Formation Of Defect Clusters Research Articles

Molecular dynamics simulations of high-energy radiation damage in hcp-titanium considering electronic effects

Abstract Titanium and its alloys are widely used as structural materials under extreme conditions due to their exceptional specific strength. However, comprehensive studies on their high-energy radiation damage remain limited. Considering electronic effects, molecular dynamics (MD) simulations were performed to explore high-energy radiation damage in hcp-titanium (hcp-Ti), focusing on displacement cascades induced by primary knock-on atoms (PKAs) with energies ranging from 1 to 40 keV. This study investigates the generation and evolution of point defects resulting from collisional cascades, particularly examining the influence of PKA energy. Additionally, the distribution and morphology of clustering defects from these events were quantitatively investigated and qualitatively visualized. The results show a significant dependence of surviving defects on PKA energies, highlighting a critical range that exhibits a shift in cascade morphology. Furthermore, it is demonstrated that PKA energy significantly influences the formation and growth of defect clusters, with both interstitials and vacancies showing increased cluster fraction and sizes at higher PKA energies, albeit with different tendencies in their formation and aggregation behaviors. Morphological analysis emphasizes the role of subcascades and provides further insights into the mechanisms of defect evolution behind high-energy radiation damage. Our extensive study across a broad range of PKA energies provides essential insights into the understanding of high-energy radiation damage in hcp-Ti.

Achieving 11.23 % efficiency in CZTSSe solar cells via defect control and interface contact optimization

The high open-circuit voltage deficit (VOC, def) caused by structural imperfections in the absorber layer and charge loss during carrier transport is a critical barrier affecting the performance of Cu2ZnSn(S,Se)4 (CZTSSe) devices. In this work, Ag was added to the DMF-based Cu+-Sn4+ system, which significantly improved the crystal morphology and electrical properties of the absorber layer. Additionally, optimizing the selenization process not only reduced surface roughness and eliminated voids at the bottom of the absorber layer but also resulted in the formation of a MoSe2 back interface layer with a more suitable thickness. These measures collectively enhanced the overall quality of the absorber layer, reducing the formation of deep-level defect clusters and effectively boosting carrier transport efficiency. Consequently, the concentration of bulk and interfacial defects decreased, and the impact of potential barriers on carrier movement was minimized. With these comprehensive improvements, the power conversion efficiency of CZTSSe solar cells increased from 8.48 % to 11.23 %. Our research demonstrates that optimizing the structure of the absorber layer can effectively enhance the performance of CZTSSe solar cells, providing valuable insights for the fabrication of high-efficiency devices in the future.

Insight into the Role of Rb Doping for Highly Efficient Kesterite Cu2ZnSn(S,Se)4 Solar Cells.

Various copper-related defects in the absorption layer have been a key factor impeding the enhancement of the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Alkali metal doping is considered to be a good strategy to ameliorate this problem. In this article, Rb-doped CZTSSe (RCZTSSe) thin films were synthesized using the sol-gel technique. The results show that the Rb atom could successfully enter into the CZTSSe lattice and replace the Cu atom. According to SEM results, a moderate amount of Rb doping aided in enhancing the growth of grains in CZTSSe thin films. It was proven that the RCZTSSe thin film had the densest surface morphology and the fewest holes when the doping content of Rb was 2%. In addition, Rb doping successfully inhibited the formation of CuZn defects and correlative defect clusters and promoted the electrical properties of RCZTSSe thin films. Finally, a remarkable power conversion efficiency of 7.32% was attained by the champion RCZTSSe device with a Rb content of 2%. Compared with that of un-doped CZTSSe, the efficiency improved by over 30%. This study offers new insights into the influence of alkali metal doping on suppressing copper-related defects and also presents a viable approach for improving the efficiency of CZTSSe devices.

Formation of an extended defect cluster in cuprous oxide

Intrinsic defects and defect clusters play an important role in the room-temperature transport of cuprous oxide. Neutralization of these defects by doping and/or modifying the synthesis process is essential to improve the room-temperature hole mobility in cuprous oxide. Toward this end, we annealed polycrystalline cuprous oxide under Cu-rich conditions, which led to the neutralization of the intrinsic acceptor defect. The concentration of both the acceptor defects ( VCu and VCusplit ) that are already present, reduces by four to five orders of magnitude. This is in accordance with the amount of possible Cu incorporation under different annealing conditions, indicating the backfilling of a large fraction of the Cu vacancies. Unforeseeably, the experimental conditions lead to the creation of yet another higher-order extended defect (3 VCu + 2Cu i ) with a defect level at ≈0.5 eV above the valence band. The formation of such a defect is also indirectly suggested by the analysis of carrier concentration vs. temperature data and first-principles calculations. Such singly ionized higher-order defects with a possibly higher capture cross-section act as more effective traps resulting in reduced hole mobility.

Grain boundary effects on chemical disorders and amorphization-induced swelling in 3C-SiC under high-temperature irradiation: From atomic simulation insight

High-temperature irradiation-induced amorphization in SiC is a crucial factor leading to swelling. This study employs atomistic simulations to clarify the effects of experimental irradiation damage (1.0 dpa) in polycrystalline 3C-SiC at 1000 K. The findings reveal amorphization and swelling thresholds at 0.45 dpa and 7.45 %, respectively. Interestingly, dose threshold for amorphization does not align with chemical bonding, volume swelling, or enthalpy thresholds. Instead, a competitive mechanism emerges between grain boundaries and the grain core in the evolution of chemical disorder and clusters. Moreover, as amorphization saturates and grain boundaries annihilate, the degradation of tensile strength and Young's modulus stabilizes at 21.1 GPa and 124.2 GPa, respectively. Notably, grain boundaries exert a significant influence on defect cluster formation during amorphous evolution and dominate the initial deterioration of mechanical properties at low damage doses. This study enhances our understanding of how grain boundaries and chemical disorder mechanisms impact amorphization SiC.

Facile Approach for Metallic Precursor Engineering for Efficient Kesterite Thin-Film Solar Cells.

Kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs) are a promising candidate for low-cost, clean energy production owing to their environmental friendliness and the earth-abundant nature of their constituents. However, the advancement of kesterite TFSCs has been impeded by abundant defects and poor microstructure, limiting their performance potential. In this study, we present efficient Ag-alloyed CZTSSe TFSCs enabled by a facile metallic precursor engineering approach. The positioning of the Ag nanolayer in the metallic stacked precursor proves crucial in expediting the formation of Cu-Sn metal alloys during the alloying process. Specifically, Ag-included metallic precursors promote the growth of larger grains and a denser microstructure in CZTSSe thin films compared to those without Ag. Moreover, the improved uniformity of Ag, facilitated by the evaporation deposition technique, significantly suppresses the formation of detrimental defects and related defect clusters. This suppression effectively reduces nonradiative recombination, resulting in enhanced performance in kesterite TFSCs. This study not only introduces a metallic precursor engineering strategy for efficient kesterite-based TFSCs but also accelerates the development of microstructure evolution from metallic stacked precursors to metal chalcogenide compounds.

A study of the effect of γ-radiation on the current-carrying mechanism in the P-CuTlS2 single crystal

The electrical conductivity and volt–ampere characteristics (VAC) of the p-CuTlS2 single crystal with specific resistance [Formula: see text] and irradiated by [Formula: see text]-quantum were studied in the range of 100–300[Formula: see text]K temperature and 10–104[Formula: see text]V/cm. It was determined that the cause of the conduction disorder observed in the CuTlS2 single crystal at low electric fields and high radiation doses is the formation of defect clusters dominated by cation vacancies. A sharp increase in current at high electric fields and temperatures occurs as a result of thermo-field ionization of the acceptor level with activation energy [Formula: see text][Formula: see text]eV and the ionization voltage decreases with increasing radiation dose. Based on the determination of the parameters ([Formula: see text]) that determine the mechanism of current flow, the dependence of the shape of the potential hole on the radiation dose was determined.

(Invited) Controlling Metal Exsolution on Oxides By External Drivers – Role of Elastic Strain and Ion Irradiation

Exsolution is an effective approach to fabricating oxide-supported metal nanoparticle catalysts via phase precipitation out of a host oxide. A fundamental understanding and control of the exsolution kinetics are needed to engineer exsolved nanoparticles to obtain higher catalytic activity toward clean energy and fuel conversion. Since oxygen release via oxygen vacancy formation in the host oxide is behind oxide reduction and metal exsolution, we hypothesize that the kinetics of metal exsolution should depend on the kinetics of oxygen release. In this work, we probe the surface exsolution kinetics both experimentally and theoretically using thin-film perovskite oxide model systems, show its relation to the oxygen evolution kinetics. We also tune defect formation and the resulting metal exsolution, by external drivers including elastic strain and ion irradiation. Using both of these external drivers, we couple to the formation of point defects and defect clusters, that serve as nucleation sites for nanoparticle exsolution. As a result, we can controllably tune size, density, composition and position of the exsolved metal nanoparticles.

MD Simulations of Collision Cascades in α-Ti. Cluster Statistics and Governing Mechanisms of Point Defect Cluster Formation

MD Simulations of Collision Cascades in α-Ti. Cluster Statistics and Governing Mechanisms of Point Defect Cluster Formation

MD Simulations of Collision Cascades in α-Ti. Statistics and Governing Mechanisms of Point Defect Cluster Formation

The outcomes of molecular dynamics simulations of primary damage formation in collision cascades initiated by primary knock-on atoms (PKA) with PKA energy 5 keV ≤Epka≤ 25 keV in α-titanium at 100 K ≤T≤ 900 K temperatures have been analysed. The fraction of vacancies, εv, and self-interstitial atoms (SIA), εi, in point defect clusters created in isolated collision cascades was evaluated. The corresponding averages ⟨εv⟩ and ⟨εi⟩ over cascade series with the same (Epka, T) parameters, the average size ⟨Nvac⟩ and ⟨Nsia⟩ of vacancy and SIA clusters, respectively, and the average vacancy ⟨Yvac ⟩ and SIA ⟨Ysia ⟩ cluster-per-cascade yield were found as well. Possible governing mechanisms have been suggested to explain ⟨εv⟩, ⟨εi⟩ , ⟨Nvac⟩, ⟨Nsia⟩, ⟨Yvac⟩ and ⟨Ysia⟩ dependence on (Epka,T).

Microstructure and defect evolution in oxygen ion-irradiated pure nickel – Insights from experimental probes and molecular dynamics simulations

Pure nickel samples were irradiated to various doses by 160 MeV oxygen ions and the change in microstructure (domain size, microstrain, dislocation density etc.) were investigated with the help of X-ray diffraction (XRD) data from synchrotron source and Transmission electron microscopy (TEM). The coherent domain size decreased, whereas the microstrain and dislocation density increased in all irradiated samples as compared to the unirradiated one. However, these parameters showed saturation with increasing dose. The analysis reveals that there is an increase in the stacking fault probability with irradiation. TEM study confirmed the presence of stacking fault tetrahedra and dislocation loops in the irradiated samples. Microhardness measurements show an increase in the hardness with irradiation due to formation of defect clusters and dislocations. The effect of irradiation on the microstructure of the pure Ni has been simulated by molecular dynamics using overlapping simulation cascades of PKA generated by oxygen ion.

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc2O3 and Y2O3 co-doping system on the thermal shock resistance of ZrO2 thermal barrier coatings, Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3–Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc2O3 and Y2O3 facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.

The Effect of Rhenium Content on Microstructural Changes and Irradiated Hardening in W-Re Alloy under High-Dose Ion Irradiation.

An amount of 100 dpa Si2+ irradiation was used to study the effect of transmutation rhenium content on irradiated microscopic defects and hardening in W-xRe (x = 0, 1, 3, 5 and 10 wt.%) alloys at 550 °C. The increase in Re content could significantly refine the grain in the W-xRe alloys, and no obvious surface topography change could be found after high-dose irradiation via the scanning electron microscope (SEM). The micro defects induced by high-dose irradiation in W and W-3Re alloys were observed using a transmission electron microscope (TEM). Dislocation loops with a size larger than 10 nm could be found in both W and W-3Re alloy, but the distribution of them was different. The distribution of the dislocation loops was more uniform in pure W, while they seemed to be clustered around some locations in W-3Re alloy. Voids (~2.4 nm) were observed in W-3Re alloy, while no void was investigated in W. High-dose irradiation induced obvious hardening with the hardening rate between 75% and 155% in all W-xRe alloys, but W-3Re alloy had the lowest hardening rate (75%). The main reasons might be related to the smallest grain size in W-3Re alloy, which suppressed the formation of defect clusters and induced smaller hardening than that in other samples.

Revealing the governing factors for long-term radiation damage evolution in multi-principal elemental alloys through atomistically-informed cluster dynamics

Multi-principal elemental alloys (MPEAs) exhibit unusual mechanical properties and irradiation resistance. It has been reported that the irradiation tolerance of MPEAs originates from the long-term defect evolution stage rather than the ballistic collision phase. However, understanding the long-term evolution of MPEAs is exceptionally challenging due to intrinsic chemical disorder. In this work, we apply atomistically-informed cluster dynamics to study defect evolution in MPEAs for the first time, which enables us to elucidate the critical features responsible for the irradiation performance of MPEAs. With suitable irradiation parameters, our obtained cluster size distributions are in good agreement with experiments. We further discuss the influences of defect migration energy heterogeneity, cluster geometry, and temperature on the produced defect cluster forms. Our results suggest that the small clusters during the collision phase in MPEAs suppress the formation of large-sized defect clusters in the long term. Contrary to the common belief, a broader migration energy distribution due to chemical disorder cannot reduce the maximal defect cluster size but increase the proportion of small clusters. Our results thus reveal that the major role of chemical complexity in MPEAs is to modify the growth mechanism of clusters by promoting small cluster formation and suppressing overall defect diffusivities.

Review of Novel High-Entropy Protective Materials: Wear, Irradiation, and Erosion Resistance Properties.

By their unique compositions and microstructures, recently developed high-entropy materials (HEMs) exhibit outstanding properties and performance above the threshold of traditional materials. Wear- and erosion-resistant materials are of significant interest for different applications, such as industrial devices, aerospace materials, and military equipment, related to their capability to tolerate heavy loads during sliding, rolling, or impact events. The high-entropy effect and crystal lattice distortion are attributed to higher hardness and yield stress, promoting increased wear and erosion resistance in HEMs. In addition, HEMs have higher defect formation/migration energies that inhibit the formation of defect clusters, making them resistant to structural damage after radiation. Hence, they are sought after in the nuclear and aerospace industries. The concept of high-entropy, applied to protective materials, has enhanced the properties and performance of HEMs. Therefore, they are viable candidates for today's demanding protective materials for wear, erosion, and irradiation applications.

Atomic-scale study of He ion irradiation-induced clustering in α-Zirconium

Zircaloy-4 alloy specimens consisting of α-Zr grains were irradiated on a tandem accelerator with 5 MeV He ions to a fluence of 5 × 1021 ions m − 2 at different temperatures. Defect clusters and He bubbles were observed in the TEM micrographs of the irradiated microstructure. The small defect clusters and He bubbles were found to exhibit spherical shapes whereas the large defect clusters showed plate-like shapes. Molecular dynamics simulations and crystallography analyses revealed that the defect clusters are mainly composed of Zr interstitials. During He-ion irradiation, the clustering of Zr dumbbell interstitials results in the formation of small and spherical defect clusters. With further irradiation, the Zr interstitials prefer to occupy the octahedral sites that align along the {12¯14} planes with other site interstitials, forming cluster sections. These initial planar clusters then grow by extending along the <101¯0> directions and by forming more cluster sections along the <12¯16> directions, leading to large plate-like defect clusters on the {1¯21¯1}planes. He bubbles nucleate by the clustering of He atoms and vacancies that are formed by kicking out the lattice atoms, and then grow by the migration and coalescence of helium-vacancy clusters. Small He atoms have many interstitial sites to occupy in the α-Zr HCP crystal, allowing He bubbles to grow to spherical shapes. In addition, grain boundaries induce the accumulation of primary point defects and He atoms and high irradiation temperature accelerates diffusion, and hence larger defect clusters and He bubbles form in the grain boundary regions and at higher irradiation temperature.

Molecular dynamics simulations of irradiated defect clusters evolution in different crystal structures

Irradiation damage is an important cause of material failure in in-service nuclear reactors. It is important to explore the resistance to irradiation of metals with different crystal structures. As the formation and evolution of point defects on the atomic scale caused by cascade collisions in the early stages of irradiation are currently difficult to observe experimentally, it is currently possible to simulate the dynamic process of irradiation damage on the atomic scale by means of molecular dynamics (MD) methods. In this paper, some atomic scale numerical simulations are performed to study the irradiation behaviour and displacement cascades in metals with different crystal structures of bcc-Fe, hcp-Ti, hcp-Zr and fcc-Ni by the MD methods. The effect of temperature and the magnitude of the primary knock-on atom (PKA) energy on the generation and evolution of point defects is mainly studied. Results show that an increase in cascade energies from 0.5 keV to 10 keV can significantly promote defect formation for different crystal structures, while ambient temperature (T) has a slight effect on the number of surviving defects. The simulations also illustrate that high-energy cascades can significantly promote the formation of defect clusters. Statistical results of the displacement cascades show that bcc-Fe produces a small number of stable defects, a small cluster size and number relative to fcc-Ni, hcp-Ti, and hcp-Zr structures, which indicates that the bcc-Fe structure has a good radiation resistance. These findings could provide an appropriate idea for obtaining potential radiation-resistant materials for nuclear reactors.

In situ study on heavy ion irradiation induced microstructure evolution in single crystal Cu with nanovoids at elevated temperature

Understanding the response of nanovoids to irradiation damage advances our capability to design radiation tolerant materials. Recent in situ studies reveal the shrinkage of hexagon-shaped nanovoids in (110) textured Cu. However, the evolution of voids with irregular geometries under irradiation is less well understood. Here, in situ Kr ion irradiation was performed at 100 °C on single-crystal Cu (112) that possesses nanovoids with high aspect ratio. In situ studies show these elongated voids fragment into smaller voids and shrink gradually with increasing dose. Phase field simulations confirm that fragmentation of the void is governed by the competing kinetics between atoms diffusing toward and away from the elongated nanovoid. Post irradiation analysis reveals the formation of high-density defect clusters.

Giant dielectric properties of terbium and niobium co-doped TiO2 ceramics driven by intrinsic and extrinsic effects

Giant dielectric properties (GDPs) of complex oxides have been widely investigated because of their potential applications in ceramic capacitors. This study describes the improvement in three crucial factors of the GDPs of TiO2 by co-doping it with Tb and Nb ions. (Tb0.5Nb0.5)xTi1-xO2 (TNTO) ceramics with x = 1–5% were prepared using a conventional mixed-oxide method. The ceramics exhibited a highly compact microstructure and microwave dielectric phases of Tb2Ti2O7 and TbNbTiO6. Notably, the temperature dependence of the dielectric permittivity (ε′) (∆ε′(T)/ε′30℃) was less than | ± 15%| from − 60–210 ℃. Furthermore, the TNTO ceramics exhibited extremely low tanδ values (0.006–0.011) and high ε′ values (4.7–5.3 ×104) at 1 kHz. The tanδ values at 200 °C were particularly low (0.033–0.057). Impedance spectroscopy revealed the presence of semiconducting grains, and the enormous resistivity of the grain boundaries and microwave dielectric phase particles. The GDPs are primarily caused by both intrinsic and extrinsic effects. The intrinsic effect was due to the formation of defect clusters (i.e., Nb25+Ti3+XTi(X=A3+/Ti3+/Ti4+)−Tb23+V0••Ti3+, 2TbTi′−VO••, TbTi′−NbTi•, 3TiTi′−VO••−NbTi•, and TiTi′−VO••−TiTi′), whereas the extrinsic effect resulted from the internal barrier layer capacitor microstructure. The presence of microwave dielectric phases can cause a decrease in tanδ and improve the temperature stability of ε′.

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.