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Carbon nanotube based advanced thin interface materials for thermal management

Authors Tse, Ka Chun
Issue Date 2007
Summary Thanks to the IT technologies, our living−standard has improved greatly. Electronic devices thus become indispensable for our daily life. Because of that, people pay more attention to the problem of heat dissipation, as the heat management becomes challenging due to the limiting size of the device. Industries normally apply thermal grease between the thermocouple and heat sink, in order to enhance the physical interaction and facilitate the heat conduction. This comes up to an idea of making use of carbon nanotubes (CNTs) as the active ingredients to prepare thermal grease. CNTs are reported to have excellent thermal (~6600 W/mK), electrical (~2000 S/cm) properties, and reasonably high stability, such materials may thus be a promising candidate for the preparation of thermal greases. A series of experiments have been carried out to optimize the parameters for the thermal resistance measurements of grease can be achieved and how the thermal grease with best performance can be prepared. CNT−based thermal greases are found to show low thermal resistances and comparable to those of the commercial products. In addition, different materials like metals, metal oxides, semi-conductors, and ionic salts with high thermal conductivity are admixed with CNTs and their effects on the thermal properties of the resulting thermal greases are studied. To improve the dispersion of CNTs, surface modification by grafting poly(ethylene glycol) and dioctylamine chains are performed. The dispersion of functionalized CNT is amazingly improved and they can be dispersed in deionized water over 6 months without sedimentation. The thermal resistance after treatment are found to lower (7 to 13%). Especially, the dioctylamine chains grafted CNT is found to aligned orderly under electric field. The electrical conductivty of the sample increases ~1000% after alignment. These results are encouraging because it widens the application of CNTs in industry because in−situ growth of CNT on SiO2 is not necessary. Thermal, electrical, and mechanical properties of materials are reported to increase significantly upon addition of small amount of CNT. The color of the products will become black due to the inherent color of CNTs. To solve this problem, wrapping CNT with polymer chains is found to give colored composites. Moreover, the hybrids may show low thermal resistances. It is found that poly(phenylacetylene)−wrapped CNTs possess good dispersion in matrice and hence show lower thermal resistance ~7%. Furthermore, the polyaniline−wrapped CNT hybrids can function as magnetic precursor and interact with γ−Fe2O3 electrostatically, this template is found to be used as magnet precursor. Absorption of γ−Fe2O3 on their surfaces furnish materials with high magnetization up to ~18.8 (emu/g). To further facilitate the thermal conduction of CNTs, we decorated silver, well−known thermal conductors to CNT peripheries. Through three different methods: (1) in−situ reduction. (2) parasitic reduction, and (3) parasitic reduction. Silver nanoparticles (~38 wt%) with sizes less than 20 nm are found to evenly dispersed on the CNT surfaces. The adorption of silver particles on the surfaces of CNTs is strong and the morphology of the hybrid materials remains unchanged after strong speed−mixing. The thermal resistances of their greases are studied.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007
Language English
Format Thesis
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