Carbon-coated Fe 3 O 4 core-shell super-paramagnetic nanoparticle-based ferrofluid for heat transfer applications.

Herein, we report the investigation of the electrical and thermal conductivity of Fe 3 O 4 and Fe 3 O 4 @carbon (Fe 3 O 4 @C) core-shell nanoparticle (NP)-based ferrofluids. Different sized Fe 3 O 4 NPs were synthesized via a chemical co-precipitation method followed by carbon coating as a shell over the Fe 3 O 4 NPs via the hydrothermal technique. The average particle size of Fe 3 O 4 NPs and Fe 3 O 4 @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 Fe 3 O 4 NPs was found to be in the range of ∼1-3 nm. The magnetic characterization revealed that the as-synthesized small average-sized Fe 3 O 4 NPs ( ca. 5 nm) and Fe 3 O 4 @C core-shell NPs ( ca. 7 nm) were superparamagnetic in nature. The electrical and thermal conductivities of Fe 3 O 4 NPs and Fe 3 O 4 @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 Fe 3 O 4 ( ca. 5 nm) and Fe 3 O 4 @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 Fe 3 O 4 ( ca. 5 nm) and Fe 3 O 4 @C core-shell ( ca. 7 nm) NPs, respectively. The exceptional enhancement in the thermal conductivity of the bare Fe 3 O 4 NP-based ferrofluid compared to that of the Fe 3 O 4 @C core-shell NP-based ferrofluid was due to the more pronounced effect of the chain-like network formation/clustering of bare Fe 3 O 4 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.