Abstract
The sintering of dried colloidal SiO2 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 SiO2 at ∼925°C, moderate heating rate (∼400°C/h), He+CI2 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 SiO2 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 SiO2 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.
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