**6.3 Microstructure analysis**

512 Sintering of Ceramics – New Emerging Techniques

Fig. 9. X Ray Diffraction of equimolar mixture Al2O3 and TiO2 with concentrated placer

The quantification of Al2TiO5 formed, was determined by the internal standard method; the

AT+Additive (%) % %TiO2 unreacted % Al2O3 unreacted %Al2TiO5 formed

V2O5 6 20.1 25.5 54.4

MnO 6 11.2 14.1 74.7

FeTiO3 .SiO2 6 5.1 6.4 88.5 (mineral) 9 5.3 628 88.5

FeTiO3 6 2.3 2.8 94,8 (pure) 9 2.1 2.0 96,0

FeSi2 6 19.8 25.1 55.1

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

3 19.2 24.3 56.5

9 22.6 28.7 48.7 3 13.1 16.6 70.4

9 9.4 11.9 78.7 3 5.5 5.8 88,7

3 1.9 2.7 95,0

3 21.3 27.1 51.7

9 16.9 21.5 61.6

ilmenite (FeTiO3.SiO2): 3, 6 and 9 wt% addition, sintered at 1450°C for 3 hours.

**6.2.1 Al2TiO5 formation phase quantification** 

achieved results are showed in Table 2.

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

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

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

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

Reactive Sintering of Aluminum Titanate 515

**Al**

**Si**

**Au Rec.**

Fig. 13. c) EDS of ternary eutectic (TE) and d) EDS of the intergranular Fe rich phase (FeP).

2011).

**O**

**Al**

**Si**

practically free of unreacted original phases.

**0.60 1.20 1.80 2.40 3.00 3.60 4.20 4.80 5.40 6.00** 

Element Wt% At% Ok 32.68 46.66 AlK 37.26 31.54 SiK 22.18 18.04 TiK 7.89 3.76 Total 100.00 100.00

**Au Rec.**

**Ti**

A fraction of free Al2O3 remains as an intragranular phase, the TiO2 reacts completely and besides the Al2TiO5 matrix phase a ternary eutectic reaction grainy phase is formed between the Al2O3-SiO2-TiO2 (as low as it is undetected by RXD), but increasing its quantity with the additive and, also a fourth intergranular phase rich in Fe is depicted (Fig.13d) (Arenas et al.

**O**

**1.10 2.00 2.90 3.80 4.70 5.60 6.50 7.40** 

**Ti**

Element Wt% At% Ok 28.81 47.95 AlK 25.75 25.41 SiK 7.96 7.55 TiK 15.35 8.53 FeK 22.14 10.56 Total 100.00 100.00

**c) d)**

**TE FeP**

**Fe**

**AT**

Pure and concentrated ilmenite (FeTiO3), additions have a beneficial effect on grain growth control (Fig. 14 and 15). The SiO2 left in the purified mineral promoted the formation of an intergranular liquid phase which could not be detected by XRD. Microstructures are

Fig. 14. a) BSE microstructural x1000 and a detail X3000, of Al2O3 and TiO2 with pure FeTiO3: 6 wt% addition, sintered at 1450°C for 3 hours. Notice the grain growth control.

Althought localized EDS analysis on the glassy phase was carried out, the Al and Ti values obtained are due to the larger electron beam action volume compared to phase size (Fig. 11b).

The microstructure of MnO added samples shows extensive Al2TiO5 phase formation, with a minor presence of liquid phase, product of the two eutectic reactions, at 1290°C and 1330°C, between MnO and TiO2. However, opposite to the V2O5 added samples, the reacting species diffusion and Al2TiO5 formation is accelerated with the MnO contents and, unreacted TiO2 is absent in the microstructure, due to the secondary reactions. EDS analysis identified the intergranular eutectic phase as 2MnO.TiO2 and MnO.TiO2 (Fig. 12). Microstructure grain size decreased with MnO contents in the sintered bodies.

Fig. 12. a)BSE microstructural detail of Al2O3 and TiO2 with MnO: 6 wt% addition, sintered at 1450°C for 3 hours. b) EDS of the intergranular MnTiO3 eutectic phase (EP). (AT: Aluminum titanate; Al: Alumina).

The FeSi2.Si modified composition has a different microstructure to that obtained with other additives (Fig.13a-d).

Fig. 13. a) BSE microstructural detail of Al2O3 and TiO2 with FeSi2.Si: 6 wt% addition, sintered at 1450°C for 3 hours. b)EDS of the Al2TiO5 matrix.

Althought localized EDS analysis on the glassy phase was carried out, the Al and Ti values obtained are due to the larger electron beam action volume compared to phase size (Fig.

The microstructure of MnO added samples shows extensive Al2TiO5 phase formation, with a minor presence of liquid phase, product of the two eutectic reactions, at 1290°C and 1330°C, between MnO and TiO2. However, opposite to the V2O5 added samples, the reacting species diffusion and Al2TiO5 formation is accelerated with the MnO contents and, unreacted TiO2 is absent in the microstructure, due to the secondary reactions. EDS analysis identified the intergranular eutectic phase as 2MnO.TiO2 and MnO.TiO2 (Fig. 12).

Fig. 12. a)BSE microstructural detail of Al2O3 and TiO2 with MnO: 6 wt% addition, sintered

**Al**

**Au Rec .**

**Al**

**1.40 2.00 2.60 3.20 3.80 4.40 5.00 5.60 6.20** 

**a) b)**

Element Wt% At% Ok 35.14 54.24 AlK 30.83 28.22 TiK 34.02 17.54 Total 100.00 100.00

**Au Rec .**

**1.00 1.60 2.20 2.80 3.40 4.00 4.60 5.20 5.80** 

**Ti**

**AT**

**Mn**

**a) b)**

**Ti** Element Wt% At% Ok 12.43 28.26 AlK 13.48 18.18 TiK 45.89 34.36 MnK 28.21 18.69 Total 100.00 100.00

**EP EP**

The FeSi2.Si modified composition has a different microstructure to that obtained with other

Fig. 13. a) BSE microstructural detail of Al2O3 and TiO2 with FeSi2.Si: 6 wt% addition,

**O**

sintered at 1450°C for 3 hours. b)EDS of the Al2TiO5 matrix.

**Al**

**FeP**

at 1450°C for 3 hours. b) EDS of the intergranular MnTiO3 eutectic phase (EP).

**AT**

(AT: Aluminum titanate; Al: Alumina).

**AT**

**Al**

additives (Fig.13a-d).

**TE**

Microstructure grain size decreased with MnO contents in the sintered bodies.

11b).

Fig. 13. c) EDS of ternary eutectic (TE) and d) EDS of the intergranular Fe rich phase (FeP).

A fraction of free Al2O3 remains as an intragranular phase, the TiO2 reacts completely and besides the Al2TiO5 matrix phase a ternary eutectic reaction grainy phase is formed between the Al2O3-SiO2-TiO2 (as low as it is undetected by RXD), but increasing its quantity with the additive and, also a fourth intergranular phase rich in Fe is depicted (Fig.13d) (Arenas et al. 2011).

Pure and concentrated ilmenite (FeTiO3), additions have a beneficial effect on grain growth control (Fig. 14 and 15). The SiO2 left in the purified mineral promoted the formation of an intergranular liquid phase which could not be detected by XRD. Microstructures are practically free of unreacted original phases.

Fig. 14. a) BSE microstructural x1000 and a detail X3000, of Al2O3 and TiO2 with pure FeTiO3: 6 wt% addition, sintered at 1450°C for 3 hours. Notice the grain growth control.

Reactive Sintering of Aluminum Titanate 517

For FeSi2.Si added samples (Fig.17), although the presence of oxidizing atmosphere, leads to the oxidation of Si and subsequent formation of ternary liquid phase between Al2O3, TiO2 and SiO2 which promotes a good densification, it has a minimal beneficial effect on stabilization. The presence of Al2TiO5 diffraction peaks is small if compared with that of Al2O3 and TiO2, product of decomposition. It might be explained as, that only a fraction of

Al2O3 TiO2 Al2TiO5

20 30 40 50 60 70

Fig. 17. X Ray Diffraction of Al2TiO5 with FeSi2.Si 6 wt% addition, heat treated for 100hours

The addition of pure FeTiO3 (ilmenite*)* clearly shows an increase in the aluminum titanate

Al2O3 TiO2 Al2TiO5

20 30 40 50 60 70

Fig. 18. X Ray Diffraction of Al2TiO5 with FeTiO3 6wt% addition, heat treated for 100hours at

2 theta

2 theta

6% FeTiO3

6% FeSi2.Si

the Fe ions from the FeSi2 react and substitute the Al+3 ions, stabilizing the material.

HT 100h 1100ºC

Intensity (A.U)

at 1100°C.

1100°C.

stabilization (Fig. 18).

HT 100h 1100ºC

Intensity (A.U)

Fig. 15. a) BSE microstructural detail X3000, of Al2O3 and TiO2 with with concentrated placer ilmenite (FeTiO3. SiO2): 6 wt% addition, sintered at 1450°C for 3 hours. b) EDS of intergranular phase due to SiO2 presence.

#### **6.4 Thermal stability**

In order to determine the compositions stability, XRD analyses were performed on samples heat treated at 1100°C for 100 hours. The temperature selection is based on industrial applications working conditions and the maximum temperature for decomposition to occur.

The samples without additives and those with V2O5 and MnO showed a complete decomposition after heat treatment, as only the diffraction peaks of Al2O3 and TiO2 were showed. (Fig. 16).

Fig. 16. X Ray Diffraction Al2TiO5 without addition, with 6wt%V2O5 and 6wt% MnO, heat treated for 100hours at 1100°C.

**a)**

Fig. 15. a) BSE microstructural detail X3000, of Al2O3 and TiO2 with with concentrated placer

**O**

In order to determine the compositions stability, XRD analyses were performed on samples heat treated at 1100°C for 100 hours. The temperature selection is based on industrial applications working conditions and the maximum temperature for

The samples without additives and those with V2O5 and MnO showed a complete decomposition after heat treatment, as only the diffraction peaks of Al2O3 and TiO2 were

No addition HT 100h

Al2O3 TiO2 Al2TiO5

20 30 40 50 60 70

Fig. 16. X Ray Diffraction Al2TiO5 without addition, with 6wt%V2O5 and 6wt% MnO, heat

2 theta

6% V205

Element Wt% At% Ok 17.61 32.28 AlK 33.60 36.53 SiK 3.02 3.15 TiK 45.78 28.04 **Al** Total 100.00 100.00

**Au Rec . Si**

**Ti**

**IP**

**b)**

6% Mn0

ilmenite (FeTiO3. SiO2): 6 wt% addition, sintered at 1450°C for 3 hours. b) EDS of

intergranular phase due to SiO2 presence.

**AT**

1100ºC

Intensity (A.U)

treated for 100hours at 1100°C.

**6.4 Thermal stability** 

decomposition to occur.

showed. (Fig. 16).

For FeSi2.Si added samples (Fig.17), although the presence of oxidizing atmosphere, leads to the oxidation of Si and subsequent formation of ternary liquid phase between Al2O3, TiO2 and SiO2 which promotes a good densification, it has a minimal beneficial effect on stabilization. The presence of Al2TiO5 diffraction peaks is small if compared with that of Al2O3 and TiO2, product of decomposition. It might be explained as, that only a fraction of the Fe ions from the FeSi2 react and substitute the Al+3 ions, stabilizing the material.

Fig. 17. X Ray Diffraction of Al2TiO5 with FeSi2.Si 6 wt% addition, heat treated for 100hours at 1100°C.

The addition of pure FeTiO3 (ilmenite*)* clearly shows an increase in the aluminum titanate stabilization (Fig. 18).

Fig. 18. X Ray Diffraction of Al2TiO5 with FeTiO3 6wt% addition, heat treated for 100hours at 1100°C.

Reactive Sintering of Aluminum Titanate 519

**Ti**

**Al**

**a) b)**

**c) d)**

**e)**

Fig. 20. a) BSE Microstructural detail x3000, Al2TiO5 with : a) V2O5 6wt% ; b) MnO 6wt%; c) FeSi2.SiO2 6wt% d) FeTiO3 6wt% and e) FeTiO3.SiO2 9wt% addition, heat treated at 1100°C

**AT**

The quantification of Al2TiO5 decomposed after heat treatment was also determined using the internal standard method being the results showed in Table 3. The values obtained corroborate BSE image analysis, observations. The stabilization addition effect being higher in the compositions with pure and concentrated mineral, the MnO and the ferrosilicon

for 100 hours. (AT: Aluminum titanate; Ti: Titania; Al: Alumina).

stabilize slightly while vanadium oxide have not any effect.

**6.5 Al2TiO5 decomposition phase quantification** 

**Ti**

**Al AT**

**Al**

**Ti**

**Al**

**Ti**

This behavior agrees with the expected solid solution formation between FeTiO3 and Al2TiO5, as depicted in the calorimetric studies (DSC). These experiments show the decomposition of ilmenite in air atmosphere to Fe2O3 and TiO2 with the formation of Fe2TiO5 (Suresh et al.,1991), followed by Al2TiO5 reaction at higher temperature. This allows the possibility of a solid solution formation between Al2TiO5 and the isostructural Fe2TiO5 by a cation replacement mechanism.

From the ionic radii concept, the structural stabilization might be explained by the incorporation of Fe+ 3 (r=0.67Å) which decreases the structure distortion, caused by the Ti+ 4: Al+ 3 radii difference (Shannon, R., 1969).

In the case of ilmenite addition, but from the concentrated mineral (Fig.19), the structural stabilization effect is evidenced only for the composition with 9% addition. This behavior might be attributed to the SiO2 contamination which, on the other hand, benefits body densification by a liquid phase sintering mechanism.

Fig. 19. X Ray Diffraction of Al2TiO5 with FeTiO3.SiO2 9wt% addition, heat treated for 100hours at 1100°C.

The microstructure study by SEM-EDS corroborated the X-ray analysis evaluation (Fig. 20). All heat treated samples showed the characteristic elongated grain shape, typical of the Al2TiO5 decomposition into its original precursors raw materials Al2O3 and TiO2. However, stabilized Al2TiO5 phase is observed in the compositions with 6% pure ilmenite, 9% concentrated mineral and in lower proportion in the samples with 6% ferrosilicon.

This behavior agrees with the expected solid solution formation between FeTiO3 and Al2TiO5, as depicted in the calorimetric studies (DSC). These experiments show the decomposition of ilmenite in air atmosphere to Fe2O3 and TiO2 with the formation of Fe2TiO5 (Suresh et al.,1991), followed by Al2TiO5 reaction at higher temperature. This allows the possibility of a solid solution formation between Al2TiO5 and the isostructural Fe2TiO5

From the ionic radii concept, the structural stabilization might be explained by the incorporation of Fe+ 3 (r=0.67Å) which decreases the structure distortion, caused by the Ti+ 4:

In the case of ilmenite addition, but from the concentrated mineral (Fig.19), the structural stabilization effect is evidenced only for the composition with 9% addition. This behavior might be attributed to the SiO2 contamination which, on the other hand, benefits body

Al2O3 lTiO2 Al2TiO5

20 30 40 50 60 70

The microstructure study by SEM-EDS corroborated the X-ray analysis evaluation (Fig. 20). All heat treated samples showed the characteristic elongated grain shape, typical of the Al2TiO5 decomposition into its original precursors raw materials Al2O3 and TiO2. However, stabilized Al2TiO5 phase is observed in the compositions with 6% pure ilmenite, 9%

Fig. 19. X Ray Diffraction of Al2TiO5 with FeTiO3.SiO2 9wt% addition, heat treated for

concentrated mineral and in lower proportion in the samples with 6% ferrosilicon.

2 theta

9% FeTiO3.SiO2.

.

by a cation replacement mechanism.

HT 100h 1100ºC

Intensity

100hours at 1100°C.

 (A.U)

Al+ 3 radii difference (Shannon, R., 1969).

densification by a liquid phase sintering mechanism.

Fig. 20. a) BSE Microstructural detail x3000, Al2TiO5 with : a) V2O5 6wt% ; b) MnO 6wt%; c) FeSi2.SiO2 6wt% d) FeTiO3 6wt% and e) FeTiO3.SiO2 9wt% addition, heat treated at 1100°C for 100 hours. (AT: Aluminum titanate; Ti: Titania; Al: Alumina).

### **6.5 Al2TiO5 decomposition phase quantification**

The quantification of Al2TiO5 decomposed after heat treatment was also determined using the internal standard method being the results showed in Table 3. The values obtained corroborate BSE image analysis, observations. The stabilization addition effect being higher in the compositions with pure and concentrated mineral, the MnO and the ferrosilicon stabilize slightly while vanadium oxide have not any effect.

Reactive Sintering of Aluminum Titanate 521

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

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

Velocity (mm/s

Relative Absortion

 (%) 0.0

Velocity (mm/s


**a)** 0.9 **b)**

**c)**


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

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.

noted that results are not accurate, with considerable dispersion.

Velocity (mm/s


 (%)

0.0

0.9

Relative Absortion

c) 6wt% FeTiO3.SiO2 additions.

**6.7 Mössbauer spectroscopy** 

ilmenites and ferrosilicon.

Relative Absortion

0.9

 (%) 0.0


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