Anybody using silicone for thermal transfer and electrical isolation?

I had some large ICs that needed to use the case as a heat sink.  I ended up using pure silicon caulking rated to 150 C to do the job.  I was contemplating fireblock silicon that was rated for very high temps but read on the label that it contained iron oxide flakes.  At that time I was concerned about maintaining the electrically isolating properties.  I have since read on wikipedia that iron oxide will not make the silicon conduct electricity and that its used in aerospace industry frequently around conduits.  This makes since since rust can conduct heat but not electricity.  Anybody have any experience?  Because a 7$ 10.1 oz tube is a better deal than a 100x100mm sheet of "elastomeric silcon" that is 20-30$.  Also more conforming to the part for possibly better heat transfer.

Hi!
Iron oxide is not "conductive" (all thing relative), at least this is a very bad electrical conductor. For example, the old radio makers used to let transformers lams rust to provide electrical insulation between them.
Other thing that comes to mind is that today, silicon is used as heat insulator : they make cooking gloves with it, so mabe not every silicon compound is recommended for heat transfer... But since your caulk is filled with rust...

Laurent.

Silicon cauling compound can and does cause corrosion on metal and electrical parts. I wouldn't use it.

when you refer to increased corrosion on metal are you referring to silicon caulking that cures using acetic acid?  I am pretty sure pure silicon by itself is fairly inert and non reactive based on the chemistry.  I have also seen several silicon based compounds for semiconductor heat transfer that were rated to super high temp like 1000F or something but they were over a hundred dollars for a small amount and a little beyond my application.  My goal was to try and work a fairly inexpensive hack using readily available materials.