Factors influencing the formation of hollow ceramic microspheres by water extraction of colloidal droplets
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Hollow ceramic microspheres of A12O3, 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.
I. INTRODUCTION Hollow ceramic/glass microspheres are currently being used as lightweight fillers in composites, fillers for reflective and luminescent coatings, and for thermal insulation and catalyst support applications.1 More advanced applications include encapsulation of nuclear fusion materials in the field of inertial confinement fusion (ICF).2'3 Recently, hollow ceramic/glass microspheres have been used to fabricate low dielectric constant materials by the introduction of controlled porosity to meet the increased demand for high performance multichip module (MCM's) substrates in the microelectronic packaging industry.4"6 As the time of flight of electronic signals is an important factor gating the design of high speed electronic systems, reducing the dielectric constant of the propagating media would reduce the time of flight for these signals. Reducing the dielectric constant by incorporating hollow ceramic/glass microspheres with diameters of the order of the grain size of the sintered ceramic (1-10 (im) would be an attractive new material for the microelectronics industry. Several manufacturing techniques have been developed for producing glass/ceramic hollow microspheres over the past decades. There are primarily two types of techniques that are commonly used by the commercial manufacturers. In one method used by the 3M Corp., glass particles containing a blowing agent are blown into shells in a gas flame.7 In the other method, either atomized liquid droplets or powders formed by gelation or spray drying are fed through a vertical shell blowing furnace in which the entrapped water vapor creates the internal cavities.8"11 Hollow microspheres produced by these methods are, in most cases, limited to glass compositions, and their sizes are typically hun84
J. Mater. Res., Vol. 10, No. 1, Jan 1995
http://journals.cambridge.org
Downloaded: 15 Mar 2015
dreds of microns in diameter. Other methods are known for obtaining hollow ceramic/glass microspheres on a laboratory scale, including a dual nozzle hollow-drop generation technique,12 a spray-pyrolysis method,13-14 and an emulsion evaporation technique.15 A water extraction sol-gel technique patented by Sowma
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