Probing the effect of R-cation radii on structural,vibrational, optical,and dielectric properties of rare earth (R=La,Pr, Nd) aluminates

The present study correlates the effect of R-cation radii on structural, vibrational, optical, and dielectric properties of rhombohedral rare earth aluminates RAlO₃ (R = La, Pr, Nd). The polycrystalline samples of RAlO₃ have been synthesized using sol-gel synthesis technique. Pure rhombohedral phase of RAlO₃ samples has been confirmed with X-ray diffraction. Systematic decrements in the lattice parameter, bond length, and bond angle have been observed, giving rise to structural distortion due to decrease in ionic radii of R-cation. The phononic properties of RAlO₃ have been investigated through Raman spectroscopy, where the degree of distortion of AlO₆ octahedra can be analyzed with the peak position of E_g and A_1g modes. An increase in the energy bandgap with decreasing R-cation radii shows an interconnection with the decrease in Al–O bond length. Interestingly, the decreasing dielectric constant with decreasing ionic radii of R-cation has been correlated with the difference in electronegativity of cation(R³⁺)-anion(O^2?) pair. Also, a positive linear relationship between dielectric constant and energy bandgap has been investigated using Penn model.     

An approach to the evaluation of multivariate data during ball end milling free-form surface fragments

We present a method for investigating the process of ball end milling, a technology widely used in tool making and moulding. We analyse the main features of free-form surfaces used in this technology, and propose a sequence of steps to identify the most suitable milling strategy. The basic idea of such a sequence lies in the definition of the tangible fragments of free-form surfaces applicable to tool making. Tangible fragments represent the parts of tooling and signed radii that can distinguish both the active and transitional surfaces of tooling. Free-form surface fragments were selected since they are capable of securing definiteness in measurement of roughness parameters and surface errors. We investigated the operation capability of solid ball end milling cutters in terms of cutting tool edge micro-geometry. Cutting edge radius (r_n) and roughness parameters of the tool edge were measured to determine the relationship between new and worn tool edges. Roughness parameters were measured at different parts of the machined surfaces, which take on typical features of dies and moulds, such as inclined wall, ridge lines, valley lines, as well as, the active surfaces defined by signed radii. Surface error such as scallop height, gouging, tolerances and actual signed radii were measured at transitive surfaces. The traditional approach of evaluating roughness parameters was used to determine the suitability of factors such as milling operation, milling strategy and direction of milling. In addition, traditional approaches such as relationships, distributions and histograms were also used. We applied Khattree–Naik’s plot, which proves its suitability to visualise all the data being measured in the same units: microns and millimetres. Characteristic features of the ball end milling process, such as tool edge micro-geometry, geometry of the machined surface, and unit length of the transitive surface were applied in Khattree–Naik’s plots. We found that this plot was capable of processing multivariate data to distinguish specific markers of the quality of machined surfaces, which are produced in ball end milling.     

Diagrammatic Separation of Different Crystal Structures of A2BX4 Compounds Without Energy Minimization: A Pseudopotential Orbital Radii Approach

The A₂BX₄ family of compounds manifest a wide range of physical properties, including transparent conductivity, ferromagnetism, and superconductivity. A 98% successful diagrammatic separation of the 44 different crystal structures of 688 oxide A₂BX₄ compounds (96% for 266 oxide‐only) is described by plotting the total radius of the A atom R_A versus the radius of the B atom R_B for many A₂BX₄ compounds of known structure types and seeking heuristically simple, straight boundaries in the R_A versus R_B plane that best separate the domains of different structure types. The radii are sums R_A = R_s(A) + R_p(A) of the quantum‐mechanically calculated “orbital radii” R_s(R_p), rather than empirical radii or phenomenological electronegativity scales. These success rates using first‐principles orbital radii uniformly exceed the success rates using classic radii. Such maps afford a quick guess of the crystal structure of a yet unmade A₂BX₄ compound by placing its atomic orbital radii on such maps and reading off its structure type.     

基于图像处理的单颗粒金刚石曲率半径测定方法研究

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采用优质天然金刚石对研、精磨和钎焊工艺制备单颗粒金刚石磨具,用于研究陶瓷等硬脆材料磨削去除、砂轮磨粒磨损及工件表面损伤形成机理。通过扫描电镜获取金刚石磨粒尖端图像,运用灰质化、直方图均衡化、中值滤波、二值化分割及Canny算子预处理并提取磨粒尖端轮廓边缘。在Photoshop软件中测量图像标尺占据的横向像素数目,绘制不同尺寸的圆形与磨粒尖端轮廓线条内切,多次操作后选择最大内切圆。统计该内切圆半径对应的像素个数,利用代数学基本原理即可求出最大内切圆半径。该方法不依赖专用的小曲率半径精密测量设备,具有较高测量精度与可操作性。
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End formation of a round tube into a square section having small corner radii

Expansion and reduction are the two common end forming processes for tubes. In the tube end expansion process using a square punch, it is difficult to obtain a small corner radii due to the stretching of the tube around the punch corners. The wall thickness around the corners is small when compared to the side wall. Hence, a tube having a poor square look is formed. In this study, a 2-stage end expansion of a round tube end into a square section having an improved square look i.e. small corner radii and increase in wall thickness around corners is developed. In the 1st stage, the tube end is flared into a cone shape using a 30° conical die by axial compression. In the 2nd stage, the conical end of the tube is drawn through a taper square die using a conical bottom square punch, and a near square section is formed. A 15% ironing ratio is applied during the drawing process to flatten the side wall of the square. Experimental and FEM simulation were performed to evaluate and to verify the forming process. Although the height of the square section increases when the punch stroke at the 1st stage is increased. However, this increase is limited by the buckling of the pipe at the circular section of the thick blank tube. Since the conical end is drawn into a square section having different radial lengths, the bottom of the square section is uneven. The uneven bottom end is trimmed off in the later process. A square section having a maximum height of 32 mm after trimming is successfully obtained from the experiment for the punch stroke, S = 44 mm using an API 5 L tube.     

顶镦压力曲研究

锅炉蛇形管小R制造中,先将管子预弯到1.5倍D半径,然后再挤压到1倍D半径。但数控顶镦弯管机可以直接弯制1倍D半径管子。     

Absence of a Competition Between Magnetism and Superconductivity in Layered Nickel Borocarbides: Common Correlations of Tc with Crystal Chemical Parameters for Magnetic and Nonmagnetic Compounds

In the present work, an analysis of the crystal chemical parameters of nickel borocarbides RNi₂B₂C (R = rare earth) is given. The reasons for the dependence of superconducting transition temperature (T _c) on crystal chemical parameters by two separate curves for magnetic and nonmagnetic R are considered. For all R, a common pattern of dependence of T _c on crystal chemical parameters similar to that existing in layered quasi two-dimensional systems (HTSC cuprates and diborides) is established. The absence of the influence on the T _c of borocarbides of magnetic properties R is also established. On the basis of the correlations found, the radii of a number of rare earths are more precisely defined, and T _c of compounds at various substitutions R are calculated.     

剪挤复合精整中凸模圆角对核心变形区应力状态的影响

目的为了提高剪挤复合精整加工零件的成形质量,研究了关键工艺参数对核心变形区应力状态的影响。方法利用理论分析的方法,确定了凸模圆角是影响核心变形区应力状态的关键工艺参数;进一步利用有限元模拟和物理实验相结合的方法,研究了不同凸模圆角半径对核心变形区应力状态的影响。结果在一定范围内,模具圆角越大,所提供的压应力也就越大。结论压应力可以抑制撕裂产生,从而降低成形面损伤,所以一定范围内增大模具圆角可以提高成形质量。     

基于协方差矩阵变换的相干源个数估计

正确地估计信号源个数是高分辨阵列信号处理的一个重要组成部分,本文提出了一种相干源个数估计的方法。这种方法是先采用空间平滑技术对信号源去相关,然后对阵列协方差进行酉变换,最后用基于协方差矩阵变换的信息论准则和盖氏半径(Gerschgorin Radii)两种方法估计信号源个数。计算机仿真结果证实了方法的可行性。     

液体温度对空泡生长和溃灭过程的影响

采用光偏转测试系统研究了不同温度纯净水中激光空泡脉动过程,通过实验获得了激光空泡在靶表面膨胀和收缩全过程,确定了空泡的最大、最小泡半径、脉动周期和泡壁运动速度。实验采用0℃到70℃的纯净水,测量了空泡的泡半径和脉动周期等特征参量变化情况。实验结果表明,液体温度是影响空泡脉动的一个非常重要的因素,随着液体温度的增加空泡的最大泡半径和溃灭周期均呈增加趋势。给出了相应的理论解释。