Defect Formation Processes Research Articles

Bridging length and time scales in predictive simulations of thermo-mechanical processes

Abstract This work introduces a theoretical formulation and develops numerical methods for finite element implementation of the formulation so as to extend the concurrent atomistic-continuum (CAC) method for modeling and simulation of finite-temperature materials processes. With significantly reduced degrees of freedom, the CAC simulations are shown to reproduce the results of atomically resolved molecular dynamics simulations for phonon density of states, velocity distributions, equilibrium temperature field of the underlying atomistic model, and also the density, type, and structure of dislocations formed during the kinetic processes of heteroepitaxy. This work also demonstrates the need of a mesoscale tool for simulations of heteroepitaxy, as well as the unique advantage of the CAC method in simulation of the defect formation processes during heteroepitaxy.

The evolution of micro-photoluminescence spectra of PECVD diamond microcrystals along the vertical growth direction and their dependence on CH4 concentration

The changes in the shape of micro-photoluminescence spectra of PECVD diamond micro-crystal measured, depending on the position of the excitation laser spot along the crystallite height, were analyzed. It was ascertained that the processes of SiV defect formation non-monotonically depend on the distance from the Si substrate. At the distances of 2–20 μm the concentration of SiV defects increases, then at distances larger than 20 μm the number of SiV defects decreases. The concentration of NV− and NV0 defects monotonically increases with the distance from the Si substrate. The predomination of SiV defect formation at the beginning stages of the crystal growth is explained by the substantial concentration of carbon vacancies required for their formation. With the increase in the distance from the substrate, the crystalline perfection increases, the concentration of carbon vacancies decreases and the processes of NV− and NV0 defect formation dominate. The increase in CH4 fraction within 0.75–6 % leads to the increase in the volume fraction of graphite-like carbon, which is the good diffusion channel for Si atoms from the substrate into the plasma. Therefore, the concentration of SiV, NV−, and NV0 defects on the surface of the crystal depends on the volume fraction of graphite-like carbon defined by CH4 content.

Real‐Time Observation of Oxygen Diffusion in CGO Thin Films Using Spatially Resolved Isotope Exchange Raman Spectroscopy

The exploitation of advanced materials for novel energy, health, and computing applications requires deep insight and fundamental understanding of physicochemical mechanisms, such as ionic and electronic conductivity, defect formation processes, and reaction kinetics. Therefore, access to the underlying constants of the functional materials via advanced but accessible and straightforward experimental techniques is key. Herein, a novel, cheap, fast, and widely applicable approach is presented to analyze oxygen tracer diffusion in thin films with unprecedented time resolution based on the novel in situ isotope exchange Raman spectroscopy (IERS) methodology. IERS utilizes the sensitivity of micro‐Raman spectroscopy to changes in the local isotopic composition. In‐plane tracer diffusion gradients are established by partially blocking the exchange at the surface followed by an isotope exchange. The isotope exchange and diffusion processes are followed via consecutive spatial and time‐resolved in situ Raman line scans. These isotopic gradients are analyzed to obtain mass transport coefficients, with an additional time component, not accessible by conventional destructive techniques. Diffusion coefficients of gadolinium‐doped ceria (CGO) thin films are reported within the range of interest for intermediate‐temperature emerging applications and confirm the validity of the measurement procedure and extracted parameters by comparison with the finite‐element method simulations and literature results.

The effect of low-intensity radiation on the speed of CMOS microcircuits

Aim. The degradation of CMOS microcircuits exposed to ionizing radiation was analysed. Three MOS defect formation processes at the Si-SiO2 boundary were examined. Methods. Using the example of test logic elements, the dependence of the time of conditional speed failure on the gamma dose rate was analysed. Findings. A critical defect at the Si-SiO2 boundary was identified. Conclusions. The approach proposed in the paper allows identifying the causes of CMOS microcircuit failures. Calculated failure times for three dose rates are presented.

Visualizing the Structure-Property Nexus of Wide-Bandgap Perovskite Solar Cells under Thermal Stress.

Wide-bandgap perovskite solar cells (PSCs) toward tandem photovoltaic applications are confronted with the challenge of device thermal stability, which motivates to figure out a thorough cognition of wide-bandgap PSCs under thermal stress, using in situ atomic-resolved transmission electron microscopy (TEM) tools combing with photovoltaic performance characterizations of these devices. The in situ dynamic process of morphology-dependent defects formation at initial thermal stage and their proliferations in perovskites as the temperature increased are captured. Meanwhile, considerable iodine enables to diffuse into the hole-transport-layer along the damaged perovskite surface, which significantly degrade device performance and stability. With more intense thermal treatment, atomistic phase transition reveals the perovskite transform to PbI2 along the topo-coherent interface of PbI2/perovskite. In conjunction with density functional theory calculations, a mutual inducement mechanism of perovskite surface damage and iodide diffusion is proposed to account for the structure-property nexus of wide-bandgap PSCs under thermal stress. The entire interpretation also guided to develop a thermal-stable monolithic perovskite/silicon tandem solar cell.

The effect mechanism of laser beam defocusing on the surface quality of IN718 alloy prepared by laser powder bed fusion

The surface quality issue of the laser powder bed fused (LPBF-ed) parts deteriorates the mechanical properties. Laser beam defocusing (LBD) can effectively improve the surface quality. To reveal the effect mechanism of LBD on the surface quality of the LPBF-ed parts, a laser powder bed fusion (LPBF) numerical model is established. Based on the calculated results, the formation process of the surface defects such as bulges, depressions, partially melted powders and spatters and the corresponding melt pool dynamic behaviors are investigated. The effects of LBD on the melt pool dynamic behaviors during the surface defects formation and the track width are discussed. The results show that LBD can improve the surface quality by reducing Marangoni force and recoil pressure to reduce bulge height and depression depth, increasing track width to decrease partially melted powders, and increasing surface tension and reducing kinetic energy to decrease spatters.

Graphene‐Based Lateral Heterojunctions for 2D Integrated Circuits

AbstractA method for patterning single‐layer graphene (SLG) and single‐layer oxidized graphene (SOG) within a continuous atomic layer to form lateral heterojunctions is presented. Raman spectroscopy is employed to investigate the evolution of defect‐related Raman peaks during excimer‐UV irradiation, facilitating the identification of structural changes and defect formation processes. Electrical transport measurements reveal that SOG‐patterned field‐effect transistors (FETs) exhibit varying characteristics depending on the degree of oxidation, thus offering the potential to tailor the electrical properties of graphene devices for specific requirements. Scanning Kelvin probe microscopy measurements reveal the surface potential and work function of the SOG regions compared with those of SLG. The effective functionality of the SOG pattern to operate as a resistor, allowing control of the electrical conductivity in the SOG‐patterned SLG channels, is demonstrated. This capability restricts the current flow while preserving the pristine electrical properties of the graphene channel. Moreover, the SOG pattern can serve as a potential barrier to constructing SLG‐SOG‐patterned integrated circuits, providing exciting opportunities for engineering advanced electronic components. This breakthrough in graphene devices simplifies the fabrication process of graphene‐based FETs and provides the foundation for developing atomically thin integrated circuits for a wide range of applications.

Comparative evaluation of the effectiveness of restorations in the cervical region of teeth by direct and indirect method

Restoration of cervical lesions of teeth is one of the most urgent problems of modern dentistry. Defects of the cervical region have different etiology, pathophysiology of the development of the defect formation process. The method of direct restoration of defects of various etiologies in the cervical region of the tooth is the most frequent method of restoration. According to many authors, this method is not always effective. One of the initial tasks in dentistry is to replace the lost tooth structure with a material whose composition and physical properties are similar to the natural tissues of the tooth. This goal can be achieved using CAD/CAM technology. The chemical stability of hybrid ceramics provides not only excellent biocompatibility, but also good physical, mechanical and optical properties.

Thermodynamic analysis of defect equilibration in highly nonstoichiometric perovskites LnBaMn2O6–δ (Ln=Nd, Sm)

Double-ordered manganites LnBaMn2O6–δ (Ln = Nd, Sm) were shown to possess a variable oxygen content from δ = 0 to 1 while maintaining a tetragonal structure within the temperature range of 973–1223 K and under partial oxygen pressure 10−22 – 1 atm. Coulometric titration technique was successfully utilized to reveal three separated phase in these materials depending on oxygen content. Thermodynamic analysis based on experimentally obtained isothermal functions δ=f(pO2) enabled one to develop a defect formation model suggesting a coexistence three main processes of defect formation in the studied oxides. Thus oxidation and partial disproportionation of manganese supplemented by intrinsic oxygen disordering were demonstrated to govern defect equilibrium. Different influence of activity coefficients involved in the thermodynamic model was considered. Moreover, a reduction in the radius of lanthanide was found to be favorable to expand oxygen homogeneity region and to decrease in the enthalpy of oxidation reaction.

Formation and annihilation of electrically driven defects in nematic liquid crystals with negative dielectric anisotropy

Orientationally ordered liquid crystals (LCs) exhibit remarkable physical anisotropy and responsiveness to external fields, which give rise to distinguished physical effects and have led to the emergence of a new generation of electric-optical applications. The LCs are also renowned for their abundance of phases and topological defects, which are of significance in studying both fundamental science and practical technology. One simple approach to generating umbilic defects involves applying an electric field to a homeotropically aligned nematic LC with negative dielectric anisotropy <inline-formula><tex-math id="M8">\begin{document}$\Delta \varepsilon $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M8.png"/></alternatives></inline-formula>. However, the influence of material properties and external conditions on the dynamic process of nematic LC defects remains unclear. Here, we select seven kinds of nematic LCs with negative dielectrically anisotropy, ranging from –1.1 to –11.5, to explore the dynamics of electric-field-induced umbilics. By using a linearly increasing electric field parallel to the molecular orientation of LC, we systematically investigate the effects of material property (dielectric anisotropy) and external conditions (temperature and electric field parameters) on the formation and annihilation of umbilic defects. The experimental results show that the dynamic process of forming the umbilic defects in nematic LCs is independent of dielectric anisotropy, temperature, and electric field frequency, but follows the Kibble-Zurek mechanism, in which the density of generated umbilic defects exhibits a power-law scaling with the change of the electric field ramp rate, with a scaling exponent of approximately <inline-formula><tex-math id="M9">\begin{document}$1/2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M9.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="5-20231655_M9.png"/></alternatives></inline-formula>. Interestingly, a stronger dielectric anisotropy leads to a higher density of umbilic defects. Additionally, a change in temperature has a significant influence on the density of umbilic defects , in which higher temperature leads to greater defect density under the same external electric field conditions. Furthermore, the annihilation rate of umbilic defects is closely related to the material properties and the ramp of the applied electric field. Specifically, the annihilation rate of umbilic defects becomes faster when dielectric anisotropy is stronger or the electric field ramp is larger. This study provides valuable insights into the relationship between the formation and annihilation of defects, material properties, and external conditions in nematic LCs with dielectrically negative anisotropy, contributing to our comprehensive understanding of the dynamic process of topological defects in soft matter.

Intelligent Online Sensing of Defects Evolution in Metallic Materials during Plastic Deformation

Metallic plastic deformation involves complex microstructural changes and defect evolution, posing challenges in predicting and controlling the quality and performance of formed parts. Therefore, a pressing demand exists for a proficient online defect‐sensing system to monitor defects evolution continuously within components during plastic deformation in real time. This article proposes an intelligent online sensing approach for detecting defects in metallic plastic forming based on acoustic emission (AE) and machine learning. A comparative analysis is conducted on AE amplitude signals, stress–strain curves, and defect evolution during the tensile process of TA15 titanium alloy specimens under different stress states. It is found that the defect formation process can be divided into four stages based on the AE amplitude signals. A convolutional neural network model for intelligent defect sensing is established. It leverages transfer learning and is grounded in the relationship between AE signals and the evolution of internal defects. The prediction accuracy using different pretrained models is investigated and compared. It is discerned that utilizing GoogleNet as the pretrained model offers the swiftest training pace with a prediction accuracy of 97.57%. This approach enables intelligent online sensing of internal defect evolution in metal plastic deformation processes.

Modeling of radiation processes of defect formation in materials irradiated with ions

The consumption of materials is growing every day, which means that we will increasingly have to cope with the problems of natural resources and supply. Therefore, humanity is forced to expand its resource base finding ways to use existing raw materials more efficiently, turn previously unusable substances into useful materials and also produce completely new materials from substances that are available in abundance. One of the ways to create new materials is to irradiate a substance with charged ions. The article discusses this technique based on the cascade-probabilistic method, the purpose of which is to obtain as well as the next ensuing use of cascade-probabilistic functions (CPF) considering energy losses for ions. The CPF computations were executed depending on the number of collisions and the depth of surveillance for various incident ions and samples. When computing cascade-probabilistic functions and spatial distributions of vacancy clusters patterns of conduct and finding the real resulting region in gold and silver alloys were obtained. Selection of step and boundaries for calculation were automated. Results of the calculations performed are illustrated in the form of graphs and tables.

Identification and extraction of surface defects on composite workpieces based on Multilayer Perceptron‐Moth Flame algorithm two‐stage neural network

AbstractIn the aerospace sector, the identification and extraction of surface defects on composite parts is particularly important as they can have a very serious impact. This paper proposes a new extraction method for surface defects of composite materials, where 3D point clouds on the surface of composite workpieces are directly manipulated to finally extract a point cloud of defects containing both morphological and spatial location information. The specific operation explores the influencing factors in the formation and extraction process of composite surface defects and summarizes them into five categories: material type, fiber direction, curvature algorithm, neighborhood size, and filtering threshold. The Multilayer Perceptron‐Moth Flame algorithm two‐stage network model was constructed, which can reveal the relationship between these five types of influencing factors and the formation and extraction of surface defects in composites in the forward direction with an accuracy of 93.07%. It can also achieve optimal parameter recommendation in the reverse direction.Highlights Automatic identification and extraction of defects on the surface of composite workpieces is achieved based on 3D point cloud and curvature calculation. The forward direction reveals the relationship between the surface defect formation process and the material type and fabric direction. The reverse direction can automatically select the best curvature algorithm, neighborhood size, and filtering condition parameters. Build a Multilayer Perceptron‐Moth Flame Algorithm network structure based on multilayer perceptron and moth‐flame algorithm for model training of defect extraction, with an accuracy of 93.07%.

Analysis, classification and remediation of defects in material extrusion 3D printing

Additive manufacturing technologies (or 3D printing) have emerged as powerful tools for creating a diverse array of objects, promising a paradigm shift in production methodologies across industries. In chemistry, it allows the manufacturing of reactors with complex topology. However, the benefits of these technologies can be diminished by the use of suboptimal parameters or inferior materials, leading to defects that significantly degrade the quality and functionality of the resulting products. The formulation of effective preventive strategies remains hampered by an incomplete understanding of defect formation. Given this, our review provides a comprehensive exploration of defects that arise during the Fused Filament Fabrication (FFF) — one of the most prevalent 3D printing methods. The defects are systematically classified according to several key characteristics, including size, type, mode of occurrence, and location. Each common defect is discussed in detail, describing its external manifestation, root causes, the impact on the properties of printed parts, and potential preventive measures. Our findings unveil the complex interplay between material properties, printing parameters, and cooling dynamics in the defect formation process. This classification has significant practical relevance, providing a solid basis for the development of strategies to minimize defects and improve the quality of 3D printed products. It provides valuable insights for a wide audience, including researchers investigating chemical processes and additive manufacturing technologies, 3D printing engineers, 3D printer operators, and quality assurance engineers involved in production quality control. In addition, our review points the way forward for future research in this area. There is a crucial need for the development of advanced machine learning and artificial intelligence models that can predict defect formation based on given printing parameters and material properties. Future investigations should also focus on the discovery of novel materials and refining of printing parameters to achieve superior quality of FFF 3D printed products. This is the first review on defect analysis, classification, and prevention methods in 3D printing. This review serves as a cornerstone for these future advances, promoting a deeper understanding of defect formation and prevention in additive manufacturing.<br> The bibliography includes 180 references .

Effects of processing parameters on pore defects in blue laser directed energy deposition of aluminum by in and ex situ observation

A single track, as the basic unit of laser directed energy deposition (L-DED) process, plays a significant role in the dimensional accuracy and mechanical performances of the ultimate products. However, there is almost no systematic investigation on the formation process and three-dimensional characteristics of the internal pore defects. Here, we used a high-speed camera, laser scanning confocal microscope (LSCM), and synchrotron radiation X-ray computed tomography (SR-CT) to study single tracks of AlSi10Mg alloy fabricated by blue laser directed energy deposition (BL-DED). A comprehensive investigation is conducted on the impact of processing parameters on the sizes, shapes, and formation mechanism of pore defects. Three types of pore defects are examined in single tracks: Type I lack of fusion, Type II spherical gas pores and Type III large irregular pores. Besides, large irregular pores are the transition between other two types. In particular, the results of SR-CT show that porosity decreases gradually with the increment of laser power and scanning speed. Therefore, high laser power accompanying with fast scanning speed will reduce the porosity. The lowest porosity of 0.074% is achieved under the power at 1600 W with scanning speed at 1080 mm/min, which has an obvious improvement over the current infrared L-DED. In addition, the mapping relationship among laser power, scanning speed and pore defects is established, which will provide a fundamental understanding of the origin of the defect and strategies for controlling the defect in L-DED towards high-quality printing.

Engineering the formation of spin-defects from first principles

The full realization of spin qubits for quantum technologies relies on the ability to control and design the formation processes of spin defects in semiconductors and insulators. We present a computational protocol to investigate the synthesis of point-defects at the atomistic level, and we apply it to the study of a promising spin-qubit in silicon carbide, the divacancy (VV). Our strategy combines electronic structure calculations based on density functional theory and enhanced sampling techniques coupled with first principles molecular dynamics. We predict the optimal annealing temperatures for the formation of VVs at high temperature and show how to engineer the Fermi level of the material to optimize the defect’s yield for several polytypes of silicon carbide. Our results are in excellent agreement with available experimental data and provide novel atomistic insights into point defect formation and annihilation processes as a function of temperature.

PROCESSES OF DEFECT FORMATION IN SILICON DIFFUSIONALLY DOPED WITH PLATINUM AND IRRADIATED WITH PROTONS

In this work, we studied the effect of technological regimes and proton implantation on the processes of defect formation in single-crystal n-type silicon (n-Si) doped with platinum using the method of impedance spectroscopy. It has been established that radiation-induced changes in the electrical conductivity of silicon depend significantly on the technological regimes of doping with impurities in silicon. Hodographs show that doping with platinum leads to a decrease in the electrical resistance of silicon samples. Irradiation with 2 MeV protons at a dose of 5.1 × 1014 particles / cm2 leads to a significant (2-3 times) increase in the electrical resistance of the silicon samples under study. It is concluded that the relatively high resistance to radiation exposure (resistance change of no more than 16%: from 55 kΩ to 65 kΩ as a result of ion implantation) of samples doped at 1200°C is presumably due to a higher concentration of impurity ions (platinum) in the samples volume compared to 1100°C.

Interplay of Mechanochemistry and Material Processes in the Graphite to Diamond Phase Transformation.

The manifestation of intramolecular strains in covalent systems is widely known to accelerate chemical reactions and open alternative reaction paths. This process is moderately well understood for isolated molecules and unimolecular processes. However, in condensed matter processes such as phase transformations, material properties and structure may influence typical mechanochemical effects. Therefore, we utilize steered molecular dynamics to induce out of plane strains in graphite and compress the system under a constant strain rate to induce phase transformation. We show that the out of plane strain allows phase transformations to initiate at small amounts of compressive strain. However, in contrast to typical mechanochemical results, the sum of compressive and out of plane work needed to form a diamond has a local minimum due to altered defect formation processes during phase transformation. Additionally, these altered processes slow the kinetics of the phase transformation, taking longer from initiation to total material transformation.

Research on the formation mechanism and fracture behavior of bifilm defects

Casting performance is seriously jeopardized by metallurgical defects, especially the long-neglected bifilm defects that have been found in various alloys. The theory of entrainment formation of bifilm defects has been commonly accepted, but their visualization of the formation process and fracture behavior are absent to date, which is explored in the present work. The vivid numerical simulation results showed that the rapid rise of the liquid level leads to violent melt fluctuation, which results in the repeated folding-entrainment-detachment process of the oxide film on the melt surface. Different motion trajectories would be produced by the bifilm defects based on their composition. The observation of metallographic samples confirmed the corresponding results. Meanwhile, the fracture behavior of bifilm defects can be studied using optical in situ tensile. The results show that the bifilm defects are the source of cracks, and the connection of their influence domains causes the cracks to expand laterally. These results will shed new light on the understanding of the formation and fracture process of bifilm defects and improve the defect control of castings.

Defect formation and migration in MAlB (M=Mo, W) and N2AlB2 (N=Cr, Fe) : A first-principles study

The exceptional properties of MAB phases (where M = transition metal, A = aluminum, and B = boron) make them a promising option for various applications. However, their usage in most fields requires an understanding of the defect formation and migration processes, which can aid in designing durable and functional materials. Through density functional theory calculations, we investigated the stability and mobility of the most prominent point and extended defects in MAlB (where M = Mo and W) and ${\mathrm{N}}_{2}{\mathrm{AlB}}_{2}$ (where N = Cr and Fe) MAB phases under different chemical conditions. Our findings indicate that ${\mathrm{V}}_{\mathrm{B}}/{\mathrm{N}}_{\mathrm{Al}}$ is the easiest defect to form under M/N-rich conditions. We observed that vacancy and antisite defects form more easily than interstitial ones in all studied systems. We also investigated different extended structural defects and stacking faults and revealed the stability of relevant compositional defects and tilt/rotational boundaries in MAlB and ${\mathrm{N}}_{2}{\mathrm{AlB}}_{2}$. Our results suggest that these systems can likely be synthesized by tuning the experimental conditions. We additionally observed grain boundary formation in ${\mathrm{N}}_{2}{\mathrm{AlB}}_{2}$ and compared our findings with relevant experiments.