Stability and thermal conductivity of nanofluids of tin dioxide synthesized via microwave-induced combustion route

SnO2 nanofluids were prepared by dispersing tin dioxide nanoparticles in deionized (DI) water as a base fluid. 4-5 nm tin dioxide crystals were synthesized via chloride solution combustion synthesis (CSCS) using SnCl4 and sorbitol as a novel precursor and the fuel, respectively. Ammonium nitrate was also used as the combustion aid. The molar ratio of sorbitol plus ammonium nitrate to SnCl4 was set at unity; whereas, the molar ratio of sorbitol-to-ammonium nitrate divided by that of stoichiometric value (Phi) was varied in the range of 0.5-1.4 in order to find the optimum values of specific surface area for the CSCS technique. Transition electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and Brunauer-Emmet-Teller (BET) techniques were employed for the characterization of the nanoparticles. Since SnO2 nanoparticles form clusters within fluids, the fluids were ultrasonicated to improve the dispersion and stability of the nanoparticles. The colloidal stability of the SnO2 nanofluids was quantitatively characterized by UV-vis spectrophotometric measurements. The results of the UV-vis experiments indicate higher dispersion together with enhanced stability for the nanofluid prepared by SnO2 nanoparticles synthesized at Phi = 1.0. After 500 h sedimentation time, the relative concentration of the nanofluid with the highest stability is remained at around 77% of the initial concentration of the fluid. A transient hot-wire apparatus was used to measure the thermal conductivities of the nanofluids. In addition, the effects of pH and temperature on the thermal conductivity were also investigated. At 353 K, for the nanofluid prepared by SnO2 nanoparticles synthesized at = 1.0 at a weight fraction of 0.024%, thermal conductivity is enhanced up to about 8.7%, with an optimal pH=8. (C) 2009 Elsevier B. V. All rights reserved.