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In article <7926@dirac.physics.purdue.edu> parks@gibbs.physics.purdue.edu (Chris Parks) writes:
>
>What is a good material (good=price + availability) which is
>an electrical insulator and a good conductor of heat?
>
>It will be used at temperatures as low as 4.2 Kelvin and it needs
>to hold up to about 200 volts / mm, heat conductivity needs to
>be comparable to copper.  Does it exist?
>
>Note: Diamond may be ideal but not practical!
>
>Chris Parks
>parks@physics.purdue.edu


Sapphire has been mentioned to me as such a material.  You do not mention in
what form: shape, size this material must be or how much of it you need.  
I believe that you can get man-made sapphire substrates.  They are probably
not cheap, but surely cheaper than diamond.

For cryogenic temperatures, you could try intrinsic silicon (at low
temperatures, it is a very poor electrical conductor (not enough
energy to put any electrons up into the conduction band).  I believe
that most of the heat conduction in silicon is carried by phonons
(lattice vibrations), however, so it should still be a fairly good
conductor of heat (I once saw figures comparing thermal conductivities
of silicon doped at different concentrations, and the thermal conductivity
did not vary much).  Silicon might be one to think of also, since it has
the diamond structure, and is just below diamond in the periodic table.

Hope this helps

John
snyder@henry.ece.cmu.edu