Doping of silica during sintering

The effect of temperature on the dopant concentration profile is studied for the case of fluorine doping of porous silica performs during sintering. The fluorine doping profile in a sintered blank is a function of relative rates of diffusion of fluorine carrying gas species through the porous preform, gas–solid reaction of fluorine species with silica preform and sintering of fluorine doped glass/soot. At low temperatures (<1225 °C), the rate of diffusion is much faster than the reaction and sintering rates and gas phase fluorine species (here taken to be SiF4) is able to easily reach the inner portions of the preform, resulting in very uniform doped profiles. However, as the temperature of the doping/sintering is increased, the rates of reaction and sintering increase at a much faster rate than diffusion. Therefore, at temperatures above 1275 °C, non-uniform fluorine doped profiles are obtained. At temperatures around 1350 °C, the sintering and reaction rates are such that fluorine cannot even reach the inner portions of the preform. A model is presented here which predicts fluorine doping behavior of silica preform during sintering. The model considers simultaneous diffusion of fluorine carrying gas species through the porous preform, gas–solid reaction of fluorine species with silica preform and sintering of fluorine doped glass. Predictions from the model are in agreement with the experimental trends reported in Deliso et al. [E.M. Deliso, C.E. Lacy, D.L. Marlatt, Method for controlling fluorine doping in soot preforms, US6263706 B1, 2001] and Dawes et al. [S.B. Dawes, A.P. Schermerhorn, V. Srikant, P. Tandon, Fluorine doping a soot preform, US Patent Application Publication No. US2002/0073740 A1, 2002].