短路过渡电弧焊电流信号的近似熵分析

依据近似熵的理论和算法，从非线性角度对纯CO₂保护气体条件下短路过渡电弧焊的电流信号进行了近似熵分析.通过对不同的送丝速度、给定电压及气体流量下电流信号的近似熵计算和比较，表明近似熵可以作为短路过渡稳定性的评判标准.近似熵越大，波动越小，焊接过程就越稳定. 关键词： 非线性 近似熵 短路过渡 稳定性     

Structural and anisotropic elastic properties of hexagonal MP (M = Ti,Zr, Hf) monophosphides determined by first-principles calculations

First-principles calculations were performed to investigate the structural properties, phase stabilities, elastic properties and thermal conductivities of MP (M = Ti, Zr, Hf) monophosphides. These monophosphides are thermodynamically and mechanically stable. Values for the bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν were calculated by Voigt–Reuss–Hill approximation. The mechanical anisotropy was discussed via several anisotropy indices and three-dimensional (3D) surface constructions. The order of elastic anisotropy is ZrP > HfP > TiP. The minimum thermal conductivities of these monophosphides were investigated using Clarke’s model and Cahill’s model. The results revealed that these monophosphides are suitable for use as thermal insulating materials and that their minimum thermal conductivities are anisotropic.     

Dispersion of nano-silicon carbide (SiC) powder in aqueous suspensions

The dispersion characteristics of nanosize silicon carbide (SiC) suspension were investigated in terms of surface charge, particle size, rheological measurement and adsorption study. Ammonium polycarboxylate has been used as dispersant to stabilize the suspension. It was found that the isoelectric point (iep) of SiC powder was pH_iep (4.9). The surface charge of powder changed significantly in presence of the ammonium polycarboxylate dispersant and iep shifted significantly towards lower acidic pH (3.6). The shift in iep has been quantified in terms of ΔG ⁰ _SP, the specific free energy of adsorption between the surface sites and the adsorbing polyelectrolyte (APC). The values of ΔG ⁰ _SP (−10.85 RT unit) estimated by the electro kinetic data compare well with those obtained from adsorption isotherms (−9.521 RT unit). The experimentally determined optimum concentration of dispersant required for maximizing the dispersion was found to be 2.4 mg/g of SiC (corresponding to an adsorbed amount of 1.10 mg/g), at pH 7.5. This is much below the full monolayer coverage (corresponding to adsorbed amount of 1.75 mg/g) of the particles surface by the dispersant. The surface charge quantity, rheological, pH, electro kinetic and adsorption isotherm results were used to explain and correlate the stability of the nanosize silicon carbide in aqueous media. At pH 7.5, where both SiC surface and APC are negatively charged, the adsorption of APC was low because of limited availability of favourable adsorption sites. In addition, the brush-like configuration of the adsorbed polymer prevented close approach of any additional dispersant; hence stabilization of the slurry happens at a comparatively lower concentration than the monolayer coverage.     

二步法大景深反射全息图

以开拓反射全息图的景深表达能力为目的,从反射全息图的共轭物像关系出发,利用反射全息图的均匀介质耦合波理论和布拉格条件,对其白光再现像模糊作了具体的分析和讨论,得到色模糊和线模糊的表达式.指出反射全息图上不同各点对任意白光再现像点的色模糊和线迷糊的影响均存在差异,给出反射全息图白光再现像的景深表达式.实验利用二步法制作了一张景深为83 cm的反射全息图,与大景深彩虹全息图再现像相比,其再现像的立体感更加强烈.理论分析和实验结果表明,光源的再现角度和观察距离对反射全息图的再现像景深大小影响显著.在再现光垂直于反射全息图平面照明情况下,反射全息图具有最好的景深表达能力.     

The stability of a non-extensive relativistic Fermi system

Based on the statistical theory of non-extensive relativity,and using theoretical analysis and numerical simulation,the non-extensive mechanical stability of ultra-relativistic free Fermi gas is investigated.The expressions of the stability conditions under high and low temperatures are given,and the mechanisms of the influences of temperature,ultrarelativistic effect,and non-extensive parameter q on stability are analysed.Our results show that at high temperature and under the condition of q < 1,the stability of a non-extensive system is weaker than that of an extensive system,and the relativistic effect reduces system stability as compared with a non-relativistic system.However,under the condition of q > 1,the stability of the non-extensive system is stronger than that of the extensive system,and the relativistic effect strengthens the system stability as compared with the non-relativistic system.In addition,under the condition of low temperature,the variation of the stability of the non-extensive system with temperature has a turning point.     

High-power high-stability Q-switched green laser with intracavity frequency doubling of diode-pumped composite ceramic Nd:YAG laser

We successfully obtain high-average-power high-stability Q-switched green laser based on diode-side-pumped composite ceramic Nd:YAG in a straight plano-concave cavity. The temperature distribution in composite ceramic Nd:YAG crystal is numerically analyzed and compared with that of conventional Nd:YAG crystal. By use of a composite ceramic Nd:YAG rod and a type-II high gray track resistance KTP (HGTR-KTP) crystal, a green laser with an average output power of 165 W is obtained at a repetition rate of 25 kHz, with a diode-to-green optical conversion of 14.68%, and a pulse width of 162 ns. To the best of our knowledge, both the output power and optical-to-optical efficiency are the highest values for green laser systems with intracavity frequency doubling of this novel composite ceramic Nd:YAG laser to date. The power fluctuation at around 160 W is lower than 0.3% in 2.5 hours.     

Adaptive synchronization of a critical chaotic system

This paper further investigates the synchronization problem of a new chaotic system with known or unknown system parameters. Based on the Lyapunov stability theory, a novel adaptive control law is derived for the synchronization of a new chaotic system with known or unknown system parameters. Theoretical analysis and numerical simulations show the effectiveness and feasibility of the proposed schemes.     

质子交换膜燃料电池动态启动特性的实验研究

实际应用时燃料电池的运行条件会随时间不断地变化,因此是一个非稳态过程.本文研究了在不同的加湿条件以及反应气流量下电池的启动特性.结果表明:随着气体流量的增大,电池启动速度加快;对电池加湿有利于提高电池的启动性能.     

Demonstration of the approximation of eliminating atomic excited populations in an atom-cavity system

Using the master equation approach to a V-type three-level atom inside a high-finesse single-mode cavity in the strong coupling condition,we demonstrate the approximation of eliminating populations of atomic excited states,which is widely used in the field of the atom-cavity systems [Hechenblaikner G,Gangl M,Horak P and Ritsch H 1998 Phys.Rev.A 58 3030];Liu L W,Tan T and Xu Y 2008 J.Mod.Opt.56 968;Cho J,Angelakis D G and Bose S 2008 Phys.Rev.A 78 062338.This is reflected in the deviation of the population δ,of which the value is 10～(-3)～10～(-2).We further find the deviation of the dipole force and demonstrate that the deviation of atomic population will not notably affect the dipole force of the atom in the strong coupling condition.A relevant experimental case is also presented.     

Structural,electronic,and optical properties of hexagonal and triangular SiC NWs with different diameters

Silicon carbide(SiC) is a wideband gap semiconductor with great application prospects,and the SiC nanomaterials have attracted more and more attention because of their unique photoelectric properties.According to the first-principles calculations,we investigate the effects of diameter on the electronic and optical properties of triangular SiC NWs(T-NWs)and hexagonal SiC NWs(H-NWs).The results show that the structure of H-NWs is more stable than T-NWs,and the conduction band bottom of H-NWs is more and more deviated from the valence band top,while the conduction band bottom of T-NWs is closer to the valence band top.What is more,H-NWs and T-NWs have anisotropic optical properties.The result may be helpful in developing the photoelectric materials.