On the energy conservation and critical velocities for the propagation of a "steady-shock" wave in a bar made of cellular material

The propagation of shock waves in a cellular bar is systematically studied in the framework of continuum solids by adopting two idealized material models, viz. the dynamic rigid, perfectly plastic, locking （D-R-PP-L） model and the dynamic rigid, linear hardening plastic, locking （D-R-LHP-L） model, both considering the effects of strain-rate on the material properties. The shock wave speed relevant to these two models is derived. Consider the case of a bar made of one of such material with initial length L 0 and initial velocity v i impinging onto a rigid target. The variations of the stress, strain, particle velocity, specific internal energy across the shock wave and the cease distance of shock wave are all determined analytically. In particular the ＂energy conservation condition＂ and the ＂kinematic existence condition＂ as proposed by Tan et al. （2005） is re-examined, showing that the ＂energy conservation condition＂ and the consequent ＂critical velocity＂, i.e. the shock can only be generated and sustained in R-PP-L bars when the impact velocity is above this critical velocity, is incorrect. Instead, with elastic deformation, strain-hardening and strain-rate sensitivity of the cellular materials being considered, it is appropriate to redefine a first and a second critical impact velocity for the existence and propagation of shock waves in cellular solids. Starting from the basic relations for shock wave propagating in D-R-LHP-L cellular materials, a new method for inversely determining the dynamic stress-strain curve for cellular materials is proposed. By using e.g. a combination of Taylor bar and Hopkinson pressure bar impact experimental technique, the dynamic stress-strain curve of aluminum foam could bedetermined. Finally, it is demonstrated that this new formulation of shock theory in this one-dimensional stress state can be generalized to shocks in a one-dimensional strain state, i.e. for the case of plate impact on cellular materials, by simply making proper replacements of the elastic and plastic constants.

Influences of Stress Wave Propagation upon Studying Dynamic Response of Materials at High Strain Rates

How the wave propagation analysis plays a key role in the studies of dynamic response of materials at high strain rates is analyzed. For the wave propagation technique, the followings are important: the loading and unloading constitutive relation presumed, the positions of the sensors embedded, the interactions between loading waves and unloading waves. For the split Hopkinson pressure bar (SHPB) technique, the assumption of one-dimensional stress wave propagation and the assumption of stress uniformity along the specimen should be satisfied. When the larger diameter bars are employed, the wave dispersion effects should be considered, including the high frequency oscillations, non-uniform stress distribution across the bar section, increase of rise time, and amplitude attenuation. The stress uniformity along the specimen is influenced by the reflection times in specimen, the wave impedance ratio of the specimen and the bar, and the waveform.

Dynamic crushingof cellular materials: a particle velocity-based analytical method and its application

Cellular material under high-velocity impacthas a typical feature oflayer-wise collapse.A cell-based finite element model is employed to simulate the direct impact of closed-cell foam, and one-dimensional velocity field distributionsareobtained to characterize thecrushing bandpropagating through a cellular material. An explicit expression of continuous velocity distribution is derivedbased on the features of velocity gradient distribution. The velocity distribution function is adopted to determine the dynamic stress-strain statesof cellular materials under dynamic loading.The local stress-strain history distribution reveals that sectional cells experience a process from the precursor of elastic behavior to the shock stress state, through the dynamic initial crushing state. A power-law relation between the dynamic initial crushing stress andthe strainrate isestablished, which confirms the strain-rate effect of cellular materials. By extracting the critical points immediately before the unloading stage on the local dynamic stress-strain history curves, the dynamic stress-strain statesof cellular materials are determined. They exhibit loading rate-dependence but are independent of the initial impact velocity.Furthermore, with the increase of relative density, the dynamic hardening behaviorof cellular specimen is enhanced and the crushing process event is advanced. The particle velocity-based analytical method is appliedto analyze the dynamic responses of cellular materials.This method is better than continuum-based shock models, since itdoes not require a pre-assumed constitutive relation.Therefore,the particle velocity-based analytical method proposed in this study may provide new ideas to carry out dynamic experimental measurement, which is especially applicable toinhomogeneous materials.

On the stress–strain states of cellular materials under high loading rates

A virtual Taylor impact of cellular materials is analyzed with a wave propagation technique, i.e. the Lagrangian analysis method, of which the main advantage is that no pre-assumed constitutive relationship is required. Time histories of particle velocity, local strain, and stress profiles are calculated to present the local stress-strain history curves, from which the dynamic stress-strain states are obtained. The present results reveal that the dynamic-rigid-plastic hardening （D-R-PH） material model introduced in a previous study of our group is in good agreement with the dynamic stress-strain states under high loading rates obtained by the Lagrangian analysis method. It directly reflects the effectiveness and feasibility of the D-R-PH material model for the cellular materials under high loading rates.

某些过渡金属钌配合物非线性光学性能的从头算和密度泛函理论研究

实验发现金属钌的某些配合物具有大的非线性光学性能 .我们的理论研究表明金属钌配合物[Ru(NH3 ) 4LDLA]n + (n =2 ,3;LD,LA=吡啶衍生物配体 )的非线性光学性能取决于推电子基团LD 的给电子能力以及拉电子基团LA 的受电子能力 ,LD 和LA 的推拉电子能力越强越有利于提高配合物的二阶非线性光学系数 β,因此带正电荷的LA 能大幅度提高 β值 .虽然增加共轭体系的长度有利于提高 β值 ,但在Ru的配体中 ,吡啶环间或吡啶环与苯环间不一定要保持共面也会有大的β值 .DFT和abinitio方法的计算结果对比表明 ,对于含过渡金属Ru的化合物 ,在HF水平上难以得到满意的结论 ,由从头算有限场方法计算得到的 β值偏小 ,而用TDDFT方法能得到可与实验值符合较好的结果 .

Measurement of Critical Impact Velocity of Copper in Tension

批评影响速度(CIV ) 没有氧高传导性(OFHC ) 铜试验性地在一个煤气的枪系统与一台新奇设备被测量。结果与用典型组成的关系的理论预言相比，并且测量 CIV 价值比预言低得多。在实验并且在理论计算的物理机制的差别被讨论。在实验的 CIV 的减小将与在需要在计算模型被考虑的张力的铜的损坏进化是相关的，这被建议。

Exact thickness-shear resonance frequency of electroded piezoelectric crystal plates

The determination of the precise thickness-shear frequency of electroded crystal plates has practical importance in quartz crystal resonator design and fabrication, especially when the high fundamental thickness-shear frequency has reduced the crystal plate thickness to such a degree that proper consideration of the effect of electrodes is very important. The electrodes effect as mass loading in the estimation of the resonance frequency has to be modified to consider the stiffness of electrodes, as the relative strength is increasingly noticeable. By following a known procedure in the determination of the thickness-shear frequency of an infinite AT-cut crystal plate, frequency equations of crystal plate without and with piezoelectric effect are obtained in terms of elastic constants and the electrode material density. After solving these equations for the usual design parameters of crystal resonators, the design process can be optimized to pinpoint the precise configuration to avoid time-consuming trial and reduction steps. Since these equations and solutions are presented for widely used materials and parameters, they can be easily integrated into the existing crystal resonator design and manufacturing processes.

Dynamic Fracture Behaviors of Selected Aluminum Alloys Under Three-point Bending

The dynamic fracture behaviors of the extruded 2024-T4 and 7075-T6 aluminum alloys are investigated by using an instrumented drop tower machine.The specimens are made from a 25 mm diameter extruded circular rod.The dynamic three-point bending tests of each alloy are carried out at different impact velocities.The initiation fracture toughness and average propagation fracture toughness of 2024-T4 and 7075-T6 are determined at different loading rates.The results show that both the initiation toughness and the propagation toughness increase with the loading rate.Further,the difference between the fracture toughness behaviors of 2024-T4 and 7075-T6 is found to be dependent on the variation of fracture mechanism.The comprehensive fractographic investigations of the fracture surfaces clearly demonstrate that the fracture mode of 2024-T4 is predominantly transgranular fracture with high density small-sized dimples,and the fracture mode of 7075-T6 is mainly intergranular fracture with many intermetallic particles in the bottom of voids located in the fracture surface.

EQUIVALENT MODEL OF EXPANSION OF CEMENT MORTAR UNDER SULPHATE EROSION

The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is results indicate that the model perfectly describes performed. The numerical and experimental the expansion of the cement mortar.

Stress Wave Dispersion in Large-Diameter SHPB and Its Manifold Manifestations

The wave dispersion due to the lateral inertia in the split Hopkinson pressure bar(SHPB) with large-(diameter) bar is numerically analyzed by means of the LS-DYNA3D code. The results show that, ① the stress distribution across the bar section is non-uniform along the radius direction and such non-uniformity depends on the material Poisson ratio and propagation distance; ② with increasing the bar diameter, the high frequency oscillations are notably enhanced and the rise time of wave front becomes longer, meanwhile the amplitude of the stress wave attenuates; ③ with decreasing the rise time of wave front, the wave dispersion markedly enhanced, particularly in the large diameter bar. All of those effects should not be neglected in order to obtain accurate results by the SHPB test..

Determination of the transport properties in 4H-SiC wafers by Raman scattering measurement

The free carrier density and mobility in n-type 4H-SiC substrates and epilayers were determined by accurately analysing the frequency shift and the full-shape of the longitudinal optic phono-plasmon coupled （LOPC） modes, and compared with those determined by Hall-effect measurement and that provided by the vendors. The transport properties of thick and thin 4H-SiC epilayers grown in both vertical and horizontal reactors were also studied. The free carrier density ranges between 2× 10^18 cm^-3 and 8× 10^18 cm^-3with a carrier mobility of 30-55 cm2/（V.s） for ntype 4H-SiC substrates and 1× 10^16 -3× 10^16 cm^-3 with mobility of 290-490 cm2/（V.s） for both thick and thin 4H-SiC epilayers grown in a horizontal reactor, while thick 4H-SiC epilayers grown in vertical reactor have a slightly higher carrier concentration of around 8.1×10^16 cm^-3 with mobility of 380 cm2/（V.s）. It was shown that Raman spectroscopy is a potential technique for determining the transport properties of 4H-SiC wafers with the advantage of being able to probe very small volumes and also being non-destructive. This is especially useful for future mass production of 4H-SiC epi-wafers.

Mechanical Yielding and Strength Behaviour of OFHC Copper in Planar Shock Waves

It is necessary to study the validation of strength models under planar shock loading in view of the fact that strength models for metals obtained at moderate strain rates are often used in the numerical simulations of shock wave phenomena. The variations of longitudinal stress, transverse stress and yield strength of oxygen-free high conductance （OFHC） copper with time under planar shock loading are obtained by using the manganin stress gauges and compared with the predicted results by the constructed seven constitutive models based on Y/G=constant and on G/B=constant （Y the yield strength, G the shear modulus, B the bulk modulus）, respectively. It seems that the pressure, density, temperature and plastic strain dependence of the yield strength for OFHC copper under planar shock loading is essential to the constitutive description.

Experimental Study on Common and Steel Fiber Reinforced Concrete Under Dynamic Tensile Stress

Split Hopkinson technique has been developed to test the strength of common concrete and steel fiber reinforced concrete under dynamic tensile stress. Two types of test methods are considered, the splitting tensile test and a modified spalling test in which a specimen is loaded under uniaxial stress. The result shows that the dynamic strength enhancement of concrete is remarkable by using the reinforcing fiber. But for the common concrete, the base of compressive strength seems to show little effect on the tensile strength under dynamic loading. The experimental results also show that the resistance to tensile fracture of the steel fiber reinforced concrete for C100-mix is higher than those of C40-mix..

Effect of CO on Characteristics of AlGaN/GaN Schottky Diode

为公司的磅 Schottky 二极管气体传感器用 AlGaN/ 被制作轧了高电子活动性晶体管(HEMT ) 结构。二极管显示出一个显著传感器信号(3 妈，在 N-2；在空中的 2mA 周围) 在 1% 公司以后的偏导的 2V 在 50 度 C 被介绍。Schottky 障碍高度分别地在二个盒子中为 36meV 和 27meV 减少。设备展览在周围的空中，但是几乎的一个慢恢复特征没有在纯 N2，它揭示那氧的背景，分子能加速公司和提议限制的解吸附作用到公司察觉。

Multi-wafer 3C—SiC heteroepitaxial growth on Si(100) substrates

Epitaxial growth of semiconductor films in multiple-wafer mode is under vigorous development in order to improve yield output to meet the industry increasing demands. Here we report on results of the heteroepitaxial growth of multi- wafer 3C-SiC films on Si（100） substrates by employing a home-made horizontal hot wall low pressure chemical vapour deposition （HWLPCVD） system which was designed to be have a high-throughput, multi-wafer （3×2-inch） capacity. 3C-SiC film properties of the intra-wafer and the wafer-to-wafer including crystalline morphologies, structures and electronics are characterized systematically. The undoped and the moderate NH3 doped n-type 3C-SiC films with specular surface are grown in the HWLPCVD, thereafter uniformities of intra-wafer thickness and sheet resistance of the 3C-SiC films are obtained to be 6%-7% and 6.7%~8%, respectively, and within a run, the deviations of wafer-to- wafer thickness and sheet resistance are tess than 1% and 0.8%, respectively.

Studies on the Coefficient of Friction During the Oblique Impact of a Hard Sphere Against a Ductile Solid Target

Following the original approach of Bowden and Tabor and introducing state variables, an effective friction coefficient μ_e for solid particle erosion is defined as a combination of shearing term and ploughing term. In the case of continuous sliding, based on considering the interaction between asperities under certain condition, it is indicated that during the oblique impact of a hardened steel sphere against a mild steel target, a possible value of μ_e is 0.05, which was chosen in all of the calculations by Hutchings for consistency with both experiments and calculations. In the case of continuous ploughing, it is shown that the value of μ_e is a function of the impact process and the initial impact angle and is greater than 0.05 on an average for Hutchings' experiments. It is suggested that the variation of sliding, rolling and ploughing state at each instant in the impact process makes “the coefficient of friction” equal to 0.05 for Hutchings' experiments, and in general, makes the effective friction coefficient during particle impact on metal far less than the friction coefficient during simple continuous sliding on an average.

Synthesis and Characterization of Nickel(II) Complex Bearing 2,9-di-tert-butyl-1,10-phenanthroline Ligand

The complex DtbpNiCl2（Dtbp = 2,9-di-tert-butyl-1,10-phenanthroline） was synthe- sized and characterized by X-ray single-crystal structure analysis. For the complex： C20 H24 Cl2 N2 Ni CH2 Cl2, Mr = 506.95, monoclinic, space group P21 /c, a = 9.5905（3）, b = 13.7587（3）, c = 17.3364（5）, β = 94.244（2）°, V = 2281.31（11）3, Z = 4, Dc = 1.476 g/cm3, λ = 1.54184, μ = 5.606 mm-1, F（000） = 1048, S = 1.079, R = 0.0402 and wR = 0.1010 for 3223 observed reflections with I 〉 2σ（I）. In complex DtbpNiCl2, the nickel adopts a distorted tetrahedral geometry coordinated by two nitrogen atoms of Dtbp and two chlorine ions. The complex is connected by intermolecular C–H…Cl hydrogen bonds to form a 1D structure in the solid state.

Microwave-assisted synthesis of CePO_4 nanorod phosphor with violet emission

Monochnic and hexagonal CePO4 nanoparticles and nanorods were successfully synthesized from Ce（NO3）36H2O and Na3PO4 121-120 solu- tions at pH 1-5 by a 180 W microwave radiation for 60 min. The products were characterized by X-ray diffraction （XRD）, Fourier transform infrared （FFIR） spectroscopy, and scanning electron microscopy （SEM）. XRD patterns revealed that the products are hexagonal CePO4 structures at pH 2-5, and monoclinic CePO4 structtLres at pH 1. SEM characterization shows that these products were nanoparticles, short nanorods, and long nanorods, controlled by the pH of the precursor solutions. Optical properties of the nanorods were also investigated by ultraviolet-visible （UV-vis） and photoluminescence （PL） spectroscopy.

A Novel Impact Tension Testing for OFHC Copper Bars under Local Strain Controlled

有特殊设备的一台新奇高速度的张力的设备(HSTF ) 被使用研究动态失败特征没有氧高传导性(OFHC ) 铜在紧张的不同层次锁。试验性的测试数字地被模仿包含空进化。在与调整 JohnsonCook 计算的 necking 区域的局部性的紧张当模特儿，这被显示， ZerilliArmstrong 模型与试验性的结果历久不渝。