Conventional and microwave sintering of condensed silica fume
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Dinesh Agrawal, Jiping Cheng, and Rustum Roy Intercollege Materials Research Laboratory, Pennsylvania State University, University Park, Pennsylvania 16802-4801 (Received 18 January 1994; accepted 22 June 1995)
Condensed silica fume, a by-product from the production of silicon alloys, was sintered by (i) conventional heating in a dilatometric furnace, both at constant heating rate and isothermal heating, and (ii) by the microwave heating. The dense products with relative density up to 95% of theoretical can be obtained only by short runs at high heating rates, preferentially accomplished by the microwave treatment. Prolonged heating leads to the devitrification of the original glassy phase to cristobalite, accompanied by an arrest of densification.
The condensed silica fume from the silicon or ferrosilicon alloys industry is a voluminous, high tonnage by-product consisting of amorphous, spherical, and submicron particles.' This is predominantly used as an active additive to cement pastes and concrete mixtures.2 The chemical composition of condensed silica fume varies according to the type of silicon alloy produced in the furnace, the design of the furnace, and the chemical composition of the raw materials. All condensed silica fumes, however, have some chemical characteristics in common. The iron content fluctuates between 0.1 and 2.0%. The alumina, lime, magnesia, alkali, and SO3 contents are quite low, being lower than 2% in each case. The carbon content is less than 1.5%.' The glassy state of the silica powder makes it a candidate for possible preparation of ceramic-shaped articles by viscous flow sintering. Though the literature on sintering of pure glassy silica or related powders of different origin is quite large,'-I0 there is no information available on sintering of silica fume used in this study. It was shown in a previous study that a commercial silica fume of high purity (Degussa Aerosil Ox 50) sinters nearly to theoretical density at 1450 "C without de~itrification.~ Samples of commercial soda-lime glass remained vitreous during ~ i n t e r i n g .The ~ , ~ crystallization of cordierite glass powder was detected only after sintering was complete to >98%.7 The presence of alkalis and ~,~ of water vapor accelerates crystallization of g l a s ~ .Asreceived diatomite, for example, devitrifies at 1000 "C, whereas leached material, composed practically of Si02, devitrifies at 1100 "C after 8 h of exposure." Leaching of alkalis from silica foam produced from commercial SiO2 sol, and stabilized by NazO, significantly retarded devitrification of the system." Devitrification occurring during the reheating of glassy powders is largely an uncontrolled process depending also upon chance effects such as the presence J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
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of foreign particles which can act as nucleation catalysts.12 The silica fume used in this study is rather an impure glassy powder. It is expected to sinter well similarly to other mentioned powders or powd
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