Preparation of Porous Oxide Beads Using Polymeric Beads to Control Bead Size and Shape
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PREPARATION OF POROUS OXIDE BEADS USING POLYMERIC BEADS TO CONTROL BEAD SIZE AND SHAPE Anne B. Hardy, Wendell E. Rhine, and H. Kent Bowen Ceramics Processing Research Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT A method was developed for preparing silica gels by silicon alkoxide hydrolysis using water-swellable polymers to control the shape. A variety of polymer beads with different water absorbencies and different bead sizes were used. Silica beads in a wide range of sizes were prepared from polymer beads. The final bead size was dependent on the size and water absorbency of the beads, on the extent of reaction, and on the final density. It was shown that porosity could be formed in two regions. The silica beads contained pores of 3-4 nm that were attributed to the intrinsic gel structure. In addition, when large amounts of polymer were removed, additional porosity on the scale of 20-50 nm was formed. It was also shown that surface areas varied dramatically with calcining. For beads containing only small pores, surface areas varied from 400 m2/g to less than 0.2 m2/g as the calcining temperature was increased from 600°C to 1000°C. Beads containing porosity formed by removal of the polymer remained porous even after calcining at 1000*C. INTRODUCTION In ceramics applications, control of morphology is often of critical importance. Because ceramics are solids with high melting points, many of the processing approaches used to control morphology in metals or polymers are not feasible. In this paper a method is presented for controlling the shape and porosity of silica gels using water absorbable polymers as a template for the final gel shape. Silicon alkoxide (TEOS) was reacted with water absorbed in the beads to form a silica gel within the polymer. Previous research has shown that acidic water droplets in a water-in-oil emulsion react with TEOS and that the silica gelation appears to take place in the water phase [1]. This method incorporates the processing flexibility of polymers into the preparation of ceramics. Water absorbable polymers are available in a variety of shapes. Polymer beads were used because the regular shape simplified comparison between the original polymer and the silica beads and also because silica gels in spherical bead form are desirable for some applications. Polymer fibers and other absorbent fibers have been used to prepare oxide fibers. In a typical formulation [2], cellulosic fibers such as Rayon imbibe an aqueous salt solution. The fibers are then heated to remove the polymer and convert the salt to the oxide. The oxide retains the shape of the original fiber. In addition to cellulose derivative fibers, a variety of novel fibers have been used including carbon fibers [3, 4] and intercalated graphite fibers [5]. Typically in these techniques absorption of the oxide precursor solution is relatively low, and when the precursor is converted to the oxide and the polymer is removed, the overall oxide yield is low. By contrast, in the technique presented here
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