Magnetic alignment of Ni-coated single wall carbon nanotubes in heat transfer nanofluids

Abstract

Thermal conductivity (TC) of heat transfer nanofluids containing magnetic-metal-coated carbon nanotubes can be significantly enhanced (>60%) by applied magnetic field. In this paper, we report the observed real images of Ni-coated single wall carbon nanotubes in water and oils (polyalphaolefin, polyol ester) under magnetic field by high speed microscopy, and correlate them with TC measurements. Initially, the nanotubes are randomly dispersed in the fluid, however, on longer holding in magnetic field the nanotubes gradually stretch and are finally aligned. The chain length in the images is found to be around 30∼150 μm, which is much longer than the real length of individual nanotubes (5∼40 μm), indicating that nanotubes are aligned and form some chains and clusters. Because of the semicontinuous nature of Ni magnetic nanoparticles, as well as the viscosity resistance of the fluid itself, it takes some time for the Ni-coated nanotubes to respond to the applied magnetic field and align. Time dependent TC experiments indicate that alignment process dominates the TC enhancement rather than microconvection. Finally, scanning electron microscopy images also show that the Ni coated nanotubes are aligned well under the influence of a magnetic field. Transmission electron microscopy images indicate that nickel remains stable and attached onto the nanotubes after the magnetic field exposure and movements.

Magnetic field alignment of template released ferromagnetic nanowires

“…This behaviour is demonstrated in the scanning electron micrographs shown in Figures 9(a) and 9(b), which show Ni nanowires lying perpendicular to each other produced by deposition in two sequential steps using high magnetic field strength orientated normal to each other. This result agrees well with the other results in literature 36,40,42 and demonstrates that the van der Waals and/or the electrostatic forces between the wires and the substrates are the dominant after deposition. This behaviour may be useful from a practical point of view in the fabrication of complex devices that needs sequential steps of nanowire alignment, for example to create memory or logic architectures.…”

“…The lengths of these chains were observed to extend over hundreds of micrometres, in agreement with the previous results. 33,40 Depending upon the concentration, the location and distance between these wires in addition to the magnetisation, the viscous drag, and the magnetic field strength, the inter-wire dipolar interactions can play an important role in the formation of chains during the deposition process.…”

“…The formation of these clusters could be attributed to the high concentration of nanowires during the deposition process, the low viscosity of the IPA, and large translational forces that result from strong applied magnetic fields. 40 These clusters of nanowires are usually undesirable in fabrication of nano-devices or for further studies on the electrical and magnetic properties of isolated nanowires. Reducing the concentration of the wires in the IPA suspension was found to reduce this effect.…”

“…Here, the efficacy of magnetic field alignment has been quantitatively assessed using magnetic nanowires; however, the results may be relevant to a wider range of functional materials where these nanostructures are capped, coated, or incorporate magnetic material as demonstrated elsewhere. 37,40,42,45 The applicability of such global magnetic alignment in technological applications depends upon the requirements of the particular application. The results show that at most $80% of nanowires were aligned within 20 of the field axis.…”

“…Therefore, a variety of techniques have been utilised to manipulate and control electrodeposited nanowires suspended in a solvent including thermocapillary motion of nanowire suspensions in microchannels, 25,26 electrostatics, [26][27][28] electric field, 26,29,30 and magnetic field [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] assisted alignment. Recently, magnetic field have been utilized by several researchers to control assembly and alignment of carbon nanotubes, 26,40 Ni, [31][32][33][34][35] semiconductor, 26 conductive polymer 26 nanowires, as well as metallic nanowires such as gold, 37,39 bismuth, 42 coppertin, 41 Pt, 43 and ZnO (Ref. 38) capped with Ni ends by manually drop casting a dilute suspension containing these nanowires between two electrodes predefined on a specimen by lithographic techniques and using two permanent magnets.…”