DSpace JSPUI

Porous silica and silica matrix composite foams reinforced with either glass or silica fibers have been developed with the pore structure and size distribution tailored through optimization of the process parameters for achieving the desirable mechanical and thermal properties. Slurry based processing route involving the use of fused silica powder, binders, long chain surfactants (surfactant stabilization route) or partially hydrophobized silica particles (particle stabilization route) and drying control additives as raw materials, as well as and the direct foaming method to generate pores through entrainment of air in the slurry have been optimized. Rheological studies have been carried out on the slurries to examine the effect of different solids loading, and volume fractions of individual additives on the viscosity, with the objective of optimizing the slurry composition and viscosity for maximum air entrainment and foam stability. In addition, the appropriate duration of air entrainment into the slurries for obtaining stable foams, and the optimum drying conditions required for obtaining defect free green bodies, have been determined experimentally. Furthermore, random dispersion of glass or silica (quartz) fibers (aspect ratio >1000 and 10 μm diameter) up to 10 wt.% has been achieved in the silica based composite foams, through the addition of partially hydrophobized silica powder. Absence of fiber entanglement in suspensions and sintered composite foams has been confirmed using X-ray radiography. The green bodies of both the porous silica and composite foams have been sintered at 1100oC. Surface densification of some of the sintered silica foams (surfactant stabilized), using silica has led to significant increase in their handling strength and hardness. Use of optical and scanning electron microscopy accompanied by image analysis has shown spherical, open and interconnected pores with size distribution of ~10-200 μm in the surfactant stabilized foams, and this has also been confirmed using mercury porosimetry. The pore volume fractions of the foams have been found to be in the range of ~38-90% using Archimedes principle. The pore size distribution and the relative densities are found to be the functions of the slurry composition and process parameters. Permeability of the foams has been found to decrease with increasing relative density. The pore size distribution of the particle stabilized foam with 85 vol.% porosity has been found to be bimodal with the pores of 4-10 μm and 50-1000 μm sizes. The structure-property relationship of the foams has been studied with emphasis on the influence of pore size and volume fraction on the mechanical properties. Although both the Young’s modulus and compressive strength increase with relative density, yet the latter property also depends on the pore sizes and strut thickness. The porous silica (surfactant stabilized) with relative densities between 0.18 and 0.62, have shown Young’s modulus and compressive strengths in the range of 120–380 MPa and 0.5–3.3 MPa, respectively. The Young’s modulus and the compressive strength of the particle stabilized foams have been found to be almost a third of those obtained in case of the surfactant stabilized foam with similar density, because of the presence of large pores and much thinner struts in the former foam. The composite foam has shown higher Young’s modulus, but much lower compressive strength than that of the surfactant stabilized silica foam. While the presence of fiber reinforcement leads to increase in Young’s modulus, the larger pores and piercing of struts by fibers cause reduction in compressive strength. The front faces of 25 mm thick porous silica tiles have been subjected to heating to 1275oC to determine thermal properties. The thermal diffusivity and conductivity calculated by fitting the back face temperature profile into a one dimensional heat flow model have been found as 0.55 m2/s and 0.18 W/m.K, each of which is 1/8th that of dense fused silica. Keywords: Silica, Fiber, Composites, Foams, Mechanical Properties, Thermal Properties.