Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

Microfibers have been playing a fundamental role in heat dissipation in composite structures like fiber-reinforced polymer and ceramic matrix composite. The growth of industrial application calls out a demand for experimental investigation on thermal properties and structure of microfibers for continued improvement on performance.

This work furthers the current understanding of structure-property relationship in microfibers through use of combined thermal characterization and structure characterization on the same samples. Human hair, ultra-high molecular weight polyethylene (UHMW-PE) microfiber and silicon carbide (SiC) microwire are sampled as representatives of natural polymer fiber, synthetic polymer fiber and ceramic fiber, respectively. Thermal characterization for them was carried out in the temperature range between 20 K and room temperature using the transient electrothermal (TET) technique. Structure analysis includes x-ray diffraction (XRD) and Raman spectroscopy.

The investigation on human hair finds that the short range order in protein (1-2 nm in size) can be revealed by the phonon life time at low temperatures. The grain boundary-induced phonon mean free path, named as structural thermal domain (STD) size in this work, is found close to the crystallite size given by XRD and comparable with the rigid domain size given by nuclear magnetic resonance. Grey hair has a higher thermal diffusivity and larger STD size than black hair, probably due to altered keratin grain size or loss of melanin.

In the study on UHMW-PE microfibers, metal-like thermal conductivity (51 W/m∙K) is achieved by heat stretching a commercially available sample, Spectra S-900. XRD analysis finds that the crystallite size and orientation has not been altered by mechanical stretching and, however, a decrease in crystallinity from 92% to 83%. Polarized Raman spectroscopy indicates improved chain alignment in amorphous region.

The investigation on SiC microwires is a comparative study for three advanced 3C-SiC microwires, including Sylramic, Hi-Nicalon S and a sample fabricated by laser chemical vapor deposition (LCVD). Nanosized grains in the microwires can be detected by STD analysis, XRD and Raman spectroscopy. Unlike the hair sample and the UHMW-PE that contain molecular chains, 3C-SiC possess compact and cross-linked molecular structure. Probably due to this, the STD size is found nearly one order of magnification smaller than the crystalline size.

DOI

https://doi.org/10.31274/etd-180810-6129

Copyright Owner

Bowen Zhu

Language

en

File Format

application/pdf

File Size

65 pages

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