过渡族金属碳化物在WC-Co硬质合金中的界面偏析与固溶行为

过渡族金属碳化物是最常见的硬质合金微观组织结构与性能调控添加剂,添加剂在硬质合金中赋存状态的研究是WC晶粒生长抑制机理研究的基础,也是硬质合金材料与工艺设计的基础。从基于第一性原理计算的理论研究、基于高分辨透射电镜和原子探针层析法的实验观察分析等2个方面,综述了过渡族金属碳化物在WC/β(钴基固溶体粘结相)相界偏析行为的研究现状。基于过渡族金属碳化物在硬质合金中晶界与相界的偏析行为、在硬质相与粘结相中的固溶行为,讨论了硬质合金中WC晶粒生长机理与晶粒生长抑制机理以及过渡族金属碳化物对硬质合金性能的影响机理,提出了硬质合金材料与工艺设计的建议。     

Structural and electrical properties of co‐doped zirconia electrolyte for intermediate temperature solid oxide fuel cell application

Structural and electrical properties of 8 yttria stabilized zirconia (YSZ) ceramics doped with varying concentrations (0.5–2.0 wt%) of Mg²⁺ ions were investigated. The result from X‐ray diffraction indicates the formation of fully stabilized cubic phase. Scanning electron microscopy observation confirms the marginal decrease in grain size with addition and increase of MgO content. However, a concentration beyond x = 0.5 wt% results in segregation along the grain boundaries. Complex impedance diagrams showed an enhancement in the grain and grain boundary conductivities with doping of magnesium and are strongly dependent on its concentration. It is observed that among the compositions investigated, 8YSZMg0.5 exhibited higher conductivity at all temperatures (600–800 °C) with a maximum conductivity of 0.0345 S/cm at 800 °C. This is further confirmed by the lowest activation energy of Ea = 0.92 eV, estimated for 8YSZMg0.5 in comparison to 1–1.01 eV observed for other compositions. Comparative evaluation of these results with standard 8YSZ electrolytes reveals the possibility of the effective use of 8YSZMg0.5 as a new electrolyte material for intermediate temperature solid oxide fuel cell applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Velocity measurements inside the concentration boundary layer during copper-magneto-electrolysis using a novel laser Doppler profile sensor

The evolution of the velocity boundary layer during the initial phase of copper electrolysis under the influence of a magnetic field is studied by using particle image velocimetry and a novel laser Doppler velocity profile sensor. With this new sensor, time-resolved velocity measurements within 400 μm of a vertically aligned cathode in an aqueous 0.05 M CuSO₄-solution are presented. In this way, the complex interaction of Lorentz force and opposing buoyancy-driven convection was studied by measuring the resulting velocity profile inside the concentration boundary layer with a spatial resolution of 15 μm. It is shown that the Lorentz force-driven convection only dominates the velocity boundary layer during the early phase of electrolysis and induces a linear velocity profile near the cathode. The linear relationship between the velocity gradient and Lorentz force is determined. With the onset of the opposing buoyancy-driven convection at the cathode, a duplex structure of the boundary layer appears. Its characteristic quantities, given by the horizontal distances, δ_max and δv=0, where the velocity reaches the maximum and where it is equal to zero, remain nearly unchanged, while the maximum velocity, vmax, in spite of the counteracting Lorentz force, increases faster as compared to pure natural convection, depending on the current density.     

An experimental study of combustion characteristics of hydrogen–Air pre-mixture on the wall boundary condition

By means of the Schlieren method, the combustion characteristics of hydrogen–air pre-mixture on wall boundary conditions have been studied in a constant volume combustion cell with ignition point located in the upper part of the cell. The results indicated that the restrictive combustion process of the pre-mixture can be divided into four stages: the laminar flame stage, the cellular flame stage, the squish flame stage and the auto-ignition stage. The squish flames appear on both sides of the main flame under most of the experimental conditions. The appearance of squish flames can cause a fluctuation in pressure change rate. The lower the equivalence ratio is, the earlier squish flames appear spatially and the less the auto-ignition is prone to present itself. The lower the initial pressure is, the more difficultly the squish flames present themselves. As the initial pressure drops, the combustion tends toward directly enter into the auto-ignition stage from the laminar flame stage. The higher the initial pressure is, the earlier the auto-ignition presents itself spatially. Squish flames experience two processes of formation and spread. The restrictive effect of the wall could be a factor that impacts on squish flames.     

A boundary equation of the principal period-2 component in the degree-n bifurcation set

The degree-n bifurcation set is a generalized Mandelbrot set for the complex polynomial P _c (z)=z ⁿ +c. The boundary of the principal period-2 component in the degree-n bifurcation set is first defined and then formulated by a parametrization of its image, which is the unit circle under the multiplier map. We investigate the boundary equation using the geometric symmetry of the degree-n bifurcation set with respect to rays of symmetry in the complex plane. In addition, an algorithm drawing the boundary curve with Mathematica codes is proposed.     

Reliability of atom column positions in a ternary system determined by quantitative high-resolution transmission electron microscopy

Employing an iterative structure refinement procedure, we have determined the atomistic structure of the Σ3 (111) grain boundary in strontium titanate (SrTiO₃) from high-resolution transmission electron microscopy (HRTEM) images. This grain boundary serves as a model system to study the effect of column occupancies on the reliability of the column positions. In this paper we introduce a method to derive confidence regions for the positions of individual atom columns at crystal defects. Based on a statistical approach we first determine the reliabilities of different types of atom columns in regions of unfaulted crystal. Next we extrapolate these reliabilities to obtain the reliabilities of individual atom columns at the grain boundary. The method accounts correctly for random errors and promises to be generally applicable provided that repetitive units of unfaulted crystal structure are contained in the HRTEM image. Under the conditions of the present study, the reliability of a column position correlates with the projected electrostatic potential of the column. Accordingly, the reliabilities of the column positions at the boundary vary with the column type: 0.008 nm for Sr–O columns, 0.014 nm for Ti columns, and 0.018 nm for O–O columns.     

Fracture of polycrystalline Fe−2.3%V−0.12%P alloy

Fracture experiments on a polycrystalline Fe-2.3mass%V-0.12mass%P alloy show that the fracture mode is directly related to the grain boundary segregation of phosphorus. Comparison of the experimental data with theoretical simulations showed that intergranular fracture spreads along those grain boundaries, which contain more than 17 at.% of segregated phosphorus.     

Optimality of Non-Restarting CUSUM Charts

Abstract

The change-point problem for the noncentral skew t distribution is studied in this article. An approach based on Schwartz information criterion (SIC) is used to detect the changes of the parameters of this distribution. Simulations are conducted to illustrate the performance of the proposed procedure. The method is successfully applied to the stock returns of several Latin American countries.     

Numerical Solutions of Diffusion Equations in Multi-Dimensional Slab Geometry by Fourier Expansions

Solutions of multi-dimensional neutron diffusion equations in Cartesian coordinates are obtained in the form of regionwise multi-dimensional Fourier series. Equations for two- and three-dimensional problems are derived. A critical and a source problem in two dimensions, and a one-regional source problem in three dimensions are numerically studied.

Two kinds of Fourier series are numerically examined from the viewpoint of rapidity of convergence of the series.

To obtain the value of the flux more accurately, a method is presented which contributes to improvement of the flux profile.