T. Ebadzadeh

Processing-microstructure-property relations in mullite-cordierite composites

The complex interrelations between processing, microstructure and electrical and mechanical properties are considered for mullite-cordierite composites. Fabrication procedures using mixed powders or routes based on sol-gel precursors are presented, along with the phase evolution, densification and microstructural formation. Final microstructures (including porosities and grain sizes) are correlated to the mechanical (hardness, toughness and strength) and electrical (dielectric constant) properties.

Thermal shock resistance of mullite–cordierite refractories

Abstract The effect of aggregate size on the strength degradation of mullite-cordierite specimens subjected to thermal shock has been studied. Strength degradation of shocked specimens is shown to depend on the aggregate size, on the preshock strength of sintered specimens, and on cordierite content. Retained strength after shocking increased in specimens prepared from small aggregates owing to the higher initial strength. These specimens contained lower cordierite content as a result of the increased dissociation of cordierite brought about by decreasing the aggregate size.

Rheological study on cordierite paste during extrusion

Abstract A cordierite compound used in the catalytic converter was synthesised with commercial materials (kaolin, ball clay and magnesite) by heat treatment at 1350°C. For increasing the open porosity of sintered body 40 wt-% of raw materials were mixed with 60 wt-% of the same calcined batch (heated at 1350°C). A paste rheology of the above mentioned body was studied with a ram extruder and extrusion was done under two different conditions, i.e.constant force and constant velocity. The rheological parameters were obtained by the Benbow–Bridgwater model. The pastes were found to have the Bingham behaviour. The effects of extrusion velocity and die geometry on the extrudated surface defects were also investigated. The results showed that while the increase in the die length reduces the surface defects of the specimen, the extrusion velocity increases the popularity of the surface flaws.

In situ reaction synthesis of Al2O3-SiC nanostructure using different carbon sources

Abstract The formation of Al2O3–SiC nanostructure was studied using three different carbon sources (charcoal activated, graphite and carbon black) mixed with colloidal silica and aluminium nitrate. All mixtures were heated at 1500°C for duration of 30, 45 and 60 min. The results showed that Al2O3–SiC powders with an average diameter of ∼220 nm and almost equiaxial geometry with aspect ratio of 1–1·2 could only be synthesised from the mixture containing carbon black (30 wt-%) at low heating time (30 min). It was found that the intensity of SiC peaks was the highest in samples containing graphite which was attributed to the higher initial density of this sample.

Thermal shock resistance of zircon/SiC composite

Abstract In the present work, thermal shock tests of zircon/silicon carbide (30 wt-%) composite specimens were evaluated up to 1000°C and compared with pure zircon specimens at nearly the same porosity content. Results confirmed that the fracture strength of the quenched specimens was not affected with the increase in quenching temperatures by incorporating SiC particles, indicating resistance to crack propagation. On the other hand, the critical temperature difference ΔT, below which material maintains its original strength, was found higher in composite rather than pure zircon specimens. X-ray diffraction investigations showed that zircon on the surface layer of composite specimens decomposes and produces a specimen comprising core and shell.

MICROWAVE SINTERING OF NANOCRYSTALLINE 8YSZ: EFFECT OF HEATING SCHEDULE ON MICROSTRUCTURE, IONIC CONDUCTIVITY, AND MECHANICAL PROPERTIES

Nanocrystalline 8 mol.% yttria-stabilized zirconia powder prepared from a combustion synthesis route has been sintered using microwave (MW) energy by two routes: Conventional one-step and a novel two-step method. The final products from the two routes have been compared regarding their microstructure, ion-conductivity, and mechanical properties. It is shown that it is possible to produce highly dense compacts with significant smaller grains using the novel two-step MW sintering method with significant smaller energy consumption with respect to conventional one-step MW assisted sintering. The novel method is able to produce sintered samples with near similar ionic conductivity at temperatures higher than 700°C and slightly lower mechanical properties at room temperature.