Contents INTRODUCTION
Polyimides with high-performance were thought highly in the past 20 years because of their outstanding characteristics such as excellent tensile strength and modulus, low thermal expansivity and dielectric constant and good resistance to organic solvents[1]. Although PI was widely used in microelectronic industry, some applications such as frequency conversion motor, highvoltage generator and high-voltage alternating current drawing motor require enhancements of corona-resistant capacity of the PI films. But polyimides are limited with regard to additional improvement in performance because of limitations inherent in all organic materials. This suggests incorporating inorganic materials such as ceramics to provide enhancements In properties. One promising approach is to use the sol-gel process[2], [3]. The major advantage of the process is that relatively low temperature is used in this type of processing of ceramics. Many factors influence the kinetics of hydrolysis and poly-condensation reaction in the sol-gel process. Some important factors are temperature, PH, nature of the solvent, and the type of ceramic precursors. The relative rates of the competing hydrolysis and condensation reactions determine the final structure of the composites films. The acid-catalyzed reaction provides relatively fast hydrolysis and slow condensation reaction. The slow condensation reaction means slow network formation, thus providing a better environment for greater interpenetration of the phases. Among the nanocluster-trapped inorganic/organic composites investigated, polyimide/-silica composites films are of particular interest[4], [5], so nanocluster-trapped polyimide/silica composites films were prepared by sol-gel technique and the corona-resistant capacity of the composites films was researched chiefly in this review.
