Sintering of gels

The low temperatures and correspondingly high viscosities involved in viscous sintering of gels (normally near the glass transition temperature Tg,... 

Very little information is available on the kinetics of sintering of gel fdms. The data of Brinker and Mukherjee [75], shown in Fig. 32, indicate that a film densifies faster than a bulk gel made from the same preparation, even though the film has lower surface area. This is contrary to the theory illustrated in Fig. 14, which predicts that a film will sinter more slowly than an unconstrained gel, since a film can shrink only in the direction perpendicular to the substrate. The experimental results suggest that the films... 

Testing. Chemical analyses are done on all manufactured abrasives, as well as physical tests such as sieve analyses, specific gravity, impact strength, and loose poured density (a rough measure of particle shape). Special abrasives such as sintered sol—gel aluminas require more sophisticated tests such as electron microscope measurement of a-alumina crystal si2e, and indentation microhardness. 

Place the appropriate amount of gel suspension in a sintered-glass funnel, remove the supernatant through suction, and resuspend the gel three times in a threefold excess of water, followed by suction (see also Section IX). 

Prepare a homogeneous gel slurry by mixing 160 g of gel from the sintered glass funnel with 160 ml of a 1 M sodium chloride solution. 

Figure 5.18. Cylindrical model for sintering. The gel is modeled as a cubic array made of intersecting cylinders with length / and radius a. (Reproduced from [52], with permission.)...

As shown in Fig. 17, the number of protons from a series of silica-alumina samples with differing BET surface areas is a nearly linear function of surface area, indicating that essentially all of the protons are situated in the surface phase of the solid. The samples corresponding to the data of Fig. 17 were prepared by steam sintering of the above-mentioned 425 meterVgram sample at various temperatures above 500° so as to reduce the surface area to the values given and then soaked in water for 4 hours, oven-dried at 110° and evacuated at 500° for 16 hours. A plot somewhat similar to Fig. 17 has been obtained for silica gel. The slope of the best straight line repre-... 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

New high-surface-area zirconia-carbon composites were prepared by a sol-gel method followed by a high-temperature treatment in inert gas.420 The samples proved to be very active and selective in the aromatization of Cg+ alkanes (n-hexane and n-octane). From n-octane, as main products, only ethylbenzene and o-xylene were formed. The catalysts have low acidity due to significant dehydration of the zirconia surface, and high surface area since the carbon matrix prevents sintering of the Zr02 particles. 

Amination of agarose beads. The washed epoxy-activated gel is suspended in 1 mL of distilled water/g of gel in a 1-L conical flask. About 1.5 mL of ammonia/ g of gel are added and the gel incubated for 12 h at 30°C in a rotary shaker. Alternatively, 1,6-diaminohexanediamine (5 eq.) can be used to aminate the gel, yielding resins with a six-carbon spacer arm. The aminated gel is washed with 40 mL of distilled water/g of gel on a sinter funnel, and stored in 20% v/v ethanol at 0-4°C. The extent of amination is determined as described in Note 3. Aminated beads can also be purchased from Amersham Biosciences. 

In commercial calcium aiuminate cements, the only phases that hydrate signilicantly at early ages are normally CA and, if present. C,A-. Some sintered cements made at relatively low temperatures also contain free lime or CH. which are very active in the hydration process. The reaction of CAj present in many white cements high in AI2O3, is very slow (B117), possibly because of gel formation. 

The wear rates of alumina and alumina-zirconia abrasive belts have been compared (43). Alumina—zirconia was found to be much freer-cutting and requires less energy7 to remove a given volume of metal. Under moderate grinding conditions the alumina —zirconia abrasive removed 10 times more metal than the alumina abrasive and under severe conditions about 30% more. The newly developed sintered sol—gel alumina abrasives are more effective in some applications than alumina-zirconia in increasing cut-rate, reducing power requirements, and increasing belt or disk life. 

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