Ca2Li2BiV3O12∶Eu3+荧光粉的制备和发光性能

采用高温固相法成功合成了一种可用于白光LED的Ca₂Li₂BiV₃O₁₂∶Eu³⁺新型红色荧光粉,使用X射线衍射仪和荧光分光光度计对合成样品进行了表征,研究了合成温度和Eu³⁺含量对合成样品相组成和发光性能的影响。结果表明,采用高温固相法在650~700 ℃能合成纯度较高、结晶度好的Ca₂Li₂BiV₃O₁₂∶Eu³⁺荧光粉,合成样品激发带覆盖200~400 nm,发射光谱的线状发射峰可归属于Eu³⁺的⁵D₀→⁷F_J(J= 1, 2, 3,4)特征锐线发射,Eu³⁺摩尔分数为14%时荧光粉的发光强度最大。     

Initial guess by improved population-based intelligent algorithms for large inter-frame deformation measurement using digital image correlation

Digital image correlation (DIC) has received a widespread research and application in experimental mechanics. In DIC, the performance of subpixel registration algorithm (e.g., Newton-Raphson method, quasi-Newton method) relies heavily on the initial guess of deformation. In the case of small inter-frame deformation, the initial guess could be found by simple search scheme, the coarse-fine search for instance. While for large inter-frame deformation, it is difficult for simple search scheme to robustly estimate displacement parameters and deformation parameters simultaneously with low computational cost. In this paper, we proposed three improving strategies, i.e. Q-stage evolutionary strategy (T), parameter control strategy (C) and space expanding strategy (E), and then combined them into three population-based intelligent algorithms (PIAs), i.e. genetic algorithm (GA), differential evolution (DE) and particle swarm optimization (PSO), and finally derived eighteen different algorithms to calculate the initial guess for qN. The eighteen algorithms were compared in three sets of experiments including large rigid body translation, finite uniaxial strain and large rigid body rotation, and the results showed the effectiveness of proposed improving strategies. Among all compared algorithms, DE-TCE is the best which is robust, convenient and efficient for large inter-frame deformation measurement.     

Sol-gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis

Nanoparticles of Zn substituted lithium ferrite (Li_0.32Zn_0.36Fe_2.32O₄) have been prepared by a sol-gel method where the ultra-sonication technique has been adopted to reduce the agglomeration effect among the nanoparticles. The samples were heat-treated at three different temperatures and the formation of the nanocrystalline phase was confirmed by X-ray diffractograms (XRD). The average particle size of each sample has been estimated from the (311) peak of the XRD pattern using the Debye-Scherrer formula and the average sizes are in the range of 10-21 nm. The average particle size, crystallographic phase, etc. of some selected samples obtained from the high-resolution transmission electron microscopy are in agreement with those estimated from the XRD patterns. Static magnetic measurements viz., hysteresis loops, field cooled and zero field cooled magnetization versus temperature curves of some samples carried out by SQUID in the temperature range of 300 to 5 K clearly indicate the presence of superparamagnetic (SPM) relaxation of the nanoparticles in the samples. The maximum magnetization of the SPM sample annealed at 500 °C is quite high (68 Am²/Kg) and the hysteresis loops are almost square shaped with very low value of coercive field at room temperature (827.8 A/m). The particle size, magneto-crystalline anisotropy, etc. have been estimated from the detailed theoretical analysis of the static magnetic data. The dynamic magnetic behavior of the samples was also investigated by observing the ac hysteresis loops and magnetization versus field curves with different time windows at room temperatures. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses, etc. were extracted from dynamic measurements. Dynamic measurements confirmed that the samples are in their mixed state of SPM and ordered ferrimagnetic particles, which is in good agreement with the results of static magnetic measurements. Mössbauer spectra of the samples recorded at room temperature (300 K) and at different temperatures down to 20 K confirmed the presence of the SPM relaxation of the nanoparticles of the samples.     

Structural transition and enhanced magnetization in Bi1−xYxFeO3

Bi_1−xY_xFeO₃ (x=0-0.2) powders were prepared to study the effect of Y substitution on their structural and magnetic properties. A structural symmetric breaking from the rhombohedral R3c to orthorhombic Pnma at around x=0.10 was identified across a ferroelectric-paraelectric phase. A parabolic dependence of the magnetization upon substitution was obtained with a maximum at the phase transition boundary and a switching behavior for x=0.20. The composition-driven magnetic structure evolution was proposed to account for the magnetic properties in Bi_1−xY_xFeO₃.     

Monte Carlo simulations for 20 MV X-ray spectrum reconstruction of a linear induction accelerator

To study the spectrum reconstruction of the 20 MV X-ray generated by the Dragon-I linear induction accelerator, the Monte Carlo method is applied to simulate the attenuations of the X-ray in the attenuators of different thicknesses and thus provide the transmission data. As is known, the spectrum estimation from transmission data is an ill-conditioned problem. The method based on iterative perturbations is employed to derive the X-ray spectra, where initial guesses are used to start the process. This algorithm takes into account not only the minimization of the differences between the measured and the calculated transmissions but also the smoothness feature of the spectrum function. In this work, various filter materials are put to use as the attenuator, and the condition for an accurate and robust solution of the X-ray spectrum calculation is demonstrated. The influences of the scattering photons within different intervals of emergence angle on the X-ray spectrum reconstruction are also analyzed.     

十字环型左手材料单元结构设计与仿真

提出了一种新型十字环型左手材料结构单元.该结构只需在介质板单侧蚀刻, 即可在二维方向上产生等效负介电常数和等效负磁导率.通过理论分析, 提出场-路结合的等效源分析法以计算等效介电常数、磁导率, 并据此对十字环型结构单元的左手特性激发机理进行了论证. 后采用Nicolson-Ross-Weir 等效参数法提取了十字环型单元阵列的相对介电常数和相对磁导率, 并通过棱镜实验对材料的负折射特性进行了验证.实验表明, 该左手材料在6.8-6.9 GHz频段具有二维入射左手特性. 此种左手材料制作工艺相对简单, 为左手材料在微波器件领域的应用提供了一种较为实用的设计方案.     

符合计数在钚的属性测量中的应用研究

中子符合计数在核材料认证和管理中有着广泛的应用．系统阐述了中子符合计数的基本原理,对“点模型”公式作了推导．利用MCNP4B程序实现了中子符合计数的数值模拟.Neutron coincidence counting is widely used in the authentication and management of nuclear material. The basic principles of coincidence counting and the derivation of "point model" formulae of "point model" are introduced in this paper. The numerical simulation of the coincidence counting is implemented by use of the MCNP4B package.     

掺溴聚-4-甲基-1-戊烯的合成研究

 液溴与聚-4-甲基-1-戊烯（PMP）的四氯化碳溶液反应，在光照条件下得到溴代PMP，通过红外与元素分析确证了其结构。并通过热重分析对其性能进行了初步的研究。研究发现，在光照条件下溴原子很容易与PMP中叔丁基上的氢原子发生取代反应，得到化学掺杂溴原子的聚合物。该聚合物在150℃以下比较稳定，在150℃以上就会失去HBr。采用此种方法，可得到掺杂均匀的低密度材料。     

Analysis of Functionally Graded Piezoelectric Cylinders in a Hygrothermal Environment

This paper presents an analytical solution for the interaction of electric potentials, electric displacement, elastic deformations, and describes hygrothermal effect responses in hollow and solid cylinders, subjected to mechanical load and electric potential. Exact solutions for displacement, stresses and electric potentials in functionally graded piezoelectric material are determined using the infinitesimal theory. The material properties coefficients of the present cylinder are assumed to be graded in the radial direction by a power law distribution. Numerical examples display the significant of influence of material inhomogeneity. It is interesting to note that selecting a specific value of inhomogeneity parameter can optimize the piezoelectric hollow and solid cylinders responses, which will be of particular importance in modern engineering designs.     

光子晶体及其应用

光子晶体是８０年代末提出的新概念和新材料,文章简单回顾了光子晶体的历史,重点阐述了其主要特征以及可能的应用,同时论述了研究光子晶体的几种理论方法。