Hollow Mullite Ceramic Microspheres by Water Extraction of Emulsified Aqueous Droplets
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Department of Materials Sciences and Engineering, University of Illinois, Urbana, Illinois 61801 ABSTRACT
This paper describes a novel colloidal route for the preparation of hollow mullite ceramic microspheres with sizes in the range of 10 gm. An emulsion technique was combined with a water extraction sol-gel process, to form an aqueous droplet dispersion and gel the resulting droplets by extracting water with a dehydrating liquid. The liquid-filled gelled microcapsules were then dried and fired to form hollow ceramic microspheres. Important processing variables controlling the microsphere morphology were determined and a formation mechanism was proposed. Due to its potential applications in microelectronic packaging, mullite was selected as the model material for the present work, but the synthesis technique may be used for making other single or multiphase ceramic compositions INTRODUCTION
There is a great deal of interest in the preparation and application of hollow microspheres in a variety of areas. Applications include their current or developing use in light-weight composite materials, thermal insulation materials, luminescent hollow powders, microsphere ceramic catalyst support, encapsulation of nuclear fusion materials, and controlled porosity dielectrics for microelectronic packaging. Technologies for1 producing hollow glass microspheres were commonly noted and have been reviewed by Downs. These technologies generally involve glass forming ingredients, gas generating species and a high temperature furnace and are generally not suitable for producing hollow microspheres of crystalline oxides. In fact, preparation of hollow ceramic microspheres is not a trivial problem and was rarely reported. In one method, hollow alumina microspheres were prepared by an emulsion evaporation technique. 2 In another
method, Sowman employed a water extraction sol-gel technique to synthesize hollow titania microspheres. 3 Further experiments with Sowman's method has recently been reported by Vernetti and Wilcox, 4 and hollow mullite microspheres with sizes in the range of several to 100 gim were obtained.
The present work on the preparation of hollow mullite microspheres has been encouraged by the studies on the controlled porosity approach to low dielectric constant materials for microelectronic packaging applications. 5 For this application, refractory hollow microspheres with sizes in the range of 1-10 jgm, compatible with the particle size of matrix powders are desired. According to the dielectric mixing theory, introducing hollow ceramic microspheres into a ceramic matrix significantly reduces the dielectric constant of the material as air (pore) has the lowest dielectric constant of 1. Theoretical studies of mullite hollow microsphere-cordierite matrix composite has demonstrated the possibility of fabricating an inorganic substrate6 material with a dielectric constant as low as 3.5 while retaining a reasonable mechanical strength. In an effort to produce smaller sized hollow ceramic microspheres, we report a synthes
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