Hollow ceramic microspheres of A1 2 O 3 , SiO 2 , and mullite have been prepared by the combination of an emulsion technique with a water extraction sol-gel method. Concentration of sol, initial droplet size, and water extraction rate of the system are found to be the important process parameters controlling the size and wall thickness of the hollow microspheres, and their influences are shown. A model that correlates the morphology of microspheres to concentration and water extraction rate is proposed and is in good agreement with the experimental observations. The capability and limitation of this process for forming hollow microspheres are demonstrated. It was shown that hollow microspheres with sizes greater than 5 fim could be readily prepared, while a limitation was met for sizes less than 1 /xm, in which case solid microspheres were normally formed.

A layered sphere Bruggeman effective-medium model was developed to describe the dielectric constant of a porous ceramic composite having potential for use as a very low dielectric constant inorganic material for microelectronic packaging. The uniqueness of this model is that it incorporates a hollow sphere geometry and thus fits closely the microstructure of the dielectric composite in a broad range of compositions. Samples of the composite were prepared by adding hollow alumina-silica alloy ceramic spheres to a cordierite matrix. Their dielectric properties were characterized as a function of volume fraction of hollow spheres. The measured results were in good agreement with the model prediction.

This paper reports the use of hollow ceramic microspheres for a controlled porosity composite application and an emulsion/extraction method to make them. Design guidelines for this application are presented in which the physical properties and geometry of the microspheres are correlated to both dielectric constant and mechanical strength of the composites. The analytical modeling indicates that small size, thin walled ceramic microspheres with thermal expansion coefficients greater than that of common ceramic substrate matrices are needed to lower the dielectric constant significantly while retaining good mechanical strength. An emulsion/water extraction method was developed to prepare such hollow ceramic microspheres. This method shows promise for making very small (< 10 μm) hollow microspheres of various ceramic compositions. The capability and limitation of this synthesis route are shown.