This paper reports on the anisotropic thermal transport in highly ordered amorphous and anatase TiO 2 nanotube arrays. Strong anisotropic thermal conductivity is observed: 0.617 W K −1 m −1 along the tube length direction and 0.077-0.102 W K −1 m −1 in the cross-tube direction for amorphous TiO 2 nanotube arrays. The anatase TiO 2 nanotube arrays are found to have a higher and anisotropic thermal conductivity, 1.12 W K −1 m −1 in the tube-length direction and 0.24 W K −1 m −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 TiO 2 nanotubes is characterized to be 15.1 and 20.6 K m 2 W −1 for the two measured amorphous samples, and 5.90 K m 2 W −1 for the anatase TiO 2 nanotubes. This quantitatively proves that annealing of amorphous TiO 2 nanotubes to transform them to anatase phase also improves the contact between nanotubes.

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This paper presents the results of molecular dynamics studies about the shock wave during laser-induced surface nanostructuring. A quasi-three dimensional model is constructed to study systems consisting of over 2 million atoms. Detailed studies are carried out about the shock wave front and Mach number, evolution of plume and ambient gas interaction zone, and energy exchange between the ambient gas and plume. Under an ambience of lower pressure or lighter molecular mass, the plume affects a larger area while the strength of the shock wave front is weaker. With the same ambient pressure, the ablated material features the same kinetic energy at the late stage regardless of the molecular weight of the ambient gas. The same conclusion holds for the energy increase of the ambient gas as well. When the ambient pressure is reduced, more kinetic energy is carried out by the ablated material while less energy is transferred to the ambient gas. It is observed that heavier ambient gas could bounce back the ablated material to the target surface.

This paper presents the results from molecular dynamics simulations that are performed to explore the properties of the shock wave during laser-assisted near field surface nanostructuring. A quasi-three dimensional model is constructed to study systems consisting of over 2 million atoms. This work includes studies on the velocity as well as pressure evolution of shock wave front with respect to different solid/gas molecular mass ratios and different ambient gas densities. The limitation on shock wave formation under the same laser fluence is also investigated. The results show that lower ratio of the solid/gas molecular weight weakens the strength of the shock wave during the nanostructuring process. Additionally, the formation and attenuation of the shock wave under different ambient gas conditions is studied in substantial detail.

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