Significantly enhanced breakdown field in Ca1-xSrxCu3Ti4O12 ceramics by tailoring donor densities

Significantly enhanced breakdown field of 24.52 kV cm^?1 as well as noteworthy nonlinear coefficient of 8.11 and low dielectric loss of 0.077 were obtained in Ca_0.6Sr_0.4Cu₃Ti₄O₁₂ ceramic. It was proved from impedance spectra that improved breakdown field was attributed to enhanced grain boundary resistance and elevated Schottky barrier height, which was further found resulting from reduced donor densities in C-V measurements. In addition, it was found that the activation energy originated from oxygen vacancies was increased, indicating the generation of oxygen vacancies was suppressed. Since oxygen vacancies acted as donors in depletion layers, it is reasonable to deduce that the reduced donor density was mainly ascribed to the decreased oxygen vacancies. In conclusion, maximum integrated action of strong solid solution effect and weak Sr-stretching effect was achieved when Sr/Ca ratio is 40/60, leading to greatly elevated potential barrier height and enhanced breakdown field consequently.     

考虑水平（切向）位移影响的垫层孔隙内土工膜顶胀

The common cushion layer used in the geomembrane face rockfill dam is made of granular materials such as gravel and non-fine concrete where bulge deformation or failure of geomembrane is very likely to occur in its surface holes. The curve intersection method based on the thin film theory is the common method used to analyze the bulge deformation of the geomembrane in the surface holes of cushion layer. However, it is inaccurate due to the neglection of horizontal (tangential) displacement of the geomembrane. Based on the FEM theory, a new method is proposed, which takes the horizontal (tangential) displacement of geomembrane into consideration, and is validated by the bulge tests on the geomembrane in a circular hole. The deformations and strains of the geomembrane in different shapes of holes are analyzed by this method, and different strain distributions and failure modes are obtained. The curve intersection method is also used to compare with the new method under the same condition. It is indicated that there is less difference in the deformation and average strain of the geomembrane when the water pressure is low. However, when the water pressure is relatively high, the influences of horizontal (tangential) displacement of the geomembrane on its strain should not be neglected.     

水下循迹航行器水动力学性能数值研究

为了研究低速水下循迹监测航行器的水动力学性能数值计算问题,采用FLUENT软件和SST剪切应力输运模型,通过雷诺时均N-S方程分析流速一定的情况下,取不同攻角、不同水平舵角作为来流条件,研究未安装推进器以及安装推进器且其安装位置不同时,航行器的升力系数、俯仰力矩系数、表面压力分布和流场速度的变化规律。结果表明:在未安装推进器以及推进器的安装位置不同时,随着攻角的变化,升力系数呈线性变化,俯仰力矩系数呈非线性变化;随着水平舵角的变化,升力系数和俯仰力矩系数呈线性变化。当推进器安装在航行器头部时,对航行器流场压力和流场速度变化影响最大;当安装在航行器尾部时,对二者影响最小。对于低速航行器,应尽量将推进器安装在中间靠后位置,以提高航行器的水动力性能。升力系数的试验结果与数值仿真结果之间最大相差7. 51%,阻力系数的试验结果与数值仿真结果最大相差5. 84%,均吻合较好。研究结果可以为低速水下循迹航行器的优化设计和发展提供理论参考。     

机械密封腔流场内粒子对密封腔壁面及波纹管冲蚀分析

为研究机械密封腔内混有固体颗粒的流体,在复杂工况下对密封腔壁面及波纹管表面冲蚀的影响,以CFD理论及方法,采用k-ε湍流模型和离散项DPM模型,对固液混合流场进行模拟分析,获得密封腔内流体中介质作用下密封腔壁面及波纹管外表面的压力场分布,分析不同工作状态下固体颗粒作用下密封腔内壁和波纹管外表面的冲蚀区域分布、固体颗粒在流场中的运动轨迹,以及机械密封工作过程中流场内固体颗粒的逃逸量、运动状态的变化。结果表明:粒子主要受颗粒阻力、浮重力、压力梯度力及波纹管和密封腔表面反作用力的影响;在旋转流场中,粒子作与旋转方向一致的螺旋方式前进;无转速时,冲蚀多发生在靠近入口处的波纹管外表面,旋转流场中,冲蚀多发生在密封腔靠后位置的壁面;随着转速增加,粒子逃逸率下降。研究结论为机械密封腔的设计布局及机械密封装置的优化提供理论依据。     

中压进汽腔的流场数值模拟比较

通过数值模拟计算,对中压对称进汽和切向进汽两种结构的流场进行了分析比较,结果表明,单一切向进汽腔的总压损失更小,出口汽流角的周向分布均匀度更好。更进一步,为整体评估中压进汽腔的流场以及对叶片级的流动影响,对中压进汽腔及第1级叶片的整体流体域流场情况进行了分析比较,结果表明,采用大几何角静叶的切向进汽腔气动性能最优;当采取切向进汽腔时,需合理选择第1级静叶几何角并耦合计算,才能实现进汽腔的气动优化。     

内插螺旋立式上行管流场及传热性能的实验研究

通过粒子图像测速流场实验与传热实验相结合，研究了内插螺旋立式上行管的螺旋节距、丝径、中径比等结构参数在不同Re下对流场、阻力及传热性能的影响。结果表明，内插螺旋能够有效扰动和混合管内流体，使管内形成多个纵向旋涡的流体结构、增大管壁附近液体涡量，有利于强化传热。当Re相同时，管内平均流速v、Nu和综合换热性能PEC均随丝径增大而增大，随中径比减小而增大；随节距增大，3种参数均出现增大的趋势，节距大于20 mm后开始减小。管内流体的阻力f随丝径和节距增大而减小，随中径比增大而增大。综合比较，在较低Re时，节距p=20 mm、丝径e=1.6 mm、中径比D/d=0.75时综合传热效果最好。     

Zr-4合金板材真空退火温度场的数值模拟

分析Zr-4合金板材真空退火过程中的传热方式,计算锆合金板材径向传热系数,利用ABAQUS对Zr-4合金板材退火温度场进行数值模拟,分析工装夹具对锆合金板材退火的影响,结果表明,工装夹具能改善锆合金板材温度场分布均匀性,使锆合金板材温度分布的最高温差由70 ℃降低至35 ℃,升温过程更加稳定,并且数值模拟结果与实测结果有较好的一致性。     

转速对齿轮泵内流场空化强度的影响

针对齿轮泵中齿轮转速在500~3652 r/min的变转速条件下工作的实际工况,该文通过实验的方法研究了转速对某型齿轮泵内流场空化强度的影响。使用现有的齿轮泵测试实验台,对在齿轮泵转速改变的条件下,齿轮泵内流场的空化强度改变进行监测。由于空化无法直接测量,该文根据空化诱导振动理论对齿轮泵的振动进行测量,得到了齿轮泵内流场空化强度随齿轮转速上升而增强的规律。通过Pumplinx软件对内流场进行建模,利用软件特有的空化损伤模块进行了仿真验证,仿真结果表明齿轮泵内流场空化强度随齿轮转速上升而增强,与实验所得规律一致。本文所总结的规律对减弱齿轮泵内流场空化强度,提升使用寿命具有一定的参考作用。     

Electron tomography: An imaging method for materials deformation dynamics

The combination of in-situ and three-dimensional (3D) in transmission electron microscopy (TEM) is one of the emerging topics of recent advanced electron microscopy research. However, to date, there have been only handful examples of in-situ 3D TEM for material deformation dynamics. In this article, firstly, the authors briefly review technical developments in fast tilt-series dataset acquisition, which is a crucial technique for in-situ electron tomography (ET). Secondly, the authors showcase a recent successful example of in-situ specimen-straining and ET system development and its applications to the deformation dynamics of crystalline materials. The system is designed and developed to explore, in real-time and at sub-microscopic levels, the internal behavior of polycrystalline materials subjected to external stresses, and not specifically targeted for atomic resolution (although it may be possible). Technical challenges toward the in-situ ET observation of 3D dislocation dynamics are discussed for commercial structural crystalline materials, including some of the early studies on in-situ ET imaging and 3D modeling of dislocation dynamics. A short summary of standing technical issues and a proposed guideline for further development in the 3D imaging method for dislocation dynamics are then discussed.