what is the thermal interface materials?

Thermal interfacial materials (TIMs) are thermally conductive materials, which are applied to increase thermal contact conductance across jointed solid surfaces, such as between microprocessors and heatsinks, in order to increase thermal transfer efficiency. These gaps are filled with air which is a very poor conductor.
They take many forms. The most common is the white-colored paste or thermal grease, typically silicone oil filled with aluminum oxide, zinc oxide, or boron nitride. Some brands of thermal interfaces use micronized or pulverized silver. Another type of TIMs are the phase-change materials. These are solid at room temperature but liquefy and behave like grease at operating temperatures. They are easy to handle and are not messy.

Thermal Interface Material Application
Without thermal interface material, the heat flow go through joint face slowly, and the thermal conductive performance is bad.Using thermal interface material to link two joint faces, the heat flow go through equally, and the thermal conductive performance is good.  

Heat transferringThree Methods of Heat Transfer:
A. Conduction
B. Convection
C. Radiation 

Thermal Management of Electronics Devices：
Most electronic such as power transistors, CPUs and power diodes produce a significant amount of heat and measures may be necessary to take account of this in order to prolong their working life and increase reliability.