Direct synthesis and characterization of a nonwoven structure comprised of carbon nanofibers
Introduction
Macroscale carbon materials composed of nanoscale features are of interest for lightweight thermal management [1], catalyst support [2], and electrical applications such as supercapacitor electrodes [3]. Currently, advanced carbon materials come in a variety of configurations such as nanotube nonwovens [4], aerogels [5], and nanoporous foams [6].
Herein, we report on a new, low cost, reliable process for creating macroscale (bulk) carbons comprised of intertwined nanoscale carbon fibers. This process includes some unique aspects: (i) the ability to grow the nonwoven material to a specific shape, (ii) the ability to control the morphology of the constituent fibers, and (iii) the ability to grow the fibers at a relatively low temperature (550 °C in the present study).
The process for creating these nanoscale nonwoven carbon materials is a direct extension of earlier work [7] where Pd was found to rapidly catalyze carbon deposition in an ethylene and oxygen environment. This technique is broadly based on the Graphitic Structure by Design (GSD) method [8] as applied to carbon nanofiber production [7], [9]. Fiber production based on this process of partial hydrocarbon oxidation generally proceeds by exposing metal catalyst particles (e.g. Ni or Pd) to a fuel-rich mixture of ethylene and oxygen (or alternatively ethylene and hydrogen) at ambient pressure and a temperature between 500 and 750 °C. The primary modification employed in the present case was to use a mold to constrain the fiber growth, thereby creating a macroscale carbon structure, comprised entirely of nanoscale fibers, of well-defined external dimensions.
The effects of process variables on the morphology of the nanofibers and the morphology’s impact on macroscale properties were also determined. Among the most important process variables are catalyst location, catalyst density in the mold, processing time and temperature program. This multi-scale property control capability results in a highly adaptable process and product.
These macroscale carbon structures were characterized for their mechanical and electric properties. The bulk material has a low density (0.12–0.40 g/cc) and has been found to be stable under moderate compressive cycling. In compression, the material was found to be elastic over multiple strain cycles and have a modest modulus of elasticity of ∼1–5 MPa. The electrical conductivity increased with compressive strain, presumably due to increased fiber contact. Modeling of both the mechanical and electrical behavior indicates that the material behaves as a nonwoven material.
Section snippets
Experimental
Nanoscale nonwoven carbon structures were generated using sub-micron Pd powder (99.9%, Sigma Aldrich) as the catalyst. This powder was placed preferentially at the edges and center of a rectangular mold (50 × 25 × 5 mm with 6 mm corner radii). The mold was heated to 550 °C while flowing nitrogen (99.999%, 100 ml/min) at atmospheric pressure. Once at temperature, a 1:1 mix of ethylene (chemically pure) and oxygen (99.99%) flowing at 15 ml/min each was added to the continuing nitrogen flow. Depending on
Results
Two samples with densities of 0.12 and 0.40 g/cc were characterized. The only process variation was the amount of Pd catalyst used (3 and 1.6 mg, respectively). The times it took for the samples to reach the pressure limit were 17 and 16 h, respectively. An example of a carbon sample is shown in Fig. 1a. Scanning electron microscopy (SEM) (Fig. 1b) shows the structure to be entirely fibrous. A focused ion beam was used to mill sections of carbon away to allow evaluation of the sample’s immediate
Discussion
The carbon materials generated for this study can be classified with nonwoven materials, which have many applications such as insulation and filtration [10]. A variety of processing techniques are available, but in general, the major steps involve web formation, web bonding, and finishing [11]. Currently, many nonwovens (mainly for the textile industry) can be produced in high-volumes economically, but there exists the potential for improving the properties and extending the applications by
Conclusions
The Constrained Formation of Fibrous Nanostructures (CoFFiN) process is a very simple and adaptable method for creating uniquely structured carbon materials. The process is accomplished via the catalytic decomposition of a gaseous carbon feedstock at atmospheric pressure, and temperatures which are typically below 700 °C. The process yields carbon comprised entirely of tightly entangled fibers. These nonwoven materials were tested in compression and the resistance was measured in situ. The
Acknowledgements
The authors gratefully acknowledge the support of the New Mexico Space Grant Consortium. This work was completed in part at the University of New Mexico Manufacturing Training and Technology Center.
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