Temperature dependence of the electrical conductivity of vapor grown carbon nanofiber/epoxy composites with different filler dispersion levels
Highlights
► The influence of dispersion of carbon nanofibers on epoxy is investigated. ► A homogeneous dispersion does not imply better electrical properties. ► The conduction mechanism has an ionic nature below the percolation threshold. ► Above the percolation threshold it is dominated by hopping between the fillers. ► The granular system theory allows explaining conduction at low temperatures.
Introduction
Epoxy resins are known due to their extensive range of applications and to their good-to-excellent properties [1] that can be tailored by incorporating nanoscale fillers. Nanoscale fillers such as carbon nanotubes (CNT) and vapor grown carbon nanofibers (VGCNF) are suitable for enhancing the epoxyʼs mechanical, thermal and electrical properties and, therefore, for extending the application range of the polymer matrix [2], [3]. The VGCNF have relatively low performance in comparison to CNT, but are less expensive and more easily available than CNT [3]. VGCNF with high aspect ratio and a diameter in the nanometer scale, such as the Pyrograf III nanofibers (Applied Sciences Inc., Ohio, USA) are a highly graphitic form of VGCNF with stacked-cup morphology [3]. It is well recognized that the electrical properties of the polymer composites are critically determined by the type, size and concentration of fillers, dispersion of fillers within the polymer matrix, but also the employed processing method [4]. In recent work, it has been demonstrated that the dispersion method can be correlated to the overall composite conductivity, and that it also influences the conduction mechanism [5], [6]. To study the composite conduction mechanism it is important to measure the temperature variation of the composite conductivity, as shown by Nakamura et al. [7] with the study of the temperature dependence of the electrical conductivity of carbon black/epoxy composites. Nakamura et al. [7] used several types of carbon black (CB) and different epoxy resins in order to study the compositesʼ conductivity in a temperature range from 173.15 to 433.13 K. That work established that a break in the conduction paths or a change in the barrier height between the fillers leads to a decrease in the composite conductivity. The composite conductivity was analyzed with the fluctuation-induced tunneling (FIT) theory [8] demonstrating that the FIT model cannot adequately explain the change of conductivity with temperature. It was also found that a modified version of the Scarisbrick model [9] can describe the conductivity change with the temperature, albeit with activation energies on the order of MeV. In a subsequent section, the relevant theoretical models that can be applied to the system under study will be reviewed. Also, for CB polymer composites, Connor et al. [10] studied the influence of the temperature from 25 to 300 K on the electrical conductivity of carbon black/polyethylene terephthalate. It was found that the FIT model can describe the variation of the composite conductivity with the temperature, although the model cannot capture the low temperature experimental behavior. The latter authors also demonstrated that the composite conductivity follows a linear relation, with ϕ being the CB volume fraction. This linear relation is characteristic of the FIT model and can also be found in a weak disorder regime where the conduction mechanism is hopping between fillers [11]. Another work that studies the influence of the temperature, from 323 to 523 K, in the composite conductivity of epoxy filled with carbon fibers, was presented by Chekanov et al. [12]. A different behavior in the conductivity was found when heating or cooling the samples. More recently, an electrospinning process was used by Wang et al. [13] to study the conductivity of a composite composed by multiwalled carbon nanotubes embedded in a poly(vinyl acetate) matrix. The latter authors measured the influence of the temperature, from 100 to 300 K, on the sample conductivity, giving strong evidence for the existence of an Arrhenius type law with an activation energy of . More closely related with our work is the study of Mei et al. [14] where the influence of the temperature, from 293 to 393 K, on the composite conductivity of carbon nanofiber/epoxy composites was addressed. The latter authors gave a qualitative explanation, based on the decrease of conductivity paths in the sample, to cope with the effect of the temperature on the composite conductivity. Mdarhri et al. [15] dispersed multiwalled carbon nanotubes (MWCNT) in two types of polymers, epoxy and polystyrene, studying the effect of the temperature, from 30 to 300 K, on the composite conductivity. The authors found that the composite conductivity follows a linear relation, with ϕ being the MWCNT volume fraction, which is in accordance with the FIT model. It was also found that the composites with 3.2 vol% follow an exponential law, predicted by the FIT model. In another study for carbon nanofiber/polyetherimide composites, Kumar et al. [16] found an exponential law, resembling the FIT model, for the temperature dependence of the conductivity. The latter study proposes that the jump in the electrical conductivity at filler concentrations below the percolation threshold is due to tunneling effect, electron hopping or to chemical bonding between CNFs and the matrix. According to this study, in the case of small filler loading, the tunneling effect and electron hoping occur at ultra-low and high temperatures, respectively.
In this work, the temperature dependence of the conductivity for VGCNF/epoxy composites prepared with different dispersion methods is studied. In the first part of this Letter, some theoretical considerations regarding conduction mechanisms are presented; in the following, an experimental section is presented, where the employed dispersion and preparation methods are described. Finally, the results and discussion are presented, followed by the key conclusions.
Section snippets
The fluctuation-induced tunneling model
In the previously mentioned works it has been shown that the FIT model is able to describe the temperature behavior of the composite conductivity. The FIT model was first presented by Sheng et al. [8] by studying the temperature dependence of the conductivity in carbon/polyvinyl chloride composites. The latter authors found that the composite conductivity obeys an exponential law: with where and with e and m being the electron charge and
Experimental
The VGCNF Pyrograf III, PR-19-LHT-XT, were supplied by Applied Sciences, Inc. (Cedarville, OH). The epoxy resin was Epikote™ Resin 862 and the curing agent was Ethacure 100 Curative, supplied by Albemarle. Samples with Epon Resin 862 from Hexion Specialty Chemicals and Epikure W from Resolution Performance Products as a curing agent were also used. The two types of resins and curing agents share the same chemical abstract service (CAS). The weight ratio of resin to curing agent was .
Results and discussion
Previous work [5], [6] has shown that high values of conductivity could be achieved by using rather simple dispersion methods, although the conductivity is strongly dependent on the dispersion method. It was also inferred that for all the methods, hopping between adjacent fillers was the main conduction mechanism responsible for the observed conductivity at room temperature. In order to assess if the main mechanism is indeed hopping between nearest fillers, a low temperature measurement must be
Conclusion
In this work, three different dispersion methods were used to fabricate VGCNF/epoxy composites. It is also shown that the dispersion method employed for preparing vapor grown carbon nanofiber/epoxy composites strongly influences the conduction mechanism due to formation of VGCNF aggregates and also by changing the physical properties of the matrix. It is also demonstrated that the main conduction mechanism has an ionic nature for concentrations below the percolation threshold and hopping
Acknowledgements
We acknowledge the Foundation for Science and Technology, Lisbon, for the financial support through the 3 Quadro Comunitário de Apoio, the POCTI and FEDER programs, projects PTDC/CTM/69316/2006, NANO/NMed-SD/0156/2007, and PEst-C/CTM/LA0025/2011. We also acknowledge the FCT grants SFRH/BD/60623/2009 (J.S.) and SFRH/BD/41191/2007 (P.C.), the Joint Luso-American Foundation (FLAD)–NSF U.S. Research Networks Program research grant (F.W.J.v.H. and D.K.). We also thank Albemarle for the hardener,
References (22)
- et al.
Compos. Sci. Technol.
(2012) - et al.
Mater. Sci. Eng. B
(2007) - et al.
Synth. Met.
(1991) - et al.
Compos. Sci. Technol.
(2011) Epoxy Resins: Chemistry and Technology
(1987)- et al.
Macromolecules
(2006) - et al.
Carbon
(2009) J. Mater. Sci.
(1993)- et al.
Nanoscale Res. Lett.
(2011) - et al.
Electron. Commun. Jpn.
(1987)
Phys. Rev. Lett.
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