The limiting behavior of the thermal conductivity of nanoparticles and nanofluids

We present experimental evidence of negative thermal conductivity enhancement in nanofluids consisting of 2 nm titania nanoparticles dispersed in 50% (w/w) water+ethylene glycol. This behavior is unlike that of other nanofluids, which have been shown to exhibit positive thermal conductivity enhancements. Our results for titania nanofluids suggest that the thermal conductivity of 2 nm titania nanoparticles is smaller than the thermal conductivity of the base fluid at the same temperature, indicating a dramatic decrease in the thermal conductivity of titania particles as the particle size becomes of the same order as the phonon mean free path. Although such a decrease has been predicted for semiconductor nanoparticles by theory and simulation, experimental evidence has hitherto been lacking. Our results provide indirect experimental evidence for this decrease in metal oxide particles, and validate our previous work on alumina nanofluids that showed an exponential decrease in the thermal conductivity of alumina particles with decreasing particle size, from a limiting value for large (micron-sized) particles. (C) 2010 American Institute of Physics. [doi:10.1063/1.3354094]