Positron annihilation spectroscopic studies of the influence of heat treatment on defect evolution in hybrid MWCNT-polyacrylonitrile-based carbon fibers

Polyacrylonitrile-based carbon fibers, embedded with multi-wall carbon nanotubes (MWCNTs) in different concentrations, have been prepared by an electrospinning technique. The samples were subjected to oxidative stabilization followed by carbonization and graphitization at temperatures from 1000 to 3000 °C. Scanning electron microscopy, X-ray diffraction, and positron annihilation spectroscopy were used to investigate the samples. Positron annihilation studies, viz. analysis of positron lifetime and Doppler broadened spectrum line shape, provide deeper insight on defect specific aspects of the material as a function of temperature. Positron lifetime spectra for all the samples give a best fit for two-lifetime components. The trapping of positrons at specific sites in the nanotubes embedded in the fibers indicates the exact regions of open volume defects in the samples. Carbonization and graphitization are enhanced by embedding nanotubes in the fibers and could be an effective way of tailoring the properties of this system.