二维原子晶体材料中的各向异性研究概述

二维原子晶体材料简称二维材料,因载流子迁移和热量扩散都被限制在二维平面内,展现出了许多奇特的性质而受到了广泛关注.二维材料的带隙可调特性在场效应管、光电器件、热电器件等领域应用广泛.另外二维材料的自旋自由度和谷自由度的可控性使得二维材料在自旋电子学和谷电子学等领域也引发了深入的研究.不同的二维材料由于晶体结构的特殊性质导致了不同的电学特性或者光学特性的各向异性,包括拉曼光谱、光致发光光谱、二阶谐波谱、光吸收谱、热导率、电导率等性质的各向异性.这些各向异性特性在偏振光电器件、偏振热电器件、仿生器件、偏振光探等领域拥有巨大的发展潜力.二维材料的各向异性还能够用于实现器件性能的最优化.文章介绍了各种二维材料的各向异性的最新研究进展.     

化学电源用泡沫镍的性能研究

作为化学电源的一种重要的电极基体材料,泡沫镍的机械特性和电学特性不仅直接影响到电池极板的综合性能,而且与电池大规模工业化生产的可行性及经济性直接相关.泡沫镍的性能参数包括各向异性、厚度、面密度、镍丝体结构、孔率、孔形等直接影响上述特性.在实际生产中可通过对泡沫基材的选择和生产过程优化控制这些参数.本文根据化学电源对基体材料机械特性和电学特性的要求,结合电池的制造工艺对泡沫镍在化学电源领域的应用进行了专题研究.     

二维原子晶体材料中的各向异性研究概述

二维原子晶体材料简称二维材料,因载流子迁移和热量扩散都被限制在二维平面内,展现出了许多奇特的性质而受到了广泛关注。二维材料的带隙可调特性在场效应管、光电器件、热电器件等领域应用广泛。另外二维材料的自旋自由度和谷自由度的可控性使得二维材料在自旋电子学和谷电子学等领域也引发了深入的研究。不同的二维材料由于晶体结构的特殊性质导致了不同的电学特性或者光学特性的各向异性,包括拉曼光谱、光致发光光谱、二阶谐波谱、光吸收谱、热导率、电导率等性质的各向异性。这些各向异性特性在偏振光电器件、偏振热电器件、仿生器件、偏振光探等领域拥有巨大的发展潜力。二维材料的各向异性还能够用于实现器件性能的最优化。文章介绍了各种二维材料的各向异性的最新研究进展。     

软模板技术制备银的各向异性纳米粒子

纳米银的光学、电学和催化活性与粒子的粒径、形貌和结构之间存在着强烈的依赖关系,研究各向异性银纳米材料的形成具有重要意义.结合软模板技术在各向异性银纳米材料制备方面的研究成果,着重讨论了银纳米棒和纳米线的制备方法和形成机理,概述了树枝状纳米银、纳米银片(盘)、纳米银立方体和三棱柱的研究进展,指出利用软模板技术可合成各种形貌的纳米银粒子,并指出了此类材料在研究中存在的不足,展望了其发展趋势.     

ReX_2(X=S,Se):二维各向异性材料发展的新机遇

二维材料因其不同于体相的超薄原子结构、大的比表面积和量子限域效应等受到了人们的广泛关注。二维各向异性材料作为二维材料家族的一员,其取向依赖的物理和化学性质,使得对该类材料性能的选择性优化成为可能。过渡金属Re基硫属化合物作为各向异性材料的典型代表,具有可调的可见光波段吸收带隙,极弱的层间耦合作用力,以及各向异性的光学、电学性能,现已成为电子和光电子领域的研究热点之一。本文主要介绍了ReX_2(X=S,Se)的晶体结构和基本性质,总结目前该材料体系主流的合成方法,研究其各向异性物理特性及优化的手段和条件,并对ReX_2的制备和发展进行了展望。     

π－共轭聚合物链取向控制

本文综述了π－共轭聚合物及其复合材料链取向控制技术。利用外场（力、磁、电场）、ＬＢ膜技术及与无机材料纳米复合，可以得到导电率高、非线性光学性强、力学性能优良、电学和光学性能高度各向异性的材料。这类材料为满足微电子、光电子、传感及生物、医疗等高技术需求提供了物质技术基础     

聚（甲基丙烯酸甲酯-丙烯酸丁酯）/碳纳米管/羰基铁粉电致形状记忆磁性复合材料的制备

采用超声分散磁场下原位聚合的方法制备了聚（甲基丙烯酸甲酯-丙烯酸丁酯）/碳纳米管/羰基铁粉电致形状记忆磁性复合材料。采用扫描电镜（SEM）、红外热像仪和电学性能测试等方法实验表征了材料的结构与性能。结果表明,在磁场下原位聚合可使羰基铁粉沿磁场方向取向,赋予材料各向异性的导电性能和磁响应性。超声辐照能使碳纳米管均匀分散在...     

新型铝锂合金板材塑性流动与各向异性性能试验研究

通过单向拉伸试验,研究了新型Al-Li-Cu-Mg合金板材的基本成形性能.针对材料显著的各向异性性能,选取屈服强度、抗拉强度、延伸率以及厚向异性指数等材料性能参数进行对比分析,绘制了7个不同取样方向的单向拉伸曲线,研究了材料各向异性的规律.基于对本构方程、各向异性屈服准则的研究及对比,建立了新型Al-Li-Cu-Mg合金的本构模型,并根据实验曲线计算得到Hill48、Barlat89屈服准则中的各向异性参数,结合各屈服准则绘制了新型Al-Li-Cu-Mg合金屈服轨迹.对比分析各试件的断口方向,并结合第一、第三强度理论,分析了材料的各向异性.利用SEM观察试样的断口形貌,分析对比试件断口的韧窝特征及带状特征.研究发现:试件的延伸率越大,其韧窝特征越明显;反之,其带状特征越明显.从微观角度印证了Al-Li-Cu-Mg合金板材存在的各向异性.     

新型稀土磁致伸缩材料Tb0.27Dy0.73Fe1.95的各向异性参数研究

全面测量了《110》轴向取向的Tb0.27Dy0.73Fe1.95(商品名为TDT-110)超磁致伸缩材料的各向异性常数.利用超声波穿透法测量在零磁场下的各向异性弹性常数Cij,用振动样品磁强计测量了该材料的各向异性磁导率μ11和μ33,用磁致伸缩测量仪测量了该材料的各向异性微分磁致伸缩应变常数d.     

塑性加工法制备各向异性稀土永磁材料的研究进展

塑性加工是一种可制备各向异性材料并可实现近终形成形的方法,已有一系列尝试将其应用于制备高性能各向异性稀土永磁材料的研究报导.介绍了用于制备各向异性磁体或磁粉的塑性加工方法,包括热压、模压、轧制、正挤压、反挤压、等径角挤压等,和用各种塑性加工方法提高永磁材料性能的基本原理、效果,以及工业化应用进展.     

Eddy currents in anisotropic composites applied to pulsed machinery

Pulsed electrical machines such as the compensated pulsed alternator (compulsator) are increasingly being considered for mobile applications. Size and weight reduction in these machines is therefore very important. Composite materials, such as carbon fiber and glass fiber epoxy composites, with high strength and low density are a natural choice for the various structural components of these electrical machines. The carbon fiber-epoxy composite (CFC) is particularly useful because of its high strength and modulus. However, this composite (CFC) is electrically conductive with greatly different electrical conductivities along the fibers and transverse to the fibers. When these materials are used in pulsed machines, it is important that the designer be cognizant of the conductivity and treat the anisotropy adequately. This paper illustrates how the eddy current distribution can be determined in carbon fiber composites in light of the anisotropy     

Anisotropy of demagnetization in rectangular electrical sheetsamples placed in a stationary magnetic field

The method of summation used for calculating magnetic field distribution was applied to determine the anisotropy of demagnetization of rectangular samples made of anisotropic and isotropic electrical sheets. The calculations refer to a standard strip used in an Epstein test and to the samples placed in homogeneous. external magnetic field. Changes in the demagnetization anisotropy were determined for different degrees of crystallographic orientation in the materials. In the case of electrical strip sheets, the results make it possible to determine the real B-H magnetization characteristic for the material, which is also very important in many cases for instrumentation     

Research on the Planar Electrical Anisotropy of Conductive Woven Fabrics

The main aim of the research is to determine electrical anisotropy of woven fabrics to describe the multidirectional dependence of the electrical resistance of woven fabrics. The van der Pauw electrodes configuration is used to determine the electrical resistance. Scanning electron microscope–energy-dispersion X-ray spectroscopy (SEM–EDS) analysis is carried out to identify the real amount of conductive elements on a fabric surface as a result of yarns or fabric metallization and to explain the electrical conductivity of the textile materials. The planar electrical anisotropy of electroconductive woven fabrics is observed. The maximum and minimum values of resistances of woven fabric are connected with the weft/warp of which direction coincides with the direction of the longitudinal and transversal axes in the sample plane. SEM–EDS analysis confirms that the electrical conductivity of fabrics depends on the samples elemental composition and the amount of metals deposited on yarns or woven fabrics. It is observed that the sample metallization with thick coating gives a better solution than weaving textiles from coated yarns from the point of view of electrical properties.     

On the fracture toughness anisotropy of mechanically poled ferroelectric ceramics

In this paper an incremental constitutive theory for the deformation due to switching in ferroelectrics is applied to predict the fracture toughness anisotropy in these materials after mechanical poling. Mechanical poling of an initially unpoled specimen differs from electrical poling in that only mechanical stresses are applied to the material. Therefore, no electrical polarization can develop. After mechanical poling, for example by a uniaxial applied stress, the fracture toughness of a ferroelectric ceramic for cracks running parallel or orthogonal to the poling direction will differ. Finite element computations of the steady crack growth process have been carried out to quantify these differences. Results are generated for a range of constitutive properties for three crack growth directions with respect to the initial mechanical poling direction. The results are discussed in relation to available experimental data and to the toughness anisotropy due to electrical poling.     

Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination: A Chemical‐Bonding Perspective

Thermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial‐and‐error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV–VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV–VI compounds with octahedral‐like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density‐of‐states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight‐binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides.     

Strong Anisotropy and Ultralow Percolation Threshold in Multiscale Composites Modified by Carbon Nanotubes Coated Hollow Glass Fiber

Inspired by biological materials, the use of combined fillers of different types and sizes has led to multiscale, hierarchical composites which are considered to be the multifunctional materials of the next generation. However, the effects of hierarchical architecture on the electrical properties and percolation behavior remain poorly understood. Here, a multiscale polymer‐based micro‐/nano‐composite with hollow glass fibers coated by carbon nanotubes (CNTs) has been produced based on a simple dip‐coating approach. Besides a significant increase in electrical performance, the composites exhibit a very strong anisotropy of electrical properties with the difference of 2–5 orders of magnitude in different directions. In the longitudinal direction of composites, an ultralow percolation threshold is found. These unique properties are shown to be related to the hierarchical morphology, which gives rise to the existence of two percolation levels with different thresholds: a local threshold in the nanoscale 2D CNT networks at the fiber‐polymer interfaces and a global threshold in 3D network formed by the fibers. This study helps to deeper understand the macroscopic electrical performance of the hierarchical composites, potentially opening up new ways for designing novel materials via flexible tailoring the orientation of fiber and the morphology of interfaces.

     

Effects of elastic anisotropy on the surface stability of thin film/substrate system

The surface stability of thin film/substrate system is an important problem both in the film synthesis and reliability of micro electrical and mechanical system (MEMS). In this work, the elastic anisotropy effect on surface stability of thin film/substrate system was considered. The theoretical analysis indicates that elastic anisotropic influence could play an important role in the surface stability of thin film/substrate system. And the anisotropy effect should be considered both in the thin film synthesis process and its service reliability. In addition, there exists an nondimensional parameter k for cubic crystalline thin film materials in evaluating the anisotropic effect. When k is larger than one unit, the surface stability will be weakened by anisotropic effect; vice versa. The method used in present work could be easy extended to multi-layered thin film/substrate system and help us to consider the elastic anisotropy effect.     

Electrical properties of hydrogenated InSe crystals

We have studied the effect of hydrogenation on the electrical properties of InSe layered crystals and their anisotropy. Hydrogenation has been shown to reduce the in-plane electron mobility in InSe and to raise its in-plane electrical conductivity and electron concentration. The decrease in anisotropy upon InSe hydrogenation is due to the marked rise in out-of-plane conductivity.     

Raman Signatures of Broken Inversion Symmetry and In‐Plane Anisotropy in Type‐II Weyl Semimetal Candidate TaIrTe4

The layered ternary compound TaIrTe₄ is an important candidate to host the recently predicted type‐II Weyl fermions. However, a direct and definitive proof of the absence of inversion symmetry in this material, a prerequisite for the existence of Weyl Fermions, has so far remained evasive. Herein, an unambiguous identification of the broken inversion symmetry in TaIrTe₄ is established using angle‐resolved polarized Raman spectroscopy. Combining with high‐resolution transmission electron microscopy, an efficient and nondestructive recipe to determine the exact crystallographic orientation of TaIrTe₄ crystals is demonstrated. Such technique could be extended to the fast identification and characterization of other type‐II Weyl fermions candidates. A surprisingly strong in‐plane electrical anisotropy in TaIrTe₄ thin flakes is also revealed, up to 200% at 10 K, which is the strongest known electrical anisotropy for materials with comparable carrier density, notably in such good metals as copper and silver.     

Probing magnetic anisotropy and spin polarization in spintronic materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.