Tuning the thermal properties of aqueous nanofluids by taking advantage of size-customized clusters of iron oxide nanoparticles

In this study, the thermal conductivity of aqueous nanofluids containing clusters of iron oxide (Fe3O4/γ-Fe2O3) nanoparticles has been investigated experimentally for the first time, with the aim of assessing the role of a controlled aggregation of nanoparticles in these final nanofluids. For that, clusters of iron oxide nanoparticles of different cluster size (46–240 nm diameter range) were synthesized by a solvothermal method and fully characterized by transmission electron microscopy, X-ray diffraction and Raman spectroscopy. The rheological behavior of the optimal nanofluids was also studied by rotational rheometry. The nanofluids were obtained by dispersing the clusters of iron oxide nanoparticles in water taking into account different solid volume fractions (from 0.50 to 1.5 wt%) and the experiments were conducted in the temperature range from 293.15 K to 313.15 K. The study reveals and quantifies enhancements in the thermal conductivity of nanofluid with increase of cluster size and temperature. Furthermore, a 0.50 wt% concentration of clusters of iron oxide nanoparticles within the whole range of proposed nanofluids offers great stability and improved thermal conductivity for heat transfer applications with an small dynamic viscosity increase. In addition, the larger the size of the clusters of iron oxide nanoparticles, the greater the increase in thermal conductivity for the designed Fe3O4/γ-Fe2O3 cluster-based nanofluids, with thermal conductivity values following a constant upward trend and reaching a maximum increase of 4.4% for the largest synthesized clusters (average size of 240 nm). These results open the door for the development of iron oxide-based nanofluids on which taking advantage of an optimized aggregation of nanoparticles by using size-customized clusters.