Synthesis and Characterization of Silica-Silica Porous Composite and Calcium Strontium Zirconium Phosphate Ceramics for Thermal Protection Applications

Abstract

A porous silica –silica composite was processed with varying fiber diameters using the slurry moulding technique. The advantage of the process was that the density of the composite could be processed to the required levels. The reinforcements used were fibers obtained by leaching E-glass cloth, imported silica fibers with diameter <1.8µ and hollow silica fibers processed using sol-gel method. All the properties depend on the density of the composite. The compressive strength was measured in the perpendicular and parallel directions. Strength was high when the load axis was along the fiber direction. The composite with fine fibers (< 1.8 µ pure silica fibers) showed higher strength compared to the leached silica fibers. The thermal conductivity measurement on these composites showed an increase with temperature owing to the domination of radiation at high temperatures. As the vacuum level was approached, the thermal conductivity showed a decrease due to the absence of the convective part of the thermal transfer process. For use as a thermal protection system, it is important to measure the thermal response of these tiles in a simulated re-entry environment. Tests were done to measure this response for a given heat flux conditions at 38W/cm2 to 75W/cm2 and the backwall temperature was measured for various types of silica -silica composites. The role of impurities like sodium and B2O3 was also studied with respect to the conversion from amorphous to crystalline forms of SiO2. The severe increase in the coefficient of thermal expansion when SiO2 converted from amorphous to α– crystoballite was also measured. CSZP CSZP which belongs to the NZP family was processed using the co-precipitation technique. The influence of substituting the ‘P’ site with ‘Si’ atom was studied for its influence on thermal expansion – both at the bulk level by dilatometry and at the intrinsic level using high temperature XRD. For many anisotropic materials micro-cracking is a serious issue while cooling from the sintering temperature. It has been previously proved that this extent of micro-cracking depends on the particle size. Smaller the particle size is therefore preferred. One of the significant results obtained in this study was the successful use of microwaves to process crack free CSZP with fine grain size. CSZP with 95% density having a grain size as small as 1µ have been processed using microwave sintering. Dielectric property evaluation namely dielectric constant, dielectric loss and temperature coefficient of resonant frequency which are vital parameters required if this material is to be used as a candidate TPS have also been measured. The thermal conductivity of the sample was measured using Laser flash apparatus and was found to be 0.9 W/mk which provides an indication that this material can be used as a successful material for TPS. Finally a composite consisting of silica fiber with CSZP as matrix was processed and tested for heat flux. The low back wall temperature indicates that this material is a potential replacement for silica tile.