Elementary Defects Research Articles

Comparative Study of Techniques and Methods Used to Characterize Intergranular Corrosion in AA2024 Aluminum Alloy

Three methodologies, i.e., optical microscope (OM) observations, analyses using tomography without synchrotron radiations, and mechanical tests, were used, and their relevancy compared to provide a quantitative description of the intergranular corrosion (IGC) damage. The study was performed on three plates of 2024 aluminum alloy, to take into account variations in the microstructure of the material, using two types of exposure conditions, i.e. continuous immersions and cyclic tests. The resolution of tomography was too low to allow an analysis at the scale of the elementary IGC defects. For thin plates with recrystallized grains, the corrosion damage corresponded mostly to intragranular corrosion, and OM observations were the most relevant technique to quantify correctly this damage. Thicker plates were mainly affected by IGC. Continuous immersion tests led to slightly branched IGC defects and low hydrogen uptake, and OM observations provided an accurate description of the IGC damage that corresponded mainly to a geometrical one. However, combining OM observations and mechanical tests was required for a full description of the IGC damage after cyclic tests that were associated with a strong branching of the IGC defects, i.e. geometrical damage, and significant hydrogen uptake, i.e., a non-negligible volume damage.

Automatic recognition method for the three-elementary woven structures and defects of carbon fabric prepregs

A vision-based automatic recognition method for the three elementary woven structures and defects of carbon fabric prepregs is proposed for the real-time detection of carbon fabric prepregs in the process of automatic placement that can effectively guarantee the quality and performance of the preformed product. The diffuse scattering of reflected light caused by the surface resin is suppressed by using this new visual illumination method, which is first reported. An Artistic Conception Drawing (ACD) revert algorithm based on the repetitive characteristics of the surface patterns is proposed to automatically identify, match, and classify the three elementary woven structures. The common defect types (such as wrinkles and bubbles) in the prepreg placement process is analyzed and the methods for defect identification, location, and classification based on the model generated by the ACD revert algorithm is discussed. More than 1600 images are captured in the actual production line to validate and compare the performance of the methods. The experimental results illustrate that the ACD revert algorithm has the highest recognition rate (96.53%) for all the samples, and the defect recognition rate of the proposed method exceeds 95%, indicating that it can effectively detect carbon fabric prepregs in the automatic placement process.

Point defects and their impact on electrochemical performance in Na0.44MnO2 for sodium-ion battery cathode application.

Sodium manganese oxide Na0.44MnO2 (NMO) in an open structure with large tunnels is of great interest for sodium-ion battery cathode materials due to its high electrode voltage and capacity. However, its practical application is limited by poor rate performance, which can be tuned and improved by controlling point defects. We herein present a comprehensive study of intrinsic point defects in NMO using density functional theory (DFT) calculations in combination with thermodynamics. Using the DFT+U approach, we determine the formation energies of elementary defects and defect complexes depending on the sets of atomic chemical potentials, corresponding to a certain thermodynamic condition for the synthesis of stable NMO. Sodium interstitials are found to have the lowest formation energies in the relevant ranges of temperature and pressure. Other intrinsic point defects such as oxygen vacancies, sodium vacancies and manganese antisites can also be formed with proper formation energies and have an impact on the cathode performance. Compared to the perfect system, oxygen vacancies lower the electrode voltage, whereas manganese vacancies and antisites increase the voltage. We find that most point defects and defect complexes improve the sodium ion diffusivity, highlighting a proper control of defect formation for enhancing the performance of sodium-ion batteries.

Resection of bilateral occipital lobe lesions during a single operation as a treatment for bilateral occipital lobe epilepsy.

BACKGROUNDNeurosurgical treatment of severe bilateral occipital lobe epilepsy usually involves two operations several mos apart. AIMTo evaluate surgical resection of bilateral occipital lobe lesions during a single operation as a treatment for bilateral occipital lobe epilepsy. METHODSThis retrospective case series included patients with drug-refractory bilateral occipital lobe epilepsy treated surgically between March 2006 and November 2015.RESULTSPreoperative evaluation included scalp video-electroencephalography (EEG), magnetic resonance imaging, and PET-CT. During surgery (bilateral occipital craniotomy), epileptic foci and important functional areas were identified by EEG (intracranial cortical electrodes) and cortical functional mapping, respectively. Patients were followed up for at least 5 years to evaluate treatment outcome (Engel grade) and visual function. The 20 patients (12 males) were aged 4-30 years (median age, 12 years). Time since onset was 3-20 years (median, 8 years), and episode frequency was 4-270/mo (median, 15/mo). Common manifestations were elementary visual hallucinations (65.0%), flashing lights (30.0%), blurred vision (20.0%) and visual field defects (20.0%). Most patients were free of disabling seizures (Engel grade I) postoperatively (18/20, 90.0%) and at 1 year (18/20, 90.0%), 3 years (17/20, 85.0%) and ≥ 5 years (17/20, 85.0%). No patients were classified Engel grade IV (no worthwhile improvement). After surgery, there was no change in visual function in 13/20 (65.0%), development of a new visual field defect in 3/20 (15.0%), and worsening of a preexisting defect in 4/20 (20.0%). CONCLUSIONResection of bilateral occipital lobe lesions during a single operation may be applicable in bilateral occipital lobe epilepsy.

Formation of complex radiation defects in irradiated materials

The principles of formation of the complex vacancy defects (V-clusters), their ensembles and patterns of formation of superlattices of the V-clusters are determined. The inclusion of the drift component of the elementary defects into the field of elastic stresses of the V-cluster in the analysis allowed describing its genesis and development adequately. The mechanisms of motion of the V-clusters in the material are described in detail, considering their interaction with each other. The authors have developed the original physical and mathematical formalism within which it has become possible to describe the order-disorder phase transition when an ensemble of clusters chaotically distributed in the irradiated solid transforms into an ordered coherent superlattice. The critical point of the phase transition and the parameters of the defect lattice itself are determined. They are confirmed by the experimental results. The ordering process in this system is understood as the motion of the undamped wave of order parameter through the material, while other configuration states of the V-cluster ensemble constitute rapidly damping fluctuations. The article also shows the mechanism of linking the symmetry of the V-cluster superlattice to the symmetry of the initial crystal.

Fusion of interfaces in Landau-Ginzburg models: a functorial approach

We investigate the fusion of B-type interfaces in two-dimensional supersymmetric Landau-Ginzburg models. In particular, we propose to describe the fusion of an interface in terms of a fusion functor that acts on the category of modules of the underlying polynomial rings of chiral superfields. This uplift of a functor on the category of matrix factorisations simplifies the actual computation of interface fusion. Besides a brief discussion of minimal models, we illustrate the power of this approach in the SU(3)/U(2) Kazama-Suzuki model where we find fusion functors for a set of elementary topological defects from which all rational B-type topological defects can be generated.

Topology and control of self-assembled domain patterns in low-dimensional ferroelectrics

Whilst often discussed as non-trivial phases of low-dimensional ferroelectrics, modulated polar phases such as the dipolar maze and the nano-bubble state have been appraised as essentially distinct. Here we emphasize their topological nature and show that these self-patterned polar states, but also additional mesophases such as the disconnected labyrinthine phase and the mixed bimeron-skyrmion phase, can be fathomed in their plurality through the unifying canvas of phase separation kinetics. Under compressive strain, varying the control parameter, i.e., the external electric field, conditions the nonequilibrium self-assembly of domains, and bridges nucleation and spinodal decomposition via the sequential onset of topological transitions. The evolutive topology of these polar textures is driven by the (re)combination of the elementary topological defects, merons and antimerons, into a plethora of composite topological defects such as the fourfold junctions, the bimeron and the target skyrmion. Moreover, we demonstrate that these manipulable defects are stable at room temperature and feature enhanced functionalities, appealing for devising future topological-based nanoelectronics.

The roles of native defects and transition metal additives in the dehydrogenation mechanism of Mg(AlH4)2

The roles of native defects and transition-metal additives (Ti, Sc and Ni) in the dehydrogenation of Mg(AlH4)2 hydride were investigated by First-principles calculations based on density functional theory. The elementary native defects including Hi−, VH+, VH−, (H2)i0, VMg2−, Mgi2+, and VAlH4+ in Mg(AlH4)2 were identified. Based on the formation and migration of dominant defects, we proposed a dehydrogenation mechanism of Mg(AlH4)2. The formation of the dominant defect VAlH4+ is the rate-limiting step in the dehydrogenation process. The highly mobile Hi− diffuses into the lattice and binds with Mg2+ to produce MgH2 phase. In the transition metal doped hydrides, interstitial defects Tii and Sci cause the Fermi-level shift to the left and lead to the decrease of the formation energy and activation energy of VAlH4+, which is beneficial to the dehydrogenation of Mg(AlH4)2. This study presents an in-depth understanding on the roles of native defects and transition metal additives in the dehydrogenation process of Mg(AlH4)2 hydride.

Degradation of CdTe SC during operation: modeling and experiment

The mechanisms of CdTe SС degradation during operation are experimentally studied. Two mechanisms of degradation of such solar cells are identified. The first is the generation of defects in the transition region, which is caused by excess charge carriers and defects. The second is the increase in the back barrier. The study of the current-voltage and voltage-capacitance characteristics of solar cells allowed proposing a model of degradation of solar cells based on CdTe. It is found that the presence of copper in the back contact is associated with better initial efficiency, but also the fastest degradation during operation. In accordance with the proposed model, the occurrence of additional elementary defects as a result of dissociation of three types of point defect complexes (Cu i + –2Cu Cd – ) – , (V Cd2 – –Cu i+ ) – , (2Cu Cd – –V Te+ ) – is explained. Shunting of the n - p heterojunction and phase transformations from the p + -Cu 2 -x Te side due to electrodiffusion of Cu Cd – with p -CdTe at the n -CdS/ p -CdTe and p -CdTe/ p + -Cu 2-x Te boundaries is considered. On the other hand, the diffusion of Cu i + (interstitial copper) into the absorber volume is possible. Electrodiffusion of defects from heterojunctions to the absorber volume is possible, which leads to the compensation of effective acceptor centers and a decrease in the lifetime of minority charge carriers and, accordingly, a decrease in J ph . In addition, there is a growth of shunting metal chains along the longitudinal grain boundaries of p -CdTe between n - p and р - р + heterojunctions and the possibility of appearance of high-resistance phases of the Cu-Te system. The proposed model explains the possibility of occurrence of the р + – Cu 2– δ S phase on the CdS/CdTe boundary, which constrains the passage of the photoactive part of the solar spectrum in p -CdTe

Direct observation of mono-vacancy and self-interstitial recovery in tungsten

Reliable and accurate knowledge of the physical properties of elementary point defects is crucial for predictive modeling of the evolution of radiation damage in materials employed in harsh conditions. We have applied positron annihilation spectroscopy to directly detect mono-vacancy defects created in tungsten through particle irradiation at cryogenic temperatures, as well as their recovery kinetics. We find that efficient self-healing of the primary damage takes place through Frenkel pair recombination already at 35 K, in line with an upper bound of 0.1 eV for the migration barrier of self-interstitials. Further self-interstitial migration is observed above 50 K with activation energies in the range of 0.12–0.42 eV through the release of the self-interstitial atoms from impurities and structural defects and following recombination with mono-vacancies. Mono-vacancy migration is activated at around 550 K with a migration barrier of EmV=1.85±0.05 eV.

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.

Physics insight and first-principles calculation of atomic conductance: implications for the future interconnects

There are still many unknowns and open questions concerning the properties of atomic wires, which can be very different from the bulk case. A physics-based compact model framework for analysis of atomic-scale conductors in the presence of point defects is described, and the results of the compact model are compared with First-principle DFT calculations. The calculations indicate that the typically more resistive bulk metals can emerge as lower-resistance options over Al and Cu at atomic dimensions. In the proposed model, the electrons are considered to be localized at their host atoms. The discussion topics include low-temperature resistance of atomic wires from different materials, and the influence of elementary point defects, such as atomic voids on electron transport. It is argued that the concept of resistivity should be used with care when applied to low-dimensional metals, for example the effective resistivity of small conductors is increased even in the absence of surface and grain-boundary scattering. The related concept of electron ‘mean free path’ needs also to be revisited. These questions could have significant implications in terms of the selection of most appropriate material systems for tera-scale metallization schemes.

Эволюция анионных и катионных экситонов в щелочно-галоидных кристаллах (О б з о р)

AbstractThe manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.

Electrical properties of montmorillonite studied together with the processes occurring under thermal activation

The results of laboratory experiments on studying the electrical conductivity σ of a clay mineral montmorillonite from different sedimentary mineral deposits of Dagestan in the temperature interval from 100 to 1000°C are presented. The general regularities in the dependence of the electrical conductivity σ of the studied samples on the absolute temperature T are accounted for by the existence of the associated complexes of elementary defects of the crystal lattice. These complexes play important role in a variety of kinetic processes under the conditions of the Earth’s interior, and their existence is demonstrated by the experiments. The activation energy of the electrical conductivity and the preexponential factors are determined for all the temperature zones. The relationship between the pattern of temperature variations in electrical conductivity and the processes of releasing interlayer water and hydroxyls from different energy sites is established. It is concluded that the anomalous change in electrical conductivity in some samples reflects the postsedimentation changes of montmorillonite manifesting themselves by the emergence of a hydromuscovite component.

Elementary defects in graphane

The main zero-dimensional defects in graphane, a completely hydrogenated single-layer graphene, having the chair-type conformation have been numerically simulated. The hydrogen and carbon−hydrogen vacancies, Stone–Wales defect, and “transmutation defect” resulting from the simultaneous hoppings of two hydrogen atoms between the neighboring carbon atoms have been considered. The energies of formations of these defects have been calculated and their effect on the electronic structure, phonon spectra, and Young modulus has been studied.

Shapes and singularities in triatic liquid-crystal vesicles

Determining the equilibrium configuration and shape of curved two-dimensional films with (generalized) liquid-crystalline order is a difficult infinite-dimensional problem of direct relevance to the study of generalized polymersomes, soft matter and the fascinating problem of understanding the origin and formation of shape (morphogenesis). The symmetry of the free energy of the LC film being considered and the topology of the surface to be determined often requires that the equilibrium configuration possesses singular structures in the form of topological defects such as disclinations for nematic films. The precise number and type of defect plays a fundamental role in restricting the space of possible equilibrium shapes. Flexible closed vesicles with spherical topology and nematic or smectic order, for example, inevitably possess four elementary strength disclination defects positioned at the four vertices of a tetrahedral shell. Here we address the problem of determining the equilibrium shape of flexible vesicles with generalized liquid-crystalline order. The order parameter in these cases is an element of , for any positive integer p. We will focus on the case , known as triatic liquid crystals (LCs). We construct the appropriate order parameter for triatics and find the associated free energy. We then describe the structure of the elementary defects of strength in flat space. Finally, we prove that sufficiently floppy triatic vesicles with the topology of the 2-sphere equilibrate to octahedral shells with strength defects at each of the six vertices, independently of the scale.

Fault Detection of a Wheelset Bearing Based on Appropriately Sparse Impulse Extraction

Convolution sparse representation (CSR) is a novel compressive sensing technique proposed in 2016 and provides an excellent framework for extracting the impulses induced by bearing faults and the unevenness of wheel tread. However, its sparsity performance on extracting impulses is sensitive to the improper penalty parameter. So, a novel fault detection method, appropriately sparse impulse extraction, is proposed based on the combination of CSR, estimating the number of atom types (ENA), and crest factor. The type of atoms embedded in vibration signals is estimated by ENA. Aiming at the different types of atoms, the impulses with different sparse characteristic are spanned by CSR with different penalty parameters. The appropriately sparse impulses are selected for fault detection based on the maximal crest factor. The simulation validation, experiment verification, and practical application are conducted to validate the effectiveness of the proposed appropriately sparse impulses extraction. These results show that the proposed appropriately sparse impulse extraction not only can obtain fault-characteristic frequency and its harmonics for fault judgment but also describes the dynamic behaviour between elementary defects and their matching surfaces. In addition, the proposed appropriately sparse impulse extraction can isolate the impulses with different types of atoms and is very suitable for detecting the wheelset bearing faults.

Evolution kinetics of elementary point defects in ZnO implanted with low fluences of helium at cryogenic temperature

Hydrothermally grown $n$-type ZnO samples, implanted with helium $({\text{He}}^{+})$ at a sample temperature of $\ensuremath{\sim}40$ K and fluences of $5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ and $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, have been studied in situ by capacitance voltage (CV) and junction spectroscopy measurements. The results are complemented by data from secondary ion mass spectrometry and Fourier transform infrared absorption measurements and first-principles calculations. Removal/passivation of an implantation-induced shallow donor center or alternatively growth of a deep acceptor defect are observed after annealing, monitored via charge carrier concentration $({N}_{d})$ versus depth profiles extracted from CV data. Isothermal anneals in the temperature range of 290--325 K were performed to study the evolution in ${N}_{d}$, revealing a first-order kinetics with an activation energy, ${E}_{a}\ensuremath{\approx}0.7\phantom{\rule{4.pt}{0ex}}\text{eV}$ and frequency factor, ${c}_{0}\ensuremath{\sim}{10}^{6}\phantom{\rule{4.pt}{0ex}}{\text{s}}^{\ensuremath{-}1}$. Two models are discussed in order to explain these annealing results. One relies on transition of oxygen interstitials $({\text{O}}_{i})$ from a split configuration (neutral state) to an octahedral configuration (deep double acceptor state) as a key feature. The other one is based on the migration of Zn interstitials (double donor) and trapping by neutral Zn-vacancy-hydrogen complexes as the core ingredient. In particular, the latter model exhibits good quantitative agreement with the experimental data and gives an activation energy of $\ensuremath{\sim}0.75$ eV for the migration of Zn interstitials.

ELECTRICAL CONDUCTIVITY OF THE KAOLIN CLAY MINERAL DEPENDING ON THE TEMPERATURE

The aim is to study temperature dependence of the electrical conductivity of the kaolin mineral on crystalchemical factors and thermal transformations. Methods. We have applied the method of investigation of geo-electric properties of high-resistivity materials. Results. We gained the results of the study of the temperature dependence of electrical conductivity in the range of 100-1000 °C of the kaolin clay mineral, which is of great scientific and practical importance. We have proved the existence of the general laws of the nature of the change in the electrical conductivity within the temperature range for all the samples studied, due to the existence of associated complexes of elementary defects in the crystal lattice of the mineral, which play an essential role in the kinetic nature in the geosphere of the Earth. Availability of the spectrum values of the activation energy in the extrinsic region characterizes the stepped nature of dehydroxylation and delocalization of protons of hydroxyl groups of non-equivalent positions of power in the crystal lattice of the mineral, which characterizes the formation of fluid regime of sedimentary strata in dehydration. Conclusions. It was found that the behavior of the electrical conductivity is interconnected with the processes caused by the existence of elementary defects associated in complexes in the crystal lattice of the mineral. The range of values of the activation energy of conductivity reflects the process of dehydroxylation and delocalization of protons and hydroxyl groups.

Dislocation depinning from nano-sized irradiation defects in a bcc iron model

At the atomic level, irradiation hardening of a body-centered-cubic (bcc) crystal of iron is investigated through the study of a dislocation interaction with nano-sized obstacles of various types, i.e. nano-voids, dislocation loops with 〈100〉 and 1/2〈111〉 Burgers vectors, and C15 clusters. The dislocation pinning is common to every type of cluster excepted for loops with 1/2〈111〉 Burgers vector, the mobility of which allows them to glide along with dislocation, thence yielding a minor contribution to hardening. At bypassing of anchoring clusters, the dislocation is found to yield a pair of jogs throughout a process of local climb due to the absorption of vacancies or of self-interstitial atoms that form the clusters. In order to rationalize our simulation results, we have employed an analytical theory proposed by Bacon and Scattergood (1982) which is extended phenomenologically to treat all anchoring defects, including C15 clusters and 〈100〉 loops. For small clusters, below 17 elementary defects, the simulation results deviate from the analytical predictions, and the different types of cluster correspond to different pinning strengths amongst those of C15 clusters appear to be the largest, emphasizing a possible important contribution to irradiation hardening in iron based alloys.