Carbon-coated FeO core-shell super-paramagnetic nanoparticle-based ferrofluid for heat transfer applications.

Herein, we report the investigation of the electrical and thermal conductivity of FeO and FeO@carbon (FeO@C) core-shell nanoparticle (NP)-based ferrofluids. Different sized FeO NPs were synthesized a chemical co-precipitation method followed by carbon coating as a shell over the FeO NPs the hydrothermal technique. The average particle size of FeO NPs and FeO@C core-shell NPs was found to be in the range of ∼5-25 nm and ∼7-28 nm, respectively. The thickness of the carbon shell over the FeO NPs was found to be in the range of ∼1-3 nm. The magnetic characterization revealed that the as-synthesized small average-sized FeO NPs ( 5 nm) and FeO@C core-shell NPs ( 7 nm) were superparamagnetic in nature. The electrical and thermal conductivities of FeO NPs and FeO@C core-shell NP-based ferrofluids were measured using different concentrations of NPs and with different sized NPs. Exceptional results were obtained, where the electrical conductivity was enhanced up to ∼3222% and ∼2015% for FeO ( 5 nm) and FeO@C core-shell ( 7 nm) NP-based ferrofluids compared to the base fluid, respectively. Similarly, an enhancement in the thermal conductivity of ∼153% and ∼116% was recorded for FeO ( 5 nm) and FeO@C core-shell ( 7 nm) NPs, respectively. The exceptional enhancement in the thermal conductivity of the bare FeO NP-based ferrofluid compared to that of the FeO@C core-shell NP-based ferrofluid was due to the more pronounced effect of the chain-like network formation/clustering of bare FeO NPs in the base fluid. Finally, the experimental thermal conductivity results were compared and validated against the Maxwell effective model. These results were found to be better than results reported till date using either the same or different material systems.