Shanthi Subramanian
,
David Muller
,
John Silcox and Stephen L.Sass (Department of Materials Science and Engineering and Department of Applied and Engineering Physics
,
Cornell University
,
Ithaca. NY 14853)
金属学报(英文版)
Local chemistry plays an important role in determining the cohesive strength of grain boundaries in Ni3Al. Doping with B increases the room temperature ductility and changes the fracture mode from intergranular to transgranular, while doping with Zr increases the ductility but leaves the fracture mode predominantly intergranular.Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray Spectroscopy (EDS) were used to probe the changes in local bonding (and hence the cohesive strength) produced by changes in local chemistry at large angle boundaries in Ni3Al.In addition , small angle tilt boundaries were studied to correlate structure with Nienrichment at the interface. B segregation to Ni-rich grain boundaries was shown to make the bonding similar to that of the bulk, thereby increasing their fracture resistance. Ni-enrichment does not occur in the presence of Zr segregation to grain boundaries. Ni-enrichment to antiphase boundaries (APB) in small angle tilt boundaries lowers the APB energy by reducing the number of high energy Al-Al interactions across the interface. Ni-enrichment to large angle boundaries is expected to produce a similar effect on energy.
关键词:
: grain boundary chemistry
,
null
,
null
,
null
Journal of Alloys and Compounds
Motivated by the ion migration mechanism proposed by David et al. [J. Am. Chem. Soc. 129 (2007) 1594-1601], we mechanically mill the Li3N/Co powder mixture to obtain defective Li3N-type structure with enhanced Li+ mobility. Compared with Li3N the hydrogen storage property of such defective phase is markedly improved, presumably due to the ion-migration-enhancing effect from the incorporation of Co and Fe (from milling utensils). During subsequent cycling, however, this effect cannot persist due to the precipitation of Co-Fe alloy upon hydrogenation. The close correlation between Co-Fe incorporation/precipitation and property variation of Li3N hydrogen storage material provides a preliminary evidence to support the ion migration mechanism. (C) 2007 Elsevier B.V. All rights reserved.
关键词:
Hydrogen absorbing materials;Nitride materials;hydrogen-storage;lithium amide;mechanism;nitride;batteries;mixtures;hydride;imides;linh2;anode
Journal of the American Ceramic Society
Search for new thermal barrier materials with high-temperature stability is very difficult because there are intractable choices among thousands of possible candidates. The present paper shows that identifying and developing new low thermal conductivity materials can be accomplished by first-principles calculations of equilibrium crystal structure and elasticity, and thereafter making theoretical prediction of minimum high-temperature lattice thermal conductivity basing on David Clarke's method for insulating materials. We highlight a new oxynitride ceramic, Y4Si2O7N2, with a low theoretical minimum thermal conductivity of 1.12 similar to W center dot m-1 center dot K-1. The theoretical result is further validated by experimental measurement of thermal conductivity of dense and bulk Y4Si2O7N2. The experimental thermal conductivity is 1.50 similar to W center dot m-1 center dot K-1 at 1300 similar to K. Furthermore, thermal properties of Y4Si2O7N2 are investigated for evaluating its potential performances.
关键词:
theoretical elastic stiffness;barrier coatings;mechanical-properties;gas;gamma-y2si2o7;crystal;y2sio5;system;phase;si3n4-sio2-y2o3
Acta Materialia
In order to achieve better understanding of the structural/property relationships of La(2)T(2)O(7) (T = Ge, Ti, Sn, Zr, Hf) pyrochlore, first-principles calculations were conducted to investigate the bonding characteristics, elastic stiffness, structural stability and minimum thermal conductivity. The results show that the relatively weak La-O bonds play a predominant role in determining the structural stability, mechanical and thermal properties of these compounds. In addition, the elastic and thermal properties are influenced when the T atom changes from Ge to Hf. When the bonding strength is enhanced by applying hydrostatic pressure, apart from c(11), c(12), and B, which normally increase at high pressures, it is found that the shear elastic moduli, c(44) and G, which relate to the shear deformation resistance, abnormally remain almost constant. The underlying mechanism may help to explain the damage tolerance of pyrochlore compounds. After comprehensive consideration of the elastic anisotropy, a modified David Clarke-type equation is used to calculate the minimum thermal conductivity of the studied pyrochlore materials, which display an extraordinary low thermal conductivity. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
关键词:
Non-binary oxides;Mechanical properties;Thermal conductivity;Density;functional theory;Local density approximations (LDA);oxide pyrochlores;a(2)b(2)o(7);simulation;disorder
郭颖
,
赵珊茸
,
刘慧芳
,
蒋宏
人工晶体学报
doi:10.3969/j.issn.1000-985X.2005.04.017
对钠长石(Ab)-钙长石(An)-透辉石(Di)三元体系跨相界线的不同成分点在不同的降温速度条件下快速结晶形成的枝状形貌进行了研究.透辉石为枝晶,斜长石为球粒晶.当降温速度慢于1℃/min后,枝状形貌变为板-柱状形貌(即近平衡形貌).我们的实验结果中没有观察到如E.Brener 和H.Muller等人提出的形貌演化图一样的演化规律.当成分点靠近同结线后,出现两晶共结形成的"中空结构".通过光学显微镜下测试其光学性质发现,透辉石沿c轴方向延长,斜长石沿a轴延长,与晶体结构中的强键链方向一致,即快速结晶时晶体沿着结构中负离子配位多面体共角顶、共棱形成的强键链方向生长形成枝体.
关键词:
Di-Ab-An体系
,
枝状形貌
,
各向异性
,
结晶学方向
何捍卫
,
周科朝
,
胡岳华
功能材料
用电化学循环伏安测试技术研究了铜在甲胺-铁氰化钾抛光液中钝化成膜的机理.通过大幅度改变电位扫描速率,分析了氧化峰电流(IApa)、氧化峰电位(EApa)及阳阴峰电流的比值(IApa/IApc)与相应电位扫描速率的关系.结果表明,氧化峰电流和氧化峰电位均与电位扫描速率的平方根成线性关系,说明成膜复盖度θ与电位扫描速率无关,成膜过程符合Muller模型.成膜过程中,电极反应存在后置化学转化,铜在去极化剂作用下发生失去一个电子的阳极溶解反应,然后再进行化学转化反应生成Cu4[Fe(CN)6]钝化膜.在CMP过程中,具有这种钝化膜的铜的腐蚀电流密度及抛光速率随抛光转速的增加而增大.
关键词:
铜
,
化学-机械抛光
,
钝化
,
机理
Physica Scripta
The impurity displacements and the g factors g(parallel to) and g(perpendicular to) of the two tetragonal Ni3+ centres (I and II) in SrTiO3 are theoretically investigated by using the perturbation formulae of the g factors for a 3d(7) ion with low spin (S = 1/2) in tetragonally distorted octahedra. In these formulae, the contributions to the g factors from the tetragonal distortion, the ligand spin - orbit coupling coefficient, and the ligand orbitals, which were usually neglected in previous studies (Muller K A et al 1969 Phys. Rev. B 186 361; Lacroix R et al 1964 Helv. Phys. Acta 37 627), are taken into account based on the cluster approach. Centre I ( or II) was ascribed to the substitutional Ni3+ at the Ti4+ site, associated with the nearest neighbour oxygen vacancy V-O (or the near neutral V-O -(- 2e)) along the C-4 axis as a compensator. The impurity Ni3+ is found to shift away from ( or towards) the compensator by about 0.033 nm ( or 0.010 nm) along the C-4 axis, respectively. As a result, the ligand octahedron around the impurity is elongated ( compressed) in centre I ( II), corresponding to the negative ( positive) anisotropy Delta g (= g(parallel to) - g(perpendicular to)). The theoretical g factors based on the above impurity displacements show good agreement with the observed values.
关键词:
atomic screening constants;transition-metal ions;paramagnetic-resonance;buffer layers;seed layers;superposition model;optical spectra;scf functions;thin-films;cr3+ ions
