Numerical-simulation of ac plasma-arc thermodynamics

A mathematical model and approximate analysis for the energy distribution of an ac plasma arc with a moving boundary is developed. A simplified electrical conductivity function is assumed so that the dynamic behavior of the arc may be determined, independent of the gas type. The model leads to a reduced set of non-linear partial differential equations which governs the quasi-steady ac arc. This system is solved numerically and it is found that convection plays an important role, not only in the temperature distribution, but also in arc disruptions. Moreover, disruptions are found to be influenced by convection only for a limited frequency range. The results of the present studies are applicable to the frequency range of 10-10(2) Hz which includes most industry ac arc frequencies. (C) 1994 Academic Press, Inc.

Impact of cathode evaporation on a free-burning arc

A free-burning, high-intensity argon arc at atmospheric pressure was modelled during the evaporation of copper from the cathode. The effect of cathode evaporation on the temperature, mass flow, current flow and Cu concentration was studied for the entire plasma region. The copper evaporates from the tip of the cathode with an evaporation rate of 1 mg s-1. The copper vapour in the cathode region has a velocity of 210 m s-1 with a mass concentration of above 90% within 0.5 mm from the arc axis. The vapour passes from the cathode toward the anode with a slight diffusion in the argon plasma. Higher temperatures and current densities were calculated in the core of the arc caused by the cathode evaporation.

四氧化二氮对卫星漫反射试片氟化镁涂层的污染和侵蚀

利用漫反射率、X射线光电能谱和质谱等测量方法,研究了固态和气态四氧化二氮(N2O4)对漫反射试片氟化镁(MgF2)涂层表面的污染情况.实验表明,固态N2O4对MgF2涂层有严重侵蚀作用,N2O4固粒污染后的涂层表面漫反射率下降了20%～30%.在一定的时间内,气态N2O4对涂层表面的影响显著地依赖它的压力.试片在压力为6.9×104 Pa和200Pa的N2O4蒸气中分别放置10 min,前者厚度为40μm的MgF2涂层基本消失,表面漫反射率下降约20%;后者涂层表面的原子组成和漫反射率变化很小.还给出了MgF2涂层表面N2O4分子吸附摩尔密度,以及与涂层表面碰撞的N2O4分子通过化学吸附过程提取MgF2的几率.

Comparative Observation of DC Arc Plasma Jets and Their Arc Root Behaviors at Reduced Pressure

Experimentally observed, results are presented for the DCarcplasmajets and theirarc-rootbehaviors generated atreduced gas pressure and without or with an' applied magnetic field. Pure argon, argon -hydrogen or argon-nitrogen mixture is used as the plasma-forming gas. A specially designed copper mirror is constructed and used for better observing the arc-root behavior on the anode surface of the DC non-transferred arcplasma torch. It is shown that for the cases without applied magnetic field, the laminar plasmajets are stable and approximately axisymmetrical. The arc-root attachment on the anode surface is completely diffusive when argon is used as the plasma-forming gas, while the arc-root attachment often becomes constrictive when hydrogen or nitrogen is added into the argon. When an external magnetic field is applied, the arcroot tends to rotate along the anode surface of the non-transferred arcplasma torch.

Evaluation of the Interfacial Adhesion between Brittle Coating and Ductile Substrate by Cross-Secitional Indention

The cross-sectional indentation method is extended to evaluate the interfacial adhesion between brittle coating and ductile substrate. The experimental results on electroplated chromium coating/steel substrate show that the interfacial separation occurs due to the edge chipping of brittle coating. The corresponding models are established to elucidate interfacial separation processes. This work further highlights the advantages and potential of this novel indentation method

Modeling of a Laminar DC Arc Plasma Torch

In recent years, stable and long laminarplasma jets have been successfully generated, and thus it is possible to achieve low-noise working surroundings, better process repeatability and controllability, and reduced metal-oxidation degree in plasma materials processing. With such a recent development in thermal plasma science and technology as the main research background, modeling studies are performed concerning the DCarcplasmatorch for generating the long laminar argon plasma jet. Two different two-dimensional modeling approaches are employed to deal with the arc-root attachment at the anode surface. The first approach is based on circumferentially uniform arc-root attachment, while the second uses the so-called fictitious anode method. Modeling results show that the highest temperature and maximum axial-velocity at the plasmatorch exit are ～15000 K and ～1100 m/s, respectively, for the case with arc current of 160 A and argon flow rate of 1.95×10{sup}(-4)kg/s.

Experimental Study and Numerical Simulation of Cellular Structures and Mach Reflection of Gaseous Detonation Wave

In this paper the Deflagration to Detonation Transition (DDT) process of gaseous H-2-O-2 mixture and Mach reflection of gaseous detonation wave on a wedge have been conducted experimentally. The cellular pattern of DDT process and Mach reflection were obtained from experiments with wedge angle theta = 10(0) similar to 40(0) and initial pressure of gaseous mixture 16kPa similar to 26.7kPa. The 2-D numerical simulations of DDT process and Mach reflection of detonation wave were performed by using the simplified ZND model and improved space-time conservation element and solution element (CE/SE) method. The numerical cellular structures were compared with the cellular patterns of soot track. Compared results were shown that it is satisfactory. The characteristic comparisons on Mach reflection of air shock wave and detonation wave were carried also out and their differences were given.

The Mitogenic and Anti-Apoptotic Activity of Tumor Conditioned Medium on Endothelium

This study was designed to observe the effect of tumor conditioned medium (TCM) on the proliferation and apoptosis of human umbilical vein endothelial cells (HUVECs). HUVECs were exposed to TCM from breast carcinoma cell line MDA-MB-231, then we measured their proliferation, apoptosis and cell cycle distribution by MTT and flow cytometery (FCM). Following the stimulation of TCM, HUVECs showed higher pro-mitogenic and anti-apoptotic ability than did the negative control group (ECGF-free medium with 20% FBS), but a similar ability to the positive control group (medium with ECGF and 20% FBS). From these results, we can conclude that breast carcinoma cell line MDA-MB-231 could secret soluble pro-angiogenic factors that induce HUVEC angiogenic switching, including cell cycle progression, proliferation and growth. The role and character of these factors remain to be further studied.

The Effect of the Thickness of Fe2 Al5 Phase Layer at Fe/Al Interface on the Mechanics Behavior of Hot Dip Aluminizing Coating

Hot Dip Aluminized Coatings with different thickness were prepared on Q235 steel in aluminum solutions with different temperature for certain time. Through tensile tests and in-situ SEM observations, the effect of the coating's microstructure on the tensile strength of the samples was studied. It was disclosed at certain aluminum solution temperature,transaction layers mainly composed of Fe2 Al5 phase got thicker with time prolonging, and this changed initial crack's extending direction from parallel with to vertical with stretching direction. The change in crack direction decreased tensile strength of samples, thus made the coating easy to break. It was concluded that the existence of thick Fe2 Al5 phase layer was the basic reason for the lowering of tensile strength of the coating.

A special method to evaluate the strong adhesion between brittle coating and ductile substrate

The evaluation of the interfacial adhesion of coating system has always been a rough task. In this paper, a special testing method of cross-sectional indentation is applied on a model coating system, i.e. electroplated chromium on a steel substrate which is generally regarded as an example of materials pair with strong adhesion. Based on fractography analysis with SEM and interfacial stress simulation with FEM, it is found that interfacial shear stress may induce coating spalling. More interestingly, spalling location is sensitive to substrate pretreatment process. This shows the feasibility of cross-sectional indentation to distinguish interfacial strength at a high level.

电弧离子镀中脉冲偏压对TiN,CrN薄膜性能的影响

在电弧离子镀技术中使用脉冲偏压电源是近年来发展起来的一项新技术。研究表明，在薄膜沉积过程中，以直流偏压为基础，迭加一个高脉冲电压，有助于改善薄膜的性能，并且能够在较低的沉积温度下获得结合力较强、内应力较小、表面光洁度也比较好的薄膜。通过正交设计和方差分析，本文首次研究了在电弧离子镀技术中使用脉冲偏压电源， 在高速钢和不锈钢基体上沉积TiN和CrN的过程中，对薄膜的表面粗糙度、表面形貌、结合力、显微硬度、沉积速率等性能的影响，发现：（1） 在考虑气氛、弧电流和脉冲电源的各种参数的情况下，气氛和频率是影响薄膜性能的主要因素；（2）在气氛、弧电流不变的条件下，脉冲电源的频率和占空比是影响薄膜粗糙度的主要因素；（3）在比较了直流偏压和脉冲偏压的实验结果之后，认为利用直流迭加脉冲的偏压方式，经过优化参数，能够比较显著地 降低薄膜的表面粗糙度，提高沉积速率。