Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA.
Nanoscale. 2015 Dec 7;7(45):19004-11. doi: 10.1039/c5nr05072c. Epub 2015 Oct 29.
The thermal properties of nano-scale materials are largely influenced by their geometry. The zero, one and quasi one dimensional forms of the same material could exhibit unique thermal transport properties depending upon the shape and nano-scale feature size. In order to gain a clear understanding of the contributions from geometrical scattering effects on thermal transport, it is required to study these nano-materials in a single isolated form rather than in clusters or films. In the past decade, titanium dioxide nanotube arrays fabricated by anodic oxidation of titanium emerged as a useful semiconductor architecture for a variety of applications, particularly for solar energy conversion. Nonetheless, the thermal properties of individual nanotubes that are important for their use in high temperature applications have not been clearly understood. Here we report the thermal transport properties of individual titania nanotubes as revealed by our preliminary study using a suspended microdevice that facilitates the thermal conductivity measurements and crystal structure investigation on the same nanotube. The nanotubes were prepared by anodic oxidation of a titanium foil in HF-DMSO electrolyte at 60 V, having outer diameters in the range of 200 to 300 nm and wall thicknesses of ∼30 to 70 nm in either amorphous or polycrystalline anatase phase. The thermal conductivity of single nanotubes was found to be very close to that of the amorphous phase (1.5 W mK(-1) and 0.85 W mK(-1) respectively) and it was only half of the thermal conductivity of the nanotube arrays in the film form. The thermal conductivity of bulk TiO2 is known to be almost six times higher. The observed thermal conductivity suppression in single nanotubes was explained using a transport model developed by considering diffuse phonon-surface scattering and scattering of phonons by ionized impurities of concentrations in the order of 10(18)-10(19) cm(-3).
纳米材料的热性能在很大程度上受到其几何形状的影响。同一种材料的零维、一维和准一维形式可能表现出独特的热输运性质，这取决于形状和纳米级特征尺寸。为了清楚地了解几何散射效应对热输运的贡献，需要以单个孤立的形式研究这些纳米材料，而不是以团簇或薄膜的形式。在过去的十年中，通过钛的阳极氧化制备的二氧化钛纳米管阵列作为一种有用的半导体结构出现在各种应用中，特别是在太阳能转换中。尽管如此，对于它们在高温应用中的使用至关重要的单个纳米管的热性能仍未得到清晰的理解。在这里，我们通过使用悬浮微器件的初步研究报告了单个二氧化钛纳米管的热输运性质，该微器件有助于对同一纳米管进行热导率测量和晶体结构研究。纳米管是通过钛箔在 HF-DMSO 电解液中在 60V 下阳极氧化制备的，具有 200 至 300nm 的外径和约 30 至 70nm 的壁厚，处于非晶或多晶锐钛矿相。发现单根纳米管的热导率非常接近非晶相（分别为 1.5W mK(-1)和 0.85W mK(-1)），仅为薄膜形式纳米管阵列热导率的一半。已知块状 TiO2 的热导率几乎高 6 倍。通过考虑扩散声子-表面散射和浓度在 10(18)-10(19)cm(-3)量级的离子杂质对声子的散射，使用开发的输运模型解释了在单根纳米管中观察到的热导率抑制。
