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High electrical conductivity, along with high piezoresistive sensitivity and stretchability, are crucial for designing and developing nanocomposite strain sensors for damage sensing and on-line structural health monitoring of smart carbon fiber-reinforced polymer (CFRP) composites. In this study, the influence of the geometric features and loadings of carbon-based nanomaterials, including reduced graphene oxide (rGO) or carbon nanofibers (CNFs), on the tunable strain-sensing capabilities of epoxy-based nanocomposites was investigated. This work revealed distinct strain-sensing behavior and sensitivities (gauge factor, GF) depending on both factors. The highest GF values were attained with 0.13 wt.% of rGO at various strains. The stability and reproducibility of the most promising self-sensing nanocomposites were also evaluated through ten stretching/relaxing cycles, and a distinct behavior was observed. While the deformation of the conductive network formed by rGO proved to be predominantly elastic and reversible, nanocomposite sensors containing 0.714 wt.% of CNFs showed that new conductive pathways were established between neighboring CNFs. Based on the best results, formulations were selected for the manufacturing of pre-impregnated materials and related smart CFRP composites. Digital image correlation was synchronized with electrical resistance variation to study the strain-sensing capabilities of modified CFRP composites (at 90° orientation). Promising results were achieved through the incorporation of CNFs since they are able to form new conductive pathways and penetrate between micrometer-sized fibers. [ABSTRACT FROM AUTHOR]

This review article gathers the most recent recycling technologies for thermoset and thermoplastic polymers. Results about existing experimental procedures and their effectiveness are presented. For thermoset polymers, the review focuses mainly on fibre-reinforced polymer composites, with an emphasis on epoxy-based systems and carbon/glass fibres as reinforcement, due to the environmental concerns of their end-of-life management. Thermal processes (fluidised bed, pyrolysis) and chemical processes (different types of solvolysis) are discussed. The most recent combined processes (microwave, steam, and ultrasonic assisted techniques) and extraordinary recycling attempts (electrochemical, biological, and with ionic liquids) are analysed. Mechanical recycling that leads to the downgrading of materials is excluded. Insights are also given for the upcycling methodologies that have been implemented until now for the reuse of fibres. As for thermoplastic polymers, the most state-of-the-art recycling approach for the most common polymer matrices is presented, together with the appropriate additivation for matrix upcycling. Mechanical, chemical, and enzymatic recycling processes are described, among others. The use of fibre-reinforced thermoplastic composites is quite new, and thus, the most recent achievements are presented. With all of the above information, this extensive review can serve as a guide for educational purposes, targeting students and technicians in polymers recycling. [ABSTRACT FROM AUTHOR]

In the present study, a series of carbon-based nanocomposites based on recycled thermoplastic polyurethane (TPU) matrix and MWCNT fillers synthesized in a laboratory environment were prepared at various loadings and assessed in terms of their functional thermal, dielectric, and rheological properties, as well as their ohmic heating capability, for self-healing applications in extrusion-based 3D printing technologies. The synthesis of nanomaterials focused on the production of two different types of carbon nanotubes (CNTs) via the chemical vapor deposition (CVD) method. A comparative assessment and benchmarking were conducted with nanocomposite filaments obtained from commercial nanomaterials and masterbatches with MWCNTs. For all the polymer nanocomposites, samples were prepared at additive contents up to 15 wt.% and filament feedstock was produced via the melt-extrusion process for 3D printing; these were previously characterized by rheological tests. The measurements of thermal and electrical conductivity resulted in a selected composition with promising ohmic heating capability. As a preliminary assessment of the self-healing ability of the above samples, artificial cracks were introduced on the surface of the samples and SEM analysis took place at the crack location before and after applying voltage as a measure of the effectiveness of the material remelting due to the Joule effect. Results indicate a promising material response with a partial restoration of artificial cracks. [ABSTRACT FROM AUTHOR]

In this work, the carbon fibres (CFs) surfaces were modified via sizing and coated with a very thin layer of a complex formulation including carbon nanotubes (CNTs). A push-out method was developed based on nanoindentation to assess the interfacial shear strength of the fibre/matrix. The mechanical properties such as indentation hardness, reduced modulus, indentation displacement and indentation creep of the composite were evaluated by means of the Oliver-Pharr method. The critical load of different composites was measured and the interfacial shear strength (IFSS) was calculated to compare the effect of the CNTs concentration in the sizing solution. Wettability evaluation of the sized fibres was performed prior to nanoindentation to investigate the adhesion of the resin. After push-out testing, characterisation by optical microscopy/SEM was carried out to ratify the results. It was found sizing with a small amount of evenly distributed nano-inclusion on CFs can increase the interfacial shear strength but large amount of sizing could lead to a decrease of the interfacial bonding due to the agglomeration of CNTs on CFs.

Carbon Fibre Reinforced Polymers (CFRPs) have spread to a broad range of sectors including automobile, aeronautics and space industry the last decades. Recently, the emergence of new requirements for improved smart properties and functionalities have been main drivers to the introduction of novel methodologies and optimization of processes. A new approach of functionalizing CFs is the in-situ grafting of carbon nanotubes (CNTs) onto the surface of fibres, through chemical vapour deposition (CVD). In this study, CFRPs were manufactured via Vacuum Assisted Resin Transfer Molding (VARTM) and characterized by microscopy techniques through their cross-section. The effect of CNTs in-situ grafting onto the CFs on the mechanical behavior of the composite was studied both in micro- and macro-scale level, through instrumented indentation technique and tensile testing. The mechanical behaviour of the composite with the CNTs in-situ grafted onto the CFs was compared with CNTs-modified composites, containing CNTs in the epoxy matrix. Comparing the nanomechanical properties with conventional mechanical testing, the enhancement of mechanical behaviour was revealed for the case of the CNTs-modified composite. Additionally, an increased interfacial adhesion between the CNTs-functionalised CFs and the polymer matrix was observed, indicating that CNTs contribute to an enhanced bonding between matrix and CFs.

This study is focused on the investigation of the morphology and the characteristics of carbon films, obtained by thermal chemical vapor deposition (T-CVD) on different substrates, during carbon nanotubes (CNTs) manufacturing. These films were examined, by means of optical and electronic microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Additional measurements were conducted related with the estimation of their nanomechanical properties. The topography is studied, in combination with the assessment of their mechanical behavior at nanoscale. Through nanoindentation, hardness and elastic modulus together with wear resistance index (H/E) were evaluated. According to this study, it may be remarked that the obtained film is classified as an amorphous carbon film.

Manufacturing of nanomaterials is an interdisciplinary field covering physics, chemistry, biology, materials science and engineering. The interaction between scientists with different disciplines will undoubtedly lead to the production of novel materials with tailored properties. The success of nanomanufacturing depends on the strong cooperation between academia and industry in order to be informed about current needs and future challenges, to design products directly transferred into the industrial sector. It is of paramount importance the selection of the appropriate method combining synthesis of nanomaterials with required properties and limited impurities as well as scalability of the technique. Their industrial use faces many obstacles as there is no suitable regulatory framework and guidance on safety requirements; specific provisions have yet to be established in EU legislation. Moreover, regulations related to the right of intellectual properties as well as the absence of an appropriate framework for patent registration are issues delaying the process of products’ industrial application. The utilization of high-quality nanomaterials is now growing and coming to the industrial arena rendering them as the next generation attractive resources with promising applications. Undoubtedly, the existing gap between basic research relating nanomaterials and their application in real life will be overcome in the coming decade.

The possibility of directly growing carbon nanofibers (CNFs) and carbon nanotubes (CNTs) on half-Heusler alloys by Chemical Vapour Deposition (CVD) is investigated for the first time, without using additional catalysts, since the half-Heusler alloys per se may function as catalytic substrates, according to the findings of the current study. As a carbon source, acetylene is used in the temperature range of 700-750 °C. The n-type half-Heusler compound Zr0.4Ti0.60.33Ni0.33Sn0.98Sb0.020.33 is utilized as the catalytic substrate. At first, a computational model is developed for the CVD reactor, aiming to optimize the experimental process design and setup. The experimental process conditions are simulated to investigate the reactive species concentrations within the reactor chamber and the activation of certain reactions. SEM analysis confirms the growth of CNFs with diameters ranging from 450 nm to 1 μm. Raman spectroscopy implies that the formed carbon structures resemble CNFs rather than CNTs, and that amorphous carbon also co-exists in the deposited samples. From the characterization results, it may be concluded that a short reaction time and a low acetylene flow rate lead to the formation of a uniform CNF coating on the surface of half-Heusler alloys. The purpose of depositing carbon nanostructures onto half-Heusler alloys is to improve the current transfer, generated from these thermoelectric compounds, by forming a conductive coating on their surface.