Number 131 (Story #2), June 3, 1993 by Phillip F. Schewe and Ben Stein PURIFIED DIAMOND HAS THE HIGHEST THERMAL CONDUCTIVITY. Natural diamond is 98.9% carbon-12 and 1.1% carbon-13. A team of scientists from Wayne State University and General Electric showed in 1990 that purifying the C-12 component of diamond to 99.9% increased its room-temperature thermal conductivity by 50%, a much bigger isotope effect than in other materials. The same scientists have now recorded the highest thermal conductivity ever observed for a solid above liquid nitrogen temperatures, 410 W/cm-K, in 99.9%-pure C-12 at 104 K. The researchers predict that the thermal conductivity for 99.999%-pure C-12 diamonds would exceed 2000 W/cm-K and that integrated circuits mounted on such diamonds (cooled in liquid nitrogen) could operate at 500 times the power density of circuits mounted on copper substrates at room temperature. (Lanhua Wei et al., Physical Review Letters, 14 June 1993.)