Microwave Drying of Borosilicate Gels
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Microwave Drying of Borosilicate Gels Srinivas Surapanani,
Michael E.Mullins,
and B.C.Cornilsen
Department of Chemistry and Chemical Engineering Michigan Technological University Houghton,
MI 49931.
Abstract Microwave processing was carried out on SiO2 -B 2 0 3 solutions and gels prepared by sol-gel methods. Monolithic gels were prepared from alcoholic solutions of trimethylborate and tetraethylorthosilicate using a two-step hydrolysis process. A novel technique of Liquid State Processing (LSP) was employed for the first time, and it was found to be faster and more effective than the conventional processing techniques. The structural evolution of the dried products was followed using FTIR. The effect of processing was examined via surface area analysis (BET), electron microscopy, and FTIR. The microwave drying has been compared with conventional oven drying and vacuum drying techniques. Shorter processing times, improved microstructures, and unique properties have been obtained. Introduction In recent years, there has been considerable growth of interest in applying microwave energy for drying ceramic materials over conventional processing methods. Reasons for this growth include: (1) significant reduction in the manufacturing costs due to shorter processing times, (2) improved product uniformity and 1 yelds, and (3) unique microstructures and properties. The applications of Mi~rowave energy in ceramic processing are four fold, and can be categorized as (1) liquid state processing, (2) solid state processing, (3) plasma processing, and (4) process control. Our study includes the first two categories. Liquid State Processing (LSP) is a novel approach of applying microwaves to synthesize the sol-gel materials from liquid solutions. Two methods are employed to prepare the solutions, TEOS/TMB and sodium silicate/TMB. A microwave transparent Teflon reaction vessel was used for this process. The alcohol and water present in the initial solution couple effectively with the microwaves generating the heat wherein the temperature and pressure are both increased in the sealed vessel, which in turn increases the gelation kinetics of the sol-gels, producing dried products within a span of a few minutes.
Mat. Res. Soc. Symp. Proc. Vol. 189. @1991 Materials Research Society
310
The use of microwave radiation for processing solid state ceramics can be divided into (1) low-temperature processing (< 500°C) which includes drying and binder/organic burnout; and (2) high temperature processing (> 5000C) which includes melting, calcining and sinteringI-2 In the present work we have studied the low temperature application of microwave energy to borosilicate gels (TEOS/TMB, two-step process) and have compared the results to conventional thermal and vacuum drying. Experimental 3 A two-step process was employed in preparing borosilicate gels from metal alkoxide precursors (method 1). In the first step, tetraethylorthosilicate (TEOS) and an acid catalyst were mixed with half-volumes of water and ethanol. After 1 hour, the second ste
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