Heterogeneous interface design to boost interfacial polarization has become a feasible way to realize high electromagnetic wave absorbing (EMA) performance of dielectric materials. However, interfacial polarization in simple structures such as particles, rods, and flakes is weak and usually plays a secondary role. In order to enhance the interfacial polarization and simultaneously reduce the electronic conductivity to avoid reflection of electromagnetic wave, a more rational geometric structure for dielectric materials is desired. Herein, a Ti 3 C 2 T x /MoS 2 self-rolling rod-based foam is proposed to realize excellent interfacial polarization and achieve high EMA performance at ultralow density. Different surface tensions of Ti 3 C 2 T x and ammonium tetrathiomolybdate are utilized to induce the self-rolling of Ti 3 C 2 T x sheets. The rods with a high aspect ratio not only remarkably improve the polarization loss but also are beneficial to the construction of Ti 3 C 2 T x /MoS 2 foam, leading to enhanced EMA capability. As a result, the effective absorption bandwidth of Ti 3 C 2 T x /MoS 2 foam covers the whole X band (8.2-12.4 GHz) with a density of only 0.009 g cm −3 , at a thickness of 3.3 mm. The advantages of rod structures are verified through simulations in the CST microwave studio. This work inspires the rational geometric design of micro/nanostructures for new-generation EMA materials.
Oxidation behaviors of carbon fiber reinforced SiC matrix composites (C/SiC) are one of the most noteworthy properties. For C/SiC, the oxidation behavior was controlled by matrix microcracks caused by the mismatch of coefficients of thermal expansion (CTEs) and elastic modulus between carbon fiber and SiC matrix. In order to improve the oxidation resistance, multilayer SiC-Si3N4 matrices were fabricated by chemical vapor infiltration (CVI) to alleviate the above two kinds of mismatch and change the local stress distribution. For the oxidation of C/SiC with multilayer matrices, matrix microcracks would be deflected at the transition layer between different layers of multilayer SiC-Si3N4 matrix to lengthen the oxygen diffusion channels, thereby improving the oxidation resistance of C/SiC, especially at 800 and 1000 °C. The strength retention ratio was increased from 61.9% (C/SiC-SiC/SiC) to 75.7% (C/SiC-Si3N4/SiC/SiC) and 67.8% (C/SiC-SiC/Si3N4/SiC) after oxidation at 800 °C for 10 h.
To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si3N4–SiCN composite ceramics were prepared from SiCl4–NH3–C3H6–H2–Ar system and then annealed at the temperatures of 1400–1700°C in N2 atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave‐absorbing properties of the ceramic were investigated. Results showed that the CVD Si3N4–SiCN ceramics gradually crystallized into nanosized SiC grains, Si3N4 grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as‐annealed CVD Si3N4–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as‐annealed CVD Si3N4–SiCN ceramic at 1600°C was −41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si3N4–SiCN ceramic (T = 1600°C) had the superior microwave‐absorbing ability.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.