Mechanical tests on individual carbon nanofibers reveals the strong effect of graphitic alignment achieved via precursor hot-drawing Sneha Chawla a , Jizhe Cai b , Mohammad Naraghi b, * a Department of Materials Science and Engineering, Texas A&M University, 3409 TAMU, College Station, TX, 77843-3409, USA b Department of Aerospace Engineering, Texas A&M University, 3409 TAMU, College Station, TX, 77843-3409, USA article info Article history: Received 4 January 2017 Received in revised form 26 February 2017 Accepted 27 February 2017 Available online 1 March 2017 abstract Electrospun carbon nanofibers (CNFs) processed via carbonization of electrospun precursors are an emerging class of nanoscale carbon-based materials with abundant sp 2 CeC bonds which can offer significant opportunities for structural light-weighting in multifunctional materials. In this work, we have studied the effect of graphitic alignment on mechanical properties of CNFs. Graphitic alignment was achieved by hot-drawing polyacrylonitrile (PAN) nanofiber precursors at temperatures above the T g of PAN which induces chain alignment. We studied several states of PAN chain alignment by varying electrospinning take-up velocity and hot-drawing ratios. Chain alignment and orientation induced crystallization was studied via polarized Fourier Transform IR spectroscopy and X-ray diffraction. IR spectroscopy revealed that the formation of crystals delays thermal stabilization and cyclization in hot- drawn PAN nanofibers. Thus, we modified the stabilization process to transform PAN chains into a ladder-like structure suitable for carbonization. The carbonization was carried out at 1100  C. MEMS- based mechanical characterization of individual CNFs revealed over 100% improvement in average strength and over 70% improvement in modulus of CNFs as a result of graphitic alignment. The CNFs obtained from hot-drawn samples demonstrated strength as high as 5.4 GPa, which is among the highest reported for this class of material. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Carbon fibers (CFs) have received continually growing attention from industry since the 1950's. Because of their remarkable specific strength and stiffness, reaching values as high as 4 GPa/g/cm 3 and 400 GPa/g/cm 3 , respectively, CFs are mainly used for structural light-weighting. Fibers from cellulose-based materials were among the first precursors of carbon fibers which have been largely replaced by wet-spun polyacrylonitrile (PAN) [1]. The extrusion of the precursors during wet-spinning exerts shear and elongational strains, inducing chain alignment. The precursor fibers are then stretched in steam or dry air to further improve molecular orien- tation, after which they are heat treated in air at 200  Ce300  C in an oxidizing environment and carbonized in inert atmosphere at temperatures >800  C. During these steps, non-carbon atoms such as hydrogen, nitrogen and other non-carbon elements are removed in the form of volatile species, and carbon fibers form. The graphitic domain alignment in CFs is responsible for their remarkable strength and modulus, which can be achieved by aligning the polymer chain backbone in the precursor with the fiber axis [2]. Despite remarkable strength and modulus of the state-of-the- art CFs [3], their mechanical properties seem to have reached a plateau [4]. Moreover, experimental studies on mechanics of indi- vidual CFs point to a pronounced diameter size effect in carbon fibers [5]. This trend is expected since the likelihood of existence of defects scales with sample volume. However, the classical view on mechanical size-effect which is highly applicable to isotropic brittle materials [6] cannot fully explain the mechanical size effects in CFs. For instance, studies relating the strength of carbon fibers to two length scales, i.e. diameter and length, have shown that the strength of CFs when expressed as a function of the sample volume is significantly more sensitive to diameter than length [7]. These results point to emergence of critical flaws in thicker carbon fibers. In this regard, Raman spectroscopy performed at various depths from the surface of carbon fibers (6e7 mm thick) by Wang et al. [8], showed a clear reduction in the graphitic order in carbon fibers at a radial distance of ~20% from the fiber surface. In other words, these results suggest that typical carbon fibers contain a skin layer which is clearly more graphitic than the core. The emergence of this skin- * Corresponding author. E-mail address: Naraghi@tamu.edu (M. Naraghi). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2017.02.095 0008-6223/© 2017 Elsevier Ltd. All rights reserved. Carbon 117 (2017) 208e219