Surface characterization of Pd/Ag23 wt% membranes after different thermal treatments

Hydrogen permeation measurements of 1.5-10 μm thick Pd/Ag23 wt% membranes before and after thermal treatments at 300 °C in air (both sides) or in the temperature range 300-450 °C in N₂ (feed side) and Ar (permeate side) were performed. Accompanying changes in surface topography and chemical composition were subsequently investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiling. For a 2 μm thick membrane, the surface roughness increased for all annealing temperatures applied, while a temperature of 450 °C was required for an increase in roughness of both membrane surfaces to occur for the 5 μm membrane. The thickest membrane, of 10 μm, showed changed surface roughness on one side of the membrane only and a slight decrease in hydrogen permeance after all heat treatments in N₂/Ar. X-ray photoelectron spectroscopy investigations performed after treatment and subsequent permeation measurements revealed segregation of silver to the membrane surfaces for all annealing temperatures applied. In comparison, heat treatment at 300 °C in air resulted in significantly increased hydrogen permeance accompanied by increasing surface roughness. Upon exposure to oxygen, Pd segregates to the surface to form a 2-3 nm thick oxide layer (PdO), with more complex segregation behavior after subsequent reduction and permeance measurements in pure hydrogen. The available permeance data for the Pd/Ag23 wt% membranes after heat treatment in air at 300 °C is found to depend linearly on the inverse membrane thickness, implying bulk limited hydrogen permeation for thicknesses down to 1.5-2.0 μm.     

A Markov chain model of mixing kinetics for ternary mixture of dissimilar particulate solids

This paper presents a simple but informative mathematical model to describe the mixing of three dissimilar components of particulate solids that have the tendency to segregate within one another. A nonlinear Markov chain model is proposed to describe the process. At each time step, the exchange of particulate solids between the cells of the chain is divided into two virtual stages. The first is pure stochastic mixing accompanied by downward segregation. Upon the completion of this stage, some of the cells appear to be overfilled with the mixture, while others appear to have a void space. The second stage is related to upward segregation. Components from the overfilled cells fill the upper cells (those with the void space) according to the proposed algorithm. The degree of non-homogeneity in the mixture (the standard deviation) is calculated at each time step, which allows the mixing kinetics to be described. The optimum mixing time is found to provide the maximum homogeneity in the ternary mixture. However, this “common” time differs from the optimum mixing times for individual components. The model is verified using a lab-scale vibration vessel, and a reasonable correlation between the calculated and experimental data is obtained.     

Progress in Research and Application of Micro-Laser-Induced Breakdown Spectroscopy

The technique of micro-laser-induced breakdown spectroscopy (μLIBS) usually refers to analyzing the surface of the sample using a compact focused laser beam in the optical microscope range. Compared to conventional laser-induced breakdown spectroscopy (LIBS), it can provide more abundant form, structure, and content information on a smaller, less sample condition. Hence, the application field is very wide. In this paper, the basic status of μLIBS in the selection of laser source and the structure of optical path system is introduced, and its application in metal, semiconductor, animal and plant fields is reviewed. The existing problems and potential development direction is pointed out.     

Growth of large dimension BaWO4 crystal by the Czochralski method

In this paper, polycrystalline materials of BaWO₄ were synthesized by solid-state phase method, and a single crystal of BaWO₄ was successfully grown along a and c-axis direction by using the Czochralski method. Up to 20×22×80 mm³ BaWO₄ crystal was obtained, and X-ray powder diffraction results show that the as-grown BaWO₄ crystal belongs to the scheelite structure. The effective segregation coefficients of Ba and W of the BaWO₄ crystal were measured by the X-ray fluorescence method, and the effective segregation coefficients of Ba and W were near 1. The rocking curve from (2 0 0) diffraction plane of as-grown BaWO₄ single crystal was measured on the High-resolution X-ray diffractometer D5005, and the full-width at half-maximum value was found to be 26.64′′ The density and hardness of the BaWO₄ crystal was measured, the measured density was in agreement with the calculated result, and the Mohs hardness was about 4.     

Segregation of particulate solids: Experiments and DEM simulations

Segregation of granular materials is a complex phenomenon, difficult to measure quantitatively and to predict. Discrete element method (DEM) can be a useful tool to predict segregation effects and to support the industrial design. In this context, a very challenging idea is the characterization of the granular solids to provide the key parameters needed for a successful DEM simulation of segregation processes. Rolling friction, sliding friction and the coefficient of restitution are the critical parameters to be studied. These microscopic simulation parameters are calibrated by comparing the macroscopic behavior of granular matter in standard bulk experiments, which have the advantage of being highly repeatable and reliable.An experimental method is presented to characterize free surface segregation. The effects of different particle properties, particularly, shape and size, on segregation of cohesionless materials were investigated. From the experiments, particle size demonstrated a stronger effect on segregation than particle shape. Finally, the corresponding DEM simulations of the segregation experiments were presented. The parameters obtained by calibration were validated by the comparison of the modeled segregation behavior with the experimental results. Thus, calibrated DEM simulations are capable of predicting segregation effects.     

Effect of the character of high-angle grain boundaries on segregation: An elastic model

We introduce a model for high-angle grain boundaries in the form of a two-dimensional array of compressive forces. Segregation profiles driven by the stress field around the boundary are calculated for several levels of translational order. We find an enhancement of segregation with increasing degree of translational disorder, in agreement with experimental observations.     

Structures of small Pd–Pt bimetallic clusters by Monte Carlo simulation

Segregation phenomena of Pd–Pt bimetallic clusters with icosahedral and decahedral structures are investigated by using Monte Carlo method based on the second-moment approximation of the tight-binding (TB-SMA) potentials. The simulation results indicate that the Pd atoms generally lie on the surface of the smaller clusters. The three-shell onion-like structures are observed in 55-atom Pd–Pt bimetallic clusters, in which a single Pd atom is located in the center, and the Pt atoms are in the middle shell, while the Pd atoms are enriched on the surface. With the increase of Pd mole fraction in 55-atom Pd–Pt bimetallic clusters, the Pd atoms occupy the vertices of clusters first, then edge and center sites, and finally the interior shell. It is noticed that some decahedral structures can be transformed into the icosahedron-like structure at 300 and 500 K. Comparisons are made with previous experiments and theoretical studies of Pd–Pt bimetallic clusters.     

Quantitative fundamental SIMS studies using O implant standards

The use of dilute ‘minor-isotope’ ¹⁸O implant reference standards for quantification of surface oxygen levels during steady-state SIMS depth profiling is demonstrated. Some results of two types of quantitative fundamental SIMS studies with oxygen (¹⁶O) primary ion bombardment and/or oxygen flooding (O₂ gas with natural isotopic abundance) are presented: (1) Determination of elemental useful ion yields, UY(X^±), and sample sputter yields, Y, as a function of the oxygen fraction c_O measured in the total flux emitted from the sputtered surface. Examples include new results for positive secondary ion emission of several elements (X = B, C, O, Al, Si, Cu, Ga, Ge, Cs) from variably oxidized SiC or Ge surfaces. (2) The dependence of exponential decay lengths λ(Au^±) in sputter depth profiles of gold overlayers on silicon on the amount of oxygen present at the sputtered silicon surface. The latter study elucidates the (element-specific) effects of oxygen-induced surface segregation artifacts for sputter depth profiling through metal overlayers into silicon substrates.     

Quantitative depth profiling of photoacid generators in photoresist materials by near-edge X-ray absorption fine structure spectroscopy

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to quantify the surface composition and depth profiling of photoacid generators in thin film photoresist materials by varying the entrance-grid bias of a partial electron yield detector. By considering model compositional profiles, NEXAFS distinguishes the surface molar excess within the top 6 nm from the bulk. A surface enriched system, triphenylsulfonium perfluorooctanesulfonate, is contrasted with a perfluorobutanesulfonate photoacid generator, which displays an appreciable surface profile within a 6 nm segregation length scale. These results, while applied to 193-nm photoresist materials, highlight a general approach to quantify NEXAFS partial electron yield data.     

基于微束X射线荧光光谱的新型铸&锻GH4096合金涡轮盘成分分布解析

铸&锻GH4096高温合金涡轮盘拥有高承温能力、高强度、低裂纹扩展速率、高抗疲劳性能等优异性能,是航空发动机的关键热端部件。但由于其合金化程度较高、零件尺寸大、制备工艺复杂等,不可避免会出现成分和组织分布不均匀,一定程度影响涡轮盘的服役性能。微区X射线荧光光谱（μ-XRF）具有微区分辨率高、分析速度快、多元素同步分析、非破坏性等优点,被广泛应用于考古、地质、生物等领域,但对大尺寸高温合金构件的成分分布的研究较少,对于材料各原始位置处的成分定量分布表征也未见报道。本试验通过选择合适的测量条件、优化仪器定量方法,建立了基于微束X射线荧光光谱的新型铸&锻GH4096合金涡轮盘成分分布定量分析方法,并引入了原位统计分析方法对涡轮盘中Cr,Co,Mo,W,Ti,Al,Nb和Ni八种主要元素进行了定量统计分布解析。发现涡轮盘厚度的中心区域内Co,Mo和Ti三种元素从轮毂至轮缘存在较明显的弧形负偏析带,而Ni和Cr两种元素存在弧形正偏析带。另外,涡轮盘径向也存在一定的成分梯度分布,Co,Cr和W三种元素含量从轮毂到轮缘的呈现逐渐降低的趋势,而Mo,Ti和Nb三种元素含量则呈现逐渐上升趋势。对各元素最大偏析度、统计偏析度、统计符合度计算分析后可知,Cr,Co,Mo,W,Ti,Nb和Ni七种元素测量区域内整体偏析程度较小、统计符合度大,在材料元素设计值允许范围内具有较好的成分均匀性。使用火花源金属原位分析仪（OPA-200）对相同测试区域的元素进行了线分布分析,其分析结果与微束荧光光谱得到的分析规律具有较好的一致性,说明大尺寸涡轮盘在热处理过程中存在温度场分布,导致了各元素扩散行为以及显微组织分布的差异,因此不同部位的成分也存在一定偏析。通过对大尺寸的涡轮盘成分进行定量统计分布解析,对于评价新型铸&锻变形GH4096高温合金涡轮盘的成分分布均匀性,解析制备工艺与大尺寸构件成分、组织结构分布的相关性具有重要意义。