Thermal Conductivity and Stability of Hydrocarbon-Based Nanofluids with Palladium Nanoparticles Dispersed by Modified Hyperbranched Polyglycerol

Palladium nanoparticles, which were prepared by modified hyperbranched polyglycerol (mHPG) as stabilizers, can be dispersed well in nonpolar organic solvents and form highly stable nanofluids. The influences of three mHPG products modified with cyclohexanethiol (CSHPG), dodecanethiol (DSHPG), and octadecanethiol (OSHPG) on the preparation and stability of the palladium nanoparticles were investigated. The stability and thermal conductivity enhancement of the hydrocarbon-based nanofluids with Pd@mHPG (Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG) compared to the corresponding base fluid were investigated at different temperatures. The average diameters of nanoparticles stabilized by CSHPG, DSHPG, and OSHPG are within 2.7–3.6 nm. The palladium nanoparticles could be dispersed well in the nonpolar base fluid such as decalin. The nanofluids with Pd@DSHPG and Pd@OSHPG could remain stable for up to 330 days at room temperature. The nanofluid with Pd@DSHPG or Pd@OSHPG could be stable for more than 24 h at 110 °C. The thermal conductivity of the nanofluids improved with increasing temperature and the mass fraction of nanoparticles compared to the corresponding base fluid. The long alkyl chain-modified HPG can give better protection for nanoparticles from agglomeration and assist metal nanoparticles in enhancing the thermal conductivity of nanofluids.