Alignment of carbon nanotubes comprising magnetically sensitive metal oxides in heat transfer nanofluids

摘要

High speed microscopy was utilized to allow real time visualization of the movement of single walled carbon nanotubes (SWNT) with magnetically sensitive nanoparticles (Fe_2O_3) and a chemical surfactant (NaDSSB) in water. Initially, entangled SWNT, Fe_2O_3 and NaDSSB mixtures were randomly dispersed in the fluid. Upon extended exposure to the magnetic field, the mixture slowly vibrated, the nanoparticles straightened and aligned with respect to the magnetic field. The aligned nanoparticle chains appeared to be continuous and unbroken, forming a combination of aligned particles and clusters. Because of the semi-continuous nature of these nanosuspensions and the inherent viscosity of the fluid, some minutes are required for the mixtures to respond to the applied magnetic field and align. Time dependent thermal conductivity experiments indicate that the alignment process dominates the thermal conductivity enhancement as opposed to micro convection. Scanning Electron Microscopy (SEM) images also show that the SWNT and Fe_2O_3 particles are well aligned under the influence of the magnetic field. Verification of the assumption that electrostatic attraction between nanotube/surfactant and metal oxides makes aggregation happen was obtained, by changing the nature of the charge of the surfactant from a negative charge (NaSDDB) to a positive charge (CTAB). Compared with the alignment of Ni coated SWNTs that contain chemical bonds between Ni and C atoms, this electrostatic force induced alignment could maintain nanotube perfect conjugate structures which result in excellent thermal, electrical, and mechanical properties. The alignment of the carbon nanotubes in nanosuspensions may offer new opportunities for the development of nanofluids. In addition, these nanosuspensions could be applicable in a wide variety of potential applications, such as thermally conductive films, reinforced polymer composites, transparent electrodes for display and solar cells, electromagnetic interference shielding, new sensors, etc.