Processing and Properties of Cellular Silica Synthesized by Foaming Sol‐Gels

Abstract

A novel process for fabricating lightweight, cellular ceramics from sols is presented. The process utilizes the rapid viscosity change during gelation to stabilize the structure of a foamed silica sol. Manipulation of gel viscosity and foaming agent concentration resulted in a minimum cell size of 90 pm at 31% density and minimum density of 17% with average cell size of 400 pm. The flexural strength compared favorably with space shuttle tiles and sintered hollow glass spheres. From -50" to 15o"C, the dielectric constant ranged from 1.51 to 1.55 for a 20% dense foam and was slightly dispersive, whereas the dielectric loss was comparable with fused silica. [

Processing Routes to Macroporous Ceramics: A Review

“…176 The well-established polymer technology used for the production of thermoplastic polystyrene foams was also recently applied by Williams and Evans 175 to fabricate ceramic foams with up to 84% porosity, as illustrated in Table IV. Foam consolidation has not been restricted only to polymeric setting systems, but has been imparted also by inorganic materials exhibiting sol-gel phase transitions in liquid medium. This approach was used by Fujiu et al 162 to prepare silica foams using Freon for gas incorporation during the condensation Gelcast setting Free radical polymerization 68,158,[187][188][189][190][191] Cross-linking of polyvinyl alcohol with organotitanates and epoxy with ethylene imine 194,200 Temperature-induced setting of polysaccharides: sucrose, 193 carrageenan gum, 195 agar 199 Thermal expansion 192,193 or mechanical frothing 68,158,165,[187][188][189][190]194,195 / agitation 164,196 in the presence of surfactants/proteins Al 2 O 3 , 68,160,164,187,188,[191][192][193][194][195]197,198 Temperature or pH-induced setting of proteins: gelatine, …”

Fabricating Porous Ceramics

“…The partly closed-cell porous ceramics was prepared by mixing silica gel, surfactants, and methanol [73], and the foaming agent of freon is added to foam and solidify, followed by sintering. The effect of solid content, high-temperature binders, granularity on the shrinkage percentage, porosity, strength, pore size/distribution, and permeability of the product is investigated based on the preparation of mixed slurry of alumina with different granularity and binders [74].…”

Tailoring Porosity and Pore Characteristics in Oxide Ceramic Foams through Controlled Processing

“…The surfactant also plays a role in stabilizing the bubbles at a later stage in the process. 58 Initially the system is maintained at a temperature below the boiling point of freon. The pH is adjusted from an initial value of 9.7 to around 5.5 to accelerate the rate of gelation.…”