Optical impurity measurements on silica glass prepared via the colloidal gel route

Abstract

The preparation of silica glass by sintering compacts of fumed silica powders is a promising method to reach a very high purity. Due to an efficient purification process the impurities can be reduced to the ppb level. Consequently, problems arise in measuring the remaining contaminations. As the detection limit of the neutron-activation analysis was reached for the transition elements, only absorption measurements on optical fibres could reveal the amount of remaining impurities. With a silicone coating acting as the optical cladding of the undoped core of sintered silica glass a minimum attenuation of 4 dB/km at 980 nm was obtained. As both methods of analysis involve much effort and high costs they are not suitable for measuring large numbers of samples. This is necessary for a characterisation and optimisation of the purification process of the compacts made from fumed silica powders. Therefore, optical absorption measurements were performed in the spectral region of the fundamental absorption modes of the impurities. These measurements could be directly made on slices of the sintered glass. The best detection limit for optical measurements was obtained with the OH mode at 2.7 μm. For a statement of the influence of the wall thickness on the purification efficiency, compacts with large diameters were prepared and examined. The spectroscopic investigations on samples up to 42 mm in diameter showed a diffusion-limited OH gradient within the samples. Furthermore, a strong influence of the particle diameter and the pore size of the porous compacts on the purification efficiency was observed.