Sensing abilities of embedded vertically aligned carbon nanotube forests in structural composites: From nanoscale properties to mesoscale functionalities

In this paper, Vertically Aligned Carbon Nanotube (VACNT) forests are embedded into two different glass fibre/epoxy composite systems to study their sensing abilities to strain and temperature. Through a bottom-up approach, performing studies of the VACNT forest and its individual carbon nanotubes on the nano-, micro-, and mesoscale, the observed thermoresistive effect is determined to be due to fluctuation-assisted tunnelling, and the linear piezoresistive effect due to the intrinsic piezoresistivity of individual carbon nanotubes. The VACNT forests offer great freedom of placement into the structure and reproducibility of sensing sensitivity in both composite systems, independent of conductivity and volume fraction, producing a robust sensor to strain and temperature

Mechanical and Electrical Characterization of Hybrid Carbon Nanotube Sheet-Graphene Nanocomposites for Sensing Applications

The unique mechanical and electrical properties of carbon nanotubes and graphitic structures have drawn extensive attention from researchers over the past two decades. The electro-mechanical behavior of these structures and their composites, in which electrical resistance changes when mechanical deformation is applied facilitates their use in sensing applications. In this work, carbon nanotube sheet- epoxy nanocomposites with the matrix modified with various contents of coarse and fine graphene nanoplatelets are fabricated. The addition of a secondary filler results in improvements of both electrical and mechanical properties. In addition, with the inclusion of the second filler, change in resistivity with mechanical deformation (manifested by gauge factor) is significantly enhanced. Nanocomposite with 5 wt. % coarse graphene platelets achieved is the most effective resistivity-strain behavior and largest gauge factor. Similar trend in variation of gauge factor variation was observed for fine graphene nanoplatelet - nanotube sheet nanocomposites. An analytical model for explaining these observations, incorporating strain and the effect of second filler, is developed. Sensors fabricated using these hybrid nanocomposites can be potentially used in damage sensing of aerospace carbon-fiber composites

Enhanced performance of direct contact membrane distillation via selected electrothermal heating of membrane surface

Membrane distillation (MD) is a thermally driven separation process with great potential, but is currently limited by low energy efficiency. Heating of the entire circulating feed represents a major source of energy consumption in MD. Here, we present electrically conductive carbon nanostructure (CNS-) coated polypropylene (PP) membranes as a possible candidate to mitigate energy consumption through selected electrothermal heating of the membrane surface. A membrane for MD was coated with CNS using a tape casting technique. The resulting CNS-PP membrane is hydrophobic, and its smaller pore size and narrow pore size distribution resulted in a higher liquid entry pressure compared to the uncoated PP membrane. An increase in surface temperature was observed when a current was passed through the conductive CNS layer. The CNS layer on the PP membrane acts as an electrothermal heater when an AC potential is applied, and the rate of heating is proportional to the amplitude of applied AC potential. We applied electrothermal heating of these membranes to desalination by direct contact membrane distillation, in conjunction with heating of the circulating feed, and compared the performance with and without application of AC bias at three feed temperatures viz. 40, 50 and 60 °C. Applying a potential across the CNS layer increased permeate flux by 75, 76 and 61% at feed temperatures of 40, 50 and 60 °C respectively, while maintaining a salt rejection of >99%. This increase in flux is accompanied by a reduction in specific energy consumption of greater than 50% for all three feed temperatures. By combining electrothermal surface heating with MD, this study paves the way for smart, low-energy MD systems

Approaching Typical Metallic Conductivities in Polymer Nanocomposites for Lightning Strike Protection

RÉSUMÉ Parmi les objectifs de performance à atteindre lors de la conception d’un nouvel aéronef, la réduction de la masse de l’appareil est au premier rang, ladite masse ayant une influence directe sur les coûts variables de l’opération de l’aéronef. À cette fin, l’industrie a graduellement introduit les matériaux composites dans les composantes structurelles, desquelles le fuselage est peut-être la plus importante. La conversion du fuselage de l’aluminium aux matériaux composites n’est pas sans générer son lot de problèmes satellites. Notamment, la diminution de la conductivité électrique de la surface de l’appareil rend celui-ci vulnérable aux effets néfastes de la foudre. En réaction à cette vulnérabilité, des grillages métalliques continus ont été ajoutés à la surface externe des panneaux composant le fuselage. Cet ajout augmentant considérablement la masse de l’ensemble sans contribuer à la rigidité, une demande pour des solutions alternatives fut créée. L’ajout graduel de charges conductrices à une matrice isolante donne lieu à une augmentation subite de la conductivité du composite et à une transition de l’état isolant à celui de conducteur à une concentration donnée, appelée le seuil de percolation. La recherche sur les composites de nanotubes de carbone a permis de mettre en évidence un seuil de percolation extrêmement bas pour les particules de haut rapport d’aspect. En réaction, de multiples études se sont succédé qui cherchèrent à repousser les limites de la conductivité électrique de tels matériaux. Malheureusement, leur performance est limitée par une importante résistance de contact à l’interface entre les particules au sein du composite. Pour qu’un composite ait une conductivité électrique assez élevée pour canaliser l’énergie de la foudre, les charges utilisées devront être caractérisées simultanément par une faible résistance de contact, par une haute conductivité intrinsèque et par une propension à former un réseau connecté dans la matrice. Une stratégie répondant à ces exigences consiste à forcer la percolation d’un réseau de charges métalliques par leur auto-organisation dans le composite. Dans cette optique, nous avons exploré la combinaison d’une dispersion aqueuse de colloïdes d’époxy et d’un précurseur liquide de l’argent, aussi à base aqueuse.----------ABSTRACT Among all performance targets a new aircraft design seeks to achieve, reduction in weight is of primary importance since it has a direct influence on the energy costs. In pursuing this target, an increasing proportion of composite materials has been introduced into modern aircrafts’ structures. Perhaps the most important structural component, the fuselage has not been spared by change. Every change requires adjustment, and thus the problem of lightning strikes to aircraft has resurfaced as the conductive aluminum skin was replaced by a lower conductivity carbon fiber/polymer composite counterpart. Bonding a continuous metallic mesh to the surface of the skin has solved the problem, only to see the quest for mass minimization resume as this additional metal along with its adhesive and matrix contributes to the weight without benefits on the structural front. A better technology is sought. Upon gradually incorporating conductive fillers in a non-conductive matrix, the material undergoes a sudden change from an insulator to a conductor at a concentration called the percolation threshold which differs for every matrix-filler combination and filler dispersion state. Research on carbon nanotubes composites has highlighted the ability of high aspect ratio fillers to reach percolation at a very low concentration. However, issues with contact resistance between individual particles inhibit the maximum performance such composites can historically reach. If a polymer composite is to fulfill the requirements of lightning strike protection, the filler used should display simultaneously a high conductivity, a low contact resistance and a propensity to form connected networks through the matrix. One strategy to obtain such a combination of properties is to force the percolation of metallic fillers through self-organization upon forming the composite. We explored such an avenue by combining an aqueous colloidal epoxy dispersion with a water-based silver precursor ink. Upon solvent evaporation, elemental silver precipitates preferentially in the continuous aqueous phase, effectively segregating the conductive phase in a connected topology and thus promoting the conductivity of the composite. A very low percolation threshold was achieved, at only 0.27 vol.%, whereas typical silver fillers percolation occurs around 20 vol. %

ELECTROMECHANICAL PROPERTIES OF POLYMER NANOCOMPOSITES CONTAINING PERCOLATED CARBON NANOMATERIAL NETWORK WITH CONTROLLED NANOSTRUCTURE AND INTERFACE

Department of Materials Science EngineeringStrain-induced resistance change, known as piezoresistivity, is one of the unique characteristics of carbon-nanomaterial-filled polymer composites and makes them a potential candidate for strain sensors. This electromechanics-based strain sensing mechanism has received much attention recently due to the distinct combined advantages provided by polymers and the percolated network formed by carbon nanomaterials. Despite the merit of distributed sensing behavior, most of the previous studies have focused on small-area, one-dimensional strain sensing. In order to overcome these limitations, we conducted our research on the aims at studying and developing a multi-faceted approach to enable distributed, large-area, multi-directional strain sensing and to “tailor” the sensing performance by controlling the following factors: (1) carbon nanomaterial geometry and hybridization; (2) carbon nanomaterial-polymer interface; and (3) microstructures including porosity, alignment and micro-domain. The effects of carbon nanomaterial geometry on piezoresistivity could be best captured by studying the electromechanical behavior of carbon nanotube buckypapers, graphene sheets, and carbon nanotubes-graphene hybrids, as they enable “isolation” of the percolated carbon nanomaterial network. The strain sensing behavior of polymer-impregnated carbon nanomaterial sheets were also studied, which provided additional advantages of highly loaded nanocomposites and easy material handling. Reduced graphene oxide was selected and coated on a polymer substrate, which enabled 2D distributed conductive network and allowed tailored sensitivity based on the interfacial strength controlled by the reduction method. A further study about interfacial bonding discussed on the effects of polydopamine-functionalized reduced graphene oxide dispersed in poly(vinyl alcohol), which served as a conductometric humidity sensor. At the same time, polydopamine functionalization resulted in remarkable simultaneous improvements in tensile modulus, strength, and percent elongation, which suggests enhanced interfacial strength as well as matrix reinforcement. Finally, the piezoresistivity of highly porous nanocomposites were investigated using graphene oxide hydrogels with controlled pore size and distribution. This self-assembled 3D architectures allowed tailoring of strain sensitivity and served as a potential alternative solution that can replace conventional pressure, vibration sensors commonly used for structural health monitoring. The conducted study covered a comprehensive approach to develop carbon-nanomaterial-enabled smart sensors, encompassing materials design and processing, understanding of the underlying physics, and applications for wide-area sensing. This is unique and significant research that bridges the gap between the exceptional properties of nano-scale materials and macro-scale sensing systems. It is anticipated that the outcome of the proposed research will make inroads into application areas where large-area strain sensing and intelligent structural health monitoring, enabled by distributed sensor network with tailored accuracy and sensitivity, are required, including aerospace, automotive, civil structures, wind turbines, and nuclear power plants.ope

Influence of Annealing and Compaction on Enhancing the Temperature and Strain Sensitivities of Multi-Walled Carbon Nanotube (MWCNT) Films

Carbon nanotubes (CNTs) possess superior thermal, electrical, and mechanical properties. When CNTs undergo particular fabrication procedures, they transform from a nanoscale form into macroscopic thin sheets referred to as buckypapers (BPs). The main idea behind using BP is to facilitate the handling of CNTs without losing their exceptional properties. Additionally, BPs showed potential for being the used material in strain and temperature applications thanks to their thermal stability, flexibility, high sensitivity, and the ability to conform to any complex structure. In the current study, the multi-walled carbon nanotube (MWCNT) thin films were prepared using the vacuum filtration technique. Following the fabrication procedure, BPs were subjected to a combination of different treatments involving annealing, exposure to a boiling solvent, and compaction. A series of experimental tests, including loading/unloading, heating/cooling, and combining strain and temperature effects at the same time, were carried out to assess the piezoresistivity as well as the temperature sensitivity of the BP. The morphology of the BPs was examined using Scanning Electron Microscopy (SEM). Moreover, the fracture morphology of the BP was obtained by the tensile stage. The results indicate that BPs are highly sensitive to temperature and mechanical strain. Moreover, CNT thin films can exhibit a higher sensitivity when subjected to specific treatments, such as annealing and compaction. The improvement was confirmed by the obtained microstructure by SEM and quantified by the obtained empirical gauge factor (GF) values and the temperature coefficient of resistance (TCR) values

Nonlinear Thermopower Behaviour of N-Type Carbon Nanofibres and Their Melt Mixed Polypropylene Composites

The temperature dependent electrical conductivity σ (T) and thermopower (Seebeck coeffi-cient) S (T) from 303.15 K (30◦ C) to 373.15 K (100◦ C) of an as-received commercial n-type vapour grown carbon nanofibre (CNF) powder and its melt-mixed polypropylene (PP) composite with 5 wt.% of CNFs have been analysed. At 30◦ C, the σ and S of the CNF powder are ~136 S m−1 and −5.1 µV K−1, respectively, whereas its PP/CNF composite showed lower conductivities and less negative S-values of ~15 S m−1 and −3.4 µV K−1, respectively. The σ (T) of both samples presents a dσ/dT 0 character, also observed in some doped multiwall carbon nanotube (MWCNT) mats with nonlinear thermopower behaviour, and explained here from the contribution of impurities in the CNF structure such as oxygen and sulphur, which cause sharply varying and localized states at approximately 0.09 eV above their Fermi energy level (EF)