Cross-Linked Carbon Nanotube Heat Spreader
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Cross-Linked Carbon Nanotube Heat Spreader Gregory A. Konesky1 1 National NanoTech, Inc., 3 Rolling Hill Rd., Hampton Bays, NY 11946, U.S.A. ABSTRACT Among the exceptional properties of isolated individual carbon nanotubes (CNTs), exceptional thermal conductivity along their axis has been demonstrated, However they have also shown poor thermal transfer between adjacent CNTs. Thick bundles of aligned CNTs have been used as heat pipes, but the thermal input and output power densities are the same, providing no heat spreading effect. We demonstrate the use of energetic argon ion beams to join overlapping CNTs in a thin film to form an interpenetrating network with an isotropic thermal conductivity of 2150 W/m K. Such thin films may be used as heat spreaders to enlarge the thermal footprint of laser diodes and CPU chips, for example, for enhanced cooling. At higher ion energies and fluence, the CNTs appear to collapse and reform, aligned parallel to the ion beam axis, and form dense high aspect ratio tapered structures. The high surface area of these structures lends themselves to applications in energy storage, for example. We consider the mechanisms of energetic ion interaction with CNTs and junction formation of two overlapping CNTs during the subsequent self-healing process, as well as the formation of high aspect ratio structures under more extreme conditions INTRODUCTION The performance of modern semiconductor devices is often limited by their ability to reject waste heat. Laser diodes and high performance computer CPU’s, for example have their output power and clock speeds, respectively, limited by their ability to efficiently dissipate waste heat, ultimately into some heat sink. This limitation derives from a given thermal junction resistance between the chip, device or package and the heat sink. A heat spreader allows the thermal footprint to be expanded over a larger area, reducing the thermal junction resistance, and allowing a laser diode to operate at higher output powers, or a CPU to operate at higher clock speeds, at a given operating temperature. Note that a heat spreader must not only transfer the thermal load laterally to a larger area, but must also equally transfer it through its thickness to the underlying heat sink. Isotropic thermal conductivity is therefore an essential requirement. Heat spreaders have been devised using Oxygen Free High Conductivity (OFHC) copper and diamond powder sintered composites with a thermal conductivity range of 400-900 W/m K, and at moderate cost. At some what higher cost, CVD diamond has been used to provide thermal conductivities on the order of 1000-1500 W/m K [1-3]. Natural type IIa diamond, with a thermal conductivity in excess of 2000 W/m K has also been employed [4], but at extreme cost.
THEORY CNTs were predicted to have an exceptionally high thermal conductivity along their axis [5], perhaps as high as 6600 W/m K as a result predominantly of ballistic phonons. An isolated MWCNT was observed to have a thermal conductivity of 3000 W/m K [6] and parallel bundle
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