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1.
Mechanically alloyed metal hydride systems   总被引:7,自引:0,他引:7  
Mechanosynthesis of metal hydrides is a new field in which important progress has been reported. In this paper, we present recent developments in mechanosynthesis of magnesium-based hydrides for storage applications. The effect of intense milling on magnesium and magnesium hydrides is presented. The influence of various additives on hydrogen-sorption properties is discussed with special emphasis on nanocomposite MgH2+5 at. % V, where hydrogen-storage characteristics, cycling properties and the mechanism of hydrogen desorption are presented. The production of novel nanocrystalline porous structures by mechanical alloying followed by a leaching technique is discussed. Hot ball-milling, as a new method for rapid synthesis of alloys, is also presented. Finally, two other methods of production of metal hydrides are discussed. One is reactive milling where metal hydrides are synthesized by mechanical alloying under hydrogen pressure, while the other is milling elemental hydrides to produce complex hydrides. Received: 15 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001  相似文献   

2.
The effect of interdiffusion on band structure and Bloch amplitudes of three dimensional superlattice composed of Gaussian shaped double quantum rings is investigated using the Fourier transformation to the momentum space. It is shown that the sequence of the principal quasimomentum vectors which correspond to the energy increase depends on superlattice symmetry. Due to interdiffusion both the first and the second minibands of conduction band shift to the region of higher energies and broaden, meanwhile the difference between the behaviors of the dispersion curves in different directions of momentum space gradually disappears. The obtained results indicate to the opportunity of purposeful manipulation of structure characteristics by means of interdiffusion both quantitively and qualitatively.  相似文献   

3.
Using ab initio total energy calculations we demonstrate that the nanotubes of germanium with atomic structure based on an alternate prism and antiprism stacking of hexagonal rings, can be stabilized by metal encapsulation. The V or Nb doped infinite nanotube is metallic. However, Mo doping leads to the formation of a metal encapsulated direct band gap semiconducting nanotube of germanium. These nanotubes with metal dependent electronic properties could prove to be vital for the development of future nanotechnologies.  相似文献   

4.
In the present paper, a theoretically study of the non-resonant laser field effect on the optical response, such as nonlinear optical rectification (NOR), second (SHG) and third harmonic generation (THG) coefficients in double finite oscillator potential (DFO) quantum wells is performed in the framework of the effective mass approximation. The obtained results reveal that, energy states and optical response is significantly affected by the non-resonant intense laser field (ILF) intensity and symmetry of the structure. Also it was found that the laser field is more effective on the optical response in the DFO potential when the asymmetric character of the confinement potential is strong. Thus, the NOR, SHG and THG coefficients with designated values can be obtained by using a properly adjusted ILF intensity and symmetry parameter of confinement potential.  相似文献   

5.
The electronic density of states and electrical conductivity are calculated for very thin metal layers with an extension of the Heine-Haydock-Kelly-method, using a tight-binding Hamiltonian fors- andd-bands includingd-band-mixing. In particular the importance of level shifts at surface atoms is considered. In the calculation of the electrical conductivity onlys-wave scattering has been considered, however otherwhise the method is exact in principle, at least fors-bands. The results depend strongly on the level shift.Based on the Thesis of K. Diebenbusch, Regensburg 1977  相似文献   

6.
本文中使用了基于密度泛函理论(DFT)的第一性原理方法,构建了手性指数(n,m)为(6,6),(10,10)硅纳米管管以及二者嵌套的双壁管结构并计算其能带结构、电子态密度以及光学反射谱与吸收谱。计算结果表明(6,6),(10,10)管表现出明显的半导体性,是直接带隙半导体。而双壁管相比于单臂本征硅纳米管其导带底与价带顶接触几乎发生交叠,表现出了轻微的金属性,其管内电子非局域性增强,导电能力提高。手性系数为(6,6)的硅纳米管对红外光、可见光以及紫外光都具有吸收能力。(10,10)管对于紫外光的吸收能力更加优秀但是对红外光的吸收能力减弱。(6,6)&(10,10)双壁管无论是吸收谱还是反射谱都最窄并出现了单峰。故可推测双壁管可作为检测一类特定范围波长的探测器的材料。  相似文献   

7.
The electronic properties of nanoclusters of transition (Ni, Co, Cr) and noble (Au, Cu) metals deposited on the surface of highly oriented pyrolytic graphite (HOPG) are studied using the method of X-ray photoelectron spectroscopy. The laws of variation of a change ΔE b in the binding energies of core-level electrons in the initial (ΔE i) and final (ΔE f) states of atoms in nanoclusters, the intrinsic widths γ of photoelectron lines, and their singularity indices α as functions of the metal cluster size d are determined. A qualitative difference in behavior of the ΔE i(d) and α(d) values in metals of the two groups (Ni, Cr versus Co, Cu) is found. The values of the final-state energy (ΔE f < 0) and the line width (Δγ > 0) in the clusters of all metals studied vary in a similar manner. It is shown that a significant contribution to E i is due to a transfer of the valence-shell electrons at the cluster-substrate interface, which is caused by the contact potential difference. The value of an uncompensated charge per nanocluster is determined as a function of the cluster size and the number of atoms in the cluster. The behavior of ΔE f(d) is controlled by the Coulomb energy of a charged cluster and by a decrease in the efficiency of electron screening, which is different in the metals studied. The broadening of photoelectron lines is determined by a spread of the cluster sizes and by lower electron screening in the final Fermi system. An asymmetry of the core-level electron spectra of nanoclusters can be explained using notions about the electron-hole pair excitation near the Fermi level. The effect of the structure of the density of electron states in the d band of transition metals on the asymmetry of photoelectron lines is considered and it is concluded that this structure near the Fermi level qualitatively changes with a decrease in the nanocluster size. The obtained results indicate that the behavior of the electron subsystem of clusters of the d-metals in a size range of 2–10 nm under consideration is close to the behavior of a normal Fermi system.  相似文献   

8.
The atomic arrangement in a solid contains a great amount of information, and observation of its structure is essential for understanding the electronic and magnetic properties of transition metal oxides at a microscopic level. Increasing interest in the surfaces and interfaces of oxide systems, which is partly driven by the anticipation of device applications, enhances the importance of structural studies of the near-surface region. We review various types of structural studies with x-ray scattering on the near-surface region of metal oxides-from thick films to surfaces-in order to clarify the structural effects on their electronic properties.  相似文献   

9.
We apply first-principles method to investigate the effect of the diameter on the stability and electronic properties of zigzag carbon nanotubes doped with iron, nickel and manganese impurity atoms. In this contribution we follow the evolution of the electronic and structural properties as a function of the nanotube diameter. As a general result, we found that the binding energy decreases with the increasing nanotube radius. Additionally, depending on the interaction of transition metal impurity with the tubular carbon structure, it is observed that the total magnetization varies with the tube diameter due to hybridization and confinement effects. It is also shown that such magnetization varies with the curvature radius, increasing for manganese impurity atoms and decreasing for iron and nickel.  相似文献   

10.
Transition metal dichalcogenides (TMDCs) have attracted various research interests as one of the priorities of materials research due to their promising properties, especially in the field of thermoelectricity. The efficiency or performance of thermoelectric devices is expressed in terms of the thermoelectric figure-of-merit (ZT) – a standard indicator of a material's thermoelectric properties for use in cooling systems. The evaluation of ZT is principally determined by the thermoelectric characteristics of the nanomaterials. In this paper, a set of investigative computations was performed to study the thermoelectric properties of monolayer TMDCs according to the semiclassical treatment of the Boltzmann transport equation. It was confirmed that the thermoelectric properties of 2D materials can be greatly improved compared with their bulk properties. Calculations show an improvement in the power factor for the TMDCs under consideration, and, thus, the ZT compared to the bulk state due to an improvement in the Seebeck modulus and electrical conductivity, without significantly affecting the thermal conductivity and negatively affecting the ZT. These materials show clear characteristic variations at room temperature, with the highest ZT values of 2.919 and 2.873 obtained for WSe2 and WS2, respectively.  相似文献   

11.
Using the first-principles technique, the electronic structures, magnetic properties and phase stability of alloyed cementite with Cr or Mn were investigated. The calculations show that the chemical and mechanical stability of alloyed cementite can be strengthened by the use of Cr/Mn-doped method. The Magnetic Moments (Ms) of Mn1Fe2C, Mn2Fe1C, Cr1Fe2C and Cr2Fe1C are 5.274, 0.941, 1.864 and 0.736 μB/f.u, respectively. The Ms of Cr in Fe2CrC (−1.374 μB) and Cr2FeC (−0.032 μB) are different due to replacing different sites Fe atoms. The magnetic behaviors of Mn are different from Cr in alloyed cementite. The Ms of Mn in Fe2MnC and Mn2FeC are 2.300 μB and −0.147 μB, respectively.  相似文献   

12.
The phase composition, structure parameters, and basic magnetic properties of several magnetically alloyed spinel-ferrite nanopowders produced from salt solutions are studied. Particle-size and surface effects on the saturation magnetization and magnetic anisotropy of the examined materials are investigated. The validity of the core-shell model for describing the magnetic properties of the nano-sized spinel ferrites under review is discussed. __________ Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 66–72, February 2007.  相似文献   

13.
In this work, beam optical focusing structures capable of double side beam focusing are proposed. The relationships between focal intensity, focal length, focal width and the distance from optical source to lens structure are investigated and numerically simulated. Furthermore, a novel structure that is steadily operating in large wavelength domain is presented, whose properties are well described by comprehensive numerical simulations.  相似文献   

14.
The local electronic structure and magnetic properties of GaAs doped with 3d transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni) were studied by using discrete variational method (DVM) based on density functional theory. The calculated result indicated that the magnetic moment of transition metal increases first and then decreases, and reaches the maximum value when Mn is doped into GaAs. In the case of Mn concentration of 1.4%, the magnetic moment of Mn is in good agreement with the experimental result. The coupling between impure atoms in the system with two impure atoms was found to have obvious variation. For different transition metal, the coupling between the impure atom and the nearest neighbor As also has different variation. Supported by the National Natural Science Foundation of China (Grant No. 10347010)  相似文献   

15.
An optical-polarization procedure is described for determining the residual stress at alloyed metal contacts on the primary crystallographic planes of semiconductors. Equations are derived for the birefringence and the angle between the stress and one of the optic axes for various directions of uniaxial stress in {100} and {110} planes. These equations can be used to calculate the mechanical stress in Td, O, and Oh cubic crystals (43 m, 432, and m3m). The stress has been determined experimentally for the cases of alloyed metal contacts on various crystallographic planes of silicon and gallium arsenide.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No.3, pp. 7–12, March, 1971.In conclusion the authors thank V. F. Konusov and V. I. Nikitenko for consultation and discussion of these results.  相似文献   

16.
The electronic structure of crystalline As2S3 and As2Se3 has been calculated in this paper. We present the energy bands, density of states (DOS) and the Compton profiles using the linear combination of atomic orbitals (LCAO) scheme based on the density functional theory (DFT). From the calculated total and partial density of states it is seen that the lone-pair p-states of sulphur/selenium contribute closest to the Fermi energy level. To interpret the theoretical data on the Compton line shape, we have measured the Compton profiles on a 100 mCi 241Am spectrometer. It is seen that the density functional theory within generalised gradient approximation gives a slightly better agreement with the experimental momentum densities. The nature of chemical bonding in arsenic chalcogenides is studied using Mulliken's population analysis and the experimentally measured equal-valence-electron-density profiles; As2S3 is found to be more ionic compared to As2Se3.  相似文献   

17.
The aim of this contribution is to show that quantum chemistry has suitable tools to extract the specific properties of small metallic and mixed non-stoichiometric clusters which cannot be obtained by extrapolation from the bulk properties to the atom. For this purpose, first the main features of the methods used for the calculations of the ground and excited states of clusters valid at zero temperature (T=0) will be sketched and the factors determining accuracy of results will be pointed out. The structural and optical response properties of cationic Na n + clusters as a function of size will be presented and compared with experimental data. The series of non-stoichiometric alkali-halide clusters containing single and multiple excess electrons will serve as prototypes to study a possible “metal-insulator transition” and “segregation into metallic and ionic parts” in finite systems. Second, an outline of ab initio molecular dynamics methods based on gradient corrected density functional approach with gaussian basis used for determination of temperature dependent ground state properties will be presented. Different temperature behavior of distinct type of structures will be illustrated on an example of Li 9 + cluster. Presented by V. Bonačić-Koutecky at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997. This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 337, Energy transfer in molecular aggregates) and the Consiglio Nationale delle Ricerche (CNR, Rome).  相似文献   

18.
为了解释Ca掺杂与Mg掺杂在影响锂离子二次电池正极材料LiCoO2体系电子输运性质方面的不同效应,采用基于密度泛函理论的第一性原理方法研究了该体系的电子结构.计算结果表明,虽然在LiCoO2体系中用Ca或Mg替代Co都会在费米能级附近产生部分占据的受主带,但两者对应的电子态都具有明显的局域化特征;此外,与Mg掺杂体系明显不同的是,Ca掺杂体系的受主带与价带之间存在清晰的带隙.这一带隙的存在正是Ca掺杂不能明显提高LiCoO2体系电导率的主要原因.此外,Ca2+与Mg2+离子半径的较大差别也是造成这两个掺杂体系的电导率存在明显差异的一个重要因素.  相似文献   

19.
本文采用密度泛函B3LYP方法在6-31G*水平上,对一系列不同宽度的锯齿型和扶手椅型六角氮化硼纳米带进行了理论研究。结果表明纳米带宽度对体系的性质有规律性的影响。随着宽度的增加,纳米带不同位置的硼氮键键长差异逐步减小从而提高了整个体系的共轭性;锯齿型纳米带的能隙单调减小而扶手椅型纳米带的能隙在减小的同时出现振荡,且振幅随宽度逐渐减小。锯齿型氮化硼纳米带的化学势在特定宽度出现了极值点。前线分子轨道的分布随着宽度的增加出现了非均匀分布,呈现出向边界偏移的现象。  相似文献   

20.
本文采用密度泛函B3LYP方法在6-31G*水平上,对一系列不同宽度的锯齿型和扶手椅型六角氮化硼纳米带进行了理论研究。结果表明纳米带宽度对体系的性质有规律性的影响。随着宽度的增加,纳米带不同位置的硼氮键键长差异逐步减小从而提高了整个体系的共轭性;锯齿型纳米带的能隙单调减小而扶手椅型纳米带的能隙在减小的同时出现振荡,且振幅随宽度逐渐减小。锯齿型氮化硼纳米带的化学势在特定宽度出现了极值点。前线分子轨道的分布随着宽度的增加出现了非均匀分布,呈现出向边界偏移的现象。  相似文献   

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