Review.1 – Magnetic alignment of Ni-coated single wall carbon nanotubes in heat transfer nanofluids

Journal of Applied Physics, 107 (2010) 104320

Authors: Mark Horton, Haiping Hong, Chen Li, Bo Shi, G. P. Peterson, and Sungho Jin

In this review, a relatively new paper on the use of magnetically stimulated nanofluids as heat transfer materials is explored.

Article Summary: Ni-coated single-walled carbon nanotubes (SWNTs) were dispersed in four fluids of varying viscosities to create magnetic nanofluids using sodium dodecylbenzene sulfonate as surfactant. Water, polyalphaolefin oil (PAO) and two types of polyol ether are used as carrier fluids for the coated nanotubes. Due to the magnetic properties of the Ni-coated SWNTs the researchers induced a magnetic field to hopefully increase the thermal conductivity of the nanofluids. It was determined experimentally that magnetic stimulation of the nanofluids increased the observed thermal conductivity of the nanofluids. The increase in thermal conductivity was caused by the alignment of the carbon nanotubes into chains and clusters due to the magnetic field. A Thermal Constants Analyzer was used to measure thermal conductivity of the different nanofluids. In addition to the thermal chracterization, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe physical properties before and after magnetic stimulation. Through the use of SEM it can easily be seen that the coated SWNTs are being aligned by the magnetic field.  The greatest increase in thermal conductivity was seen with the use of water as a carrier fluid and its value increased from 0.63 W/mK to 1.10 W/mK during magnetic field stimulation. The other nanofluids showed increases in thermal conductivity, but not to the extent of the Ni-coated SWNT/water nanofluid.  It was observed that the lower the viscosity of the carrier fluid used, the higher the thermal conductivity during magnetic stimulation. The thermal conductivity peaked after exposure to the magnetic field for 4 minutes and then subsequently decreased after the 4 minute mark.

Some interesting points to be taken from this article include:

• Viscosity of carrier fluid has an effect on thermal conductivity during magnetic stimulation. A lower viscosity fluid allows the coated SWNTs to move around more freely.
• Time dependence of magnetic stimulation on thermal conductivity. An ideal magnetic stimulation time of 4 minutes was found, where thermal conductivity peaks. Before and after this time, the thermal conductivity decreases as time moves away from the 4 minute mark.
• The use of SEM and TEM really adds to the article, as we get a look at the structure of the nanotubes on a µm and nm scale.

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