Fabrication and Properties of Three-dimensional Braided Carbon Fiber Reinforced SiO2-rich Mullite Composites

In order to enhance the fracture toughness of mullite, three-dimensional braided carbon fiber reinforced mullite(C/mullite) composites were prepared using the Al2O3-SiO2 sol with a high solid content as raw material. Mullitization behavior of the sol was characterized. Then, the microstructure, mechanical properties and oxidation resistance of C/mullite composites were investigated. It is found that the SiO2-rich mullite with desirable sintering shrinkage can be synthesized at 1 300 ℃ from the sol with an Al2O3/SiO2 mass ratio of 1:1. The C/mullite composites with a total porosity of 21.5% were fabricated by repeating 18 cycles of vacuum impregnation-drying-heat treatment, showing a flexural strength of 234.5 MPa and a fracture toughness of 13.1 MPa·m1/2. Since carbon fibers were protected by compact matrix, the C/mullite composites show favorable oxidation resistance during 1 200 ℃-1 600 ℃ even if an open porosity of 10.3% was detected.

Effect of Sintering Additives on Properties of Si_3N_4BN Composites Fabricated via Die Pressing and Precursor Infiltration and Pyrolysis Route

The Si3N4-BN composites have been prepared via die pressing and precursor infiltration and pyrolysis route using borazine as precursor, and the effect of sintering additives on properties of the composites has been investigated. After sintering additives are adopted, the a to β phase transition of Si3N4 and the mechanical properties of the composites at both room temperature and high temperature are all increased with small extent. When using Y2O3＋Al2O3 as additives, the phase transition of Si3N4 and the mechanical properties of the composites have better results. The β-Si3N4 content is 17.47%. The flexural strength, elastic modulus and fracture toughness of the composites are 188.74 MPa, 84.34 GPa and 2.96 MPa.m1/2, respectively. After exposed at 1 000 ℃ in the air for 15 min, the flexural strength of the composites is 154.62 MPa with a residual ratio of 81.92%. The elongated β-Si3N4 grains appear in all composites with different sintering additives. Relatively more rod like β-Si3N4 grains can be observed in composites with Y2O3＋Al2O3 as additives, making it to possess better mechanical properties.

Numerical Study on the Behaviour of Hybrid FRPs Reinforced RC Slabs Subjected to Blast Loads

The safety of civilian and military infrastructure is a concern due to an increase in explosive risks,which has led to a demand for high-strength civil infrastructure with improved energy absorption capacity.In this study,a Finite Element(FE)numerical model was developed to determine the effect of hybrid Fibre Reinforced Polymer(FRP)as a strengthening material on full-scale Reinforced Concrete(RC)slabs.The reinforcing materials under consideration were Carbon(CFRP)and Glass(GFRP)fibres,which were subjected to blast loads to determine the structural response.A laminated composite fabric material model was utilized to model the failure of composite,which facilitates the consideration of strain rate effects.The damaged area of the laminate is determined in the FE model,and it is in good agreement with the corresponding experimental results in the literature.Models containing different stacking sequences were built to demonstrate their efficiency in resisting blast loads.In general,the damaged area was reduced when a hybrid reinforcement with CFRP as the top layer was used.

A thin radar-infrared stealth-compatible structure:Design,fabrication,and characterization

A thin radar-infrared stealth-compatible structure with reflectivity below -10 dB in the whole radar X wave band and infrared emissivity less than 0.3 in the infrared region of 8μm-14 μm is reported. The designed stealth-compatible structure consists of metallic frequency selective surface （MFSS）, resistive frequency selective surface （RFSS）, and metal backing from the top down, and it is only 2. l-mm thick. The MFSS is made up of some divided low infrared emissivity metal copper films, and the RFSS consists of a capacitive array of square resistive patches. They are placed close together, working as an admittance sheet because of a mutual influence between them, and the equivalent admittance sheet greatly reduces the thickness of the whole structure. The proposed stealth-compatible structure is verified both by simulations and by experimental results. These results indicate that our proposed stealth-compatible structure has potential applications in stealth fields.

Fabrication and oxidation resistance of silicon nitride fiber reinforced silica matrix wave-transparent composites

Wave-transparent ceramic matrix composites for the high temperature use should possess excellent oxidation resistance. In this work, Si3N4f/SiO2 composites with different fiber content were fabricated by filament winding and sol gel method. The oxidation resistance was investigated by tracking the response of flexural strength to the testing temperature. The results show that the flexural strength and toughness of the composites with fiber content of over 37% can reach high levels at around 175.0 MPa and 6.2 MPa m^1/2, respectively. After 1 h oxidation at 1100℃, the flexural strength drops a lot but can still reach 114.4 MPa, which is high enough to ensure the safety of structures. However, when the oxidation temperature rises to 1200–1400℃, the flexural strengths continue to fall to a relatively low level at 50.0–66.4 MPa. The degradation at high temperatures is caused by the combination of over strong interfacial bonding, the damage of fiber and the crystallization of silica matrix.

Infrared Thermochromic Properties of VO_2 Thin Films Prepared through Aqueous Sol-gel Process

The stoichiometric vanadium（IV） oxide thin films were obtained by controlling the temperature, time and pressure of annealing. The thermochromic phase transition and the IR thermochromic property of 400 nm and 900 nm VO2 thin films in the 7.5 μm-14 μm region were discussed. The derived VO2 thin film samples were characterized by Raman, XRD, XPS, AFM, SEM, and DSC. The resistance and infrared emissivity of VO2 thin films under different temperature were measured, and the thermal images of films were obtained using infrared imager. The results show that the VO2 thin film annealed at 550 ℃ for 10 hours through aqueous sol-gel process is pure and uniform. The 900 nm VO2 thin film exhibits better IR thermochromic property than the 400 nm VO2 thin film. The resistance of 900 nm VO2 film can change by 4 orders of magnitude and the emissivity can change by 0.6 during the phase transition, suggesting the outstanding IR thermochromic property. The derived VO2 thin film can control its infrared radiation intensity and lower its apparent temperature actively when the real temperature increases, which may be applied in the field of energy saving, thermal control and camouflage.

Preparation and Ablation Properties of W/TaC Cermet via in-situ Reaction Sintering Process

An attractive way to prepare W/TaC based cermet at a relatively low temperature was proposed and confirmed experimentally. The thermodynamics calculations indicated that the reaction between WC and Ta2O5was feasible at as low a temperature as 1 400 ℃. The experimental results showed that W/TaC cermet could be fabricated by in-situ reaction sintering process at 1400 ℃ for 2 h in vacuum. The open porosity and bulk density of the W/TaC cermet were 15.3% and 13.4 g/cm3. Further, the microstructural features revealed that W, TaC, and Ta2WO8were identified to be the main constituents of the W/TaC cermet. The mass lose rate and linear recession rate of the W/TaC cermet during an oxyacetylene torch test were 0.0048 g/s and 0.0233 mm/s, respectively. The high porosity, the presence of Ta2WO8phases within W/TaC and evaporation of WO3on the surface of the composite contributed to the decrease of ablative property when comparing with pure W.

Robust,fire-resistant,and thermal-stable SiZrNOC nanofiber membranes with amorphous microstructure for high-temperature thermal superinsulation

Ceramic nanofibers with robust mechanical properties,high-temperature resistance,and superior thermal insulation performance are promising thermal insulators used under extreme conditions.However,developing of ceramic fibers with both low solid thermal conductivity(λs)and low infrared radiation thermal conductivity(λr)is still a great challenge.Herein,according to the Ioffe-Regel limit theory,we report a novel SiZrNOC nanofiber membrane(NFM)with a typically amorphous structure by combining the electrospinning method and high-temperature pyrolysis technique in a NH3 atmosphere.The prepared SiZrNOC NFM has a high tensile strength(1.98±0.09 MPa),excellent thermal stability(1100℃in air),and superior thermal insulation performance.The thermal conductivity of SiZrNOC NFM was 0.112 W·m^(−1)·K^(−1) at 1000℃,which is obviously lower than that of the traditional ceramic fiber membranes(>0.2 W·m^(−1)·K^(−1) at 1000℃).In addition,the prepared SiZrNOC NFM-reinforced SiO2 aerogel composites(SiZrNOCf/SiO2 ACs)exhibited ultralow thermal conductivity of 0.044 W·m^(−1)·K^(−1) at 1000℃,which was the lowest value for SiO2-based aerogel composites ever reported.Such superior thermal insulation performance of SiZrNOC NFMs was mainly due to significant decreasing of solid heat conduction and thermal radiation by the fancy amorphous microstructure and high infrared shielding compositions.This work not only provides a promising high-temperature thermal insulator,but also offers a novel route to develop other high-performance thermal insulating materials.

具有(100)择优取向的高致密LaB_(6)多晶材料的制备与表征

本文以六硼化镧(LaB_(6))纳米立方体粉体为原料,经放电等离子烧结(SPS)制备了具有(100)择优取向的高致密LaB_(6)多晶材料。详细研究了LaB_(6)多晶材料的微观组织结构和热电子发射性能,结果表明LaB_(6)多晶材料的相对密度高达95.8%,而且在垂直于SPS烧结压力方向上的表面形成了(100)择优取向。具有择优取向的LaB_(6)多晶材料表面的电子逸出功仅为2.73 eV,与LaB_(6)单晶材料(100)晶面的理论电子逸出功接近。分析了LaB_(6)多晶材料具有择优取向的原因。

Design and realization of a magnetic-type absorber with a broadened operating frequency band

In this paper, we present an efficient method to obtain absorbers with broadened operating frequency bands. They are accomplished by using conventional magnetic absorbing materials （MAMs） in the forms of array and mesh structures, which are similar to those in the case of a frequency selective surface. The proposed approach is verified not only by simulations but also by experimental results under the normal incidence at microwave frequencies. Moreover, the wideband absorber is lighter than the conventional magnetic absorber. These results indicate that our proposed absorbing structures can be used for designing good electromagnetic absorbers.

Oxidation Performance of Ytterbium Disilicate/Silicon Environmental Barrier Coating via Optimized Air Plasma Spraying

Environmental barrier coatings (EBCs) play a critical role in mitigating the degradation of SiCf/SiC ceramic matrix composites (CMCs) in complex combustion environment, and improve the service life of thermal engine components. In this paper, by adjusting the parameters of atmospheric plasma spraying (APS), the spraying process of ytterbium disilicate (Yb2Si2O7) under a lower power has been optimized. A two-layer EBC system consisting of ytterbium disilicate and silicon is prepared on the SiCf/SiC composite substrate by using optimized technological parameters. The thermal resistance and water oxygen corrosion resistance of such two-layer EBC system are investigated. The results indicate that the current ytterbium disilicate/silicon EBC system exhibits good phase stability, excellent water vapor and oxygen corrosion resistance. However, the exposed silicon bonding layer tends to generate an excessive thermal growth oxide (TGO) layer known as SiO2, leading to an early spallation of the coating.

Effect of Deposition Time on Microstructures and Growth Behavior of ZrC Coatings Prepared by Low Pressure Chemical Vapor Deposition with the Br2-Zr-C3H6-H2-Ar System

ZrC coatings were deposited on graphite substrates by low pressure chemical vapor deposition（LPCVD） with the Br2-Zr-C3H6-H2-Ar system. The effects of deposition time on the microstructures and growth behavior of ZrC coatings were investigated. ZrC coating grew in an island-layer mode. The formation of coating was dominated by the nucleation of ZrC in the initial 20 minutes, and the rapid nucleation generated a fine-grained structure of ZrC coating. When the deposition time was over 30 min, the growth of coating was dominated by that of crystals, giving a column-arranged structure. Energy dispersive X-ray spectroscopy showed that the molar ratio of carbon to zirconium was near 1：1 in ZrC coating, and X-ray photoelectron spectroscopy showed that ZrC was the main phase in coatings, accompanied by about 2.5mol% ZrO2 minor phase.

Research Progress of Ceramic Metallization Technology

Due to the wide application of ceramics in electronic device packaging,the performance of ceramic metallization layer directly determines the performance of the whole package device.This paper introduces the main preparation methods of ceramic metallization,discusses the influence of Mo powder size,metallization formula,sintering temperature and other factors on the performance of ceramic metallization layer prepared by activated Mo-Mn method,and introduces several kinds of methods that can be tested to test the performance of ceramic metallized sealing samples.A new research direction of Ceramic Metallization Technology in the advanced field is put forward.

基于水性气相二氧化硅浆料制备柔性二氧化硅气凝胶复合毡

二氧化硅气凝胶由于其低导热率在隔热应用方面具有巨大的潜力.然而,它们通常具有较差的机械性能,需要在保持低热导率的同时增强机械性能.本研究以商业化气相二氧化硅和甲基三甲氧基硅烷为硅源,并以水和乙醇为溶剂形成浆料.基于此,二氧化硅气凝胶块体(SAMs)可通过常压干燥进行制备,且无需额外的表面改性或溶剂置换.制备的SAMs保持了典型的纳米孔结构,具有低密度(0.24 g cm^(-3))、收缩率(4%)和热导率(0.046 W m^(-1)K^(-1)).通过辊压将浆料浸渍到纤维毡中,并通过浆料热固化和常压干燥制备出二氧化硅气凝胶毡(SABs).制备的SABs具有良好的柔韧性和机械性能,便于安装和隔热应用,并显著减少了生产周期和成本.此外,基于SAMs的纳米孔结构和低收缩率,通过调控遮光剂的粒径和质量分数进一步降低了SABs的高温热导率,优化后的SABs在800℃的热导率低至0.054 W m^(-1)K^(-1).

Online preparation of high-quality BN coatings with atomic diffusion based on carbon-free water-soluble precursor

Efficient and environmentally friendly production of high-quality continuous fiber coatings using current preparation methods is highly challenging due to issues such as scale and batch processing restrictions,low deposition rate,high energy consumption,and utilization of multiple environmentally hazardous steps.To address these challenges,we propose a stable and efficient wet chemical deposition coating method for high-throughput online continuous preparation of boron nitride(BN)coatings on ceramic fibers under an ambient environment.Our process involves surface modification,in-situ wet chemical deposition,and heat treatment,and all seamlessly connecting with the ceramic fiber preparation process through continuous stretching.Hydrophilic groups were introduced via surface modification enhancing wettability of the fiber surface with impregnating solution.An in-situ reaction and atom migration improve uniformity and binding of the coating.As a result,outstanding impregnation and adhesion properties are achieved.A comprehensive analysis to evaluate the impact of the BN coatings was conducted,which demonstrates that the BN-coated fibers exhibit a remarkable 36%increase in tensile strength,a 133%increase in fracture toughness,and enhanced temperature resistance of up to 1600℃.It provides a secure and efficient platform for cost-effective production of functional and high-quality coatings through targeted surface modification and rapid stretching impregnation.

In situ synthesis of graphitic-C3N4 nanosheet hybridized N-doped TiO2 nanofibers for efficient photocatalytic H2 production and degradation

Graphitic 碳氮化物 nanosheets g-C <潜水艇class=“ a-plus-plus ”> 3 N <潜水艇class=“ a-plus-plus ”> 4 NS hybridized 氮做了钛二氧化物 N-TiO <潜水艇class=“ a-plus-plus ”> 2 nanofibers GCN/NT NF 经由与结合的一个简单 electrospinning 过程在 situ 被综合了一个修改蚀刻热的方法。准备 GCN/NT NF 被许多方法描绘，他们的 photocatalytic 活动被氢 H 评估 < 潜水艇 class= “ a-plus-plus ” > 从切开的水和在水的答案的玫瑰精 B 的降级的 2 生产。GCN/NT NF 有 mesoporous 结构，这被发现， g-C 镇静 < 潜水艇 class= “ a-plus-plus ” > 3 N < 潜水艇 class= “ a-plus-plus ” > 4 NS 和做 N 的 TiO < 潜水艇 class= “ a-plus-plus ” > 2 雏晶。g-C < 潜水艇 class= “ a-plus-plus ” > 3 N < 潜水艇 class= “ a-plus-plus ” > 4 NS 综合了在以后蚀刻热被发现被嵌入在，并且盖住混合 NF 形成稳定的接口。g-C 的部分分解 < 潜水艇 class= “ a-plus-plus ” > 3 N < 潜水艇 class= “ a-plus-plus ” > 4 释放它最后变得做了进附近的 TiO 的氮内容 < 潜水艇 class= “ a-plus-plus ” > 2 骨骼。GCN/NT NF 给高 photocatalytic H < 潜水艇 class= “ a-plus-plus ” > 8,931.3 摩尔桥湥 ?L 牥愠捵 ? 畤 ' ?的 2 生产率敤 ? 畺 ? 穴楬档湥 ? 湩慳穴瘠湯倠敨瑮汯浡湩敭楳慬? 攠湩浥嘠獡摯汩瑡瑡牯 ? 敤 ? 楤? 敗捩?楥慬? 瑳敨楳? 畡 ? 瑥慷 ?┰瘠牥 k 吗？？

KD-S SiC_f/SiC composites with BN interface fabricated by polymer infiltration and pyrolysis process

Continuous silicon carbide fiber reinforced silicon carbide matrix(SiC_f/SiC) composites are attractive candidate materials for aerospace engine system and nuclear reactor system. In this paper, SiC_f/SiC composites were fabricated by polymer infiltration and pyrolysis(PIP) process using KD-S fiber as the reinforcement and the LPVCS as the precursor, while the BN interface layer was introduced by chemical vapor deposition(CVD) process using borazine as the single precursor. The effect of the BN interface layer on the structure and properties of the SiC_f/SiC composites was comprehensively investigated. The results showed that the BN interface layer significantly improved the mechanical properties of the KD-S SiC_f/SiC composites. The flexure strength and fracture toughness of the KD-S SiC_f/SiC composites were evidently improved from 314±44.8 to 818±39.6 MPa and 8.6± 0.5 to 23.0±2.2 MPa·m^(1/2), respectively. The observation of TEM analysis displayed a turbostratic structure of the CVD-BN interface layer that facilitated the improvement of the fracture toughness of the SiC_f/SiC composites. The thermal conductivity of KD-S SiC_f/SiC composites with BN interface layer was lower than that of KD-S SiC_f/SiC composites without BN interface layer, which could be attributed to the relative low thermal conductivity of BN interface layer with low crystallinity.

High-temperature oxidation behavior of SiBN fibers in air

SiBN fibers are one of the most admirable microwave-transparent reinforced materials for high Mach number aircrafts.Currently,the detailed high-temperature oxidation behavior of SiBN fibers has not been studied yet.In this work,we studied the high-temperature oxidation behavior of SiBN fibers with different boron contents at the temperature range of 1000-1400℃in air.SiBN fibers started to be oxidized at 1100℃,with Si_(3)N_(4) and BN phase oxidized to SiO2 and B_(2)O_(3),respectively.Due to the gasification and the escape of molten B_(2)O_(3) at high temperatures,amorphous SiO_(2) could be remained at the fiber surface.As the fiber further oxidized,the molten B_(2)O_(3) at the inside may infiltrate into the fiber interior to react with Si_(3)N_(4),causing the precipitation of hexagonal boron nitride(h-BN)nanoparticles and the formation of SiO_(2)/BN layer.Finally,complex oxidation layers with two distinct concentric sublayers accompanied with two transition sublayers could be formed after the oxidizing treatment.

Highly conductive wear resistant Cu/Ti3SiC2(TiC/SiC) co-continuous composites via vacuum infiltration process

The MAX phase Ti3SiC2 has broad application prospects in the field of rail transit,nuclear protective materials and electrode materials due to its excellent electrical conductivity,selflubricating properties and wear resistance.Cu–Ti3SiC2 co-continuous composites have superior performance due to the continuous distribution of 3 D network structures.In this paper,the Cu/Ti3SiC2(Ti C/Si C)co-continuous composites are formed via vacuum infiltration process from Cu and Ti3SiC2 porous ceramics.The co-continuous composites have significantly improved the flexural strength and conductivity of Ti3SiC2 due to the addition of Cu,with the conductivity up to 5.73×10^5 S/m,twice as high as the Ti3SiC2 porous ceramics and five times higher than graphite.The reaction between ingredients leads to an increase in the friction coefficient,while the hard reaction products(Ti Cx,Si C)lower the overall wear rate(1×10^–3 mm^3/(N·m)).Excellent electrical conductivity and wear resistance make co-continuous composites more advantageous in areas such as rail transit.

N-doped graphene grown on silk cocoon-derived interconnected carbon fibers for oxygen reduction reaction and photocatalytic hydrogen production

Carbon-based metal-free catalysts are a promising substitute for the rare and expensive platinum （Pt） used in the oxygen reduction reaction. We herein report N-doped graphene （NG） that is exquisitely integrated into highly conductive frameworks, simultaneously providing more active sites and higher conductivity. The NG was in situ grown on carbon fibers derived from silk cocoon （SCCf） using a simple one-step thermal treatment. The resulting product （NG-SCCf）, possessing a meso-/macroporous structure with three-dimensional （3D） interconnected networks, exhibits an onset potential that is only 0.1 V less negative than that of Pt/C and shows stability and methanol tolerance superior to those of Pt/C in alkaline media. Moreover, in the absence of Pt as co-catalyst, NG-SCCf shows a photocatalytic H2 production rate of 66.0 ~tmol-h l.g 1, 4.4-fold higher than that of SCCf. This outstanding activity is intimately related to the in situ grown NG, hierarchically porous structure, and 3D interconnected networks, which not only introduce more active sites but also enable smooth electron transfer, mass transport, and effective separation of electron-hole pairs. Considering the abundance of the green raw material in combination with easy and low-cost preparation, this work contributes to the development of advanced sustainable catalysts in energy storage/conversion fields, such as electro- and photocatalysis.