Abstract: A filler powder comprised of an alumina powder coated with a silicon containing coating, wherein the silicon containing coating at most partially covers the surface of said coated alumina powder and the filler powder, when mixed with a thermosetting epoxy resin, has an average spiral flow length that is at least about 1.1 times greater than a comparable filler powder containing uncoated alumina powder mixed with the thermosetting resin. A preferred process for making the filler powder comprises: heating, simultaneously, an alumina powder with a second powder coated with an organo-silicate to a temperature for a time sufficient to volatilize, deposit and pyrolyze at least a portion of the organo-silicate on the alumina powder to form a silicon containing inorganic coating thereon.