No description available.

No description available.

No description available.

Thesis (M.S.)--Ohio University, 2002. / Title from PDF t.p.

Thesis (Ph.D. in Materials Engineering) -- University of Dayton. / Title from PDF t.p. (viewed 06/23/10). Advisor: Liming Dai. Includes bibliographical references (p. 136-162). Available online via the OhioLINK ETD Center.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1994. / Abstract. Includes bibliographical references. Also available via the Internet.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the Degree of Doctor of Philosophy. J o h a n n e s b u r g , 2 0 1 3. / Structured carbon nanomaterials have attracted considerable interest because of their unique structures and outstanding properties. Among other structured carbon nanomaterials, carbon nanofibers (CNFs) have been the subject of study for several decades with particular interest having been paid towards their synthesis and application. However, control over the size and shape of these materials still remains a challenge. Three main components necessary for the synthesis of CNFs are the catalyst or template, the carbon source and the source of energy/power. It has been noted that catalyst morphology and the carbon source plays an important role in controlling CNF growth and morphology. As such one of the main challenges is to produce the catalyst particles that would yield the desired CNF morphology. In this study, we investigated methods for controlling the size and morphology of CNFs by synthesizing Ni and Cu catalysts of particular morphology, while using C2H2 and trichloroethylene (TCE) as a carbon source for the synthesis of CNFs. A mixture of TCE/C2H2 was also employed as a carbon source for comparison. The catalysts and synthesized CNFs were characterized by different techniques such as TEM, XRD, TPR, TGA, Raman spectroscopy, IR spectroscopy, etc. The synthesis of Ni nanoparticles (NPs) was achieved by reduction of Ni(acetate)2 with hydrazine (35%). CNFs were synthesized by deposition of TCE, C2H2 and their mixtures using a chemical vapor deposition technique (CVD) in the temperature range 400-800 oC. N2 and CO2 were used as carrier gases. TEM analysis of the Ni particles as a function of time revealed that the Ni underwent a morphological change with time. Further, as the temperature of the reaction changed, so did the shape of the carbon materials. The shapes changed from structures showing bilateral growth at T = 400 oC to tripod-like structures and multipod-like structures at T = 450 oC and T = 500 oC respectively. Irregular shaped materials were observed at T > 500 oC. It was also found that when acetylene or an acetylene/trichloroethylene mixture was used at T = 450 oC, helical (> 80%) and linear fibers were produced respectively. It was also demonstrated that the flow rate of H2, N2 and CO2 had a dramatic influence on the morphology of CNFs. CO2/TCEwas found to produce linear fibers with controlled sizes at 800 oC. The results demonstrated that the formation of tripod CNFs only occurs in a very narrow parameter regime. Manoko S. Maubane The preheating of the TCE prior to its deposition over a Ni particle catalyst was achieved using a double stage CVD reactor. TCE was subjected to high temperatures prior to its deposition at low temperatures. Results showed that the decomposition temperature was the key parameter in the synthesis of CNFs. It was found that during the decomposition, TCE breaks down into different species/radicals which then adsorb onto the catalyst particle to give CNFs of different morphology. Raman studies revealed that the synthesized CNFs showed an increase in graphitic nature when the temperature in the first reactor of a two stage reactor was increased. Decomposition of C2H2 was also performed over Cu NPs, and Cu modified catalysts (Cu@SiO2 and Cu/SiO2) with different silica coatings at 300 oC. These catalysts were prepared by reduction of Cu(acac)2 with hydrazine (35%). TEM images revealed that coiled CNFs were only produced from Cu/SiO2 grown in the presence of H2 (> 90 %; d = 60-70 nm). IR spectra of all the CNFs indicated the presence of surface C=C, C=O, CH3 and CH2 moieties, and that the ratios of peak intensities of C=O/CHx and C=C/CHx species indicated the variable CNF surface that was produced by the gases and the Cu particles used. It was thus revealed that the CNFs produced by different Cu catalysts have different chemical and physical properties and that these properties correlate with catalyst particle size and the gas mixtures used. CuO and SrO modified Cu catalysts (with different Cu/Sr ratios) were also employed using the CVD method for the synthesis of CNFs at 300 oC. These catalysts were prepared by a coprecipitation method. The TEM images of the CNFs revealed a mixture of straight and coiled CNFs with a broad diameter distribution (50-400 nm) dependent on the Cu/Sr ratio of the catalyst used. IR and TGA analysis revealed that the chemical composition of the CNFs changed as the SrO content changed. The SrO content also affected the Cu particle size and influenced the morphology of the Cu particles from which the CNFs grew.

No description available.

The focus of the research is to develop a consistent and repeatable method to evaluate the coefficient of thermal expansion (CTE) of carbon fibers at high temperatures. Accurate measurement of the CTE of carbon fibers is essential to understand and develop optimal processing procedures as well as computational simulations to predict properties and allowables for fiber-reinforced composites. The mismatch between the coefficient of thermal expansion of the fiber and the matrix has a profound impact on the development of residual stresses and the subsequent damage initiation and progression, potentially diminishing the performance of composite structures. In situ transmission electron microscopy (TEM) is selected to perform the experimental work on account of the high resolution and the capability of evaluating both the longitudinal and transverse CTE. The orthotropy in the CTE is tested by rotating the fibers through 45° about their axis. The method is validated by testing standard tungsten filaments of known CTE. Additionally, the microstructure of the fibers is studied in a field emission scanning electron microscope as well as through selected area diffraction patterns in a TEM to observe presence of any potential orthotropy. The pitch based P55 fiber revealed a cylindrically orthotropic microstructure, but the PAN based IM7 and T1000 fibers did not reveal any orthotropy. Finite element models of hexagonally arranged IM7 fibers in a 977 epoxy matrix are developed using PATRAN and analyzed using the commercial FEA code ABAQUS 6.4. The fiber properties were considered temperature independent where as the matrix properties were varied linearly with temperature. The lamina properties evaluated from the finite element modeling are in agreement with the experimental results in literature within 10% in the temperature range of room temperature to the stress free temperature of the epoxy, however at cryogenic temperatures the difference is greater. The residual stresses developed during processing of the composite indicated a potential location for fiber matrix debonding to be in the matrix dominant regions.

Thesis(M.S.)--Case Western Reserve University, 2009 / Title from PDF (viewed on 2010-01-28) Department of EMC - Mechanical Engineering Includes abstract Includes bibliographical references and appendices Available online via the OhioLINK ETD Center