Novel Nanofluids Based on Magnetite Nanoclusters and Investigation on Their Cluster Size-Dependent Thermal Conductivity

To probe the effect of particle size on thermal conductivity (<i>k</i>) enhancement in nanofluids, especially in a very large particle size range, we study the cluster size-dependent <i>k</i> in novel nanofluids containing magnetite (Fe₃O₄) nanoclusters. The Fe₃O₄ nanoclusters in the size range of 115 to 530 nm were synthesized by a facile and cost-effective solvothermal approach. The structural, surface, and magnetic characteristics of Fe₃O₄ nanoclusters were investigated by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and vibrating sample magnetometer (VSM). Thermal conductivity studies in diethylene glycol (DEG)-based Fe₃O₄ cluster nanofluids showed an enhancement in <i>k</i> with an increase in nanocluster size. With a fixed volume fraction (ϕ) = 0.0193, the <i>k</i> enhancement was about 5.3% and 12.6%, respectively, for nanofluids having cluster size of 115 and 530 nm. The observed increase in nanofluid <i>k</i> with increase in cluster size being contrary to the microconvection hypothesis confirms the less prominent role of Brownian motion-induced microconvection on the <i>k</i> enhancements of nanofluids. The increase in nanofluid <i>k</i> with increase in cluster size is attributed to the growth of clusters into fractal-like aggregates in the suspensions which was confirmed by optical microscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM) studies. Furthermore, the experimental <i>k</i> data fall within the upper and lower Maxwell bounds for homogeneous systems, confirming the classical nature of thermal conduction in nanofluids. The nanofluids developed in the present study are promising candidates for heat transfer applications because of their improved thermal conductivity and long-term stability. The present study can provide new insights for engineering efficient nanofluids containing nanoclusters with superior thermal conductivity for heat transfer applications