The Bradford Research group focuses its efforts on the synthesis of ultra-high aspect ratio carbon nanotubes (CNTs) and production of textile like structures from those unique CNTs. The CNT textiles are currently being explored in applications such as composites, sensors, electrodes and filtration. We are grateful to have received research funding in these areas from the Air Force Office of Scientific Research, American Chemical Society, Eastman Chemical, and the North Carolina Space Grant.
Carbon nanotube synthesis is considered by some as both a science and an art. The Bradford research group works constantly to increase our knowledge of the fundamentals of CNT growth processes since we utilize our final product in all of our other research areas. Our main area of expertise is in the growth of high quality, multi-walled carbon nanotube arrays. We have experience in arrays grown using catalyst deposited by both physical vapor deposition and vapor phase deposition techniques. Our lab currently is outfitted with a 3” diameter tube furnace that can used to conduct CNT synthesis in both atmospheric pressure and low pressure growth environments with a variety of source gases.
Due to their diameter and shape, carbon nanotubes have the propensity to bundle together. This bundling is detrimental to many applications that utilize them and is very hard to reverse. Carbon nanotube arrays are unique CNT structures where, for the most part, the CNTs are separate and aligned. Through special synthesis conditions, our research group can produce aligned CNT sheet structures (seen to the right) which can be taken up continuously from the CNT arrays at high speeds. These CNT sheets are the foundation for producing textile like nonwoven fabrics which retain the CNT alignment and individualized CNT structure.
To maximize the mechanical properties of any composite, there are 4 essential design considerations: 1) the fibers should be as long as possible, 2) the fibers should be arranged precisely in defined directions, 3) the fiber volume fraction should be as high as possible and 4) the matrix should surround each fiber individually. While this has been common practice with traditional high performance fibers in the composites industry for many decades, it is difficult to achieve for CNT structures. The focus of our group’s work in CNT composites is to achieve all of the desirable features through the use of aligned CNT fabric structures and high performance matrices.
The diameter of carbon nanotubes is much smaller than any nanofibers produced in the textile industry. Fiber size, specific surface area and fabric density are three of the most crucial parameters that determine the barrier and surface properties of a fabric structure. The CNT fabrics produced in the Bradford lab have shown the capability to be excellent filters of aerosol nano-particles. Fabrics produced with an areal density of 0.1 g/m2 can acheive HEPA level filtration properties while remaining super hydrophobicity.
Carbon nanotubes have been extensively studied as energy storage materials. While they perform at a low level in their as-grown state, they are an ideal scaffold for “active” electrode materials. In our lab, we produce CNT fabrics which are highly electrically conductive and have a high specific surface area. We then deposit an inorganic electrochemically active layer conformally on the surface of the CNTs. Those thin coatings, along with the aligned structure of the CNT fabrics have produced stable lithium-ion battery electrodes with high energy capacity.