**6.6 Grain size**

Since the best stabilyzing behaviour after heat treatment was achieved by the ilmenites addition, these samples in the as sintered condition, were selected to determine the effect of additive contens on grains size. There were no significant variations in the grain size obtained with the two additives; however, there is a slight decrease in size as the percentage of the additive increases (in both cases), determined by image analysis of grain size (Table 4). The grain size varies between 9 and 12μm. In other words, the presence of second phase slightly inhibits the growth of grain.


Table 4. Effect of FeTiO3 (pure and concentrated mineral) on the grain size of sintered samples at 1450°C/ 3hours.

#### **6.7 Mössbauer spectroscopy**

520 Sintering of Ceramics – New Emerging Techniques

AT+Additive (%) % %TiO2 decomposed % Al2O3 decomposed %Al2TiO5 left

V2O5 6 43.29 55.10 1.61

MnO 6 32.40 41.22 26.38

FeTiO3 .SiO2 6 23.06 29.30 47,64

FeTiO3 6 10.28 13.01 76.71

FeSi2 6 36.26 46.13 17.61

Since the best stabilyzing behaviour after heat treatment was achieved by the ilmenites addition, these samples in the as sintered condition, were selected to determine the effect of additive contens on grains size. There were no significant variations in the grain size obtained with the two additives; however, there is a slight decrease in size as the percentage of the additive increases (in both cases), determined by image analysis of grain size (Table 4). The grain size varies between 9 and 12μm. In other words, the presence of second phase

FeTiO3 (%) Tg ( μm) FeTiO3.SiO2(%) Tg ( μm)

3 11.96 3 11.18

6 11.60 6 10.52

9 9.18 9 9,31

Table 4. Effect of FeTiO3 (pure and concentrated mineral) on the grain size of sintered

Table 3. Al2TiO5 % Phase decomposition after heat treatment at 1100°C/ 100hours, by

internal standard quantification method.

slightly inhibits the growth of grain.

samples at 1450°C/ 3hours.

**6.6 Grain size** 

3 43.86 55.82 0.32

9 43.71 55.63 0.66 3 35.69 45.41 18.90

9 32.90 41.84 25.26 3 26.53 26.97 46.50

9 23.10 30.12 48.78 3 17.17 21.78 61.05

9 8.65 10.93 80.42 3 38.01 48.36 13.63

9 33.20 42.24 24.56

It has confirmed the presence of the Fe+3 ions in all compositions with Fe added, i.e., the ilmenites and ferrosilicon.

The Mössbauer spectrum for material with 6% of ferrosilicon addition can be adjusted to two doublets (Fig.21a). The first doublet corresponds to the ferrous cation (Fe+2) with a resonance that fits the hyperfine splitting with an isomer shift: IS = 1.01 ± 0.002 mm/s and a quadrupole splitting: QS= 0.664 ± 0.003 mm/s. The second doublet corresponds to the resonance of the ferric cation (Fe+3) with a IS = 0. 323 ± 0.003 mm/s and a QS = 0. 520 ± 0.004 mm/s. For composition with 6% pure ilmenite addition (Fig. 21b.), it is revealed a consistent doublet with the ferric state (Fe+3), with a IS = 0. 323 + 0.003 mm/s and QS = 0.520 ± 0.004 mm/s. The spectrum for the sample with 6% of mineral ilmenite (Fig. 19c), one could guess the doublet corresponds to both states ferrous (Fe+2) and ferric (Fe+3), however should be noted that results are not accurate, with considerable dispersion.

Fig. 21. Mössbauer Spectroscopy of Al2TiO5 with: a) 6wt% FeSi2.SiO2 ; b) 6wt% FeTiO3 and c) 6wt% FeTiO3.SiO2 additions.

These results corroborate the possible replacement of the Al+3 ions by ion Fe+3; there is higher stabilization in samples with pure ilmenite addition, where all Fe ions are in ferric state. However, as it was found in previous research (Barrios de Arenas & Cho, 2010) , the presence of Fe+2 ions, also represent the possibility of Al+3 ions substitution, with the creation of defects, which in turn promotes the diffusion in solid state.

Reactive Sintering of Aluminum Titanate 523

Composition α25-1000ºC x10-6 ºC-1 α25-1450ºC x10-6 ºC-1

3 0.62 0.96

6 0.76 1.05

9 0.86 1.16

3 0.83 1.11

6 0.94 1.22

9 1.02 1.40

The evident effect of FeTiO3 additions on Al2TiO5 was established. An increase of the addition leads to a sensitive decrease on decomposition, this is due to the expected solid solution formed between Al2TiO5 and the isostructural Fe2TiO5, the latter being the product

The ilmenites additions produce a slightly increase in the thermal expansion coefficients being more important with the concentrated mineral addition. Although the values remain

The MnO increases densification by the presence of a localized liquid phase, which allows the rearrangement of particles in the first stage of sintering and also slightly decomposition controls.The ferrosilicon (FeSi2.SiO2), SiO2 reacts with Al2O3 and the TiO2 forming a liquid

Asbrink, G. & Magneli, A. (1967) "X-Ray Studies on Some Mixed Oxides Systems of

Bachmann, J. L. (1948) "Investigations of Properties of Aluminium Oxide and Some

Barrios de Arenas I. & Cho S–A. (2011) "Efecto de la adición de ferrosilicio - FeSi2 en la

microestructura y estabilidad del titanato de aluminio- Al2TiO5." *Revista* 

phase, allowing rearrangement of particles; however, it lacks of stabilizing effect.

Aluminuos Materials"; *Ph.D Thesis*, *Pennsylvania State University*.

*Latinoamericana de Metalurgia y Materiales* 2011; 31 (1): 11-19.

Pseudobrookite Structure", *Acta Chem. Scand* 21.

Table 5. Effect of FeTiO3 (pure and concentrated mineral) addition, on Al2TiO5 Thermal

Al2TiO5 -0.55 0.87

FeTiO3 (%)

FeTiO3.SiO2 (%)

of ilmenite decomposition in the oxidizing conditions used.

Expansion.

acceptable.

**8. References** 

**7. Conclusion** 

#### **6.8 Thermal expansion**

Some authors have directly related the area of hysteresis in thermal expansion curves to sample microcracks density (Lingenberg W. 1985). In this study, after comparison of the ilmenite added and pure Al2TiO5 materials results (Fig.22), show an evident reduction in the area of hysteresis in the formers, being even more important in the samples with concentrated mineral FeTiO3 addition (fig.22 b).

In both FeTiO3-added samples, the property values are antagonist to addition contents.

Temperature ºC

Fig. 22. Thermal Expansion betweeen 20 -1450°C of Al2TiO5 with a) FeTiO3: 6wt% and b) FeTiO3.SiO2 6wt% addition.

Table 5, lists the experimental values obtained for the thermal expansion coefficients between 25-1000ºC and 25-1450°C. The Al2TiO5 without additives has a value slightly negative α25-1000°C= -0.55 x 10-6°C- 1. Both additions turn this characteristic value into very low positive ones augmenting with the additive content in each case. This behavior could be explained by the reduction in grain size which causes a microcracking inhibition, as grains boundaries surface increases, i.e., it is necessary a higher energy for cracking (Yoleva et al. 2010).

Some authors have directly related the area of hysteresis in thermal expansion curves to sample microcracks density (Lingenberg W. 1985). In this study, after comparison of the ilmenite added and pure Al2TiO5 materials results (Fig.22), show an evident reduction in the area of hysteresis in the formers, being even more important in the samples with

In both FeTiO3-added samples, the property values are antagonist to addition contents.

0 200 400 600 800 1000 1200 1400

Fig. 22. Thermal Expansion betweeen 20 -1450°C of Al2TiO5 with a) FeTiO3: 6wt% and

Table 5, lists the experimental values obtained for the thermal expansion coefficients between 25-1000ºC and 25-1450°C. The Al2TiO5 without additives has a value slightly negative α25-1000°C= -0.55 x 10-6°C- 1. Both additions turn this characteristic value into very low positive ones augmenting with the additive content in each case. This behavior could be explained by the reduction in grain size which causes a microcracking inhibition, as grains boundaries surface increases, i.e., it is necessary a higher energy for cracking (Yoleva et al.

3% 6% 9%

AT

3% 6% 9%

AT

AT+FeTiO3SiO2

Temperature ºC

AT+FeTiO3

**6.8 Thermal expansion** 

concentrated mineral FeTiO3 addition (fig.22 b).

1 2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2

**a)**

**b)**


Thermal

Thermal

(

b) FeTiO3.SiO2 6wt% addition.

2010).

l/lo)x100

Expansion

 (%)

(

l/lo)x100

Expansion

 (%)


Table 5. Effect of FeTiO3 (pure and concentrated mineral) addition, on Al2TiO5 Thermal Expansion.
