Effects of structural characteristics on microwave dielectric properties of low-loss (Zn1-xNix)ZrNbTaO8 ceramics

Low-loss (Zn_1-xNi_x)ZrNbTaO₈ (0.02?≤?x?≤?0.10) ceramics possessing single wolframite structure are initiatively synthesized by solid-state route. Based on the results of Rietveld refinement, complex chemical bond theory is used to establish the correlation between structural characteristics and microwave performance in this ceramic system. A small amount of Ni²⁺ (x?=?0.06) in A-site with the fixed substitution of Ta⁵⁺ in B-site can effectually raise the Q?×?f value of ZnZrNb₂O₈ ceramic, embodying a dense microstructure and high lattice energy. The dielectric constant and τ_f are mainly affected by bond ionicity and the average octahedral distortion. The (Zn_0.94Ni_0.06)ZrNbTaO₈ ceramic sample sintered at 1150?°C for 3?h exhibits an outstanding combination of microwave dielectric properties: ε_r =?27.88, Q?×?f?=?128,951?GHz, τ_f =?–39.9?ppm/°C. Thus, it is considered to be a candidate material for the communication device applications at high frequency.     

Possible origin of ferromagnetism in antiferromagnetic orthorhombic-YFeO3: A first-principles study

Rare-earth orthoferrites (RFeO₃) are well-known for the antiferromagnetic ground state. However, some of the recent experimental results suggest that the few members of RFeO₃ family possess ferromagnetism. In the present investigation we report the possible origin of ferromagnetism in antiferromagnetic YFeO₃ using density functional theory. For this purpose, we have considered pure as well as self-doping in YFeO₃ i.e. by considering the point defect at Y, Fe and O sites. Our finding suggests that the point defects in YFeO₃ results in the mixed-valence state of Fe, which may result in ferromagnetism through Zener double exchange mechanism.     

柱塞副微观表面对间隙油膜压力的影响

根据分形理论中的W-M函数建立柱塞与缸体孔表面三维形貌数学模型,并利用MATLAB软件编写程序,分析分形维数对表面微观形貌的影响。将柱塞副间隙油膜沿轴向展成平面,建立油膜厚度及压力分布计算模型,采用有限体积法求解二维雷诺方程,分析粗糙度对柱塞副间隙油膜压力分布的影响。结果表明:随着分形维数的增加,柱塞表面粗糙度呈增加趋势,且表面粗糙度越大,压力峰值越大。     

溢出效应和网络贴合度扰动下的二级供应链网络广告投放策略分析

为研究溢出效应和网络贴合度扰动下,二级供应链的网络广告投放策略问题,以强势制造商和两个网络零售商组成的二级供应链为研究对象,考虑产品的网络贴合度和网络广告计费特征,构建基于溢出效应的期望需求模型,通过博弈论方法给出了完全不合作、合作、半合作、战略联盟等4种模式下的均衡解,并对以上4种均衡结果进行比较分析。研究结果表明:无论在何种模式下,制造商批发价格与产品的网络贴合度成反比;固定位网络广告投资额和网络广告投资总额与产品的网络贴合度成正比,固定位网络广告投资额与溢出效应成反比;合作状态下的利润总额通常情况下较低;当产品的网络贴合度超过一定阈值范围时,采用战略联盟使得供应链成员利润最大化。     

神经网络全局势函数在多相催化中的应用

当今的多相催化研究需要新的技术和方法从原子尺度上表征活性中心结构和反应中间体。本文作者课题组近期开发了理论模拟新技术来探索催化剂活性位点结构，即基于神经网络势函数的大规模原子模拟（LASP）软件中实现的全局神经网络势函数计算方法。本文介绍了该方法可以显著降低催化体系的计算代价，而维持与密度泛函理论同一级别的计算精度，从而解决多相催化中的许多复杂问题。本文对神经网络势函数方法的实现细节和目前已实现的应用场景进行了详细介绍。神经网络势函数可以用来预测材料晶体结构，理解高压氢化条件下TiO₂表面的结构演化和确定三元氧化物ZnCrO晶相中合成气制甲醇活性位点。最后文章分析了神经网络势函数的局限性和今后可能的三个研究方向，即材料性质预测、多元素体系神经网络势函数构造和化学反应拟合。     

Machining accuracy improvement for a dual-spindle ultra-precision drum roll lathe based on geometric error analysis and calibration

This paper performs a comprehensive analysis and calibration on the geometric error of the ultra-precision drum roll lathe with dual-spindle symmetrical structure and cross slider layout. Firstly, the volumetric error model which contains all geometric errors of the dual-spindle ultra-precision drum roll lathe (DSUPDRL) is developed based on the combination of the homogenous transfer matrix (HTM) and multi-body system (MBS) theory. Secondly, sensitivity analysis for the volumetric error model is conducted to identify the sensitive geometric error components of the DSUPDRL using an improved Sobol method based on the quasi-Monte Carlo algorithm. The result of sensitivity analysis laid the foundation for the subsequent geometric error calibration. Then, some sensitive error components along the X and Z directions are calibrated using a laser interferometer and a pair of inductance displacement probes. Besides the volumetric error model, the concentricity error caused by dual-spindle symmetrical structure is proposed and calibrated by the on-machine measurement using a classic reversal method. Finally, a large-scale roller mold with a diameter of 250 mm and a length of 600 mm is machined using the DSUPDRL after calibration. The experimental result shows that 1.4 μm/600 mm generatrix accuracy is obtained, which validate the effectiveness of the geometric error analysis and calibration.     

Statistics of ceramic strength: Use ordinary Weibull distribution function or Weibull statistical fracture theory?

“Weibull statistics” for strength distribution analysis refers to either the ordinary Weibull distribution function or the Weibull statistical fracture theory. The ordinary Weibull distribution function is an empirical distribution function on an equal footing with other type of classical empirical distributions such as normal and log-normal distributions for fitting the statistical data of various random variables nonexclusive to materials strength. It has no explicit physical meaning and cannot be used for size scaling and prediction of strength. The Weibull statistical fracture theory is a weakest-link statistical fracture model for a solid with the strength distribution of an elemental volume being described by the ordinary Weibull distribution function. It has the capability of size scaling and prediction of strength for specimens with different geometries and different loading modes. The three-parameter Weibull statistical fracture theory in uniaxial flexure of prismatic beams is reformulated and validated by both numerical and real strength experiments of different ceramics.     

高速冲压生产线上下料冗余机械臂的运动学分析

在应用于高速冲压生产线冗余机械臂姿态规划和轨迹优化过程中,以兼顾上下料效率与轨迹的平稳性为目的,对冗余机器臂的加加速度进行约束,基于旋量理论建立运动学模型,得到正运动学方程,采用遗传算法对冗余机械臂进行逆运动学分析,结合五次B样条插值方法,对末端执行器运动轨迹的加加速度进行优化。结果表明,该方案可有效提高上下料效率,保证冗余机械臂上下料过程运行平稳。     

汽车路况网的进一步研究

汽车路况网的概念源于数字城市的概念,通过研究网络、实时数据库和汽车智能,实现安全的交通网络。研究的语言一般应用Lisp语言,数据库为实时数据库。     

现代机械设计方法研究及其创新

科学技术高速发展,对机械设计尤其是工业设计提出了更高的要求,对现代机械设计的理论进行了探讨,分析了机械设计的方法与过程,对机械创新设计的步骤过程进行了详细的论述,将现代机械设计的创新方法进行了分类研究和探讨。