A Study of the Cathode-Spot Operation on Tungsten Point Cathodes in the Range of Threshold Arc Currents

The paper reports some experimental results on measuring the lifetime and the current density of the cathode spot initiated on the tip of point cathodes. The point taper angle was varied in the range of 13-80°, the arc current was 0.5-4 A. The generator total voltage was not above 50 V. It is shown that the cathode-spot lifetime is independent of both the point taper angle and the current and is in the range of 50-250 ns. By the moment of arc extinction, the current density amounts to (0.3-2) × 108 A/cm2 at the voltage across the arc gap 20-30 V. The experimental results are compared with those calculated using a Joule model that takes into account the displacement of molten metal from the emission region under the action of the pressure of evaporating atoms. Some possible reasons are given for the disagreement of experimental results with theoretical predictions.     

Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

Phänomenologie und Energiebilanz von Lichtbogenkatoden bei niedrigen Drücken und hohen Stromstärken

The behaviour of an arc cathode in a coaxial gas stream is investigated experimentally in a pressure and current range of 15 to 120 mms Hg and 400 to 1000 A, respectively. Argon and hydrogen are used as ambient test gases. Particular phenomena occurring during the short instationary ignition period are described. Stationary operation characterized by a diffused cathode attachment covering a relatively large area is more closely studied. Measurements of the extension of the arc attachment and the distribution of the cathode temperature are made and evaluated quantitatively. Average current densities in the attachment are presented, as functions of pressure and current, by empirical expressions. Using a quasi-onedimensional approach, an interior energy balance of the cathode is set up and heat entering the cathode in the attachment region is calculated. Finally, under further simplifying assumptions, the cathode drop voltage and the fraction of current carried by ions are estimated and the so-called Richardson constant is determined assuming electron emission according to the Richardson-Schottky-equation. All data discussed depends — in general, strongly — on ambient pressure and current, and the type of gas has a substantial qualitative and quantitative influence.     

Anode Melting in a Multicathode-Spot Vacuum Arc

Melting of the anode surface in a multicathode-spot vacuum arc is expected when the incident energy flux is not balanced. The anodic energy influx is proportional to the arc-current collected by the anode and melting of the anode should be observed when peak arc-current exceeds a critical value. In this work, the critical peak arc-current Ipt was measured, and its dependence on anode and cathode materials was determined. The arc was sustained between two parallel cylindrical electrodes, 14 mm in diameter and spaced 4 mm apart. The almost critically damped current pulse lasted for 30 ms with a 6-ms rise time to peak value. Peak currents were in the range of 500-2300 A. In most of the experiments the anode material differed from that of the cathode. In the runs where the cathode-anode materials were Cu-Al or Mo-Cu, respectively, the time dependence of a spectral line intensity radiated by the anode atoms located in the plasma near the anode surface was recorded. We found that Ipt depended on both the anode and cathode materials. Thus for an Al anode and Al and Cu cathodes, Ipt equaled to 1100 and 900 A, respectively. In arcs with a peak current larger or equal to Ipt, a sudden jump of the spectral line intensity was observed. In all experiments, even when strong melting of the anode was observed, the arc-voltage stayed quiescent and in the range 15-35 V, suggesting that no anode spot was formed.     

药芯焊丝脉冲TIG电弧增材制造电弧特性研究

对药芯焊丝脉冲TIG电弧增材制造电弧特性展开研究。利用高速摄像拍摄不同熔敷层脉冲电流条件下的电弧与熔滴过渡图片,对高速摄像图片进行分析,发现焊丝熔化过程存在“滞熔”现象,导致熔滴过渡存在渣桥过渡与液桥过渡两种接触过渡方式,在脉冲峰值电流较小的50/100 A电流参数下,出现熔滴断续的渣桥过渡的频率最高。熔滴过渡影响电弧温度场与药粉成分在电弧中的分布,利用光谱诊断分析熔敷过程中在不同脉冲峰值电流与脉冲基值电流条件下电弧温度场及药粉成分在电弧中的分布。利用点阵法测量得到各点光谱数据,根据Boltzmann图法计算各点温度,将各点温度拟合得到完整电弧温度场,结果表明,焊丝从钨极轴线前（左）侧送入,吸收电弧热量并且对电弧有扰动作用,电弧前侧温度低于电弧后（右）侧,电弧前侧尺寸稍小于后侧;随着熔敷层数增加,降低峰值电流,电弧收缩,高温区面积相对减小,低温区面积相对增大。电弧最高温度区域出现在钨极下方1~2 mm的范围,大约为13 000~15 000 K,脉冲峰值电流越大则最高温度区域面积越大。在脉冲基值电流时期,由于电流小,电弧面积相比于峰值时期要小得多,焊丝与电弧相互作用减弱,电弧温度场基本关于钨极轴线对称分布。选择药芯焊丝中特有的Na元素的NaⅠ589.6 nm谱线对其分布点进行标记,拟合绘出不同脉冲峰值电流与基值电流下药粉元素在电弧中的分布情况,结果表明,电流越小,药粉运动高度越低,在不同的脉冲峰值电流下药粉均没有沾染到钨极上,在不同的脉冲峰值电流与脉冲基值电流下 Na元素均偏电弧后侧分布,说明焊丝自电弧前侧送入熔池后,在电弧前侧的电弧中没有出现药粉强烈的喷发现象,而是进入熔池进行冶金反应。接触过渡解决了碱性焊丝工艺性差的问题,电弧较为稳定,避免药粉喷发损伤钨极,熔敷过程稳定进行。     

Arc voltage of a fast triggered vacuum gap

At high current, the performance of triggered vacuum gaps (TVGs) is limited by constriction of the vacuum arc. Several concentrated modes can be defined, i.e., foot point, anode spot, and intense arc mode. In all cases, small, luminous, high-temperature spots (from melting to boiling temperature) appear on the anode surface. In accordance with the anodic mode, the arc voltage has different characteristics (quiet and low or with high-frequency noise). The arc voltage is measured for different electrode configurations for a conduction time of 27 μs and for peak current up to 45 kA. For small gap distances (1-1.5 mm), the arc voltage is quiet and low (20 to 30 V) and almost independent of the peak current. For greater distances, the arc voltage increases with the distance and the peak current. If the peak current is higher than the threshold interruption current, the arc voltage is high and noisy. These overvoltages, with a frequency of about one megahertz, can reach more than 1000 V. These overvoltages disappear completely after about 15 μs, and the voltage decreases     

Evolution of Copper Vapor from the Cathode of a Diffuse Vacuum Arc

The metal-vapor evolution from the cathode of a diffuse vacuum arc is described by a model that starts from the expression for the collisionless expansion into vacuum of atoms originating from an instantaneous point source. The velocity distribution is assumed to be Maxwellian at the moment the atoms are released from the source. By convolution with the vapor generation rate, which is given by an effective erosion rate and the waveform of the arc current, this expression is generalized to yield the atomic density at an arbitrary distance from a point source which emits atoms for a finite period. The result is integrated over the cathode surface which is treated as an extended homogeneous source of vapor. The copper-vapor density was calculated for a vacuum arc driven by a sinusoidal 50-Hz current half-cycle of 500 A rms, for the center of the contact gap. With a vapor temperature of 2000 K and an effective copper-vapor erosion rate of 3 ?/C, the model well describes the measured decay of the copper-vapor density from about 5 × 1017 m-3 at 300 ?s before current-zero to 5 × 1014 m-3 at 400 ?s after current-zero. Comparison with calculations based on the assumption that metal vapor is generated predominantly by molten droplets evaporating in flight indicates that before current-zero the contribution of the droplets to the vapor density is negligibly small, while after current-zero both vapor generation mechanisms produce vapor at the same order of magnitude.     

高密度氦相变的分子动力学研究

采用分子动力学方法结合对关联函数分析计算了0–1000 K范围内氦的固–液相变曲线, 与实验数据的对比显示, 在0–500 K之间与实验数据符合很好, 500 K以上还没有相应的实验数据. 另外, 计算了钛金属中不同尺寸氦泡的压强, 并与高密度氦的固–液相变曲线进行了对比. 结果显示, 在低温条件下, 随着温度的降低, 钛晶体中可能会出现固态氦泡; 在300 K以上不会存在固态氦泡.     

悬浮区域熔炼法制备LaB6单晶体与发射性能研究

采用区域熔炼法成功制备出了高质量,高纯度,大尺寸的LaB₆单晶体. 系统分析了制备过程中每个参数对LaB₆单晶生长的影响,确定了晶体生长最佳工艺为:样品转速为30 r/min,生长速度为8-10 mm/h. 分析了单晶LaB₆ (100) 晶面的热电子发射性能,结果表明,当阴极温度为1873 K时,最大热发射电流密度为44.36 A/cm²;利用 Richardson 直线法求出了绝对零度逸出功和有效逸出功分别为1.99和2.59 eV. 场发射测试结果表明,单晶LaB₆场发射单尖最大场发射电流密度达到4.9×10⁶ A/cm²,场发射因子为41500 cm^-1,表现出良好的场发射性能. 因此单晶LaB₆作为热阴极和冷阴极都具有很广阔的应用前景. 关键词： 区域熔炼法 6')" href="#">单晶LaB₆ 热发射性能 场发射性能     

Surface and bulk melting of small metal clusters

We present an analytical solution to the two-parabola Landau model, applied to melting of metal particles with sizes in the nanoscale range. The results provide an analytical understanding of the recently observed pseudo-crystalline phase of nanoscale Sn particles. Liquid skin formation as a precursor of melting is found to occur only for particles with radii greater than an explicitly given critical radius. The size dependences of the melting temperature, and of the latent heat, have been calculated, and a quantitative agreement is found with the experiment on tin particles.     

硫酸甲醇体系中钽的电解动力学

 为了探索钽的电解动力学行为,在硫酸甲醇电解液中对钽进行了电解抛光的实验研究,测定了电解液配比、电压、搅拌速率对钽薄膜减薄速率的影响;通过测定温度对反应速率的影响,计算了阿仑尼乌斯（S.A.Arrhenius）活化能。研究结果表明：温度在0 ℃,硫酸甲醇体积比为1∶7时,钽的电解行为符合典型金属极化行为规律;在电压10~20 V的范围内,搅拌速率大于8 m/s时,减薄速率稳定为11.45 μm/min;当搅拌速率小于8 m/s时,扩散传质控制电解过程,当大于8 m/s后,电解过程为非扩散控制。温度对反应速率的影响表明,钽在硫酸甲醇电解液中的电解行为符合S.A.Arrhenius公式,活化能为15.77 kJ·mol^-1。     

Energy mechanism during machining process by high-power continuous CO2 laser

The high-power continuous CO₂ laser (4 KW) can provide an energy capable of causing melting or even, with special treatment of surface, vaporizing an XC42 - iron sample. During the laser-metal interaction, the energetic machining mechanism takes place according to the following assumptions: Laser energy absorbed by metal is maximal for a p-polarization. The melting front precedes the laser beam. The beam interacts with a preheated surface whose temperature is near the melting point. In such conditions one finds that mean average absorptive power (A), calculated through Maxwell's equations at fusion temperature, is around 25%, which enables us to calculate the laser energy absorbed by the metal. The available thermal models provide a lot of information concerning thermal diffusion but are unable to describe the physical process of the groove. Hence practical information required for industrial applications cannot be obtained. So in this work we have established a model able to calculate the characteristic parameters of the groove (or cut) as a function of laser energy and beam impact diameter (D). This model is based on writing down the balance of exchanged heat during the time of laser-material interaction ((t). This new procedure makes it possible to determine the machining parameters (laser power P, impact diameter D, and machining speed V) which one has to use during the machining process in order to implement an optimum groove (or cut) with predetermined characteristics (width L_s, and groove depth P_r).     

Molecular beam surface ionization detection: I. Absolute ionization coefficients and work function of “low work function” Pt(8%W) surfaces

Absolute ionization coefficients β have been measured for K on a “low work function” PtW surface by the field reversal method. The modified Langmuir-Saha equation has been fitted by a least-squares procedure to the static surface ionization values as a function of temperature, and ionization coefficients have been computed from the best fit parameter values. The agreement between the two methods is satisfactory at low temperatures, within 9% at β = 0.7–0.8, with the largest error probably lying in the coefficients from static ionization. By a comparison with other methods used for determinations of β < 1 the field reversal method is shown to be as accurate as the apparently most reliable method used for such work, the microbalance method, first used by Schroen 1963. The Richardson plots for the probably graphite coated metal surface are curved. It is shown, that the curvature does not depend on the choice of the spectral emissivity or other possible experimental errors. The only explanation for the curved plots, which appears to be consistent with the experiments, is a true non-linear temperature dependence of the work function, probably due to the semiconducting surface layer. Non-linear leastsquares fits have been used to fit the theoretical expressions to the electron emission current, the surface ionization current and also to a combined quantity, in which the temperature dependence of the work function has been effectively removed. The nature of the PtW surface and some earlier, inconsistent investigations of this surface are also discussed.     

Instabilities and Statistical Behavior of Low Current Cold Cathode Arcs

This paper presents a study of instabilities and of spontaneous extinctions which occur in low current dc arcs between metal electrodes. A theoretical model for the behavior of emitting sites on the cathode surface is proposed. This model explains experimental results obtained in arc duration measurements and arc noise correlation. The model yields figures for site life time and elementary current of emitting sites. These values are compared with those given by other authors. The proposed model can be applied to explain arc duration and related erosion occuring in telecommunication relays and circuit breakers.     

双面电弧焊接的传热模型

将等离子焊接(PAW)电弧和钨极氩弧焊(TIG)电弧串接,相对作用于工件的正反面形成双面电弧焊接(DSAW)系统,可以引导焊接电流沿工件厚度方向流过小孔,补偿等离子电弧穿透工件时消耗的能量,以有效地提高等离子弧的穿透能力.综合考虑影响双面电弧焊接正反面熔池几何形状的力学因素,建立了熔池表面变形的控制方程,以此为基础并采用帖体曲线坐标系建立了DSAW焊接传热的数学模型,分析了DSAW,PAW焊接传热的差异,从传热的角度解释了DSAW焊接熔深增加的原因.焊接工艺实验表明,计算结果与实测结果吻合良好. 关键词： 双面电弧焊接(DSAW) 传热模型 熔池表面变形模型     

GaN基蓝光发光二极管正向电压温度特性研究

对GaN基蓝光发光二极管（LED）正向电压温度特性进行了研究,发现在温度较高时,正向电压随温度的变化系数逐渐减小,直至出现拐点,正向电压随温度的变化系数由负数变为正数.此时若继续升高温度,则正向电压随温度升高迅速增加,并常常伴随有器件失效的现象发生.在小电流情况下,这种现象不很明显,随着电流的增加,现象表现得越来越明显,拐点出现的温度也越来越低,而且温度超过拐点之后,正向电压值增加得更快.通过与相同封装的另一组器件测试结果对比,排除了封装材料玻璃转换温度的影响.分析认为,这一现象的出现是由器件等效串联电阻 关键词： 发光二极管 氮化镓 正向电压 温度系数     

Das optische Emissionsvermögen des Titans

The spectral emissivity of titanium has been measured over a temperature range from 1100 °K to approximately the melting point. Discrepancies in the data as reported by several authors are discussed with respect to the surface conditions of the metal investigated.     

The anode surface temperature of CuCr contacts at the limit ofcurrent interruption

This paper estimates anode surface temperature at the current interruption limit by measuring the melting time of a contact material after current interruption and simulating the anode surface temperature. As a result, the minimum anode surface temperature of CuCr(50/50) contact material was about 1750 K. We also calculated the metal vapor density between electrodes with a simplified model. The calculation results showed that the critical vapor density was about 3×10²⁰ atoms/m³. This vapor density is equivalent to the averaged pressure of 8 Pa, which is close to the value of the Paschen minimum     

Effect of voids and pressure on melting of nano-particulate and bulk aluminum

Molecular dynamics simulations are performed using isobaric–isoenthalpic (NPH) ensembles to study the effect of internal defects in the form of voids on the melting of bulk and nano-particulate aluminum in the size range of 2–9 nm. The main objectives are to determine the critical interfacial area required to overcome the free energy barrier for the thermodynamic phase transition, and to explore the underlying mechanisms for defect-nucleated melting. The inter-atomic interactions are captured using the Glue potential, which has been validated against the melting temperature and elastic constants for bulk aluminum. A combination of structural and thermodynamic parameters, such as the potential energy, Lindemann index, translational-order parameter, and radial-distribution functions, are employed to characterize the melting process. The study considers a variety of void shapes and sizes, and results are compared with perfect crystals. For nano aluminum particles smaller than 9 nm, the melting temperature is size dependent. The presence of voids does not impact the melting properties due to the dominancy of nucleation at the surface, unless the void size exceeds a critical value beyond which lattice collapse occurs. The critical void size depends on the particle dimension. The effect of pressure on the particulate melting is found to be insignificant in the range of 1–300 atm. The melting behavior of bulk aluminum is also examined as a benchmark. The critical interfacial area required for the solid–liquid phase transition is obtained as a function of the number of atoms considered in the simulation. Imperfections such as voids reduce the melting point. The ratio between the structural and thermodynamic melting points is 1.32. This value is comparable to the ratio of 1.23 for metals like copper.     

Macroparticle Dynamics during Multi-Cathode-Spot Vacuum Arcs

Macroparticle dynamics in multi-cathode-spot (MCS) vacuum arcs were studied by utilizing laser Doppler anemometry (LDA) methods for in situ measurement of the cathodic macroparticle velocities and relative emission rates. Arc current pulses having peak values of 1-2 kA at either 6 or 1 ms after arc initiation were investigated. Systematic dependence of the macroparticle dynamics (i.e., speed and direction of flight) on cathodic thermophysical properties, location of the measurement probe in the interelectrode region, instantaneous value of the arc current, arc current waveform, and macroparticle size was determined. It was found that the macroparticle velocity increased with the melting temperature of the cathode metal, distance from the cathode surface, and the instantaneous value of the arc current, and decreased with macroparticle size and the rise time of the current waveform. All the above dependencies may be understood as direct indications of the plasma-macroparticle interaction during the discharge. The measured instantaneous relative emission rates were found to peak later than the arc current but before the peak average cathode surface temperature, which was estimated using a semi-empirical model. This result may be an indication of the dependence of cathodic erosion in the form of molten metal droplets on the average cathode surface temperature.