Formation Mechanism Of Defects Research Articles

Unveiling the Effect of Cooling Rate on Grown-in Defects Concentration in Polycrystalline Perovskite Films for Solar Cells with Improved Stability.

Numerous efforts are devoted to reducing the defects at perovskite surface and/or grain boundary; however, the grown-in defects inside grain is rarely studied. Here, the influence of cooling rate on the point defects concentration in polycrystalline perovskite film during heat treatment processing is investigated. With the combination of theoretical and experimental studies, this work reveals that the supersaturated point defects in perovskite films generate during the cooling process and its concentration improves as the cooling rate increases. The supersaturated point defects can be minimized through slowing the cooling rate. As a result, the optimized FAPbI3 polycrystalline films achieve a superior carrier lifetime of up to 12.6 µs and improved stability. The champion device delivers a 25.47% PCE (certified 24.7%) and retain 90% of their initial value after >1100 h of operation at the maximum power point. These results provide a fundamental understanding of the mechanisms of grown-in defects formation in polycrystalline perovskite film.

BRIDGING DATA GAPS: A FEDERATED LEARNING APPROACH TO HEAT EMISSION PREDICTION IN LASER POWDER BED FUSION

Abstract Deep learning has impacted defect prediction in Additive Manufacturing (AM), which is important to ensure process stability and part quality. However, its success depends on extensive training, requiring large, homogenous datasets–remaining a challenge for the AM industry, particularly for small- and medium-sized enterprises (SMEs). The unique and varied characteristics of AM parts, along with the limited resources of SMEs, hampers data collection, posing difficulties in the independent training of deep learning models. Addressing these concerns requires enabling knowledge sharing from the similarities in the physics of the AM process and defect formation mechanisms while carefully handling privacy concerns. Federated learning (FL) offers a solution to allow collaborative model training across multiple entities without sharing local data. This paper introduces an FL framework to predict section-wise heat emission during Laser Powder Bed Fusion (LPBF), a vital process signature. It incorporates a customized Long Short-Term Memory (LSTM) model for each client, capturing the dynamic AM process's time series properties without sharing sensitive information. Three advanced FL algorithms are integrated–FedAvg, FedProx, and FedAvgM–to aggregate model weights rather than raw datasets. Experiments demonstrate that the FL framework ensures convergence and maintains prediction performance comparable to individually trained models. This work demonstrates the potential of FL-enabled AM modeling and prediction where SMEs can improve their product quality without compromising data privacy.

A neural-network potential for aluminum

Aluminum and its alloys are most often used as structural materials due to their specific properties, such as low weight, low energy consumption for remelting and the possibility of almost complete processing. This paper utilizes machine learning, specifically the Behler-Parrinello neural network scheme, to develop a powerful potential for studying the underlying mechanisms of deformation, fracture, and defect formation. By surpassing the limitations of first principles calculations, the application of machine-learned potentials (MLP) becomes highly advantageous for describing pure aluminum (Al) in its solid and liquid phases. Specifically, from the generated potential equilibrium, thermodynamic, elastic, and vibrational properties of face-centered cubic (fcc) Al are obtained in very good agreement especially with density functional theory (DFT) results as well as with previous calculations using existing semi-empirical potentials, such as EAM and MEAM, recent machine-learned potentials, and experimental data. Furthermore, our potential proves to accurately reproduce defect formation energies such as previously computed and measured stacking-fault energy curves. Finally, stacking fault profiles as well as key quantities of the liquid phase such as the melting point at ambient pressure, temperature-dependent densities, and radial distribution functions are also calculated in very good agreement with the results from previous theoretical and experimental investigations. Nevertheless, our investigation goes beyond previous studies in proving excellent agreement with experimental data especially of the specific heat and the melting point at very high pressures. The competitive analysis performed in this work thus clearly demonstrates the validity and accuracy of the generated machine-learned potential to describe a wide range of properties of Al at various temperatures and pressures and thereby lays ground for future applications.

Processing parameter effect, formation mechanism and inhibition method of micro-hole exit defects in rotary ultrasonic pecking drilling of SiCf/SiC composites

High brittleness and anisotropy of ceramic matrix composites lead to great challenges for micro-hole machining. Micro-holes with large depth-to-diameter ratio (10:1) of SiCf/SiC composites were machined by rotary ultrasonic pecking drilling process. Based on the mechanical theory and structural characteristics of SiCf/SiC, a critical thrust force model was established. The effects of processing parameters on machining accuracy and machining quality of micro-hole exits were analyzed and parameters were optimized. The formation mechanism of exit defects includes interfacial debonding, fiber bending and matrix fracture. A method based on the surface morphology to inhibit exit defects by increasing the support stiffness of the material was proposed, under which the exit roundness all reached IT8 level and chipping factors decreased by 53 %–67 %, and only a small amount of chipping and micro-cracks was at the exit. The proactive control of micro-hole exit defects was realized, and a better machining quality was obtained.

Formation mechanism and elimination method of joint defects in lattice structures printed by metal droplet deposition manufacturing

Benefiting from the dense inner microstructure and smooth bead-like surface morphology, metal droplet deposition manufacturing (MDDM) is advantageous in manufacturing small metallic lattice structures, which have great potential in advanced aerospace, energy, and robotics applications. However, the improper top-layer droplet distance leads to joint defects in the printed lattices and deteriorates the forming accuracy. Moreover, if the substrate temperature and inclined angle increase, joint defects may occur even if the top-layer droplets are at an ideal distance due to droplet oscillation. This work proposes a novel method to eliminate joint defects by re-placing the joint region droplets to an ideal center distance. To this end, the dynamics of top-layer droplets (impact, spread, recoil, and oscillation) were investigated by combining morphology analysis and high-speed photography (Ts = 673 K, θ = 45°). A theoretical model was established to obtain the parameter range of defect-free joints through a series of bifurcate lattice printing experiments. Then, the morphology and generation mechanism of cumulative joint defects in the lattices at higher temperatures and angles (Ts = 773 K, θ = 60°) were analyzed. A parameter window of joint morphology with different values of inclined angle, substrate temperature, and top-layer droplet distance was established for predicting potential defects in the printing trace. Next, a novel method was proposed to remove redundant droplets and re-place top-layer droplet coordinates into the ideal center distance. Furthermore, the printing strategy was expanded to multi-truss lattices using three-dimensional transformation matrixes. Finally, multiple cells bifurcate, triangular, and pyramid lattices were successfully printed with perfect joints. The height deviation of joint regions is <5 % of the droplet diameter, verifying the feasibility of the proposed strategy. This work represents a further step towards MDDM printing of high-quality truss lattice structures and other complex 3D architectures.

A reactive neural network framework for water-loaded acidic zeolites

Under operating conditions, the dynamics of water and ions confined within protonic aluminosilicate zeolite micropores are responsible for many of their properties, including hydrothermal stability, acidity and catalytic activity. However, due to high computational cost, operando studies of acidic zeolites are currently rare and limited to specific cases and simplified models. In this work, we have developed a reactive neural network potential (NNP) attempting to cover the entire class of acidic zeolites, including the full range of experimentally relevant water concentrations and Si/Al ratios. This NNP has the potential to dramatically improve sampling, retaining the (meta)GGA DFT level accuracy, with the capacity for discovery of new chemistry, such as collective defect formation mechanisms at the zeolite surface. Furthermore, we exemplify how the NNP can be used as a basis for further extensions/improvements which include data-efficient adoption of higher-level (hybrid) references via Δ-learning and the acceleration of rare event sampling via automatic construction of collective variables. These developments represent a significant step towards accurate simulations of realistic catalysts under operando conditions.

Numerical and experimental analysis of edge wave defect control during TA2 circular tube cold roll forming

AbstractThe increasing application of TA2 titanium profiles in marine and petrochemical industries has spurred in a growing demand for diverse forms, including U‐shaped, rectangular, and tubular thin‐walled profiles. Traditional methods like mechanical subtractive processing and extrusion, despite their prevalence, suffer from high production costs and low efficiency. As a metal sheet forming technology, roll forming stands out for its efficiency, accuracy, and capability of producing complex shapes continuously. Nevertheless, the application of cold rolling to TA2 profiles is challenging primarily due to its low elastic modulus and high yield strength. In view of this, this study employed finite element simulation to analyze the stress and strain distribution during the TA2 roll forming process, aiming to have a better understanding of edge wave defect formation mechanism. Orthogonal experiments were performed to assess the influence of frame spacing, forming speed, roll gap, and downhill amount, on edge wave defects. The findings revealed a predominant influence of the downhill amount. Maintaining the downhill volume at 0.6 times the tube diameter kept the longitudinal strain below 0.9%, effectively mitigating edge wave defects. Implementation of these optimized parameters in an actual TA2 roll forming process confirmed the reliability of the simulations. This study establishes a solid foundation for advancing the TA2 tube cold roll forming process, enhancing the production efficiency of titanium profiles, and shedding some light on current energy conservation and emission reduction.

Microstructure, solidification defects and mechanical properties of high-modulus and high-strength SiC/AlSi10Mg composites fabricated by selective laser melting

Since the aluminum matrix composites prepared by additive manufacturing are prone to defects such as pores and cracks, the properties of the formed composites cannot be further improved. Therefore, the preparation of SiC/AlSi10Mg composites with high modulus and high strength by selective laser melting technology is of great significance for the wide application of aluminum matrix composites. This work will further investigate the defect formation, microstructure evolution, and solidification mechanism of the composites. The matrix microstructure of the composites can be efficiently refined by adding SiC. In addition, the 5 wt.% SiC/AlSi10Mg composites showed improvements in the tensile yield strength, microhardness, bending strength, elastic modulus, and ultimate tensile strength as compared to the AlSi10Mg alloy. These increases were 21.1 %, 18 %, 8.7 %, 23.2 % and 9.2 % respectively. The strengthening mechanisms at room temperature include fine grain strengthening, thermal mismatch strengthening, solid solution strengthening and precipitation strengthening. It is also mentioned that the primary causes of the decrease in toughness are stress concentration and the existence of the brittle precipitation phase (Al4C3). The increase in high-temperature strength is attributed to the pinning role of SiC and precipitate phases relative to grain boundary and dislocation climbing.

Interface investigations of 2A14Al/Nb explosively welded composites with 1060Al and Ag interlayers: Morphologies, grains, and mechanical properties

Explosive welding (EW) with an interlayer is considered a suitable technology for producing Al/Nb layered composite, but the fundamental process behaviors of this method remain insufficient discussion. Here, contrast EW experiments respectively using 1060Al and Ag sheets as the interlayers were carried out to prepare 2A14Al/Nb composites, and the complex interfacial phenomena and the underlying mechanisms were thoroughly examined by advanced characterizations and theoretical analyses. The microstructure results indicated that the interlayer materials had significant influence on the bonding morphologies, but the defects of the two composites shared the same geometry and distribution characteristics. Combining the microstructures and thermodynamic history, the formation mechanisms of the different defects were revealed. The 2A14Al/Ag/Nb and 2A14Al/1060Al/Nb composites had the advantages of no intermetallic compounds and few defects, respectively, but the latter showed a higher bond strength, suggesting that the geometric defects had more negative effects on mechanical properties than intermetallic compounds. The multi-view EBSD data provided insights into the intricate grain evolutions induced by the EW process, including equiaxed 2A14Al grains and columnar 1060Al grains at the 2A14Al/1060Al interface, and nanoscale Al3Nb grains at the 1060Al/Nb interface. Finally, the correlations between the grain structures and mechanical properties were established by nanoindentation tests.

Reliability evaluation of spacecraft power generation performance with competitive failure processes under irradiation

AbstractThe performance of space power systems is crucial for space products as it determines the operational capabilities, endurance, and efficiency of satellites, spacecraft, and other extraterrestrial devices. Unlike reliability analysis in aerospace systems, studying spacecraft power generation performance requires consideration of both hardware and software aspects. Existing failure models do not fully capture the self‐recovery process of control programs. Therefore, this study presents an impact degradation model for space power systems that incorporates competitive failures under irradiation conditions. The model analyzes solar arrays and power controllers to derive a performance degradation model by considering the defect formation mechanism of amorphous semiconductor materials. Additionally, two shock types are defined based on redundancy backup in power controllers and scrubbing frequency in field‐programmable gate array (FPGA) units. Within the case analysis section, the research meticulously investigates and elucidates the correlation probabilities among varying proton irradiation doses, scrubbing frequencies, and the aforementioned shock types.

Laser additive manufacturing of TiB2-modified Cu15Ni8Sn/GH3230 heterogeneous materials: Processability, interfacial microstructure and mechanical performance

Cu/Ni heterogeneous materials integrate excellent thermal conductivity and high-temperature mechanical properties, enabling them to be widely used in the aerospace domain. Differences in the thermal and physical properties of the Cu and Ni materials, however, make them difficult to be processed using the laser powder bed fusion (LPBF) additive manufacturing process. This study systematically examines the effects of various LPBF process parameters on microstructure, element diffusion, bonding strength and microhardness at the Cu/Ni interface, as well as investigating the mechanisms of defect formation within Cu/Ni heterogeneous materials. The results indicate that a reasonable control of laser energy input (<100 J/mm3) facilitates the Cu/Ni components through strong interfacial metallurgical bonding without pore defect formation. Compared to single-material Cu alloy, the ultimate tensile strength (UTS) of the horizontally bonded Cu/Ni specimen increased by 55.25 %, without significant reductions in elongation. The vertically bonded Cu/Ni tensile specimen fractured in the middle of the Cu region rather than the interfacial region, indicating superb interfacial bonding strength. Another advantage lies in the enhancement of thermophysical properties, with a 109.5 % increase in thermal conductivity achieved in the LPBF-fabricated Cu/Ni heterogeneous materials compared to the single-material Ni alloy. Quasi-static compression experiments indicated that the Cu/Ni lattice structure could absorb more energy when compressed parallel to the build direction (BD), compared to being perpendicular to the BD. This study provides guidance for the design and manufacture of high-performance Cu/Ni heterogeneous components via LPBF.

Degradation Mechanisms of Electrodes Promotes Direct Regeneration of Spent Li-Ion Batteries: A Review.

The rapid growth of electric vehicle use is expected to cause a significant environmental problem in the next few years due to the large number of spent lithium-ion batteries (LIBs). Recycling spent LIBs will not only alleviate the environmental problems but also address the challenge of limited natural resources shortages. While several hydro- and pyrometallurgical processes are developed for recycling different components of spent batteries, direct regeneration presents clear environmental, and economic advantages. The principle of the direct regeneration approach is restoring the electrochemical performance by healing the defective structure of the spent materials. Thus, the development of direct regeneration technology largely depends on the formation mechanism of defects in spent LIBs. This review systematically details the degradation mechanisms and types of defects found in diverse cathode materials, graphite anodes, and current collectors during the battery's lifecycle. Building on this understanding, principles and methodologies for directly rejuvenating materials within spent LIBs are outlined. Also the main challenges and solutions for the large-scale direct regeneration of spent LIBs are proposed. Furthermore, this review aims to pave the way for the direct regeneration of materials in discarded lithium-ion batteries by offering a theoretical foundation and practical guidance.

Investigation on the formation mechanism of collapse defects, microstructural evolution and mechanical properties in full penetration laser welding of thick-section steel

In this paper, we described full penetration laser welding (FPLW), a process capable of achieving single-pass welding of thick-section steel. This work summarized a welding process window encompassing various typical formations, including those well-formed and collapse defects. Furthermore, collapse defects were categorized as “collapse formation with backside humping” and “collapse formation with underfill”. High-speed camera (HSC) images reveal that the formation mechanisms of these two defects are attributed to the accumulation of liquid metal at the rear edge of the bottom molten pool and the ejection of a large amount of liquid metal from the weld in a spatter form, respectively. A computational fluid dynamics (CFD) numerical simulation model well-aligned with the experimental results has been established. The analysis of the molten pool flow behavior and keyhole evolution revealed that during well-formed welding, the keyhole status in the welding process exhibits a periodic closure-collapse mode. This periodic mode can instantaneously release the pressure at the bottom of the keyhole, inhibiting the excessive accumulation of liquid metal at the bottom of the weld and thus inhibiting the formation of collapse defects. The cooling rate in the upper part of the weld exceeds that in the middle and lower parts, primarily resulting in the formation of fine acicular ferrite and martensite. The increased tensile strength exhibited by the welded joint relative to the base material predominantly emanates from the mechanisms of grain boundary reinforcement and dislocation strengthening, precisely caused by the refined microstructure.

Characterization and formation mechanism of short step-bunching defects on 4H-SiC thick homoepitaxial films

Short Step-Bunching (SSB) are a kind of linear defects consisting of convex and concave undulations on the surface of 4H-SiC homoepitaxial layer. In this work, we report the characterizations of SSBs on 12 μm, 50 μm and 85 μm thick SiC epilayers by using optical microscopy (OM), micro-photoluminescence spectra (PL), atomic force microscopy (AFM), chemical mechanical polishing (CMP), molten KOH etching (10 min, 500 ℃) and high resolution transmission electron microscope (HRTEM). It is found that these SSBs are 200 ∼ 250 μm long, perpendicular to the step-flow [11–20] direction, leading to local rugged surfaces of epilayer. No other poly-types are identified on and around these SSBs. H2 etching the substrate reveals the existence of short-line defects on the substrate with one or more dislocations (TSD or TED) or an amorphous bump locating in center of most of them. Presumably, the amorphous bump was a kind of carbon inclusion. Based on the above results, we proposed that the SSBs develop from the short-line defects generated when etching the substrates prior to epilayer growth. Elimination of short-line defects by proper etching process is an effective route to reducing the SSBs in 4H-SiC homoepitaxial layers.

Room Temperature Crystallized Phase-Pure α-FAPbI3 Perovskite with In-Situ Grain-Boundary Passivation.

Energy loss in perovskite grain boundaries (GBs) is a primary limitation toward high-efficiency perovskite solar cells (PSCs). Two critical strategies to address this issue are high-quality crystallization and passivation of GBs. However, the established methods are generally carried out discretely due to the complicated mechanisms of grain growth and defect formation. In this study, a combined method is proposed by introducing 3,4,5-Trifluoroaniline iodide (TFAI) into the perovskite precursor. The TFAI triggers the union of nano-sized colloids into microclusters and facilitates the complete phase transition of α-FAPbI3 at room temperature. The controlled chemical reactivity and strong steric hindrance effect enable the fixed location of TFAI and suppress defects at GBs. This combination of well-crystallized perovskite grains and effectively passivated GBs leads to an improvement in the open circuit voltage (Voc) of PSCs from 1.08V to 1.17V, which is one of the highest recorded Voc without interface modification. The TFAI-incorporated device achieved a champion PCE of 24.81%. The device maintained a steady power output near its maximum power output point, showing almost no decay over 280h testing without pre-processing.

Surface defects formation mechanism of Mg2Si/Al composites by micro cutting

Mg2Si/Al composites have wide application prospects in aerospace and automotive industries. However, many defects often remain on the machined surface after micro cutting. In this paper, the formation mechanism of surface defects of Mg2Si/Al composites by micro cutting was studied by finite element (FE) simulations and machining experiments. Considering the real microstructure characteristics of Mg2Si particles, 2D FE simulations of Mg2Si/Al composites in micro cutting were carried out and then verified by corresponding cutting experiments. The simulation results and experimental data reveal the relationship between damage and deformation behavior of Mg2Si particles and formation mechanism of surface defects. In addition, the effects of cutting parameters such as depth of cutting (DoC), cutting speed and volume fraction of particles on residual defects of machined surfaces were studied. The results show that with lower DoC and higher cutting speed, the residual defects and surface roughness are significantly reduced, especially machining the composites with smaller particle size and lower volume fraction. The research findings provide a theoretical basis for reasonable selection of machining parameters and improving the machinability of Mg2Si/Al.

Insights into structural defect formation in individual InP/ZnSe/ZnS quantum dots under UV oxidation

InP/ZnSe/ZnS quantum dots (QDs) stand as promising candidates for advancing QD-organic light-emitting diodes (QLED), but low emission efficiency due to their susceptibility to oxidation impedes applications. Structural defects play important roles in the emission efficiency degradation of QDs, but the formation mechanism of defects in oxidized QDs has been less investigated. Here, we investigated the impact of diverse structural defects formation on individual QDs and propagation during UV-facilitated oxidation using high-resolution (scanning) transmission electron microscopy. UV-facilitated oxidation of the QDs alters shell morphology by the formation of surface oxides, leaving ZnSe surfaces poorly passivated. Further oxidation leads to the formation of structural defects, such as dislocations, and induces strain at the oxide-QD interfaces, facilitating In diffusion from the QD core. These changes in the QD structures result in emission quenching. This study provides insight into the formation of structural defects through photo-oxidation, and their effects on emission properties of QDs.

Drop and hump behaviors in robotic arc-directed energy deposition with vertical position

Arc-directed energy deposition (Arc-DED) with the vertical position offers significant potential for in-situ fabrication of overhang structures. However, the vertical position application is seriously hindered by drop and hump defects since Arc-DED possesses serious heat accumulation resulting from multi-layer depositions. The novelty of this study is to reveal the formation mechanisms of drop and hump defects in vertical Arc-DED of low-carbon steel walls. Force models of the molten pool in vertical-down and vertical-up Arc-DED are established. Two indicators, i.e., the molten pool area and the molten pool dimensional ratio, are proposed to evaluate the probability of drop and hump occurrence, respectively. Furthermore, how the wire feed speed (WFS), travel speed (TS), and the constant ratio of WFS to TS affect the molten pool behavior is explored and discussed. In the vertical-down deposition, decreasing the TS or increasing the WFS will increase the possibility of drop defects occurring when the molten pool area exceeds 45 mm². In the vertical-up deposition, increasing the TS or the WFS will increase the possibility of hump defects occurring when the molten pool dimensional ratio exceeds 4.1. Maintaining the WFS/TS constant, the probabilities of hump and drop defects increase as the WFS and the corresponding TS increase. This study offers a better understanding of the formation mechanisms underlying the drop and hump phenomena and lays a solid foundation for the practical application of vertical Arc-DED to fabricate overhang features.

Study on the Characteristics of Circumferential and Longitudinal Flow of Vault Concrete during Tunnel Lining Pouring Processes.

With a large number of railroad and highway tunnels opening for operation, the diseases caused by hidden lining defects are increasing. The study of flow characteristics of freshly mixed concrete during tunnel lining casting is the key to revealing the formation mechanism of hidden defects. This paper revealed the location of blank lining formation by investigating the circumferential and longitudinal flow characteristics of concrete in the vault during tunnel pouring to provide suggestions for improving the quality of tunnel lining pouring for the various projects. This paper adopted the method of indoor testing, selected the suitable working conditions and flow parameters, validated the accuracy of the test with a numerical simulation, and simulated the secondary lining pouring process of the tunnel arch from the circumferential direction and longitudinal direction. This revealed the flow characteristics of the freshly mixed concrete in the process of pouring the arch lining. The flow of concrete in the arch lining was basically characterized by two major features which were similar to the flow in the pumping pipe and the layered flow. It also revealed the relationship between the concrete flow rate, flow distance, and the location of the formation of the blank lining risk zone with the slump of the concrete, the pumping pressure, and the radius of the tunnel.

Effect of process parameters on the densification of SLM-produced Mg–Y–Sm–Zn–Zr alloy

In this study, Mg–Y–Sm–Zn–Zr samples were prepared using the selective laser melting technique. The influence of process parameters on the densification and mechanical properties of samples was systematically studied from a single-melt track to double melt tracks to block, and the formation mechanism of defects was analysed. Research results show that the appropriate laser power and scanning speed resulted in excellent single-melt path formability. When the scanning spacing was 80 μm, adjacent trajectories were closely connected without spheroidised particles and porosity defects. The densest samples were obtained at an energy density of 125 J/mm3 with maximum microhardness (86.8 ± 1.5 HV), and the compressive yield strength and ultimate strength reached 301.8 ± 5 MPa and 467.9 ± 5 MPa.