Field induced anisotropic thermal conductivity of silver nanowire dispersed-magnetic functional fluid

Abstract

Nanoparticles (NPs) or nanowires (NWs) dispersed suspensions with high thermal conductivity have gained much attention as thermal interface materials in developing thermal engineering applications. Here, a novel silver (Ag) NW - dispersed magnetic functional fluid (NWD-MFF), whose thermal conductivity is actively tunable with an external magnetic field, has been proposed. The NWD-MFFs were prepared by dispersing either Ag NWs with an average diameter of 70nm and aspect ratios ranging between 29 and 162 or Ag NPs with an average diameter of 70nm in water-based magnetic suspension composed of 5.0vol% of magnetic NPs. The influence on thermal conductivity enhancement in the above fluids due to the orientation of non-magnetic Ag NWs and magnetic NPs under external magnetic field was measured using the transient hot-wire method. The Ag NWs performed better than Ag NPs, when their thermal conductivities were measured under an external magnetic field. Furthermore, the thermal conductivities exhibited a linear increase for higher Ag NW concentrations and higher aspect ratios ranging between 0.029–0.110vol% and 29–162, respectively for the external magnetic field applied parallel to the direction of thermal gradient. The maximum thermal enhancement of 7% was recorded for the NWD-MFF dispersing a mere 0.110vol% of Ag NWs with an aspect ratio of 162. On the other hand, a 7% decrease in thermal conductivity was recorded when the external magnetic field was applied perpendicular to the thermal gradient direction. The reason for the enhancement and decrease in thermal conductivity was considered due to the orientation of NWs in the magnetic field direction, which was experimentally confirmed using the dark field microscopy. The above results suggested that the novel NWD-MFF could be used as an efficient thermal interface material for effective heat dissipation as well as heat accumulation that can be easily manipulated using external magnetic field.