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J. Phys. Chem. C 2007, 111, 1646-1650
Novel Synthesis and Thermal Conductivity of CuO Nanofluid Hai T. Zhu,* Can Y. Zhang, Ya M. Tang, and Ji X. Wang Institute of Material Science and Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China ReceiVed: September 11, 2006; In Final Form: December 4, 2006
CuO nanofluid was synthesized by transforming an unstable Cu(OH)2 precursor to CuO in water under an ultrasonic vibration, followed by microwave irradiation. The influences of the type of precursor, ultrasonic vibration, microwave irradiation, and dispersant were studied. The thermal conductivity of CuO nanofluid was investigated. The results show that well-dispersed CuO nanofluid with high volume fractions can be synthesized using the current method. The synthesis mechanism is proposed. The current CuO nanofluid has a higher thermal conductivity than those prepared by the dispersing method.
Introduction Nanofluid is a novel heat transfer fluid prepared by suspending solid nanoparticles in a traditional fluid. As compared to the traditional fluid, the nanofluid exhibits an enhanced thermal conductivity. For example, the thermal conductivity of a nanofluid, consisting of ethylene glycol (EG) and only 0.3 vol % Cu nanoparticles, increased by up to 40% that of pure EG.1 It is expected that nanofluid will be the next generation of heat transfer fluid and have great potential applications in many important fields, such as micro-electronics, aerospace, transportation, and medicine. Different methods have been developed to prepare nanofluids, such as the dispersing method,2-6 physical vapor condensation,1,7 and one-step chemical method,8 etc. In case of the dispersing method, synthesized nanoparticles are dispersed in liquid under stirring or ultrasonic vibration. For example, Lee and Choi prepared CuO nanofluid by dispersing commercial CuO nanoparticles in water under ultrasonic vibration.2 It is a two-step method, in which the preparation of the nanoparticles and the preparation of the nanofluid are separated. Therefore, the nanoparticles may agglomerate during the drying, storage, and transportation process, leading to difficulties in the following dispersion stage. Consequently, the stability and thermal conductivity of nanofluid are not ideal. In addition, the production cost is high. To reduce the agglomeration of the nanoparticles, Choi et al. developed a one-step physical vapor condensation method to prepare Cu/EG nanofluid,1,7 in which Cu vapor was directly condensed into nanoparticles by contacting with a flowing ethylene glycol. The prepared nanofluid showed high stability as well as high thermal conductivity. However, it is difficult to synthesize nanofluid in large scale by this method, and the cost is also high. Recently, we developed a one-step chemical method to prepare Cu nanofluid by reducing CuSO4 with a reducing agent in EG.8 The obtained Cu nanofluid also had high thermal conductivity and stability. Yet, the solid volume fraction was usually low (
