Title: Study of the Stress Relaxation Mechanisms Enabling Shape Preserving Silica Magnesiothermic Reduction

Shape preserving silica magnesiothermic reduction makes use of gas-silica displacement chemistry to convert intricately shaped, hierarchically ordered, three-dimensional silica structures into porous silicon replicas while maintaining complex features of the starting silica template down to ~15 nm.  This technique has been used to convert a number of starting silica templates into porous silicon replicas including diatom frustules, inverse opals, nanospheres, micropatterned structures, etc., which exhibit enhanced surface area and porosity due to the nature of the silicon conversion chemistry. 

Despite the progress made in applying silica magnesiothermic reduction to a wide range of silica templates, understanding of the mechanisms enabling shape preservation is still poor.  In particular, large compressive stresses (~2-5 GPa) are evolved in the products of silica magnesiothermic reduction due to large changes in molar volume upon reaction (~20-60% depending upon silica polymorph/crystallinity).  Despite these stresses, geometric distortion between the silica templates and porous silicon replicas is only very modest.  This implies that a stress relaxation mechanism operates concurrently with the reaction, which enables the process to be shape preserving.  This investigation hopes to gain a better understanding of the nature of this stress relaxation mechanism.  Silica wafers will be magnesiothermically treated to produce thin product films on the surface which will be characterized using XRD (sin2ψ method) to quantify the compressive stress evolved in these product films.  Annealing treatments at the reaction temperature will then be applied to induce stress relief in the product films.  Microstructural changes in the product films will then be characterized to discern correlations between the microstructural evolution of the product films and the rate of stress relief during annealing, revealing the nature of the stress relaxation mechanism operating during silica magnesiothermic reduction.  It is hoped that this work will provide a better understanding of the silica magnesiothermic reduction process which can be used to attain better replication of starting silica templates as the complexity of these templates increases.