Date of Award

Summer 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

Zafar Iqbal

Second Advisor

N. M. Ravindra

Third Advisor

Trevor Tyson

Fourth Advisor

Marek Sosnowski

Fifth Advisor

Frank J. Owens

Abstract

The first part of this thesis will be focused on developing a facile and scalable solid — solid chemical vapor deposition (CVD) process using a solid boron precursor to synthesize crystalline boron nanotubes (BNTs) mixed with small amounts of boron nanofibers (BNFs). The synthesis involves the use of solid boron precursors - magnesium boride and magnesium borohydride, magnesium and nickel boride as co-catalysts, and porous MCM-41 zeolite as the growth template. Experimental parameters, such as temperature, grinding of constituents and starting precursor-catalyst-template compositions, were studied and optimized. Detailed characterization have been carried out using field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) coupled with energy dispersive x-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS) and Raman spectroscopy. The BNTs mixed with BNFs produced have a diameter of 10 nm and below and up to 1 micrometer in length. Lattice fringes were found by high resolution TEM (HRTEM) imaging, and the lattice spacing from the fringes is consistent with a recent theoretical calculation.

The second part of the thesis will focus on laboratory scale and scaled-up in-situ synthesis of high strength metal and ceramic-carbon nanotube (CNT) composites extending prior work in our group of iron-carbon nanotubes composites. Both laboratory scale and scaled up setups were developed and were successfully used in producing the uniformly nanotube filled composites. The synthesis of composites involved the use of pure iron, stainless steel with two different particle sizes, aluminum and boron carbide ceramic. The starting powders were dry-coated with nano-sized Fe2O3 as catalyst of multiwalled carbon nanotubes growth. The samples produced were characterized by FE-SEM, together with EDS, Raman spectroscopy, and mechanical measurements, which include compressive stress-strain and hardness measurements. Compressive stress-strain measurements on nanotube composites of stainless steel mixed with some iron showed a yield strength increase of 153% (relative to a similarly produced pellet without nanotubes) with 5.2 wt% of infiltrated multiwalled carbon nanotubes (MWNTs). Rockwell hardness increase of 157% for stainless steel infiltrated with MWNTs relative to a similarly produced pellet without MWNTs, was observed.

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