Vacancy-type Defects Research Articles

Dislocation Substructure Analysis in γ′-Precipitated Ni-Based Alloy by X-Ray Diffraction Combined with Positron Annihilation

The systematic changes in the dislocation density and characteristics of γ′-precipitated Ni-based model alloys that develop under cold rolling are studied as simulated deformations, to examine the fundamental dislocation behavior in terms of the dislocation substructure formation. In particular, the dislocation density is quantified through X-ray line profile analysis (XLPA), which is effective for quantifying the dislocation density and characteristics, as well as positron annihilation lifetime (PAL) measurements, which are sensitive to vacancy-type lattice defects. Similar tendencies are obtained for the strain dependency of the dislocation density analyzed using XLPA and PAL. Hence, the influence of the γ/γ′ coherent interface and γ′ precipitation on the dislocation substructure and vacancies is shown by comparing with a Ni-Cr solid solution. These results help to understand the interaction of solute atoms, vacancies, and dislocation with regard to substructure formation in Ni-based alloys.

Study on vacancy-type defects in SIMP steel induced by separate and sequential H and He ion implantation

To provide a basic understanding of the synergy effect of hydrogen and helium on structural materials for future advanced nuclear systems, the vacancy-type defects in SIMP steel induced by separate and sequential H and He implantation at room temperature was investigated using positron annihilation Doppler broadening spectroscopy (DBS) and transmission electron microscopy (TEM). The DBS results indicated that, when implanted by H and He separately, the ΔS parameter of the H-implanted sample was greater than that of He, although the damage level (that is, the displacement per atom) induced by H was lower than that induced by He. This could indicate that He is more easily trapped by vacancy than H and prefers to occupy the center of the vacancy. When sequentially co-implanted by He and H, whatever the implantation sequence, the ΔS parameter was lower than that of H-only implantation. In addition, the ΔS parameter of sequential He + H implantation was greater than that of H + He implantation when the dose of H was sufficiently high. Combined with the TEM results, the synergistic effects of He and H on vacancy-type defect evolution are discussed.

Investigations of Ti-substituted CuFeO2 ceramics on the structure, defects, the local electron density and magnetic properties

Abstract CuFe1−xTixO2 (0 ≤ x ≤ 0.08) ceramics are synthesized by solid state reaction method to investigate the effect of Ti substitution on the microstructure, vacancy-type defects, the local electron density, and magnetic properties. The XRD and Raman spectra results show that all experimental samples possess the characteristic peak of CuFeO2 ceramics. SEM observations indicate that Ti substitution has obvious influence on the morphologies of experimental ceramics, and the microvoids increases significantly with increasing Ti content. Positron annihilation lifetime and Doppler broadening spectra results indicate that the vacancy concentration increases rapidly and then reaches a higher platform (>35%) as x > 0.01, and Ti substitution does not change the defects species in CuFe1-xTixO2 ceramics. The electron density decreases with increasing x and then reaches a low plateau (≈5.7) at x = 0.08. The χ - T curves reveal that the TN1 is independent of Ti content, while, the TN2 decreases with increasing x at x = 0.00–0.02 and even disappears in the magnetization curves as x > 0.02. The M-H curves indicate that the coexistence of weak ferromagnetism and antiferromagnetism for all the samples could be found. The possible reasons for the above observations are discussed in detail.

Features of the Temperature Dependence of the Specific Contact Resistance of Au–Ti–Pd–n+–n-Si Diffusion Silicon Structures

The temperature dependences of the specific contact resistance of silicon ρc with a doping step are measured experimentally and described theoretically. The measurements are performed in the temperature range from 4.2 to 380 K. It is established that the contacts of the studied Au–Ti–Pd–n+–n-Si structures are ohmic. It is shown that minimal ρc is implemented at T = 75 K. Its value rises both with a decrease in temperature (due to the freezing effect) and with an increase in temperature (due to the electron-enriched layer at the boundary with the bulk material). It is established that the bulk electron concentration strongly decreases in the near-contact region in a layer with a thickness on the order of one micron due to the compensation of silicon by deep acceptors appearing because of the formation of a rather high vacancy concentration during stress relaxation and the appearance of a high dislocation density, as well as due to their diffusion from the contact after heating to 450°C. The data on the occurrence of vacancy-type defects are confirmed by X-ray measurements. The dislocation density in the studied structures is also estimated from X-ray measurements.

Point-Defect Distribution and Transformation Near the Surfaces of AlGaN Films Grown by MOCVD

The characteristics of vacancy-type point defects near the surface and inside the AlGaN films were studied by positron annihilation spectroscopy (PAS) and fluorescence spectra. Two types of AlGaN films were grown by varying the growth temperature and the Si doping to maintain the volume of the threading dislocation densities (TDs). The PAS results showed that the point-defect types of the AlGaN films with approximate TDs differed. In the detailed PAS results, the W–S curves of AlGaN films showed two types of point-defect distribution forms. One type was uniform in the AlGaN film, namely, the major point defects inside the bulk and near the surfaces are almost the same as that of the VIII–ON complex, which was the probable candidate. The other type was nonuniform, suggesting that the major point defects were VGa/VAl inside the bulk and VIII–nVN complexes near the surfaces of the AlGaN films. The evolution of the PAS and fluorescence spectra also suggested that a point-defect rearrangement occurred near the surfaces of the AlGaN films after 10 MeV high-energy e-beam irradiation. This rearrangement was likely related to the transformation of the point-defect type from the oxide-substituted nitride (ON) to the oxide vacancy (VO, equaling to VN) based on our growth recipes.

Defect Creation in the Root of Single-Crystalline Turbine Blades Made of Ni-Based Superalloy.

An analysis of the defects in the vicinity of the selector–root connection plane occurring during the creation of single-crystalline turbine blades made of CMSX-6 Ni-based superalloy was performed. X-ray diffraction topography, scanning electron microscopy, and positron annihilation lifetime spectroscopy were used. Comparing the area of undisturbed axial growth of dendrites to the area of lateral growth concluded that the low-angle boundaries-like (LAB-like) defects were created in the root as a result of unsteady-state lateral growth of some secondary dendrite arms in layers of the root located directly at the selector–root connection plane. Additional macroscopic low-angle boundaries (LABs) with higher misorientation angles were created as a result of concave curvatures of liquidus isotherm in platform-like regions near selector–root connections. Two kinds of vacancy-type defects, mono-vacancies and vacancy clusters, were determined in relation to the LABs and LAB-like defects. Only mono-vacancies appeared in the areas of undisturbed axial growth. Reasons for the creation of macroscopic LABs and LAB-like defects, and their relationships with vacancy-type defects were discussed.

The effect of dislocations on MnNi-rich clusters in self-ion irradiated FeMnNi alloy

The effect of dislocations on irradiation-induced MnNi-rich clusters in self-ion irradiated FeMnNi alloy was investigated using X-ray diffraction (XRD), nanoindentation, positron annihilation Doppler broadening spectroscopy (PADBS), and atom probe tomography (APT). The nanoindentation results showed that irradiation-induced hardening decreased with dislocation density increasing. The results of APT and PADBS revealed that high-energy Fe-ion irradiation generates high-density MnNi-rich clusters and large vacancy-type defects in irradiated FeMnNi alloy. Note that dense dislocations suppress the formation of large vacancy-type defects and MnNi-rich clusters. By analyzing the effect of dislocations on point defects (PDs), diffusion of Mn and Ni atoms, and nucleation of MnNi-rich clusters, we found that nucleation mechanism of MnNi-rich clusters is strongly dependent on the flux of PDs. However, high-density dislocations can facilitate to annihilate irradiation-induced PDs and cause the decline of defect-solute coupling fluxes, which are responsible for a reduction in number density and mole fraction of MnNi-rich clusters.

Accumulation of Defects in Austenitic Stainless Steels with Phosphorus and Titanium Additions upon Electron Irradiation at 573 К Investigated Using Positron Annihilation Spectroscopy

Using the positron annihilation spectroscopy method, the accumulation of vacancy-type defects in steels Kh16N15М3 and Kh16N15М3T1 alloyed with phosphorus has been studied at early stages of irradiation at a temperature of 573 К. The obtained data have showed that vacancies in the steels interact with phosphorus atoms at this temperature with the formation of immobile and low-mobile vacancy—impurity complexes, which results in the accumulation of vacancy-type defects. In the steel Kh16N15М3Т1 under irradiation, the formation of nanosized particles of intermetallic Ni3Ti precipitates occurs, which intensify recombination of point defects and reduce their accumulation as compared to the steel Kh16Н15М3. As this occurs, the presence of phosphorus in the steel Х16Н15М3Т1 likewise leads to the intensification of the accumulation of vacancy defects.

Deciphering Z-scheme Charge Transfer Dynamics in Heterostructure NiFe-LDH/N-rGO/g-C3N4 Nanocomposite for Photocatalytic Pollutant Removal and Water Splitting Reactions

A series of heterostructure NiFe LDH/N-rGO/g-C3N4 nanocomposite were fabricated by combining calcinations-electrostatic self-assembly and hydrothermal steps. In this method, negatively charged N-rGO was electrostaticaly bonded to the self-assembled interface of n-n type g-C3N4/NiFe LDH hybrid. XRD and AFM results revealed successful formation of heterostructure nanocomposite due to the coupling effect of exfoliated NiFe LDH nanosheets with N-rGO and g-C3N4. Among the as synthesized heterostructure, CNNG3LDH performed superior photocatalytic activities towards 95 and 72% mineralization of RhB and phenol. Furthermore, CNNG3LDH could achieve the highest photocatalytic H2 evolution rate of 2508 μmolg−12h−1 and O2 evolution rate of 1280 μmolg−12h−1 under visible light irradiation. The CNNG3LDH possess lowest PL intensity, reduced arc of the Nyquist plot (43.8 Ώ) and highest photocurrent density (−0.97 mA cm−2) which revealed effective charge separation for superior photocatalytic activities. TRPL spectral results reveal the synergistic effect of layered component in CNNG3LDH for achievable higher life time of excitons of ~16.52 ns. In addition, N-rGO mediator based Z-scheme charge transfer mechanisms in CNNG3LDH were verified by the ESR and TA-PL studies. Enriched oxygen vacancy type defects in NiFe LDH and N-rGO mediated Z-scheme charge transfer mechanistic path strongly manifest the superior photocatalytic activities of the heterostructure materials.

Study of damage in binary Fe85Cr15 alloys irradiated by ions and the effect of an external magnetic field during irradiation

Understanding radiation-induced damage in iron-chromium alloys is critical for the use of advanced steels in future fusion reactors. However, the role of the strong magnetic fields present in such magnetically confined plasma devices on material damage, though considered important, has not been thoroughly explored to date. In this work, irradiation experiments of Fe85Cr15 alloy (15% Cr content) by heavy (Fe+) and light (He+) ions have been carried out in order to characterize the damage and, additionally, the influence of an external magnetic field (B = 0.4 T) has been taken into account. The analysis of the data has been done with different techniques, the distribution of the Cr and/or vacancies around probe nuclei has been explored by Mössbauer spectroscopy and the vacancy profile by Slow Positron Annihilation Spectroscopy (SPAS). Comparison between samples irradiated by He+ and Fe+, in absence of B, reveals significant differences in the average hyperfine magnetic field (<ΔH> = 0.8 T). This can be attributed to changes in the local environment around the probe nuclei (57Fe) where either vacancies or the Cr distribution play a role. From depth-profiling by SPAS in Fe+ irradiated samples, a large concentration of vacancy-type defects is found with a profile that extends deeper than that predicted by computer simulations. Furthermore, sequential irradiation by Fe+ and He+ ions indicates that the He atoms tend to fill the vacancy clusters just in the depth where the He+ is implanted. On the other hand, small but significant differences in vacancy-type defect profiles are found between the samples irradiated both in the presence and absence of a magnetic field (B = 0.4 T) both independently of single and sequential irradiation. Detailed Mössbauer spectroscopy and SPAS analysis seem to point out that external magnetic fields could be a non-negligible parameter in the material damage due to ion irradiation.

Characterization of proton-irradiated Chinese A508-3-type reactor pressure vessel steel by slow positron beam, TEM, and nanoindentation

Evolution of matrix defects and hardening of Chinese A508-3-type reactor pressure vessel steel irradiated to the doses of 0.05 dpa, 0.1 dpa, and 0.2 dpa by 110-keV proton at room temperature were investigated by slow positron beam, TEM, and nanoindentation. Slow positron beam results showed that proton irradiation-induced vacancy-type defects were observed and only one type of vacancy-type defects was characterized. Size and concentration of vacancy-type defects increased with the dose. TEM images revealed that dislocation loops were produced in the proton-irradiated samples. The mean size of the dislocation loops was unchanged but the number density of the dislocation loops was increased with the dose. An obvious hardening phenomenon of proton irradiated samples was detected by nanoindentation test. The irradiation hardening was mainly attributed to irradiation-induced matrix defects such as vacancy clusters, H-vacancy complexes, and dislocation loops.

Numerical Simulation of the Structure and Mechanical Properties of Silicene Layers on Graphite during the Lithium Ion Motion

The molecular dynamics method is applied to study structural and mechanical effects appearing during the lithium ion motion in a dc electric field along a planar channel formed by perfect silicene sheets and sheets containing vacancy-type defects. Mono-, di-, tri-, and hexavacancies of rather densely and uniformly filled silicene sheets are arranged one above the other on a graphite substrate. The times of Li+ ion passage through silicene channels with various gaps are determined. The construction of Voronoi polyhedra and truncated polyhedrons, whose centers coincide with the moving ion position allowed revealing the structural features inherent to the two-dimensional layered structure. The nature of stresses appearing in silicene sheets most critical to ion motion over the channel is determined.

Mechanical Properties of Vacancy Tuned Carbon Honeycomb.

Carbon honeycomb (CHC) has great application potential in many aspects for the outstanding mechanical properties. However, the effect of both defects and temperature on the mechanical properties are far from reasonable understanding, which might be a huge obstacle for its promising applications as engineering materials. In this work, we investigate the effect of vacancy-type defect, which is inevitably exists in material, on the mechanical properties of CHC via reactive molecular dynamics simulations. The mechanical strength is anisotropic and decreases with the increasing temperature. CHC yield in cell axis direction since the break of C–C bonds on the junction. Vacancies weaken CHC by reducing the strength and failure strain. The effect of single vacancy on strength of CHC becomes more obvious with reducing temperature and is sensitive to the location and bonding of the vacancies. The maximum reduction of strength in cell axis direction is with vacancy on the middle of the wall of CHC where sp2 bonds are removed. The strength is reduced by 8.1% at 500 K, 11.5% at 300 K and 12.8% at 100 K. With 0.77% defect concentration, the strength reduces 40.3% in cell axis direction but only 18.7% in zigzag direction and 24.4% in armchair direction.

Proton irradiation induced defects in T92 steels: An investigation by TEM and positron annihilation spectroscopy

In order to investigate proton irradiation damage on ferritic/martensitic T92 steels, both the unaged and aged (650 °C for 15,000 h) T92 steels were irradiated with 250 keV protons to 0.01, 0.05 and 0.20 dpa at room temperature due to the lower dose rate of protons compared with heavy-ions. The microstructural evolution induced by thermal aging and proton irradiation was studied by transmission electron microscopy and positron annihilation spectroscopy, and the corresponding micromechanical property changes were investigated by nano-indentation. After 0.20 dpa proton irradiation, the dominant irradiation-induced dislocation loops were a0100 type loops for both the unaged and aged samples. The dislocation-type defects in the aged T92 sample were larger in size and higher in number density, compared with those in the unaged samples. Less vacancy-type defects induced by protons were detected in the aged than the unaged T92 samples under the same irradiation conditions. The higher number density of dislocation-type defects led to more severe irradiation hardening in the aged T92 samples.

Structural disorder and in-gap states of Mg-implanted GaN films evaluated by photothermal deflection spectroscopy

Both structure of the valence band maximum (VBM) and deep-level defects for box-profile Mg-ion implanted (4 × 1019 cm−3) GaN samples are characterized by photothermal deflection spectroscopy (PDS). Compared with the results evaluated by positron annihilation spectroscopy, the variations caused by the thermal annealing is discussed with respect to Urbach energy, defect levels in the band gap, and photoluminescence. Forming Urbach tail gradually at the VBM at the 1000 °C annealing, vacancy-type defects clusters without drastic decrease of deep-level defects. Green and yellow luminescence emits slightly as the PDS signal at the deep-level is decreased by the annealing at 1100 °C due to the decrease of non-radiative recombination centers. The Urbach energy is improved by the further annealing so that the luminescence becomes intense due to the energy transfer through the phonon from Urbach region. The shift of Fermi level towards the valence band at the 1300 °C annealing, the sign of p-conduction, is confirmed from the valence band spectra. Compared to the Mg-doped GaN with p-conduction, it is considered that the improvement of Urbach energy is crucial for p-type activation.

First-Principles Assessment of the Structure and Stability of 15 Intrinsic Point Defects in Zinc-Blende Indium Arsenide

Point defects are inevitable, at least due to thermodynamics, and essential for engineering semiconductors. Herein, we investigate the formation and electronic structures of fifteen different kinds of intrinsic point defects of zinc blende indium arsenide (zb-InAs ) using first-principles calculations. For As-rich environment, substitutional point defects are the primary intrinsic point defects in zb-InAs until the n-type doping region with Fermi level above 0.32 eV is reached, where the dominant intrinsic point defects are changed to In vacancies. For In-rich environment, In tetrahedral interstitial has the lowest formation energy till n-type doped region with Fermi level 0.24 eV where substitutional point defects In A s take over. The dumbbell interstitials prefer &lt; 110 &gt; configurations. For tetrahedral interstitials, In atoms prefer 4-As tetrahedral site for both As-rich and In-rich environments until the Fermi level goes above 0.26 eV in n-type doped region, where In atoms acquire the same formation energy at both tetrahedral sites and the same charge state. This implies a fast diffusion along the t − T − t path among the tetrahedral sites for In atoms. The In vacancies V I n decrease quickly and monotonically with increasing Fermi level and has a q = − 3 e charge state at the same time. The most popular vacancy-type defect is V I n in an As-rich environment, but switches to V A s in an In-rich environment at light p-doped region when Fermi level below 0.2 eV. This study sheds light on the relative stabilities of these intrinsic point defects, their concentrations and possible diffusions, which is expected useful in defect-engineering zb-InAs based semiconductors, as well as the material design for radiation-tolerant electronics.

Electrical Doping Effect of Vacancies on Monolayer MoS2

Doping of transition-metal dichalcogenides (TMDCs) is an effective way to tune the Fermi level to facilitate the band engineering required for different types of devices. For TMDCs, a controversy abounds with regard to the doping role played by vacancy-type defects. Here, we report a detailed study based on first-principles calculations proposing that the native sulfur vacancies (VS) can significantly alter the electrical doping level in MoS2 and tune the material to exhibit conventional n- or p-type semiconductor characteristics. In particular, we reveal that the lower concentration of the single VS (2.8 and 6.3%) yields p-type characteristics, whereas the higher concentration of the single VS or a cluster of VS (12.5, 18.8, and 25.0%) yields n-type characteristics. The trend is consistent with previous X-ray photoelectron spectroscopy and scanning tunneling microscopy results. Employing this method of tuning the electron doping level, we modeled a commonly used metal–semiconductor interface to demonstra...

Operando monitoring of charging-induced defect formation in battery electrodes by positrons

The defect specific technique of positron–electron annihilation is utilized in an appropriate electrochemical cell for operando monitoring of vacancy-type defect formation which occurs in battery electrodes upon charging, using the cathode material LiNi1/3Mn1/3Co1/3O2 as a case study. The variation of the positron lifetime with charging-induced Li ion extraction indicates the formation of divacancies and vacancy agglomerates in a progressive amount as well as a reordering of vacancy agglomerates to one-dimensional vacancy chains at the end of charging. A remarkable correlation of the charging-induced variation of the positron lifetime with that of Li ion diffusion data in the literature has been found.

Characterisation of atomic-scale defects in ODS ferritic alloys by positron annihilation

ABSTRACTThe vacancy-type defects and their local chemical environment in different ODS alloys produced in the USA (14YWT), China (K5) and Russia (ODS EP-450) are studied. The Angular Correlation of Annihilation Radiation (ACAR), which is one of the positron annihilation spectroscopy method, was used. It was shown that in all alloys, except 14YWT, the dominant type of positron traps are vacancy-like defects, localised in matrix or associated with dislocations and/or interfaces of the incoherent particles. In the case of 14YWT alloy, which contains Y–Ti–O nanoclusters of a high density, the positrons confine and annihilate at O-vacancy pairs or complexes within nanoclusters. It is testified by enhanced electron density in annihilation sites and neighbourhood of Ti and Y atoms. These results, obtained by the ACAR method, indicate that the vacancies play an essential role in the formation of nanoclusters in ODS 14YWT alloy as it was theoretical predicted by first-principle calculations.

Компьютерное моделирование структуры и механических свойств слоев силицена на графите при движении иона лития

AbstractThe molecular dynamics method is applied to study structural and mechanical effects appearing during the lithium ion motion in a dc electric field along a planar channel formed by perfect silicene sheets and sheets containing vacancy-type defects. Mono-, di-, tri-, and hexavacancies of rather densely and uniformly filled silicene sheets are arranged one above the other on a graphite substrate. The times of Li^+ ion passage through silicene channels with various gaps are determined. The construction of Voronoi polyhedra and truncated polyhedrons, whose centers coincide with the moving ion position allowed revealing the structural features inherent to the two-dimensional layered structure. The nature of stresses appearing in silicene sheets most critical to ion motion over the channel is determined.