Thermal conductivity of nanoparticle suspensions

Abstract

We describe an optical beam deflection technique for measurements of the thermal diffusivity of fluid mixtures and suspensions of nanoparticles with a precision of better than 1%. Our approach is tested using the thermal conductivity of ethanol-water mixtures; in nearly pure ethanol, the increase in thermal conductivity with water concentration is a factor of 2 larger than predicted by effective medium theory. Solutions of ${\mathrm{C}}_{60}–{\mathrm{C}}_{70}$ fullerenes in toluene and suspensions of alkanethiolate-protected Au nanoparticles were measured to maximum volume fractions of 0.6% and $0.35\mathrm{vol}\%$ , respectively. We do not observe anomalous enhancements of the thermal conductivity that have been reported in previous studies of nanofluids; the largest increase in thermal conductivity we have observed is $1.3\%\pm 0.8\%$ for $4\mathrm{nm}$ diam Au particles suspended in ethanol.