Date Awarded

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Applied Science

Advisor

Hannes C Schniepp

Committee Member

David Kranbuehl

Committee Member

Michael Kelley

Committee Member

Tarek M Abdel-Fattah

Abstract

Boron nitride nanotubes (BNNTs) are a fascinating material that has a lot of potential applications due to its outstanding physical and chemical properties. BNNTs synthesized using different high- and low-temperature methods, unfortunately, contain 30-70% impurities. To maximize the potential of BNNTs, new characterization techniques and purification methods are needed, especially with respect to industrial applications. Research groups have used different characterization techniques to monitor the synthesis and purity of BNNTs. Quantitative and qualitative characterization of the tubes for quality or purity control has not been previously described in the literature. In this work, we created a comprehensive set of thermogravimetric and spectroscopic analysis techniques to detect and quantify the different types of BNNT impunities. Based on our thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy results, we demonstrated that it is possible to monitor the presence of boron, boron oxide, and boron nitride impurities quantitatively in BNNTs and to verify their removal after purification. Although there are several reported purification methods in the literature, none of them succeeded in purifying BNNTs from boron nitride (BN) impurities without damaging the tubes and resulting in a low yield. Here we introduce a non-aggressive, non-destructive, high-yield purification method using heptane at the moderate temperature T=90 °C. Our method effectively removes ≥99.8% of the hexagonal boron nitride (h-BN) and a significant amount of other BN impurities. This is a substantial advancement over all previously reported methods that rely on more aggressive treatments. Field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) images, as well as X-ray diffraction, Raman, and FTIR spectra, were used to support our purification results. The latter three spectroscopic techniques characterize a macroscopic region of the sample and are thus more representative than imaging selected areas using electron microscopy. Finally, we studied the effect of the BNNT loading and composite morphology - networked versus dispersed BNNTs - on the composite's thermal and structural properties. BNNT composites at different loadings showed that the thermal conductivity sharply increases with the increase in the BNNT loading. Loading polystyrene with 18% BNNTs increased the thermal conductivity by a factor of three. The thermal conductivity in the radial direction was higher than the axial direction for all composites. The difference in thermal conductivity between the radial and the axial direction is believed to be due to the hot-pressing step introducing radial tube orientation and alignment. Regarding the tensile properties of the BNNT composites, the Young's modulus was increased 50-105% at 5%-7.5% loadings compared to the neat polystyrene, and tensile strength increased by around 40%. Higher loadings led to more brittle composites. Impregnation of the BNNT network with polystyrene led to better mechanical properties than BNNT dispersion. The FESEM images showed favorable wetting of the tubes by polystyrene and a good matrix/tubes load transfer.

DOI

http://dx.doi.org/10.21220/s2-sybw-3328

Rights

© The Author

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