Jaime Silva

Low percolation transitions in carbon nanotube networks dispersed in a polymer matrix: dielectric properties, simulations and experiments

The low concentration behaviour and the increase of the dielectric constant in carbon nanotubes/polymer nanocomposites near the percolation threshold are still not well understood. In this work, a numerical model has been developed which focuses on the effect of the inclusion of conductive fillers in a dielectric polymer matrix on the dielectric constant and the dielectric strength. Experiments have been carried out in carbon nanotubes/poly(vinylidene fluoride) nanocomposites in order to compare to the simulation results. This work shows how the critical concentration is related to the formation of capacitor networks and that these networks give rise to high variations in the electrical properties of the composites. Based on numerical studies, the dependence of the percolation transition on the preparation of the nanocomposite is discussed. Finally, based on numerical and experimental results, both ours and from other authors, the causes of anomalous percolation behaviour of the dielectric constant are identified.

Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/multiwall carbon nanotubes composites

Composites of styrene–butadiene–styrene (SBS) block copolymer with multiwall carbon nanotubes were processed by solution casting to investigate the influence of filler content, the different ratios of styrene/butadiene in the copolymer and the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical properties of the composites. It was found that filler content and elastomer matrix architecture influence the percolation threshold and consequently the overall composite electrical conductivity. The mechanical properties are mainly affected by the styrene and filler content. Hopping between nearest fillers is proposed as the main mechanism for the composite conduction. The variation of the electrical resistivity is linear with the deformation. This fact, together with the gauge factor values in the range of 2–18, results in appropriate composites to be used as (large) deformation sensors.

LOGIC CONTROLLERS DEPENDABILITY VERIFICATION USING A PLANT MODEL

Abstract This paper focuses on usefulness of a plant model for model-checking of untimed properties of logic controllers. Verification results obtained on a case study by using the symbolic model-checker NuSMV and three methods: verification of the only controller, constraints-based verification, in which the plant is simply modeled as a set of physical constraints, and model-based verification, that relies on a detailed model of the plant, are presented. The results yielded by these approaches enable to draw up application rules for formal verification of logic controllers.

Applying complex network theory to the understanding of high aspect ratio carbon filled composites

This work demonstrates that the theoretical framework of complex networks typically used to study systems such as social networks or the World Wide Web can be also applied to material science, allowing deeper understanding of fundamental physical relationships. In particular, through the application of the network theory to carbon nanotubes or vapour-grown carbon nanofiber composites, by mapping fillers to vertices and edges to the gap between fillers, the percolation threshold has been predicted and a formula that relates the composite conductance to the network disorder has been obtained. The theoretical arguments are validated by experimental results from the literature.

The influence of the dispersion method on the electrical properties of vapor-grown carbon nanofiber/epoxy composites

The influence of the dispersion of vapor-grown carbon nanofibers (VGCNF) on the electrical properties of VGCNF/Epoxy composites has been studied. A homogenous dispersion of the VGCNF does not imply better electrical properties. In fact, it is demonstrated that the most simple of the tested dispersion methods results in higher conductivity, since the presence of well-distributed nanofiber clusters appears to be a key factor for increasing composite conductivity.PACS: 72.80.Tm; 73.63.Fg; 81.05.Qk

Advances in Thermoplastic Matrix Towpregs Processing

The present work reports the development of new technologies to fabricate long and continuous-fiber-reinforced composite structures from low-cost thermoplastic matrix semiproducts. These semiproducts, thermoplastic matrix towpregs and preconsolidated tapes (PCTs), were produced in a purposely-built prototype machine, by deposition of a thermoplastic polymer in powder form on continuous fibers. The work also presents the advances made in the processing of these materials into composites by conventional techniques, such as pultrusion, filament winding and compression molding. Finally, it describes the investigation of the optimal processing conditions that maximize the mechanical properties of the composites. These properties are good enough for the composites to be used as engineering materials in many structural applications.

Finite-size effects in the absorption spectra of a single-wall carbon nanotube

The determination of the optical spectrum of single-wall carbon nanotubes (SWCNTs) is essential for the development of optoelectronic components and sensors with application in many fields. Real SWCNTs are finite, but almost all the studies performed so far use infinite SWCNTs. However, the spectra of finite and infinite systems are different. In this work the optical spectrum of finite (3,3) and (5,5) SWCNTs is calculated as a function of nanotube length. For the (3,3) SWCNTs, the calculated absorption spectra for light polarized both parallel and perpendicularly to the nanotube axis are in good agreement with experimental results. However, our results indicate that the lowest energy peak present in the experimental results for light polarized parallel to the nanotube axis can be attributed to a surface-plasmon resonance that is a consequence of the finite nature of the SWCNTs and not to the presence of SWCNTs with other chiralities, as claimed by the previous theoretical works. The surface-plasmon reson...

Applications of the Graph Theory to the Prediction of Electrical and Dielectric Properties of Nano-filled Polymers

The addition of carbon nanofibers to a polymeric matrix is known to affect its mechanical and electrical properties, although the mechanisms responsible for the changes are not sufficiently understood. Particularly, there are currently no adequate predictive methods that allow the creation of knowledge-based structures tailored for specific electrical response. We have developed a method for predicting the electric and dielectric properties of nanofiber-reinforced polymer matrices based on the application of the graph theory and circuit laws. We consider the individual properties of the polymeric matrix and the complex nanofiber network (including fiber orientation, concentration, and size), under an applied external electric field, and from the analysis we obtain information such as perlocative pathways, breakdown voltage, and impedance of the overall system. Simulations for two-phase systems consisting of a dielectric matrix and randomly oriented nanofibers have shown that the concentration and the length of the fibers affect the properties. Increased concentrations or longer fibers both result in networks for which it is easier to establish conducting paths through breakdown mechanisms.

A Computational Method to Explore the Breakdown Process of Conductive Fillers in a Lossless Dielectric Nanocomposite

In this paper we introduce a numerical model to study breakdown mechanisms in nanocomposites comprised of conductive fillers in a lossless dielectric. The model is not dependent on the filler geometry and thus can be easily generalized. We also present several results of the application of this model to specific nanofiller networks. We demonstrate that the breakdown voltage decreases with conductive filler concentration and that the dielectric strength of the nanocomposite follows a linear trend with the average minimum distance between the fillers.

Modeling Carbon Nanotube Electrical Properties in CNT/Polymer Composites

In this work it is demonstrated that the capacitance between two cylinders increases with the rotation angle and it has a fundamental influence on the composite dielectric constant. The dielectric constant is lower for nematic materials than for isotropic ones and this can be attributed to the effect of the filler alignment in the capacitance. The effect of aspect ratio in the conductivity is also studied in this work. Finally, based on previous work and by comparing to results from the literature it is found that the electrical conductivity in this type of composites is due to hopping between nearest fillers resulting in a weak disorder regime that is similar to the single junction expression.