Electron correlation effects in electron–hole recombination and triplet–triplet scattering in organic light emitting diodes

Using a time-dependent model many-body formulation, we follow the electron–hole recombination between a pair of oppositely charged polyene chains and triplet–triplet (T–T) scattering in π-conjugated systems. Electron correlations reduce the overall yields and also lead to large differences between the singlet and triplet yields. External electric field has a strong influence on the recombination products. The fraction of singlet yield, η, increases with the chain length. Heteroatoms also change this ratio significantly, as has been observed experimentally. The outcome of the triplet–triplet scattering is also significantly affected by electron correlations.     

The mechanism of climb in dislocation–nanovoid interaction

Techniques of atomistic simulation have been employed to determine the mechanism of the newly observed process of climb of a dislocation line during depinning at a nanosized void. The analyses prove that, in contrast to the conventional notion of dislocation climb, this unique process is not a diffusion controlled phenomenon. Instead, the gradient of structural energy of the system alone suffices to produce the climb motion. The energetics of this process has been investigated through molecular statics and dynamics computations and the causes of discrepancies are discussed. In order to further explore these concepts in a quantitative manner we have developed a robust simulation strategy to accurately measure the reduction in energy of the nanovoid during dislocation climb. The results explain the lowering of the critical depinning load and the effect of thermal facilitation on void-induced climb. Moreover, we highlight the key role played by the curvature of the dislocation line in mediating this mechanism.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Influence of sulfur on the solubility of graphite in Fe–C–S melts: optimization of interaction parameters

We report a Monte Carlo simulation study of a molten Fe–C–S system at 1600°C with the aim of developing an atomic model with optimum interaction parameters. This model should account for key features of the system, e.g., homogenous nature of Fe–FeS solution, separation of Fe–C–S liquid in two immiscible layers and a decrease in solubility of graphite in iron melts with the addition of sulfur. The atoms in the ternary Fe–C–S system were arranged on a graphitic hexagonal lattice and pair-wise interactions between them were assumed to be short ranged. The simulations were carried out using a combination of canonical and grand canonical ensembles for a range of interaction strengths, carbon and sulfur concentrations. The strength of ionic Fe–S interaction was one of the significant parameters and could lead to electronic distortions around the Fe atom. Local modifications to various interactions due to these distortions were represented by three parameters: ε (Fe–Fe), δ (Fe–S) and δ (Fe–C) and their effect on the Fe–C–S system was systematically analyzed. Optimum modulation parameters for this system, which simultaneously simulate key features of a molten Fe–C–S system are: ε (Fe−Fe)=1.0, δ (Fe−S)=1.0, δ (Fe−C)=1.0 and 1.5. Even though a slight increase in Fe–C repulsion gives optimum results, our high temperature simulation results clearly show that electronic distortions around Fe due to strong Fe–S bond do not play a significant role in the molten Fe–C–S system.     

Fluorene–fullerene dyads: Modulation of interaction

Two new fullerene[C₆₀]–fluorene dyads, N-methyl-2-(9-fluorenyl)-3,4-fulleropyrrolydine and N-methyl-2-(2-fluorenyl)-3,4-fulleropyrrolydine, were synthesized and investigated. Cyclic voltammetry, fluorescence experiments, electronic spectra and Density Functional Theory (DFT) calculations, all indicate ground and excited state interactions when fluorene is linked at carbon 9, whereas linkage through position 2 does not affect the ground and excited state of the fulleropyrrolidine moiety. As a consequence, only changing the position of the fluorene chromophore linkage, opens the way to modulate interactions, choosing between a simple spacer and an active unit.     

First-principles calculation of optical absorption spectra in conjugated polymers: role of electron–hole interaction

Experimental and theoretical studies have shown that excitonic effects play an important role in the optical properties of conjugated polymers. The optical absorption spectrum of trans-polyacetylene, for example, can be understood as completely dominated by the formation of exciton bound states. We review a recently developed first-principles method for computing the excitonic effects and optical spectrum, with no adjustable parameters. This theory is used to study the absorption spectrum of two conjugated polymers: trans-polyacetylene and poly-phenylene-vinylene (PPV).     

Polariton effects in π-conjugated molecular solids: single crystal and nanocrystalline films of sexithiophene

Strong excitons of π-conjugated molecules crystallize in anisotropic solids of peculiar optical properties, which are analysed in detail for α-sexithiophene. The main absorption peak is observed in single crystals either at 2.6 eV in transmission or at 3.4 eV in reflectance spectra. Nanocrystalline films show much stronger but almost featureless absorption with little correspondence to a richly structured electroabsorption spectrum or the spectra of the single crystal. It will be shown that these differences arise from the polarization field of coherently driven excitons, which are coupled, into a polariton mode.     

The competing crystalline and amorphous solid solutions in the Ag–Cu system

Upon nonequilibrium processing using vapor quenching or mechanical alloying, the supersaturated fcc solid solution predominates over the amorphous solution in the Ag–Cu system. The thermodynamic and kinetic origins of this phase selection are explored. The enthalpy of formation of both solutions has been determined as a function of composition using calorimetry measurements and molecular dynamics (MD) simulations. The enthalpy of the fcc solution is found to be lower than that of the competing amorphous phase. The preference for the fcc crystalline state is enhanced by the low kinetic barrier to crystallization of the amorphous solution, which occurred during quenching even when high cooling rates were employed or during annealing at low temperatures. Consequently, the retention of an amorphous Ag–Cu solution required kinetic trapping using ultrahigh quenching rates achievable only under stringent vapor deposition conditions or in MD simulations. However, transmission electron microscopy revealed the presence of local regions of amorphous Ag–Cu after cold rolling of multilayers of elemental Ag and Cu foils at room temperature. This result of partial amorphization demonstrates the possibility of mechanically driven solid-state amorphization in a system with a positive heat of mixing.     

Local atomic arrangements and their topology in Ni–Zr and Cu–Zr glassy and crystalline alloys

Different experimental techniques (X-ray diffraction, neutron diffraction with isotopic substitution, extended X-ray absorption spectroscopy) and theoretical methods (reverse Monte-Carlo simulation, molecular dynamics modelling, Voronoi analysis) were applied to elucidate the atomic structure of Ni–Zr and Cu–Zr alloys in glassy and crystalline states and to explain differences in the glass-forming abilities of the Ni₆₄Zr₃₆ and Cu₆₅Zr₃₅ compositions. Both glasses show similar strong topological ordering, but it is established that the degree of chemical ordering is much more pronounced in Ni₆₄Zr₃₆ glass than in Cu₆₅Zr₃₅ glass. The short-range atomic order and topology in the glassy and crystalline structures are remarkably different, and these differences are presumed to hinder crystal nucleation and growth, hence promoting glass formation upon fast cooling of the Ni₆₄Zr₃₆ and Cu₆₅Zr₃₅ liquid alloys. The larger differences observed for the Cu₆₅Zr₃₅ alloy in glassy and crystalline states are suggested to play a decisive role in increasing its bulk-glass-forming ability.     

N–N interaction and nitrogen activity in the iron base austenite

The Monte Carlo computer simulation of the f.c.c. Fe–N alloy is performed using the data of abundances of different iron sites in the austenite lattice as obtained by means of Mössbauer spectroscopy. The validity of the interpretations of available Mössbauer spectra was tested based on the data of calculation of N–N interaction energies in the two first coordination spheres on the interstitial sublattice, which could satisfy the experimental data of nitrogen distribution in austenite derived from Mössbauer spectra. It is shown that no values of N–N interaction energies exist that could be consistent with the data of conversion electron Mössbauer spectroscopy (CEMS), where a thin surface layer only contributes to the Mössbauer spectra. A strong N–N repulsion (>0.14 eV) in the first coordination sphere and a soft N–N repulsion (<0.07 eV) in the second one was found to be consistent with the studies performed by means of transmission Mössbauer spectroscopy (TMS), according to which single nitrogen atoms and 180° N–N pairs exist in the nitrogen austenite. A possibility of the long-range order-like Fe₄N is shown at the energy values determined for two coordination spheres, provided a small N–N repulsion in the third coordination sphere occurs. It is also observed that the available data of nitrogen activity in Fe–N austenite are not consistent with the values of N–N interaction energies determined from the analysis of Mössbauer spectra, i.e. the available activity data do not correspond to the nitrogen distribution in the iron austenite as it follows from Mössbauer data. The concentration dependence of nitrogen activity and an effect of the N₂ gas pressure on the nitrogen solubility in Fe–N austenite are calculated using the values of N–N interaction energies in two coordination spheres.     

The interaction of dissolved H with internally oxidized Pd–Rh alloys

Homogeneous fcc Pd–Rh alloys have been internally oxidized in the atmosphere at several temperatures from 1023 to 1123 K forming oxide precipitates within a Pd matrix. As shown from electron diffraction patterns of the internally oxidized Pd_0.97Rh_0.03 alloy, the oxide that forms is a mixed oxide, PdRhO₂. Internally oxidized Pd–Rh alloys can be reduced with H₂ (573–623 K) forming PdRh precipitates within the Pd matrix. This is a technique for segregating components of a substitutional solid solution binary alloy. The segregated alloy can be returned to the homogeneous Pd–Rh alloy by annealing at an elevated temperature and thus, in contrast to the internal oxidation of, e.g., Pd–Al alloys, the process can be readily reversed. The oxidation and (reduction+H₂O loss) were monitored from weight changes.After internal oxidation/reduction “diagnostic” hydrogen isotherms (323 K) were measured to follow the extent of oxidation and to determine the extent of H trapped at the internal interfaces from the positive intercepts along the H/Pd axis in the dilute phase. The H capacities in the steeply rising hydride region of the H₂ isotherms for the internally oxidized alloys are smaller than for Pd unless the Pd in the PdRh precipitates is considered to be inactive for H₂ absorption for the calculation of H/Pd values.In the two-phase region the absorption plateau pressures were significantly greater than for Pd–H, however, they were much closer to the Pd plateau p_H2 than to that of the unoxidized Pd–Rh alloys. The desorption plateaux were very close to those of Pd–H for X_Rh=0.01 to 0.05 alloys and therefore it can be concluded that the matrix is pure Pd whose absorption plateaux are affected by the presence of the precipitates.The techniques of TEM, SEM and SANS (small angle neutron scattering) were employed to examine the alloys after internal oxidation both before and after reduction. SANS confirmed directly that internal precipitates form from the internal oxidation and that they are rather large with an appreciable fraction having diameters greater than 100 nm.     

Oxygen–molybdenum interaction with dislocations in Nb-Mo single crystals at elevated temperatures

A study using a transmission electron microscope was performed for the deformation microstructures of Nb-20mol%Mo single crystals containing from 0.03 to 0.65 mol% oxygen and a Nb-40mol%Mo-0.05mol%O single crystal, compressed to approximately 4% plastic strain at 298 K, 973 K and 1473 K. At 298 K, screw dislocations are predominant in all alloys. At 973 K, in Nb-20Mo-0.03O and Nb-40Mo-0.05O, the dislocation microstructure contains the same amount of edge dislocations and small edge dipoles that tend to aggregate into clusters. In Nb-20Mo-0.65O, however, the arrangement consists of long glide loops parallel to the screw direction. At 1473 K, edge dislocations are dominant in all alloys. From the results, the following conclusions are made: (1) Oxygen atoms impede screw dislocation motion at 298 K and 973 K; (2) The influence of oxygen on screw segments is significantly strong at 973 K; (3) At 1473 K, oxygen atoms impede edge dislocation motion. It is inferred that O-Mo atmosphere is formed around dislocations.     

Microstructure–bifilm interaction and its relation with mechanical properties in A356

Cooling conditions and inclusions are of the most important factors that affect mechanical properties of cast aluminium alloys. This study investigated the effect of secondary dendrite arm spacing (SDAS) on the shape of pores (i.e. bifilm unravelling) and the mechanical properties of cast A356 alloy. Different cooling conditions were established by electrolytic copper chill, H13 steel chill and insulated ceramic that was placed in the mould cavity. SDAS and shape of pores were investigated by optical microscopy. The fracture surfaces of tensile test samples were analysed by SEM and EDX. Weibull two-parameter statistical method was used to assess the tensile properties. Results show that mechanical properties were dominantly affected by pore morphology that was formed by bifilms. Increasing the cooling rate (i.e. decreasing SDAS), the unravelling of bifilms delayed which decreased porosity formation significantly.     

Numerical analysis of solid–liquid interface shape during largesize single crystalline silicon with Czochralski method

Numerical analysis is an effective tool to research the industrial Czochralski(CZ) crystal growth aiming to improve crystal quality and reduce manufacturing costs. In this study, a set of global simulations were carried out to investigate the effect of crystal-crucible rotation and pulling rate on melt convection and solid–liquid(SL) interface shape. Through analyses of the simulation data, it is found that the interface deformation and inherent stress increase during the crystal growth process The interface deflection increases from 7.4 to 51.3 mm with an increase in crystal size from 150 to 400 mm. In addition, the SL interface shape and flow pattern are sensitive to pulling rate and rotation rate. Reducing pulling rate can flat SL interface shape and add energy-consuming Interface with low deflection can be achieved by adopting certain combination of crystal and crucible rotation rates The effect of crystal rotation on SL interface shape is less significant at higher crucible rotation rates.     

Analysis of phases in a Cu–Cr–Zr alloy

Phases of a Cu–0.31%Cr–0.21%Zr alloy were analyzed by scanning electronic microscope and energy dispersive X-ray spectroscopy (EDXS) and transmission electronic microscope (TEM). The EDXS results showed that there are three types of phases in the alloy, Cu-matrix, chromium-rich and zirconium-rich phases; coarse phases mainly consist of zirconium-rich phase. TEM result showed that fine chromium distributed in matrix and Cu₅₁Zr₁₄ phase was found in matrix.     

Degradation behavior of stainless steel in PbO–CaO–SiO2 slag in the presence of sulphur

Five stainless steel grades are subjected to PbO–CaO–SiO₂–S slag at 1200 °C. The degradation phenomena are identified as liquid slag and liquid metal corrosion, oxidation and sulphidation. The relation between sulphidation and steel and slag composition is discussed. For the slag with the lowest PbO/SiO₂ ratio, sulphidation is mainly recognized through (Fe, Cr)_xS_1?x at the surface and in the subsurface of the steel, especially for the steel grades with the lowest Cr content. For the slag with the highest PbO/SiO₂ ratio, sulphidation is mostly pronounced in the steel grades with the highest Ni content through the formation of a liquid (Ni, Pb, S) phase.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.