Journal of Applied Physics
The effect of 3d transition elements (such as V, Cr, Mn, Fe, Co, and Ni additions) on material properties in L1(0) TiAl has been studied using the first-principles DMol and discrete variational method within the framework of the density-functional theory. The transfer energy calculation indicates that all these 3d elements show a tendency to substitute for Al sites in the stoichiometric TiAl alloy, with the increasing order V < Cr < Mn < Ni < Fe < Co. Our results show that the local distortion of crystal lattice around an impurity atom is nonuniform, and it is found that the important influencing factors on the local lattice distortion are the impurity atom size and the site preference. Based on the effects of the doped alloying elements on the structural parameters and mechanical properties, we conclude that the larger bond strength and the smaller axial ratio benefit the ductility. (c) 2007 American Institute of Physics.
关键词:
alloying elements;site preference;electronic-structure;mn;substitutions;base alloys;compound;nb;deformation;occupancies;aluminides
Guozhen SHEN
,
Di CHEN
材料科学技术（英文）
One-dimensional (1-D) nanostructures have been attracted much attention as a result of their exceptional properties, which are different from bulk materials. Among 1-D nanostructures, 1-D heterostructures with modulated compositions and interfaces have recently become of particular interest with respect to potential applications in nanoscale building blocks of future optoelectronic devices and systems. Many kinds of methods have been developed for the synthesis of 1-D nanoscale heterostructures. This article reviews the most recent development, with an emphasize on our own recent efforts, on 1-D nanoscale heterostructures, especially those synthesized from the vapor deposition methods, in which all the reactive precursors are mixed together in the reaction chamber. Three types of 1-D nanoscale heterostructures, defined from their morphologies characteristics, are discussed in detail, which include 1-D co-axial core-shell heterostructures, 1-D segmented heterostructures and hierarchical heterostructures. This article begins with a brief survey of various methods that have been developed for synthesizing 1-D nanoscale heterostructures and then mainly focuses on the synthesis, structures and properties of the above three types of nanoscale heterostructures. Finally, this review concludes with personal views towards the topic of 1-D nanoscale heterostructures.
关键词:
Nanostructures
,
Heterostructures
,
One-dime
Xiaosheng FANG
材料科学技术（英文）
One-dimensional (1D) nanomaterials and nanostructures have received much attention due to their potential interest for understanding fundamental physical concepts and for applications in constructing nanoscale electric and optoelectronic devices. Zinc sulfide (ZnS) is an important semiconductor compound of II-VI group, and the synthesis of 1D ZnS nanomaterials and nanostructures has been of growing interest owing to their promising application in nanoscale optoelectronic devices. This paper reviews the recent progress on 1D ZnS nanomaterials and nanostructures, including nanowires, nanowire arrays, nanorods, nanobelts or nanoribbons, nanocables, and hierarchical nanostructures etc. This article begins with a survey of various methods that have been developed for generating 1D nanomaterials and nanostructures, and then mainly focuses on structures, synthesis, characterization, formation mechanisms and optical property tuning, and luminescence mechanisms of 1D ZnS nanomaterials and nanostructures. Finally, this review concludes with personal views towards future research on 1D ZnS nanomaterials and nanostructures.
关键词:
One-dimensional
,
ZnS
,
Nanomaterials
,
Nanost
Nanostructured Materials
The Landauer electrical resistivity R of one-dimensional (1D) layered nanostructures is calculated. The analysis of the logarithm of the resistance as a function of grain size reveals that there is a minimum value of resistivity R(min) when the grain size takes a value of d(c), where d(c) depends on the barrier height and excess volume of interface. R increases in the region of d(cl) > d > d(c), where d is the grain size, and d(cl) is another critical value of grain size. The mechanism of the anomalous resistivity behavior is discussed briefly.
关键词:
nanocrystalline;resistance;alloys
Wenhui MA
,
Gang XIE
,
Hua WANG
材料科学技术（英文）
The thermal expansion behavior of La1-xSrxMn1-yCoyO3-d (x=0.2~0.4, y=0.1~0.3) perovskites in air has been investigated. The average linear thermal expansion coefficients increased with increasing Sr content up to 40 mole fraction or Co content up to 30 mole fraction. The expansion is generally attributed to an increase in the average cation radius as some of the cations in the perovskite are reduced in valence when oxygen ions are removed from the structure.
关键词:
Thermal expansion
,
null
,
null
Zhaoyang ZENG
,
Lide ZHANG
,
Xiaohong YAN
材料科学技术（英文）
Ballistic transport of Q1D electrons through multiple magnetic barriers is investigated. It is shown that ballistic conductance peaks disappear progressively with the increase of the oscillation frequency of the confining parabolic potential.
关键词:
Journal of Physical Chemistry B
Size-controlled gold nanocrystals were conveniently synthesized through direct electroreduction of bulk AuCl4- ions in the presence of poly(N-vinylpyrrolidone) (PVP). PVP greatly enhanced the gold particle formation process and also significantly retarded the gold electrodeposition process, allowing the electrochemical synthesis of gold nanocrystals to be carried out in the form of simple electroreduction. This novel electrochemical method may be extended to synthesis of other noble metal nanoparticles with controllable size on a large scale. The PVPK90-protected gold nanocry.stals spontaneously self-assembled into nearly ordered 2D close-packed arrays and interesting 1D nanostructures. The aggregation of unstable PVPK17-protected gold nanocrystals resulted in the formation of ultrathin single-crystalline films. PVP plays multifunctional roles in controlling the size and shape of gold nanocrystals and in inducing individual gold nanocrystals to construct ID nanostructures. The nanoparticle self-assembling technique based on PVP offers a simple, but effective, path to organize individual gold nanoparticles into various 1D and 2D nanostructured materials.
关键词:
shape-controlled synthesis;uniform silver nanowires;optical-properties;size control;nanoparticles;nanorods;transition;mechanism;nanomaterials;microscopy
Journal of Alloys and Compounds
The bonding characteristics of A(3)B L1(2) intermetallics have been studied by the discrete variational X(alpha) cluster method. Two concepts, i.e. similarity of bonding strength and delocalization of bonding electrons, are introduced to explain the ductility or brittleness of the materials. The analysis of bonding characteristics shows that the strength of the A-A bond is similar to that of the A-B bond in A(3)B L1(2) intermetallics, and the delocalized degree of bonding electrons is different between the ductile and brittle intermetallics. This is a dominant factor in governing the ductile/brittle behavior of the materials. Environmental embrittlement in some intermetallics is related to the reactive centers in grain boundaries, which is due to the directional bonding and a small amount of charge transfer, and the reactive centers weaken the ability against the attack of the environmental species. The effect of stoichiometry on ductility contributes to the degree of delocalized bonding electrons. Mechanical strength of the materials is correlated with the covalent bond strength.
关键词:
intermetallics;mechanical properties;bond strength calculations;intergranular hydrogen embrittlement;grain-boundary strength;ll2;ordered alloys;environmental embrittlement;fracture;ductility;co3ti
Nabil S. Youssef
,
K.H.Hegab
材料科学技术（英文）
Some transition metal chelates of two ligands L1 and L2 were prepared and characterized by elemental analysis. The IR and 1H NMR spectra of several chelates of two ligands L1 and L2 showed the involvement of the azo group in chelation with the transition metal ions, in most of the studied chelates, and that the two ligands L1 and L2 showed the involvement of the azo group in chelation with the transition metal ions, in most of the studied chelates, and that the two ligands L1 and L2 were coordinated either in the enol or the keto form. IR spetra also showed that Fe(III)-, Ni(II)-, Mn(II)-, VO(II)-L1 and Cu(II)-L2 chelates behaved in a bidentate manner, in contrast with the two (1:1) M:L cobalt chelates with the two ligands are tridentate. TG analysis indicated the presence of three to twelve water molecules of hydration eliminated on heating up to 150°C and one or two coordinated water molecules removed at 150-180°C. The octahedral structure is proposed for all the chelates, except Cu(II)-L2 and Ni(II)-L1 chelates which have square planar geometry, based on their electronic spectra.
关键词:
