Advanced Materials Based on Carbon Nanotube Arrays, Yarns and Papers

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Title: Advanced Materials Based on Carbon Nanotube Arrays, Yarns and Papers
Author: Bradford, Philip David
Advisors: Yuntian Zhu, Committee Chair
Jon-Paul Maria, Committee Member
Maurice Balik, Committee Member
Alexander Bogdanovich, Committee Member
Abstract: Carbon nanotubes have hundreds of potential applications but require innovative processing techniques to manipulate the microscopic carbon dust into useful devices and products. This thesis describes efforts to process carbon nanotubes (CNTs) using novel methods with the goals of: 1) improving the properties of energy absorbing and composite carbon nanotube materials and 2) increasing understanding of fundamental structure – property relationships within these materials. Millimeter long CNTs, in the form of arrays, yarns and papers, were used to produce energy absorbing foams and high volume fraction CNT composites. Vertically aligned CNT arrays were grown on silicon substrates using chemical vapor deposition (CVD) of ethylene gas over iron nano-particles. The low density, millimeter thick arrays were tested under compression as energy absorbing foams. With additional CVD processing steps, it was possible to tune the compressive properties of the arrays. After the longest treatment, the compressive strength of the arrays was increased by a factor of 35 with a density increase of only six fold, while also imparting recovery from compression to the array. Microscopy revealed that the post-synthesis CVD treatment increased the number of CNT walls through an epitaxial type radial growth on the surface of the as-grown tubes. The increase in tube radius and mutual support between nanotubes explained the increases in compressive strength while an increase in nanotube roughness was proposed as the morphological change responsible for recovery in the array. Carbon nanotube yarns were used as the raw material for macroscopic textile preforms with a multi-level hierarchical carbon nanotube (CNT) structure: nanotubes, bundles, spun single yarns, plied yarns and 3-D braids. In prior tensile tests, composites produced from the 3-D braids exhibited unusual mechanical behavior effects. The proposed physical hypotheses explained those effects by molecular level interactions and molecular hindrance of the epoxy chains with individual carbon nanotubes occupying about 40% of the composite volume. Dynamic Mechanical Analysis was used to study the molecular transitions of neat epoxy resin samples and their corresponding CNT yarn composite samples with varying matrix properties. Dramatic effects on the intensity and temperature at which α-transitions occurred were recorded, as well as a marked effect on the smaller segmental motions, or β-transitions. These changes in the matrix assist in explaining the previously reported tensile property data and the proposed physical explanation of those data. Electrical conductivity of carbon nanotube yarns and hybridized 3-D braided composites consisting of CNT yarns and insulating glass fibers were also investigated. The innovative hybridized structure provided electrical conductivity to the otherwise insulating preforms and composite structures. Finally, a new processing method of “shear pressing†was developed to produce CNT buckypapers. Tall aligned carbon nanotube arrays were converted into aligned CNT buckypaper preforms for composite fabrication. These preforms contained the desired characteristics of millimeter long CNTs, high CNT volume fraction, high CNT alignment, small diameter MWNTs and fast processing speed, which have been challenging to achieve simultaneously and are crucial for obtaining the optimum composite tensile properties. Alignment of CNTs in the buckypaper preforms was confirmed through SEM analysis of the shear pressed films in their as-pressed state and of failure surfaces of a tensile specimen. Mechanical properties of the composite were very promising as they were higher than other CNT-epoxy composites with similar volume fractions. Tensile strength of the composites reached 400 MPa. Electrical conductivity of the composites reached 77 S/cm, proving that they may be useful for composite applications where electrical conductivity is important.
Date: 2010-04-20
Degree: PhD
Discipline: Materials Science and Engineering

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