Regulatable thermal conductivity and excellent mass transport of water-filled carbon nanotube as capillary wicks

Abstract

• The heat and mass transfer of single wall carbon nanotubes as capillary wicks are studied by molecular dynamics simulations. • The regulatable thermal conductivity can be beneficial to both start-up and overheating operation of heat pipe. • The thermal conductivity of carbon nanotube is reduced due to the thermal motion of water molecules and vdW force. Passive liquid cooling of ultrathin heat pipe has received considerable attention due to its excellent performance of removing heat from hot spot of electronic devices. However, achieving controllable thermal conductivity of the capillary wick materials to reduce the start-up time and total thermal resistance remains a challenge. Here we propose the capillary wick of carbon nanotube array to regulate its heat transfer performance via the water filling ratio. Molecular dynamics simulation results found that the thermal conductivity of water-filled carbon nanotubes is reduced by 17.8%-23.8%, which is benefit to start-up operation. At overheating operation, the high thermal conductivity of empty carbon nanotube can reduce the temperature of the endothermic surface. Importantly, the thermophoretic movement speed of water can reach up to 74 m/s and radial mass exchange occurs driven by temperature gradient. Phonon spectral analysis and heat flow decomposition revealed that the decrease of water-filled carbon nanotube thermal conductivity originates from the low-energy coupling of van der Waals force between water molecules and carbon nanotube and the thermal motion of two water molecules layers. The regulatable thermal conductivity and excellent mass transport performance demonstrate the promising potential of carbon nanotube as next-generation capillary wick of ultrathin heat pipe.