Optimum stabilization processing parameters for polyacrylonitrile-based carbon nanofibers and their difference with carbon (micro) fibers

Carbon nanofiber webs have high electrical and thermal conductivity, porosity and surface area, etc. making them favorable in many applications. In this paper, three types of polyacrylonitrile (PAN) copolymers are electrospun (average diameter of 150-500 nm) and carbon nanofibers are produced (average diameter of 110-300 nm). The effects of chemical composition and processing parameters on the formation of graphitic structure, morphology and electrical conductivity of the carbon nanofibers are studied. Unlike carbon fibers in micro scale, using PAN without acidic comonomers is suitable for production of carbon nanofibers. Common processing parameters for stabilization of PAN microfibers are not applicable to PAN nanofibers. Nanofibers stabilized using common microfibers procedure cannot tolerate high carbonization temperatures. While, thermal stabilization at higher temperature (300 °C) results in proper stabilized structure with ability to tolerate carbonization conditions. Progress of stabilization reactions higher than 98% is inappropriate for obtaining electrically conductive nanofiber webs, whereas 85-90% progress is considered adequate for development of proper structure during carbonization to obtain optimum electrical conductivity. Formation of nanofiber mats in shape of an interconnected sponge-like structure is believed to be necessary for obtaining much higher electrical conductivity (17-26 S/cm compared to 1-8 S/cm). Reducing fiber diameter from micro to nanoscale, the effect of processing parameters as well as the rate of thermochemical reactions can be different.