Effect of Size and Morphology on Stability and Thermal Conductivity of ZnO Nanofluid

ZnO nanofluids with different crystallite size (25, 17.4, 11.3 and 8.6 nm) have been prepared by chemical route using different solvent. XRD pattern confirms that the samples have hexagonal Wurtzite structure. SEM image shows that 25 nm crystallite size particles are of rod shape whereas 17.4 nm crystallite size are star shape particles. 11.3 nm size particles are almost spherical aggregates whereas 8.6 nm crystallite size particles show mixed shape of spherical as well as elongated rod shape particles. The change in morphology is due to the different reaction rate of solvents used in the reaction to produce ZnO nanoparticles. Nanofluid thus prepared in distilled water was found highly stable and the stability is confirmed using the steady state photograph as well as UV-Vis spectral. DLS measurement shows the formation of aggregation over a period of time though the nanofluid is stable. Relative intensity variation analysis also confirms the non-sphericity of particles. The thermal conductivity increases with increase in volume fraction for all samples. It is observed that the maximum 3.126 W/m-K thermal conductivity is observed for smallest sized ZnO sample (8.6 nm) at 0.5465 volume fraction. The high value of thermal conductivity can be a collective effect due to the aggregation phenomena, morphology and conduction-convection mechanism. The result promises the possibility of enhanced heat transfer characteristics of ZnO nanofluid.