Enhanced heat transfer of laser-fabricated copper nanofluid at ultra-low concentration driven by the nanoparticle surface area

As solar thermal energy systems are an important pillow toward green energy production, the enhancement of their thermophysical properties using nanofluids is a highly relevant topic. However, when nanofluids are designed by the addition of nanoparticles (NPs), their colloidal stability is frequently impaired during hightemperature processing, a phenomenon related to particle size, morphology, and concentration. In this work, we synthesized nanofluids composed of ligand-free colloidal CuNPs dispersed in ethylene glycol by continuousflow, picosecond-pulsed laser ablation in liquids synthesis, yielding monomodal-CuNPs with mean diameters of 2.5 and 4.8 nm. The nanofluids' thermal conductivity (knf) was measured using a guarded-hot-plate method in the temperature range from 298 to 318 K. We observed a nanoparticle surface area-dependent enhancement of the knf up to 30 % at ultra-low volume concentration of 20 ppm. This corresponds to 30 times higher concentration-normalized knf in comparison to the state-of-the-art, while the resulting nanofluids retain their rheological properties. The findings are matched with Yu-Choi's theoretical model calculations, indicating that heat transfer at the nanoparticle-solvent interface is driven by an interfacial layer of solvent molecules. These findings highlight the suitability of laser-fabricated ligand-free CuNPs as additives for heat transfer fluids, maximizing performance in mid-temperature heat transfer applications like solar thermal collectors.

Automated summary

As an important method for green energy production, the use of nanofluids to enhance the thermophysical properties of solar thermal energy systems is a highly relevant topic. However, the colloidal stability of nanofluids is often impaired during high-temperature processing when nanoparticles are added, which is related to particle size, morphology, and concentration. In this study, we synthesized nanofluids composed of ligand-free colloidal CuNPs dispersed in ethylene glycol through a laser ablation in liquids synthesis method, and observed a nanoparticle surface area-dependent enhancement of thermal conductivity. These findings highlight the suitability of laser-fabricated ligand-free CuNPs as additives for heat transfer fluids in mid-temperature heat transfer applications.