Abstract:
Magnetic nanofluids have enormous potential to improve thermal conductivity under the influence of magnetic fields. Magnetic field induced thermal transport capabilities of metallic magnetic nanoparticles viz. Fe, Ni and Co based stable magnetic colloids under the influence of magnetic field has been reported for the first time (detailed survey of literature supports the claim). Experimental investigations reveal highly enhanced thermal conductivity of such colloids under the influence of external magnetic field. The highest magnitude of thermal conductivity enhancement ∼106% and 284% is achieved for the Fe/HTO magnetic-nanofluids w.r.to the base nanofluid and pristine base fluid (in the absence of magnetic field) respectively at 0.05 T magnetic fields and 7.0 vol.% particle concentration. Ni and Co based nanofluids demonstrate less enhancement in the thermal conductivity magnitude compared to Fe based nanofluids due to lower values of saturation magnetic moments. However, the reduction in performance is not as drastic as expected since Ni and Co possesses better thermal conductivities than Fe, leading to compensation of the reduced field response. The underlying mechanism of enhanced conduction has been explained based on the formation of stable nanoparticle chains along the magnetic field lines which act as ‘short circuits’ for the thermal waves to travel faster. The enhancement in the thermal conductivity drops gradually beyond a critical magnetic field due to zippering/self-aggregation of the chained structure. The magnetic fluids have also been observed to be reversible with low thermal hysteresis and prove to be potential candidates as smart fluids in micro-scale devices.
