Degree Type

Dissertation

Date of Award

2011

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

This dissertation first presents the work on measuring thermal properties of MgH2 nanostructures at microscale by using laser-based method -- photothermal technique. In order to investigate the thermal properties at nanoscale, a scanning photothermal microscopy (SPTM) system is proposed. In the SPTM, the most important part is the complex tip-laser interaction. Systematical work has been done to in this field. For the first part, the photothermal technique is used to measure the effective thermal conductivity and density of the vanadium-doped MgH2 nanostructures which is extensively used in energy storage applications. A multilayer physical model is used to fit the experimental data. Our results show that the effective thermal conductivity of the hydrogenated V-doped Mg nanostructures is in the range of 1.16 2.40 W/(m∙K), and the density falls in the range of 878 1320 kg/m3. The measured density agrees well with the estimation from electron micrograph observation. Variation in the measurements indicates strong non-uniformity of the sample structure and thickness. Based on the measured density and effective thermal conductivity, the thermal conductivity of bulk V-doped Mg hydrides is also evaluated using Maxwell's correlation.

For the second part, scanning photothermal microscopy (SPTM) system is designed to diagnose the defects on sub-surface under nanoscale range. This system consists of a conventional AFM and an external laser. Due to the complex interaction between the sharp tip and the laser, the thermal response of tungsten tip was first studied. At first step, we systematically report on a study of highly enhanced optical field and its induced thermal transport in nanotips under laser irradiation. Our Poynting vectors study clearly shows when a laser interacts with a metal tip, it is bent around the tip and concentrated under the apex, where extremely high field enhancement appears. This phenomenon is more like a liquid flow being forced/squeezed to go through a narrow channel. How the geometry of the tip, the tip-substrate distance, polarization angle and incident angle of laser affect the field enhancement are systematically presented. The thermal transport inside the nanoscale tungsten tips due to absorption of incident laser is also explored. As the polarization angle or apex radius increases, the peak apex temperature decreases. The peak apex temperature goes down as the cone angle increases, even though the mean laser intensity inside the tip increases, revealing a very strong effect of the cone angle on thermal transport.

Correspondingly, experimental work on the laser-tip interaction was reported. The temperature rise of an ordinary AFM silicon tip under laser irradiation was measured. Laser heated AFM tip can act as both thermal probe and thermal heating source in nanoscale thermal heating process. The elevated temperature of the tip can be acquired from the relationship between Raman shift vs. temperature, which, however, is different compared to the Raman shift-temperature curve for bulk Si. The influences of focal position and energy flux level on the Raman shift have been discussed intensively. We substantiate our Raman measurement by theoretically modeling the electric field distribution as well as the heating processing through finite element method and a one-dimensional model, respectively.

DOI

https://doi.org/10.31274/etd-180810-643

Copyright Owner

Xiangwen Chen

Language

en

Date Available

2012-04-06

File Format

application/pdf

File Size

119 pages

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