Removal of oxygen from silicon by electron beam melting

Small amounts of multicrystalline silicon were melted in an electron beam furnace in different experimental conditions in order to investigate the oxygen evaporation behavior during the electron beam melting (EBM) process. The oxygen content level before and after EBM was determined by secondary ion mass spectroscopy. The oxygen content was reduced from 6.177 to 1.629 ppmw when silicon was melted completely at 15 kW with removal efficiency up to 73.6 %. After that, it decreased continually to <0.0517 ppmw when the refining time exceeded 600 s with a removal efficiency of more than 99.08 %. During the melting process, the evaporation rate of silicon is 1.10 × 10^?5 kg/s. The loss of silicon could be reduced up to 1.7 % during oxygen removal process to a desirable figure, indicating EBM is an effective method to remove oxygen from silicon and decrease the loss of silicon.     

High-temperature impedance spectroscopy of BaFe0.5Nb0.5O3 ceramics doped with Bi0.5Na0.5TiO3

Bi_0.5Na_0.5TiO₃ (BNT)-doped BaFe_0.5Nb_0.5O₃ (BFN) ceramics were synthesized by a two-step solid-state reaction. Temperature dependence of dielectric properties measured at different frequencies was investigated over broad temperature and frequency ranges. Impedance spectroscopy and universal dielectric response were employed to study the relaxation behavior and conductivity mechanism of the ceramics in a frequency range from 40 Hz to 100 MHz and a temperature range from 300 K to 800 K. The complex plane impedance data revealed the bulk and grain boundary contributions toward conductivity processes in the form of semicircular arcs. The high-temperature conductivity of ceramics is attributable to thermally activated second ionized oxygen vacancy.     

高辐射效率超宽谱脉冲辐射天线的设计与实验

设计了一种采用共面TEM馈电结构的抛物反射面超宽谱脉冲辐射天线,分析了该种馈电结构的阻抗特性与辐射特性。实验表明当馈入电压为3kV、前沿180ps的超宽谱脉冲时,该天线主轴辐射场测试距离与峰值场强之积平均值达到18.3kV,相较于常用的TEM喇叭馈电的超宽带反射面天线,其辐射效率有了显著的提高。     

Three-dimensional ghost imaging based on periodic diffraction correlation imaging

We experimentally demonstrate a three-dimensional （3D） ghost imaging method based on period diffraction correlation imaging. Compared with conventional ghost imaging, our method can easily retrieve the images of different focal planes. Due to the correlation between the disturbed object beam and the reference beams which do not pass through any scattering, the clear images can be periodically obtained in the uncovered zones even through a scattering medium. The analysis of the 3D imaging resolution reveals that the proper resolution for actual demand can be achieved by designing our devices. The implementation of this experiment is quite simple and low-cost. It facilitates the practical applications of ghost imaging.     

A study on defect formation and magnetic properties of nitrogen-doped ZnO nanowires by the first principles

Theoretical calculation based on density functional theory (DFT) and generalized gradient approximation (GGA) has been carried out in studying defect formation energies, ionizing energies and ferromagnetism of nitrogen-doped ZnO nanowires. The result shows that N_O is deep acceptor, which make it hard to ionize. Ferromagnetic (FM) and antiferromagnetic (AFM) coupling between N atoms are also investigated. The results show that FM coupling between N atoms is more stable than AFM coupling. The FM coupling mechanism is explained by the interaction of N energy level. In addition, zinc and oxygen vacancies affecting FM coupling is also discussed. It is found that zinc and oxygen vacancies are unfavorable for stabilizing FM coupling of nitrogen-doped ZnO nanowires.     

Fluctuations in apparent mass at the bottom of granular columns due to different arrangements

We report the fluctuations in apparent mass at the bottom of granular columns due to various configurations. It is found that the fluctuations decrease with the increase in the ratio of diameters of silo to grain. For the arrangement of different grain layers in a column, the higher fluctuations appear when the larger grains are stacked at the bottom layer while reversing the order of grain-layers leads to smaller fluctuations. We attribute this behavior to the randomness in the direction of frictional forces between the grains and the confining wall. Moreover, due to polydisperse media, the development of inhomogeneous force transmission in grains may cause this to happen.     

Admissible consensus and consensualization of high-order linear time-invariant singular swarm systems

Admissible consensus analysis and consensualizing controller design problems for high-order linear time-invariant singular swarm systems are investigated. Firstly, by projecting the state of a singular swarm system onto a consensus subspace and a complement consensus subspace, a necessary and sufficient condition for admissible consensus is presented in terms of linear matrix inequalities (LMIs). An approach to decrease the calculation complexity is proposed, by which only three LMIs independent of the number of agents need to be checked. Then, by using the changing variable method, sufficient conditions for admissible consensualization are shown. An explicit expression of the consensus function is given, and it is shown that the modes of the consensus function can be arbitrarily placed if each agent is R$R$-controllable and impulse controllable and the interaction topology has a spanning tree. Finally, theoretical results are applied to deal with cooperative control problems of multi-agent supporting systems.     

Complex object 3D measurement based on phase-shifting and a neural network

An accurate phase-height mapping algorithm based on phase-shifting and a neural network is proposed to improve the performance of the structured light system with digital fringe projection. As phase-height mapping is nonlinear, it is difficult to find the best camera model for the system. In order to achieve high accuracy, a trained three-layer back propagation neural network is employed to obtain the complicated transformation. The phase error caused by the non-sinusoidal attribute of the fringe image is analyzed. During the phase calculation process, a pre-calibrated phase error look-up-table is used to reduce the phase error. The detailed procedures of the sample data collection are described. By training the network, the relationship between the image coordinates and the 3D coordinates of the object can be obtained. Experimental results demonstrate that the proposed method is not sensitive to the non-sinusoidal attribute of the fringe image and it can recover complex free-form objects with high accuracy.     

Optimization of photonic bandgap fiber long period grating refractive-index sensors

Long period gratings in low-index contrast solid-core photonic bandgap fibers are a promising platform for fiber-based fluid refractive index sensing with very low detection limits. We provide a comprehensive investigation of the possibilities for refractive index sensing using that principle in a commercial photonic crystal fiber filled with a fluid: using an acoustic grating, we map out the cladding bands, and use this data to optimize a long period grating’s sensitivity. We then implement the optimized long period grating, again using an acoustic grating, and directly measure its sensitivity to refractive index. We demonstrate a sensitivity of 17,900 nm/RIU (6.94 nm/°C) which corresponds to a smallest detectable index change of the fluid of 8.4 × 10⁻⁶.     

Diode pumped active-mirror Yb:YAG ceramic lasers

Lasers from Yb:YAG ceramic at different temperature were reported in this paper. The Yb:YAG ceramic laser with active-mirror structure was end-pumped by a laser diode whose central wavelength was 940 nm. At 80 K, with doping concentration of 5 at % Yb:YAG ceramic slab, output energy of 0.68 J at 10 Hz repetition rate was obtained under the pump energy of 2.49 J, corresponding to a slope efficiency of 40% and a conversion efficiency of 27.3%, respectively.