As吸附在InP(110)表面电子性质的理论研究

利用形式散射的格林函数方法，研究了As在InP(110)表面不同吸附结构的电子性质.体材料采用实用的经验紧束缚近似方法的哈密顿，分别计算了As-P交换作用和形成外延连续层结构表面态的性质，指出了表面态和表面共振态产生的原因.在计算中，一些表面紧束缚相互作用参数进行了调整，所得结果好于其他理论方法. 关键词：     

Theory of electron states at random alloy surfaces - surface states on Cu-Ni (111)

We present first principles calculations (based on the KKRCPA) of the angle-resolved photocurrent emitted from the (111) surfaces of single crystals of $\text{Cu}$-Ni random alloys, and compare the results with new experimental data. Surface states close to the Fermi level are observed, even for concentrated alloys, and their behaviour as a function of composition and $\text{k}$_∥ is correlated with features in the bulk spectral density. Calculations for alloys with a non-uniform concentration profile at the surface (surface segregation) are described, and the effect on the surface states is discussed.     

Oxygen-ion conducting electrolyte materials for solid oxide fuel cells

Oxygen-ion conducting solid electrolyte systems have been reviewed with specific emphasis on their use in solid oxide fuel cells. The relationships between phase assemblage, electrolyte stability and ionic conductivity have been discussed. The role of parameters such as sintering temperature and atmosphere which influence the segregation of impurities, present in the starting ceramic powders, at grain boundaries and at the external surface of the electrolyte compacts has been emphasised. The stability of various electrolyte materials in contact with other fuel cell components and in fuel environments has been discussed in detail. The ageing behaviour at fuel cell operating temperatures has been described. Data on ionic conductivity, mechanical and thermal properties have been presented for a number of electrolyte materials.     

Yttrium segregation and surface phases of yttria-stabilized zirconia (1 1 1) surface

Surface segregation of yttria-stabilized zirconia (YSZ) was studied via first-principles computations and thermodynamics. For the cubic YSZ (1 1 1) surface, yttrium can segregate only to a subsurface layer, and these segregation phases are terminated at the surface by defective oxygen layers with honeycomb structure. The segregation is independent of the bulk yttrium concentration at high oxygen partial pressures or low temperatures. At very low oxygen partial pressures and high temperatures there is no surface yttrium segregation and the surface is terminated by O–Zr. Our results provide a reasonable explanation for previous experimental work, and also a framework for extending our understanding of cation segregation in oxide surfaces.     

First-principles calculations of interfacial and segregation energies in α-Cr2O3

The interfacial energies of three twin boundaries with low-index boundary planes: prismatic (101?0), basal O-terminated (0001), and basal Cr-terminated (0001), and the segregation energies of five doping elements (Ce, Hf, La, Y and Zr) have been calculated as a function of temperature. The static energies at 0 K were obtained through first-principles calculations and the energies at finite temperatures were derived based on the Debye model. The calculation results show that both the interfacial and segregation energies decrease as temperature increases and the segregation energies are found to be proportional to the ionic size mismatch and the interfacial energy. Our combined approaches suggest an efficient and less computationally intensive way to derive grain boundary energetics at finite temperatures.     

Thermal conductivity of micro/nano-porous polymers: Prediction models and applications

Micro/nano-porous polymeric material is considered a unique industrial material due to its extremely low thermal conductivity, low density, and high surface area. Therefore, it is necessary to establish an accurate thermal conductivity prediction model suiting their applicable conditions and provide a theoretical basis for expanding their applications. In this work, the development of the calculation model of equivalent thermal conductivity of micro/nano-porous polymeric materials in recent years is summarized. Firstly, it reviews the process of establishing the overall equivalent thermal conductivity calculation model for micro/nanoporous polymers. Then, the predicted calculation models of thermal conductivity are introduced separately according to the conductive and radiative thermal conductivity models. In addition, the thermal conduction part is divided into the gaseous thermal conductivity model, solid thermal conductivity model and gas–solid coupling model. Finally, it is concluded that, compared with other porous materials, there are few studies on heat transfer of micro/ nanoporous polymers, especially on the particular heat transfer mechanisms such as scale effects at the micro/nanoscale. In particular, the following aspects of porous polymers still need to be further studied: micro scaled thermal radiation, heat transfer characteristics of particular morphologies at the nanoscales, heat transfer mechanism and impact factors of micro/nanoporous polymers. Such studies would provide a more accurate prediction of thermal conductivity and a broader application in energy conversion and storage systems.     

Carbon segregation to single crystal surfaces of Pt,Pd and Co

Results are reported on the surface segregation behaviour of carbon from dilute solid solutions in Pt, Pd and Co. With Pt(100) no preferential surface segregation was observed; this is similar to previous results for Pt(111). For Pd(lOO), Pd(111) and Co(0001) segregation was observed with evidence for a surface phase transition of the type previously reported for Ni(111). These observations suggest that the strong carbon-carbon interactions within a graphite monolayer are of more importance in producing the transition than a good epitaxial fit to the substrate. A comparison of the kinetics of carbon segregation to Co(0001) with those predicted by a simple diffusion model suggest that surface processes such as nucleation or lateral diffusion may play important roles.     

Electron dynamics in H/Na/Cu(1 1 1) collisions

Surface adsorbates induce strong local perturbations in the electronic structure and potentials in their surroundings. Consequently, charge transfer processes between projectiles and adsorbate-covered surfaces are strongly affected. The theoretical calculations and experiment measurements reported herein are focused on the H⁻/Na/Cu(1 1 1) system. The electron dynamics at the Na/Cu(1 1 1) surface and the influence of Na adsorbates on the H⁻-Cu(1 1 1) charge transfer are treated and discussed in detail. The ion fractions are mainly influenced by the ion exit trajectories. At low Na coverage, they exhibit a maximum near the 60° exit angle from surface. The calculations and experimental data are in good agreement.     

Reversed surface segregation in palladium-silver alloys due to hydrogen adsorption

It is well known that silver segregates to the surface of pure and ideal Pd–Ag alloy surfaces. By first-principles band-structure calculations it is shown in this paper how this may be changed when hydrogen is adsorbed on a Pd–Ag(1 1 1) surface. Due to hydrogen binding more strongly to palladium than to silver, there is a clear energy gain from a reversal of the surface segregation. Hydrogen-induced segregation may provide a fundamental explanation for the hydrogen or reducing treatments that are required to activate hydrogen-selective membrane or catalyst performance.     

Catalysis and corrosion: the theoretical surface-science context

Numerous experiments in ultra-high vacuum as well as (T=0 K, p=0) theoretical studies on surfaces have been performed over the last decades in order to gain a better understanding of the mechanisms, which, for example, underlie the phenomena of catalysis and corrosion. Often the results achieved this way cannot be extrapolated directly to the technologically relevant situation of finite temperature and high pressure. Accordingly, modern surface science has realized that bridging the so-called pressure gap (getting out of the vacuum) is the inevitable way to go. Of similar importance are studies in which the temperature is changed systematically (warming up and cooling down). Both aspects are being taken into account in recent experiments and ab initio calculations.

In this paper we stress that there is still much to learn and important questions to be answered concerning the complex atomic and molecular processes which occur at surfaces and actuate catalysis and corrosion, although significant advances in this exciting field have been made over the years. We demonstrate how synergetic effects between theory and experiment are leading to the next step, which is the development of simple concepts and understanding of the different modes of the interaction of chemisorbed species with surfaces. To a large extent this is being made possible by recent developments in theoretical methodology, which allow to extend the ab initio (i.e., starting from the self-consistent electronic structure) approach to poly-atomic complexes with 10,000 and more atoms, time scales of seconds, and involved statistics (e.g., ab initio molecular dynamics with 10,000 and more trajectories). In this paper we will

1. sketch recent density–functional theory based hybrid methods, which bridge the length and time scales from those of electron orbitals to meso- and macroscopic proportions, and

2. present some key results on properties of surfaces, demonstrating their role in corrosion and heterogeneous catalysis. In particular we discuss

◦ the influence of the ambient gas phase on the surface structure and stoichiometry,

◦ adsorbate phase transitions and thermal desorption, and

◦ the role of atoms' dynamics and statistics for the surface chemical reactivity.

Surface segregation of Ge at SiGe(001) by concerted exchange pathways

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.     

Surface segregation in bimetallic Pt3M (M = Fe, Co, Ni) alloys with adsorbed oxygen

Surface segregation of Pt₃M (M = Fe, Co, and Ni) alloys under oxygen environment has been examined using periodic density functional theory. The segregation trend at a (1 1 1) surface is found to be substantially modified by the adsorbed oxygen. Our calculations indicate that under 1/4 monolayer O coverage both the Pt-segregated and M-segregated surfaces are less stable than the non-segregated one. Further analysis reveals that segregation energy under adsorption environments can be expressed as the sum of the segregation energy under vacuum conditions and the adsorption energy difference of the segregated and non-segregated alloy systems. Therefore, the surface segregation trend under adsorption conditions is directly correlated to the surface-adsorbate binding strength.     

Refining Fermi surface topologies from ab initio calculations through momentum density spectroscopies

It is well known that the agreement between the Fermi surface topologies predicted by ab initio electronic structure calculations and experiment can often be brought into much better agreement through small rigid-band-like shifts. A new method for refining these calculations using experimental data containing Fermi surface information, based on a rigid-band-like fitting approach is presented. In this method, experimental data from different methods can be combined to refine and deliver a ‘tuned’ bandstructure, allowing an investigation of FS nesting properties, a quantitative comparison between experiment and calculation, and highlighting the origin of inconsistencies. Results of the application of this method to positron annihilation experiments in vanadium are presented, showing significant improvement over the ab initio calculation. In order to demonstrate the versatility of this fitting method, it has been applied to a combination of positron annihilation measurements and magnetic Compton scattering experiments in ferromagnetic nickel.     

Theory of surface polaritons associated with the extreme anomalous skin effect in normal and superconducting metals

Surface polaritons associated with the extreme anomalous skin effect in metals have been theoretically investigated for the normal and superconducting states. The Pippard expression for the current density was used to obtain the nonlocal conductivity tensor for the extreme anomalous limit. From the resultant components of the dielectric tensor, the specular reflection-mirror image technique of Kliewer and Fuchs was used to obtain the surface polariton dispersion relation; calculations were performed for aluminium. For frequencies above the superconductor gap frequency but much less than the plasma frequency, the surface polariton behaviour is nearly the same for the normal and superconductor (T = 0 K) cases. For frequencies below the gap frequency, the surface polariton damping is zero for the superconductor case, and is nonzero and decreases rapidly with frequency for the normal conductor case.     

The relationship between grain-boundary structure and segregation in a rapidly solidified Fe-P alloy

It is well known that the amount of solute segregation can vary from one grain boundary to another. Though it is accepted that this variation is due to differences in boundary structure and crystallography, direct correlation between the degree of segregation and specific boundary structural characteristics is not well documented. In the present paper, P grain-boundary segregation in rapidly solidified Fe was studied by X-ray mapping (XRM) in a scanning transmission electron microscope (STEM). The boundary structure was characterized by convergent beam electron diffraction (CBED). To explore the relationship between the degree of segregation and boundary structure, a parameter?β?is introduced, which describes how well the two crystal planes on either side of a grain boundary match each other in a manner similar to the long-established plane matching theory. The introduction of this parameter enables us to relate the degree of segregation to boundary structure in a consistent way, e.g., both small-angle and low?Σ?symmetric boundaries correspond to low angle of β, leads to a low degree of segregation.     

Segregation of tin on (111) and (100) surfaces of copper

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (√3 × √3)R30° structure for the (111) surface and a p(2 × 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to ΔH = ?(53 ± 5) kJ/g-atom.     

Stargazing Basics: Getting Started in Recreational Astronomy,by P.E. Kinzer

Soil has its origin in physical processes of comminution and abrasion of rock masses and in chemical processes of solution and recrystallization of rock minerals. After formation it is transported and distributed by a variety of agencies before subaerial or subaqueous deposition. Generally, soils experience a number of cycles of sedimentation, uplift, erosion and redeposition. Their mechanical behaviour under the stresses imposed by engineering structures varies with the type of soil. Both the deformation and failure of soils are of interest in the design of structures and these properties must be studied by special laboratory tests. The solution of a soil engineering problem therefore involves a field investigation and sampling programme to define the boundaries of the soil involved, laboratory testing of the soil, and analytical or computer calculations to determine the displacements and stresses which will occur in the soil and structure. Problems of particular interest include slope failures, the design of earth dams, retaining walls and piled structures, the displacement and failure of soil under buildings and the determination of soil properties on the surface of the moon and other planets.     

Direct observation of the surface segregation of Cu in Pd by time-resolved positron-annihilation-induced Auger electron spectroscopy

Density functional theory calculations predict the surface segregation of Cu in the second atomic layer of Pd which has not been unambiguously confirmed by experiment so far. We report measurements on Pd surfaces covered with three and six monolayers of Cu using element selective positron-annihilation-induced Auger electron spectroscopy (PAES) which is sensitive to the topmost atomic layer. Moreover, time-resolved PAES, which was applied for the first time, enables the investigation of the dynamics of surface atoms and hence the observation of the segregation process. The time constant for segregation was experimentally determined to τ=1.38(0.21) h, and the final segregated configuration was found to be consistent with calculations. Time-dependent PAES is demonstrated to be a novel element selective technique applicable for the investigation of, e.g., heterogeneous catalysis, corrosion, or surface alloying.     

Structures of CoAl(1 1 1) surface: A first principles study

The structures of the CoAl(1 1 1) surface are studied by first principles calculations. Our calculations show that the surface layer is always occupied by pure Al for all concentrations studied here, which indicates the dominant role of the Al segregation tendency. This is different from the CoAl(0 0 1) surface, where a number of Co anti-sites are found on the top most layer. The calculated surface phase diagram of ground states shows that there are three stable structures. The diffusion barriers of the metastable structure evolving to the stable structure are also calculated. The high diffusion barrier can explain the appearance of metastable structures at low temperature in experiment.