I need to increase thermal conductivity between Steel and Aluminum plates at +500C temps, using a thing semi-flexible sheet without gluing it to the surfaces of the metal plates.
Contact manufacturers of heat transfer paste.
I have used both MgO and BN for high temp heat transfer and they work fairly well.
At your temps a silicone or Krytox grease loaded with a high heat transfer filler would do the trick.
I have used 611HTC from Timtronics which is rated up to 360C with good results. At the time, this was the highest temperature rating I could find that was still a grease as opposed to a cement. [URL unfurl="true"]https://timtronics.com/high-temperature/[/url]
Graphite is a good heat conductor - graphite is used in compact heat exchangers in some exotic applications - see if this helps -flexible graphite sheets