Mutual Recombination Research Articles

Optically generated small electron and hole polarons in nominally undoped and Fe-dopedKNbO3investigated by transient absorption spectroscopy

Transient light-induced absorption in nominally undoped and Fe-doped ${\text{KNbO}}_{3}$ crystals is observed in the visible and infrared spectral ranges after single pulse illumination with $\ensuremath{\lambda}=532\text{ }\text{nm}$. For nominally undoped ${\text{KNbO}}_{3}$ the decay of the light-induced absorption in a single step can be explained by incoherent hopping transport of optically generated small bound ${\text{O}}^{\ensuremath{-}}$ hole and small free ${\text{Nb}}^{4+}$ electron polarons and their mutual recombination. Iron doping causes an additional slow decay component and, remarkably, accelerates the initial decay process. A consistent model for the formation, hopping, and recombination paths of hole and electron polarons is deduced from the experimental data set for both nominally undoped and Fe-doped ${\text{KNbO}}_{3}$. The decrease in the polaron hopping-transport length in Fe-doped samples is attributed to the increased number densities of optically generated hole polarons by additional one-quantum excitations.

Optimum grouping in a modified genetic algorithm for discrete-time, non-linear system identification

The genetic algorithm approach is widely recognized as an effective and flexible optimization method for system identification. The flexibility of a genetic algorithm allows various strategies to be applied to it. One of the strategies applied is the modified genetic algorithm which relies on, among other things, the separation of the population into groups where each group undergoes mutual recombination operations. The strategy has been shown to be better than the simple genetic algorithm and conventional statistical method, but it contains inadequate justification of how the separation is made. The usage of objective function values for separation of groups does not carry much flexibility and is not suitable since different time-dependent data have different levels of equilibrium and thus different ranges of objective function values. This paper investigates the optimum grouping of chromosomes by fixed group ratios, enabling more efficient identification of dynamic systems using a NARX (Non-linear AutoRegressive with eXogenous input) model. Several simulated systems and real-world timedependent data are used in the investigation. Comparisons based on widely used optimization performance indicators along with outcomes from other research are used. The issue of model parsimony is also addressed, and the model is validated using correlation tests. The study reveals that, when recombination and mutation are used for different groups, equal composition of both groups produces a better result in terms of accuracy, parsimony, speed, and consistency.

The model of deformation-induced segregation near a moving grain boundary in the Fe-Cr-Ni alloy

A model is proposed of diffusion-induced segregation at a moving grain boundary in a three-component alloy upon severe plastic deformation with allowance for generation and absorption of point defects and their mutual recombination. Redistribution of atoms of alloying elements and formation of near-boundary segregates during severe plastic deformation in stable austenitic alloys of the Fe-12Cr-30Ni type is studied. It has been shown by numerically solving an appropriate set of equations that the motion of a grain boundary leads to the formation of a deformation-induced segregation with an asymmetrical profile of the component concentrations. The boundary is being enriched with nickel and depleted of the other components.

Multiscale Modelling of bcc-Fe Based Alloys for Nuclear Applications [PowerPoint Submission

AbstractUnderstanding the basic mechanisms that determine microstructure changes in neutron irradiated steels is vital for a safe lifetime management of existing nuclear reactors and a safe design of future nuclear options. Low-alloyed ferritic steels containing Cu, Ni, Mn and Si as principal solute atoms are used as structural materials for current reactor vessels. The microstructural evolution under irradiation in alloys is decided by the interplay between defect formation and thermodynamic driving forces, together determining the appearance of phase transformations (precipitation, segregation, …) and favouring or delaying the nucleation and growth of point-defect clusters, their diffusion and their mutual recombination or removal at sinks. A reliable description of the production, evolution and accumulation of radiation damage must therefore start from the atomic level and requires being able to describe multicomponent systems for timescales ranging from few picoseconds to years. This goal demands firstly the fabrication of interatomic potentials for alloys that must be both consistent with the thermodynamic properties of the system and capable of reproducing correctly the characteristic solute-point defect interactions, versus ab initio or experimental data. Secondly the performance of extensive molecular dynamics (MD) simulations, to grasp the main mechanisms of defect production, diffusion, mutual interaction, and interaction with solute atoms and impurities. Thirdly, the development of simulation tools capable of describing the microstructure evolution beyond the timeframe and lengthscale of MD, while reproducing as much as possible the atomic-level origin of the mechanisms governing the evolution of the system, including phase changes.In this presentation the results of recent efforts made in this direction in the case of Fe-Cu, Fe-Cr and Fe-Ni alloys, as basic model alloys for the description of steels of technological relevance, are highlighted. In particular, advanced techniques to fit interatomic potentials consistent with thermodynamics are proposed and the results of their application to the mentioned alloys are presented. Next, the development of advanced methods, based on the use of artificial intelligence, to improve both the physical reliability and the computational efficiency of kinetic Monte Carlo codes for the study of point-defect clustering and phase changes beyond the scale of MD, is reported. These recent progresses bear the promise of being able, in the near future, of producing reliable tools for the description of the microstructure evolution of realistic model alloys under irradiation.

A new formulation of sink strengths under steady irradiation: recombination and interference effects

The rate theory of irradiation effects in crystalline solids rests on the balance between the production and the annihilation of defects. The annihilation takes place either by mutual recombination or by absorption on sinks. The absorption rate is a function of the sink strength, a factor mainly of geometrical origin attached to each type of sink. Following Doan and Martin [Phys. Rev. B 67 134107 (2003)], an alternative definition of the sink strength is used which, at variance with the classical one, depends on the irradiation conditions and ensures, by construction, a correct partitioning of defect elimination between recombination and absorption on sinks. The correction factors for individual sink strengths can reach several orders of magnitude. Interference effects between sinks can also be introduced and calculated analytically.

Elimination of irradiation point defects in crystalline solids: Sink strengths

The rate theory of irradiation effects in crystalline solids rests on a set of two ordinary differential equations which, for each type of point defect (vacancy and self-interstitial), describe the balance between the production of defects on the one hand and their annihilation on the other. The latter process occurs either by mutual recombination, a bimolecular reaction, or by elimination on point defect sinks, a first-order reaction. The elimination rate is proportional to the defect concentration times the defect diffusion coefficient times a geometrical factor, the ``sink strength.'' The classical expression of sink strengths is obtained by solving the diffusion equation of point defects in a cell, which contains the sink, and ensuring that the mean value of the defect concentration in the cell equals the concentration in the rate theory. We propose an alternative criterion. Since the amplitude of the irradiation effects of practical relevance is dictated by the partitioning of the defect annihilation between mutual recombination and elimination on sinks, we propose that the value of the sink strength should give the correct value for the latter partitioning. The sink strengths so defined, scaled to their classical value, are evaluated for sink geometries of practical interest and expressed as a function of one dimensionless parameter, which is a function of the irradiation flux and temperature. Depending on the irradiation conditions, the correcting factors for individual sink strengths may be large (several orders of magnitude). When several types of sinks compete, we further impose that the partitioning of the elimination among the various types of sinks has the correct value. The sink strengths, as defined in this work, are additive, at variance with the classical ones. According to our definition, the dislocation bias, which measures the relative difference between the sink strengths of dislocations respectively for interstitials and vacancies, is shown to increase with the strength of neutral sinks around the dislocation. It ranges from zero when the dislocations are the only sinks to several ${10}^{\ensuremath{-}1}$ when the neutral sinks have a strength much larger than that of dislocations. The computation of the correcting factor is presented in such a way that it can be easily incorporated into the rate theory of irradiation effects.

Sinks strengths revisited

The rate theory of irradiation effects in crystalline solids rests on the balance between the production and the annihilation of defects. The annihilation takes place either by mutual recombination or by absorption on sinks. The elimination rate is a function of the sink strength, a geometrical factor attached to each type of sink. We propose a alternative definition of the sink strength, which, at variance with the classical one, ensures by construction a correct partitioning of defect elimination by recombination and by absorption on sinks. The correction factors for individual sink strengths can reach several orders of magnitude. Interference effects between sinks can be introduced and calculated.

The primary origin of dose rate effects on microstructural evolution of austenitic alloys during neutron irradiation

The effect of dose rate on neutron-induced microstructural evolution was experimentally estimated. Solution-annealed austenitic model alloys were irradiated at ≃400 °C with fast neutrons at seven different dose rates that vary more than two orders difference in magnitude, and two different doses were achieved at each dose rate. Both cavity nucleation and growth were found to be enhanced at lower dose rate. The net vacancy flux is calculated from the growth rate of cavities that had already nucleated during the first cycle of irradiation and grown during the second cycle. The net vacancy flux was found to be proportional to (dpa/s) 1/2 up to 28.8 dpa and 8.4×10 −7 dpa/s. This implies that mutual recombination dominates point defect annihilation in this experiment, even though point defect sinks such as cavities and dislocations were well developed. Thus, mutual recombination is thought to be the primary origin of the effect of dose rate on microstructural evolution.

Morphology and thermochromic phase transition of merocyanine J-aggregate monolayers at the air–water and solid–water interfaces

Thermal changes of the merocyanine dye (MD) monolayer at the air–water interface were investigated under various subphase conditions in order to elucidate the formation mechanism and to control morphological and chromatic properties of two-dimensional MD J-aggregate crystallites (JC) formed in the monolayer. The dissociation temperature (Td) of the JC to the monomer MD was measured for different counterions of MD molecules in the subphase. The JC size was found to be dependent on the subphase temperature; it becomes larger when the JC is formed at a temperature closer to Td. This phenomenon is qualitatively reproduced by the numerical simulation of the Cahn–Hilliard equation. In the case of the MD monolayer on the subphase which contains two kinds of counterions, it exhibits a reversible thermochromic transition between two different JC states. The chromatic change is discrete, and is attributed to the structural phase transition of the JC induced by the mutual recombination of two kinds of counterions to MD molecules. The structural difference between the high and low temperature JC states is examined by the point dipole model. The transition temperature and thermal hysteresis width can be varied by the fraction of 2 counterions. In situ observations using a multipurpose nonlinear optical microscope revealed that the transition is of first order and the nucleation and growth process of the low temperature phase in the high temperature matrix was observed. The JC size of the low temperature phase became much larger through the recrystallization process. For future application of this phenomenon, an airtight cell consisting of two monolayers at the solid–water interface and the subphase was developed. In the cell, the same reversible transition occurs, but with a slow relaxation.

Structure and dynamics of the He 2 * (a 3Σ u + molecular complex in condensed phases of helium

An investigation is made of the absorption spectra of triplet metastable helium molecules in the a(3) Sigma (+)(u)state in liquid He-4 and He-3 at various pressures and in dense He-3 gas. An analysis of the spectrum corresponding to the a(3)Sigma (+)(u) --> c(3)Sigma (+)(g) transition confirms the conclusion that there is a microscopic bubble surrounding the molecule in liquid helium. A simple approximation is proposed for the wave function of the valence electron of the molecule and the parameters of the bubble are determined for various experimental conditions. The coefficient of molecular recombination in liquid He-3 and He-4 was determined experimentally at various pressures and in dense cold He-3 gas. The results show good agreement with the theory of mutual recombination limited by molecular diffusion under conditions of strong van der Waals interaction. It is shown that in the condensed phases of helium the polarization of the molecules under the action of the magnetic field does not lead to suppression of their mutual recombination, and this is confirmed experimentally. (C) 2000 MAIK "Nauka/Interperiodica".

Structural phase transition of merocyanine J-aggregate induced by ion-recombination in the aqueous sub-phase

By using the sub-phase, which contains two different kinds of counter-ions, we found a reversible thermochromic transition between different J-aggregate states of amphiphilic merocyanine dye (MD) molecules in the monolayer at the air–water interface. This chromatic change is attributed to the structural phase transition of MD J-aggregate crystallites induced by the mutual recombination of different counter-ions to MD molecules. The drastic morphological change of the MD monolayer during the transition is revealed by the in-situ observation using a multipurpose non-linear optical microscope.

Microstructural evolution and radiation stability of steels and alloys

Results of a systematic investigation of structure–phase transformations in the main FSU (Former Soviet Union) construction steels and alloys of ferritic and austenitic classes irradiated in reactors and in heavy ion accelerators are presented. Features in the dislocation structure evolution for these steels related to differences in stacking fault energy are considered and the role of cold deformation in swelling behaviour is investigated. The mechanisms of infiltration and compulsory alloying of second phase precipitates during irradiation as a result of mutual recombination at structural defects in interphase boundaries are discussed. The role of second phase formation and evolution on swelling behavior processes is investigated. The influence of undersized and oversized alloying elements (B, Si, Ti, Nb, Mo, Sc) on different aspects of structure–phase transformations, which are generally defined as radiation stability over a wide interval of irradiation temperature and dose, is considered.

Modelling of irradiation-induced reordering in Ni 3Al on the basis of the migration of point defects

The kinetics of reordering under irradiation was investigated using a chemical reaction model in the case of vacancy- or interstitial-controlled mechanisms. We have calculated the theoretical temperature dependence of the steady-state degree of order in L1 2 Ni 3Al using the results of recent investigations of point defect properties, in various regimes of point defect annihilation. In a given annihilation regime, the temperature dependence of the steady-state degree of order shows a clear difference between interstitial and vacancy mechanisms. An important result is that a significant reordering can only be related to vacancy diffusion, except when the mutual recombination is the main annihilation process.

Study of the thermally stimulated glass structure reconstruction in extruded Li 2O · 2SiO 2 glasses by the method of radiation color center chronospectroscopy

Longitudinal (LS) and transverse (TS) Li 2O · 2SiO 2 glass samples cut along and across the axis of cylindrical rods produced by means of extrusion were investigated. All the glasses contained iron impurities (∼80 ppm). Spectra of radiation induced absorption (RIA) and kinetic curves, Δ a( E)= f( t), where E is the energy of a band and t is time, of the RIA relaxation were measured in the LS and TS before and after their heat treatment at 465°C (∼5°C above the glass transition temperature, T g). It turned out that, at smallest irradiation doses, Fe 3+ ions capture free electrons excited by X-rays. As a result, intrinsic hole color centers (CCs) with absorption bands at 2.1, 2.8, and 3.9 eV and impurity electron CCs [radiation reduced ions (Fe 3+) −] are formed. Radiation induced conversion Fe 3+ → (Fe 3+) − causes negative RIA with the maximum at ∼5.7 eV. Centers (Fe 3+) − and intrinsic hole CCs do not participate in the process of mutual recombination. With a dose increase, intrinsic electron CCs are created with absorption band at ∼5.5 eV which compensate the above negative RIA. These CCs recombine with intrinsic hole CCs. Before the heat treatment, rates of CCs recombination decay for the LS and TS differ but they are approximately equal after annealing for 4 h. As these rates must depend on distances between recombining electron and hole CCs and these distances in axial and radial directions of extruded glasses appear to be different, we conclude that the heat treatment stimulates reconstruction of the extruded glass anisotropic structure to an isotropic state similar to that of a non-extruded glass.

Calculation of marker distortion at elevated temperatures under low energy ion bombardment

The contributions of radiation-enhanced diffusion and ion induced collisional mixing to the distortion of the sputter depth profile of a near surface pseudo-marker under 100 eV Ar ion bombardment of a Cu (100) surface at 500 K have been evaluated using a simple semi-infinite model of radiation — enhanced diffusion via vacancies and interstitials. The ion mixing coefficient and the production of ion-induced point defects have been obtained from Molecular Dynamics calculations which were discussed in a previous paper [G. Kornich, G. Betz, Nucl. Instr. and Meth. B 117 (1996) 81]. The distortion of such a pseudo-marker is mainly caused by radiation-enhanced diffusion and only weakly due to collisional mixing. Steady-state concentrations of point defects with depth have been obtained. It was found that they depend strongly on the efficiency of defect sinks at the surface. The influence of internal sinks and of mutual recombination of defects is much weaker.

Optical spectra of the triplet molecules4He 2 * in superfluid helium in a magnetic field

We report study of absorption spectra fora→b anda→c transitions of the triplet excimer molecules4He 2 * in superfluid helium in the temperature range 1.4–2.1 K and in magnetic fields of 0–5.5 T. The lifetime of the excimers was determined by the rate of their mutual recombination and was 2 order of magnitude longer than in previous measurements. It was found that even at the maximum lifetime of 50 ms the molecules were not completely thermalized with respect to the rotational and vibrational degrees of freedom. The vibrational relaxation time was determined for the first time. It was found that the rotational relaxation is significantly faster than the vibrational relaxation. No expected suppression of the excimer recombination due to the molecules polarization in the magnetic field was found.

Effect of He on void formation and radiation-induced segregation in dual-beam irradiated Fe-Cr-Ni

To investigate the effect of helium on void formation and radiation-induced segregation, dual-beam irradiation with electrons and helium ions has been carried out for an Fe-Cr-Ni alloy by using a 1000 keV HVEM and a 300 keV ion-accelerator. From the measurement of dislocation loop growth, the vacancy migration energy was estimated at different injection rates of helium. The value increased with increasing injection rate of helium, which means strong trapping between vacancies and helium atoms. As a result the mutual recombination of point defects is retarded and the formation of secondary defects can be enhanced. Namely, on increase the densities of voids and dislocations was observed, as compared with the result without helium. With respect to the radiation-induced segregation, depletion of Ni and enrichment of Cr in the irradiated area were detected. The segregation of Ni and Cr was suppressed with increasing helium. From these results the role of helium on radiation-induced segregation is discussed on the basis of vacancy trapping and internal sinks.

LASER PHOTOLYSIS STUDY ON POLY(N‐VINYLCARBAZOLE) ADSORBED ON CELLULOSE SUBSTRATE

Abstract Triplet state and photoinduced charge separation dynamics under adsorption was Confirmed for the first time for the polymer without or with coadsorbed electron acceptor, respectively. The absorption spectrum of the triplet polymer is similar to the Tn← T1 band of N‐ethylcarbazole, showing no appreciable interchromophoric interaction. Absorption spectra of the coadsorbed systems comprise the polymer cation and electron acceptor anion. The ion radicals on the cellulose substrate undergo mutual recombination, and simple uni‐ and bimolecular decay kinetics do not hold. This mechanism was compared with that for poly(N‐vinylcarbazole) film.

A study of sink strength of dislocations through kinetics of loop formation under electron irradiation

The investigation of the sink strength of dislocations, Z E, fot interstitials is experimentally performed through 1.6 × 10 −13 J (1 MeV) electron irradiation around interface dislocations between β Nb and β Zr phases in a Nb-40.0wt.%Zr alloy. The value of Z E is determined using the fraction of interstitials annihilated by interface dislocations, which is derived from the decrement of the density and size of interstitial loops around the interface. The sink strength is successfully analyzed in terms of the dislocation array model [1—3] with the modification for the mutual recombination between interstitials and vacancies around the interface.

A theory of dislocation loop formation in binary alloys under irradiation

Abstract A model for dislocation loop formation under irradiation in a face-centered cubic (fcc) A-B alloy is presented. Besides reactions, such as defect production by irradiation, mutual recombination of an interstitial and a vacancy, dislocation loop nucleation, and their growth, conversion among the three types of interstitial dumbbells composed of A- and B-atoms is newly introduced in the formulation, considering the configurations and movements of the dumbbells. The present model is confirmed to give similar loop nucleation behavior to that derived by the conventional model in the case of a dilute alloy. Sample calculations are performed for a concentrated alloy, varying the strength of the binding between A- and B-atoms. Conversion among interstitial dumbbells is demonstrated to play an important role in the loop nucleation process. The atom that dominates loop nucleation varies with the binding of A- and B-atoms. As a result, kinetics of loop nucleation also differ.