Effect of size of multiwalled carbon nanotubes on thermal conductivity and viscosity of ethylene glycol-based nanofluids for solar thermal applications

The paper investigates the influence of the length of multi-walled carbon nanotubes (MWCNTs) dispersed as an additive in solar thermic fluids to enhance thermal conductivity. Here, pure ethylene glycol was chosen as solar thermic fluid due to its low viscosity, high boiling point, excellent chemical stability, and compatibility with the materials used in solar thermal systems. Pristine multi-walled carbon nanotubes are ball milled for 5, 10, and 20 h to reduce the length due to attrition. The effect of ball milling duration on the defect formation and damage to the tubular structure is assessed using Raman spectroscopy. Furthermore, the ball-milled MWCNTs were oxidized to separate amorphous carbon produced during ball milling, increasing their purity. The pristine long nanotubes and ball-milled nanotubes are mixed in pure ethylene glycol in 0.25 wt. %, and the stability of liquids is estimated using Ultraviolet-Visual (UV-Vis) spectroscopy for two months. The stability of the fluids and thermal conductivity have considerably enhanced with the dispersion of shortened MWCNTs. The dispersion of short MWCNTs in ethylene glycol resulted in a 20%-30% increase in thermal conductivity compared to pure ethylene glycol. It was also found that higher ball milling times resulted in ultra-high stability but the properties deteriorated due to the destruction of the MWCNTs' tubular structure, making them useless.