Density functional study of the adsorption of K on the Cu(111) surface

The adsorption of potassium on the Cu(111) surface in a (2×2) pattern has been simulated with all-electron full-potential density functional calculations. The top site is found to be the preferred adsorption site, with the other highly symmetric adsorption sites being nearly degenerate. The bond length from potassium to the nearest copper atom is computed to be 2.83 ?. Population analysis and density of states indicate that there is no evidence for covalent bonding so that the binding mechanism appears to be a metallic bond. Received 11 April 2001     

Screen Printable Sol-Gel Materials for High-Throughput Borosilicate Glass Film Production

The use of sol-gel materials can simplify the industrial fabrication of high-efficiency silicon solar cells if a suitable deposition method is established. In this work, we investigate the possibilities to adapt a borosilicate glass sol-gel to provide a stable screen printing process. This material has previously been used as a boron dopant source for silicon solar cells. We now use an adjusted synthesis process, with an increased gelling time and different additives. This changes the rheological properties (i.e., the elastic and viscous moduli G′ and G″) in a way that avoids the dripping of paste through the screen and that stabilizes the material transfer in subsequent printing steps. Using this synthesis process, we were able to show a printing process with long-term stability of more than 500 prints. When comparing the adjusted to the initial paste, we show that, after thermal treatment, the obtained thin films are very similar in terms of their constitution, with a refractive index between n = 1.47 (initial) and n = 1.55 (adjusted). We also show that they provide the same amount of doping under the tested conditions (950 °C, 30 min), resulting in sheet resistances of R_□ = (42.5 ± 2.6) Ω/□ (initial) and R_□ = (46.4 ± 3.6) Ω/□ (adjusted).     

Atomistic modeling of thermodynamic equilibrium and polymorphism of iron

We develop two new modified embedded-atom method (MEAM) potentials for elemental iron, intended to reproduce the experimental phase stability with respect to both temperature and pressure. These simple interatomic potentials are fitted to a wide variety of material properties of bcc iron in close agreement with experiments. Numerous defect properties of bcc iron and bulk properties of the two close-packed structures calculated with these models are in reasonable agreement with the available first-principles calculations and experiments. Performance at finite temperatures of these models has also been examined using Monte Carlo simulations. We attempt to reproduce the experimental iron polymorphism at finite temperature by means of free energy computations, similar to the procedure previously pursued by Müller et al (2007 J. Phys.: Condens. Matter 19 326220), and re-examine the adequacy of the conclusion drawn in the study by addressing two critical aspects missing in their analysis: (i) the stability of the hcp structure relative to the bcc and fcc structures and (ii) the compatibility between the temperature and pressure dependences of the phase stability. Using two MEAM potentials, we are able to represent all of the observed structural phase transitions in iron. We discuss that the correct reproductions of the phase stability among three crystal structures of iron with respect to both temperature and pressure are incompatible with each other due to the lack of magnetic effects in this class of empirical interatomic potential models. The MEAM potentials developed in this study correctly predict, in the bcc structure, the self-interstitial in the (110) orientation to be the most stable configuration, and the screw dislocation to have a non-degenerate core structure, in contrast to many embedded-atom method potentials for bcc iron in the literature.     

Sterically Active Electron Pairs in Lead Sulfide? An Investigation of the Electronic and Vibrational Properties of PbS in the Transition Region Between the Rock Salt and the α-GeTe-Type Modifications

Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree-Fock and density functional theory to globally search for possible thermodynamically stable and metastable structures. The perhaps most fascinating observation was that besides the experimentally known modification exhibiting the rock salt structure a second minimum exists close-by on the landscape showing the low-temperature α-GeTe-type structure. In the present study, we investigate the possible reasons for the existence of this metastable modification; in particular we address the question, whether the α-GeTe-type modification might be stabilized (and conversely the rock salt modification destabilized) by steric effects of the non-bonding electron pair.     

The quasihole aspect of hole strength distributions in odd potassium and calcium isotopes

The ${}^{\mathrm{40,42,44,48}}\text{Ca}\text{(}\text{d}\text{, ρ)}{}^{\mathrm{39,41,43,47}}\text{K and}{}^{\mathrm{40,44}}\text{Ca(d, t)}{}^{\mathrm{39,43}}$ reactions have been studied at 52 MeV. Angular distributions have been taken and spectroscopic factors have been extracted by means of a DWBA analysis for states with excitation energies up to typically 10 MeV. Analog relations are proposed for mass-39 and mass-43 nuclei. A discussion of proton occupation numbers for various shells above and below the Fermi surface shows a substantial and about equal core excitation for ⁴²Ca and ⁴⁴Ca. Typically 70 % of the $\text{1}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ hole strengths have been located. The corresponding energy averaged strength distributions show quasihole structures whose widths are dominated by phonon-hole coupling to the first 2⁺ state in the target nucleus.     

Eigenschaften der Lösungen der nichtlinearisierten Ginsburg-Landau-Gleichungen in Zylindersymmetrie

Solutions of the nonlinear Ginzburg-Landau equations in cylindrical symmetry have been computed for a type I superconductor. From these solutions the behaviour of a circular cylinder of infinite length in a magnetic field parallel to its axis has been deduced. For a series of values of the magnetic field solutions are given in two cases. The first case was calculated with the assumption of no fluxoid frozen in (fluxoid quantum number n=0), whereas in the second case a vortex with fluxoid quantum numbern=1 was assumed on the axis of the cylinder. For both series of solutions investigation of the thermodynamic stability was carried out. This and further thermodynamic considerations led to the result that in a gedankenexperiment the transition from the normal to the superconducting state and vice versa can be performed in a reversible manner. The expulsion of the magnetic field from the sample during the reversible transition to the superconducting state (Meissner-Effect) is also described by the solutions. Further results are the existence of a supercooled state down to a magnetic fieldH _c2=κ√2H_cb and of a superheated state up to a fieldH _c1>H _cb. The value ofH _c1 depends on the radius of the cylinder. If a condensation to the superconducting state takes place at a fieldH ₀ whereH _c2<H ₀<H _cb, condensation withn=0 seems to be preferred in comparison to that withn=1.     

The polymorphism of poly(vinylidene fluoride). I. The effect of head-to-head structure

The phenomenon of polymorphism in poly(vinylidene fluoride) has been observed recently by several authors. It has also been reported that high-resolution NMR measurements demonstrate the presence in this polymer of head-to-head linkages, resulting from the “backward” addition of from 5-6% of the monomer units. Since the van der Waals radii of fluorine (1.35 Å) and hydrogen (1.1-1.2 Å) are similar, the cocrystallization in a polymer chain of units that differ only by the substitution of fluorine atoms for hydrogen atoms is not unexpected. The two polymorphic forms of poly(vinylidene fluoride), examined in this investigation, have different chain conformations. Chains in phase I have a planar zigzag conformation, while chains in phase II are assumed to exhibit a 2₁ helical conformation. The incorporation into the polymer chain of small amounts of tetrafluoroethylene or trifluoroethylene comonomer favored the crystallization of phase I. This is in accord with the relative abilities, deduced from consideration of atomic size, of these comonomers to cocrystallize with vinylidene fluoride units in the two indicated chain conformations of the polymer. Since tetrafluoroethylene units are present in the head-to-head structure in the homopolymer, it can be concluded that the elimination of the head-to-head structure will eliminate or restrict crystallization in phase I.     

LiFePO<Subscript>4</Subscript> synthesized via melt synthesis using low-cost iron precursors

LiFePO₄/C material has been prepared using fast-melt synthesis method followed by grinding and carbon coating. The low-cost iron ore concentrate (IOC) and purified iron ore concentrate (IOP) were used as iron precursors in the melt process to reduce significantly the cost of LiFePO₄/C. The same product was also synthesized using pure Fe₂O₃ under similar conditions as reference. The physical-chemical and electrochemical properties of samples were investigated. The X-ray Diffraction (XRD) results confirm the formation of an olivine structure of LiFePO₄ with a minor amount of Li₃PO₄ and Li₄P₂O₇ impurities for all the samples but no Fe₂P. The power performances of LiFePO₄/C using low-cost iron precursors were close to the sample using pure Fe₂O₃ precursor although capacity in mAh g^?1 is somewhat lower. With the inherent presence of silicon and other metals species, multi-substitution may take place when using IOC as source of iron leading to a Li(Fe_1-yM_y)(P_1-xSi_x)O₄ general composition. Multi-substitution, however, allows a better cycling stability. Therefore, these iron precursors present a promising option in this field to reduce the cost of a large-scale synthesis of LiFePO₄/C for Li-ion batteries application.     

Reactivity between CuxV2O5 and AgyV2O5 bronzes studied by spark plasma sintering

A method based on the analysis of reaction layers that form in Cu_xV₂O₅–Ag_yV₂O₅ interdiffusion couples annealed by spark plasma sintering to quickly explore the Cu–Ag–V₂O₅ ternary system at high pressure is presented. Through use of microanalysis profiling, the phases occurring in this system have been obtained much faster than by conventional techniques of solid-state chemistry. Microdiffraction profiling has also been used to properly identify the Cu_0.5Ag_0.5V₂O₅ phase in the reaction layer between CuV₂O₅ and Ag_0.8V₂O₅. The stability domains of the phases have been approximately determined and interpreted. In most cases, reaction kinetics occurs quickly, as expected by the high diffusion coefficient of Cu and Ag in V₂O₅. Though the experiments have been carried out under high pressure (75 MPa), the same phases are obtained than with sealed quartz tubes experiments.