Enhancement of thermal conductivity and volumetric behavior of FexOy nanofluids

Homogeneous and stable magnetic nanofluids containing iron oxide nanoparticles, alpha-Fe2O3 (hematite) and Fe3O4 (magnetite) in ethylene glycol, were prepared at concentrations up to 25% in mass fraction. Commercial Hexagonal Scalenohedral-shaped alpha-Fe2O3 nanoparticles were selected while Fe3O4 nanoparticles were synthesized using a coprecipitation method. The products were characterized by transmission and scanning electron microscopy and x-ray diffraction. The thermal conductivity of both nanofluids was measured as a function of volume fraction and temperature. The results illustrate that the enhanced thermal conductivity of the nanofluids increases with volume fraction but is temperature independent. The experimental results show that both types of nanoparticles in this base fluid present no significant aggregation. These experimental values were also compared with theoretical models. Moreover, the density of these nanofluids was measured as a function of volume fraction, temperature, and pressure. The volumetric behavior of nanofluids containing hematite is closer to the ideal behavior than those using magnetite. (C) 2011 American Institute of Physics. [doi:10.1063/1.3603012]