Several years ago the semiconductor boron arsenide was predicted to have a very high thermal conductivity and now three separate groups have finally prepared crystals pure and uniform enough to confirm that in three recent Science papers. The crystals are a few mm across and the measured thermal conductivity ranges from about 1000 to 1300 W/m-K. For comparison, silicon is about 150, copper 400, and diamond 2000. Now they just have to make perfect 300 mm wafers with good transistors. Should only take another week or two :-).
Regards,
Carl Ijames