Improvement in colloidal sol-gel SiO2 glass by attrition of aggregates in sols

The two-step colloidal sol-gel process for forming SiO2 glasses has been improved by the use of attrition in ball mills consisting of borosilicate glass jars with fused SiO2 cylinders as milling media. The average size of aggregates was reduced from more than 40 μm to less than 3 μm by milling up to 64 h depending on the hardness of the dried gel. Data on pore size distribution in the twice dispersed and dried gels show that 24 to 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 SiO2 particles). These large pores cause a bimodal size distribution with the secondary peak at 3 to 4 μm with no milling and the position of the secondary peak decreases 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 the SiO2 glass after sintering.