Degree Type

Dissertation

Date of Award

2012

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

Thermal transport in low-dimensional structure has attracted tremendous attentions because micro/nanoscale materials play crucial roles in advancing micro/nanoelectronics industry. The thermal properties are essential for understanding of the energy conversion and thermal management. To better investigate micro/nanoscale materials and characterize the thermal transport, pulse laser-assisted thermal relaxation 2 (PLTR2) and transient electrothermal (TET) are both employed to determine thermal property of various forms of materials, including thin films and nanowires.

As conducting polymer, Poly(3-hexylthiophene) (P3HT) thin film is studied to understand its thermal properties variation with P3HT weight percentage. 4 P3HT solutions of different weight percentages are compounded to fabricate thin films using spin-coating technique. Experimental results indicate that weight percentage exhibits impact on thermophysical properties. When percentage changes from 2% to 7%, thermal conductivity varies from 1.29 to 1.67 W/m*K and thermal diffusivity decreases from 10-6 to 5×10-7 m2/s. Moreover, PLTR2 technique is applied to characterize the three-dimensional anisotropic thermal properties in spin-coated P3HT thin films. Raman spectra verify that the thin films embrace partially orientated P3HT molecular chains, leading to anisotropic thermal transport. Among all three directions, lowest thermal property is observed along out-of-plane direction. For in-plane characterization, anisotropic ratio is around 2 to 3, indicating that the orientation of the molecular chains has strong impact on the thermal transport along different directions.

Titanium dioxide (TiO2) thin film is synthesized by electrospinning features porous structure composed by TiO2 nanowires with random orientations. The porous structure caused significant degradation of thermal properties. Effective thermal diffusivity, conductivity, and density of the films are 1.35~3.52×10-6 m2/s, 0.06~0.36 W/m*K, and 25.8~373 kg/m3, respectively, much lower than bulk values. Then single anatase TiO2 nanowire is synthesized to understand intrinsic thermophysical properties and secondary porosity. Thermal diffusivity of nanowires varies from 1.76 to 5.08 × 10-6 m2/s, while thermal conductivity alters from 1.38 to 6.01 W/m*K. SEM image of TiO2 nanowire shows secondary porous surface structure. In addition, nonlinear effects are also observed with experimental data. Two methods, generalized function analysis and direct capacitance derivation, are developed to suppress nonlinear effects. Effective thermal diffusivities from both modified analysis agree well with each other.

DOI

https://doi.org/10.31274/etd-180810-2711

Copyright Owner

Xuhui Feng

Language

en

File Format

application/pdf

File Size

161 pages

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