TPG, thermal pyrolytic graphite, is a material pioneered by Momentive Performance Materials. With a thermal conductivity > 1500 W/mK and a density < 2.4 gm/cc, compared to copper TPG has ~4 the conductivity at ~1/4th the weight; a 16 fold weight specific thermal performance advantage. Copper can be too heavy, aluminum does not conduct well enough, and heat pipes can freeze or boil. TPG can be encapsulated with various materials to give structure, and to match thermal expansion.
See us in Booth 800 at EuMW 2009 in Rome.