Mechanical,dielectric and thermal properties of porous boron nitride/silicon oxynitride ceramic composites prepared by pressureless sintering

Porous boron nitride/silicon oxynitride (BN/Si₂N₂O) composites were fabricated by pressureless sintering at 1650 °C with Li₂O as sintering aid. The influence of Li₂O and hexagonal boron nitride (h-BN) contents on phase, microstructure, mechanical, dielectric and thermal properties of the resulting porous BN/Si₂N₂O composites was investigated. Increasing Li₂O content facilitated densification and decomposition of Si₂N₂O into Si₃N₄. The apparent porosity of the composites increases with the h-BN content increases and Si₂N₂O grain growth was restrained by the dispersed h-BN particles. The dielectric properties and thermal conductivities (TC) were affected mainly by porosity. Porous BN/Si₂N₂O ceramic composites with 4 mol% Li₂O and 25 mol% BN exhibit both low dielectric constant (3.83) and dielectric loss tangent (0.008) with good mechanical and thermal performance, suggesting possible use as high-temperature structural/functional materials.     

High temperature erosion behavior of spark plasma sintered ZrB2-SiC composites

Damage of structural components of hypersonic vehicles by atmospheric particles demands thorough understanding on their wear behavior. In the present work, dense ZrB₂-SiC (10, 20, and 30 vol%) composites are prepared by spark plasma sintering at 55 MPa in two stages: 1400 °C for 6 min followed by 1600 °C for 2 min. With increase in SiC content, microstructures of sintered composites reveal strongly bonded ZrB₂ grains with SiC particles. A combination of maximum hardness of 23 GPa, elastic modulus of 398 GPa and fracture toughness of 5.4 MPa m^1/2 are obtained for the composite containing 30 vol% SiC particles. It is found that cracks are bridged or deflected by SiC particles in the composites. When the composites are subjected to SiC particle erosion at 800 °C, a 14% decrease in erosion rate is obtained with increase in SiC content from 10 to 30 vol%. The formation of large extent of boro-silicate rich viscous surface on eroded surfaces is attributed to reduced fracture or removal of ZrB₂ grains of the composites with increased SiC content.     

CuO nanosheets/ZnO nanorods synthesized by a template-free hydrothermal approach and their optical and magnetic characteristics

Nanostructured CuO/ZnO heterojunctions were fabricated via a template-free hydrothermal reaction. The prepared CuO/ZnO nanocomposites are composed of one dimensional (1D) hexagonal ZnO nanorods and two-dimensional (2D) monoclinic CuO nanosheets. The single crystal nature of both ZnO and CuO was confirmed. Optical spectra show the widened absorption range from UV to visible light, which suggests the potential of the obtained CuO/ZnO composites for visible-light-driven photocatalyst. The point defects at the interface play the leading role in triggering ferromagnetism of CuO/ZnO composites. Fundamentally, the ferromagnetism can be understood by the charge-transfer mechanism according to Stoner theory. The findings can give a further insight into the ferromagnetic origin of nonmagnetic composites and the integration of ferromagnetic and photocatalytic properties into an identical sample.     

A novel B–Si–Zr hybridized ceramizable phenolic resin and the thermal insulation properties of its fiber-reinforced composites

A novel B–Si–Zr hybridized ceramicizable resin(BSZ-PR) was fabricated by chemical reaction of boric acid, zirconium hydroxyl-containing polyhedral oligomeric silsesquioxane(Zr-POSS) and phenolic. The incorporation of boric acid and Zr-POSS improved the thermal stability of the resin effectively, and the residual carbon rate increased to 72.63% at 800 °C under nitrogen atmosphere. The flexural strength of carbon fiber/BSZ-PR and high silica fiber/BSZ-PR composites were increased by 25.7% and 175.5%, and linear ablation rates were reduced by 37% and 44.75%, respectively. It was discovered that the ceramic structures such as SiO₂, ZrO₂ and SiC can be formed at high temperatures as well as under extreme ablative conditions from both BSZ-PR and its fiber-reinforced composites, which may be the key to the improved thermal, ablative properties.     

Controllable synthesis of FeSe2/rGO porous composites towards an excellent electromagnetic wave absorption with broadened bandwidth

Due to the escalating demand for electronic dependability and defense security, there has been a surge in research into broadband and lightweight microwave absorbers. Porous composites that are lightweight and plentiful in interfaces have the potential to be high-performance absorbers due to their ability to attenuate waves in a balanced manner and match impedance. “Using a solvothermal technique we generated FeSe₂/rGO composites with a porous topology. By varying the weight of rGO, the electromagnetic properties of FeSe₂/rGO composites may be finely tuned. Impedance matching and attenuation capability are both improved as a direct result of the porous structure and the appropriate electromagnetic parameters. FeSe₂/rGO composites benefit from the tunable composition, porous structure, and strong synergistic effect between FeSe₂ and rGO sheets and display outstanding microwave absorption performance with an ultrabroad bandwidth approaching 5.2 GHz with a thin thickness of 1.6 mm which covers 75% of the studied frequency range. At the same thickness, a significant reflection loss of ?43.7 dB is attained. This work not only enables the tuning of electromagnetic parameters but also expands the use of high-performance microwave absorption devices. Remarkable microwave absorption ability, of the porous composites FeSe₂/rGO can be utilized as a high-performance microwave absorber.”     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.     

无机纳米粒子与聚合物基复合材料的研究进展

综述了纳米粒子／聚合物基复合材料的制备方法及应用，并指出纳米粒子在聚合物改性中的设计思想，最后就纳米粒子／聚合物基复合材料设计发展提出一些参考意见。     

Use of organosolv lignin in phenol-formaldehyde resins for particleboard production: II. Particleboard production and properties

The objective of this work was to demonstrate the utility of lignin-based resins designed for application as an adhesive in the production of particleboard. Bond qualities of lignin-phenol-formaldehyde resins, phenolated-lignin-formaldehyde resins and commercial phenol-formaldehyde (PF-com) resin were assessed by using an automatic bonding evaluation system, prior to production of particleboards. In order to evaluate the quality of lignin-based resins, particleboards were produced and physical and mechanical properties were investigated. These physical properties included internal bond, modules of rupture and modulus of elasticity. Thickness swell and water absorption properties of particleboards bonded with lignin-based resins were also determined. The lignin-based resins have been reported previously in Part I of this study. The results showed that particleboards bonded with phenolated-lignin formaldehyde resins (up to 30% lignin content) exhibited similar physical and mechanical properties when compared to particleboards bonded with PF-com. The work has indicated that phenolated-lignin formaldehyde resins (up to 30% substitution level) can be used successfully as a wood adhesive for constructing particleboard. The performance of these panels is comparable to those of boards made using PF-com resin.     

Polysilazane derived micro/nano Si3N4/SiC composites

The pyrolised polysilazanes poly(hydridomethyl)silazane NCP 200 and poly(urea)silazane CERASET derived Si–C–N amorphous powders were used for preparation of micro/nano Si₃N₄/SiC composites by hot pressing. Y₂O₃–Al₂O₃ and Y₂O₃–Yb₂O₃ were used, as sintering aids. The resulting ceramic composites of all compositions were dense and polycrystalline with fine microstructure of average grain size <1 μm of both Si₃N₄ and SiC phases. The fine SiC nano-inclusions were identified within the Si₃N₄ micrograins. Phase composition of both composites consist of , β modifications of Si₃N₄ and SiC. High weight loss was observed during the hot pressing cycle, 12 and 19 wt.% for NCP 200 and CERASET precursors, respectively. The fracture toughness of both nanocomposites (NCP 2000 and CERASET derived) was not different. Indentation method measured values are from 5 to 6 MPa m^1/2, with respect to the sintering additive system. Fracture toughness is slightly sensitive to the SiC content of the nanocomposite. Hardness increases with the content of SiC in the nanocomposite. The highest hardness was achieved for pyrolysed CERASET precursor with 2 wt.% Y₂O₃ and 6 wt.% Yb₂O₃, HV 23 GPa. This is a consequence of the highest SiC content as well as the chemical composition of additives.     

ZrO2-CaO-BN复合材料抗氧化性的研究

为解决浸入式水口的结瘤问题 ,采用热压烧结方法制备了ZrO2 -CaO -BN复合材料 ,讨论了其氧化动力学过程 ,推导了氧化速度和氧化温度的数学关系式 ,并研究了其抗氧化性能及氧化层产物的显微结构。实验表明 :ZrO2 -CaO -BN复合材料在 90 0℃以下有很好的抗氧化性 ,而在 12 0 0℃以上氧化明显加剧 ;135 0℃时 ,材料的氧化产物主要由Ca3 B2 O6组成 ;ZrO2 -CaO -BN材料的氧化规律服从反应控速 -混合控速 -扩散控速 3段模型。