Effect of phonon focusing on Knudsen flow of phonon gas in single-crystal nanowires made of spintronics materials

Effect of anisotropy of elastic energy on the phonon propagation in single-crystal nanowires made of Fe, Cu, MgO, InSb, and GaAs materials that are used to fabricate spintronics devices in the regime of the Knudsen flow of phonon gas has been studied. A new method of analyzing the focusing of quasi-transverse modes has been suggested, which made it possible to determine the average values of the densities of phonon states in the regions of focusing and defocusing slow and fast quasi-transverse modes. The effect of phonon focusing on the anisotropy of heat conductivity and lengths of the phonon free paths has been analyzed for all acoustic modes that exist in spintronics nanostructures. It has been shown that for all the nanowires investigated the angular dependences of the free paths of fast and slow transverse modes in the {100} and {110} planes correlate with the angular dependences of the densities of phonon states for these modes. Directions of the heat flux that ensure the maximum and minimum phonon heat conductivity in the nanowires have been determined.     

气流再压缩等离子弧焊接电弧行为

电弧等离子体行为对焊接接头组织结构和性能具有决定性作用,开展气流再压缩等离子弧特性研究对于指导先进材料的气流再压缩等离子弧焊接工艺和提高焊接接头质量具有重要意义. 针对气流再压缩等离子弧焊接新工艺,基于流体动力学和电磁理论,建立气流再压缩等离子弧数值分析模型,采用ANYSYS Fluent软件,通过C语言进行二次开发,定量计算等离子弧温度分布、流场分布、电势分布,分析压缩气对等离子弧温度场、流场、电弧电压的影响规律. 模拟结果表明,压缩气对喷嘴内的等离子弧温度分布基本没有影响,压缩气对喷嘴外的等离子弧具有拘束压缩作用;压缩气对等离子弧流场分布基本没有影响;压缩气能够提高电弧电压. 相同电流条件下,与常规等离子弧焊接相比,气流再压缩等离子弧焊接电弧穿透能力有望提高.     

Influence of gas flow on argon microwave plasma jet at atmospheric pressure

Many kinds of an atmospheric-pressure plasma jet have been developed and used for widespread applications such as a surface treatment and modified. This study focused on the argon atmospheric-pressure microplasma jet generated by discharging of RF power of 2.45 GHz microwave. The plasma jet shows sensitivity to surrounding environment: pressure, temperature and gaseous species. It is therefore absolutely imperative that a nature of atmospheric-pressure plasma jet should be understood from a point of fluid dynamics. This study, therefore, focused on the interrelationship between the plasma jet and the working gas. Motion of the plasma jet and the working gas was evaluated by velocity measurement and fast photography. As a result, the unsteady sinusoidal waving motion in the radial direction of a torch was observed. Advection velocity of the plasma in just downstream region of the torch exit increases with the supplying flow rate, and the velocity ratio is in the range of 0.75-0.87.     

锂离子电池高镍单晶正极材料的合成与性能

通过高温固相焙烧法合成高镍单晶正极材料LiNi0.88Co0.09Al0.03O2。采用XRD、XPS、TEM等技术研究初始样品和脱锂化高镍单晶材料的物理化学性能;采用长循环测试、循环伏安法和原位阻抗分析表征其电化学性能。研究结果表明:在高温处理过程中,样品的内部和表面发生锂氧损失和相转变现象,材料在固相合成反应过程中会加剧相转变,并在单一颗粒上形成多相共存状态;高温可以促进一次颗粒生长,但对单晶颗粒的稳定层状结构有一定的损害。     

Influence of gas flow rate on the microstructure and mechanical properties of hydroxyapatite coatings fabricated by gas tunnel type plasma spraying

Gas tunnel type plasma torch was used to spray hydroxyapatite (HA) coatings on 304 stainless steel substrate at different plasma gas (Ar) flow rates (100-170 l/min). Microstructure was observed by scanning electron microscope and phase analysis by X-ray diffraction. The mechanical properties such as hardness and abrasive wear resistance of HA coatings sprayed at different gas flow rates were investigated. The results showed that the gas flow rates affect greatly the microstructure and mechanical properties of HA coatings. Crystallinity increased and porosity decreased as the gas flow rate increased. Hardness and abrasion resistance increased as the gas flow rates increased.     

Influence of platform position on stray grain nucleation in Ni-based single-crystal dummy blade clusters

Stray grains are the most severe of the solidification defects that occur in the industrial single-crystal blade preparation process. In this study, a single-crystal dummy blade cluster with different crystal orientations controlled by the seeding method was prepared, and the influence of the position of the circular platform (relative to the sample and furnace body) on stray grain nucleation was investigated. Results show that the microstructure of the circular platforms could be divided into the center, expansion, and stray grain regions. The inside of the circular platform facing the center of the cluster is more prone to stray grain formation than the outside of the circular platform facing the furnace body. With an increase in the distance between the circular platform and the bottom of the dummy blade cluster, the stray grain region expands, whereas the expansion region narrows. The stray grain is slightly aggravated with increase of the misorientation. Finally, the mechanism underlying the influence of platform position on the formation of stray grains in single-crystal dummy blade clusters is discussed based on the temperature evolution during directional solidification.

     

Influence of withdrawal rate on the microstructure of Ni-base single-crystal superalloys containing Re and Ru

The influence of elevated withdrawal rate on the microstructure and segregation behavior of Ni-base single-crystal superalloys containing Re and Ru is investigated. The experimental superalloys are processed under a high thermal gradient of approximately 250 K/cm and withdrawal rates between 10 and 500 μm/s. With increasing withdrawal rate, the dendritic structures and γ′ precipitates in as-cast microstructures are apparently refined. Electron-probe microanalyzer (EPMA) results indicate that the degree of segregation for the constituent elements (e.g. Al, Ta, W, Re etc.) increases initially and then decreases with increasing withdrawal rate. In addition, the Re and Ru additions obviously increase the amounts of γ-γ′ eutectic and the tendency of segregation for Al and Ta.     

Influence of deformation on the retention of helium in materials

The influence of deformation on the retention of helium in samples of the chromium–nickel austenitic steel uniformly saturated with helium using bombardment with α particles in a cyclotron has been studied. It has been shown that, under certain conditions, helium atoms can be carried away by moving dislocations, which can have a significant impact on the removal of helium from material, exit to the grain boundaries and redistribution it over the sample volume.     

Low-temperature plasma carburizing of AISI 420 martensitic stainless steel: Influence of gas mixture and gas flow rate

Low-temperature plasma carburizing was studied aiming to determine the effect of the gas mixture and flow rate on the surface properties of AISI 420 martensitic stainless steel samples. Plasma carburizing was carried out for gas mixtures of 20% Ar + 80% H₂ comprising CH₄ contents between 0.25 and 1.00%, and gas flow rates ranging from 1.67 × 10^− 6 to 6.68 × 10^− 6 Nm³ s^− 1. The modified layers were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. The plasma was also characterized by optical emission spectrometry. Results indicate the presence of a hard and thin outer layer and a carbon-enriched martensite diffusion layer. It is shown that gas mixture composition plays an important role in the process kinetics. Spectroscopic characterization of the glow discharge shows that the variation of the CH₄ content in the gas mixture leads to a variation of the emission lines intensity but does not significantly alter the relative peak intensities. It suggests a variation on the plasma density and no significant variation on the active species. It also indicates that, for the studied conditions, the emission spectroscopy cannot be applied as a tool for process control.     

Influence of GaCl carrier gas flow rate on properties of GaN films grown by hydride vapor-phase epitaxy

The influence of GaCl carrier gas flow rate on GaN films grown by hydride vapor-phase epitaxy (HVPE) was investigated. The symmetric (0 0 0 2) and asymmetric (10-12) ω scans were detected to estimate the quality of GaN films. Optical properties were studied by room temperature photoluminescence spectra. Raman spectroscopy was employed to analyze the residual stress in the samples. The surface morphology of the GaN films was investigated by atomic force microscopy (AFM). On the basis of process optimization the optimal GaCl carrier gas flow rate for growth of high quality GaN films in our system was obtained as 1.3 L/min.     

Modelling the abrasive flow machining process on advanced ceramic materials

Abrasive flow machining (AFM) is a unique machining method used to achieve high surface quality on inner, difficult-to-access and on outside contours. Using AFM, it is possible to realise predefined edge rounding on any brittle or hard material. AFM is easy to integrate in an automated manufacturing environment. The abrasive medium applied during AFM is a fluid consisting of a polymer which carries silicon carbide or super-abrasive grains. With a specified pressure and temperature, this fluid flows in alternating directions along the contours of the workpiece resulting in an abrasive effect. AFM is also well suited to process advanced ceramic materials. Especially advanced ceramics are playing increasingly a significant role as a substitute for metals. However the high costs for the inevitable finishing process on ceramics prevent a more frequent use. This paper represents the technological results of a research-project discovering the fundamental principles of AFM on advanced ceramic materials such as a correlation between flow processes, surface formation and edge rounding. Furthermore an insight into a process model is given, which was developed using modern simulation techniques. The overall objective of this approach is to anticipate work results like surface quality and edge rounding on any user-defined geometry.     

Influence of gas flow on thermal field and stress during growth of sapphire single crystal using Kyropoulos method

The professional modeling software package CrysVUn was employed to study the process of a large sapphire single crystal growth using Kyropoulos method.The influence of gas pressure on thermal field, solid-liquid interface shape, gas velocity field and von Mises stress were studied for the first time.It is found that the root of the seed melt when gas pressure equals to one atmosphere or more than one atmosphere, especially during the seeding period, this result is consistent with the experimental observation, and this paper presents three ways to solve this problem.The temperature gradient and stress decreases significantly as the gas pressure increases.The convexity of the solid-liquid interface slightly increases when the gas pressure increases.Numerical analysis was used to optimize the hot zone design.     

Anisotropy in the plastic flow properties of single-crystal α titanium determined from micro-cantilever beams

Single-crystal micro-cantilever beams were manufactured from a polycrystalline commercially pure Ti sample using a focused ion beam. The cantilevers were approximately 5 μm wide and 30 μm long. A nano-indenter was then used to conduct micro-bending tests. Slip systems are selectively activated in these α-Ti beams by varying the crystal orientation along the beam. Increasing end deflections, from 1 to 8 μm, were applied to a series of similarly oriented cantilevers to show the progressive development of deformation. Load drops associated with strain bursts were seen in the mechanical response of some of the plastically deforming cantilevers. These appear to correlate with the formation of intense slip bands in the cantilevers. A crystal plasticity-based finite element model was applied to simulate the bending behaviour of single-crystal beams in the tested crystal orientations. Critical resolved shear stresses of 181 MPa for a on the prismatic, 474 MPa for c + a on the pyramidal and 209 MPa for a on the basal planes were determined via the reverse process of fitting the model load–displacement curves to experimental ones.     

超声振动对材料流变行为的影响机制

设计了材料局部超声波与压力载荷耦合的压痕实验装置,以7050铝合金和哈氏C-276镍基合金为实验材料进行挤压实验,研究超声振动挤压对材料单点流变行为的影响规律及传能机理。研究表明,超声能对材料单点受载的塑性变形具有降载、提效双重效应;经分析论证,在局部流变的界面上叠加超声场时,变形区金属以脉冲方式产生滑移流变;同时滑移脉冲激励滑移体内部粒子的谐振响应,产生高能的短波声子,传播到变形区高密度位错晶界,导致这些区域粒子能量的跃迁,并最终转化为材料的塑性变形能。     

Influence of ultrasonic vibration on the shear formability of metallic materials

The flow stress reduction of metallic materials due to superimposed ultrasonic vibration is an intensively researched phenomenon. So far, however, only a few studies have analysed the vibrational influence on forming limits. Existing findings are inconsistent, suggesting both a formability extension and a decrease. Ultrasonic-assisted shear tests constitute a novel experimental approach for the fundamental investigation of predominant cause–effect–relationships. The test results reveal a distinct correlation between the application of ultrasonic vibration and earlier material failure. This effect is attributed to enhanced crack initiation and propagation caused by localised stress and strain concentration as well as intermittent high strain rates.     

Knudsen Effect on the Estimation of the Effective Thermal Conductivity of Thermal Barrier Coatings

A numerical model based on the use of cross-sectional micrographies and a 3D image of thermal barrier coatings for the estimation of the material effective thermal conductivity is presented. The case of a YSZ thermal spray coating consisting of a 2 phase network, namely, the coating material and pores, is considered. The variation of the thermal conductivity of pores caused by their small size was considered by taking the Knudsen effect into account. The quantification of this effect on the effective thermal conductivity of the coating was achieved with the help of image analysis combined with an in-house program coded in C language. Finite-difference (FD) and finite-element (FE) models were applied using both 2D images and a 3D image. Despite the differences in the computed values obtained with these two numerical methods, the decrease of the computed thermal conductivity caused by the Knudsen effect was found to remain quite moderate for both methods (i.e., about 3-5% for the 3D results).