Degree Type

Dissertation

Date of Award

2017

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

To realize their potential applications in electronic, energy, environmental and medical devices, new nanostructured carbon materials have been synthesized and studied. In this work, the excellent thermal properties of four typical new nanostructured carbon materials including graphene foam, graphene aerogels, graphene paper with different reduction level, and carbon nanotube bundles have been studied in detail by using phonon scattering mechanisms analysis. The effect of low temperature, different nanostructures and thermal strain are the focus.

Specifically, for the first time, the defect level in graphene foam is identified by evaluating the thermal reffusivity at the 0 K limit. The ideal thermal diffusivity and conductivity of graphene presented in the 3D graphene foam structure in the range of 33-299 K is also reported and discussed. We synthesized ultra-light graphene aerogels material, although its density is about two times higher than air, its k at room temperature is about 80% lower than that of air. At low temperatures, its k even reaches a lower level of 2×10-4 -4×10-4 W m-1 K-1, which is the lowest ever reported. The mechanism of this extremely low k is explored by studying the temperature variation of k, α, and specific heat (cp) from RT to as low as 10.4 K. A high k switch-on phenomenon in high-purity graphene paper when its temperature is reduced from RT to 10 K is investigated and reported. The switch-on behavior is attributed to the thermal expansion mismatch among pure graphene flakes and the impurities embedded flakes. By conducting comparison studies with pyrolytic graphite, graphene oxide paper and partly-reduced graphene paper, the whole physical pictures are illustrated clearly. The sequential process of current-induced thermal annealing on improving the structure, electrical and thermal conductivity of CVD grown CNTs is studied for the first time. By combining large current annealing in vacuum environment and in-situ afterwards transient electro-thermal characterization, the dynamic electrical and thermal properties of the same sample annealed at different temperatures are obtained. The future work about the parallel heat transport in the carbon nanotube bundles and a super-insulation material is also discussed at the end of this work.

Copyright Owner

Yangsu Xie

Language

en

File Format

application/pdf

File Size

141 pages

Included in

Engineering Commons

Share

COinS