Preparation of High‐Silica Glasses from Colloidal Gels: II, Sintering

The sintering of dried colloidal SiO 2 gels, whose preparation and properties are reported in Part I, is described. The effects of various sintering parameters were studied and the conditions for achievement of the best optical quality include the use of: a pretreatment of the SiO 2 at ∼925°C, moderate heating rate (∼400°C/h), He+CI 2 atmosphere, 1500° to 1600°C sintering temperature, and 1 to 4 h sintering time. Dynamic sintering kinetic studies (heating rate=400°C/h) show that this SiO 2 sinters to nearly theoretical density by about 1380°C. However, optical transparency is achieved by removal of minor residual porosity at above 1500°C. Isothermal sintering data fit to a model assuming interconnecting cylinders of SiO 2 predict the proper activation energy for the viscosity if initial stages of sintering are considered. Residual porosity in sintered glasses is related to large interstices in the unsintered gel.