Questioning the Assumptions of the Parabolic Model for Superconductive Cuprates

The assumptions of the parabolic model are questioned. These assumptions pertain to an expectation of universal T _c optima for cuprates at an experimental hole concentration of p = 0.16n, where n is the number of CuO₂ planes. This model was developed based on the T _c maximum for La_{2 – x} Sr _x CuO₄ at x = 0.16. However, a variety of cases are presented for higher optimal hole concentrations, including La₂CuO_4.16, where it is twice as high. Also, the success of a charge order model in universally predicting optimal T _c at formal stoichiometric holes, h = 0.5n, suggests a need for expansion of the parabolic model. By quantitatively taking into account the deleterious effect of the blocking layer, optimal T _c can be absolutely calibrated at a uniform optimal charge order with alternate holes.     

Estimation of the Surface-d to Bulk-s Crossover in the Macroscopic Superconducting Wavefunction in Cuprates

The concept of a surface d- and bulk s-symmetry in cuprate superconductors is applied to recent small-angle neutron-scattering results. These show a change of hexagonal to square vortex lattice as a function of the magnetic field along the c-axis. Identifying the hexagonal lattice with s- and the square with d-symmetry, the crossover distance from the surface d to the bulk s perpendicular to the c-axis is estimated to be 35 nm for LSCO and roughly 7 nm for YBCO, both at optimum doping. The crossover along the c-axis has to be of only a few layers distance to reconcile tunneling, photoemission, and pulsed femtosecond reflectivity experiments. These estimates are compatible with -rotation, NMR, and other experiments.     

Instanton Sector of Correlated Electron Systems as the Origin of Populated Pseudo-gap and Flat “Band” Behavior: Analytic Solution

Finite temperature instantons between meta-stable vacua of correlated electronic system are solved analytically for quasi one-dimensional Hubbard model. The instantons produce dynamic symmetry breaking and connect metallic state with the dual vacua: superconducting (SC) and spin-density wave (SDW) states. The instantons spread along the Matsubara’s imaginary time and possess the structure similar to the coordinate-space solitonic lattices previously discovered in quasi one-dimensional Peierls model. On the microscopic level the inter-vacua excursion is described by mutual transformations between the “resonating quartets” of the couples of electron–hole and Cooper pairs. Spectral properties of the electrons in the “instantonic crystal” reveal pseudo-gap (PG) behavior, with finite fermionic density of states in the center of the PG and “flat-band” outside of it. Analytically derived inverse temperature scaling of the pseudo-gap and the densities of the SC and SDW condensates is discussed in the context of ARPES and STM data in high-T_c cuprates.     

Two Domains of the Parameter Space of the 2D Hubbard Model for High-Tc Superconductors

High-T _c superconducting cuprates have two types of Fermi surfaces: simple-2D-tight-binding-band type (LSCO type) and the much deformed one (Bi2212 type). The difference is attributed to that of band parameter values, i.e., t′ ∼ −0.1 and t″ ∼ 0 versus t′ ∼ −0.3 and t″ ∼ 0.2 in terms of the second- and third-neighbor transfer energies t′ and t″, respectively (energy unit is the nearest-neighbor transfer energy t). Assuming a moderate value of on-site Coulomb energy U ∼ 6 and performing the variational Monte Carlo computation, we found that the two superconducting parameter domains exist in fact around these parameter sets, respectively, in which superconductivity predominates over spin density wave (SDW) due to the latter being at the brink of vanishing. Stripes were obtained in the first domain but tend to disappear in the second. In the latter domain there seems to exist parameter sets for which superconductivity appears without doping.     

Simple measurement of the AC transport current loss for HTS conductors using an active power detection method

Reducing AC loss in a superconducting apparatus is one of the most important issues, and the precise measurement and estimation of AC losses are essential to reduce them. The four-terminal method is universally used as an electric measurement method of AC losses for superconducting tapes and wires. In this method, noise and inductive voltage superposed on the terminal voltage of the superconductor are eliminated by a lock-in amplifier and cancel coil, respectively, and then measurement of very small resistive voltage is achieved. However, using this conventional method, a plurality of measuring instruments and apparatuses are needed, and therefore the measuring system becomes complicated and much time is consumed in the calibration process. In this paper, we present a simple and precise measurement system based on an active power detection method, which is proposed as a quench detection method. The proposed system consists of a small number of instruments and apparatus and is less susceptible to noise. Its usefulness is verified by comparing the proposed method and the conventional four-terminal method in measuring the AC transport current loss of a Bi2223/Ag tape.     

An Effective Emissivity Model for Rapid Thermal Processing Using the Net-Radiation Method

A reflective shield has been placed in the lower chamber of some rapid thermal processing (RTP) systems so that the temperature of the silicon wafer can be accurately measured in situ with light-pipe radiometers. Better knowledge of the effective emissivity of the wafer reduces the uncertainty in the temperature measurement. This paper describes an enclosure model based on the net-radiation method for predicting the effective emissivity of the wafer. The model treats the surfaces in the enclosure as diffuse emitters, with a reflectivity that may include a diffuse component and a specular component. Using this model, a parametric study is performed to investigate the influence of the geometric arrangement, surface temperature and properties, and wavelength on the effective emissivity. The algorithm developed in this work may serve as a tool to improve radiometric temperature measurement in RTP systems.     

一类奇异椭圆方程正解存在性

研究了一类奇异椭圆方程。利用变分方法,证明了奇异椭圆方程正解的存在性和唯一性,包含和推广了已有的些结果。     

基于Lattice-Boltzmann方法的填充型复合导热材料的导热性能数值分析

陶瓷被广泛用作催化燃烧的催化剂载体。为及时将反应热导出,避免催化剂烧结、失活,需提高陶瓷导热能力。针对通过填充高导热性的金属或无机填料的方法制成的陶瓷复合导热材料,引入二维九速度不可压格子多相Lattice-Boltzmann模型,对构造的5种规则填充形状(圆形、正八边形、正六边形、正四边形和正三角形)的陶瓷基复合材料进行了完整的二维导热过程的数值分析。结果表明,在相同的填料和填充体积分数下,三角形填充的复合材料有效导热系数最大,填充形状愈趋近圆形则有效导热系数越小。同时模拟了各填充形状下,填充材料与基体材料的导热系数之比kp/km对复合材料有效导热系数的影响。研究结果表明,孤立的导热填料对于复合材料导热系数的提高作用是有限的,当kp/km增大到一定程度,有效导热系数不再明显增加。     

A Revisit of the Semi-Adaptive Method for Singular Degenerate Reaction-Diffusion Equations

This article discusses key characteristics of a semi-adaptive finite difference method for solving singular degenerate reaction-diffusion equations. Numerical stability, monotonicity, and convergence are investigated. Numerical experiments illustrate the discussion. The study reconfirms and improves several of our earlier results.     

Application of the Extended Langmuir Model for the Determination of Lyophobicity of 1-Propanol in Acetonitrile

Surface tensions (σ) of binary liquid mixtures of acetonitrile (ACN) with 1-propanol (PrOH) were measured over the entire composition range at eight different temperatures, 278.15 K, 283.15 K, 288.15 K, 293.15 K, 298.15 K, 303.15 K, 308.15 K, and 313.15 K. The lyophobicities (β) of the surfactant PrOH relative to that of ACN as well as the surface mole fractions () of PrOH at various temperatures were derived using the extended Langmuir model (Langmuir 17, 4261, 2001). The β values indicate the greater affinity of PrOH for the surface, and this trend slightly increases with rising temperature. The determined values indicate that the surface concentration of PrOH is always higher than its bulk concentration and consequently confirm that the surface is enriched with PrOH.     

颗粒增强复合材料热机疲劳性能的模拟分析

针对颗粒增强复合材料，基于杂交应力元的基本原理，推导考虑塑性应变、热应变和蠕变应变的Voronoi单元。利用新单元模拟分析机械疲劳载荷和热疲劳载荷的相位关系及夹杂的拓扑结构对结构的热机疲劳迟滞回线影响。     

On the Homogenization Analysis of Electromagnetic Properties for Irregular Honeycombs

Honeycombs are widely used in aerospace structures due to their low density and high specific strength. In this paper, effective electromagnetic properties of irregular honeycombs are investigated, by using the three dimensional homogenization theory and corresponding computational procedure. This homogenization method, being the extension of two-scale asymptotic approach, is employed to determine the expressions of the effective dielectric permittivity, magnetic permeability and electrical conductivity. To verify and validate the proposed model and procedure, effective permittivities of a typical irregular honeycomb are studied and compared with those of semi-empirical formulae. Moreover, the effect of geometry of honeycomb's unit cell on effective permittivities is also examined. Compared to semi-empirical estimations, the two-scale asymptotic homogenization method can be used to achieve more accurate results of effective electromagnetic properties for honeycombs in the scope of numerical modeling, and it can be also extended for estimation of effective electromagnetic tensors for various periodic composites.     

颗粒填充二元复合材料等效介电特性的修正通用有效介质计算公式

介电特性在复合材料的电磁效应研究和材料设计中具有重要的作用。本工作在研究传统通用有效介质(GEM,General Effective Medium)公式的局限性基础上,提出了用于预测和计算颗粒填充二元复合材料等效介电特性的修正通用有效介质(MGEM,Modified General Effective Medium)公式。运用MC-FEM(Monte Carlo-Finite Element Method)方法分析计算各种参数条件下颗粒随机填充二元复合材料的等效介电特性,并与MGEM公式计算结果进行比较,验证MGEM公式的正确性和有效性。此外,还将MGEM的预测结果与部分经典理论公式的计算结果、部分文献报道的实验测量数据进行了比较。研究表明,在不同介电常数比(1/50~50)和不同体积分数(0~1)的情况下,MGEM公式预测结果与MC-FEM模型结果完全吻合,与实验测量结果基本一致,为颗粒填充二元复合材料等效介电性能分析提供了一种具有较高计算精度的理论计算方法。     

A Multiscale Model for the Effective Thermal Conductivity Tensor of a Stratified Composite Material

Thermal modeling of composites has three essential objectives: (i) comprehension of their thermal behavior; (ii) composite scaling in order to satisfy specific requirements; and (iii) optimal analysis of experimental results from thermal characterization. For a complete study of the material, each of these three points must be taken into account at the fiber scale ( 10m), the yarn scale ( 1 mm), and the composite scale ( 10 cm). This work presents multi-scale modeling of the effective thermal conductivity tensor of a stratified composite material made from carbon fibers, phenolic resin, and carbon loads. The longitudinal and transverse thermal conductivities of the yarn are computed from optical microscopic imaging of the material. The isotropic thermal conductivity of the loaded matrix is computed by the Bruggeman model. Then, the thermal conductivity tensor is determined by a finite element method taking into account the morphology of the fabric. Computed values are close to experimental values measured by classical methods. Finally, analytical relations are proposed to obtain an efficient model which can be used in a multiphenomenon simulation of the composite structure.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.     

梁的离散模型的模态反问题

采用集中质量法或有限差分法对梁进行离散,得到横向振动梁的弹簧-质点-刚杆模型,其质量矩阵为对角矩阵而刚度矩阵为对称五对角矩阵。已知(λ,x)、(μ,y)为梁的两个特征对,W为梁的总质量,考虑由这些数据构造离散梁的物理参数,得出构造具有正的质量和刚度的真实系统的充要条件。如果这些条件得到满足,则系统的构造是唯一的。因为构造梁的离散系统需要的数据可由测试得到,所以其结果适用于模态分析应用。     

Evaluation of the effective emissivity of reference sources for the radiometric emissivity measurements

The effective emissivity of cavities or interreflecting surfaces is formulated in terms of the effective absorptivity using Kirchhoff s law and the reciprocity of the bidirectional reflectivity distribution function. It is shown that, according to the formulation, the effective emissivity can be evaluated by analyzing absorption processes of radiation. Several types of reference sources are evaluated statistically and analytically: cylindro-cones, lateral holes formed on a sample tube, and opaque samples covered by a hemispherical mirror. The radiative properties of the associated surfaces are characterized by the isotropic specular-diffuse model.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Additive Heredity Model for the Analysis of Mixture-of-Mixtures Experiments

Abstract

The mixture-of-mixtures (MoM) experiment is different from the classical mixture experiment in that the mixture component in MoM experiments, known as the major component, is made up of subcomponents, known as the minor components. In this article, we propose an additive heredity model (AHM) for analyzing MoM experiments. The proposed model considers an additive structure to inherently connect the major components with the minor components. To enable a meaningful interpretation for the estimated model, the hierarchical and heredity principles are applied by using the nonnegative garrote technique for model selection. The performance of the AHM was compared to several conventional methods in both unconstrained and constrained MoM experiments. The AHM was then successfully applied in two real-world problems studied previously in the literature. Supplementary materials for this article are available online.     

Three Phase Composite Cylinder Assemblage Model for Analyzing the Elastic Behavior of MWCNT-Reinforced Polymers

Evolution of computational modeling and simulation has given more emphasis on the research activities related to carbon nanotube (CNT) reinforced polymer composites recently. This paper presents the composite cylinder assemblage (CCA) approach based on continuum mechanics for investigating the elastic properties of a polymer resin reinforced by multi-walled carbon nanotubes (MWCNTs). A three-phase cylindrical representative volume element (RVE) model is employed based on CCA technique to elucidate the effects of inter layers, chirality, interspacing, volume fraction of MWCNT, interphase properties and temperature conditions on the elastic modulus of the composite. The interface region between CNT and polymer matrix is modeled as the third phase with varying material properties. The constitutive relations for each material system have been derived based on solid mechanics and proper interfacial traction continuity conditions are imposed. The predicted results from the CCA approach are in well agreement with RVE-based finite element model. The outcomes reveal that temperature softening effect becomes more pronounced at higher volume fractions of CNTs.     

计算复合材料有效弹性模量的重心有限元方法

采用几何法构造出任意边数多边形单元的重心插值形函数, 应用Galerkin法提出了求解弹性力学问题的重心有限元方法。用重心有限元方法对SiC/Ti和B/Al 2种纤维复合材料横向截面的有效弹性模量进行了预报。计算模型取纤维呈六边形排列且为各向同性的代表性单胞, 对其杨氏模量、 剪切模量和体积模量在较大的体积分数范围内进行了数值模拟。通过与解析公式和传统有限元的计算结果对比, 重心有限元方法的计算结果符合解析公式解的趋势, 与传统有限元的计算结果吻合较好。与传统有限元方法相比, 重心有限元方法的单元划分不受三角形或四边形的形状限制, 能够再现材料的真实结构。由于单元较大且数目较少, 本文方法具有很高的计算效率。