Improvement of sintering,nonlinear electrical,and dielectric properties of ZnO-based varistors doped with TiO_2

The effects of TiO_2 on sintering and nonlinear electrical properties of(98.5-x)ZnO–0.5MnO_2–0.5Co_2O_3-0.5Bi_2O_(3–x)TiO_2(x = 0.3,0.5,0.7,0.9 mol%) ceramic varistors prepared by the ceramic technique are investigated in this work.The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages.The optimum sintering temperature is found to be 1200℃,at which each of the samples shows a maximum firing shrinkage and minimum water absorption.Also minimum water absorption appears in a sample of x = 0.9 mol%.Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering.The crystal size of ZnO decreases with increasing TiO_2 doping.The addition of TiO_2 improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO_2,further addition negatively affects it.A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz–200 kHz.The temperature and composition dependences of the dielectric constant and AC conductivity are also studied.The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.     

Investigations of thickness-shear mode elastic constant and damping of shunted piezoelectric materials with a coupling resonator

Shear-mode piezoelectric materials have been widely used to shunt the damping of vibrations where utilizing surface or interface shear stresses. The thick-shear mode(TSM) elastic constant and the mechanical loss factor can change correspondingly when piezoelectric materials are shunted to different electrical circuits. This phenomenon makes it possible to control the performance of a shear-mode piezoelectric damping system through designing the shunt circuit. However, due to the difficulties in directly measuring the TSM elastic constant and the mechanical loss factor of piezoelectric materials, the relationships between those parameters and the shunt circuits have rarely been investigated. In this paper, a coupling TSM electro–mechanical resonant system is proposed to indirectly measure the variations of the TSM elastic constant and the mechanical loss factor of piezoelectric materials. The main idea is to transform the variations of the TSM elastic constant and the mechanical loss factor into the changes of the easily observed resonant frequency and electrical quality factor of the coupling electro–mechanical resonator. Based on this model, the formular relationships are set up theoretically with Mason equivalent circuit method and they are validated with finite element(FE) analyses. Finally, a prototype of the coupling electro–mechanical resonator is fabricated with two shear-mode PZT5 A plates to investigate the TSM elastic constants and the mechanical loss factors of different circuit-shunted cases of the piezoelectric plate. Both the resonant frequency shifts and the bandwidth changes observed in experiments are in good consistence with the theoretical and FE analyses under the same shunt conditions. The proposed coupling resonator and the obtained relationships are validated with but not limited to PZT5 A.     

Positron Characteristics in Cadmium and Zinc Chalcogenides

Electron energy levels and positron states have been calculated for cadmium and zinc chalcogenide compounds within the pseudo-potential approach and the independent particle model.Furthermore,the present contribution deals with the electron and positron chemical potentials allowing the calculation of the positron affinity to different materials of interest and hetero-structures formed by these materials.Besides,we here determine the positron diffusion constant by means of the positron deformation potential.An attempt has been made to scale positron affinity and diffusion constant with the lattice constant and the band gap energy,respectively.Such scaling is found to be not possible.The information gathered by the present study is of prime importance for a better understanding of positron trapping at interfaces and precipitates and should be useful in slow positron beam experiments.     

Mobility of large clusters on a semiconductor surface:Kinetic Monte Carlo simulation results

The mobility of clusters on a semiconductor surface for various values of cluster size is studied as a function of temperature by kinetic Monte Carlo method. The cluster resides on the surface of a square grid. Kinetic processes such as the diffusion of single particles on the surface, their attachment and detachment to/from clusters, diffusion of particles along cluster edges are considered. The clusters considered in this study consist of 150–6000 atoms per cluster on average.A statistical probability of motion to each direction is assigned to each particle where a particle with four nearest neighbors is assumed to be immobile. The mobility of a cluster is found from the root mean square displacement of the center of mass of the cluster as a function of time. It is found that the diffusion coefficient of clusters goes as D = A(T)Nαwhere N is the average number of particles in the cluster, A(T) is a temperature-dependent constant and α is a parameter with a value of about-0.64 α -0.75. The value of α is found to be independent of cluster sizes and temperature values(170–220 K)considered in this study. As the diffusion along the perimeter of the cluster becomes prohibitive, the exponent approaches a value of-0.5. The diffusion coefficient is found to change by one order of magnitude as a function of cluster size.     

Preferred Binding of Carboxylates by Chiral Urea Derivatives Containing α‐Phenylethyl Group

An efficient, simple protocol for the synthesis of a new family of chiral ureas 1 – 4 is described. The binding properties of 1 – 4 toward different anion (acetate, benzoate, fluoride, and chloride) have been studied by ¹H‐NMR titration and have been observed in the case of 4 is a selective receptor for acetate. The theoretical calculation M06/6‐311+G(d,p) helped us explain the binding properties observed. The most interesting observation is that this calculated structure is consistent with expected, based on the concept of allylic 1,3‐strain (A^1,3 strain). When chiral caboxylates were studied, urea 1 was the best in discriminating between enantiomers.     

大气湍流对轨道角动量态复用系统通信性能的影响

通过数值仿真,以多个相位屏模拟大气湍流,研究大气湍流对轨道角动量(Orbital Angular Momentum,OAM)态复用通信系统质量的影响.改变大气湍流折射率结构常量C2n和传输距离z,分析其对OAM态复用系统归一化接收功率、误码率等的作用.研究结果表明当大气湍流强度增加和传输距离增长时,复用系统归一化接收功率下降、误码率增加.当C2n增大到5×10-13 m-2/3时,系统归一化接收功率下降到0.2以下,误码率增大到0.3以上;当传输距离增大到10 000m,系统归一化接收功率下降到0.3以下,误码率增大到0.35以上.同时,对比不同方位角指数l取值的两个OAM态复用系统受大气湍流的影响,发现:l取值越接近的系统,受大气湍流影响越大.在等同光学信噪比下大气湍流导致OAM态复用系统误码率性能降低2至3个数量级.该研究对OAM态复用系统的实用化具有积极的参考作用.     

基于PN结传感器的数字式温度计实验研究

利用数字电压表单元模块、恒流源、AD620等设计测温实验电路,设计制作出0~100℃的数字温度计,它具有线性度高、热响应速度快、测温精度较高、重量轻、易制作、成本低等优点,可以测量固体、液体和气体的温度,具有广泛的用途。     

一种基于场景的红外图像非均匀性校正算法

提出了一种基于场景的红外图像非均匀性校正算法。该算法结合了两点定标校正算法和基于场景的改进的恒定统计算法,将两点校正算法的校正系数作为恒定统计算法的系数初值,并引入阈值进行运动状态检测,对运动场景和非运动场景分别进行系数更新。实验表明,该算法可以实现对红外图像非均匀性的校正,对于本文实验中的视频图像,在100帧时算法收敛,其收敛时间优于其他传统基于场景的非均匀性校正算法,并一定程度上抑制了"鬼影"现象。     

The electron affinity of Al13H cluster: high level ab initio study

Al₁₃H clusters have been considered candidates for cluster assembled materials. Here we have carried out benchmark calculations for the Al₁₃H cluster, both neutral and anionic, with the aim of verifying the nature of stationary points on the potential energy surface, studying dynamics of H atom and determining an adiabatic electron affinity. A range of correlated methods applied include second-order perturbation theory (MP2), spin-component-scaled MP2, coupled electron pair (CEPA) and coupled cluster singles and doubles with perturbative triple corrections (CCSD(T)). These methods are used in combination with the correlation consistent basis sets through aug-cc-pVTZ including extrapolation to the complete basis set (CBS) limit. Performance of several different flavours of density functional theory (DFT) such as generalised gradient approximation (GGA), hybrid GGA, meta-GGA and hybrid-meta-GGA is assessed with respect to the ab initio correlated reference data. The harmonic force constant analysis is systematically performed with the MP2 and DFT methods. The MP2 results show that for neutral Al₁₃H only the hollow structure is a potential energy minimum, with the bridged structure being a transition state for the H shift from the hollow site to the adjacent hollow site. The CCSD(T)/aug-cc-pVTZ (CCSD(T)/CBS) estimate of the energy barrier to this H shift is 2.6 (2.9) kcal/mol, implying that the H atom movement over the Al₁₃H cluster surface is facile. By contrast, the DFT force constant analysis results suggest additional terminal and bridged minima structures. For the anion Al₁₃H^?, exhibiting ‘stiffer’ potential energy surface compared to the neutral, the existence of the hollow and terminal isomers is consistent with the earlier photoelectron spectroscopy assignment. The adiabatic electron affinity of Al₁₃H is determined to be 2.00 and 1.95 eV (the latter including the ΔZPE correction) based on the CCSD(T) energies extrapolated to the CBS limit, whereas the respective CCSD(T)/CBS thermodynamic EA values are 2.79 and 2.80 eV.