 

**1.00 1.70 2.40 3.10 3.80 4.50 5.20 5.90 6.** 

**Ti**

**GP**

**Au Rec . V**

**Ti**

**AT**

Fig. 10. a)BSE microstructure of Al2O3 and TiO2 without addition, sintered at 1450°C for 3 hours, b) EDS of the matrix with an exact atomic relationship: 25 at% Al, 12.5 at% Ti and 62.5

The addition of the low melting point V2O5 (678°C) is evidenced in the microstructure with the presence of an abundant glassy intergranular phase, which constitutes a physical barrier

**Al**

Fig. 11. a)BSE microstructure detail of Al2O3 and TiO2 with V2O5: 6 wt% addition, sintered at 1450°C for 3 hours. b) EDS of the intergranular glassy phase (GP), appearing due to V2O5

at% O. (AT: Aluminum titanate; Ti: Titania; Al: Alumina).

**GP**

**Ti**

**AT**

between Al2O3 and TiO2, retarding the Al2TiO5 formation (Fig. 11a).

**Al Ti**

low melting point. (AT: Aluminum titanate; Ti: Titania; Al: Alumina).

**6.3 Microstructure analysis** 

it was found by: Wohlfromm et al., (1991).

Fig. 9. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with concentrated placer ilmenite (FeTiO3.SiO2): 3, 6 and 9 wt% addition, sintered at 1450°C for 3 hours.

### **6.2.1 Al2TiO5 formation phase quantification**

The quantification of Al2TiO5 formed, was determined by the internal standard method; the achieved results are showed in Table 2.


Table 2. Al2TiO5 % phase formation by sintering at 1450°C/3hours.

It can be seen that the best results are obtained firstly for the pure ilmenite, secondly the mineral ilmenite, then the MnO, vanadium oxide and ferrosilicon additions respectively.
