Electrical conductivity of compacts of graphene, multi-wall carbon nanotubes, carbon black, and graphite powder
Graphical abstract
Investigation of the electrical and mechanical conductive behaviors of different carbon powders during compression.
Introduction
The discovery of graphitic nanoparticles with exceptional electrical transport properties, like high conductivity and high charge mobility, has incredibly broadened the range of potential applications of this class of materials, thus unleashing a revolution in the electronic device industry. Two of the most important members of this new generation of materials are undoubtedly carbon nanotubes and graphene [1], [2], [3], [4], [5], [6].
Particularly in composite science and technology, current studies have shown that the incorporation of these two materials into polymeric matrices is capable of enhancing the electrical conductivity of polymers by several orders of magnitude without compromising other important features, such as the mechanical and optical properties [7], [8].
Perhaps the biggest challenge to be faced at this stage is how to manipulate these nanoparticles in order to bring effectively their remarkable electrical properties onto the macroscopic level. Since the conductivity of a composite is directly related to the formation of a conductive network through the polymer matrix [9], [10], its understanding depends, at least partly, on the knowledge of the electrical behavior of the nanoparticles agglomerates, here called bulk powder.
Traditionally, due to its simplicity and reproducibility for many systems, the electrical behavior of both metallic and non-metallic powders [11], [12], [13], [14], [15], [16] is characterized by monitoring the electrical conductivity of these powders under compression. This method has also been employed recently to study the electrical resistivity of carbon microtube compacts as a function of filament diameter and graphitization technique [17].
Recent studies with filtered dispersions of carbon nanotubes produced highly oriented films in which some of the favorable intrinsic features of these nanomaterials, such as their electronic and thermal transport properties, are duly reflected. These films, known as buckypapers, consist of paper-like structures in which the nanoparticles are joined together by van der Waals interactions and present promising materials for investigating their properties macroscopically, not only for the nanotubes [18] but also for graphene [19]. Similar structures can be made from other carbon fillers and we refer to these generically as papers or if a specific carbon filler such as graphene is used as graphene paper.
The electrical conductivity of a bulk powder is generally lower than that of the individual particles, since the interface between the particles offers extra resistance to charge transport. The application of pressure increases the conductivity basically by enlarging the contact area between the particles; some elastic and plastic deformation also may happen. In the final stage, which corresponds to the theoretical maximum degree of compaction (i.e. in principle 100% relative density), single particle conductivity is generally not reached, since the contact effects cannot be completely eliminated [20], [21].
In this work, four different carbon fillers have been studied: multi-walled carbon nanotubes (MWCNTs), graphene, graphite and carbon black (CB). Except for CB, which consist of a mixture of sp2- and sp3-hybridized carbon atoms, all other materials are mainly formed from a sp2 honeycomb network. Interpreting the conductivity of these materials is a challenging task. In literature [2], [3], [4], [5], [6], [7] their intrinsic conductivity is studied. In this article the powder (bulk) and paper conductivity are measured, related to structure and intrinsic conductivity and their relevance in the field of composites processing technology discussed.
Section snippets
Experimental
In order to study the electrical conductivity of carbon-based materials, two different processing conditions were applied. The first one consists of monitoring the electrical conductivity during the compaction of powders, whereas the second involves the preparation of paper films and measuring their conductivity. The conductivities of both compact and paper were studied as a function of the bulk density, defined by ρ = m/Al, where m is the mass of material, A is the area and l is the thickness of
Amount of powder for pressing
The amount of powder used to fill the die plays an essential role in the success of a compression experiment. Indeed a certain minimum number of particles are required in order to achieve representative results. Moreover, small amounts of material are more susceptible to edge effects like particle orientation, since the proportion of material in contact with the piston and chamber walls is relatively high. On the other hand, a too large amount of material causes a less homogeneous pressure
Conclusions
We studied four forms of carbonaceous materials: MWCNTs, graphene, graphite and CB. The differences in their electrical behavior, as observed using powder compaction and paper formation, reflect their distinct morphologies.
With powder compaction for all the materials, the bulk conductivity depends basically on the packing density. In a first pressing stage, the density is controlled by rearrangement and fragmentation of agglomerates, followed by a second regime where the elastic and plastic
Acknowledgments
This work forms part of the research program of the Dutch Polymer Institute (DPI), project number 648. We would like to acknowledge Dr. Kangbo Lu for the TEM images, Huub van de Palen for helping with the powder pressing set up development and Dr. Adolphe Foyet for the support with the conductivity measurements.
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