Nonequilibrium Point Defects Research Articles

Multi–length scale characterization of point defects in thermally oxidized, proton irradiated iron oxides

A key for the success of safe nuclear power generation system is to consider structural materials that are economical, meet mechanical property needs, possess good corrosion resistance, and are radiation tolerant. Nevertheless, fundamental insights that elucidate the details of radiation damage on materials corrosion performance are lacking. This includes the behavior of surface oxides which often regulate corrosion. For example, it is unclear how non-equilibrium point defects, oxide structure, mass transport in oxides, and subsequent oxidation behavior are altered by the radiation. In this work, some of the effects of proton irradiation on the attributes of point defects, iron oxide microstructures, and the physical nature of the oxidation product were correlated with corrosion behavior. Iron oxides, fabricated by thermal oxidation in air at 400°C and 800°C for 1 h, were subjected to 200 keV, 0.03 dpa (displacements per atom) of proton irradiation, and subjected to corrosion reactivity assessment using AC and DC electrochemical methods. Experimental methods that target materials information at different length scales, such as positron annihilation spectroscopy (atomistic), transmission electron microscopy (mesoscopic), and electrochemical methods (macroscopic), were coupled to shed light on the impact of radiation-induced defect modifications and structural changes in oxides on corrosion reactivity which ultimately affects durability in harsh environments.

Dose rate dependent cation & anion radiation enhanced diffusion in hematite

Irradiation induced non-equilibrium point defect populations influence mass transport in oxides, which in turn affects their stability and performance in hostile environments. In this study a strong dose rate dependence is observed.

Radiation-Enhanced Anion Transport in Hematite

The influence of radiation-induced (1 MeV energy H+ to ∼0.1 displacements per atom (dpa) at 450 °C), nonequilibrium point defect populations on mass transport is studied with an integrated campaign...

Two-level Nanostructural States in Metallic BCC-Materials after High-pressure Torsion in the Bridgman Anvils

The results of an electron-microscopy investigation of two-level nanostructural states (submicrocrystals, measuring about 100 nm in size with the inner nanostructure and nanocrystal size about 10 nm, and crystal lattice curvature of hundreds of deg/μm) formed in the specimens of unalloyed Ta and V and Mo–Re-based alloys in the course of high-pressure torsion in the Bridgman anvils are generalized. A mechanism of their formation is proposed – quasiviscous motion of nanodipoles of partial disclinations, which is controlled by the flows of nonequilibrium point defects in the fields of high local pressure gradients. It is shown that the microstructure evolution with the increasing deformation degree consists in an increase in the volume fraction of a two-level nanostructural state, which results in a 3–4-fold increase in the microhardness of the deformed specimens, its maximum values being in the range Hμ ≈ (E/27–E/32). The principal physical factors and conditions of formation of these states upon plastic deformation of submicro- and nanocrystals are discussed.

Nondislocational Mechanisms of Strain Localization in Nickel Nanocrystals During Deformation by High-Pressure Torsion in Bridgman Anvils

Using the methods of transmission electron microscopy, new aspects of formation of reorientation nanobands with partial involvement of nondislocational deformation mechanisms are investigated in nickel nanocrystals under the conditions of its severe plastic deformation by high-pressure torsion in Bridgman anvils: local reversible (FCC→BCC→FCC) transformations of the martensitic type and quasi-viscous mass transfer by the flows of nonequilibrium point defects in the fields of high local pressure gradients. The features of disclinational structure and elastically stressed state at the nanoband propagation front are studied. A theoretical analysis of the rate of plastic deformation via the mechanisms of quasi-viscous mass transfer by the flows of nonequilibrium point defects is performed. A possibility of simultaneous realization of the martensitic and quasi-viscous deformation modes at the front of nanoband propagation is established. An analysis of the conditions and mechanisms where these modes are involved is performed as a function of the type of point defects (vacancies and interstitial atoms), deformation temperature, and peculiarities of disclination structure and elastically stressed state.

Structural Transformations in (Bi, Sb)2Te3 Solid Solutions Grown by Spark Plasma Sintering

We have studied the dependence of the thermoelectric properties of the bulk (Bi,Sb)2Te3 material on the temperature of spark plasma sintering (SPS). For analysis of the experimental results we took into account the regularities of structure formation in the material. The average crystallite size decreases with an increase in the SPS temperature to above 400 °C. Transmission electron microscopy showed that at above 400 °C the bulk and boundaries of the initial grains that are several micrometers in size contain a large number of nanosized grains (approx 10 – 20 nm) having the same composition. We show that the dependence of the thermoelectric properties of the material on SPS temperature correlates with changes in the fine structure of the material which is controlled by the redistribution of the intrinsic point defects. Our results suggest that, along with the well-known nanostructure formation processes, there is one more high-temperature self-organizing process of the formation of the nanostructural material based on the redistribution and change in the form of occurrence of the nonequilibrium point defects. This information can be used to broaden the possibilities of controlling the properties of the bulk thermoelectric material on the basis of (Bi,Sb)2Te3.

THEORETICAL EVIDENCE FOR STATIONARY SIZE DISTRIBUTION OF OXIDE NANOPARTICLES IN DISPERSION STRENGTHENED STEEL UNDER CASCADE PRODUCING IRRADIATION

A model developed in this paper describes the transport of nonequilibrium (produced by irradiation) point defects across a coherent interface in a heterophase medium. In the framework of this model we derive a kinetic equation for the distribution function of spherical nanoparticles of the second phase in a solid solution, which accounts for the flow and diffusion of nanoparticles in the dimension space as well as their dissolution in atomic collision cascades. We obtain an analytical form and study the stationary solution of this equation. The result obtained fits well to experimental data [A. Certain et al. Journal of Nuclear Mate 2013) 434, 311] on distribution of Y-Ti-O nanoparticles in the oxide dispersion strengthened ferritic steel 14YWT, irradiated with nickel ions up to 100 dpa at different temperatures. We conclude that in this case irradiation affects the distribution of fine oxide nanoparticles by creating nonequilibrium point defects rather than by cascade mixing.

The short-range clustering in Fe–Cr alloys enhanced by severe plastic deformation and electron irradiation

ABSTRACTThe short-range clustering enhanced by migration of non-equilibrium point defects generated under the high-pressure torsion in Bridgman anvils and ball milling is detected by means of Mössbauer spectroscopy in Fe100−cCrc (c = 12–20.6) binary alloys. The degree of short-range clustering is increased with increasing the temperature of severe plastic deformation. The short-range clustering enhanced by severe plastic deformation is similar to that obtained after the thermal annealing and electron irradiation at close temperatures.

Effect of Point Defects on the Interface Strength of Joint Materials

The paper proposes a nonequilibrium thermodynamic model of quasi-static crack growth at the interface of perfectly elastic materials which are either free of defects or contain nonequilibrium point defects like vacancies or interstitial/substitutional impurity atoms. Considering that the interface of joint materials is a point-defect adsorber from their bulk, the model establishes a crack growth criterion in terms of the defect adsorbability at the interface of two materials and at their free surfaces formed by an open crack, work of reversible interface separation dependent on the defect concentration in the materials, and crack growth rate. The criterion suggests that varying the defect concentration can provide one or another crack mode: growth or healing. The model is supported by a detailed mathematical analysis of the crack growth in the presence of nonequilibrium vacancies. Assuming that only one of the joint materials contains vacancies, the analysis gives an equation for the vacancy concentration which can change the crack mode, shows its analytical solution for two limiting concentrations, and demonstrates how the crack mode is changed as the vacancy concentration increases. The analysis is supplemented with quantitative estimates of the range of external and internal parameters (temperature, tensile stress, parameters of two joint materials, vacancy adsorbability at their interface and free surfaces) that admit of the two limiting cases in actual multilayer structures under typical operating conditions. The research results can be useful for modeling and optimizing the adhesion characteristics and assessing the service life of multilayer structures employed in modern micro- and nanoelectronics.

Lattice Curvature and Mesoscopic Strain-Induced Defects as the Basis of Plastic Deformation in Ultrafine-Grained Metals

Here, in the context of space, time, and energy, we analyze the nanoscale mesosubstructure of ultrafinegrained nickel and copper after equal channel angular pressing and subsequent rolling and its changes after lowtemperature annealing. The analysis, including scanning tunnel microscopy and positron lifetime spectroscopy, shows that the basis for plastic deformation in such materials is provided by their lattice curvature and associated nanoscale mesoscopic strain-induced defects. Under equal channel angular pressing and rolling, for example, these structural elements increase the role of nonequilibrium point defects, plastic distortion, and low-angle subboundaries. We also analyze the energy of internal interfaces (grain boundaries) estimated from dihedral angles of etch grooves of different scales and their relative energy from cumulative energy distribution functions. In ultrafinegrained nickel, the integral energy distribution function is Gaussian both after equal channel angular and rolling and after further low-temperature annealing, and this is because of the presence of low-angle subboundaries. In ultrafine-grained copper, the integral energy distribution function is Gaussian after equal channel angular pressing and rolling, and after low-temperature annealing it assumes a power form because of the absence of lattice curvature and low-angle subboundaries. Both metals reveal vacancy clusters due to their lattice curvature and to dissolved low-angle subboundaries. In ultrafine-grained copper at T> 180°C, dynamic recrystallization occurs as nonequilibrium low-angle subboundaries inside nanograins are dissolved. It is the lattice curvature that controls the formation and evolution of mesoscopic substructures on different scales under low-temperature annealing.

Мультимасштабне моделювання самоорганізації нерівноважних точкових дефектів в опромінюваному $\alpha$-цирконії

Мультимасштабне моделювання самоорганізації нерівноважних точкових дефектів в опромінюваному $\alpha$-цирконії

Growth of Oxygen Precipitates and Dislocations in Czochralski Silicon

The impact of oxygen precipitates and dislocations on carrier recombination is investigated on thick silicon slabs cut vertically from a Czochralski-grown silicon ingot. Using a combination of photoluminescence imaging, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy, we investigate the impact of pre-anneal on their recombination activity. We show that the vacancy concentration during precipitate growth affects the recombination activity of oxygen precipitates. Finally, we demonstrate the impact of nonequilibrium point defect concentrations on precipitate and dislocation growth.

Scale invariance of structural transformations in plastically deformed nanostructured solids

The scale-invariant mechanical behavior of a nanostructured solid is associated with plastic distortion as a major mechanism of nano- and microscale structural transformations. Active grain boundary sliding in a deformed material (microscale) within its highly developed planar subsystem (nanograin boundaries) causes a progressive increase in lattice curvature and plastic distortion of atoms which produces nonequilibrium vacant sites in the nanostructure. The motion of nonequilibrium point defects in nanostructure curvature zones provides conditions for noncrystallographic plastic flow, dissolution or dispersion of initial phases, and formation of nonequilibrium phases in a deformed material. The possibility of reversible structural phase transformations in the presence of high lattice curvature opens the way to greatly increase the fatigue life of surface nanostructured polycrystalline materials.

Multiscaling of lattice curvature on friction surfaces of metallic materials as a basis of their wear mechanism

A comprehensive structural study has been performed to explore deformation and wear debris formation on friction surfaces of metallic materials. A hierarchy of structural scales of plastic deformation and failure during wear has been established. The nanoscale plays the major role in the hierarchical self-organization of multiscale debris formation processes. On this scale, bifurcational interstitial states arise in zones of local lattice curvature, with plastic distortion and motion of nonequilibrium point defects which determine the nonlinear dynamics of structure formation and wear of surface layers. Nonequilibrium vacancies on lattice sites form microporosity through the coalescence mechanism under plastic distortion. The microporosity is a precursor of meso- and macroscale plastic shearing that defines wear debris formation.

Patterning and pattern selection in a surface layer: Feedback between point defects population and surface layer temperature variations

Abstract We study dynamics of pattern formation in a prototype system of nonequilibrium point defects in thin foils under sustained nonequilibrium conditions. A reaction–diffusion model describing spatio-temporal behaviour of both vacancy population and local temperature of a surface layer is used. It is shown that pattern selection processes caused by coupling between defect population and local temperature of a surface are realized. Associated oscillatory dynamics of main statistical moments of both vacancy concentration field and surface layer temperature is analysed in detail. It is found that during the system evolution spatial distribution of local temperature variations of the surface layer relates to vacancy population distribution. It is shown that the mean size of vacancy clusters (from 30 nm up to 300 nm) evolves in oscillatory manner due to pattern selection processes. Morphology of defect complexes can be controlled by defects generation rate.

Microscopic mechanism responsible for radiation-enhanced diffusion of impurity atoms

Modelling of radiation-enhanced diffusion (RED) of boron and phosphorus atoms during irradiation of silicon substrates respectively with high- and low-energy protons was carried out. The results obtained confirm the previously arrived conclusion that impurity diffusion occurs by means of the ‘impurity atom – intrinsic point defect’ pairs and that the condition of the local thermodynamic equilibrium between substitutional impurity atoms, nonequilibrium point defects created by irradiation, and the pairs is valid. It is shown that using RED, one can form a special impurity distribution in the semiconductor substrate including retrograde profiles with increasing impurity concentration in the bulk of the semiconductor. In addition, modelling of radiation-induced segregation of nitrogen implanted in stainless steel modified by titanium is carried out. It is shown that vacancy-impurity complexes are responsible for nitrogen diffusion in an implanted layer excluding the ‘tail’ region. The calculations performed give clear evidence in favour of further investigation of various doping processes based on RED, especially the processes of plasma doping, to develop a cheap method for forming specific impurity distributions in the near surface region.

ТЕРМОАКТИВАЦІЙНІ ПРОЦЕСИ У РАДІАЦІЙНО ЗАБАРВЛЕНИХ КРИСТАЛАХ CAF2-NA+

У моделі лінійного іонного кристала досліджено радіаційні параметри кристалів CaF2-Me+ після їх повторного опромінення іонізуючою радіацією. Показано, що досліджені кристали містять два типи дорадіаційних дефектів – термічно рівноважні дефекти дипольного типу [(Me+)-(Va)+ – (Me+)-(Va)+] та термічно нерівноважні електрично заряджені дефекти [(Me+)-– (Va)+(Me+)-(Va)+], де (Me+)- – іон лужного металу, (Va)+ – аніонна вакансія. Розраховано граничні концентрації центрів забарвлення (FA, FA(1), MA+, VK, VKA, VKA(1)-центрів), досліджено їх термічну стабільність та термоіндуковані перетворення.

Silicon germanium interdiffusion in SiGe device fabrication: A calibrated TCAD model

SiGe alloys are used in many types of electronic devices. For upcoming FinFET technologies, there is interest in the use of SiGe heterostructures in the complete mole fraction range (0–100% Ge atoms). For high Ge mole fraction and small device dimensions, interdiffusion of Si and Ge atoms can lead to critical changes in the distribution of Si and Ge atoms, impacting channel strain and carrier mobility. A process model for Si–Ge interdiffusion has been developed which covers the needs for actual device fabrication processes. Interdiffusion is understood to be correlated with the diffusion of vacancies, self-interstitials, and dopant-defect pairs. We consider the impact of non-equilibrium point defect concentrations (such as interstitial super-saturation after ion implantation), the impact of compressive or tensile strain, and a twofold impact of doping: first, doping determines the local electron concentration and thereby the abundance of charged point defects, and second, the diffusion of dopant-defect pairs can contribute to the interdiffusion of Si and Ge atoms. The model covers the full range of possible Ge mole fractions (0–100%). For undoped SiGe, it has been calibrated against a broad range of published experimental data for radio-tracer diffusion in SiGe and for interdiffusion in biaxially strained SiGe heterostructures grown on Si, stress-relaxed SiGe, or Ge. The extensions for doped SiGe are presented with a reference to few interdiffusion data for heavily doped SiGe heterostructures. Finally, the model has been consistently integrated into a set of models for the continuum process simulation of point defects and dopants in pure Si, SiGe, and pure Ge, in Sentaurus Process.

Electrical and optical properties of ceramic barium lead titanate containing nonequilibrium oxygen vacancy concentration

The electrical properties of stoichiometric and nonstoichiometric samples of barium lead titanate ferroelectric ceramics with the initial composition Ba0.95Pb0.05TiO3 have been studied by impedance spectroscopy in weak electric fields at temperatures from 295 to 620 K and frequencies from 0.3 to 103 kHz. In addition, the optical properties of the samples have been investigated in the infrared spectral region at room temperature. The measurements were made before and after vacuum quenching, which helped to produce nonequilibrium point defects: oxygen vacancies.

Molecular-beam epitaxy of heterostructures of wide-gap II–VI compounds for low-threshold lasers with optical and electron pumping

The paper presents basic approaches in designing and growing by molecular beam epitaxy of (Zn,Mg)(S,Se)-based laser heterostructures with multiple CdSe quantum dot (QD) sheets or ZnCdSe quantum wells (QW). The method of calculation of compensating short-period ZnSSe/ZnSe superlattices (SLs) in both active and waveguide regions of laser heterostructures possessing the different waveguide thickness and different number of active regions is presented. The method allowing reduction of the density of nonequilibrium point defects in the active region of the II–VI laser structures has been proposed. It utilizes the migration enhanced epitaxy mode in growing the ZnSe QW confining the CdSe QD sheet. The threshold power density as low as P thr ∼ 0.8 kW/cm2 at T = 300 K has been demonstrated for laser heterostructure with single CdSe QD sheet and asymmetric graded-index waveguide with strain-compensating SLs.