Degree Type

Dissertation

Date of Award

2013

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

Thermal transport in micro/nano materials is very critical, not only because the thermal property may have important influence on the performance of the materials, but also because the change of this property can reflect the inner structure change. Due to the complicated structure and small scale, the thermal properties of biomaterials, such as silkworm silk, spider silk and human head hair, are still not fully understood.

Experiments are conducted on silkworm silk, spider silk and human head hair to explore their thermal properties and potential applications. This work reports on the first time study of thermal transport in the axial direction of single silkworm silks. The measured thermal diffusivity of relaxed silkworm silk and thermal conductivity are 0.39 × 10-6∼2.03 × 10-6 m2 s-1 and 0.54∼6.53 W m-1 K-1, respectively. The thermal diffusivity of silkworm silk increases up to 263% upon elongation up to 63.8%. For one of the samples studied (sample 5), its thermal conductivity goes up to 13.1 W m-1 K-1 after elongation of 68.3%, surpassing many other polymers. Three factors combine together to give rise of the remarkable thermal diffusivity increase: alignment improvement of β–sheets blocks, straightening of α– and random coils under stretching, and structural transformation from α–helices and random coils to β–sheets crystal by elongation (confirmed by our Raman spectroscopy study). Thermal path break-down is observed when elongation is beyond 63.8%. Our Raman spectroscopy study confirms this speculation: after 60% elongation, the Raman frequency started to increase, indicating the internal stress has been released due to internal structure break-down.

Through series of experiments, a linear relationship between the effective thermal diffusivity and pressure-which has an effect on the effective thermal diffusivity in the form of gas conduction-is discovered and proved. By testing samples with different length, the effect of radiation and gas conduction can be eliminated. In the second part, this work reports on the much more accurate characterization of thermal transport in the axial direction of single silkworm silk, spider silk and human head hair (three parts: at the root, in the middle, and at the tip). The measured real thermal diffusivity of silkworm silk, spider silk and human head hair is 3.68 × 10-7 m2 s-1, 3.53 × 10-7 m2 s-1, 1.53 × 10-7 m2 s-1 (at the root), 1.40 × 10-7 m2 s-1 (in the middle) and 1.49 × 10-7 m2 s-1 (at the tip), respectively. The thermal conductivity and effective emissivity of the materials can also be calculated with the given value of volume-based specific heat (ρcp).

After characterizing the original samples, the study of thermal transport in the axial direction of single filaments of silk (Bombyx mori) fibroin before and after heat treatment is performed. The measured thermal diffusivity of the original silk fibroin fiber ranges from 4.05 × 10-7 to 4.65 × 10-7 m2 s-1. After heat treatment (from about 140 C to about 220 C) and subtracting the gold and radiation effect, the real thermal diffusivity of silk fibroin type 1, 2 and 3, increase by 38.12%, 20.72% and 21.35%, respectively. The sample diameter change is almost negligible which is proved by checking the diameter of the sample at the same place before and after heat treatment by SEM. Raman analysis was performed on the original and heat-treated (heated at about 147 C and 179 C) samples. After the heat treatment at 147 C, the peaks at 1081, 1230 and 1665 cm-1 become slightly sharper, which is a probable sign of structural transformation from amorphous region to crystalline region. According to the literature, a model composed of amorphous regions, crystalline regions and laterally ordered regions is proposed to explain the structural changes induced by heat treatment. Due to the close packing of the more adjacent laterally ordered regions, the number and size of the crystalline regions of Bombyx mori silk fibroin increased by heat treatment. Thus the thermal properties of the samples are significantly improved.

Copyright Owner

GUOQING LIU

Language

en

File Format

application/pdf

File Size

95 pages

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