Graphene Nanoplatelet (GNPs) Doped Carbon Nanofiber (CNF) System: Effect of GNPs on the Graphitic Structure of Creep Stress and Non-Creep Stress Stabilized Polyacrylonitrile (PAN).

Annas Bin Ali,Ralf Sindelar,Christoph Tegenkamp,Julian Koch,Franz Renz

doi:10.3390/nano10020351

Annas Bin Ali, Ralf Sindelar + Show 3 more

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https://doi.org/10.3390/nano10020351

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Abstract

Improving the graphitic structure in carbon nanofibers (CNFs) is important for exploiting their potential in mechanical, electrical and electrochemical applications. Typically, the synthesis of carbon fibers with a highly graphitized structure demands a high temperature of almost 2500 °C. Furthermore, to achieve an improved graphitic structure, the stabilization of a precursor fiber has to be assisted by the presence of tension in order to enhance the molecular orientation. Keeping this in view, herein we report on the fabrication of graphene nanoplatelets (GNPs) doped carbon nanofibers using electrospinning followed by oxidative stabilization and carbonization. The effect of doping GNPs on the graphitic structure was investigated by carbonizing them at various temperatures (1000 °C, 1200 °C, 1500 °C and 1700 °C). Additionally, a stabilization was achieved with and without constant creep stress (only shrinkage stress) for both pristine and doped precursor nanofibers, which were eventually carbonized at 1700 °C. Our findings reveal that the GNPs doping results in improving the graphitic structure of polyacrylonitrile (PAN). Further, in addition to the templating effect during the nucleation and growth of graphitic crystals, the GNPs encapsulated in the PAN nanofiber matrix act in-situ as micro clamp units performing the anchoring function by preventing the loss of molecular orientation during the stabilization stage, when no external tension is applied to nanofiber mats. The templating effect of the entire graphitization process is reflected by an increased electrical conductivity along the fibers. Simultaneously, the electrical anisotropy is reduced, i.e., the GNPs provide effective pathways with improved conductivity acting like bridges between the nanofibers resulting in an improved conductivity across the fiber direction compared to the pristine PAN system.

Highlights

Carbon fibers are by far the most prominent commercialized product in carbon and carbon reinforced materials
Stabilization at 250 ◦C for 10 h was performed without application of creep stress but with mechanical fixing of nanofiber mat ends for both pristine PAN and PAN doped with graphene nanoplatelets (GNPs) at four different carbonization temperatures
Our results revealed that the doping of carbon nanofibers (CNFs) with GNPs improves the crystalline structure and graphitization degree of polyacrylonitrile as confirmed by the decrease of the ID/IG ratio, full width at half maximum (FWHM) for G peak and increased crystallite size La and at all carbonization temperatures

Summary

Introduction

Carbon fibers are by far the most prominent commercialized product in carbon and carbon reinforced materials. Since the last decade, efforts are being made to employ nano-carbons as a reinforcement or property agent for an improved structure that would replicate into better mechanical, electrical and electrochemical properties. Polyacrylonitrile (PAN) is the most employed one for the production of carbon fibers [2,9] In this top down approach, the cyclization process in the carbon nanofiber production is of extreme importance as during this step the formation of graphene-like ribbon structures take place. Without this process, a so-called ladder structure is avoided and PAN cannot withstand the carbonization process. Forth, the ability to improve the graphitic structure at lower temperatures is highly desirable with regards to economic and application perspective

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