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Advances in core—shell engineering of carbon-based composites for electromagnetic wave absorption
Electromagnetic (EM) absorption is paving the way to overcome the challenges related to conventional shielding strategy against EM pollution through sustainable energy dissipation. As characteristic functional media that can interact with electric or magnetic field branch, EM wave absorption materials (EWAMs) have received extensive attention and realized considerable development in the past two decades, where carbon-based composites are always considered as promising candidates for high-performance EMAWs due to their synergetic loss mechanism as well as diversified composition and microstructure design. Recent progress indicates that there is more and more interest in the fabrication of carbon-based composites with unique core—shell configuration. On one hand, core—shell configuration usually ensures good chemical homogeneity of final products and provides some positive protections for the components with susceptibility to corrosion, on the other hand, it creates enough heterogeneous interfaces between different EM components, which may bring enhanced polarization effect and intensify the consumption of EM energy. In this review, we firstly introduce EM wave absorption theory, and then highlight the advances of core—shell engineering in carbon-based composites in terms of built-in carbon cores and built-out carbon shells. Moreover, we also show some special core—shell carbon-based composites, including carbon/carbon composites, assembled composites, and decorated composites. After analyzing EM absorption performance of some representative composites, we further propose some challenges and perspectives on the development of core—shell carbon-based composites.
Journal Article
Microstructure-Based Thermochemical Ablation Model of Carbon/Carbon-Fiber Composites
The microstructure of carbon fiber–reinforced carbon-matrix composites (carbon/carbon composites) has important effects on its ablation performance. However, the traditional macro-ablation methods have underestimated the ablation recession rate and ignored the influence of microstructure. To simulate the ablation of large-sized structures while accounting for the influence of microstructure, it is necessary to modify these methods. In this work, a thermochemical ablation model for carbon/carbon composites is proposed based on the evolution behavior of their microstructure. The ablation recession rate and surface temperature predicted by this model are in good agreement with the experimental results. Through numerical analysis, we found that the ablation recession rate of the material without carbon fibers is much greater than that of the material containing carbon fibers. The ablation recession rate is influenced by the fiber orientation due to the change in thermal conductivity. The anti-ablation efficiency of carbon/carbon composites can be improved by increasing their fiber radius, radiation coefficient, specific heat capacity, interphase density, and thermal conductivity coefficient. The thermochemical ablation model provides a guide for the design of better anti-ablation carbon/carbon composites.
Journal Article
Metal oxide-based carbon nanocomposites for environmental safety and remediation
\"This book focusses on nanotechnology for the preparation of metal oxide-based carbon nanocomposite materials for environmental remediation. It analyses the use of nanomaterials for water, soil, and air solutions, emphasizing on environmental risks of pollution. It further explores how magnetic and activated carbon nanomaterials are being used for sustainable environmental protection of water and soil, and detection of harmful gases. Status and major challenges of using carbon-based nanomaterials on a large scale are explained supported by relevant case studies. Features: Exhaustively covers nanotechnology, metal oxide- carbon nanocomposites and their application in soil, water and air treatments. Explores pollutants nano-sensing and their remediation towards environmental safety. Includes economics analysis and environmental aspects of metal oxide materials. Describes why properties of oxide-carbon based nanomaterials useful for environmental applications. Discusses current cases studies of remediation technologies. This book is aimed at graduate students and researchers in nanotechnology, environmental technology and remediation\"-- Provided by publisher.
Book
Native functionality in triple catalytic cross-coupling: sp³ C–H bonds as latent nucleophiles
The use of sp³ C–H bonds—which are ubiquitous in organic molecules—as latent nucleophile equivalents for transition metal–catalyzed cross-coupling reactions has the potential to substantially streamline synthetic efforts in organic chemistry while bypassing substrate activation steps. Through the combination of photoredox-mediated hydrogen atom transfer (HAT) and nickel catalysis, we have developed a highly selective and general C–H arylation protocol that activates a wide array of C–H bonds as native functional handles for cross-coupling. This mild approach takes advantage of a tunable HAT catalyst that exhibits predictable reactivity patterns based on enthalpic and bond polarity considerations to selectively functionalize α-amino and αoxy sp³ C–H bonds in both cyclic and acyclic systems.
Journal Article
Antioxidant Behavior of Carbon/Carbon Composites with Hot Dip Plating and Electroplating for Single-Crystal Furnaces
In the Czochralski single-crystal silicon manufacturing industry, single-crystal furnaces often experience corrosion from silicon vapor, which reduces their operational lifespan. However, the preparation of metal coatings on the surface of C/C composites is challenging due to their low coefficient of thermal expansion and the intricate structure of carbon fibers. To address this issue and achieve high-quality alloy coatings, Ni-Al and Ni-Al/Si composite coatings are successfully prepared on the surface of C/C composites through a combination of electroplating and hot-dip plating, and their oxidation behavior at elevated temperatures is thoroughly investigated. The experimental results indicate that the Ni-Al composite coatings exhibit superior antioxidant properties compared to Ni coatings following thermal shock experiments, thereby significantly enhancing the antioxidant performance of C/C composites. This improvement is attributed to the preferential oxidation of surface aluminum, which forms a dense Al2O3 layer in aerobic and high-temperature environments, effectively preventing oxygen from reaching the underlying matrix. During the oxidation process, coating elements migrate outward along the concentration gradient, while oxygen molecules diffuse inward. Simultaneously, aluminum atoms diffuse inward, and Ni atoms diffuse outward, where they partially dissolve with oxygen. The inner coating’s Ni enhances the bonding of the coating by connecting the substrate to the outer layer. Meanwhile, the added Si in the Ni-Al/Si composite coating further improves the antioxidant properties of the coating.
Journal Article
Evaluating Extrusion Deposited Additively Manufactured Fiber-Reinforced Thermoplastic Polymers as Carbon/Carbon Preforms
Although development of high char-yielding polymers has reduced the manufacturing costs of carbon/carbon composites associated with multiple densification cycles, manufacturing highly customized complex-shaped carbon/carbon composites can still be expensive due to molds/tooling surfaces used by traditional preform production techniques. In this study, we explored whether extrusion deposition additive manufacturing (EDAM) could be used as a mold-less approach to manufacturing complex-shaped carbon/carbon composites. The thermogravimetric analysis and coupon distortion results of several short carbon fiber-reinforced thermoplastic polymers used for 3D printing were investigated, including polyphenylene sulfide, polyetherimide, poly sulfone, polyether ether ketone, and polyether sulfone. Although polyetherimide had the highest char yield 57wt.%, carbon fiber-reinforced polyphenylene sulfide was the best preform for manufacturing complex shapes because of its dimensional stability, with carbonized strains of -4.18×10-2 and 1.82×10-1 at 1 ∘C/min in the 1- and 3- direction, respectively, after heat treating to 900∘C. The distortion results of more complex shapes showed that EDAM can be a practical alternative over more traditional preform production techniques for manufacturing complex-shaped carbon/carbon composites.
Journal Article
Transition metal–catalyzed alkyl-alkyl bond formation
Chemical reactions such as Heck and Suzuki coupling facilitate access to an enormous range of relatively flat molecules. This geometrical constraint is associated with the comparative ease of linking together aryl and alkenyl carbons. Choi and Fu review recent developments in forming bonds between the more abundant alkyl carbon centers that underlie diverse molecules with complex three-dimensional structures. Nickel catalysis in particular has emerged as a powerful method to access individual mirror-image isomers selectively and thereby tune the biological properties of the targeted products. Science , this issue p. eaaf7230 Because the backbone of most organic molecules is composed primarily of carbon-carbon bonds, the development of efficient methods for their construction is one of the central challenges of organic synthesis. Transition metal–catalyzed cross-coupling reactions between organic electrophiles and nucleophiles serve as particularly powerful tools for achieving carbon-carbon bond formation. Until recently, the vast majority of cross-coupling processes had used either aryl or alkenyl electrophiles as one of the coupling partners. In the past 15 years, versatile new methods have been developed that effect cross-couplings of an array of alkyl electrophiles, thereby greatly expanding the diversity of target molecules that are readily accessible. The ability to couple alkyl electrophiles opens the door to a stereochemical dimension—specifically, enantioconvergent couplings of racemic electrophiles—that substantially enhances the already remarkable utility of cross-coupling processes.
Journal Article
Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization
Practical, high-yield lignin depolymerization methods could greatly increase biorefinery productivity and profitability. However, development of these methods is limited by the presence of interunit carbon-carbon bonds within native lignin, and further by formation of such linkages during lignin extraction. We report that adding formaldehyde during biomass pretreatment produces a soluble lignin fraction that can be converted to guaiacyl and syringyl monomers at near theoretical yields during subsequent hydrogenolysis (47 mole % of Klason lignin for beech and 78 mole % for a high-syringyl transgenic poplar). These yields were three to seven times those obtained without formaldehyde, which prevented lignin condensation by forming 1,3-dioxane structures with lignin side-chain hydroxyl groups. By depolymerizing cellulose, hemicelluloses, and lignin separately, monomer yields were between 76 and 90 mole % for these three major biomass fractions.
Journal Article