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

Abstract

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.