钙钛矿铁电体原子势参数的灵敏度分析及优化

原子相互作用势参数在分子动力学模拟中非常关键.本文针对钙钛矿铁电体原子势参数较多的特点,提出了一种高效的优化参数的方法:首先应用灵敏度分析方法从众多参数中找到对结构和性能影响最为明显的主要参数,忽略影响很小的次要参数,大大降低优化问题的维数;然后应用遗传算法对主要参数进行优化.优化结果表明,用本文得到的势参数模拟BaTiO₃ 和SrTiO₃,其结构和物理性能更接近实验值.     

利用电子结构数据拟合H2分子的电子壳模型势函数参数

电子壳模型势函数在离子晶体的原子级计算机模拟中有广泛应用,其势参数主要通过拟合晶体的实验数据或电子结构数据得到.提出了通过拟合双原子分子的量子化学从头计算电子结构数据来获得该势函数的方法,并由H₂分子的电子结构数据建立了H原子间的电子壳模型势函数.此外,还应用该势函数对H⁺₂分子离子进行了计算.该势函数拟合方案更适合于共价键型的分子. 关键词： 电子壳模型势 参数拟合 共价键 2分子')" href="#">H₂分子     

金红石结构TiO2晶体点缺陷形成能的经验途径计算

基于经验参数化途径,通过对晶体结构、晶格形成能、介电性质和弹性实验数据拟合确定金红石结构氧化物晶体TiO₂的电子壳模型参数和非Coulomb互作用势参数.计算点 缺陷形成能.论证Schottky缺陷是金红石结构TiO₂中的本征缺陷. 关键词： 2')" href="#">金红石TiO₂ 点缺陷形成能 电子壳模型 经验途径计算     

单母体遗传算法优化(H2O)n(n≤14)的结构

在传统遗传算法的基础上提出了单母体遗传算法(single-parent genetic algorithm, SPGA)，通过对母体团簇实施两种不同的变异操作对结构进行优化，给出了分子团簇结构优化的算法实现. 结合TIP3P模型势函数，研究了水分子团簇(H₂O)_n(n≤14)的稳定结构. 优化结构和已有理论及实验结果一致. 计算结果表明当n<8时，平均结合能随n增加较快；当n≥8时有小的起伏. n=4,8,10,12的团簇结构具有较高对称性，比较稳定. 关键词： 单母体遗传算法 水分子团簇 结构优化     

GaMnN材料红外光谱中洛伦兹振子模型的遗传算法研究

利用红外反射光谱研究了蓝宝石衬底上用金属有机物化学气相淀积方法生长的稀磁半导体GaMnN材料的晶格振动特性. 并成功地将改进的遗传算法应用于其红外反射光谱洛伦兹振子模型参数的提取. 通过与GaN薄膜的洛伦兹振子模型参数的对比研究发现,GaN掺入Mn后,ω_TO向高频方向移动,γ,ε_∞和ε_s均增加,而ω_LO基本保持不变. 文中同时分析和讨论了Mn对晶格振动特性的影响及 关键词： 稀磁半导体GaMnN材料 遗传算法 洛伦兹振子模型 参数提取     

雷达回波资料反演海洋波导的算法和抗噪能力研究

利用海洋环境中大气波导的参数化模型及电磁波传播的地型抛物方程,建立电磁波传播正问题,获得电磁波传播功率损耗.结合正演模式,通过遗传算法反演出大气波导参数,并对遗传算法参数的设置进行了讨论,对反演系统进行了抗噪能力的分析.发现当误差小于10%时,遗传算法仍能保持较高的反演精度;当误差大于10%时,该算法的精度开始受到显著的影响.研究结果可能会对雷达仪器设计和应用提供参考. 关键词： 波导 电磁波传播方程 遗传算法 反演     

基于遗传小波神经网络的机器人腕力传感器动态建模研究

提出一种基于改进遗传算法进化小波神经网络用于机器人腕力传感器动态建模的新方法,介绍了该算法原理.该方法利用腕力传感器的动态标定数据,用改进的遗传算法来优化小波神经网络结构和参数,建立腕力传感器的动态模型.结果表明,采用遗传小波神经网络进行腕力传感器动态建模,能克服误差反向传播算法存在易陷入局部极小点的缺点,网络的复杂度、收敛性和泛化能力得到了好的综合,建模的速度和精度得到提高. 关键词： 腕力传感器 动态建模 小波神经网络 遗传算法     

高功率太赫兹脉冲半导体探测器的分析与设计

设计了一种基于半导体热电子效应的0.14 THz高功率脉冲探测器.首先根据探测器的结构特点,分析了探测器的工作原理,并推导了探测器的相对灵敏度表达式.接着采用三维电磁场时域有限差分法,模拟计算了探测器的电压驻波比和线性区的相对灵敏度.在优化的结构参数下,探测器在0.14 THz波段的电压驻波比不大于1.3,相对灵敏度约为0.6 kW^-1,且在0.13—0.16 THz频带内波动不超过10%.然后讨论了焦耳热效应对探测器的影响,考察了太赫兹脉冲宽度与输出电压变化率的关系.最后对探测器的 关键词： 高功率太赫兹脉冲 探测器 热电子 灵敏度     

基于delta-P1近似模型的空间分辨漫反射一阶散射参量灵敏度研究

以delta-P1近似光学模型为基础, 推导了双点源近似下空间分辨漫反射一阶散射参量μ_s'灵敏度的解析式, 并进行了数值分析和比较. 研究表明, 与混合漫射近似模型和漫射近似模型相比, delta-P1 近似模型能更好地描述强散射较强吸收情况下近光源区域生物组织漫反射光子的分布, 且在有效反照率a'>0.83时, 获得最佳优化距离ρ_opt, ρ_opt 随μ_s' 的增大而减小, 且在距光源约2.7—4个输运平均自由程处μ_s'的变化对测量吸收的影响最小. 这项研究对于优化传感器几何结构以及生物组织光学参量的测量具有重要意义.     

基于粒子模拟和并行遗传算法的高功率微波源优化设计

提出了基于粒子模拟和并行遗传算法的高功率微波源优化设计方法, 以全电磁粒子模拟软件UNIPIC模拟的高功率微波器件输出功率作为适应度函数, 采用浮点数编码的遗传算法对高功率微波源器件进行优化. 采用该算法, 对相对论返波管的布拉格反射器位置以及高度进行了浮点数编码,然后在巨型机上进行参数的全局优化, 获得了该返波管布拉格反射器的全局最优参数. 关键词： 并行遗传算法 相对论返波管 粒子模拟 高功率微波源     

Electromagnetic properties and microwave absorption properties of BaTiO3-carbonyl iron composite in S and C bands

BaTiO₃ powders are prepared by sol-gel method. The carbonyl iron powder is prepared via thermal decomposition of iron pentacarbonyl. Then BaTiO₃-carbonyl iron composite with different mixture ratios was prepared using the as-prepared material. The structure, morphology, and properties of the composites are characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron microscopy (SEM), and a network analyzer. The complex permittivity and reflection loss of the composites have been measured at different microwave frequencies in S- and C-bands employing vector network analyzer model PNA 3629D vector. The effect of the mass ratio of BaTiO₃/carbonyl iron on the microwave loss properties of the composites is investigated. A possible microwave absorbing mechanism of BaTiO₃-carbonyl iron composite has been proposed. The BaTiO₃-carbonyl iron composite can find applications in suppression of electromagnetic interference, and reduction of radar signature.     

Modelling of the phase transitions sequence in KNbO3 and BaTiO3

Interatomic potentials are determined in the framework of the nonlinear oxygen polarizability model to describe the structural behavior of KNbO₃ and BaTiO₃ as a function of temperature. To this purpose, the adiabatic potential is evaluated for different ferroelectric distortions and the model potential parameters are improved by comparing with total energies from full-potential LMTO calculations. Phonon dispersion curves are computed to test if the model reproduces the pronounced two-dimensional character instabilities found by first-principles lattice dynamics using a linear response approach. Finally, the phase diagram for KNbO₃ is obtained through a constant-pressure molecular dynamics simulation.     

Simulation of domain patterns in BaTiO3

Computer simulations of domain structure were performed within the continuous phenomenological time-dependent Ginzburg–Landau–Devonshire model including electrostatic long-range interactions. Calculations are done on cube or rectangle area blocks with periodic boundary conditions, employing the previously proposed method consisting in eliminating the elastic field using Euler's equations and solving the kinetic equations in Fourier space. The authors demonstrate that both strong anisotropy of the Ginzburg gradient interaction and realistic estimation of elastic and electrostatic long-range interactions are crucial for correct domain wall properties of BaTiO₃-type ferroelectrics. Domain architecture obtained from simulations performed with the authors' model parameters for BaTiO₃ is found to be in reasonable agreement with experiment.     

Electrophoretic deposition of BaTiO3/CoFe2O4 multiferroic composite films

Electrophoretic deposition was utilized for preparation of BaTiO₃/CoFe₂O₄ multiferroic composite thick films on indium-tin oxide substrates. The suspensions for electrophoretic experiments were prepared by dispersing BaTiO₃ and CoFe₂O₄ nanoparticles with different molar ratios into solvents composed of ethanol and acetylacetone. Polyvinyl butyral was added to the suspensions in order to enhance the adhesion and strength of deposit and prevent cracking. The zeta potential values of BaTiO₃/CoFe₂O₄ suspensions were measured to be 26.4-36.9 mV. The experiment results showed that deposited films were obtained only when the applied electric field was larger than a certain critical value. XRD and SEM analysis depicted the presence of constituent phases in composite films. The percolation threshold of composite films was improved through dispersing ferromagnetic phase into ferroelectric phase. Therefore, the ferroelectric properties of composite thick films were maintained when the ferromagnetic properties were enhanced significantly with increasing CFO content.     

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO₃ content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO₃, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO₃. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.     

BaTiO3半导体陶瓷从PTC特性向边界层电容效应过渡问题探讨——晶界势垒模型的应用

对作者提出的晶界势垒模型作定量分析．由理论模型得到的结果定量解释了BaTiO₃半导体陶瓷从ＰＴＣ特性向边界层电容效应过渡的有关现象；由此对BaTiO₃半导体陶瓷边界层电容器和ＰＴＣ电阻器的性能作出了设计，设计值明显优于目前的实验值；对提高器件性能的方法作了探讨，提出了相应的措施．本文的结果为BaTiO₃半导体陶瓷器件的设计、生产和性能提高提供了理论基础     

Electronic,optical, vibrational and EFG properties of tetragonal BaTiO3 under pressure: By first-principles study

The First principles study on structural, electronic, Electron Field Gradient (EFG), optical and vibrational properties of tetragonal BaTiO₃ have been done in the framework of Density Functional Theory (DFT). Obtained structural properties are in agreement with others, and also the electronic study shows that the tetragonal BaTiO₃ has an indirect energy gap (E_g) about 1.864 eV by GGA and 2.6 eV by GGA-mbj at equilibrium state which E_g shifts toward small values by decreasing pressure. By increasing pressure to 31.479 GPa, the EFG decrease for O atom, and increase for Ba and Ti atoms that is a sign to piezoelectric property for tetragonal BaTiO₃. The optical parameters were studied under pressure, such as the real and imaginary parts of the dielectric function, Loss function, reflection index, absorption coefficient, conductivity and reflection. Moreover, by applying hydrostatic pressure, the roots of the real part of the dielectric function shift toward higher energies, and the energy loss and absorption peak intensity were reduced. Finally, lattice vibration survey indicates the stability of tetragonal BaTiO₃ under pressure.     

Computer simulation of intrinsic defects in YAlO3 single crystal

Computer simulation techniques were used to investigate intrinsic defects in YAlO₃ single crystal. A set of short-range potential parameters were derived using a relaxed fitting procedure incorporating with the known crystal properties. These parameters were then applied within the framework of the shell model. The simulation results reveal that oxygen Frenkel disorder and the antisite defect of Al ion substituting the Y ion dominate the intrinsic defects in YAlO₃. An analysis of redox reactions corroborate that the oxidation is most likely to occur via forming interstitial oxygen, while the oxidation via filling oxygen vacancies and reduction reaction may predominate at high temperature. The activation energy of oxygen vacancy migration on conduction was also studied.     

Different temperature dependences of photorefractive parameters of Ce-doped and Rh-doped BaTiO3

eff and the electro-optic coefficient r₃₃ in Ce-doped and Rh-doped BaTiO₃ were determined by two-beam coupling measurements. It was found that the effective trap density N_eff of BaTiO₃:Ce increases whereas that of BaTiO₃:Rh decreases with increasing temperature. The electro-optic coefficient r₃₃ of both crystals increases with temperature. The photorefractive response times were also measured and found to decrease with different rates as temperature increases in the two crystals. The results were discussed by using the two-centre model for BaTiO₃:Ce and three-charge-state model for BaTiO₃:Rh. We found that the different temperature dependence of N_eff in the two crystals was due to the fact that the deep- and shallow-trap levels in BaTiO₃:Ce are caused by different impurity centres whereas those in BaTiO₃:Rh are caused by different charge states of the same impurity centre. Received: 16 July 1998/Revised version: 26 October 1998