Characterisation of a laser droplet formation process by acoustic emission

The aim of this article is to describe an application of acoustic emission to characterise a process of laser droplet formation from a metal wire. Laser droplet formation is a crucial process in new laser droplet welding technology, where parts are joined by means of the heat content of a liquid metal droplet deposited onto the parts to be joined. A laser beam is used for heating and melting the wire tip, and for detaching the molten pendant droplet. Depending on the process parameters, three different outcomes of the process can be observed: (1) no droplet formed; (2) a droplet formed but not detached; (3) a droplet formed and detached from the wire. It is shown that AE can be used to monitor the process and to indicate the different process outcomes.     

ITER用内锡法Nb3Sn超导线材的不可逆温度研究

本文采用超导量子干涉仪(SQUID)测定了一种国际热核聚变实验堆(ITER)项目用内锡法Nb3Sn超导线材的不可逆温度,测量方法是在恒定磁场下循环温度,即将温度以一定间隔从10K上升到20K,然后再返回到10K,测定磁矩的偏离温度.所得结果可用于从生成最佳Nb-Sn相组成方面来优化A15相成相热处理制度.本研究得出的结论是,对于像ITER使用的高场磁体超导线来说,鉴于其需要在较高磁场下有高的临界电流密度,就需要将超导线的热处理温度适当提高一些.本实验所用Nb3Sn超导线材的最适宜热处理制度为675°C/128小时,这样可以得到最佳不可逆温度特性,即最佳的A15相组成.     

Characterization of laser droplet formation by reflected light

The subject of this experimental investigation is a novel laser droplet formation (LDF) process. During the LDF process, droplets are formed by melting the tip of a wire using a laser pulse. The goal of our research is to develop a non-contact method for characterization of the LDF process. For this purpose we employ the signals of the laser light that is reflected from the tip of the wire. The experiments were performed with nickel and tin-alloy wire. Results show that in-process characterization of various phases of the LDF process, like melting of the wire tip and formation of a molten pendant droplet is possible regardless of the wire material. In addition, a method for post-process detection of droplet detachment has been developed. Using the statistically defined threshold value, 99.2% and 97.8% detection reliability of droplet detachment was achieved in the case of nickel and tin-alloy wire, respectively.     

Mechanism of microweld formation and breakage during Cu-Cu wire bonding investigated by molecular dynamics simulation

Currently,wire bonding is the most popular first-level interconnection technology used between the die and package terminals,but even with its long-term and excessive usage,the mechanism of wire bonding has not been completely evaluated.Therefore,fundamental research is still needed.In this study,the mechanism of microweld formation and breakage during Cu-Cu wire bonding was investigated by using molecular dynamics simulation.The contact model for the nanoindentation process between the wire and substrate was developed to simulate the contact process of the Cu wire and Cu substrate.Elastic contact and plastic instability were investigated through the loading and unloading processes.Moreover,the evolution of the indentation morphology and distributions of the atomic stress were also investigated.It was shown that the loading and unloading curves do not coincide,and the unloading curve exhibited hysteresis.For the substrate,in the loading process,the main force changed from attractive to repulsive.The maximum von Mises stress increased and shifted from the center toward the edge of the contact area.During the unloading process,the main force changed from repulsive to attractive.The Mises stress reduced first and then increased.Stress concentration occurs around dislocations in the middle area of the Cu wire.     

Understanding the mechanism of nano-aluminum particle formation by wire explosion process using optical emission technique

Nano-aluminum particles are produced by a wire explosion process in different inert gas ambience. It is observed that generated particles have different sizes and it follows log–normal probability distribution. The particle size produced by the wire explosion process varies depending on the thermal conductivity of the medium and the operating pressure of the gas. To understand the mechanism of nano-particle formation, the optical-emission spectroscopic technique is used for measuring characteristics of plasma generated during the wire explosion process. Strong emission lines were observed from the species formed during the wire explosion process. Plasma temperatures are estimated based on local thermal equilibrium principle and using Al emission lines. Plasma temperature of more than 8000 K is observed in an Ar ambient. The optical emission study clearly indicates that the intensity of plasma increases with an increase in the ambient pressure. Further, it is observed that an increase in the pressure of the gas, the plasma temperature also increases. The study shows that the plasma temperature in the He gas is lesser than in the Ar gas. The plasma temperature due to the discharge plays a significant role on nano-particle formation. In addition, it is observed that irrespective of polarity, emission characteristics are almost the same.     

Electron transport across a quantum wire in the presence of electron leakage to a substrate

We investigate electron transport through a mono-atomic wire which is tunnel coupled to two electrodes and also to the underlying substrate. The setup is modeled by a tight-binding Hamiltonian and can be realized with a scanning tunnel microscope (STM). The transmission of the wire is obtained from the corresponding Green’s function. If the wire is scanned by the contacting STM tip, the conductance as a function of the tip position exhibits oscillations which may change significantly upon increasing the number of wire atoms. Our numerical studies reveal that the conductance depends strongly on whether or not the substrate electrons are localized. As a further ubiquitous feature, we observe the formation of charge oscillations.     

Formation of extended directional breakdown channels produced by a copper wire exploding in the atmosphere

Experimental data for switching initiated by the electrical breakdown of air gaps up to 1.9 m long with an arbitrary geometry that are produced by an exploding copper wire 90 μm in diameter are presented. At an initial voltage of 11 kV, the stored energy equals 100–2100 J. Two channel formation conditions are possible: explosion of a wire without electrical breakdown and electrical breakdown in a channel produced by an exploding wire with a delay (current pause) no longer than 250 μs. Current and voltage waveforms across the discharge gap, as well as the resistivity values, under the electrical breakdown conditions are shown. Mechanisms and conditions for streamer initiation at a mean electric field strength in the discharge gap of 5.3–17.0 kV/m are discussed. The geometrical dimensions of plasma objects in the forming channel, the run of the electrical current under breakdown, and the formation mechanism of wire explosion products are found from color microphotographs. The formation mechanism of large aerosols in the form of tiny spherical copper and copper oxide (CuO, Cu₂O) particles under wire explosion conditions is discussed.     

Numerical simulation of the initial plasma formation and current transfer in single-wire electrical explosion in vacuum

In this paper, a computational model is constructed to investigate the phenomenon of the initial plasma formation and current transfer in the single-wire electrical explosion in a vacuum. The process of the single-wire electrical explosion is divided into four stages. Stage Ⅰ: the wire is in solid state. Stage Ⅱ: the melting stage. Stage Ⅲ: the wire melts completely and the initial plasma forms. Stage IV: the core and corona expand separately. The thermodynamic calculation is applied before the wire melts completely in stages Ⅰ and Ⅱ. In stage Ⅲ, a one-dimensional magnetohydrodynamics model comes into play until the instant when the voltage collapse occurs. The temperature, density, and velocity, which are derived from the magnetohydrodynamics calculation, are averaged over the distribution area. The averaged parameters are taken as the initial conditions for stage Ⅳ in which a simplified magnetohydrodynamics model is applied. A wide-range semi-empirical equation of state, which is established based on the Thomas-Fermi-Kirzhnits model, is constructed to describe the phase transition from solid state to plasma state. The initial plasma formation and the phenomenon of current transfer in the electrical explosion of aluminum wire are investigated using the computational model. Experiments of electrical explosion of aluminum wires are carried out to verify this model. Simulation results are also compared with experimental results of the electrical explosion of copper wire.     

New method of creation of a rearrangeable local Coulomb potential profile and its application for investigations of local conductivity of InAs nanowires

We present a new technique to create a reconfigurable Coulomb potential profile. The potential profile on the sample surface covered with residual polymethyl methacrylate (PMMA) layer as charge accumulation substance is performed with a low DC voltage applied to conductive probe tip of scanning microscope. To characterize the resulted Coulomb potential profile Kelvin probe technique is used. The effectiveness of this method is demonstrated by performing measurements of the local conductivity of InAs quantum wires. These investigations revealed an inhomogeneous conductivity of the wires and the formation of a potential barrier in the wire at the contact pad interface when the electronic system of the wire is depleted.     

Z箍缩丝阵烧蚀的简单模型数值模拟

丝阵烧蚀在丝阵Z箍缩动力学中扮有重要角色,透彻理解丝阵烧蚀对于丝阵Z箍缩动力学的研究至关重要。采用简单模型数值模拟了烧蚀过程,该模型物理上采用绝热、单流体、单相、磁流体力学近似,数值上采用欧拉法结合保单调输运格式来离散。数值结果很好地展示了烧蚀过程的主要特征,例如芯-冕结构、径向等离子体流、轴线上先驱等离子体柱的形成、静止丝芯的长寿命以及延长的烧蚀期,一些关键时间参数定量或定性地相符于文献报道的实验和数值结果,例如冕等离子体角向融合的时刻、等离子体首次到达轴线的时刻、丝芯静止持续的时间、最后的箍缩滞止时刻,但某些方面略有差异。     

X-ray backlighting of two-wire Z-pinch plasma using X-pinch

Two 50-μm Mo wires in parallel used as a Z-pinch load are electrically exploded with a pulsed current rising to 275 kA in 125 ns and their explosion processes are backlighted using an X-pinch as an x-ray source.The backlighting images show clearly the processes similar to those occurring in the initial stages of a cylindrical wire-array Z-pinch,including the electric explosion of single wires characterised by the dense wire cores surrounded by a low-density coronal plasma,the expansion of the exploding wire,the sausage instability (m=0) in the coronal plasma around each wire,the motion of the coronal plasma as well as the wire core toward the current centroid,the formation of the precursor plasma column with a twist structure something like that of higher mode instability,especially the kink instability (m=1).     

丝阵负载Z箍缩内爆动力学研究

丝阵负载内爆动力学行为基本可以分为以下四个过程:1)丝的烧蚀;2)壳层的形成;3)内爆;4)滞止.利用所研制的可见光分幅相机和X射线分幅相机在"强光一号"加速器上对多种型号的丝阵负载Z箍缩内爆动力学行为进行了实验研究,获得了从早期单丝烧蚀到等离子体柱崩毁全过程图像,并对实验结果进行了分析,主要研究成果如下:1)发现存在较长时间的丝烧蚀过程,且单丝烧蚀在轴向上并不均匀;实验得到的内爆轨迹与唯像模型计算结果较为一致.2)不论早期的可见光图像还是中后期的软X射线图像都存在明显的阴极发射,内爆后期在阴极附近存在明 关键词： 丝阵 内爆动力学 Z箍缩 脉冲功率技术     

Compressibility measurements of quasi-one-dimensional quantum wires

We report measurements of the compressibility of a one-dimensional quantum wire, defined in the upper well of a GaAs/AlGaAs double quantum well heterostructure. A wire defined simultaneously in the lower well probes the ability of the upper wire to screen the electric field from a biased surface gate. The technique is sensitive enough to resolve spin splitting of the subbands in the presence of an in-plane magnetic field. We measure a compressibility signal due to the 0.7 structure and study its evolution with increasing temperature and magnetic field. We see no evidence of the formation of the quasibound state predicted by the Kondo model, instead our data are consistent with theories which predict that the 0.7 structure arises as a result of spontaneous spin polarization.     

Meissner effect in superconducting microtraps

We report on the realization and characterization of a magnetic microtrap for ultracold atoms near a straight superconducting Nb wire with circular cross section. The trapped atoms are used to probe the magnetic field outside the superconducting wire. The Meissner effect shortens the distance between the trap and the wire, reduces the radial magnetic-field gradients, and lowers the trap depth. Measurements of the trap position reveal a complete exclusion of the magnetic field from the superconducting wire for temperatures lower than 6 K. As the temperature is further increased, the magnetic field partially penetrates the superconducting wire; hence the microtrap position is shifted towards the position expected for a normal-conducting wire.     

Moderate micro-explosion during the combustion of iron wire in atmospheric air

The combustion of a thin iron wire in atmospheric air was investigated through the in-situ high-speed videography and the sampled product characterization. During the iron wire burning, a hot spherical ball was found to form at the wire tip and propagate with a speed of around 3.5 cm/s along the wire. Meanwhile, small bright droplets were randomly ejected from the burning ball. In fact, the burning spherical ball is hollow to be a ferric bubble according to the SEM image of sampled product. The high-speed videography further indicates that the small droplet ejection occurs largely owing to the bubble bursting, that is called the moderate micro-explosion to be different from the conventional micro-explosion process. For this bubble bursting process, the volume expansion rate inside the burning bubble needs not to be very fast but the bubble size should be adequate. If the bubble is too small (e.g. <50 μm), the droplet ejection during bubble bursting may be difficult to occur. Furthermore, the bubble formation is mainly attributed to the fact that carbon dissolved in iron can be preferentially oxidized to produce CO₂, which nucleates to generate a large bubble inside the ferric ball. It is noted that less than 0.05%wt of carbon content is enough for the bubble formation. It thereby suggests that carbon content should be accurately measured to predict the micro-explosion phenomena during iron combustion.     

丝电爆过程的电流导入机理

丝电爆制备纳米粉时, 电流从电极导入金属丝的过程直接影响电极烧损和粉末中微米级大颗粒产生. 分别通过接触和气体放电两种方式导入电流进行电爆试验. 结果表明, 光测量装置检测到的丝端部光电流几乎与回路放电电流同时产生, 而中间位置的光电流则要滞后一段时间; 由探针收集的产物确定, 金属丝端部主要形成熔融粒子, 中间部分主要形成气相粒子. 分析可知, 接触方式导入电流时, 丝端部也存在气体放电现象, 大电流主要通过气体放电形成的等离子体导入. 等离子体对电流的旁路作用会阻碍能量向金属丝沉积, 这是产生微米级大颗粒和"积瘤"主要原因. 通过气体放电方式导入电流时, 电极烧损明显减轻, 并可以避免"积瘤"产生.     

丝阵负载Z箍缩可见光图像诊断系统

 主要介绍了为“强光一号”加速器丝阵负载Z箍缩实验设计的可见光图像诊断系统,系统时间分辨约为5 ns,空间分辨约为6.5 lp/mm,光谱响应范围365~750 nm。系统能够满足从早期单丝电离到等离子体柱飞散整个Z箍缩过程的诊断要求, 给出了系统在Z箍缩实验中获得的丝阵负载内爆图像序列,并对其反映的内爆现象进行了初步的唯象分析。     

Spatial and temporal evolution of high-energy density plasmas inthe composite pinch on GIT-4 generator

A series of collaborative experiments on complex plasma loads has been carried out on the large inductive energy storage generator GIT-4. The aim of the experiments is to explore the different configurations for the formation of ultrahigh-energy density plasmas in high-voltage pulsed-power systems by direct electromagnetic energy coupling. In this paper, we present some of the underlying philosophy on these experiments and the results obtained. Particular emphasis is placed on the pulsed-power aspects and the effect of source-load coupling for the different studied Z-pinch loads. Resulting radiative properties of the classical exploding wire and liner are experimentally compared with those of the composite pinch scheme in which an intermediate low-density shell is used for staged energy transfer onto a micron-sized wire     

Investigation of THz Sommerfeld wave propagation on single-wire at different temperature

In this paper, THz Sommerfeld wave propagation on dielectrically coated cylindrical metal wire and conical metal wire is presented. The propagation characteristics of single wire are largely related to the characteristics of material being used, which may change significantly with the temperature variation. Then, the surface wave propagation along the THz wire waveguide is investigated by the numerical calculation from 0.1 to 1 THz at different temperature. The influences of different conductivity and different temperature on the propagation characteristics of the dielectrically coated cylindrical metal wire and conical metal wire are discussed, including electric field distribution, propagation loss and energy coupling characteristic. The analysis results release a fundamental characteristic for describing the THz surface wave propagation on single wire at different temperature, and moreover, the analysis results also suggest that the single wire is a promising THz waveguide and very useful for the space sensing at cryogenic temperature in future.     

等离子体断路开关装置上的多丝Z箍缩初步实验

 开展了基于等离子体断路开关的脉冲功率源驱动多丝Z箍缩负载初步实验,实验中采用了2根或4根钨丝组成的环形阵列,其中钨丝的直径分别为7 mm和20 mm。利用高速扫描摄影获取钨丝电爆炸和箍缩过程中等离子体自发光的物理图像。实验结果表明：导通电流为105 kA的等离子体断路开关将67%~78%的电流转换至金属丝阵负载上,负载电流上升沿为84~110 ns。高速扫描相机观察到了钨丝电爆炸形成晕等离子体及其向轴线箍缩和后期向外膨胀的物理过程。