Processing Of Zirconia Based Honeycombs And Evaluation Of Thermo Mechanical Properties
Saha, Bhaskar Prasad
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Ceramic cellular solids, mainly honeycombs and foams, are a novel class of composite materials where one phase is an interconnected network of solid struts or plates and the other one an empty phase or possibly a fluid. Honeycombs are an array of two dimensional prismatic cells whereas in foams the arrangements of cells are three dimensional polyhedral cells. Unlike solids, the properties of honeycombs are based on three major variables i.e. a) relative density (p* /p s where p* is the density of the cellular material and ps that of the solid of which it is made) b) cell wall material and c) geometry of the cells. Because of the flexibility in tailoring these variables, cellular solids can be engineered to exhibit a unique combination of mechanical and thermal properties for diversified thermostructural applications. Ceramic based honeycombs fabricated out of cordierite (2MgO.2Al2O35SiO2), mullite (3Al2O32SiO2), cordierite: mullite (2MgO.2Al2O35SiO2) with specific configurations are the leading candidates for many of the applications such as substrates for catalytic converters, molten metal filters, air heaters and heat exchangers etc. Zirconia by the virtue of its high fracture toughness and low thermal conductivity and high refractoriness is an interesting ceramic material and explored for versatile applications. However, no significant efforts have been reported to produce zirconia/alumina and their composite based honeycomb structures and also they have not been explored for their thermo-mechanical and energy absorption based applications. In the present study, looking at the possible potential applications of the honeycombs of Zirconia/alumina and their composites such as solid oxide fuel cells, high temperature filters, blast protection tiles etc., attempts are made to fabricate honeycomb structures. Chapter 1 of the thesis describes the detailed literature survey that has been carried out using advanced search packages regarding the evolution of ceramic honeycomb structures and their properties followed by the advantages of zirconia/alumina and their composites as candidate materials for targeted applications. Literature survey also covers the various processing techniques, characterization procedures with special emphasis on the thermo-mechanical properties. Chapter II describes attempts on developing an optimum scheme of processing of zirconia honeycomb which includes selection of precursor oxides, mixing of formulations, dough making based on viscosity measurements, shaping by extrusion, microwave drying, debinding and sintering to obtain the defect free monolithic structures keeping in view of the scale up possibilities. The chapter also describes a specially developed die fabrication process with innovative machining procedures. (Patent no. 198045). Sintered honeycombs were also characterized for their critical physiochemical properties. In chapter III mechanical characterization of honeycomb samples is reported after subjecting them to compression testing with varying cell channel orientation, compositions and configurations. It is observed that all honeycombs, irrespective of the composition and configuration exhibited anisotropic behavior. In addition, the anisotropy increases with the rib thickness and decreases with increase in the unit cell length. Thermal conductivity measurement studies of the honeycombs are reported in chapter IV. Two types of measurement techniques viz. laser diffraction and monotonic heating technique have confirmed the reduction of thermal conductivity of the honeycomb samples as compared to their solid counterpart. It is observed that the finer channel honeycombs offer low thermal conductivity as compared to the coarser channel when tested across the channel direction. For equivalent relative density, the thermal conductivity value for triangular channel is found to be more as compared to the square channel. Also, the thermal conductivity values were found less when measured across the channel as compared to the values when measured along the channels. The thermal conductivity value for fine channel zirconia-alumina composite honeycombs was found much less than the thermal conductivity of the alumina matrix. Chapter V summarises the implications of the study, conclusions and the target applications.
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