非化学计量配比La0.67Sr0.33-x□x MnO3的结构和输运性质的研究

利用固相反应法制备了非化学计量配比的类钙钛矿锰氧化物La0.67Sr0.33-x□xMnO3(0相似文献   

(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构制备及表征

采用射频磁控溅射的方法在SrTiO3(001,基片上制备了(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构.X射线衍射分析证明(La0.7Sr0.3MnO3)m(BiFeO3)n具有明显的超晶格结构.电流垂直于薄膜表面测得的电阻-温度关系表明.La0.7Sr0.3MnO3)m(BiFeO3)n超晶格薄膜在290 K有金属-绝缘体转变,略低于单层La0.7Sr0.3MnO3薄膜的转变温度.电流在0.01-10 mA范围内,观察到薄膜的峰值电阻随电流增大而减小,峰值变化率远大于单层La0.7Sr0.3MnO3薄膜,且随着超晶格周期厚度的增加而增大.低温下,电流-电压曲线表明其导电机制应主要为空间载子限制,且显示较大的电压偏置,表现出肖特基结的特性.     

红外测温法研究Al-Mg合金中的Portevin-Le Chatelier效应

试件塑性变形过程伴随着机械能向热能的转化.利用红外测温法,通过分析红外热像仪采集的温度场图像,系统研究了Al-Mg合金中的Portevin-Le Chatelier （PLC）效应.在不同应变率下,实验得到了三类锯齿形应力-应变曲线,分析了相应情况下试件温度变化曲线的异同及其原因,探讨了三种类型PLC变形带的空间传播特性.研究发现,试件表面的温升随着应变率的增加而增加;PLC带的倾角转向发生在试件的两端或者带外的温度最高处. 关键词： Portevin-Le Chatelier效应 红外测温 Al-Mg合金     

Identification of lagrangian coherent structures in the turbulent boundary layer

Using Finite-Time Lyapunov Exponents (FTLE) method, Lagrangian coherent structures (LCSs) in a fully developed flat-plate turbulent boundary layer are successfully identified from a two-dimensional (2D) velocity field obtained by time-resolved 2D PIV measurement. The typical LCSs in the turbulent boundary layer are hairpin-like structures, which are characterized as legs of quasi-streamwise vortices extending deep into the near wall region with an inclination angle θ to the wall, and heads of the transverse vortex tube located in the outer region. Statistical analysis on the characteristic shape of typical LCS reveals that the probability density distribution of θ accords well with t-distribution in the near wall region, but presents a bimodal distribution with two peaks in the outer region, corresponding to the hairpin head and the hairpin neck, respectively. Spatial correlation analysis of FTLE field is implemented to get the ensemble-averaged inclination angle θ _R of typical LCS. θ _R first increases and then decreases along the wall-normal direction, similar to that of the mean value of θ. Moreover, the most probable value of θ saturates at y ⁺=100 with the maximum value of about 24°, suggesting that the most likely position where hairpins transit from the neck to the head is located around y ⁺=100. The ensemble- averaged convection velocity U _c of typical LCS is finally calculated from temporal-spatial correlation analysis of FTLE field. It is found that the wall-normal profile of the convection velocity U _c(y) accords well with the local mean velocity profile U(y) beyond the buffer layer, evidencing that the downstream convection of hairpins determines the transportation properties of the turbulent boundary layer in the log-region and beyond. Supported by the National Natural Science Foundation of China (Grant Nos. 10425207 and 10832001)     

Investigation of hot electrons and hot phonons generated within an AlN/GaN high electron mobility transistor

We review our recent results obtained on an AlN/GaN-based high-electron-mobility transistor. The temperature of the electrons drifting under a relatively-high electric field is significantly higher than the lattice temperature (i.e., the hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. We have also measured the decay time constant for LO phonons to be about 4.2 ps. An electric field present in a GaN/AlN heterostructure can bring both the first-order and second-order Raman scattering processes into strong resonances. The resonant Stokes and anti-Stokes Raman scattering results in the increase and decrease of non-equilibrium longitudinal-optical phonon temperatures, respectively. Moreover, the phonon temperature measured from the Raman scattering is increased with an applied electric field at a much higher rate than the lattice temperature due to the presence of field-induced non-equilibrium longitudinal-optical phonons.     

耦合系统的混沌相位同步

通过引入混沌运动的相位定义分析了线性和非线性耦合参数对两个主共振子系统之间的混沌相位同步的影响.讨论了在近似于主共振条件下,两子系统不同步、不完全相位同步和完全相位同步之间的演化过程,揭示了不同状态相互转化与Lyapunov指数变化之间的关系,指出随着线性耦合力的增加,相位同步效应增强,然而随着非线性耦合力的增加,相位同步效应减弱. 关键词： 相位同步 Rssler振子 耦合 Lyapunov指数     

镍基单晶高温合金相界面错配位错网络的演化

运用分子动力学方法,研究了镍基单晶高温合金γ/γ′相界面错配位错网络的特征.通过对界面位错的形成、位错的反应、位错网络的演化等现象的分析发现,在温度场影响下,位错网络将由弛豫初期的十四面体演化成最终的正六面体. 关键词： 镍基单晶高温合金 相界面错配位错 位错网络演化 分子动力学     

Color image encryption based on orthogonal composite grating and double random phase encoding technique

A new method of color image encryption based on orthogonal composite grating and double random phase encoding technique is proposed. The red (R), green (G) and blue (B) components of a color image to be encrypted is modulated into an orthogonal composite grating. The deformed composite grating is subsequently encrypted using double random phase encoding technique. At the decoding end, the deformed composite grating is decrypted through double random phase decoding system. By filtering in frequency domain and phase demodulating, the modulated RGB components can be recovered. Computer simulation experiments have proved the validity of the new method. The proposed method is also applicable to encrypt three color images simultaneously after they have been transformed respectively into indexed formats.     

氢化物发生-原子荧光光谱法测定灌木中的总汞

采用氢化物发生-原子荧光光谱法对5种常见灌木中的总汞进行分析,比较其结果,发现：相同的生长环境下,各种植物对汞吸收能力不同,汞含量有很大的差别,并呈现出叶〉枝的趋势,而且汞在植物中的富集作用明显。     

基于高光谱技术的玉米种子可视化鉴别研究

种子纯度是衡量种子品质的重要指标。提出一种基于近红外(874～1 734 nm)高光谱技术实现玉米种子可视化鉴别的方法。采集4个品种共384个玉米种子样本的高光谱图像数据,随机选择288个样本作为建模集,剩余96个样本作为预测集。对玉米种子光谱曲线进行分析后,通过连续投影算法（SPA）选取7个特征波段作为输入,结合偏最小二乘法判别分析(PLS-DA)模型,对预测集进行预测,获得较好的分类效果,其中R_C=0.917 7,RMSECV=0.444 2; R_CV=0.911 5,RMSECV=0.459 9,建模集和预测集的总体鉴别率分别为78.5%和70.8%。通过图像处理技术提取高光谱图像中每个玉米颗粒的平均光谱数据,输入建立的SPA-PLS-DA模型,在计算生成的鉴别图中以不同颜色标识不同类别,实现了混杂玉米种子样本的可视化鉴别。对3份不同组成的混杂种子样本进行鉴别,达到了较好的可视化效果。结果表明,通过可视化鉴别技术,可以直观方便地观察混杂种子样本中不同品种种子的分布和数量,为农业生产中种子的纯度鉴别和筛选提供了帮助。