We present a study of powder agglomeration and thermal conductivity in copper-based nanofluids. Synthesis of the copper powders was achieved by the use of three different surfactants (polyvinylpyrrolidone, oleic acid, and cetyl trimethylammonium bromide). After careful determination of morphology and purity, we systematically and rigorously compared all three of the surfactants for the production of viable copper-based nanofluids using dynamic light scattering. Our results show that the use of surfactants during synthesis of copper nanopowders has important consequences on the dispersion of the powders in a base fluid. The oleic-acid-prepared powders consisted of small particles of ∼100 nm that did not change with the addition of dispersant. The CTAB-prepared powders exhibited the best dispersion characteristics, as they formed small particles of approximately 80 nm in the presence of SDBS. The thermal conductivity enhancement in our nanofluids exhibited a linear relationship with powder loading for an average particle size of ∼100 nm and similar particle size distributions that range from ∼50 to 650 nm, but independent of crystallite size and with all other factors maintained constant (surface area, surface additives, levels of oxidation) such that a 0.55 vol % loading results in a thermal conductivity enhancement of 22% over water and a 1.0 vol % loading results in a thermal conductivity enhancement of 48% over water. This study is the first to decouple the effect of a carefully characterized particle size distribution using dynamic light scattering versus crystallite size from X-ray line broadening on the thermal conductivity enhancement of a nanofluid.

We present an analysis of the dispersion characteristics and thermal conductivity performance of copper-based nanofluids. The copper nanoparticles were prepared using a chemical reduction methodology in the presence of a stabilizing surfactant, oleic acid or cetyl trimethylammonium bromide (CTAB). Nanofluids were prepared using water as the base fluid with copper nanoparticle concentrations of 0.55 and 1.0 vol.%. A dispersing agent, sodium dodecylbenzene sulfonate (SDBS), and subsequent ultrasonication was used to ensure homogenous dispersion of the copper nanopowders in water. Particle size distribution of the copper nanoparticles in the base fluid was determined by dynamic light scattering. We found that the 0.55 vol.% Cu nanofluids exhibited excellent dispersion in the presence of SDBS. In addition, a dynamic thermal conductivity setup was developed and used to measure the thermal conductivity performance of the nanofluids. The 0.55 vol.% Cu nanofluids exhibited a thermal conductivity enhancement of approximately 22%. In the case of the nanofluids prepared from the powders synthesized in the presence of CTAB, the enhancement was approximately 48% over the base fluid for the 1.0 vol.% Cu nanofluids, which is higher than the enhancement values found in the literature. These results can be directly related to the particle/agglomerate size of the copper nanoparticles in water, as determined from dynamic light scattering.

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