Solution processing of boron nitride nanotubes and Bending behavior of carbon nanotube fibers

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Boron Nitride Nanotubes (BNNTs) are allotropes of boron nitride (BN). Unlike carbon nanotubes (CNTs), BNNTs have a high band-gap and stronger chemical resistance making them stable to temperatures around 1000 °C. Additionally, BNNTs possess high thermal conductivity, high mechanical strength, a high neutron absorption cross-section with density comparable to water. These extraordinary properties make BNNTs viable for applications in the aerospace and automotive industries to fabricate low-density structures with high mechanical strength suitable for extreme temperature and corrosive environments. However, BNNTs have not had research exposure as much as CNTs due to difficulty in their synthesis and purification. Chapter 2 reviews the different synthesis processes to produce BNNTs and purification methods available to isolate BNNTs respectively. Chapter 3 investigates the dissolution behavior of BNNTs in chlorosulfonic acid (CSA). Cryo-TEM showed dissolution of BNNTs along with non-BNNT species, a behavior also observed with CNTs. Additionally, EELS and IR spectroscopy revealed no apparent functionalization of BNNTs after exposure to CSA. Finally, a macroscopic film fabricated from BNNT-rich phase is demonstrated. Purification methods in the literature yield only milligram quantities of BNNTs. Chapter 4 presents a novel, continuous purification process to chemically etch h-BN and elemental boron from as-received raw BNNTs. The process yields ~5 wt.% BNNTs exclusively.

CNT fibers spun from CNT/CSA solutions have specific properties on par with metals making them the strongest soft conductors available. However, no bending stiffness data exist to quantify the softness of CNT fibers. A simple method is developed and discussed in Chapter 5 to determine the bending stiffness of CNT fibers using different models. Chapter 6 investigates the scaling factors that scale bending stiffness within experimental errors of ~15%. Bending stiffness of CNT fibers scaled with aspect ratio of nanotubes with an exponent of 1.5±0.3 and the fiber diameter of 2.1±0.1. Additionally, a simple model was constructed to describe the bending behavior of CNT fibers. Chapter 7 describes a novel device built to measure the bending stiffness of metal microwires and their fatigue performance subjected to repeated bending within hours at high strains. SEM images proved that metal microwires break by catastrophic rupture caused by repeated bending instead of tension. Preliminary tests indicate that CNT fibers can undertake much higher cyclical stresses compared to metals.

Doctor of Philosophy
Nanotechnology, Boron Nitride Nanotubes, Bending behavior, Purification, Fibers, Carbon Nanotubes

Adnan, Mohammed. "Solution processing of boron nitride nanotubes and Bending behavior of carbon nanotube fibers." (2016) Diss., Rice University.

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