SWNT and MWNT from a Polymeric Electrospun Nanofiber Precursor
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SWNT and MWNT from a Polymeric Electrospun Nanofiber Precursor John D. Lennhoff, Ph.D. Physical Sciences, Inc., 20 New England Business Center, Andover, MA 01810, U.S.A. ABSTRACT Carbon nanotubes (CNT) are expected to revolutionize a range of technologies because of their unique mechanical and electrical properties. Using nanotubes in structural materials holds significant promise due to their extremely high modulus and tensile strength, however their cost, production rate and integration into a fiber form severely limit the current structural application opportunities. The high cost of CNT is tied to their slow, batch synthesis using vapor phase, vacuum processes. We report the investigation of the formation of carbon nanotubes from a polymeric precursor using an electrospinning production process. Electrospinning generates nanofibers at velocities up to 10 m/s from a single nozzle without a vacuum requirement, with the potential to generate CNT appropriate from structural and electrical applications. Our CNT formation concept is based upon Reactive Empirical Bond order calculations that show carbon nanofibers have a thermodynamic preference for the cylindrical graphite conformation. Simulations suggest that for small diameter carbon fibers, less than about 60 nm, the single wall and multi wall nanotubes (SWNT and MWNT) phases are thermodynamically favored relative to an amorphous or planar graphitic nanofiber structure. We have developed a novel process using continuous electrospun polyacrylonitrile (PAN) nanofibers as precursors to continuous SWNT and MWNT. The process for converting PAN nanofibers to SWNT's and MWNT's follows the process for typical carbon fiber manufacture. The PAN nanofibers, of 10 to 100 nm in diameter, are crosslinked by heating in air and then decomposed to carbon via simple pyrolysis in inert atmosphere. The pyrolyzed carbon nanofibers are then annealed to form the more energetically favorable SWNT or MWNT phase, depending upon the precursor diameter. We will discuss the process and characterization data. INTRODUCTION Carbon fibers are broadly used on advanced aerospace systems in applications such as rocket motors and aircraft wings. Significant cost effective enhancements to the mechanical properties of carbon fibers would have a tremendous impact on the aerospace and defense industries. Conventional continuous fiber spinning is a decades old technology that involves the pressurized feed of a polymer solution or polymer melt through a spinerette followed by precipitation and/or drying steps. Fiber diameter is limited by the size of the holes in the spinerette and complications arising from the mechanics of operation. Minimum conventional fiber diameters are around 5 microns for pressure fed spinerette based processes. Novel solution based self-assembly processes for nanofibers have been reported, but do not provide continuous nanofibers or ready process scalability. Electrospining uses a type of conventional spinerette with a high electrostatic field to drop the fiber diameter as it exit
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