Size effect on thermal conductivity and stability of TiO2/MWCNT-based hybrid nanofluids synthesized via probe ultrasonication

Abstract

This study reports the enhancement of thermal conductivity in hybrid TiO₂ grafted onto multi-wall carbon nanotubes (MWCNTs) dispersed in an ethylene glycol nanofluid synthesized by a scalable probe-ultrasonication process. The hybrid nanofluids were formulated at ultra-low loadings; MWCNT = 0.001 wt% (fixed) and TiO₂ = 0.001–0.01 wt% (15 nm and 30 nm). The 15 nm TiO₂ sample at 0.01 wt% achieved 16.7% thermal conductivity enhancement at 70 °C while maintaining >4 weeks stability. To the best of our knowledge, this is the first report achieving double-digit conductivity improvement at ≤0.01 wt% solids using a surfactant-free, scalable probe-ultrasonication route. Homogeneous and stable TiO₂/MWCNT nanofluids were produced using a surfactant-free approach, and their performance was validated through Raman spectroscopy, Zetasizer, TEM, and UV-Vis analyses. Formulations with ultra-low loadings, MWCNT = 0.001 wt% (fixed) and TiO₂ = 0.001–0.01 wt% (15 or 30 nm), were investigated. The sample containing 15 nm TiO₂ at 0.01 wt% exhibited a reproducible 16.7% thermal-conductivity enhancement at 70 °C and maintained colloidal stability for over four weeks. Such a high enhancement at extremely low solid content in an ethylene glycol matrix, achieved through a surfactant-free and scalable ultrasonication route, has not been previously reported.