5. Summary

were carried out according to ASTM C1198–91 using the resonance bar technique (Scanelastic

Figure 16 depicts the in situ elastic modulus evolution (first and second cycles) up to 1400C, in addition to results of the as-sintered foamed Al2O3 at 1500C/4 h. Table 5 presents the crystalline phase changes obtained by X-ray diffraction as a function of temperature for

The foaming and casting processes of Al2O3 suspension were carried out at room temperature (~25C). As reported by the literature [72], the main cement hydrate phase formed at room temperature is CAH10 (CaO.Al2O3.10H2O). When the temperature increases, this phase partially dehydrates ~110C into a mixture of gibbsite, AH3 (Al2O3.3H2O) and tricalcium aluminate hydrate, C3AH6 (CaO.Al2O3.6H2O). This suggests that the drop of

0 200 400 600 800 1000 1200 1400

green (1st cycle) 2nd cycle after sintering

Temperature (°C)

Figure 16. In situ elastic modulus of macroporous Al2O3: blue curve corresponds to the first cycle up to 1400C of the green sample; the red curve is the measurement after the first cycle, and the green curve corresponds to the measurement after sintering up to 1500C/4 h. The arrows indicate the discontinuities in the curve caused by decomposition or

Crystalline phases 110C 1000C 1200C 1400C 1500C α-Al2O3 \*\*\*\*\*\* \*\*\*\*\*\* \*\*\*\*\*\* \*\*\*\*\*\* \*\*\*\*\*\*

CaO.2Al2O3, CA2 \* \*\*\* \*\*\*\* \* CaO.6Al2O3, CA6 \*\*\*

The concentration of phases is qualitatively defined by the number of asterisks (\*) displayed [48].

Table 5 Phase changes obtained by X-ray diffraction in alumina composition containing 5 wt% of high alumina cement.

equipment, ATCP, Brazil).

192 Recent Advances in Porous Ceramics

ceramic composition.

formation reactions of specific ceramic phases [48].

CaO.Al2O3, CA \*\*\* \*

Al2O3.3H2O, AH3 \*\* CaO. Al2O3.6H2O, C3AH6 \*\*

**Elastic modulus (GPa)**

This chapter reviews the mechanical properties of porous ceramics with special interest on the mechanical strength, fracture toughness and elastic modulus of these materials.

One of the more significant findings to emerge from analysis of mechanical strength section is that, in addition to the porosity of porous ceramics, the number of connecting struts between the cells/pores in the microstructure plays a fundamental role in their mechanical strength behavior. Data from the literature support that once the connecting struts are lost in the porous structure, it is impossible to recover the original mechanical strength of it by merely increasing the struts thickness.

Regarding to fracture toughness of porous ceramics, two factors appear to control this property: the presence of surrounding pores at the crack front and the interaction of cracks with the pores in the microstructure.

Finally, the in situ hot elastic modulus analysis appears as an important method to better understand the processing steps as well as for predicting the life operation of this class of ceramic materials.

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