Effect of particle size and viscosity on thermal conductivity enhancement of graphene oxide nanofluid

In this study, first, graphene oxide nanosheets were synthesized based on the modified Hummers methods. The physicochemical properties of fabricated graphene oxide were characterized using X-ray diffraction analysis (XRD), a scanning electron microscope (SEM), and UV-Vis spectrophotometry. Second, graphene oxide nanofluids were prepared at different concentrations (0.01, 0.05, 0.1 and 0.5 wt.%) in water (base fluid). Particle-size distribution and stability of the colloidal solution of the graphene oxide nanofluids were investigated using dynamic light scattering and zeta potential techniques. Also, Theological behavior of graphene oxide nanofluids was studied at different temperatures (25 degrees C, 40 degrees C, and 60 degrees C) and different shear rates (10-100 1/s). Results show that both particle size and viscosity of graphene oxide nanofluids increased linearly by increasing the graphene oxide concentration from 0.01 to 0.1 wt%, but there was a very sharp increment on average particle size and viscosity by increasing the concentration to 0.5 wt.%. The thermal conductivity of graphene oxide nanofluids was measured at different temperatures. Thermal conductivity of graphene oxide nanofluids depends on both particle-size distribution of graphene oxide and viscosity of graphene oxide nanofluids. All graphene oxide nanofluids showed enhanced thermal conductivity compared to the base fluid, water. Increasing the graphene oxide concentration from 0.01 wt.% to 0.1 wt.% resulted in 8.7% and 18.9% thermal conductivity enhancement at 25 degrees C, respectively. However, further concentration increment to 0.5 wt.% increased thermal conductivity to 19.9%. This behavior shows that there is an optimal concentration of graphene oxide at which particle size and viscosity of graphene oxide nanofluids exhibit significant thermal conductivity enhancement, and further concentration increments have no significant effect on thermal conductivity enhancement. (C) 2016 Elsevier Ltd. All rights reserved.