Xinggang Chen

The escalating complexity of maritime security has driven the demand for high-performance protective structures capable of withstanding underwater impulsive loads. Sandwich structures have attracted considerable attention in blast-resistant buildings, naval vessels, and military protective systems because of their high specific strength, lightweight characteristics, and superior energy-absorption capabilities under extreme dynamic loading conditions. Sandwich systems comprising fiber-reinforced polymer (FRP) face-sheets and polymer foam cores exhibit exceptional specific strength and broadband energy dissipation capabilities. As a result, they have emerged as a pivotal solution for deep-sea protective applications. However, a systematic review of the dynamic response mechanisms and failure modes of these structures under extreme underwater impact is still lacking. This research systematically reviews the dynamic response behaviors and recent advances of foam core sandwich structures subjected to underwater impulsive loading. Initially, the response principles of these materials in impact environments are elucidated, focusing on the propagation characteristics, loading modes, and dynamic damage mechanisms associated with three distinct types of underwater impulsive loads. Subsequently, the key factors governing the impact resistance of the structures are systematically analyzed from two primary dimensions: loading conditions and structural parameters. Furthermore, a comparative analysis is conducted on the impact characteristics of typical core materials, including polymethacrylimide (PMI), polyvinyl chloride (PVC), styrene-acrylonitrile (SAN), and aluminum foams, and the current application status of impact-resistant foam sandwich materials is outlined. Finally, the limitations of existing research are summarized. It is proposed that future efforts should focus on developing multi-field coupled numerical simulation methods. Such advancements aim to further enhance the impact resistance and service life of these materials in complex marine environments. This review serves as a critical theoretical benchmark for the optimized design and standardized evaluation of future underwater protective structures.

Against the backdrop of the global energy crisis and environmental pollution, efficient and sustainable electrochemical energy storage materials have attracted considerable attention. Carbon materials are widely used due to their superior conductivity and chemical stability; however, they face limitations such as poor surface wettability, low specific capacitance, and insufficient active sites. In recent years, heteroatom doping—especially the co-doping of nitrogen (N) and sulfur (S)—has become a research hotspot, utilizing synergistic effects to significantly enhance the electrochemical performance of carbon materials. Despite progress in the synthesis, performance optimization, and mechanism study of N/S co-doped carbon materials, challenges remain, including complex synthesis methods, unclear doping mechanisms, and scalability issues for industrial production. This review systematically summarizes synthesis strategies (e.g., template-assisted methods, direct carbonization) for polymer-derived N/S co-doped carbon materials, critically analyzing their advantages and limitations. Furthermore, it elucidates the mechanistic impact of N/S co-doping on electrochemical properties, focusing on electron redistribution and active site modulation. Additionally, the application of density functional theory (DFT)-based computational simulations in material design is discussed, revealing the electronic structure modifications induced by N/S co-doping. The work aims to provide theoretical insights and experimental guidelines for advancing N/S co-doped carbon materials in electrochemical energy storage applications.

Bio-based phthalonitrile resins were synthesized using a two-step process. Initially, vanillin underwent a nucleophilic reaction with nitro and potassium carbonate to form intermediates, which were then dissolved in N, N-dimethylformamide. The intermediate was divided into two portions, with 4,4’-oxydianiline and 4,4’-methylenedianiline added to each portion, respectively, to produce two different vanillin-based phthalonitrile monomers: OVA monomer and DVA monomer. The successful synthesis of these two bio-based phthalonitrile monomers was confirmed using Fourier transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy. Differential scanning calorimetry was employed to study the processability of the monomers, revealing that OVA monomer and DVA monomer had a wide processing window of 110°C and 69°C, respectively, which were advantageous for resin processing. Dynamic mechanical analysis showed that the storage modulus of the OVA and DVA resins reached 4818 MPa and 3526 MPa, respectively, with a 24-h water absorption rate of only 1.03% and 0.90%. This study presents a viable method for synthesizing bio-based phthalonitrile resins, which can be widely utilized as functional and structural materials in daily life due to their renewability, degradability, and minimal environmental impact.

To improve the shock resistance of personal protective equipment and reduce casualties due to shock wave accidents, this study prepared four types of carbon fiber/polymethacrylimide (PMI) foam sandwich panels with different face/back layer thicknesses and core layer densities and subjected them to quasi-static compression, low-speed impact, high-speed impact, and non-destructive tests. The mechanical properties and energy absorption capacities of the impact-resistant panels, featuring ceramic/ultra-high molecular-weight polyethylene (UHMWPE) and carbon fiber/PMI foam structures, were evaluated and compared, and the feasibility of using the latter as a raw material for personal impact-resistant equipment was also evaluated. For the PMI sandwich panel with a constant total thickness, increasing the core layer density and face/back layer thickness enhanced the energy absorption capacity, and increased the peak stress of the face layer. Under a constant strain, the energy absorption value of all specimens increased with increasing impact speed. When a 10 kg hammer impacted the specimen surface at a speed of 1.5 m/s, the foam sandwich panels retained better integrity than the ceramic/UHMWPE panel. The results showed that the carbon fiber/PMI foam sandwich panels were suitable for applications that require the flexible movement of the wearer under shock waves, and provide an experimental basis for designing impact-resistant equipment with low weight, high strength, and high energy absorption capacities.

A novel quinoxaline-based phthalonitrile monomer, namely, 4,4′-(((quinoxaline-2,3-diylbis (oxy))bis (3,1-phenylene))bis (oxy))di-phthalonitrile (QDOP), was successfully synthesized by nucleophilic substitution reaction, which exhibits a low melting point (85°C) and a wide processing window (150°C). The QDOP monomer was cured with 4-aminophenoxy phthalonitrile (APPH) as catalyst by different temperature programs, and the curing process and kinetics were discussed in detail by non-isothermal differential scanning calorimetric (DSC). The QDOP polymer mainly forms triazine, isoindoline and phthalocyanine structure, as revealed by Fourier transform infrared (FTIR) spectroscopy, and its properties improved with the increase of post-curing temperature and curing time. After post-curing at 380°C, the polymer exhibited high storage modulus (3731 MPa), high glass-transition temperature (>400°C), and outstanding thermal stability and thermal oxidation stability.

MXenes are a family of two‐dimensional (2D) layered transition metal carbides/nitrides that show promising potential for energy storage applications due to their high‐specific surface areas, excellent electron conductivity, good hydrophilicity, and tunable terminations. Among various types of MXenes, Ti₃C₂T_x is the most widely studied for use in capacitive energy storage applications, especially in supercapacitors (SCs). However, the stacking and oxidation of MXene sheets inevitably lead to a significant loss of electrochemically active sites. To overcome such challenges, carbon materials are frequently incorporated into MXenes to enhance their electrochemical properties. This review introduces the common strategies used for synthesizing Ti₃C₂T_x, followed by a comprehensive overview of recent developments in Ti₃C₂T_x/carbon composites as electrode materials for SCs. Ti₃C₂T_x/carbon composites are categorized based on the dimensions of carbons, including 0D carbon dots, 1D carbon nanotubes and fibers, 2D graphene, and 3D carbon materials (activated carbon, polymer‐derived carbon, etc.). Finally, this review also provides a perspective on developing novel MXenes/carbon composites as electrodes for application in SCs.