Preparation of high-purity silica glasses by sintering of colloidal particles

Abstract

The preparation of very pure silica glass with low OH content exceeds the limits of the conventional melting process. Two alternatives are the well-known technologies for the production of optical fibers which meet the highest purity requirements, and the sol-gel process. In this paper a third route (powder ceramic) is presented which enables the preparation of green bodies with high densities (40 to 50 % of the density of compact silica glass). Different forming processes which are known from ceramic technology to be suitable for the production of rods and tubes (extrusion, colloidal gel, colloidal filtering, electrophoretic deposition) are discussed and experimental results obtained with fumed silica (Degussa Aerosil OX 50) as starting material are given. Subsequently, the shrinkage of the compact during drying is discussed. Although the linear shrinkage is relatively high (up to 9 %) , conventional drying is possible and crack-free green bodies can be obtained. Afterwards ah interfering impurities (especially transition elements and OH groups) are removed down to the ppb level at 400 to 1100 °C in an atmosphere containing chlorine. For sintering the samples are moved through a hot zone (temperature: 1500 °C, velocity: 10 mm/min) in helium atmosphere with a small addition of chlorine which leads to transparent shaped bodies of silica glass with glossy surfaces. The kinetics of sintering are studied by mercury porosimetry and shrinkage measurements.