Zahra Negahdari

Development of Novel Functionally Graded Al2O3-Lanthanum Hexaaluminate Ceramics for Thermal Barrier Coatings

Lanthanum hexaaluminate (LHA) has superior thermo-chemical stability at temperatures higher than 1000 °C and is a promising competitor to Y-ZrO2-based thermal barrier coatings (TBCs). The yet unresolved problem is control of microstructure of high LHA content ceramics and adjustment of porosity upon, to arrive at a material exhibiting low thermal conductivity at high temperature combined with structural reliability. Therefore, a functionally graded alumina/lanthanum hexaaluminate (FGLHA) with a gradient in composition was developed. The thermal diffusivity and thermal conductivity of FGLHA were compared with the one of the monolithic composite ceramics. The alumina-rich composites showed excellent mechanical properties whereas the LHA-rich composites presented lower thermal conductivity.

Comparison of Microwave and Conventional Sintering of LHA Ceramics and Functionally Graded Alumina-LHA Ceramics

The low thermal conductivity of Lanthanum hexaaluminate, abbreviated as LHA, combined with high structural reliability of alumina matrix ceramics attracted our attention to develop a new functionally graded layered LHA-Al2O3-composite, with a LHA and a porosity gradient along the thickness of a bulk oxide ceramic. LHA is formed by in–situ reaction during sintering of the alumina/LHA composite. The high sintering temperature required for completion of LHA formation in LHA-rich layers causes grain growth and a degradation of mechanical strength in alumina-rich layers. Therefore, microwave hybrid heating was investigated as a method to enhance the reaction rate without excessive grain growth. Comparison of conventionally and microwave assisted sintered homogenous composite ceramics with 20–80 volume percent LHA showed that utilization of microwave heating could enhance the solid–state reaction and densification in samples containing more than 20 volume percent LHA. Enhanced microwave absorption in LHA rich layers assisted the sintering of a functionally graded composite at lower temperatures, enabling LHA formation without any abnormal grain growth in alumina rich layers.

Influence of Slurry Formulation on Mechanical Properties of Reticulated Porous Ceramic

Increased processing pressure has resulted in need for the industry to optimize mechanical properties of ceramic foam filters used for molten metal processing. This paper will summarize the effect of ceramic slurry formulation on mechanical properties of reticulated porous ceramics processed by replica method. Commercial polyurethane sponge materials were chosen as a template to produce the RPC (reticulated porous ceramic) from a shear thinning multi component aqueous slurry composed of alumina, SiC, colloidal silica and some different additives. Thermal transformation sequence of the mixtures, resultant macrostructure and microstructure, relative density, grain morphology, phase composition, and mechanical strength of the sintered porous ceramics were investigated. Mechanical properties of struts were investigated by a theoretical model. The RPC had its maximum strength when Alumina/SiC weight ratio was 4.4. Such materials sintered at 1650° C in air, due to their excellent functional properties, are suitable for application as molten metal filter.

Microwave-Material Interaction in Oxide Ceramics with Different Additives

A systematic study upon microwave (2.45 GHz frequency) and conventional heating (resistance heating furnace) was undertaken on porosity, grain growth and densification of commercial grade Al2O3-ceramics doped with MgO (aliovalent doping), with ZrO2 (grain boundary pinning) and with additives promoting elongated grain growth (LaAlO 3 and La2O3, AlPO4). Processes accompanied by a strong non-equilibrium situation, e.g., dissolution, segregation, vacancy formation, are influenced by the presence of the microwave field during heat treatment, visible on microstructure differences of microwave as compared to conventionally sintered samples. Regular grain growth is almost not affected by the microwave field, but the on-set of exaggerated grain growth and pore coalescence is delayed and occurs at higher density as compared to conventional sintering. Such microstructure differences seem to be more pronounced if the surface to volume ratio of the samples is low, therefore volumetric heating has a larger contribution to transport phenomena as compared to small samples.