Synthesis of Compact and Porous SiO₂ Nanoparticles and Their Effect on Thermal Conductivity Enhancement of Water-Based Nanofluids

In this work, compact and porous SiO₂ nanoparticles (NPs) were synthesized using the Stöber and the modified Stöber methods. Water-based nanofluids were characterized by dynamic light scattering, scanning electron microscopy, transmission electron microscopy, energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction, and the porosity of the compact and porous SiO₂ samples was measured by N₂ adsorption–desorption isotherms. Thermal wave resonator cavity and inverse photopyroelectric configuration novel techniques were used for the first time to obtain the porous nanoparticles thermal diffusivity (D) and effusivity (e), respectively. Thermal conductivity (k) was obtained from the relationship between them k=eD. An increase in the thermal conductivity of the porous SiO₂ NPs was obtained compared to the thermal conductivity of the compact NPs fluids with an enrichment of 14.7 %. Our results are supported by a theoretical model with a thermodynamic approach adapted for the thermal conductivity of porous SiO₂ governed by the parameters of the porosity and nanoparticle size.