Metal nanoparticles have unique properties making them attractive for optical, electrical, magnetic, and heat and mass transfer applications. Stable suspensions made of metal nanoparticles dispersed in a host fluid (so-called nanofluids) are used as heat transfer fluids, ferromagnetic fluids, and as gain media in random lasers. In some applications, suspensions using pure metal nanoparticles are desired since such particles offer enhanced properties when compared to their respective oxides. Unfortunately, due to their extreme reactivity, metal nanoparticles require special processing and handling conditions, are highly sensitive to their environment, and agglomerate. Surfactants can be added to the suspension for its stabilization though the use of surfactants complicates the chemistry of the produced fluid and of the process. Additionally, surfactants might not be stable under real conditions of use (ex. high temperature heat transfer applications) and might affect the properties of the nanofluids (ex. optical absorptivity).

We recently developed a dry, semi-continuous and scalable synthesis process for the production of metal nanoparticles coated with an ultrathin layer, and which layer is chemically compatible with the host fluid ensuring the stabilization of the suspension. The process combines two scalable plasma sources mounted in series and operating under a controlled environment: a cathodic metal vapor arc source capable of producing pure metal nanoparticles having diameters ranging from approximately 5 to 50 nm, and ii) an in-flight plasma polymerization source capable of coating the produced metal nanoparticles with an ultrathin (2 to 10 nm) and dense coating from a gaseous precursor. A gas-liquid contactor is used in some applications for the direct synthesis of nanofluids.

We successfully demonstrated the suitability of this process for the synthesis of copper nanoparticles coated with an ultrathin organic layer from the in-flight plasma polymerization of C₂H₆. TEM and FESEM observations of the coated nanoparticles confirmed the presence of an ultrathin layer covering the metal nanoparticles, while the FTIR analysis revealed the organic nature and the polymer-like structure of this layer. The same nanoparticles, after being exposed to air for post-oxidation and then dispersed into ethylene glycol, remained stably suspended for more than 6 hours. Work is in progress for the direct in-flight plasma polymerization using the vapor of ethylene glycol (EG) and the direct dispersion of the produced nanoparticles in the same fluid. The details of this novel synthesis process along with the UV-VIS absorptivity and heat transfer properties of the produced Cu-EG nanofluids will be reported.