Effect of Ultrasonication Time on Microstructure, Thermal Conductivity, And Viscosity of Ionanofluids with Originally Ultra-Long Multi-Walled Carbon Nanotubes

Abstract

Ionanofluids (INFs) combine the advantages of ionic liquids (ILs) and nanofluids. They are characterized by higher thermal conductivity than pure ILs, as well as non-flammability and enhanced thermal stability than conventional nanofluids. Such an augmentation leads to efficient and safe heat transfer fluids. The stability along with thermal and rheological characteristics of these hybrid systems profoundly depend on the protocol of preparation. Therefore, in this work, the effect of ultrasonication time on microstructure, thermal conductivity, and viscosity of INFs containing 0.2 wt% of originally ultra-long multi-walled carbon nanotubes (MWCNTs) and four different ILs, namely 1-propyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-butyl-1-methylpyrrolidinium is(trifluoromethylsulfonyl) imide, 1-ethyl-3-methylimidazolium thiocyanate, or 1-ethyl-3-methylimidazolium tricyanomethanide, was studied. The INFs were obtained by a two-step method usingan ultrasonic probe. The ultrasonication process was performed for 1, 3, 10, or 30 min ata constant nominal power value of 200 W. The obtained results showed that for the shortest sonication time, the highest thermal conductivity enhancement of 12% was obtained. The extended sonication time from 1 to 30 min caused the cutting of MWCNTs and breaking the nanoparticle clusters, leading to a decrease in the average length of the nanotube bundles by approx. 70%. This resulted in a decline in thermal conductivity even by 7.2% and small deviations from Newtonian behavior of INFs.