Ab initio modeling study of boron diffusion in silicon

We present investigations of boron diffusion in crystalline silicon using ab initio calculations (W. Windl et al., Phys. Rev. Lett. 83 (1999) 4345). Based on these results, a new mechanism for B diffusion mediated by Si self-interstitials was proposed. Rather than kick-out of B into a mobile channel-interstitial, one- or two-step diffusion mechanisms have been found for the different charge states. The predicted activation energy of 3.5–3.8 eV, migration barrier of 0.4–0.7 eV, and diffusion-length exponent of −0.6 to −0.2 eV are in excellent agreement with experiment. We also present results of ab initio calculations for the structure and energetics of boron-interstitial clusters in Si. We show how these first-principles results can be used to create a physical B diffusion model within a continuum simulator which has strongly enhanced predictive power in comparison to traditional diffusion models.     

Investigation of the interface problem between high temperature superconductor and ionic conductors

The charge carriers transfer process across the interface between a superconductor and an ionic conductor, around T _c is investigated. Low temperature electrochemical measurements are carried out on the interfaces between different polycrystalline high-T_c superconductors (HTSC) and RbAg₄l₅ and Ag⁺ ion-conducting glass. The experiments cover a temperature range down to 10 K in the de-frequency range. Atransient technique in the time domain and electrochemical impedance spectroscopy (EIS) in the frequency domain are used to study the silver (Ag⁺) deposition as the Faradaic charge transfer process at that interface. The results show significant enhancement of the charge transfer observed around the critical temperature. This was indicated by either an admittance peak in the transient measurements or a corresponding decrease of the polarization resistance (R _p) in EIS measurements. This enhancement of the charge transfer is correlated to the formation of Cooper pairs at T T _c and interpreted on the basis of a band structure model as a quantumelectrochemical phenomenon with the tunnelling of Cooper pairs through the electrochemical double layer.     

Crystal structure and ionic conductivity of Li14Zn(GeO4)4 and other new Li+ superionic conductors

This paper reports the synthesis and characterization of a number of new Li⁺ superionic conductors with the type formula Li_16?2xD_x(TO₄)₄, where D is a divalent cation (Mg²⁺ or Zn²⁺), T is a tetravalent cation (Si⁴⁺ or Ge⁴⁺), and 0 < x < 4. One of these materials, Li₁₄Zn(GeO₄)₄, has a resistivity of 8 ω-cm at 300°C, lower than that of any Li⁺-ion conductor so far reported. The structure of this compound, which we have named LISICON (for $\text{Li s}$uper$\text{io}$nic $\text{con}$ductor), has been determined by single-crystal x-ray analysis. The space group is Pnma, with cell parameters $\text{a}\text{=10.828}\text{A}\text{?}$, $\text{b}\text{=6.251}\text{A}\text{?}$, $\text{c}\text{=5.140}\text{A}\text{?}$, and z=1. The structure has a rigid three-dimensional network of Li₁₁Zn(GeO₄)₄. The three remaining Li⁺ ions have occupancies of 55 and 16%, respectively, at the 4c and 4a interstitial positions. Each 4c position is connected to two 4a positions and vice versa. The bottlenecks betweenthese positions have an average diameter that is larger than twice the sum of the Li⁺ and O^2? ionic radii, thus satisfying thegeometrical condition for fast Li⁺ -ion transport. Moreover, all four sp³ orbitals of the O^2? ion are shared by strong tetrahedral covalent bonds with the network cations. Therefore, the anion charge is polarized away from the interstitial Li⁺ ions, weakening the Li1bO bond and increasing the Li⁺-ion mobility.     

Ab initio tensile testing simulation of aluminum and aluminum nitride ceramics based on density functional theory

The ideal strength and mechanical behavior of aluminum, aluminum nitride, and their composites at zero temperature are investigated using ab initio norm conserving pseudopotential methods based on local density approximation. The total energy and stress of supercells which contain these atomistic models are calculated under uniaxial tensile strain. Estimated various physical properties of aluminum and aluminum nitride at equilibrium state are in good agreement with the experiment. The tensile test of the composite model shows a failure of the composite in the aluminum matrix. The ideal strength of these models is also estimated.     

Ab initio study of reactions between halogen atoms and various fuel molecules by Gaussian-2 theory

Ab initio calculations by using Gaussian-2 theory have been carried out for the reactions between halogen atoms and various fuel molecules, i.e. fluorine, chlorine, and bromine atoms vs. hydrogen, methane, ethane, ethylene, acetylene, ammonia, silane, dichlorosilane and phosphine. The activation energy for the reaction between a halogen atom and a fuel molecule seems to indicate whether the reaction between the fuel gas and the corresponding halogen gas occurs spontaneously when they are brought into contact to each other at room temperature.     

Ab initio study of phase stability in doped TiO2

Ab initio density functional theory calculations of the relative stability of the anatase and rutile polymorphs of TiO₂ were carried out using all-electron atomic orbitals methods with local density approximation. The rutile phase exhibited a moderate margin of stability of ~ 3 meV relative to the anatase phase in pristine material. From computational analysis of the formation energies of Si, Al, Fe and F dopants of various charge states across different Fermi level energies in anatase and in rutile, it was found that the cationic dopants are most stable in Ti substitutional lattice positions while formation energy is minimised for F^? doping in interstitial positions. All dopants were found to considerably stabilise anatase relative to the rutile phase, suggesting the anatase to rutile phase transformation is inhibited in such systems with the dopants ranked F?>?Si?>?Fe?>?Al in order of anatase stabilisation strength. Al and Fe dopants were found to act as shallow acceptors with charge compensation achieved through the formation of mobile carriers rather than the formation of anion vacancies.     

Ab initio identified design principles of solid-solution strengthening in Al

Abstract

Solid-solution strengthening in six Al–X binary systems is investigated using first-principle methods. The volumetric mismatch parameter and the solubility enthalpy per solute were calculated. We derive three rules for designing solid-solution strengthened alloys: (i) the solubility enthalpy per solute is related to the volumetric mismatch by a power law; (ii) for each annealing temperature, there exists an optimal solute–volume mismatch to achieve maximum strength; and (iii) the strengthening potential of high volumetric mismatch solutes is severely limited by their low solubility. Our results thus show that the thermodynamic properties of the system (here Al–X alloys) set clear upper bounds to the achievable strengthening effects owing to the reduced solubility with increasing volume mismatch.     

Evaluation criteria about parasitics in impedance measurements of ionic conductors

Impedance spectrum measurements of an ionic conductor material constitute a well known method for the optimization of these materials. Today the most adopted method consists, essentially, in the vectorial comparison between two sinusoidal signals, one taken as reference, over a wide range of frequencies. In this work, after some considerations about practical electrical connections, an evaluation criterion to eliminate the so-called ‘parasitic impedance’, noticeable chiefly at high frequencies (10 kHz), is proposed. A method is also reported which permits the voltage at the extremes at the measuring cell to remain constant.     

Ab initio calculations of the hydroxyl impurities in BaF2

OH^? impurities in BaF₂ crystal have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Three different configurations of OH^? impurities were investigated and the (1 1 1)-oriented OH^? configuration is the most stable one. Our calculations show that OH^? as an atomic group has a steady geometrical structure instead of electronic properties in different materials. The studies on band structures and density of states (DOS) of the OH^?-impurity systems indicate that there are two defect levels induced by OH^? impurities. The two superposed occupied OH^?-bands located 1.95 eV above the valance bands (VB) at Γ point mainly consist of the O p orbitals, and the H s orbitals do the major contribution to the empty defect level located 0.78 eV below the conduction bands (CB). The optical absorption due to the doped OH^? is centered around 8.61 eV.     

Molecular dynamics simulations of oxygen ion diffusion and superionic conduction in ytterbia-stabilized zirconia

Superionic conduction of oxygen ions in 10 mol% ytterbia-stabilized zirconia (YbSZ) at different temperatures is studied employing molecular dynamics simulations. Eventhough discrete hopping of one or two oxygen ions starts at about 675 K, onset of superionic conduction occurs at about 1200 K when almost all the oxygen ions participate in the hopping process. The activation energy for oxygen ion diffusion is found to be 53.25 kJ mol⁻¹. At this temperature and above, oxygen ionic conductivity exceeds 0.1 Ω⁻¹ cm⁻¹ thereby confirming superionic conduction in the YbSZ material. For 675 K < T < 1200 K, the material acts as a normal ionic conductor. The ionic conductivity values, obtained through our simulation compare well with experimental results. But activation energy for oxygen ion conduction, found from the Arrhenius plot of our simulation is 52.71 kJ mol⁻¹ which is 35% less than experimental value. Radial distribution functions, g(r) show that there is no sharp structural phase transition and no oxygen ion sub-lattice melting in YbSZ material at superionic transition. However, the reduction in, broadening and shifting of the peaks in g(r) for all ionic pairs, at higher temperatures, indicate a volume expansion of the crystal.     

从头算法研究HEDP中质子传导机理

采用软件Hperchem中的从头算法研究HEDP(羟基亚乙基二膦酸)中质子的传递过程。本文选用了STO-3G、3-21G、6-31G、6-31G*、6-31G**为基组来优化HEDP的分子结构,实验结果表明6-31G为最优基组。利用该最优基组优化结构,得到HEDP单重态稳定结构。通过H+与磷酸中P=O、P-O、P-O缔合得到HEDP的双重态稳定结构。分析H~+与上述三种环境中氧原子的结合能,结果表明H+与C-O中氧原子结合时,分子中多个氢原子处于被激活状态,具有较高能量,容易脱离体系形成H~+,这个过程有利于质子的传递。     

Ab initio molecular dynamics simulation of pressure-induced phase transformation in BeO

Ab initio molecular dynamics (MD) method has been used to study high pressure-induced phase transformation in BeO based on the local density approximation (LDA) and the generalized gradient approximation (GGA). Both methods show that the wurtzite (WZ) and zinc blende (ZB) BeO transforms to the rocksalt (RS) structure smoothly at high pressure. The transition pressures obtained from the LDA method are about 40 GPa larger than the GGA result for both WZ → RS and ZB → RS phase transformations, and the phase transformation mechanisms revealed by the LDA and GGA methods are different. For WZ → RS phase transformations both mechanisms obtained from the LDA and GGA methods are not comparable to the previous ab initio MD simulations of WZ BeO at 700 GPa based on the GGA method. It is suggested that the phase transformation mechanisms of BeO revealed by the ab initio MD simulations are affected remarkably by the exchange–correlation functional employed and the way of applying pressure.     

Brownian diffusion of ultrafine particles to an air–water interface

Fluorescent core-plain shell silica particles of different sizes were synthesized in order to investigate the interaction of ultrafine particles with bubbles. The morphology, surface chemistry and rate of diffusion of these synthesized particles were characterized by various techniques. The collection efficiency of particles by bubbles was measured in single bubble flotation experiments. Results show that particle-bubble collection efficiency increased with decreasing particle size and increasing particle surface hydrophobicity. These results are interpreted by taking into account the influence of particle size and surface hydrophobicity on particle-bubble collision and attachment efficiencies.     

Ab initio Calculations of the Formation Energies of Lithium Intercalations in SnSb

SnSb has attracted a great attention in recent investigations as an anode material for Li ion batteries. The formation energies and electronic properties of the Li intercalations in SnSb have been calculated within the framework of local density functional theory and the first-principles pseudopotential technique. The changes of volumes, band structures, charge density analysis and the electronic density of states for the Li intercalations are presented. The results show that the average Li intercalation formation energy per Li atom is around 2.7 eV.     

Ab initio study of phonon-induced dephasing of electronic excitations in narrow graphene nanoribbons

Vibrational dephasing of the lowest energy electronic excitations in the perfect (16,16) graphene nanoribbon (GNR) and those with the C2-bond insertion and rotation defects is studied with ab initio molecular dynamics. Compared to single-walled carbon nanotubes (SWCNTs) of similar size, GNRs shows very different properties. The dephasing in the ideal GNR occurs twice faster than that in the SWCNTs. It is induced primarily by the 1300 cm (-1) disorder mode seen in bulk graphite rather than by the 1600 cm (-1) C-C stretching mode as in SWCNTs. In contrast to SWCNTs, defects exhibit weaker electron-phonon coupling compared to the ideal system. Therefore, defects should present much less of a practical problem in GNRs compared to SWCNTs. The predicted optical line widths can be tested experimentally.     

Approximate solution of an internal problem in laminar boundary layer theory

Laminar flow of a liquid in a channel of arbitrary shape is examined with account for interaction of the boundary layer with the core. A simple approximate method is described for calculating the boundary layer in a channel with arbitrary generator.     

Ab initio study of phosphorus donors acting as quantum bits in silicon nanowires

A phosphorus (P) donor has been extensively studied in bulk Si to realize the concept of Kane quantum computers. In most cases the quantum bit was realized as an entanglement between the donor electron spin and the nonzero nuclei spin of the donor impurity mediated by the hyperfine coupling between them. The donor ionization energies and the spin-lattice relaxation time limited the temperatures to a few kelvin in these experiments. Here, we demonstrate by means of ab initio density functional theory calculations that quantum confinement in thin Si nanowires (SiNWs) results in (i) larger excitation energies of donor impurity and (ii) a sensitive manipulation of the hyperfine coupling by external electric field. We propose that these features may allow to realize the quantum bit (qubit) experiments at elevated temperatures with a strength of electric fields applicable in current field-effect transistor technology. We also show that the strength of quantum confinement and the presence of strain induced by the surface termination may significantly affect the ground and excited states of the donors in thin SiNWs, possibly allowing an optical read-out of the electron spin.     

The dependence of structure and conductivity in three-dimensional phosphate ionic conductors on composition

Two series of solid solutions, Na _x Ca_(1−x)/2Zr₂(PO₄)₃ (NCZP(x), 0⩽x⩽1) and Na _x Nb_1-x Zr_1+x (PO₄)₃(NNZP(x), 0⩽x⩽1), were synthesized. They were examined by powder X-ray diffraction, infra-red (i.r.) absorption and Raman scattering. Ionic conductivities of graphite coated samples were measured. A complete series of solid solutions was formed for NCZP(x), while a second phase was found forx<0.1 for NNZP(x). The i.r. and Raman spectra of their solid solutions consistently showed the formation of PO₄ tetrahedra with different geometries. The composition dependence of conductivity is interpreted on the basis of a structural change around Na⁺.