关键词： 填充方钴矿   石墨烯   纳米复合   热电材料   热电转换性能  

摘要： 填充方钴矿是目前最具应用前景的一种中温区热电材料。方钴矿(SKD)结构中的正二十面体孔隙可填入稀土、碱土、碱金属等原子，产生有效的声子共振散射抑制晶格热导率，并可优化电输运性能。相比于多填方钴矿，单填方钴矿成分简单，合成工艺易控，在工业生产中有望放量生产降低成本，因而受到较多关注。目前方钴矿器件制备中使用较多的单填方钴矿分别为n型YbyCo4Sb12(n-SKD)及p型CeyFe3CoSb12(p-SKD)，其热电优值(ZT)分别约为1.1-1.2及0.8-0.9，相应器件转换效率～7％。提高材料ZT值对于提高器件转换效率极为关键。此外，p-SKD力学性能相比于n-SKD较差，成为方钴矿器件结构失效的薄弱环节，提高p-SKD的力学性能对高性能热电器件的设计集成尤为迫切。\n 纳米复合是优化热电材料性能的一种有效途径。引入均匀分散的纳米颗粒被证明可通过所引入界面及纳米相散射声子降低热导率。但由于纳米颗粒极易团聚，其含量与所引入界面量受限于较低水平，因而限制了热导率的进一步抑制。此外，纳米氧化物颗粒复合体系中载流子散射对电性能的恶化往往不可忽略。有理论研究指出，纳米层状第二相均匀包覆基体相的核壳结构或三维包裹结构比纳米颗粒第二相复合可以更有效地散射声子。实验上在块体材料中实现该三维网状结构需要高柔性且电性能良好的层状第二相。我们以n-SKD及p-SKD为基体材料，以石墨烯为第二相，较系统研究了方钴矿/石墨烯复合材料的制备、微结构、热电性能及其相互关系。此外，以石墨烯为增强相引入陶瓷基复合材料被证明可有效提高陶瓷力学性能，本工作同时研究了石墨烯复合对p-SKD力学性能的影响。全文主要取得如下结果:\n 一、提出以氧化石墨烯(GO)为前驱体，利用其良好的水溶分散性，通过液相分散获得了网状GO对基体颗粒的包覆与均匀分散。利用放电等离子烧结(SPS)，利用其热还原作用削弱GO中的含氧基团，获得C原子Sp3杂化结构向Sp2结构的转换，同时实现了致密化和对GO的原位还原，获得了还原氧化石墨烯(rGO)三维网状包覆的方钻矿基纳米复合材料。\n 二、系统地研究了石墨烯复合对n-SKD及p-SKD热电性能的影响。复合适量石墨烯时，复合材料中厚度为～2-5nm三维网状rGO可强烈散射声子抑制热导率，同时几乎不影响电输运，使得热电优值得到大幅优化，n-SKD及p-SKD的ZT值分别达到1.5及1.06，分别提高～25％及20％。复合含量升高，界面效应逐渐转为体积效应，热电性能逐步恶化。通过建立SKD/rGO复合材料的热传导模型发现，复合rGO后晶界热阻提高～3-5倍，界面声子散射得到有效增强。\n 三、系统地研究了石墨烯复合对p-SKD力学性能的影响。均匀分布于p-SKD基体晶界的网状rGO具有桥接作用，有效阻碍了裂纹的扩展，p-SKD/rGO复合材料的抗弯强度和断裂韧性分别得到提高～40％及25％。\n 四、通过一体化烧结、钎焊等工艺集成了基于n-SKD/rGO及p-SKD/rGO复合材料的尺寸为20mm×20mm×16mm的热电发电器件，测试并评价了其输出特性。由8对发电组构成器件在温差为577K时转换效率达到～8.4％，相比于无rGO复合的SKD器件提高～25％。

关键词： Viscoelastic damping material   Microstructure design   Modal loss factor   Topology optimization   Homogenization  

摘要： The geometric layout and physical properties of a viscoelastic damping material have a significant influence on the damping performance of a passive constrained layer damping (PCLD) structure. This paper presents a two-scale optimization method and aims to find the optimal microstructural configuration of the viscoelastic material (i.e., the optimal effective properties of the material) with maximum modal loss factors of the macrostructures. The modal loss factor is obtained by using the Modal Strain Energy (MSE) method. The material microstructure is assumed to be homogeneous in the macro-scale, i.e., the macrostructure is composed of periodic unit cells (PUC). In the optimization formulation, the relative densities are introduced as the design variables for the material microstructure design, based upon the idea of the Solid Isotropic Material with Penalization (SIMP) method of topology optimization. The modal loss factor of the structure is assigned as the objective function. All the sensitivities of the modal loss factor with respect to the design variables are derived analytically and the optimization problem is solved by Method of Moving Asymptote (MMA) method. Several examples of the design optimization of viscoelastic cellular materials are presented to demonstrate the validity of the method. The effectiveness of the design method is illustrated by comparing a solid and an optimized cellular viscoelastic material as applied to a cantilever beam with the PCLD treatment.

关键词： Stochastic FEM   Computational homogenization   Random geometry  

摘要： In the current work we examine the application of the stochastic finite element method (SFEM) to the modeling of representative volume elements for heterogeneous materials. Uncertainties in the geometry of the microstructure result in the random nature of the solution fields thus requiring use of the stochastic version of the finite element method. For considering large differences in the material properties of matrix and inclusions a standard SFEM approach proves not stable and results in high numerical errors compared to a brute-force Monte-Carlo evaluation. Therefore in order to stabilize the SFEM we propose an alternative Gauss integration rule as resulting from a truncation of the probability density function for the random variable. In addition we propose new basis functions substituting the common polynomial chaos expansion, resulting in higher accuracy for the standard deviation in the homogenized stress at the macro scale.

关键词： Reconstruction   2-point statistics   Approximate correlations   Phase recovery  

摘要： A novel method for the 3D reconstruction of a microstructure from limited statistical information provided by 2D cross sections is developed. In the proposed approach, first full-set statistical information (two-point correlation functions) are extracted from 2D cross sections, and then an approximate 3D microstructure is realized based on them. The proposed method relies mainly on conditional probability theorem to establish explicit functional forms between two-point correlation functions extracted from 2D cross sections and full-set 3D statistics. For 3D realization, a novel phase-recovery algorithm is developed that captures prominent attributions of the microstructure. The salient feature of the proposed realization scheme is the ability to fully reconstruct 3D microstructures from statistical information provided by just one cross section for isotropic microstructures and two perpendicular cross sections for anisotropic ones. A number of illustrative examples are provided to demonstrate the accuracy and the versatility of the proposed scheme. The application of the method for the 3D realization of microstructure using an experimental dataset is demonstrated. Finally, the accuracy of the method in capturing and retaining essential features including volume fractions and characteristic attributions as well as the state of anisotropy and percolation of the phases is discussed. (C) 2015 Elsevier B.V. All rights reserved.

关键词： Gd2Ti2O7   Nanomaterials   Optical-Properties   NIR Reflective Pigment   Micro-Structural Features  

摘要： In this interesting article, the synthesis of nanocrystalline Gd2Ti2O7, material by ball milling followed by annealing, characterization and its application are reported. The Rietveld structural refinement data confirm the cubic pyrochlore Gd2Ti2O7 phase with lattice parameter, a = 1.022(9) nm. The particle size observed by HRSEM image concurs with the average crystalline sizes of particles extracted from XRD data. The optical properties of pyrochlore nanocrystalline Gd2Ti2O7 material are explored from UV-visible, FT-IR, NIR, PL, and Raman spectroscopic measurements. From the present investigation, it is identified that the Gd2Ti2O7 nanopowder may be used as solar reflective as well as color pigment due to its high NIR reflectivity (85-95%) under the wavelength of 750-2500 nm.

关键词： 水泥基   水化硬化   水化机理   微结构   土壤结构   研讨会  

摘要： 为了促进学科发展,充分发挥中国硅酸盐学会的学术交流优势和《硅酸盐学报》作为开展学术交流、学术争鸣重要阵地的作用,定于2016年8月14日;6日在山东省济南市举办水泥基材料水化硬化机理与微结构形成研讨会,研讨内容为:水泥基材料的组成、水化硬化机理、微结构形成及其与物理力学性能的关系。一、论文征集1.征文范围（1）新型水泥基材料的组成与性能;（1）水泥基材料的水化机理

关键词： COMPRESSIVE strength   MINERALS   POWDERED glass   POZZOLANIC reaction   HYDRATION   MECHANICAL behavior of materials  

摘要： The compressive strength of complex binders containing two or three blended mineral admixtures in terms of glass powder (GP), limestone powder (LP), and steel slag powder (SP) was determined by a battery solution type compressive testing machine. The morphology and microstructure characteristics of complex binder hydration products were also studied by microscopic analysis methods, such as XRD, TG-DTA, and SEM. The mechanical properties of the cement-based materials were analyzed to reveal the most appropriate mineral admixture type and content. The early sample strength development with GP was very slow, but it rapidly grew at later stages. The micro aggregate effect and pozzolanic reaction mutually occurred in the mineral admixture. In the early stage, the micro aggregate effect reduced paste porosity and the small particles connected with the cement hydration products to enhance its strength. In the later stage, the pozzolanic reaction of some components in the complex powder occurred and consumed part of the calcium hydroxide to form C-S-H gel, thus improving the hydration environment. Also, the produced C-S-H gel made the structure more compact, which improved the structure's strength.

关键词： 五模材料   微结构设计   各向异性   水声斗篷  

摘要： 五模材料是一种具有固体特征的复杂流体,可通过超材料技术由固体材料经过微结构精心设计近似得到.可调的模量各向异性和固体特征赋予五模材料优越的水声调控能力,在降低水下物体目标强度等领域有着重要潜在应用,因此受到了国内外工程和学术界关注.本文就五模材料基本概念、微结构设计、声波调控、加工制备等方面对该类材料的研究进展进行详细介绍,并对五模材料在工程中应用存在的问题进行了讨论,以期为后续相关研究者提供参考.

关键词： 植物短纤维   拉伸性能   CZM界面模型   纤维微结构   有限元分析   纤维/基体界面  

摘要： 随机植物短纤维增强聚丙烯复合材料(如短云杉/PP和剑麻/PP)质轻、断裂韧性高、环保、可循环使用,是新型绿色环保结构材料,日益广泛地应用于汽车、高速列车和民用飞机的内承力构件。研究其纤维微结构和纤维基体界面性能对复合材料宏观有效性能的影响,对该材料及其结构件的优化设计具有重要的理论和工程应用意义。采用均匀化方法,研究了材料尺度对随机短纤维复合材料宏观有效弹性性能的影响。建立三维随机短纤维复合材料变尺度代表体有限元模型,考虑不同纤维体积含量、纤维长细比,以及短纤维位置、取向和材料各向异性主方向的随机性影响。用自编的有限元前后处理程序,对代表体模型施加9种周期性边界条件,计算了材料正交各向异性有效弹性模量。结果表明,在细观纤维尺度,随机短纤维复合材料有效性能呈现为随机各向异性;随尺度增加,其有效性能逐渐趋于宏观各向同性。采用内聚区模型(Cohesive Zone Model,CZM),表征短植物纤维和基体界面的力学行为,研究界面性能对随机短纤维增强复合材料宏观拉伸性能的影响。建立了含界面相二维随机短纤维增强复合材料代表体单元有限元模型,用自编的有限元前后处理程序,嵌入cohesive界面单元,考虑界面刚度,初始破坏位移和完全破坏位移等参数影响,同时考虑不同纤维体积含量、纤维长细比、尺寸,以及短纤维位置、取向和材料各向异性主方向的随机性影响。模拟了不同纤维含量复合材料的实验拉伸曲线,进而确定CZM模型的界面参数,结果表明,界面刚度影响复合材料有效弹性模量。用多个数值试件的拉伸实验,得到统计平均的纤维基体界面刚度与复合材料有效弹性模量的关系曲线,即Ec-Ki曲线,并给出数值预测公式。数值结果表明,同一复合材料的不同纤维含量的Ec-Ki曲线存在一个临界交点,在交点的强界面刚度一侧,复合材料有效模量随纤维含量增加而提高;在交点弱界面刚度一侧,有效模量随纤维含量增加而减少。交点的位置与纤维、基体的力学性能有关。研究了纤维基体界面切向刚度对复合材料有效弹性模量的影响。

摘要： The automotive industry strive towards minimizing fuel consumption which is commonly done by reducing the weight for all ingoing components within a vehicle without jeopardizing the functionality. In the process of developing improved cast iron components, a research project CCSIM2 (Closing the Chain of SIMulation for cast components part 2) has been performed at Jonkoping University, JU, in collaboration with the automotive industry in Sweden. The researchers at JU have been studying the topic of virtual and computational modeling of materials, trying to connect the microstructure of materials with the mechanical properties. When working with finite element analysis (FEA), material definitions are traditionally defined only by one material property for a whole component, assuming homogeneous properties. In order to predict the local mechanical behavior throughout a whole cast component and incorporate this into FEA for structural analysis a simulation strategy was formulated by J. Olofsson. In order to verify the software simulations, three truck engine cylinder heads were cast consecutively from the same melt with the intention to experimentally test the local material properties. The thesis work presented aims towards gathering deeper knowledge about the casting process and the effect it has on the local mechanical and physical properties. Also to find an improved way to simulate the material behavior of a cast component before sending it to the manufacturing process in order to fit the industry's demands for developing components at a lower cost and in shorter time. Investigations were made on the variations in mechanical as well as physical properties within the third cylinder head with the intent to validate the mechanical properties collected from the two previous examined cylinder heads. Also trying to connect the local microstructural variations of the physical properties throughout the cast component and implement these into a simulation methodology which