Drying, sol-gels

H. Weetall, Retention of bacteriorhodopsin activity in dried sol-gel glass. Biosens. Bioelectron. 11, 327-333 (1996). 

FIGURE 16J26 Relative linear shrinkage versus HintAring fima for a dried sol-gel TiOj powder (a) 1020°C, (b) 1060°C, (c) 1100°C, and (d) 1160 C. Taken from Barringer [53]. 

Another ceramic processing technique that involves drying as a key step is that of the sol-gel production of ceramic powders. Sol-gel precursors consist of small particles weakly bound together, permeated with networks of fine, interconnected pores. These pores typically are filled with either an alcohol-based or a water-based liquid. Microwave heating has been successfully applied to dry sol-gel precursors. ... 

This method allows the sol-gel siliea NR latex eompound to be moulded into the desired shape. TESPT was eo-mixed with TEOS and eoneentrated NR latex. Ammonia which functioned as base catalyst was added into the concentrated NR latex. The silica-TESPT-NR latex compound was then subjected to heat to complete the sol-gel silica conversion process. The dried sol-gel silica-NR mixture was compounded as per normal mixing procedure. A good dispersion of silica particles of the size between 100 and 500 nm was achieved. Using the two-level factorial design, it was concluded that the mechanical properties, i.e. tensile properties and tear strength, were significantly affected by the TEOS loading. It was also found that the amount of ammonia present in the concentrated latex, i.e. 0.7% (w/w) was sufficient to convert TEOS into silica. 

Many methods were developed for the synthesis of perovskites materials with the target to obtain pure phase and also to increase the specific surface area. However the conventional methods used such as spray pyrolysis, freeze-drying, sol-gel.. . requiring high calcination temperature to generate the perovskite phase (>700 °C), lead to solid with specific surfece area below lOm /g. 

Near Infrared Reflectance Spectrum of a Nearly dried Sol Gel Body... 

Figure 2-17. The pseudo absorption spectra measured by NIRR of a nearly dried sol-gel sample from 1200 to 2400 nm.

Aravind, P.R., Shajesh, P., Mukundan, P. et al. (2009) Silica-titania aerogel monoliths with large pore volume and surface area by ambient pressure drying. /. Sol-Gel Set TechnoL, 52 (3), 328-334. 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

Overview. Three approaches are used to make most sol—gel products method 1 involves gelation of a dispersion of colloidal particles method 2 employs hydrolysis and polycondensation of alkoxide or metal salts precursors followed by supercritical drying of gels and method 3 involves hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient atmospheres. 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

Sol-Gel Techniques. Sol-gel powders (2,13,15,17) are produced as a suspension or sol of coUoidal particles or polymer molecules mixed with a Hquid that polymerizes to form a gel (see Colloids SoL-GELtechnology). Typically, formation of a sol is foUowed by hydrolysis, polymerization, nucleation, and growth. Drying, low temperature calciaation, and light milling are subsequently required to produce a powder. Sol-gel synthesis yields fine, reactive, pseudo-crystalline powders that can be siatered at temperatures hundreds of degrees below conventionally prepared, crystalline powders. 

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