Mohammadmehdi Choolaei - Research Institute of Petroleum Industry (RIPI), Azadi sport complex West Blvd., Tehran,
Amir Yadegari - Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, University of Tehran, Tehran, Iran
Amideddin Nouralishahi - Energy, Environment and Nanostructure Materials, Caspian Faculty of Engineering, University of Tehran, Rezvanshahr, Iran

SiO2-carbon nanotubes (SiO2-CNTs) were synthesized via two-step synthesis method using carbon nanotubes in the presence of silica nanoparticles at different concentrations. The morphological features characterized by field-emission scanning electron microscopy, X-ray diffraction, and N2 adsorption/desorption. The thermal conductivity tests were conducted on different weight percent of SiO2, and SiO2-CNTs on water/ethylene glycol base fluid. It was found that the combination of CNTs and SiO2 nanoparticles as a hybrid nanostructure performed excellent stability and thermal conductivity tests in comparison to the base fluids in the temperature range of 20-40 oC. It was also found that the co-presence of carbon nanotubes with SiO2 nanoparticles, i.e. in the form of SiO2-CNTs hybrid nanoparticles, can increase the effective thermal conductivity, comparing with that of SiO2 based nanofluids (without CNTs). It was also found that the calcination process, during the synthesis of hybrid nanoparticles, has a negative impact on the final conductivity of the resulted nanofluids, due to the elimination of surface functional groups from CNTs side. The results prove the crucial role of surface properties and nanoparticles/base fluids interfacial interactions in improving of overall thermal conductivity of nanofluids

Hybrid Nanoparticles, Silica, Carbon Nanotubes, Effective Thermal Conductivity