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The authors have presented a literature review on computational and experimental studies on the thermal conductivity of nanostructure enhanced phase change materials (nePCMs). It was found that a wide range of nanostructures have been investigated, from carbon nanostructures and carbon nanotubes to metal and metal oxide nanoparticles. The carbon nanostructures and nanotubes were found to generally enhance the thermal conductivity of the PCMs by a greater amount than metal or metal oxide nanoparticles due to their high aspect ratio in comparison to the spherical or agglomerated nanoparticles.
The thermal transport properties of nanofluids containing single-walled carbon nanotubes (SWCNTs) in ethylene glycol and (poly)-alpha olefins (PAO) were investigated. It was found that the addition of SWCNTs enhanced the thermal conductivity of each of the two base fluids, with this enhancement increasing as the loading of SWCNTs increased. The authors were able to create an accurate predictive model of the thermal conductivity for these nanofluids by characterizing the morphology of the nanotubes after dispersal in the base fluids and incorporating the effects of a thermal interface layer.
Multi-walled carbon nanotube (MWCNT)/poly (methyl methacrylate) composites were prepared and their mechanical and physical properties were examined. An amphiphilic copolymer was used to aid in dispersing the MWCNTs throughout the composite. It was found that the use of this copolymer resulted in a good dispersal of the MWCNTs without drastically affecting the molecular weights of the composites, so it was concluded that the observed improvements in mechanical properties of the composites were due to the MWCNTs. Since the nanotubes that were used were short in length, the electrical and thermal conductivities were not as high as desired.
This paper proposes conducting carbon aerogels, composed of graphene and multi-walled carbon nanotubes, as an option for a thermoelectric material. The researchers found that this combination of materials increased the electrical conductivity and the Seebeck coefficient due to bridging effects, and decreased the thermal conductivity through high porosity and a uniquely structured 3-D skeleton. Atomic force microscopy and X-ray diffraction were used to quantify the physical structure of the aerogels. Results indicated that the open cell nature of the material produced depressed the thermal conductivity as the solid structure was not suited for good heat transport capabilities. The thermal conductivity of the nanocomposite aerogels was low, reaching a minimum of 0.056 W/mK. This, coupled with the increase in the Seebeck coefficient and the electrical conductivity, indicates that these could be a potential low cost thermoelectric material.
Carbon nanotubes (CNTs) have extremely high thermal conductivity and can raise the thermal conductivity of a polymer matrix when used as an additive. Previous research has shown that random CNT orientation in polymer matrices gives a much lower thermal conductivity value than a specific alignment direction. This project tested the thermal conductivity of oils and polymers. Results were encouraging, with the thermal conductivity of the composites increasing linearly with the amount of CNTs added. A spontaneous alignment of the CNTs in liquid crystal polymer matrix caused a large increase in thermal diffusivity. The researchers concluded that CNTs are an effective filler for use in increasing the thermal conductivity of composites and improving their thermal performance.
