Novel fibrillar carbon nanotube heat transfer gels with enhanced thermal conductivities
Lubricants and cooling agents such as oil, ethylene glycol, and water are often used as traditional heat transfer fluids (HTFs) in engines, radiators, heat pumps, and other equipment which require cooling and/or energy maintenance. The United States of America (USA) spends over $80 billion on energy maintenance and increasing the thermal efficiency of HTFs by 25% could annually save over $20 billion. This study was aimed at improving the thermal conductivity (TC) of synthetic and vegetable oil heat transfer fluids (HTFs) by using Single-Walled Carbon Nanotubes (SWNTs) additives that resulted in a semi-solid lubricant that was a nanotube-HTF (n-HTF). The n-HTFs were processed with nitrogen containing additives that coupled with toluene and acetone solvent processing promoted the gelation of the carbon nanotube fibers in the oil. Such processing improved the nanotube dispersion due to hydrogen bonding (H bonding) and micelle formation of the amine groups around the carbon nanotube rod-like fibers and the additive-oil matrix. This allowed for high weight percent (Wt %) loadings of carbon nanotubes in the oil. Characterization via thermal graviometric analysis (TGA) and Fourier Infrared Transform (FTIR) showed that high temperature radical mechanisms breakdown both the oil and nanotubes, and optical microscopy showed that sonication-homogenizing-mixing affects the coagulation-flocculation-aggregation-agglomeration of the nanotubes. Additionally we used two new TC instruments, the Mathis-Hot Disk and KD2 systems, to provide accurate and reproducible data with a 10% and 4% error margin for the Mathis and KD2, respectively. Raman and FTIR spectroscopies suggest that the TC enhancements result from SWNT phonon mechanisms, these being phonon-phonon, phonon-defect, and phonon-interface, all of which are present at room temperature with the absence of ballistic and superconductivity phenomena. Additional vibrations in the oil that occur due to Brownian motion and electron-phonon and H bonding from the additives would have also contributed to the TC mechanism and were evidenced via Raman spectroscopy and FTIR. The optical microscopy, Raman, FTIR, and TC values indicated that the n-HTFs had a three-dimensional (3D) SWNT networked structure due to the inclusion of the oleylamine additive and the toluene-acetone processing. This would have resulted in the formation of oleylamine-nanotube micelles that were suspended in the oil-additive mixture. Raman spectroscopy evidenced a percolation effect that was coupled with fluid vibrations and Brownian motion, which led to the absence of the one- or two-dimensional (1D, 2D) ballistic or superconductivity phenomena that is often associated with aligned and single SWNTs and other 1D or 2D media, and this was reflected in the TC values. Nevertheless, the TC of the resulting n-HTFs was improved by over 80-96% when compared to other HTFs and the dispersion of the nanotubes in the oil-additive mixture was greatly enhanced. Applications of the n-HTFs include: mechanical-frictional damping, semiconductor packaging, thin-films, hydraulic oil lubricant use, thermoelectric power, thermo-sensing, fuel cells, additive-antioxidant-viscosity modifiers, and filtration.
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