Improvement in colloidal Sol Gel SiO(2) glass by attrition of Sols.

The two step collodial sol gel process for forming SiO(2) glasses has been improved by the use of attrition to break the aggregates formed after a sol prepared from fumed silica is dried and redispersed in water. The attrition was performed in ball mills consisting of borosilicate glass jars with fused SiO (2) cylinders as milling media. Measurements of the aggregate sizes in the sol showed that the average size of aggregates was reduced from >40microns to 3 microns by milling up to 64h depending on the hardness of the once dried gel. Data on pore size distribution in the twice dispersed and dried gels show that regardless of milling times about 24-30 vol % of the pores are due to interaggregate spaces (pores whose sizes are larger than those represented by the average 16 nm pores between primary SiO(2) particles). These large pores cause a bimodal size distribution with the secondary peak at 3-4 microns with no milling and the position of the secondary peak decreases in average size with milling until it is no longer resolved from the primary peak. The average size of the interaggregate pores is more than an order of magnitude smaller than the average size of the aggregates due to the efficiency of packing the aggregates of a broad size distribution. Milling is shown to markedly improve the transparency of sintered SiO(2) at the expense of increased drying shrinkage.