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Nanoscale Polymer Coatings for Enhanced Bonding and Thermal Conductance in Carbon Nanotube Array Interfaces
Keywords: Carbon nanotubes, Thermal interface materials, Polymer bonding
Carbon nanotube (CNT) arrays are chemically stable and mechanically robust materials that have shown great promise as low-resistance thermal interface materials (TIMs). In certain configurations, these materials have been demonstrated to produce thermal resistances that are comparable to soldered joints. Measurements capable of resolving component resistances in CNT arrays have revealed that the resistance between free CNT ends and opposing substrates comprises most of the resistance produced by the entire interface. This bottleneck to heat transfer at the interface to free CNT ends has proven difficult to overcome. Furthermore, bonding is often weak at this interface, limiting many applications where both low thermal resistances and mechanically robust joints are required. We developed a simple method to apply nanoscale coatings of soluble conjugated polymer to free CNT ends as a novel approach to bond the free ends to substrates and reduce thermal resistance at the contacts. Polymer solutions were spray coated in controlled amounts on CNT arrays grown on silicon and copper substrates without significant change to the morphology of the CNT arrays. Dry polymer-coated CNT arrays were then placed in contact with interface surfaces coated with a solvent that dissolves the polymer, and the interface was allowed to dry under moderate pressure to increase the contact area at the interface to CNT free ends. The thermal resistances of CNT array interfaces with and without nanoscale polymer coatings were measured with a photoacoustic (PA) technique (at 20 psi) to be 1-4 mm2K/W and 12-20 mm2K/W, respectively, which demonstrates a 10-fold reduction in thermal interface resistance by adding the polymer coating. The polymer-bonded interfaces were separated after testing and CNTs were observed to transfer from the growth substrate to the substrate in direct contact with free CNT ends, which demonstrates the strength of the polymer bonds created here.
Baratunde A. Cola, Assistant Professor
Georgia Institute of Technology
Atlanta, GA

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