There is a great discussion going on right now at the
Linked In/Thermal Management Group.
The discussion is specifically about the thermal conductivity
of thermal management material, about the amount of pressure necessary to make such material work properly and a consideration about carbon materials for that work.  We highly recommend taking a look.  Just click this link:  Thermal conductivity of thermal interface material discussion on Linked-In.

For a tutorial on thermal interface material, checkout this article in the coolingZONE Library, “Thermal Interface Materials: An Introduction” and “Reduce Contact Thermal Resistance with Thermal Interface Material“.