Mechanical and structural characterization of electrospun PAN-derived carbon nanofibers
Introduction
Carbon nanofibers, like other quasi-one-dimensional nanostructures such as nanowires and nanotubes, have recently been receiving increased attention. This is due to their potential application as heat-management materials, for composite reinforcement, high-temperature catalysis, membrane-based separation, and as components for nanoelectronics and photonics [1], [2], [3].
Carbon fibers are typically produced either by pyrolyzing fibers spun from an organic precursor (e.g., polyacrylonitrile (PAN), or alternatively pitch), or by chemical vapor deposition (CVD) [4]. The spinning method can only produce microscale carbon fibers (diameter >5 μm). CVD can synthesize carbon fibers with diameters ranging from several microns down to 10 nm [5], [6].
Recently, carbon fibers were produced by pyrolyzing electrospun nanofibers from PAN [7], [8], [9] and from pitch [10] with typical diameters of few hundreds of nanometer and several microns, respectively. However, the structure and the mechanical properties of carbon nanofibers produced from an electropsun polymer precursor are largely unknown. The purpose of this paper is to characterize the structure and to explore the modulus and strength of electropsun PAN-derived carbon nanofibers.
Fibers can be electrospun from polymer solutions in a fairly cost-effective manner [11], [12]. Electrospun fibers undergo huge elongation and thinning with a strain rate of ∼1000 s−1 and the drawing ratio is often as high as 104[13]. The aspect ratio of the fibers, L/d, is in the range of 1000, with diameters (d) of 10–400 nm and lengths (L) up to several centimeters. Recent observations show that both the molecular orientation and the degree of crystallinity of the electrospun fibers are high [14]. The spatial orientation of the as-spun nanofibers can be controlled with an electrostatic field leading to control of deposition geometry. This has potential application in the fabrication of one-dimensional devices or the reinforcement of composite materials [15], [16].
As in other fiber processing techniques, the final properties of the carbon fibers are largely determined by the precursor material, and the conditions used to form the precursor fiber. Post-treatment steps (e.g., stretching and carbonization) merely refine and perfect the as-spun structure. Hence, we speculate that the fundamental structure and orientation of the fiber are established during the electrospinning process. Therefore, to obtain high performance carbon nanofibers, it is important to understand the processing parameters. Due to the special properties of the electrospun PAN precursor and the typical dimensions of the carbonized nanofiber, high modulus and strength are each worthwhile goals to attempt to achieve. Carbonized microfibers are brittle, which suggests they usually fail under mechanical load at critical flaws [17]. Due to the high L/d ratio of the nanofibers, reinforcement of composite materials would be expected to be effective based on the Halpin–Tsai model [18].
This paper initially presents characterization of the structure and chemical composition of the as-spun and then carbonized nanofibers. The investigation of the bending modulus and stress failure of individual carbonized nanofibers is then presented.
Section snippets
Experimental procedure
All reagents were used without further purification. Polyacrylonitrile (PAN) with an average molecular weight of Mw = 150,000 g/mol (Aldrich) was dissolved in slightly heated N,N-dimethylformamide (DMF) to yield an 8 wt.% solution. The polymer solution was electrospun from a 5 ml syringe with a hypodermic needle with an inner diameter of 0.1 mm. A pressure of 150 mbar of air was applied to the solution to control the flow rate. A copper electrode was placed in the polymer solution and the extruded
Materials and structural characterization
Fig. 2 shows SEM micrographs of electrospun polymer, and carbonized, nanofibers at different magnifications. After a collection time of 1 h electrospun polymer fibers form a dense mat with a porosity of ∼30% and a thickness of 50 μm as shown in Fig. 2a and b. Individual fibers, Fig. 2b, have a uniform cross-section with an average diameter of 220 ± 60 nm, although in some cases beads appear; these are due to capillary instability [25]. In certain instances, a connection between the collected fibers
Conclusions
In summary, this work describes the fabrication of electrospun PAN-derived carbon nanofibers that were then carbonized, with diameters ranging from 50 to 250 nm. The production of such nanofibers represents an important step for study of their structure and properties, and for further developing them for potential use in composites or as individual elements. A high degree of orientation of the as-spun nanofiber was obtained, which can be explained by the structure formed during the
Acknowledgments
This work was funded by the Office of Naval Research under grant N000140210870. E. Zussman was supported by the National Science Foundation under grant 0200797 (K. Chong, Program Director). D. Dikin and X. Chen were supported in part by the National Science Foundation (NIRT program, Grant No. 0304506, K. Chong, Program Director), and by the NASA University Research, Engineering and Technology Institute on Bio Inspired Materials (BIMat) under award No. NCC-1-02037. We are grateful to the NUANCE
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