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Abstract: With the aim to design a particular material for low and high frequency cooperative electromagnetic absorption at high temperature, a multiscale design is proposed by combining the microstructure and meta‐structure in one material. The SiCf/Si3N4 composite is prepared via the chemical vapor infiltration technique with SiCf as the EM wave absorbing phase and Si3N4 as the wave‐transparent ceramic matrix. The crossing grooved meta‐structure is designed and fabricated to further improve its absorbing properties and to guarantee its absorbing capacity stability at high temperature. A minimum reflection loss of −15.3 dB and −14.8 dB can be reached at 8 and 18 GHz with a total thickness of 5 mm. The temperature‐dependent reflection loss of the designed meta‐structure keeps relative reliable high temperature absorbing performances from room temperature to 500°C. This effective enhanced EM wave absorbing property is believed to be a consequence of multiscale effect induced by combining the traditional EM absorbing materials with metamaterial structure. [ABSTRACT FROM AUTHOR]

A kind of chemical vapor infiltration ( CVI) Si3N4- BN-Si CN composite ceramic with excellent electromagnetic wave ( EMW) absorbing properties is obtained by CVI BN interface and Si CN matrix on porous Si3N4 ceramics, and then annealed at high temperatures (1200°C-1500°C) in N2 atmosphere. The crystallization behavior, EMW absorbing mechanism and mechanical properties of the composite ceramics have been investigated. Results showed CVI Si CN ceramics with BN interface were crystallized in the form of nanograins, and the crystallization temperature was lower. Moreover, both EMW absorbing properties and mechanical properties of CVI Si3N4- BN-Si CN composite ceramics firstly increased and then decreased with the increase in annealing temperature due to the influence of BN interface on the microstructure and phase composition of the composite ceramics. The minimum reflection coefficient ( RC) and maximum effective absorption bandwidth ( EAB) of the composite ceramics annealed at 1300°C were −47.05 dB at the thickness of 4.05 mm and 3.70 GHz at the thickness of 3.65 mm, respectively. The flexural strength and fracture toughness of the composite ceramics annealed at 1300°C were 94 MPa and 1.78 MPa/m1/2, respectively. [ABSTRACT FROM AUTHOR]

Novel single-source-precursors (SSPs), namely carbon nanotube modified poly (methylvinyl) silazane (CNTs-HTT 1800), were synthesized via amidation reaction of poly (methylvinyl) silazane (HTT 1800) with carboxylic acid functionalized carbon nanotubes (CNTs-COOH) at the assistance of ZnCl2 catalyst, which was confirmed by means of Fourier transform infrared spectra ( FT IR) and transmission electron microscopy ( TEM). Besides, the TEM results unambiguously show the homogeneous distribution of the CNTs in the matrix of SSPs while serious aggregation of the CNTs in the matrix of physically-blended-precursor. Crack-free monolithic silicon carbonitride modified by carbon nanotubes ceramic nanocomposites ( CNTs-Si CN) were prepared through pyrolysis of the obtained SSP green bodies at 1000°C. Due to the strong influence of polymer structure on the microstructure of final ceramics, the SSP-derived CNTs-Si CN nanocomposites clearly show the homogeneous distribution of the CNTs in the Si CN matrix while the physically-blended-precursor derived CNTs-Si CN nanocomposites exhibit serious aggregation and entangling of the CNTs in the Si CN matrix. With the same CNT content in the feed, the SSP-derived CNTs-Si CN nanocomposites possess significant improvements of electromagnetic ( EM) absorbing properties compared to those from physically-blended-precursors, due to the quality of the dispersion of CNTs in the ceramic matrices. [ABSTRACT FROM AUTHOR]

To tailor a new electromagnetic wave ( EMW) absorbing material with lower reflection coefficient ( RC) and larger operating frequency band, the CVD Si3N4-Si CN 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-Si CN 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-Si CN 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-Si CN 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-Si CN ceramic ( T = 1600°C) had the superior microwave-absorbing ability. [ABSTRACT FROM AUTHOR]

In this paper, a low-temperature densification process of Al-Si alloy infiltration was developed to fabricate C/SiC-Ti3Si(Al)C2, and then the microstructure, mechanical, and electromagnetic interference ( EMI) shielding properties were studied compared with those of C/SiC-Ti3SiC2 and C/SiC-Si. The interbundle matrix of C/SiC-Ti3Si(Al)C2 is mainly composed of Ti3Si(Al)C2, which can bring various microdeformation mechanisms, high damage tolerance, and electrical conductivity, leading to the high effective volume fraction of loading fibers and electrical conductivity of C/SiC-Ti3Si(Al)C2. Therefore, C/SiC-Ti3Si(Al)C2 shows excellent bending strength of 556 MPa, fracture toughness 21.6 MPa·m1/2, and EMI shielding effectiveness of 43.9 dB over the frequency of 8.2-12.4 GHz. Compared with C/SiC-Si and C/SiC-Ti3SiC2, both the improvement of mechanical properties and EMI shielding effectiveness can be obtained by the introduction of Ti3Si(Al)C2 into C/SiC, revealing great potential as structural and functional materials. [ABSTRACT FROM AUTHOR]

In this paper, a low-temperature densification process of Al-Si alloy infiltration was developed to fabricate C/SiC-Ti3Si(Al)C2, and then the microstructure, mechanical, and electromagnetic interference ( EMI) shielding properties were studied compared with those of C/SiC-Ti3SiC2 and C/SiC-Si. The interbundle matrix of C/SiC-Ti3Si(Al)C2 is mainly composed of Ti3Si(Al)C2, which can bring various microdeformation mechanisms, high damage tolerance, and electrical conductivity, leading to the high effective volume fraction of loading fibers and electrical conductivity of C/SiC-Ti3Si(Al)C2. Therefore, C/SiC-Ti3Si(Al)C2 shows excellent bending strength of 556 MPa, fracture toughness 21.6 MPa·m1/2, and EMI shielding effectiveness of 43.9 dB over the frequency of 8.2-12.4 GHz. Compared with C/SiC-Si and C/SiC-Ti3SiC2, both the improvement of mechanical properties and EMI shielding effectiveness can be obtained by the introduction of Ti3Si(Al)C2 into C/SiC, revealing great potential as structural and functional materials. [ABSTRACT FROM AUTHOR]

Owing to the widespread presence of electromagnetic interferences, it is necessary to develop new materials with excellent high-temperature electromagnetic wave ( EM) absorption properties. In the present work, ZnO is infiltrated into porous ZrSiO4 substrates to form ZnO/ ZrSiO4 composite ceramics using sol-gel process. The doping of aluminum results in the improvement of electrical conductivity and the significant change in the morphology of ZnO. With the increase in environment temperature during measurement, the permittivity of the composite ceramics increases first and then decreases dramatically, which is attributed to the change in conductive loss. The electrical conductivity increases with increasing measurement temperature. However, the concentration of oxygen vacancies decreases under air atmosphere when the measurement temperature increases continuously, which results in the reduction in conductivity. Therefore, permittivities of the undoped and doped ceramics measured at 673 K are higher than the ones at the other temperatures. The composite ceramics maintain a relatively high EM absorption coefficient, low reflection coefficient ( RC), and wide effective absorption bandwidth at environment temperatures up to 773 K. As a result, we conclude that the ZnO/ ZrSiO4 composite ceramics exhibit a promising prospect as a kind of high-temperature EM absorbing material. [ABSTRACT FROM AUTHOR]

A thermodynamic calculation on the chemical vapor deposition of the SiCl4- NH3- CH4- H2- Ar system was performed using the FactSage thermochemical software databases. Predominant condensed phases at equilibrium were SiC, Si3 N4, graphite, and Si. The equilibrium conditions for the deposition of condensed phases in this system were determined as a function of the deposition temperature, dilution ratio (δ), and reactant ratios of CH4/ SiCl4 and NH3/ SiCl4. The CVD phase diagrams were used to understand the reactions occurring during the formation of Si- C- N from the gas species and determine the area of SiC- Si3 N4. The concentration of condensed-phase products was used to determine the deposition conditions of CVD SiC- Si3 N4. The present work was helpful for further experimental investigation on CVD Si- C- N. [ABSTRACT FROM AUTHOR]

In this study, Al-doped ZnO particles are infiltrated into porous ZrSiO4 substrates to form an Al-doped ZnO/ ZrSiO4 composite ceramic using sol-gel process. The doping of aluminum results in the high-concentration carriers, leading to the improvement of polarization capability and electric conductivity. When the Al content increases to 2.5 at.%, particle shape is changed from spherical into flower-like one, and the electric conductivity of ZnO/ ZrSiO4 composite ceramics is further improved. Therefore, both dielectric loss and microwave absorption properties of ZnO/ ZrSiO4 composite ceramics increase with the increase of Al content. The further increase of Al content leads to the production of ZnAl2 O4, which is a low-permittivity ceramic, so the dielectric constant and dielectric loss of ZnO/ ZrSiO4 composite ceramics show a decreasing tendency. Owing to the higher dielectric loss, the 2.5 at.% Al-doped ZnO/ ZrSiO4 composite ceramics exhibit the best microwave absorption properties over the frequencies ranging from 8.2 to 12.4 GHz ( X-band). [ABSTRACT FROM AUTHOR]

Pyrolytic carbon-silicon nitride ceramics with dense silicon nitride coating ( PyC-Si3N4/Si3N4) were fabricated by chemical vapor infiltration of PyC into porous Si3N4 ceramics combined with chemical vapor deposition of Si3N4 at their surface. The electromagnetic shielding effectiveness ( SE) of PyC-Si3N4/Si3N4 increases with the increase of PyC content and specimen thickness. When the content of PyC is 11.7 vol% and the thickness of specimen is 2.8 mm, the mean total SE of PyC-Si3N4/Si3N4 reaches 43.2 dB in the frequency range of 8.2-12.4 GHz. Due to the excellent sealing effect of dense Si3N4 coating, the mean total SE of PyC-Si3N4/Si3N4 remains unchanged even after oxidation at 1100°C for 10 h in air. [ABSTRACT FROM AUTHOR]