Carbon Nanofiber Applications & Properties

David Burton, Patrick Lake, Andrew Palmer

Applied Sciences, Inc., Cedarville, OH

Properties and Applications of Carbon Nanofibers (CNFs) Synthesized using Vapor-grown Carbon Fiber (VGCF) Manufacturing Technology

Introduction

Pyrograf®-III vapor-grown carbon nanofibers are within the class of materials termed multi-walled carbon nanotubes (MWCNTs), and are produced by the floating catalyst method. Carbon nanofibers (CNFs) are discontinuous, highly graphitic, highly compatible with most polymer processing techniques, and they can be dispersed in an isotropic or anisotropic mode. CNFs have excellent mechanical properties, high electrical conductivity, and high thermal conductivity, which can be imparted to a wide range of matrices including thermoplastics, thermosets, elastomers, ceramics, and metals. Carbon nanofibers also have a unique surface state, which facilitates functionalization and other surface modification techniques to tailor/engineer the nanofiber to the host polymer or application. Carbon nanofibers are available in a free-flowing powder form (typically 99% mass is in a fibrous form).

Product Description and Specifications

Pyrograf®-III vapor-grown carbon nanofibers possess a unique morphology (Figure 1) not currently available from other nanomaterial producers. The individual nanofiber is precipitated from a catalyst particle, and has a hollow core that is surrounded by a cylindrical fiber comprised of highly crystalline, graphite basal planes stacked at about 25 degrees from the longitudinal axis of the fiber. This morphology, termed "stacked cup" or "herringbone", generates a fiber with exposed edge planes along the entire interior and exterior surfaces of the nanofiber. These edge sites are reactive, relative to the basal plane of graphite, and facilitate chemical modification of the fiber surface for maximum incorporation and mechanical reinforcement in polymer composites. This open architecture also facilitates rapid intercalation and de-intercalation by heterogeneous atoms, useful for tuning conductivities.

HRTEM micrographs of PR-25 carbon nanofiber showing exposed edge sites forming the inner and outer surfaces of the nanofiber wall.

Figure 1.HRTEM micrographs of PR-25 carbon nanofiber showing exposed edge sites forming the inner and outer surfaces of the nanofiber wall.

The carbon nanofibers offered have average diameters ranging from 125 to 150 nm depending upon the grade, and have lengths ranging from 50 to 100 µm. The nanofibers are much smaller in diameter than conventional continuous or milled carbon fibers (5-10 nm) and significantly larger than carbon nanotubes (1-20 nm), yet offer many of the same benefits. The carbon nanofibers are treated after production in order to impart various properties on the surface state. Three types of nanofibers are available. Product number 719811 is pyrotically stripped to remove surface hydrocarbons and generate a pristine surface for chemical bonding. This product also serves as a precursor for the other two listings. Product number 719803 is thermally treated to 1500 °C to provide the best combination of mechanical and electrical properties, while product number 719781 is thermally treated to 2900 °C to generate a catalyst free product and maximize thermal conductivity properties in composites. Typical physical properties of each product are listed in Table 1.

Table 1Select Properties of Pyrograf Carbon Nanofibers

Properties and Applications

Electrical Conductivity

Endo et al. first reported the intrinsic conductivity of highly graphitic vapor-grown carbon fiber at room temperature to be 5 x 10-5 Ω.cm, which is near the resistivity of graphite. Since virtually all of the electrical conductivity in carbon nanofiber/polymer composites is through the network of carbon nanofibers, it is clear that good fiber dispersion and maintenance of fiber length will aid in achieving high composite electrical conductivity at even a low fiber loading. Due to their high electrical conductivity and high aspect ratio, CNF can impart equivalent electrical conductivity to a composite at lower loadings than conventional conductive fillers. Also, by controlling the loading, one can produce composites with different electrical resistivity values. This is of particular importance for applications that require a resistivity in different ranges such as electrostatic dissipation (ESD) {106 – 108 Ω-cm}, electrostatic painting {104 – 106 Ω.cm}, EMI shielding {103 – 101 Ω.cm}, and lightning strike protection {< 10 Ω.cm}.

The following figure represents percolation curves possible with different CNF loading levels and shear conditions. Higher shear levels during composite processing lead to higher percolation thresholds.

Volume electrical resistivity of the composites made with CNF as a function of fiber weight loading.

Figure 2.Volume electrical resistivity of the composites made with CNF as a function of fiber weight loading.

Mechanical Reinforcement

Direct measurement on individual nanometer scale fibers has only recently been achieved and only reproducibly in limited quantities. Ozkan et al. performed5 careful tensile strength measurements directly on individual carbon nanofibers and measured the true strengths. Based on the annular cross-sectional area, strengths were found to be as high as 8.7 GPa, which approaches the strength of graphite microfibers. The modulus of the carbon nanofiber is inferred to be 600 GPa based on direct measurements of the parent classes of carbon nanofiber, or macroscopic vapor-grown carbon fibers.6 When incorporated into polymeric composites, the carbon nanofiber can increase the tensile strength, compression strength, Young’s modulus, interlaminar shear strength, fracture toughness, and vibration damping of the base polymer. The extent of improvement is dependent upon the type of polymer, the degree of dispersion, and processing history.7-12

Overview of the mechanical properties of CNF-based composite materials.

Figure 3.Overview of the mechanical properties of CNF-based composite materials.

Thermal Properties

The thermal conductivity of the carbon nanofiber can be inferred to be 2000 W/m-K again, based on direct measurements of the parent classes of carbon nanofibers, or macroscopic vapor-grown carbon fibers. Of the three carbon nanofiber types, only the thermally treated nanofiber (to 2900+ °C, Product No. 719781) provides a significant boost to the thermal conductivity of the polymer composite. Lafdi and Matzek were able to achieve13 an increase of thermal conductivity from 0.2 W/m-K for epoxy resin to 2.8 W/m-K for a 20 wt% Vapor Grown CNF composite. These results indicate that, unlike strength or stiffness, good coupling to the matrix is not necessary to achieve high thermal conductivity, making compounding less critical.

Other researchers14-15 have focused on the fire retardant properties of the carbon nanofibers in thermoplastic materials. Composites loaded with carbon nanofibers and exposed to a flame exhibited delayed and lower peak heat release rates, lower smoke emissions, and no dripping or pooling of molten polymer.

Links describing the performance of CNF as a flame retardant additive in polymeric composites are available at NIST’s (National Institute of Standards and Technology) website:

CNFs in Flexible Polyurethane Foams -

CNFs in Clay Foams –

CNFs Cut Flammability of Upholstered Furniture -

Enhanced Fire Retardancy of CNFs vs Talc and Clays. Used with permission from NIST: Polymer for Advanced Technologies, June 2008

Figure 4.Enhanced Fire Retardancy of CNFs vs Talc and Clays. Used with permission from NIST: Polymer for Advanced Technologies, June 2008

Given that graphite has a low thermal expansion, polymeric composites loaded with carbon nanofibers were not only expected but have been shown to have substantially lower coefficients of thermal expansion of than the neat matrix.16

A graph to show the reduced coefficient of thermal exapansion (CTE) of a 15 vol % CNF composite vs the neat polymer material.

Figure 5.A graph to show the reduced coefficient of thermal exapansion (CTE) of a 15 vol % CNF composite vs the neat polymer material.

Materials
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References

1.
The HRTEM images were provided by Oak Ridge National Laboratory's High Temperature Materials Laboratory, and the microscopy was sponsored by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program..
2.
Afzal M. 2004. Heuristic Model for Conical Carbon Nanofiber. . [dissertation]. University of Toledo.: Toledo OH.
3.
Endo M, Koyama T, Hishiyama Y. 1976. Structural Improvement of Carbon Fibers Prepared from Benzene. Jpn. J. Appl. Phys.. 15(11):2073-2076. http://dx.doi.org/10.1143/jjap.15.2073
4.
Leer C. 2010. Carbon Nanofibers Thermoplastic Nanocomposites: Processing – Morphology – Properties Relationships. . [dissertation]. Minho, Portugal: University of Minho.
5.
Ozkan T, Chen Q, Naraghi M, Chasiotis I. Symposium Proceedings. In 53rd International SAMPE; 07 Sep 2008; Memphis, TN
6.
applied sciences. [Internet].[cited 09 Mar 2011]. Available from: http://apsci.com/
7.
Finegan IC, Tibbetts GG, Glasgow DG, Ting J, Lake ML. 2003. 38(16):3485-3490. http://dx.doi.org/10.1023/a:1025109103511
8.
Kumar S, Doshi H, Srinivasarao M, Park JO, Schiraldi DA. 2002. Fibers from polypropylene/nano carbon fiber composites. Polymer. 43(5):1701-1703. http://dx.doi.org/10.1016/s0032-3861(01)00744-3
9.
Tibbetts GG, McHugh JJ. 1999. Mechanical properties of vapor-grown carbon fiber composites with thermoplastic matrices. J. Mater. Res.. 14(7):2871-2880. http://dx.doi.org/10.1557/jmr.1999.0383
10.
Sadeghian R, Minaie B, Gangireddy S, Hsiao K. 2005. Symposium Proceedings. In 50th International SAMPE ; 30 Apr 2005; Long Beach, CA
11.
Li B, Wood W, Baker L, Sui G, Leer C, Zhong W. 2010. Effectual dispersion of carbon nanofibers in polyetherimide composites and their mechanical and tribological properties. Polym Eng Sci. 50(10):1914-1922. http://dx.doi.org/10.1002/pen.21717
12.
Gou J, O'Braint S, Gu H, Song G. 2006. Damping Augmentation of Nanocomposites Using Carbon Nanofiber Paper. Journal of Nanomaterials. 20061-7. http://dx.doi.org/10.1155/jnm/2006/32803
13.
Lafdi K, Matzek M. 2003. Symposium Proceedings. In 48th International SAMPE; 10 May 2003; Long Beach, CA
14.
Joseph K. 2010. Polymer Nanocomposites: Processing, Characterization and Applications. McGraw Hill Professional.
15.
Zammarano M, Krämer RH, Harris R, Ohlemiller TJ, Shields JR, Rahatekar SS, Lacerda S, Gilman JW. 2008. Flammability reduction of flexible polyurethane foams via carbon nanofiber network formation. Polym. Adv. Technol.. 19(6):588-595. http://dx.doi.org/10.1002/pat.1111
16.
Chen Y, Ting J. 2002. Ultra high thermal conductivity polymer composites. Carbon. 40(3):359-362. http://dx.doi.org/10.1016/s0008-6223(01)00112-9