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Unformatted text preview: Enhanced thermal conductivity by aggregation in heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes Jesse Wensel, Brian Wright, Dustin Thomas, Wayne Douglas, Bert Mannhalter, William Cross, Haiping Hong, a ! and Jon Kellar Department of Material and Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, USA Pauline Smith and Walter Roy Army Research Laboratary, Aberdeen Proving Ground, Maryland 21005, USA s Received 18 November 2007; accepted 19 December 2007; published online 17 January 2008 d An approximately 10% increase in the thermal conductivity s TC d of heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes has been determined with very low percentage loading s around 0.02 wt % d of these two nanomaterials. These fluids are very stable and the viscosity remains approximately the same as water. A possible explanation for these interesting results is the aggregation of metal oxide particles on the surface of nanotubes by electrostatic attraction and form the aggregation chain along the nanotube. Time dependant magnetic results demonstrate that, under the influence of a strong outside magnetic field, the TC value decreases. Also, the TC value decreases when the p H is shifted from 7 to 11.45. 2008 American Institute of Physics . f DOI: 10.1063/1.2834370 g The discovery of carbon nanotubes s CNTs d has insti- gated considerable research efforts in recent years due to their promising thermal, electrical, mechanical, and func- tional properties. For example, single wall carbon nanotubes exhibit thermal conductivity s TC d as high as 20006000 W / m K. 1 under ideal circumstances. By con- trast, typical heat transfer fluids, such as water and ethylene glycol, have TC values of only 0.6 and 0.34 W / m K, respec- tively. There is a great need to increase the thermal conduc- tivity significantly, while maintaining a desirably low fluid viscosity. Fluids containing carbon nanotubes s called nanofluids hereafter d should exhibit substantially improved TC values 2 8 and could be useful for a variety of heat transfer related applications including coolants and lubricants. However, nanofluids utilizing a simple composite structure do not enhance the TC effectively. For instance, nanofluids with low percentage nanotube loading showed no significant improvements in TC, while at loading of 1 vol % CNTs s, 1.4 wt % d , resulted in about 10%20% TC increase. 9 , 10 Unforunately, at such a high concentration, the fluid became mudlike, thus, making the fluid much less useful for coolant and lubricant applications. A possible explanation for this unexpectedly small TC increase may be attributed to a lack of nanotube-nanotube physical contacts in the fluids. The carbon nanotubes are irregularly positioned in the fluids leading to infrequent con- tacts between the CNTs, therefore, only very high concentra- tions of CNTs produce noticeable TC improvements.tions of CNTs produce noticeable TC improvements....
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