Monte Carlo simulations of electron dynamics in N2/CO2 mixtures

Motivated by experimental investigations of electrical discharges in N₂/CO₂/H₂O, Monte Carlo (MC) electron dynamics simulations in atmospheric N₂/CO₂ mixtures were performed. The goal was to obtain electron energy distribution functions (EEDFs), mean free path, drift velocity, collision frequency and mean energy of electrons, rate coefficients of electron-impact reactions, ionisation and attachment coefficients, as functions of the reduced electric field strength (E/N) and of the concentration of individual gas components. The results obtained by MC simulations were fitted with polynomials of up to the 3rd order with reasonable accuracy for E/N above 80 Td. The studied parameters below 80 Td were strongly non-linear as functions of E/N. This is mostly due to the influence of elastic collisions of electrons with CO₂ molecules prevailing in CO₂-dominant mixtures for E/N < 40 Td, and vibrational excitation collisions of N₂ species prevailing in N₂-dominant mixtures for E/N from 40 to 80 Td. The effect of these electron-impact processes was specific for each of the studied parameters.     

Magnetic properties of the ordered and disordered double perovskite Sr<Subscript>2</Subscript>Fe<Subscript>1+x</Subscript>Mo<Subscript>1−x</Subscript>O<Subscript>6</Subscript> (−1 ≤ x ≤ 1/3)

We have studied the effect of cationic disorder on the spin polarization of the double perovskite system Sr₂Fe_1+x Mo_1−x O₆ with  −1 ≤ x ≤ 1/3. The composition x = 0 corresponds to the well-known double-perovskite Sr₂FeMoO₆, which is expected to have complete spin polarization, however all samples present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. We consider an electronic model within the renormalized perturbation expansion Green’s functions, consisting in a correlated electron picture with localized Fe-ions and itinerant electrons interacting with the local spins via a double-exchange type mechanism. Our results show the influence of disorder on the density of states and the ground-state properties, particularly on the spin polarization over the whole range of x.     

Pressure dependent mechanical and thermodynamical properties of Hg<Subscript>0.91</Subscript>Mn<Subscript>0.09</Subscript>Te semiconductor

The mechanical, thermodynamical and elastic properties of Hg_0.91Mn_0.09Te compound are calculated by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb, three body force parameter, the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure have revealed reasonably good agreement with the available experimental data on the phase transition pressure P _t = 11.5 GPa and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zincblende (B3) to rock salt (B1) structure. Later on, the Poisson’s ratio ν, the ratio R _S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From Poisson’s ratio and the ratio R _S/B it is inferred that Hg_0.91Mn_0.09Te is brittle in nature in both B3 phase and B1 phase. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_0.91Mn_0.09Te compounds and still awaits experimental confirmations.     

Porous In(OH)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> hollow nanocubes: low-temperature reaction design,shape evolution,growth mechanism,and photoluminescent property

Porous sulfur-doped In(OH)₃ (In(OH) _x S _y ) hollow nanocubes were fabricated for the first time by taking advantage of a facile solution-phase approach using thioacetamide as the sulfur source at a temperature as low as 80 °C. The phase structures, composition, and morphologies of resulting products were investigated by powder X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. The characterization results indicated that the addition of thioacetamide in synthetic solution could not only accelerate the hollowing process and tune the size of nanocubes but also facilitate the doping of sulfur in In(OH)₃. We proposed that the cooperative combination of oriented attachment and Ostwald ripening as well as chemical-etching process governed the crystal growth, resulting in the formation of the porous sulfur-doped In(OH)₃ hollow nanocubes in this study. We also found these novel In(OH)₃-based hollow nanostructures showed evolutional room temperature photolumincence emissions at visible-light region, suggesting their potential application in the optical and photocatalytic fields.     

Application of R-matrix method to electron-H<Subscript>2</Subscript>S collisions in the low energy range

Electron-H₂S collision process is studied using the R-matrix method. Nine low-lying states of H₂S molecule are considered in the R-matrix formalism to obtain elastic integral, differential, momentum transfer and excitation cross sections for this scattering system. We have represented our target states using configuration interaction (CI) wavefunctions. We obtained adequate representation of vertical spectrum of the target states included in the scattering calculations. The cross sections are compared with the experiment and other theoretical results. We have obtained good agreement for elastic and momentum transfer cross sections with experiment for entire energy range considered. The differential cross sections are in excellent agreement with experiment in the range 3–15 eV. A prominent feature of this calculation is the detection of a shape resonance in ²B₂ symmetry which decays via dissociative electron attachment (DEA). Born correction is applied for the elastic and dipole allowed transition to account for higher partial waves excluded in the R-matrix calculation. The electron energy range is 0.025–15 eV.     

Single enzyme nanoparticle for biomimetic CO<Subscript>2</Subscript> sequestration

Nanoparticle technology is being increasingly used in environmental sciences. We prepared single enzyme nanoparticle (SEN) by modifying the surface of carbonic anhydrase (CA) with a thin layer of organic/inorganic hybrid polymer. SEN-CA appears to be improving the stability of free enzyme. CA, as ubiquitously found enzyme, is involved in gaseous CO₂ sequestration and is being looked as a promising candidate for combating global warming. We report here physical characterization of SEN-CA using transmission electron microscope (TEM), Fourier-transform infrared analysis (FTIR), X-ray diffraction analysis (XRD), and energy dispersive X-ray (EDX). Average size of SEN-CA particles appears to be in the range of 70–80 nm. We also report the effect of SEN formation on the kinetic parameters of free CA such as Michaelis–Menten constant (K _m), maximum reaction velocity (V _max), and storage stability of free CA and SEN-CA. The V _max of SEN-CA (0.02857 mmol/min/mg) and free enzyme (0.02029 mmol/min/mg) is almost similar. K _m has decreased from 6.143 mM for SEN-CA to 1.252 mM for free CA. The stabilization of CA by SEN formation results in improved the half-life period (up to 100 days). The formation of carbonate was substantiated by using gas chromatography (GC). The conversion of CO₂ to carbonate was 61 mg of CaCO₃/mg of CA and 20.8 mg of CaCO₃/mg of CA using SEN-CA and free CA, respectively.     

Preparation and characterization of polybithiophene/<Emphasis Type="BoldItalic">β</Emphasis>-MnO<Subscript>2</Subscript> composite electrode for oxygen reduction

A composite material of polybithiophene (PBTh) and β-MnO₂ was prepared by electrodeposition of organic conducting polymer on β-MnO₂ surface in 0.1 M LiClO₄/0.01 M BTh/CH₃CN. Synthesized material was characterized by using various techniques, i.e., X-ray diffractometry (XRD), scanning electron microscopy (SEM), and magnetic measurements (SQUID). Electrochemical features of oxygen reduction reaction were investigated using cyclic voltammetry on β-MnO₂ and PBTh/β-MnO₂ electrode, and chronopotentiometry tests were carried out at different currents. The results show that peak current and potential of oxygen reduction are changed for β-MnO₂ modified by polybithiophene.     

II. Electron-impact dissociative excitation of C<Subscript>2</Subscript>H<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> and C<Subscript>2</Subscript>D<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>

Absolute cross-sections for electron-impact dissociative ionization of C₂ H₂⁺ and C₂ D₂⁺ to CH⁺, C⁺, C₂⁺ , H⁺, CH₂⁺ and C₂D⁺ fragments are determined for electron energies ranging from the corresponding threshold to 2.5 keV. Results obtained in a crossed beams experiment are analyzed to estimate the contribution of dissociative ionization to each fragment formation. The dissociative ionization cross sections are seen to decrease for more than an order of magnitude, from CH⁺ (5.37±0.10) × 10^-17 cm² over C⁺ (4.19± 0.16) × 10^-17 cm², C₂D⁺ (3.94±0.38) × 10^-17 cm², C₂⁺ (3.82±0.15) × 10^-17 cm² and H⁺ (3.37±0.21) × 10^-17 cm² to CH₂⁺ (2.66±0.14) × 10^-18 cm². Kinetic energy release distributions of fragment ions are also determined from the analysis of the product velocity distribution. Cross section values, threshold energies and kinetic energies are compared with the data available from the literature. Conforming to the scheme used in the study of the dissociative excitation of C₂H₂⁺ ( C₂ D₂⁺ )\left( {\rm C}_2 {\rm D}_2^+ \right), the cross-sections are presented in a format suitable for their implementation in plasma simulation codes.     

Structural evolutions of the mechanically alloyed Al<Subscript>70</Subscript>Cu<Subscript>20</Subscript>Fe<Subscript>10</Subscript> powders

Elemental mixtures of Al, Cu, Fe powders with the nominal composition of Al₇₀Cu₂₀Fe₁₀ were mechanically alloyed in a planetary ball mill for 80 h. Subsequent annealing of the as-milled powders were performed at 600–800°C temperature range for 4 h. Structural characteristics of the mechanically alloyed Al₇₀Cu₂₀Fe₁₀ powders with the milling time and the heat treatment were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and differential thermal analysis (DTA). Mechanical alloying of the Al₇₀Cu₂₀Fe₁₀ did not result in the formation of icosahedral quasicrystalline phase (i-phase) and a long time milling resulted in the formation of β-Al(Cu,Fe) solid solution phase (β-phase). The i-phase was observed only for short-time milled powders after heat treatment above 600°C. The β-phase was one of the major phases in the Al₇₀Cu₂₀Fe₁₀ alloy. The w-Al₇Cu₂Fe₁ phase (w-phase) was obtained only after heat treatment of the short-time milled and unmilled samples. The present investigation indicated that a suitable technique to obtain a large amount of quasicrystalline powders is to use a combination of short-time milling and subsequent annealing.     

Processes on the target surface under vanadium reactive sputtering in Ar-O<Subscript>2</Subscript> mixtures

Experimental realization of the hysteresis-free mode of vanadium reactive magnetron sputtering in Ar-O₂ mixtures made it possible to perform detailed measurements of the discharge current-voltage characteristics for the first time. They appeared to be not smooth, as predicted by the existing model of reactive sputtering, but having a kink. The experimental data can be interpreted on the assumption that the target surface is oxidized at a depth not less than two monolayers at low ion current densities.     

Specific features of the electronic structure of the CeCu<Subscript>6</Subscript>, CePd<Subscript>3</Subscript>, CeSi<Subscript>2</Subscript>, and CeF<Subscript>3</Subscript> systems

The electronic structure of cerium systems, the hybridization of 4 f and outer-shell electrons, and the influence of the position of the localized 4 f level with respect to the Fermi level E _F in the conduction band have been investigated. The CeCu₆, CePd₃, CeSi₂, and CeF₃ systems have been studied using X-ray photoelectron spectroscopy. The densities of states have been calculated by the tight-binding linearized muffin-tin orbital method within the atomic sphere approximation, which takes into account the covalent character of bonds and the nonspherical distribution of the electron density. The results obtained from the calculations of the total density of states are in good agreement with the valence band X-ray photoelectron data for the systems under investigation. It has been shown that the differences in the properties of the cerium systems are determined by the specific features of their electronic structure. A strong interatomic interaction is characteristic of heavy-fermion systems.     

High-pressure neutron diffraction study of BaFe<Subscript>2</Subscript>As<Subscript>2</Subscript>

The crystal structure of BaFe₂As₂ was studied by high-pressure neutron powder diffraction in the pressure range from ambient to 6.5 GPa as well as in the temperature range from 12 K to 293 K at 4.4 GPa and no pressure or temperature induced phase changes were observed. The compression mechanism of BaFe₂As₂ was found to be anisotropic as the a- and c-axes are reduced by 2.49 and 3.66%, respectively at 6.5 GPa. Within the FeAs layers the Fe-As and Fe-Fe bonds decrease by 2.49 and 3.66%, respectively. The Ba-As distance decreases by 3.70% while the As-As inter-atomic distance along the c-axis exhibits a complex pressure dependence. The bulk modulus B ₀ and its pressure derivative B ₀' were determined to be B ₀ = 59(2) GPa and B ₀' = 6.1(7) at ambient temperature.     

Physical properties,crystal and magnetic structure of layered Fe<Subscript>1.11</Subscript>Te<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductors

The physical and structural properties of Fe_1.11Te and Fe_1.11Te_0.5Se_0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe_1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe_1.11Te_0.5Se_0.5 compound, the superconducting transition with T _c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H _c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe_1.11Te_0.5Se_0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe_1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.     

Photoluminescence,thermoluminescence and electron spin resonance studies on Eu<Superscript>3+</Superscript> doped Ca<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript> red phosphors

Tricalcium aluminate doped with Eu³⁺ was prepared at furnace temperatures as low as 500°C by using the convenient combustion route and examined using powder X-ray diffraction, scanning electron microscope and photoluminescence techniques. A room-temperature photoluminescence study showed that the phosphors can be efficiently excited by UV/Visible region, emitting a red light with a peak wavelength of 616 nm corresponding to the ⁵D₀–⁷F₂ transition of Eu³⁺ ions. The phosphor exhibits three thermoluminescence (TL) peaks at 195°C, 325°C and 390°C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the defect centres responsible for the TL process. Room-temperature ESR spectrum of irradiated phosphor appears to be a superposition of three distinct centres. One of the centres (centre I) with principal g-value 2.0130 is identified as O⁻ ion while centre II with an axially symmetric principal values g _∥=2.0030 and g _⊥=2.0072 is assigned to an F⁺ centre (singly ionized oxygen vacancy). O⁻ ion (hole centre) correlates with the TL peak at 195°C and the F⁺ centre (electron centre), which acts as a recombination centre, is also correlated to the 195°C TL peak. F⁺ centre further appears to be related to the high temperature peak at 390°C. Centre III is also assigned to an F⁺ centre and seems to be the recombination centre for the TL peak at 325°C.     

Rectifying and photovoltage characteristics of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript>/Nb-doped SrTiO<Subscript>3</Subscript> heterojunction

A heterojunction was fabricated by growing a layer of Bi₂Sr₂Co₂O _y thin film on the 0.7 wt% Nb-doped SrTiO₃ substrate. Such heterojunction showed good rectifying characteristics over a wide temperature range, and its transport mechanism under the forward bias can be attributed to a space charge limited conduction mechanism via defects near the interface of the heterojunction. Photovoltaic properties of the heterojunction were studied by using both continuous-wave and pulsed irradiations and the results can be well explained by the photovoltaic effect of a p–n junction.     

Molecular structure of tris(acetylacetonato)scandium Sc(C<Subscript>5</Subscript>H<Subscript>7</Subscript>O<Subscript>2</Subscript>)<Subscript>3</Subscript>

Molecular structure of tris(acetylacetonato)scandium, Sc(C₅H₇O₂)₃, is investigated by gas-phase electron diffractometry. The main structural parameters of the molecule are evaluated. The average intemuclear distances and angles correspond to C₃ symmetry. The chief structural motif is trigonal antiprisms of six oxygen, carbon, and hydrogen atoms with a scandium atom at the center. It is found that r_g[Sc-O) = 204.1(8) pm and r_g(C−O) = 124.7(4) pm Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 633–639, July–August, 1998.     

Electron-impact ionization and dissociation of C<Subscript>2</Subscript>D<Superscript>+</Superscript>

Absolute cross sections for electron-impact single ionization, dissociative excitation and dissociative ionization of the ethynyl radical ion (C₂D⁺)^+) have been measured for electron energies ranging from the corresponding reaction thresholds to 2.5 keV. The animated crossed electron-ion beam experiment is used and results have been obtained for the production of C₂D²⁺, C²⁺, C₂⁺_2^+ , CD⁺, C⁺ and D⁺. The maximum of the cross section for single ionization is found to be (2.01 ± 0.02) × 10^-17 cm², at the incident electron energy of 105 eV. Absolute total cross sections for the various singly charged fragments production are observed to decrease by a factor of almost three, from the largest cross-section measured for C⁺, over C₂⁺_2^+ and CD⁺ down to that of D⁺. The maxima of the cross sections are obtained to be (14.5 ± 0.5) × 10^-17 cm² for C₂⁺_2^+, (12.1 ± 0.1) × 10^-17 cm² for CD⁺, (27.7 ± 0.2) × 10^-17 cm² for C⁺ and (11.1 ± 0.8) × 10^-17 cm² for D⁺. The smallest cross section is measured to be (1.50 ± 0.04) × 10^-18 cm² for the production of the doubly charged ion C²⁺. Individual contributions for dissociative excitation and dissociative ionization are determined for each singly-charged product. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes. Kinetic energy release distributions of dissociation fragments are seen to extend from 0 to 6 eV for the heaviest fragment C₂⁺_2^+, up to 11.0 eV for CD⁺, 14.2 eV for C⁺ and 11.2 eV for D⁺ products.     

Low energy electron attachment to C<Subscript>60</Subscript>

Low energy electron attachment to the fullerene molecule (C₆₀) and its temperature dependence are studied in a crossed electron beam–molecular beam experiment. We observe the strongest relative signal of C₆₀ anion near 0 eV electron energy with respect to higher energy resonant peaks confirming the contribution of s-wave capture to the electron attachment process and hence the absence of threshold behavior or activation barrier near zero electron energy. While we find no temperature dependence for the cross-section near zero energy, we observe a reduction in the cross-sections at higher electron energies as the temperature is increased, indicating a decrease in lifetime of the resonances at higher energies with increase in temperature.     

Electron momentum density in Cu<Subscript>0.9</Subscript>Al<Subscript>0.1</Subscript>

A reconstruction technique based on the solution of the Radon transform in terms of Jacobi polynomials is used to obtain the 3D electron momentum density, ϱ(p), from nine high-resolution Compton profiles (CPs) for a Cu_0.9Al_0.1 disordered alloy single crystal. The method was also applied to theoretical CPs computed within the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) first-principles scheme for the same nine orientations of the crystal. The experimental ϱ(p) is in satisfactory agreement with the theoretical ϱ(p), shows most details of the Fermi surface (FS) and exhibits electron correlation effects. We comment on the map of the FS obtained by folding the reconstructed ϱ(p) into the first Brillouin zone, which yields the occupation number density, ϱ(k). A test of the validity of data via a consistency condition (within our reconstruction algorithm) as well as the propagation of experimental noise in the reconstruction of both ϱ(p) and ϱ(k) are investigated. Received: 24 October 2001 / Accepted: 20 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +48-71/344-10-29, E-mail: samsel@int.pan.wroc.pl