Anisotropic thermal transport in highly ordered TiO2 nanotube arrays

Abstract

This paper reports on the anisotropic thermal transport in highly ordered amorphous and anatase TiO2 nanotube arrays. Strong anisotropic thermal conductivity is observed: 0.617WK−1m−1 along the tube length direction and 0.077–0.102WK−1m−1 in the cross-tube direction for amorphous TiO2 nanotube arrays. The anatase TiO2 nanotube arrays are found to have a higher and anisotropic thermal conductivity, 1.12WK−1m−1 in the tube-length direction and 0.24WK−1m−1 in the cross-tube direction. The experimental results show that the density of the nanotube arrays is much lower than the value estimated from the geometry of the nanotube array, largely due to the existence of defects and loose contact among nanotubes. The thermal contact resistance between TiO2 nanotubes is characterized to be 15.1 and 20.6Km2W−1 for the two measured amorphous samples, and 5.90Km2W−1 for the anatase TiO2 nanotubes. This quantitatively proves that annealing of amorphous TiO2 nanotubes to transform them to anatase phase also improves the contact between nanotubes.