Carbon-coated Fe3O4 core–shell super-paramagnetic nanoparticle-based ferrofluid for heat transfer applications

Herein, we report the investigation of the electrical and thermal conductivity of Fe3O4 and Fe3O4@carbon (Fe3O4@C) core–shell nanoparticle (NP)-based ferrofluids. Different sized Fe3O4 NPs were synthesized via a chemical co-precipitation method followed by carbon coating as a shell over the Fe3O4 NPs via the hydrothermal technique. The average particle size of Fe3O4 NPs and Fe3O4@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 Fe3O4 NPs was found to be in the range of ∼1–3 nm. The magnetic characterization revealed that the as-synthesized small average-sized Fe3O4 NPs (ca. 5 nm) and Fe3O4@C core–shell NPs (ca. 7 nm) were superparamagnetic in nature. The electrical and thermal conductivities of Fe3O4 NPs and Fe3O4@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 Fe3O4 (ca. 5 nm) and Fe3O4@C core–shell (ca. 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 Fe3O4 (ca. 5 nm) and Fe3O4@C core–shell (ca. 7 nm) NPs, respectively. The exceptional enhancement in the thermal conductivity of the bare Fe3O4 NP-based ferrofluid compared to that of the Fe3O4@C core–shell NP-based ferrofluid was due to the more pronounced effect of the chain-like network formation/clustering of bare Fe3O4 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.