Formation Of Defect Clusters Research Articles

Understanding the effect of irradiation temperature on microstructural evolution of 20MnMoNi55 steel

In this study, the effect of irradiation temperature on microstructural evolution of Indian RPV steel is reported. This study, by virtue of helium ion irradiation at 77, 300 and 573 K, could bring out the effect of the irradiation induced defects on microstructural and mechanical property changes at different stages of their existence starting from the state of cascade damage till the point of their free migration. Irradiation experiments were performed with varying ion energies to achieve nearly uniform irradiation damage of 0.05, 0.2 and 3 dpa in a ~ 300 nm wide region. Irradiated samples were characterized using GIXRD, PAS, TEM and nanoindentation. Unirradiated samples showed predominant presence of a combination of di- and tri-vacancy type of defects. Most of the dislocations present in unirradiated samples were screw dislocations, while mixed type was noticed upon irradiation irrespective of the irradiation temperature. PAS study showed formation of distinct defect types at different irradiation temperatures. TEM study confirmed formation of dislocation loops and defect clusters on irradiation. Higher irradiation temperatures resulted in the extension of the width of the damage region owing to increased migration of defects.

Experimental study and DFT calculations of improved giant dielectric properties of Ni2+/Ta5+ co-doped TiO2 by engineering defects and internal interfaces

High-performance ceramics with chemical formula (Ni1/3Ta2/3)xTi1−xO2 with excellent dielectric properties are demonstrated. The dopants of Ni2+ and Ta5+ in TiO2 caused the formation of oxygen vacancies and free electrons. The (Ni1/3Ta2/3)xTi1−xO2 exhibited low loss tangent of 0.046 and a high dielectric permittivity of 3.5–4.5 × 104 with a very weak dependence on temperature (−60 to 200 °C). Broadband dielectric spectroscopy shows at least four dominant sources in the dielectric relaxation response in the temperature range of − 253–210 °C. DFT calculations indicate the formation of defect clusters, which are the largest contributors to the dielectric response, and these are found to be dominant even at temperatures down to − 253 °C. Both grain boundary and surface layer mechanisms in the ceramics contribute to the dielectric response at the relatively high temperatures. The sample–electrode contact effect associated with oxygen vacancy diffusion is dominant at high temperatures above 150 °C.

Ab initio calculations for void swelling bias in α - and δ -plutonium

Void swelling can develop in materials under persistent irradiation when nonequilibrium vacancy and self-interstitial populations migrate under sufficiently asymmetric interaction biases. In conventional metals, the propensity is determined to the first approximation by comparing point-defect relaxation strains. We thus present DFT-based calculations of structures and formation energies and volumes of point defects in the $\ensuremath{\alpha}$ and the $\ensuremath{\delta}$ phases of plutonium. We discuss the pros and cons of various levels of electronic structure theory: spin polarization, spin-orbit coupling, and orbital polarization. Our results show that lattice defects in $\ensuremath{\delta}$-Pu, in contrast to most fcc metals, have surprisingly small formation volumes. Equally unexpected are the large defect formation volumes found in the low-symmetry $\ensuremath{\alpha}$-Pu phase. Both these unusual properties can be satisfactorily explained from defect-induced spin/orbital moment formation and destruction in the Pu phases. Surprisingly, the point defects in $\ensuremath{\alpha}$-Pu are found to induce far larger transformation of the local electronic structure than in $\ensuremath{\delta}$-Pu. When we use the calculated defect properties to estimate the classic void swelling bias in each of the phases, we find it to be unusually small in $\ensuremath{\delta}$-Pu but likely much larger in $\ensuremath{\alpha}$-Pu. Hence, swelling rates and mechanisms can diverge dramatically between the different phases of Pu. Especially in the transient regime before the formation of large defect clusters, the swelling rate of $\ensuremath{\alpha}$-Pu can reliably be expected to be much larger than $\ensuremath{\delta}$-Pu. However, accurate forecasts over longer times will require the conventional void-swelling theory to be modified to handle the complexities presented by the different Pu phases. As a case in point, we show the possible anomalous temperature dependence of vacancy properties in $\ensuremath{\delta}$-Pu, caused by entropic contributions from defect-induced spin-lattice fluctuations. Such complications may affect defect-defect interactions and thus alter the void swelling bias.

Post irradiated microstructure and mechanical properties of pure V

Proton irradiation has been carried out on pure vanadium samples and the changes in the microstructure and mechanical properties have been evaluated as a function of dose. The microstructural parameters such as domain size and microstrain have been assessed along different crystallographic planes using the X-ray diffraction line profile analysis methods such as simplified breadth and the modified Rietveld technique. No significant change in the dislocation density is observed as a function of dose from the XRD results. However, the results of positron annihilation spectroscopy in terms of positron lifetime and S-parameter clearly indicated the formation of small defect clusters with irradiation. Transmission electron microscopy investigation confirmed the presence of small dislocation loops in the irradiated samples. Molecular dynamics simulations carried out to model the formation of irradiation-induced defects clusters with irradiation dose support the experimental observation of small defect clusters and increasing cluster size with dose. The yield strength and tensile strength showed systematic increase with irradiation dose corroborating with the results of microhardness measurement. The fractography of the tensile samples clearly showed the characteristics of brittle fracture in the irradiated samples.

Surface structure and quenching effects in BiFeO3‐BaTiO3 ceramics

AbstractThe structure and properties of Mn‐doped 0.67BiFeO3‐0.33BaTiO3 ceramics are systematically investigated with respect to the effects of annealing prior to rapid cooling by quenching in air. Air‐quenching induces a change in crystal structure from pseudo‐cubic to rhombohedral, with higher quenching temperatures leading to an increased rhombohedral distortion. These structural changes are correlated with the appearance of more well‐defined ferroelectric domain configurations. It is shown that the surface preparation procedures for XRD measurements can induce significant changes in the peak profiles, indicating differences in crystal structure between the surface and bulk regions. Frequency dispersion in the temperature‐dependent relative permittivity for the as‐sintered sample is significantly reduced after quenching, accompanied by enhancement of the Curie point and improved temperature‐stability of piezoelectric properties. It is proposed that the formation of defect clusters by A‐site cation diffusion during cooling is circumvented by quenching, leading to the observed modification of structural distortion and ferroelectric properties.

Growth and Photovoltaic Device Application of Cu2BaGe1–xSnxSe4 Films Prepared by Selenization of Sequentially Deposited Precursors

Toward suppressing the formation of anti-site defects and related defect clusters in kesterite Cu2ZnSnS4–xSex (CZTS) absorber films, Cu2–II–IV–X4 (II = Sr, Ba; IV = Ge, Sn; X = S, Se) compounds hav...

Investigation of irradiation resistance characteristics of precipitation strengthened high-entropy alloy (CoCrFeNi)95Ti1Nb1Al3 using slow positron beam

A high-entropy alloy (HEA), (CoCrFeNi)95Ti1Nb1Al3, with a tensile strength approximately 60% higher than that of the equiatomic high-entropy alloy CoCrFeNi was developed. The increase in tensile strength results from the formation of Ni, Ti, Nb, and Al-rich precipitates, having a face-centered cubic (FCC) L12 type crystal structure, in the alloy during aging. To investigate the irradiation resistance of this alloy, it was irradiated up to 1.5×1019 ions/m2, corresponding to 1.65 displacements per atom (dpa) at the peak damage, at 300, 573, and 773 K using high-energy 2.5 MeV Fe ions. Applying a slow positron beam, the formation of defect clusters induced by ion irradiation and the thermal stability of defects formed at 300 K were investigated. Vacancy clusters formed after irradiation with 5.0×1018 ions/m2 at 300 K. Irradiation above this fluence at 300 K, and at the same fluence at 573 K did not significantly affect the formation of vacancy clusters. However, no vacancy clusters were formed after irradiation at 5.0×1018 ions/m2 at 773 K. The high-temperatures irradiation of this HEA hinders the formation of vacancy clusters. The irradiation resistance of the (CoCrFeNi)95Ti1Nb1Al3 HEA was excellent.

Formation and propagation of solitonlike defect clusters in confined active nematics with chiral anchoring

The authors show that active nematics confined in channels with chiral anchoring can generate cohesive topological defect clusters, which undergo periodic structural reconfiguration locally and energy oscillations and are transported directionally in a soliton-like form.

Probing the defects and trap distribution in MgAl2O4 nanocrystals through electron spin resonance and thermoluminescence

Trap distribution in MgAl2O4 nanocrystals has been investigated by analyzing thermoluminescence (TL) glow curves upon UV-irradiation. To probe various defects and trap distribution with the growth of spinel phase and crystallinity, three different fuels viz. monoethanolamine (MEA), glycine and urea, were employed in the combustion synthesis and annealing was performed at 700 ∘C and 900 ∘C. MEA and glycine resulted in crystalline spinel phase in annealed samples, however, urea resulted in minor secondary phase of MgO with the spinel phase in pristine as well as annealed samples. The x-ray photoelectron spectroscopy informed the presence of minor concentration of C and N in addition to Mg, Al and O, which are expected to influence the various defects of MgAl2O4 samples. The TL glow curve broadening in pristine and 700 ∘C annealed samples inferred the presence of multiple closely distributed traps. Shallow traps were more populated with urea, while higher concentrations of deep traps were observed with MEA and glycine. Computerized glow curve deconvolution of the TL glow curves revealed various trapping parameters. The observed shallow and deep traps have been attributed to various electron and hole centres such as antisite defects, O− and F+ centres. The significant high temperature shift of glow curve maxima in 700 ∘C annealed samples, synthesized using MEA and glycine, might be due to the reorganization of the local energy levels with the growth of spinel phase. Quenching of TL intensity at 900 ∘C anticipated the defect annealing accompanied by clustering of existing defects. Electron spin resonance analysis probed the existence of F+, V− centres and the spin–spin interaction of electron/hole spins trapped at these centres with the nearby cations, and estimated the formation of paramagnetic defect clusters at 900 ∘C annealing. A schematic band model is proposed to illustrate the TL behaviour in MgAl2O4 nanocrystals.

Microstructural characterisation of 160 MeV oxygen irradiated niobium

ABSTRACT Microstructural changes due to 160 MeV oxygen ion irradiation on pure Nb have been characterised using X-ray Diffraction Line Profile Analyses. Microstructural parameters such as coherent domain size, microstrain within the domain and dislocation density are evaluated from the homogeneously damaged region. It is observed that the domain size decreases along with an increase in microstrain and dislocation density as a function of dose. The local changes in the microstructure have also been evaluated in terms of band contrast, mean angular deviation and local misorientation from electron backscattered diffraction data at the surface and an intermediate depth in the homogenously damaged region of selected irradiated samples and in the unirradiated sample. An increase in mean angular deviation and local misorientation is observed in the highest dose sample, which may be an indication of dislocation loop formation. Defect clusters and dislocation loops are observed using transmission electron microscopy in the highest dose sample. Microhardness measurement has also been carried out on the unirradiated and irradiated samples to correlate the microstructural changes with the mechanical property as a function of dose. It is observed that the hardness of the material increases as a function of dose due to the formation of defect clusters and dislocation loops that pin the movement of dislocations. Finally, a comparison of the microstructures in proton and heavy ion irradiated samples at approximately similar dpa-rate has been carried out to understand the effect of the ion species on the defect morphology.

Advanced Data Encryption ​using 2D Materials.

Advanced data encryption requires the use of true random number generators (TRNGs) to produce unpredictable sequences of bits. TRNG circuits with high degree of randomness and low power consumption may be fabricated by using the random telegraph noise (RTN) current signals produced by polarized metal/insulator/metal (MIM) devices as entropy source. However, the RTN signals produced by MIM devices made of traditional insulators, i.e., transition metal oxides like HfO2 and Al2 O3 , are not stable enough due to the formation and lateral expansion of defect clusters, resulting in undesired current fluctuations and the disappearance of the RTN effect. Here, the fabrication of highly stable TRNG circuits with low power consumption, high degree of randomness (even for a long string of 224 - 1 bits), and high throughput of 1 Mbit s-1 by using MIM devices made of multilayer hexagonal boron nitride (h-BN) is shown. Their application is also demonstrated to produce one-time passwords, which is ideal for the internet-of-everything. The superior stability of the h-BN-based TRNG is related to the presence of few-atoms-wide defects embedded within the layered and crystalline structure of the h-BN stack, which produces a confinement effect that avoids their lateral expansion and results in stable operation.

Analysis of a SiGe Mach-Zehnder modulator for 400G data communication.

In this paper, the performance of a silicon (Si) Mach-Zehnder modulator (MZM) is enhanced by implanting germanium (Ge) on Si, forming a graded Si-Ge (SiGe) core. A process simulation study is done, and the effect of substrate temperature, implantation energy, and pre-amorphization on the Ge composition and developed in-plane stress is observed. The dependence of active dopant concentration and defect-cluster formation on the annealing conditions is discussed. A comparison of the process simulated SiGe phase shifter with a Si phase shifter shows 2.27× higher phase shift at -5V for 1550 nm wavelength of operation. A dual-arm drive with quadrature operation is investigated for both SiGe and Si MZMs. A traveling-wave electrode is used to enhance the modulation bandwidth. The SiGe MZM achieves better performance in terms of modulation bandwidth, modulation speed, fiber transmission length, energy per bit, and dispersion tolerance compared to the Si MZM. The dual-arm driven SiGe MZM can achieve 62 Gbps error-free 2 km fiber transmission with energy per bit of 1.92 pJ/bit and 3 dB bandwidth of 62.8 GHz at -2.5V bias using 3Vpp on-off keying modulation.

Molecular dynamics simulations of radiation cascade evolution near cellular dislocation structures in additively manufactured stainless steels

The dislocation dense cellular structures observed in additively manufactured (AM) stainless steels can allow these materials to have enhanced strength and ductility compared to conventionally manufactured materials. Effective design of radiation resistant materials often necessitates creating microstructures containing of a high density of dislocations that act as sinks for radiation induced defects. This work uses molecular dynamics simulations to study the impact of pre-existing defects on radiation damage in stainless steel 316L fabricated by the laser powder bed fusion process. The evolution of synthetically generated dislocations in a crystal system in response to multiple radiation collision cascades suggest that dislocation dense regions reduce the probability of surviving point defects forming clusters. This finding seems to result from the dislocation cores’ ability to absorb point defects, thereby limiting the formation of clusters during the recombination phase. However, after successive cascades, the ability of the dislocation entanglement to prevent defect formation is diminished and defect cluster formation trends to an equilibrium. Accordingly, it is hypothesized that in an experimental setting the dislocation cells will initially act as neutral sinks for point defects, potentially delaying the onset of radiation damage effects; but, these structures will be degraded due to radiation-enhanced diffusion and lose their effectiveness at doses above 1.5–2 displacements per atom. Nevertheless, the ability to produce complex and feature-specific microstructures suggests promise for the design of radiation tolerant materials using AM methods.

Distortion energy-electronic energy compensation determines the nature of solute interactions with irradiation induced vacancies in ferritic steel

Fundamental knowledge of vacancy-solute atom (in particular, Cu and Ni) interactions at the electronic level is of utmost importance to understand experimentally observed Cu-precipitation in reactor pressure vessel (RPV) steel. In the present investigation, using first-principles electronic structure calculations within the framework of density functional theory (DFT), we unravel the nature of such interactions between a vacancy (V) or di-vacancy and solute atoms (mainly Cu and Ni) in the bcc-Fe lattice. One of the very novel features of the present investigation is that we demonstrate the importance of distortion energy-electronic energy compensation in stabilizing the formation of vacancy-Cu and vacancy-Ni clusters in ferritic steel. Further decomposition of the electronic energy contribution into different bonding contributions in conjugation with differential charge density analyses clearly reveals the origin of stability as a consequence of mutual compensation of different energy modes. For both Cu-Cu and Ni-Ni interactions, the presence of a vacancy leads to a more attractive interaction, implying that such vacancies generated due to irradiation make solute aggregation easier compared with the case of model steel with no defects. We have also demonstrated that the formation of CumNin clusters (m, n = 1, 5) is energetically favorable in addition to demonstrating that the stability increases with an increasing number of Cu or Ni atoms. The rate of increase of stability with the addition of solute atoms is higher in the case of the addition of Cu atoms into a Ni cluster than it is for adding Ni atoms into a Cu cluster. The present investigation thus provides a deeper electronic level understanding of solute-point defect interaction and cluster formation probability for Cu and Ni atoms in the ferritic steel.

Tensile properties of powder-metallurgical-processed tungsten alloys after neutron irradiation near recrystallization temperatures

The tensile properties of powder-metallurgical-processed Pure W, K-doped W, W-3%Re, and K-doped W-3%Re were examined after neutron irradiation up to 0.7 dpa at 910–1020 °C with a thermal neutron shield in the High Flux Isotope Reactor (HFIR). After irradiation, recrystallized Pure W (R) exhibited a brittle fracture mode, while recrystallized K-doped W-3%Re (R) exhibited a ductile fracture mode at 500 °C. K-doped W-3%Re (R) has fine grains, and hence, contains a considerable number of grain boundaries that act as sinks for irradiation defects. Solid solute Re in the W matrix could improve not only the mechanical properties of W, but also its resistance to neutron irradiation. At 500 °C, the ductility of K-doped W-3%Re after irradiation was significantly higher than that of Pure W. The irradiation at ~1000 °C did not induce hardening of stress-relieved (SR) W materials, but SR W materials tended to exhibit a decrease in the ultimate tensile strength (UTS) and an increase in total elongation (TE). The softening due to the recovery and recrystallization of SR W materials and the hardening due to the formation of irradiation defect clusters were balanced during irradiation at ~1000 °C, and ductility was exhibited without an increase in strength.

Synergistic effects of applied strain and cascade overlap on irradiation damage in BCC iron

Irradiation-induced damage accumulation in BCC iron under an applied mechanical strain was investigated using molecular dynamics (MD) simulations. We found that the application of tensile and compressive strains affects defect accumulation and cluster formation, albeit according to different mechanisms. At low doses, radiation damage in the form of point defect formation was dominant, while at high doses, large cluster formation dominated, in which the diffusivity D and binding energy Eb of the defects became important. The applied strain was found to result in monotonic decrements of the formation energy Ef of the Frenkel pair, leading to an increment in defect production with an increase in the applied strain at low doses. However, at high doses, both the compressive and tensile strains enhanced damage accumulation and promote the clustering process through competition between defect formation, diffusion, and binding. We believe our results will aid in providing a fundamental mechanistic understanding of the effects of strain on damage accumulation under high-dose irradiation.

Photoluminescence properties of implanted Praseodymium into Gallium Nitride at elevated temperatures

Since Rare-Earth (RE) doped Gallium Nitride (GaN) is expected to be used as electrically driven single photon source at room temperature, low dose RE-ion implantation and their activation as luminescent centers are of interest. This paper reports photoluminescence (PL) properties of Praseodymium (Pr) implanted GaN at different temperatures ranging from room temperature to 1200 °C. All the Pr-implanted GaN samples are thermally annealed after implantation and show strong PL emissions at 650.2 nm and 652.0 nm, originated from 3P0→3F2 transition in 4f-shell of Pr3+ ions. It is shown that the PL intensity originating from Pr3+ ions is reduced as the implantation temperature increases for the Pr-implanted samples annealed at 1200 °C. This result suggests that Pr3+ ions quench due to the formation of complex defects and defect clusters. The effect of high temperature implantation on the recovery of GaN crystallinity is discussed in terms of critical dose and displacement damage.

Over 10% Efficient Pure CZTSe Solar Cell Fabricated by Electrodeposition with Ge Doping

The fabrication of kesterite CZTSe solar cells via an electrodeposition method provides an attractive approach for the low‐cost and environment‐friendly energy supply, yet the highest conversion efficiency for such solar cells (around 9%) is still far away from the highest efficiency of Cu‐based kesterite solar cells. Herein, a 10.54% efficient CZTSe solar cell (0.28 cm2 active sized area, without an antireflection layer) is developed by introducing electrodeposited Cu–Ge alloy layer at the bottom of metal precursor. It is found that the presence of Ge element within the bottom of the film can promote downward diffusion of Sn element. Consequently, the distribution of Sn is relatively homogeneous during the annealing process; thus, the formation of undesirable defect clusters is inhibited and the band alignment of the CdS/CZTSe interface is optimized. As a result, the conversion efficiency of CZTSe thin‐film solar cells is increased from 6.74% to 10.54%, which is the highest efficiency reported for electrochemically fabricated CZTSe solar cells.

Atomistic simulation study of clustering and evolution of irradiation-induced defects in zirconium

Zirconium (Zr) alloys have been widely used as structural materials for in-core components in water-cooled nuclear reactors. During normal operation, these materials are exposed to a neutron flux in the core of the reactor, resulting in material degradation such as irradiation-induced anisotropic growth, thus affecting their performance in the long-term. Experimental and theoretical studies have shown that irradiation-induced defects in zirconium lead to the formation of defect clusters and loops. The anisotropy in the migration of defects has been suggested to play an important role in irradiation growth in pure Zr and its alloys. However, the mechanisms that govern the microstructural evolution that lead to the observed anisotropic growth of Zr is still unclear. In the present work, we perform a molecular dynamics simulation study of irradiation-induced lattice defects in Zr to investigate the formation of clusters and loops. Irradiation-induced damage is modeled by constrained stochastic formation of vacancies and self-interstitial atoms in bulk Zr. Using this approach, the formation and evolution of defect clusters and loops were determined. The dynamic properties of lattice defect structures were investigated through the evaluation of their migration and diffusivity. We found that the diffusivity of vacancy and interstitial clusters is anisotropic and slow, while the diffusivity of large loops is relatively high and confined to the ⟨a⟩ plane.

Sensing Behaviour Investigation of Amperometric Electrochemical NH3 Sensor: Optimization of Lanthanum Gallate Based Electrolyte Composition

To delineate the ammonia sensing mechanism by amperometric sensors a series of lanthanum gallate based electrolytes having nominal compositions of La0.8Sr0.2Ga0.8Mg0.2−xNixO3 (0 ≤ x ≤ 0.2) and La0.8Sr0.2Ga1−yFeyO3 (0.01 ≤ y ≤ 0.1) were prepared by solution combustion technique. Phase purity of electrolyte pellets was achieved by high temperature sintering process. Aforementioned electrolyte compositions were investigated for NH3 sensing performance in the temperature range of 300 °C–550 °C. Exposing the sensors alternately to the base gas (5% O2 + N2) and 3–40 ppm NH3 in base gas. Effect of oxygen pumping current on sensing mechanism was studied. The electrolytes with 10 mol% Ni and 5 mol% Fe doping showed the highest sensitivity of 35 and 43 μA decade−1 at 400 °C and 450 °C respectively. Conductivity measurements showed conductivity of 526 × 10−5 at 400 °C and 1080 × 10−5 S cm−1 at 450 °C for La0.8Sr0.2Ga0.8Mg0.1Ni0.1O3 and La0.8Sr0.2Ga0.95Fe0.05O3 respectively. A correlation between conductivity and gas sensing was established. Finally, a mechanism based on the hypothesis of defect cluster formation due to aliovalent doping that enhanced conductivity coupled with oxygen pumping current was proposed.