There is a metal based O-ring shaped seal that has temperature ratings to 850C. Link follows: http://www.tss.trelleborg.com/com/www/media/downloads/catalogs/wills_rings_gb.pdf
Application very unfavorable for O-ring. Many possible failure modes including explosive decompression, compression set, nibbling at extrusion gap.
You are going to require a high filled PTFE Variseal PTFE spring energized seal c/w PEEK back-ups or equivalent. Moveover request to have seal...
Based on your radial application parameters you are going to experience seal failure through wear and by the Joule Effect. The only viable solution you have is with a spring energized PTFE based seal.
With radial spring energized PTFE for rotary you have to select a spring with a very flat...