Effects of electron heating and surface rippling on Rayleigh–Taylor instability in radiation pressure acceleration

The acceleration of ultrathin targets driven by intense laser pulses induces Rayleigh–Taylor-like instability.Apart from laser and target con-figurations,we find that electron heating and surface rippling,effects inherent to the interaction process,have an important role in instability evolution and growth.By employing a simple analytical model and two-dimensional particle-in-cell simulations,we show that the onset of electron heating in the early stage of the acceleration suppresses the growth of small-scale modes,but it has little influence on the growth of large-scale modes,which thus become dominant.With the growth of surface ripples,a mechanism that can significantly influence the growth of these large-scale modes is found.The laser field modulation caused by surface rippling generates an oscillatory ponderomotive force,directly modulating transverse electron density at a faster growth rate than that of ions and eventually enhancing instability growth.Our results show that when surface deformation becomes obvious,electron surface oscillation at 2ω0(whereω0 is the laser frequency)is excited simultaneously,which can be seen as a signature of this mechanism.

广泛耐药革兰阴性菌感染的实验诊断、抗菌治疗及医院感染控制:中国专家共识

广泛耐药革兰阴性杆菌(XDR-GNB),是指除1~2类抗菌药物(主要指多黏菌素和替加环素)外,几乎对所有类别抗菌药物均不敏感的革兰阴性杆菌。XDR-GNB常见于肠杆菌科细菌、鲍曼不动杆菌、铜绿假单胞菌和嗜麦芽窄食单胞菌等。近年来,中国XDR-GNB的发生率呈不断上升趋势,由于缺乏有效的治疗药物,XDR-GNB感染成为公共卫生安全的一大威胁。我国有关感染的临床、微生物学及临床药理学专家们就XDR-GNB感染的实验室诊断、临床诊疗和医院感染控制等问题深入讨论,形成此共识。其中细菌药敏试验的抗菌药物品种及结果判定标准遵循美国临床和实验室标准化协会(CLSI)、欧洲抗菌药物敏感性试验委员会(EUCAST)或美国食品与药物监督管理局(FDA)的指南。研究提示长疗程使用广谱抗菌药物是引发XDR-GNB感染最重要的危险因素。根据现有的临床研究和实验室数据,共识提供治疗各种XDRGNB感染的联合用药推荐方案,其中常用的抗菌药物有替加环素、多黏菌素、碳青霉烯类、氨基糖苷类和磷霉素等。同时建议实行严格的感染控制措施,包括手卫生、接触隔离、主动筛查、环境消毒、去定植和抗菌药物应用管理等遏制XDRGNB感染的传播。

Microstructures and mechanical properties of the age hardened Mg-4.2Y-2.5Nd-1Gd-0.6Zr(WE43)microalloyed with Zn

The effect of trace addition of 0.2 wt.%Zn on the microstructures and mechanical properties of the age-hardening Mg-4.2Y-2.5Nd-1Gd-0.6Zr(wt.%)(WE43)alloy has been investigated.As compared with the WE43 alloy after solid solution treatment at 525 ℃,the block-like Zn-Zr phase was still observed in the WE43-0.2Zn alloy.However,the time for WE43-0.2Zn alloy to get peak hardness at 250 ℃ was two hours,a half earlier than that in WE43 alloy,meaning a accelerated age precipitation kinetics has been achieved due to the addition of 0.2 wt.%Zn.Microalloyed with 0.2 wt.%Zn enhanced the ultimate tensile strength(UTS)slightly and ductility significantly both in the solutionized and peak aged condition.The enhancement in strength and ductility is possible associated with the larger volume fraction of precipitation phases due to a reduction of the solubility of rare earth elements(RE)in theα-Mg matrix,the larger aspect ratio(length to width)of precipitates and a decrease in stacking fault energy by addition of Zn.

Ductility enhancement of EW75 alloy by multi-directional forging

In this study,the Mg-7Gd-5Y-1Nd-0.5Zr alloy can reach a high ductility by the process of multi-directional forging,and the evolution of the microstructure,texture and the mechanical properties were discussed systematically.The results show that after the solutionized sample was multi-forged at 500℃,its grain size can be refined from 292 um to 58 um.As the forging temperature decreased,fine particles precipitated in the matrix.The volume fraction of the particles increased with the forging temperature decreasing,so the nucleation and growth of crystallization were strongly restricted.There was no recrystallization as the forging temperature fell to 410℃,and the severe deformed grains distributed as streamlines perpendicular to the final compression axis.The texture intensity decreased with increasing forging passes.The sample with best ductility was obtained after compressed at 470℃,with an elongation to failure of 21%at room temperature,which is increased by 200%,in comparison with that of the samples in solutionized condition.EBSD results revealed that the mean grain size was 15 um.Refined grains as well as the weakened texture were the key factors to its high ductility.

Effect of heat treatment on the microstructures and mechanical properties of the sand-cast Mg-2.7Nd-0.6Zn-0.5Zr alloy

The tensile testing bars of the Mg-2.7Nd-0.6Zn-0.5Zr(wt.%)alloy were prepared by sand casting.The effect of solution temperature and aging time on the microstructures and mechanical properties were investigated.The as-cast alloy was composed ofαmagnesium matrix and Mg12Nd eutectic compounds.After solution treatment at 500℃ for 18 h,the volume fraction of eutectic compounds decreased from∼7.8%to∼2.3%,and some small Zr-containing particles were observed to precipitate at grain interiors.As the solution temperature increased to 525℃ for 14 h,most of the eutectic compounds dissolved into the matrix.Peak-aged at 200℃ for 12 h,fineβ″particles was the dominant strengthening phase.The yield strength,ultimate tensile strength and elongation in the peak-aged condition were 191 MPa,258 MPa and 4.2%,respectively.Moreover,the Mg-2.7Nd-0.6Zn-0.5Zr alloys under different heat treatment conditions exhibited different tensile fracture modes.

EFFECTS OF TEST TEMPERATURE AND STRAIN RATE ON THE MECHANICAL PROPERTIES IN AN INTERCRITICALLYHEAT-TREATED BAINITE-TRANSFORMED STEEL

Larger amount of austenite could be retained in an intercritically heat-treated bainite- transformed steel. The elongation and the strength-ductility balance of the steel could be enhanced considerably due to strain-induced martensite transformation and transformation- induced plasticity (TRIP) of retained austenite. The effects of test temperature and strain rate on the mechanical properties and strain induced transformation behavior of retained austenite in the steel were investigated. Total elongation and strength-ductility balance of the specimen reached maximum when it strained at a strain rate of 2.8×10-4s-1 and at 350℃. The relation between test temperature and tensile properties showed the same tendency at three kinds of strain rates. Flow stress increased considerably with decreasing the strain rate.

Characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids: The role of bremsstrahlung and radiation reactions

In this work, characteristics of X/γ-ray radiations by intense laser interactions with high-Z solids are investigated by means of a newlydeveloped particle-in-cell (PIC) simulation code. The PIC code takes advantage of the recently developed ionization and collision dynamicsmodels, which make it possible to model different types of materials based on their intrinsic atomic properties. Within the simulations, bothbremsstrahlung and nonlinear Compton scatterings have been included. Different target materials and laser intensities are considered forstudying the parameter-dependent features of X/γ-ray radiations. The relative strength and angular distributions of X/γ ray productions frombremsstrahlung and nonlinear Compton scatterings are compared to each other. The threshold under which the nonlinear Compton scatteringsbecome dominant over bremsstrahlung is also outlined.

High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

This paper investigated the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy with different stress concentration factor(Kt),under different stress ratio(R),and along different loading direction.The smooth specimen(Kt=1),under R=0.1 and along longitude direction,shows a high fatigue strength of 162 MPa at 107 cycles.The fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy exhibits a high sensitive to the notch.Moreover,change of stress ratio from 0.1 to−1 may also result in a bad fatigue property.The flux inclusions were elongated along longitude direction and/or transverse direction during the forging process of the Mg-7Gd-5Y-1Nd-0.5Zr alloy.The interface between the flux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,leading to the deterioration of the fatigue property along thickness direction and a high anisotropic fatigue behavior between longitude direction and thickness direction.

Proton beams from intense laser-solid interaction:Effects of the target materials

We report systematic studies of laser-driven proton beams produced with micrometer-thick solid targets made of aluminum and plastic,respectively.Distinct effects of the target materials are found on the total charge,cutoff energy,and beam spot of protons in the experiments,and these are described well by two-dimensional particle-in-cell simulations incorporating intrinsic material properties.It is found that with a laser intensity of 8×10^(19) W/cm^(2),target normal sheath acceleration is the dominant mechanism for both types of target.For a plastic target,the higher charge and cutoff energy of the protons are due to the greater energy coupling efficiencies from the intense laser beams,and the larger divergence angle of the protons is due to the deflection of hot electrons during transport in the targets.We also find that the energy loss of hot electrons in targets of different thickness has a significant effect on the proton cutoff energy.The consistent results obtained here further narrow the gap between simulations and experiments.

Investigation of Optimal Split Ratio of 6-Slot/2-pole High Speed Permanent Magnet Motor with Toroidal Winding

Split ratio,i.e.the ratio of stator inner diameter to outer diameter,has a closed relationship with electromagnetic performance of permanent magnet(PM)motors.In this paper,the toroidal windings with short end-winding axial length are employed in the 6-slot/2-pole(6s/2p)PM motor for high speed applications.The split ratio is optimized together with the ratio of inner slot to outer slot area,i.e.slot ratio,considering stator total loss(stator iron loss and copper loss).In addition,the influence of maximum stator iron flux density and tooth-tip on the optimal split ratio,slot ratio,and average torque is investigated.The analytical predictions show that when the slot ratio is 0.5,the maximum torque can be achieved,and the optimal split ratio increases with the decrease of slot ratio,as confirmed by the finite element(FE)analyses.Finally,some of predicted results are verified by the measured results of 6s/2p prototype motor with 0.5 slot ratio.

A negative deviation from Porod’s law in SAXS of organo—MSU—X

Small-angle X-ray scattering(SAXS)using synchrotron radiation as X-ray source has been employed to charactcizc the microscopic structrure of organo-modified mesoporous molecular sieves(organo-MSU-X)prepared by a one-pot template-directed synthesis.It is shown that the SAXS profile is hardly constant with Porod’s law showing a negative slope,i.e.,negative deviation.This suggests that there is diffuse interfacial layer located between the pores and the matrix.This suggests that the organic groups remain covalently linked to the matrix,as indicated by ^29SiCP MAS NMR and FT-IR.The average thickness of the interfacial layer was found to be about 1nm for each of the three samples with different kinds and the same amounts(20? of organic groups.This kind of material has also been proved to possess both surface and mass fractal structure of the amophous porous silica materials.2001 Elsevier Science B.V.All rights reserved.

Classical imaging with undetected photons using four-wave mixing in silicon core fibers

Undetected-photon imaging allows for objects to be imaged in wavelength regions where traditional components are unavailable. Although first demonstrated using quantum sources, recent work has shown that the technique also holds with classical beams. To date, however, all the research in this area has exploited parametric downconversion processes using bulk nonlinear crystals within free-space systems. Here, we demonstrate undetectedphoton-based imaging using light generated via stimulated four-wave mixing within highly nonlinear silicon fiber waveguides. The silicon fibers have been tapered to have a core diameter of915 nm to engineer the dispersion and reduce the insertion losses, allowing for tight mode confinement over extended lengths to achieve practical nonlinear conversion efficiencies(-30 dB) with modest pump powers(48 m W). Both amplitude and phase images are obtained using classically generated light, confirming the high degree of spatial and phase correlation of our system. The high powers(>10 nW) and long coherence lengths(>4 km) associated with our large fiber-based system result in high contrast and stable images.

Enhanced tensile properties in a Mg-6Gd-3Y-0.5Zr alloy due to hot isostatic pressing(HIP)

Hot isostatic pressing (HIP) was applied to Mg-6Gd-3Y-0.5Zr (GW63) alloy to reduce shrinkage porosity, thus, to enhance the integrity and reliability of castings. During HIP process, shrinkage porosity was closed by grain compatible deformation and subsequent diffusion across the bonding interface. The amount of initial shrinkage porosity was the key factor for shrinkage porosity closure. HIP was testified to be effective on shrinkage porosity reduction in GW63 alloy due to its relatively narrow solidification range and resultant low content of initial shrinkage porosity in most sections, leading to higher tensile properties both in as-cast and cast-T6 condition. The improvement in tensile properties was mainly because of shrinkage porosity reduction and resultant effective rare-earth (RE) elements homogenization and precipitation strengthening.

The microstructure evolution and tensile properties of medium-Mn steel heat-treated by a two-step annealing process

The martensitic hot-rolled 0.3 C-6 Mn-1.5 Si(wt%)steel was annealed at 630℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃for 10 min.The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines.A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally,proved the above process is feasible."Austenite memory"was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite,leading to lath-shaped morphology of austenite and ferrite grains."Austenite memory"disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization,resulting in globular microstructure and refinement of both austenite and ferrite grains.The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation,but the dislocation strengthening played a decisive role on the yielding behavior.The tensile curves change from continuous to discontinuous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement.A method by controlling the cold-rolled reduction is proposed to avoid the Lüders strain.

Dynamic Recrystallization and Texture Evolution of GW94 Mg Alloy During Multi-and Unidirectional Impact Forging

Multi- and unidirectional impact forgings were successfully applied to a （GW94） Mg-RE alloy. The microstructure and texture evolution were investigated systematically. The obtained results indicated that during unidirectional impact forging, a bimodal chain deform microstructure was sustained till last forging pass, whereas { 10-12} extension twins-assisted continuous dynamic recrystallization took place during the multidirectional impact forging （MDIF）. The coalescence and intersection of {10-12} extension twins during MDIF efficiently refined the original coarse grains and led to an almost recrystallized homogeneous microstructure. The texture analysis demonstrated that unidirectional impact forging yielded out the strong basal texture; however, MDIF resulted in non-basal texture, which was attributed to the cooperative effects of continuous DRX, twinning, and MDIF itself during the deformation process.

Effect of solute atom concentration and precipitates on serrated flow in Mg-3Nd-Zn alloy

Influence of solute atom concentration and precipitates on serrated flow, i.e., Portevin-Le Chatelier effect, was studied in Mg-3Nd-Zn alloy by tensile test at 250℃ with a strain rate of l x 10^-3 s^-1 Microstructure and tensile property of the Mg-3Nd-Zn alloy in solution and aging conditions were also investigated. Results indicate that the serrated flow was weakened with aging time, and geometry of the serrations changed from sharp to rounded corner. Through analyzing the mechanism of the interactions between dislocations and solute atoms, it was identified that the precipitates did not only weaken the serrated flow due to the decrease in the concentration of solute atom, but also regulate the serration type by restraining the movement of dislocations during high temperature deformation.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.

Microstructure and texture optimization by static recrystallization originating from {10-12} extension twins in a Mg-Gd-Y alloy

During the deformation of Mg alloys,{10-12} extension twin often contributes to the formation of basal texture but rarely assists the nucleation of recrystallization,i.e.,effective grain refinement,therefore it seems to make against the improvement of formability and mechanical properties.In this work,{10-12} extension twin has been creatively utilized as a preference nucleation site for static recrystallization(SRX),achieving grain refinement and orientation randomization in a Mg-Gd-Y alloy using multi-directional impact forging(MDIF)and subsequent annealing treatment.Effect of {10-12} extension twin on SRX behavior has been investigated by annealing treatment at 450℃ using quasi-in-situ optical microscopy(OM)and quasi-in-situ electron back-scattering diffraction(EBSD).The microstructural evolution during annealing shows that several SRX gains can nucleate from the grain boundary of untwinned grains,but they only have few influences on the final microstructure due to their limited volume faction and sluggish growth.In contrast,a large number of SRX gains can initiate from {10-12} extension twin and grow up without the confine of twin boundaries.Finally,they consume their parent grains and make the main contribution to grain refinement.This should be attributed to those pinned {10-12} twin boundary,by interacting with various dislocation slips during the MDIF process,which can operate like grain boundary,store enough strain energy,and promote the nucleation of SRX during annealing.On the other hand,SRX grains usually keep initial random orientation and further randomize the forging texture during annealing treatment.

On-Load Voltage Distortion Compensation Method Using Disturbance Observer for SPM Machines with Closed Slot

This paper proposes an on-load voltage distortion compensation method,in which the voltage distortion is mainly produced in the surface-mounted permanent magnet machines having closed slot.Apart from the voltage distortions caused by the inverter,such as dead time and inverter non-linearity,the on-load voltage distortion has relatively large magnitude dependent of machine speed.As a result,it causes a significant distortion of current waveforms and deteriorates control performance.Hence,a voltage disturbance compensation method using a disturbance observer is proposed and implemented.Based on the machine model,the disturbance observer is chosen which helps to observe a voltage disturbance,while the error between measured and estimated currents is minimized.The disturbance observer only compensates the sixth-multiple harmonic components of the inverter voltage in synchronous reference frame.The harmonic compensator with multiple narrow-band pass based adaptive filter is proposed and implemented in the feedback loop with current controllers.The observed voltage disturbances are compensated to the reference voltages.In addition,robust performance of the proposed method is shown against the parameter uncertainties.The effectiveness of the proposed compensation scheme is validated by the experiments.