Rapid Heating of Silicon Carbide Fibers under Radio Frequency Fields and Application in Curing Preceramic Polymer Composites

Self-Healing of Microcracks and Scratches in a Carbon-Fiber Reinforced Epoxy Vitrimer by Conventional or Remote Heating

This study addresses the extension of the service life of carbon-fiber reinforced epoxies by inducing thermal healing of microcracks through the use of a vitrimer as a polymeric matrix. Our aim was to explore the feasibility of using a blend of selected carboxylic acids (citric, glutaric, and sebacic acids) and commercial monomers to design a matrix specifically developed for technological implementation in composites with the ability of intrinsic repair of microcracks under moderate (even remote) heating treatments. The selection of the formulation (the acid blend, catalysts, and monomers) was the result of an exhaustive prescreening analysis of processing requisites and final properties. The glass transition temperature of the cured vitrimer composite measured by differential scanning calorimetry (DSC) is 94 °C, a value lying in the range required for several technological applications, whereas stress relaxation to (1/e) of the initial value took ∼4.7 h at 180 °C and only 1.1 h at 200 °C. Composites containing 50 vol % of carbon fibers could be successfully prepared by compression molding. Acoustic emission tests proved the formation and partial healing of microcracks during tensile tests performed until 350 MPa. Surface scratches could also be healed by remote activation using near-infrared irradiation (NIR). These first results under nonoptimized thermal cycles are a proof of concept that microcrack and scratch healing can be produced in high glass-transition temperature epoxy-based carbon-reinforced composites.

A Review on Synthetic Fibers for Polymer Matrix Composites: Performance, Failure Modes and Applications

In the last decade, synthetic fiber, as a reinforcing specialist, has been mainly used in polymer matrix composites (PMC's) to provide lightweight materials with improved stiffness, modulus, and strength. The significant feature of PMC's is their reinforcement. The main role of the reinforcement is to withstand the load applied to the composite. However, in order to fulfill its purpose, the reinforcements must meet some basic criteria such as: being compatible with the matrix, making chemical or adhesion bonds with the matrix, having properties superior to the matrix, presenting the optimal orientation in composite and, also, having a suitable shape. The current review reveals a detailed study of the current progress of synthetic fibers in a variety of reinforced composites. The main properties, failure modes, and applications of composites based on synthetic fibers are detailed both according to the mentioned criteria and according to their types (organic or inorganic fibers). In addition, the choice of classifications, applications, and properties of synthetic fibers is largely based on their physical and mechanical characteristics, as well as on the synthesis process. Finally, some future research directions and challenges are highlighted.

Radio frequency heating and material processing using carbon susceptors

Carbon nanomaterials have been shown to rapidly evolve heat in response to electromagnetic fields. Initial studies focused on the use of microwaves, but more recently, it was discovered that carbon nanomaterial systems heat in response to electric fields in the radio frequency range (RF, 1-200 MHz). This is an exciting development because this range of radio frequencies is safe and versatile compared to microwaves. Additional RF susceptor materials include other carbonaceous materials such as carbon black, graphite, graphene oxide, laser-induced graphene, and carbon fibers. Such conductive fillers can be dispersed in matrices such as polymer or ceramics; these composites heat rapidly when stimulated by electromagnetic waves. These findings are valuable for materials processing, where volumetric and/or targeted heating are needed, such as curing composites, bonding multi-material surfaces, additive manufacturing, chemical reactions, actuation, and medical ablation. By changing the loading of these conductive RF susceptors in the embedding medium, material properties can be customized to achieve different heating rates, with possible other benefits in thermo-mechanical properties. Compared to traditional heating and processing methods, RF heating provides faster heating rates with lower infrastructure requirements and better energy efficiency; non-contact RF applicators or capacitors can be used for out-of-oven processing, allowing for distributed manufacturing.

Free Radical Chemistry of Phosphasilenes

Understanding the characteristics of radicals formed from silicon-containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe2 substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron-donor NMe2 and the bulkiest Tip-substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur-substituted phosphasilene forms two radicals (Si- and P-centred); and c) Mes-substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.