At 600 or 800 degrees you are probably stuck with a dry joint design.. The fabrication and design of your joining methods will likely dominate the thermal resistance of your design. These joints in general show significant variability. Some factors that affect the thermal resistance include the hardness, finish and flatness of the faying surfaces in addition to joining pressure. The objective is to minimize the gaps that inevitably occur in real life joints.
Assuming you can solve the joint design problem and we focus strictly on potential solids meeting your thermal requirements, you might consider the following:
OFHC copper (possible interfacial corrosion products, very soft-a good thing)
graphite (you're at its upper use temperature in air, watch out for anisotropy especially in pyrolytic grades)
boron nitride(barely meets requirement,soft & machinable, at upper use temp., anisotropic, CTE mismatch)
aluminum nitride (hard-requires diamond tooling, low thermal expansion coefficient)
beryllium oxide (hard, TOXIC, low CTE)
The ceramics will be available for smaller area joints only. Minimizing the physical extent of the joint design will be desirable regardless of the material choice.
So, design your joint as small as possible, polish the faying surfaces, make sure all surfaces are flat, uniform clamping pressure, use as soft a material as possible, design for minimum warpage with temperature excursion and match thermal expansion coefficients as best you can.
Good luck,
Bruce
Bruce
