Structural evolution of sol-gel derived ceramics during sintering

The objective of the study was to improve our understanding of the structure, energetics and physical properties of sol-gel derived ceramics and glasses. The gel to ceramic evolution of colloidal derived sol-gels and alkoxide derived sol-gels of silica was followed by photoacoustic Fourier-transform infrared spectroscopy, high temperature reaction calorimetry, thermal analysis and physical characterizations.; The effects of binder and firing atmosphere in the gel to ceramic transition were examined for colloidal derived gels. Polyvinyl alcohol was an effective binder that promoted sintering and crystallization by enhancing the silica framework with hydrogen-bonding. Without a binder, colloidal silica sinters at 1150{dollar}spcirc{dollar}C only in an water vapor saturated firing atmosphere.; For the alkoxide silica system, the effects of (a) catalysts or pH, (b) drying control chemical additives (DCCA's), and (c) supercritical drying were examined. Alkoxide gels resulted in a material with small pores and high surface area. The materials were sintered to glass and energetically equivalent to fused silica by 800{dollar}spcirc{dollar}C. The base-catalyzed gels were composed of larger primary particles, lower surface area, and greater extent of Si-O-Si order analogous to the colloidal gels. These materials contained strained siloxane bonds. Relaxation of these strained bonds gave off substantial energy releases during annealing and sintering. The larger pores prevented the materials from sintering at 1000{dollar}spcirc{dollar}C.; The DCCA's facilitated drying, however, DCCA's delayed densification from 800{dollar}spcirc{dollar}C to 1000{dollar}spcirc{dollar}C. The pH variation caused by the DCCA was important to the resulting structure and energetics of the two-step processed gels. Oxalic acid and glycerol were acidic and neutral DCCA's, respectively. They gave rise to small pore sizes. Formamide was a basic DCCA that produced larger pore sizes that expedited drying but challenged sintering. More strained siloxane bonds from rapid condensation were found in the resulting gel, making the material less thermo-dynamically stable and more difficult to transform to fused silica.; Large monolithic alkoxide gels were obtained by supercritical drying. Gels with large pores and small surface areas result from supercritical drying. Higher Si-O-Si order existed in these gels, indicating the supercritical drying process consolidated the silica backbone. The pores were persistent and difficult to collapse, and sintering initiated in the supercritically dried two-step processed gels only above 1000{dollar}spcirc{dollar}C. (Abstract shortened with permission of author.)...