Preparation of silica glass microspheres by sol-gel processing
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A method for making glass microspheres by the sol-gel process has been developed. Porous silica microspheres were produced at temperatures as low as 500 °C and densified silica microspheres were prepared at 800 °C. The size of the microspheres was controlled by adjusting the feed rate of the raw materials and the frequency of the droplet generator so that uniform spheres were obtained. Glass microspheres prepared by this method were characterized by Nuclear Magnetic Resonance (NMR), Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and optical microscopy.
I. INTRODUCTION
Glass microspheres are widely used in industrial and consumer products. Solid glass microspheres are well known for their use in reflective materials for paints and screens.1 Microspheres have been used recently for weight savers and fillers in plastics,2 laser fusion targets,1'3 and the in vivo irradiation of organs in humans.4 Commercial processes for producing glass spheres were reviewed by Hendricks,1 who also developed a liquid drop process for making hollow glass microspheres whose quality can meet the requirements for inertialconfinement fusion targets (ICF).4 Hendricks used a droplet generator to improve the size uniformity of the spheres, coupled with a vertical tube furnace to dehydrate and melt the glass microspheres. This process yielded hollow spheres of uniform size and wall thickness. Glasses synthesized by the sol-gel process, especially those prepared by metal alkoxides, have been reported to be purer and to require lower processing temperatures than glasses produced by conventional melting.5"9 In this study, an apparatus was built to produce glass microspheres from tetraethyl orthosilicate (TEOS). A droplet generator was used to disperse an alcohol solution of metal alkoxides partially prehydrolyzed by water and catalyzed by acid. The gelmicrospheres were solidified below 250 °C, dehydrated into porous spheres between 500 and 800 °C, and densified above 800 °C. The transition of sol-to-gel and the conversion of gel-to-glass were characterized by NMR, DTA, and TGA. Advantages of this process include the production of porous and solid glass spheres of uniform diameter without melting and low equipment cost. Combining the sol-gel process with the liquid drop process offers the possibility of preparing glass microspheres of compositions that cannot be made by conventional a)
Current address: Fiber Optic Materials Research Center, Rutgers University, Piscataway, New Jersey 08855-0909. J. Mater. Res., Vol. 6, No. 1, Jan 1991
168 http://journals.cambridge.org
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melting. The present work was also part of a program to develop glass microspheres for use in controlled drug delivery application. II. EXPERIMENTAL PROCEDURE A. Equipment
The apparatus for producing microspheres by the liquid drop process is shown in Fig. 1. The flow system, electronic system, and heating system for the liquid To Frequency Generator
Liquid Inlet
Strobe Light
n
TTrE
-To Vacuum Pump
'. Telescope A
Heating Zon
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