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排序方式: 共有7112条查询结果,搜索用时 31 毫秒
71.
《Journal of the European Ceramic Society》2021,41(14):6859-6868
Electrospun unidirectional SiC fibers reinforced SiCf/SiC composites (e-SiCf/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiCf/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiCf/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiCf/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiCf/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiCf/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future. 相似文献
72.
《Journal of the European Ceramic Society》2021,41(14):7005-7013
The lead-free piezoelectric material sodium bismuth titanate (NBT, Na0.5Bi0.5TiO3) has attracted considerable attention owing to its promising dielectric, piezoelectric, and electrical properties. However, the literature on the binary subsystems is contradictory and there are only limited data for the ternary system. The present work surveys all of the reports of the binary subsystems Bi2O3 – TiO2 and Na2O – TiO2 and synthesizes these data into inclusive revised versions. The compatibilities for the ternary system Na2O – Bi2O3 – TiO2 were determined experimentally, thus enabling the construction of a complete isothermal section at 800 °C. The compatibilities associated with the problematic binary subsystem Na2O – Bi2O3, which experiences extreme volatilisation, were determined through the generation of the absent standard-state thermodynamic functions for the relevant binary and ternary phases, thus providing a full suite of thermodynamic data for this system. The thermodynamic stability diagrams for Na2O, Bi2O3, and TiO2 thus were calculated. The isothermal section also addresses the contradictions in the literature concerning the formation of solid solutions of Bi12TiO20-x / Bi12-xTi1+xO20+0.5x, pyrochlore (Bi2Ti2O7 / NawBi2-xTi2-yO7-z), BTO (Bi4Ti3O12 / NaxBi4Ti3O12+0.5x), and NBT (Na0.5Bi0.5TiO3 / Bi1±xNaxTiO3.5±x). Further, it was observed that the congruent melting point of NBT, which was determined to be 1225 °C, was preceded by the onset of gradual structural destabilization at 940 °C. Also, the NBT rhombohedral → tetragonal phase transformation was observed at an onset temperature of ∼250 °C. The present work thus provides platform data for the fabrication and reactivities of materials in the ternary system Na2O – Bi2O3· TiO2 and its binary subsystems. 相似文献
73.
《Journal of the European Ceramic Society》2021,41(13):6196-6206
Creep properties of 2D woven CVI and PIP SiC/SiC composites with Sylramic™-iBN SiC fibers were measured at temperatures to 1650 °C in air and the data was compared with the literature. Batch-to-batch variations in the tensile and creep properties, and thermal treatment effects on creep, creep parameters, damage mechanisms, and failure modes for these composites were studied. Under the test conditions, the CVI SiC/SiC composites exhibited both matrix and fiber-dominated creep depending on stress, whereas the PIP SiC/SiC composites displayed only fiber-dominated creep. Creep durability in both composite systems is controlled by the most creep resistant phase as well as oxidation of the fibers via cracking matrix. Specimen-to- specimen variations in porosity and stress raisers caused significant differences in creep behavior and durability. The Larson-Miller parameter and Monkman-Grant relationship were used wherever applicable for analyzing and predicting creep durability. 相似文献
74.
《Journal of the European Ceramic Society》2021,41(14):7375-7379
New generation wireless communication systems require characterisations of dielectric permittivity and loss tangent at microwave and terahertz bands. La2Ti2O7 is a candidate material for microwave application. However, all the reported microwave dielectric data are average value from different directions of a single crystal, which could not reflect its anisotropic nature due to the layered crystal structure. Its dielectric properties at the microwave and terahertz bands in a single crystallographic direction have rarely been reported. In this work, a single crystal ferroelectric La2Ti2O7 was prepared by floating zone method and its dielectric properties were characterized from 1 kHz to 1 THz along one single direction. The decrease in dielectric permittivity with increasing frequency is related to dielectric relaxation from radio frequency to microwave then to terahertz band. The capability of characterizing anisotropic dielectric properties of a single crystal in this work opens the feasibility for its microwave and terahertz applications. 相似文献
75.
XU Tao GAO Huawei LIU Jianhua YU Cuiyan 《稀有金属(英文版)》2007,26(4):398-402
The effect of ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in HC1 solution was studied and the corrosion mechanisms were analyzed. It is found that ultlasonic vibration reduces the dechromisation incubation time, accelerates the dechromisafion corrosion rate, decreases the temperature and concentration of HC1 solution, and when the dechromisation occurs it seriously weakens the microstmcture of dechromisation layer. It is concluded that ultrasonic vibration can accelerate destruction of the passivation film on the Cr surface and increase the activities of Cl^- and Cr. 相似文献
76.
采用溶胶-凝胶与自蔓延燃烧相结合的方法制备出碳.纳米铁氧体复合材料,应用X射线衍射仪、扫描电镜分别对产物的晶体结构和微观形貌进行了表征分析,着重对比研究了超细碳材料加入前后的变化。利用小型烟箱试验测出产物在军用红外波段质量消光系数大于1.1m^2/g;利用矢量网络分析仪测试其在2~18GHz的电磁参数,并得到损耗角正切值随频率变化的曲线。结果表明:在纳米铁氧体中添加超细碳材料,铁氧体原有的磁损耗不变,介电损耗值有了显著提高,从而增加了电磁波的总损耗。由此证实超细碳-纳米铁氧体复合材料具有宽频吸波特性。 相似文献
77.
基于弹性模型的三角网格曲面优化展开 总被引:1,自引:0,他引:1
根据材料力学中杆件的变形及弹性变形能分析,提出了曲面展开的弹性变形模型。根据边的变形情况,将曲面展开分为约束展开和无约束展开,并对每种展开方法进行分析。特别是约束展开时产生的变形能,根据曲面展开时的判断准则,对弹性变形能进行释放,得到优化的曲面展开。最后给出了曲面展开的算法步骤及实例。该方法中曲面展开的局部精度较易控制,因此较之以前的方法,更能有效地解决CAD/CAGD中曲面展开问题。 相似文献
78.
介绍了两种新型的多机构组合加工试验机的结构和功能特点。其中多机构组合加工自动测量机能连续实现对工件的装夹、钻孔、镗孔和自动测量四个工序;另一种多机构组合加工螺栓扭紧机能连续实现对工件的装夹、钻孔、攻丝、自动扭紧螺母的工序。两种试验机在结构上采用了多种平面机构来实现各加工工序的快速进给、回退动作。试验机具有多功能、高效率、自动化程度高、动作可靠、成本较低的显著优点,适用于机械设计、机械制造以及机床自动化、自动检测等相关专业课程的实践教学环节。 相似文献
79.
80.