Investigation of the devitrification and microwave penetrating properties of fused silica

Four kinds of fused silica (SiO₂) bulk materials were prepared by the hot-pressing method. Bulk SiO₂ made from sol-gel powders was optically transparent and exhibited the best microwave penetrating properties. Commercial SiO₂ obtained from melted quartz increased crystallization. The addition of polyvinyl alcohol (PVA) (aqueous solution) blocked the crystallization of commercial SiO₂ to a limited extent. In contrast, PVA powders accelerated the heterogeneous nucleation and growth of crystalline silica. In this case, the bulk material of SiO₂-PVA was visibly darkened and exhibited the worst microwave penetrating properties. X-ray diffraction patterns and electron probe X-ray microanalysis confirmed that crystallization was correlated with the optical transmission loss. Crystallized areas randomly dispersed in the continuous amorphous body of fused SiO₂, served as scattering centers which affected the optical transparency and also served as microwave absorbing or scattering centers which limited the microwave penetration. The connection between devitrification and microwave penetrating properties of fused SiO₂ could therefore be used as a reference to produce other microwave penetrating materials with better performance.