Thermal interface materials (TIMs) are often inserted between the surfaces of a contact pair to reduce the thermal contact resistance. Although they typically have lower thermal conductivity than the substrate, they are highly compliant and hence under the application of relatively small contact pressures, deform to conform to the geometry of the adjacent rough surfaces. A part of the low thermal conductivity air present is thus replaced by a higher conductivity material. This leads to a decrease in the constriction of the heat flow lines, and hence, an increase in the contact conductance.
Conductivity and Compliance:
The two most desirable properties of a Thermal Interface Material are high thermal conductivity and high compliance. Since relatively few homogeneous materials possess both these properties, TIMs are typically composite materials with metallic or ceramic fillers in a polymeric matrix. Typically used fillers such as alumina (Al O ) or boron nitride (BN) are characterized by relatively high thermal conductivity and low compliance. Most matrix materials, e.g., silicone, have low thermal conductivity but high compliance. In view of practical applications, optimal volume fractions and geometric distributions of filler and matrix materials are sought at which the contact conductance assumes a maximum value.
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