Identification of a new oxidation/ dissolution mechanism for boria-accelerated SiC oxidation

Boria effects on accelerated SiC oxidation kinetics were investigated by conducting thermogravimetric analysis on SiC substrates coated with sol-gel derived borosilicate glass isothermally exposed to dry O₂ and argon at 800°C and 1200°C for 100 hours. Boria concentrations in the glass coatings were 0, 14-38, and 92-94 mol%, balance silica. Accelerated weight gain was observed for SiC exposures in dry O₂ at 800°C when boria concentrations were ≥ 92 mol%, corroborated by oxide thickness ranging from 3.5 to 10 µm. The oxide thickness predicted for pure SiC exposed to these conditions in the absence of boria is 0.15 µm. Microstructural analysis of SiC surfaces after oxide removal revealed that boria etched the underlying SiC substrate. Oxidation exposures at 1200°C in dry O₂ suppressed boria effects on accelerating SiC oxidation kinetics due to rapid boria volatilization coupled with the formation of a protective thermally grown silica scale. Accelerated weight gain or oxide growth did not occur with argon exposures at either temperature. A new mechanism for boria-accelerated SiC surface-reaction kinetics is presented based on evidence for boria etching of SiC.     

SiOC modified carbon-bonded carbon fiber composite with SiC nanowires enhanced interfibrous junctions

Carbon-bonded carbon fiber (CBCF) composites are promising lightweight and high efficient thermal insulators to be applied in aerospace area, but their practical applications are usually restricted by the low mechanical performance and poor oxidation resistance. To overcome these drawbacks, many efforts have been made in the fabrication of ceramic coated CBCF composites. However, the densities of these modified composites are usually very high, which would result in the reduction in their thermal insulation performance. Herein, we prepared a CBCF composite with SiC nanowires enhanced interfibrous junctions and SiOC ceramic coated carbon fibers (SiCNWs-SiOC-CBCF). Similar to CBCF, the SiCNWs-SiOC-CBCF exhibits a low density of 0.35 g/cm³ and an anisotropic and highly porous architecture. The SiCNWs-SiOC-CBCF possesses a compressive strength of 3.8 MPa and a compression modulus of 195.7 MPa in the X (or Y) direction, ~26.7% and 150% higher than those of CBCF respectively. It can also suffer from an isothermal treatment in air at 900°C for 120 minutes. The combination of these properties makes the SiCNWs-SiOC-CBCF a good candidate for thermal insulator to be applied in extreme conditions.     

激光熔覆碳化钛增强钛基复合涂层研究进展

钛合金具有密度低、比强度高、耐腐蚀性强等显著优点，在航空航天、海洋工程等领域具有广阔的应用前景。然而，钛合金硬度低、耐磨性差，严重制约其在摩擦工况下的使用寿命。激光熔覆技术具有生产效率高、热影响区窄、结合强度高、组织致密等优势，被广泛用于钛合金零部件表面改性和熔覆修复。高硬、高模量碳化钛的热物性参数与钛合金基材相近，常被选作激光熔覆钛基复合涂层的增强相，以提高其耐磨性。介绍了碳化钛的晶体结构、生长形态和性能特点。综述了碳化钛增强钛基激光熔覆材料体系以及工艺参数对熔覆层成形质量、宏观形貌和微观组织的影响。重点从碳化钛增强相的分布、数量、尺度以及相结构等方面，论述了碳化钛增强钛基激光熔覆层的组织特征，同时阐述了碳化钛强化机制，讨论了碳化钛增强钛基激光熔覆层组织特征与耐磨性能的内在关联性。最后提出了目前激光熔覆碳化钛增强钛基复合涂层研究中存在的问题与展望。     

微波烧结法制备含钪扩散阴极及其发射性能研究

采用溶胶凝胶结合氢还原法制备出Sc均匀掺杂的钨粉,随后采用微波烧结法成功制备出浸渍型含钪扩散阴极。对Sc掺杂钨粉特性、Sc掺杂钨海绵基体微观结构、阴极发射性能和阴极表面活性物质分布进行了分析,结果显示:Sc以Sc2O3的形态均匀分布于平均粒径1μm的钨粉中,海绵体骨架主要由大小均匀的亚微米级准球形颗粒构成,表面孔结构良好,孔分布均匀,平均孔径在0.46μm左右,Sc_2O_3均匀分布于基体之中。电子发射测试结果表明,该阴极950℃b时脉冲偏离点电流密度Jdiv为137.59 A/cm~2,发射斜率1.431。激活后的阴极表面Ba:Sc:O的原子比例为1.8:1:2.2,同时阴极表面存在大量的纳米粒子,对阴极发射有促进作用。     

Wetting behavior and brazing of titanium-coated SiC ceramics using Sn0.3Ag0.7Cu filler

By coating active titanium, Sn0.3Ag0.7Cu (SAC) filler wetted SiC effectively, as the contact angle decreased significantly from ~145° to ~10°. Ti₃SiC₂ and TiO_x (x ≤ 1) reaction layers were formed at the droplet/SiC interface, leading to the reduction of contact angle. Reliable brazing of SiC was achieved using titanium deposition at 900°C for 10 minutes, and the typical interfacial microstructure of Ti-coated SiC/SAC was SiC/TiO_x + Ti₃SiC₂/Sn(s,s). Comparing to direct brazing, Ti–Sn compounds in the brazing seam were effectively reduced and the mechanical property of joints was dramatically improved by titanium coating. The optimal average shear strength of SiC joints reached 25.3 MPa using titanium coating- assisted brazing, which was ∼62% higher than that of SiC brazed joints using SAC-Ti filler directly.     

Combined role of SiC particles and SiC whiskers on the characteristics of spark plasma sintered ZrB2 ceramics

In this research work, the effects of silicon carbide (SiC) as the most important reinforcement phase on the densification percentage and mechanical characteristics of zirconium diboride (ZrB₂)-matrix composites were studied. In this way, a monolithic ZrB₂ ceramic (as the baseline) and three ZrB₂ matrix specimens each of which contains 25 vol% SiC as reinforcement in various morphologies (SiC particulates, SiC whiskers, and a mixture of SiC particulates/SiC whiskers), have been processed through spark plasma sintering (SPS) technology. The sintering parameters were 1900 °C as sintering temperature, 7 min as the dwell time, and 40 MPa as external pressure in vacuum conditions. After spark plasma sintering, a relative density of ~96% was obtained (using the Archimedes principles and mixture rule for evaluation of relative density) for the unreinforced ZrB₂ specimen, but the porosity of composites containing SiC approached zero. Also, the assessment of sintered materials mechanical properties has shown that the existence of silicon carbide in ZrB₂ matrix ceramics results in fracture toughness and microhardness improvement, compared to those measured for the monolithic one. The simultaneous addition of silicon carbide particulates (SiC_p) and whiskers (SiC_w) showed a synergistic effect on the enhancement of mechanical performance of ZrB₂-based composites.     

高导热高介电BT/SiC/PA6复合材料的制备及性能研究

以聚酰胺(PA6)为基体,氮化硅(SiC)为导热填料,钛酸钡(BT)为介电填料,通过热压法制备出系列复合材料;研究了不同粒径填料的搭配对材料导热与介电性能的影响。结果表明:在填充量较低时,使用混合粒径导热填料能产生一定的级配效应,从而提高复合材料的导热性能。总填充量为26%时,以4∶1的比例,用粒径为0.5~0.7μm和3μm的SiC共同填充PA6,制备获得了最高导热系数为0.9198W/(m·K)的复合材料,而不同粒径、不同功能的混合功能填料还能产生协同效应,进一步提升材料的导热性能并使材料同时获得较好的介电性能,当SiC填充量为20%,BT填充量为20%时,复合材料的导热系数达到1.1110W/(m·K),介电常数到达16(100Hz),损耗保持在0.075(100Hz)左右。     

WS2纳米结构的构筑及其储能性能研究

针对市场对于高性能储锂、储钠负极材料的巨大需求和解决金属硫化物存在的关键制约问题，设计并开发了一系列纳米棒、纳米块和微米球等不同形貌和纳米结构的硫化钨，研究了形貌和石墨烯表面修饰对硫化钨储能性能的影响。研究结果表明：相比于WS₂纳米块和WS₂微米球，WS₂纳米棒比表面积更大、结晶性更好，展现了更好的储能性能。进一步通过冷冻干燥法，在WS₂纳米棒表面包裹上一层石墨烯，有效地提升了所制硫化钨的循环稳定性和倍率性能。在500 mA·g^-1下循环500圈，其储钠放电容量仍保持在65.9 mAh·g^-1，在1 000 mA·g^-1下循环500圈，其储锂放电容量可保持在288.3 mAh·g^-1。     

Zirconia ultrafiltration membranes on silicon carbide substrate: microstructure and water flux

This study reported the preparation of ZrO₂/SiC ceramic membrane with silicon carbide as the substrate and intermediate layers and zirconia as the selective layer. The substrate and intermediate layers were sintered by evaporation-condensation process at 2200 and 1900 ℃, respectively. After sintering, the intermediate layer presented layer thickness of 50 μm, pore size of 0.87 μm and pure water permeability of 2140 L/(m²·h). The selective lay was deposited on the silicon carbide substrate by dip-coating method and then sintered in the temperature range from 800 to 1000 ℃. For the membrane coated by one dip-coating cycle and sintered at 800 ℃, it presented average pores of 82 nm and water flux of 850 L/(m²·h). Due to the exclusion of low-melting oxides during sintering, the ZrO₂/SiC ceramic membrane can satisfy the separation and purification of chemical corrosion and high temperature wastewater.     

氦离子预注入对钨中氘滞留行为的影响

利用高能离子注入机和直线等离子体模拟装置,本文研究了高能氦离子预注入对氘等离子体辐照后钨中氘滞留行为的影响。采用FIB-SEM、TEM、GD-OES和TDS等分析方法,分析了氦离子预注入对钨中氘滞留行为的影响。结果表明：氦离子预注入在辐照损伤区域形成大量氦泡,钨经过氘等离子体辐照后,表面的氘泡数量明显低于未经过氦离子预注入的样品。GD-OES分析中可以看到在氦捕获位处氘滞留浓度明显升高,同时氦离子预注入增加了氘在钨中的扩散深度,结合TDS分析可知氦离子预注入增加了氘在钨中的滞留总量,这是由于氦离子预注入后,形成的缺陷又为钨中氘的俘获提供大量新的位点,从而导致钨中的氘滞留量明显提高。