G. Urretavizcaya

Electrical properties and thermal expansion of cordierite and cordierite-mullite materials

Abstract Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. The employed cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller size component. Mullite powder employed in composites was a 7 h-attrition milled one. The dielectric constant (e), dielectric loss tangent (tan δ), resistivity (ρ) and thermal expansion coefficient (α) were measured. The influence of the porosity, mullite and glassy phase contents and grain size in the electrical parameters was analyzed. The thermal expansion coefficient as a function of the composition was studied.

Thermal Evolution of Alumina Prepared by the Sol-Gel Technique

The thermal evolution of an alumina gel synthesized by hydrolysis of aluminium alkoxide (sol-gel technique) was studied by thermal analysis (DTA and TGA), X-ray diffraction, FTIR and NMR spectroscopies, and specific surface area measurements. Between 400 and 900°C, γ- and δ-aluminas were formed showing aluminium vacancies preferentially located in tetrahedral sites. The atomic rearrangements produced during α-alumina formation are oriented to the progressive elimination of tetrahedral aluminium in the ultimate phase. The evolution of the specific surface area during heating is explained by changes in structure and microstructure.

Mechanical behavior of cordierite and cordierite–mullite materials evaluated by indentation techniques

Abstract Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. Cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller component. Mullite powder employed in composites was the 7 h-attrition milled one. The Vickers hardness (HV) and fracture toughness (KIC) were measured by Vickers indentation techniques. The Knoop hardness (HK) and Youngs modulus (EK) were estimated by Knoop indentation method. Fracture surfaces of sintered materials were analyzed using a scanning electron microscope (SEM). The indentation load to determinate the fracture parameters was previously determined. The influence of the porosity, increasing mullite and glassy phase contents and grain size on the mechanical parameters were analyzed and a fracture mechanism was proposed.

Elongated mullite crystals obtained from high temperature transformation of sillimanite

The high temperature transformation of sillimanite to mullite has been studied by means of dilatometry, X-ray diffraction, FT-IR and scanning electron microscopy (SEM-EDS) techniques. The results obtained showed that total transformation of sillimanite to mullite takes place between 1550 and 1600°C. A rapid nucleation occurs above 1550°C due to the resemblance of sillimanite and mullite structures. As a consequence of the high temperature of mullitization of sillimanite, the grain growth of elongated mullite crystals takes place almost simultaneously with the nucleation stage. Finally, a step of acid leaching was employed to remove the glassy phase and high-grade purity mullite powders were obtained.

Pressureless sintering of sol–gel alumina matrix composites

Abstract The effect of the temperature and the conditions of the reducing atmosphere on the sintering behaviour of pressureless-sintered sol–gel alumina matrix composites are studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), and density measurements. The results are compared with those obtained with fine commercial alumina/SiC W composites, and analyzed by means of a thermodynamic study of the system. A notable improvement in sintering behaviour and a more extensive grain growth as the sintering temperature increases are determined for sol–gel alumina matrix samples. The amount of melt observed in the microstructural analysis of sol–gel alumina matrix materials is lower than that detected in fine alumina matrix samples. The mechanisms of formation of the glassy phase due to the reducing conditions of the atmosphere can operate in both series of samples, but the higher content of impurities in fine alumina matrix materials produces a more extensive liquid formation. The presence of glassy phase, higher starting densities, and lower weight losses during sintering, contribute to the higher final densities of fine alumina matrix composites.

Densification improvement of Al2O3SiCw composites by impregnation

Abstract The effect of the impregnation of Al 2 O 3 SiC w composites (0, 5, 10 and 15 vol% SiC) was studied by Hg porosimetry, density measurements and SEM analysis. Samples were prepared by slip casting, precalcined, impregnated with aluminium nitrate, treated with NH 3 and pressureless-sintered in reducing atmosphere. Higher fired densities were obtained because of the lower pore volume and higher green densities resulting from the impregnation. Also, the high reactivity of the small particles filling the pores in the impregnated samples contributed to the densification improvement.

Pressureless sintering of Al2O3/SiCw materials: Effect of the reducing atmosphere

Abstract The effect of the temperature and the conditions of the reducing atmosphere on the sintering behavior of pressureless-sintered Al 2 O 3 /SiC w composites was studied by SEM, XRD and density measurements and analyzed throughout a thermodynamic study of the system. Different partial pressures of oxygen were estimated for two ways of arrangement of the samples, between graphite disks or packed in an SiC bed, the former being lower. A higher amount of glassy phase was observed when pressureless sintering was conducted with samples between graphite disks and at the highest sintering temperature studied (1800 °C). Taking into account the reactions that can occur at the partial oxygen pressures thermodynamically estimated, the formation of a high amount of liquid was explained throughout two mechanisms: formation of a melt with the composition of the eutectic of the Al 2 O 3 -Al 4 C 3 binary system and liquid production by reaction of SiC and Al 2 OC. The higher densities reached at 1800 °C than at 1700 °C were attributed to a liquid-phase assisted mechanism of sintering.