Properties Of Grain Boundaries Research Articles

Growth defects in heteroepitaxial diamond

In focus of this report are the mechanisms of formation, propagation, and interaction of growth defects in heteroepitaxial diamond films along with their impact on the optical emission properties of N- and Si-vacancy (NV and SiV) color centers. Here, we analyze and discuss the properties of incoherent grain boundaries (IGBs) and extended defects in a nitrogen- and boron-doped heterodiamond nucleated and grown on Ir(001) via bias-enhanced nucleation and chemical vapor deposition techniques. We show that the low-angle IGBs alter the structural and optical emission properties of NV and SiV complexes by subduing NV emission and supporting the formation of interstitial Si-vacancy complexes dominating in the faulted IGB regions. We also demonstrate that the IGB-confined threading dislocations are responsible for the vertical transport and incorporation of Si impurities in thick layers, leading to an enhanced SiV emission from the IGBs.

Universal prediction of strain footprints via simulation, statistics, and machine learning: low-Σ grain boundaries

Interface engineering usually imposes a trade-off between catalytic activity, product selectivity, and property tunability, facilitating multiple product yields. Due to their structural disorder, metal grain boundaries (GBs) can be constructed to satisfy all of these conditions. Though experiments show GB catalytic footprints are determined by dislocation-induced elastic strain, the atomistic structural and energetic origins of GB catalytic performance are not currently known. In former models solely reviewing metastable GBs defined by macroscopic degrees of freedom (DOFs), exceptional low-Σ GBs anticipated by the coincident site lattice model cannot be consistently discerned. Herein, a structure-energy correlation linking changes in atomistic properties to physical models is developed, distinguishing exceptional GBs from unexceptional high-angle GBs over structural DOFs. This work constructs particular GB elastic responses by varying three microscopic DOFs, comparing low-Σ distinguished and other GB responses via a directed statistical approach. Such elastic properties successfully distinguish low-Σ GBs from high-angle grain boundaries (HAGBs) by their strain-related footprints without failure of a single unique GB structure and composition combination. A priori application of machine learning classification is completed using metastable GB and elastic response related properties. This classification not only validates distinguished strain footprints, but also identifies GB subgroups with footprints unexpected by former models.

Structure‐composition-property correlations of symmetrical tilt grain boundaries in copper-based binary alloys

The thermodynamic stability and mechanical strength of grain boundaries (GBs) govern many key properties of nanocrystalline or polycrystalline metals. In order to design novel Cu-based alloys with extraordinary properties and broaden the application scopes of Cu alloys, it is essential to clarify the effects of GB structure and composition on the stability and cohesion of Cu GBs. Firstly, the GB energies and works of separation of 12 Cu symmetrical tilt GBs with various GB orientations were figured out by using first-principles calculations, to link the GB properties with structures. Our results shown that the GB energies and works of separation of 12 Cu GBs are notably anisotropic. The GB energies for these GBs have an inverted and linear relationship with their works of separation. As to GB composition, the segregation tendencies of 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) in the Cu Σ5 [001](210) symmetrical tilt GB and their cohesive and embrittling effects on the GB were studied. It was found that all considered solutes excluding the element Ni can segregate to the Cu GB and thus play stabilizing roles into the Cu GB. Among 8 solutes, only the Zr and Cr segregations can improve the thermodynamic stability and fracture strength of Cu GB simultaneously. The GB strengthening effects of Zr and Cr segregations was attributed to their significant chemical contributions to the embrittling potencies, i.e., the presence of covalent-like bonding features in newly-formed Cu–Zr/Cr bonds. The reduction/increase of anti-bonding/bonding states below the Fermi level in Cu–Zr/Cr atomic pairs should be responsible for the GB stabilizing effects of Zr and Cr segregations.

Defect states and passivation mechanism at grain boundaries of zinc-blende semiconductors

Grain boundaries (GBs) are significant in determining the electrical properties of polycrystalline semiconductors. However, the electronic structures and passivation mechanisms of polycrystalline semiconductors remain poorly understood. In this study, we systematically investigated the Σ3 (112) GB properties of several typical zinc-blende semiconductors via first-principles density functional calculations. We found significant differences of Σ3 (112) GB structures and properties between IV/III and V types, where dangling atoms formed new covalent bonds, and II–VI/I–VII types, where dangling atoms formed no new bonds. These different bonding configurations lead to different origins of defect states at GBs. We successfully designed a targeted doping approach to passivate such defect states for different types of semiconductors. We demonstrated the validity of the proposed approach in Σ3 (112) GB of the zinc-blende semiconductors. This work elucidates the defect states at GBs in common zinc-blende semiconductors, rationalizes diverse post-treatment approaches reported in previous experiments, and provides general guidance for defect passivation at the GBs of polycrystalline semiconductors.

Grain boundary structure-property model inference using polycrystals: The underdetermined case

Historically, attempts to correlate grain boundary (GB) structure and properties have relied on experimental measurements and computational calculations performed on bicrystals. While the resulting data can be precise, it is usually severely limited in quantity due to the cost and/or computational expense of sampling the full 5D GB character space. As a result, existing data is concentrated around highly-symmetric sub-regions. In contrast, polycrystals are ubiquitous, inexpensive, and contain a broad range of GB characters. However, extracting GB properties from the effective properties of polycrystals requires the solution of an inverse problem. In this work, we present a Bayesian framework, that we call GB property localization, to address both of these challenges by facilitating the inference of GB structure-property models from indirect polycrystal measurements and severely limited data (i.e. when the system is strongly underdetermined). The method naturally provides uncertainty quantification for the resulting inferences. We present the results of validation tests using 1DOF and 3DOF structure-property models for GB diffusivity in 2D (or columnar) microstructures, and we investigate the influence of the number of observations and the number of grains in a polycrystal on the inference quality. We identify a transition in the preferred type of data as a function of the number of observations: when data is abundant bicrystals lead to more accurate inferences; however when data is severely limited, polycrystals may provide more accurate results.

Thermodynamic stability and electronic structure of pristine wurtzite ZnO{0001} inversion domain boundaries

Recently, it was demonstrated that the conductivity of wurzite (wz) ZnO bicrystal samples containing ${0001}$ inversion domain boundaries (IDB) can be massively and reversibly tuned by mechanical loading. As a step towards a detailed microscopic understanding of this effect, we systematically investigate the atomic structure and chemical composition of such IDBs using density functional theory calculations. In total, 92 model geometries that differ in structure and/or chemical composition are constructed, optimized, and compared thermodynamically. The lack of higher symmetries in wz ZnO prohibits a straightforward calculation of individual grain boundary (GB) excess energies. However, we show that, in nonperiodic slab models of wz ${0001}$ IDBs, the additional surface contribution to the total energy may be approximated by that of corresponding zincblende (zb) surfaces; the latter can be obtained by a series of prism calculations. Subtracting these surface energies allows us to construct absolute GB energy diagrams for wz IDBs and compare their thermodynamic stability with other GBs known from the literature. We find that thermodynamically favored IDBs are characterized by fully (4-fold) coordinated atoms and possess relatively low excess energies that range from 45 to $95\phantom{\rule{4pt}{0ex}}\mathrm{meV}/{\AA{}}^{2}$, depending on the termination (Zn/Zn or O/O) of the IDB and the exchange-correlation functional used in the calculation (LDA, GGA, or $\text{GGA}+U$). The electronic properties of the GB deviate only weakly from those of the bulk and are rather insensitive towards compressive and tensile strains. Our results thus indicate that experimentally observed piezotronic properties of wz bicrystals are not an intrinsic property of the pristine GB itself, but originate, for example, from externally supplied trapped charges, defects, impurities, or dopants. Low-energy structure models identified here may also be transferable to other wz- or zb-type IDBs (e.g., GaN, AlN, SiC, etc.).

Diffusion temperature dependent microstructure and magnetic properties of TbHx grain boundary diffused Nd–Fe–B magnets

Commercial N50 sintered Nd–Fe–B magnets were grain boundary diffusion (GBD) processed with TbHx powders. The diffusion temperature and time dependent microstructure and magnetic properties of the GBD processed magnets were investigated. High remanence and coercivity were obtained when the magnet was GBD processed at 875 °C for 14 h; the coercivity of the magnet increased from 12.57 kOe to 22.65 kOe. The relationships between microstructure and coercivity, remanence, and thermal stability of the GBD processed magnets were analyzed and discussed.

Solute segregation induced stabilizing and strengthening effects on Ni Σ3 [110](111) symmetrical tilt grain boundary in nickel-based superalloys

Grain boundary segregation engineering (GBSE) is a promising approach for accurately manipulating chemical composition, structure and properties of grain boundaries (GBs). With the aim of attenuating or even removing disadvantages of GBs in nickel-based superalloys, the GB segregation behaviors of 3d (Ti–Co), 4d (Zr–Rh) and 5d (Hf–Ir) transition-metal (TM) solutes X and their effects on the thermodynamic stability and fracture strength of Ni Σ3 [110](111) symmetrical tilt grain boundary were systematically investigated, using first-principles total-energy calculations based on density functional theory. We found that all TM solutes considered herein can segregate towards the Ni Σ3 [110](111) GB. For TM elements from same row of the periodic table, their segregation energies show a concave-up parabolic-like dependency on the atomic number of TM elements. The GB segregations of all considered solutes can lead to a reduction in GB energy and render the Ni GB more stable, and the GB energies of X-segregated GBs vary linearly with the GB segregation energies of solutes X. Except elements Zr, Hf and Rh, the GB segregations of the other 15 TM alloying elements increase the GB fracture strength. The Mn and Re segregations improve the GB fracture strength and thermodynamic stability to a maximum extent, respectively. The underlying mechanisms for segregation-induced GB stabilizing and strengthening effects were further revealed at the electronic level by analyzing the crystal orbital Hamiltonian populations and deformation charge densities. Our work could provide new insights into designing Ni-based superalloys with desired performances based on the concept of GBSE.

Diffusion transport over grain-boundary barriers as the origin of N1 deep level transient spectroscopy signal in [formula omitted] solar cells

In this work, we explore the hypothesis that the N1 peak in Cu(In,Ga)Se2 (CIGS) solar cells is caused by secondary barriers. We indicate, through numerical modelling of the corresponding Deep Level Transient Spectroscopy (DLTS) signal, that these barriers may be located at grain boundaries. Moreover, the proper reconstruction of the N1 signal using the double-barrier model requires an assumption that corresponding capacitance transients are due to the diffusion limited transport of holes over grain boundaries. Only then we were able to achieve sufficiently small values of the saturation current for realistic values of material parameters. We show that the position of the N1 DLTS peak and its activation energy depends on the effective density and the effective mobility of holes which can be calculated based on grain size and the properties of grain boundaries. This interpretation opens a room for the explanation of the unusual properties of the N1 signal in CIGS solar cells.

Microscopic and Macroscopic Characterization of Grain Boundary Energy and Strength in Silicon Carbide via Machine-Learning Techniques.

Predicting the properties of grain boundaries poses a challenge because of the complex relationships between structural and chemical attributes both at the atomic and continuum scales. Grain boundary systems are typically characterized by parameters used to classify local atomic arrangements in order to extract features such as grain boundary energy or grain boundary strength. The present work utilizes a combination of high-throughput atomistic simulations, macroscopic and microscopic descriptors, and machine-learning techniques to characterize the energy and strength of silicon carbide grain boundaries. A diverse data set of symmetric tilt and twist grain boundaries are described using macroscopic metrics such as misorientation, the alignment of critical low-index planes, and the Schmid factor, but also in terms of microscopic metrics, by quantifying the local atomic structure and chemistry at the interface. These descriptors are used to create random-forest regression models, allowing for their relative importance to the grain boundary energy and decohesion stress to be better understood. Results show that while the energetics of the grain boundary were best described using the microscopic descriptors, the ability of the macroscopic descriptors to reasonably predict grain boundaries with low energy suggests a link between the crystallographic orientation and the resultant atomic structure that forms at the grain boundary within this regime. For grain boundary strength, neither microscopic nor macroscopic descriptors were able to fully capture the response individually. However, when both descriptor sets were utilized, the decohesion stress of the grain boundary could be accurately predicted. These results highlight the importance of considering both macroscopic and microscopic factors when constructing constitutive models for grain boundary systems, which has significant implications for both understanding the fundamental mechanisms at work and the ability to bridge length scales.

Electric-field-assisted processing of ceramics: Nonthermal effects and related mechanisms

Field-assisted processing methods, such as spark plasma sintering and flash sintering, have considerably expanded the toolbox of ceramic engineering. Depending on the conditions, substantial electric currents may flow through the material resulting in fast heating rates due to Joule heating. Here, we focus on nonthermal effects induced by electric fields during processing of fluorite- and perovskite-based ceramics. The fundamentals of how a field can directly modify defect formation and migration in crystals are discussed. In addition, the interplay of ion transport and electrical conductivity is considered, this interplay being crucial to understanding nonthermal effects caused by electric fields (as in memristive switching). Electrochemical reactions leading to new phases or reduction are also described, as are densification rates and sintering parameters that are significantly affected even though the sample temperature is held constant. Finally, as grain-boundary properties and segregation are changed by ion transport, we describe how both retardation and acceleration of grain growth can be achieved including graded microstructures.

Semi-analytical approach to transport gaps in polycrystalline graphene.

Electron transport in graphene is dominated by its Dirac-like charge carriers. Grain boundaries add a geometric aspect to the transport behavior by coupling differently oriented grains. In the phase coherent limit this aspect allows to relate the transport properties to two factors: the electronic structure of individual grains around the Dirac points and the orientation relation of the Dirac cones within the grain boundary Brillouin zone. Based on this picture it is possible to quantify the size and strain modulation of transport gaps without the need for explicit transport calculations within the non-equilibrium Green functions formalism. In this work we present a semi-analytical method that exploits this picture. Our method can explore arbitrary grain misorientations in the presence of an external strain providing valuable information about the electronic properties of individual grain boundaries.

Spectrum of embrittling potencies and relation to properties of symmetric-tilt grain boundaries

Grain boundaries in metallic materials can exist in a wide range of stable and metastable structures. In addition, the properties of a grain boundary may be altered through solute segregation. In this work, we present a formulation that combines the spectrum of embrittling potencies associated with solute segregation with site-occupancy statistics. As a prototype problem, we illustrate the relation between segregation and embrittlement in the case of S segregation to grain boundaries in Ni. To obtain a population of site segregation energies, we perform molecular statics calculations on 378 different symmetric-tilt grain boundaries and their free surface equivalents, using an embedded-atom method interatomic potential developed specifically for studying embrittlement. Our results show that it is important to consider both the energies associated with embrittlement and the probability of occupancy to describe the general embrittling nature of a grain boundary. When analyzed in isolation, certain grain boundaries show large embrittling potencies; however, that effect is diminished when the probability of S segregation to that grain boundary is considered within a polycrystal. We propose a new quantity, the embrittling estimator, which not only categorizes grain boundaries as embrittling or strengthening, but also considers site occupancy probabilities, so that the embrittlement behavior of grain boundaries within a network of grain boundaries can be compared. Finally, we examine the relationship between embrittlement behavior and innate grain boundary properties, such as the free volume, and find statistical evidence that the complex nature of embrittlement cannot be explained by linear correlations with excess volumes or energies. Ultimately, this combined approach provides a theoretical tool to assist grain boundary engineering of metastable alloys.

Segregation-assisted spinodal and transient spinodal phase separation at grain boundaries

Segregation to grain boundaries affects their cohesion, corrosion, and embrittlement and plays a critical role in heterogeneous nucleation. In order to quantitatively study segregation and low-dimensional phase separation at grain boundaries, here, we apply a density-based phase-field model. The current model describes the grain-boundary thermodynamic properties based on available bulk thermodynamic data, while the grain-boundary-density profile is obtained using atomistic simulations. To benchmark the performance of the model, Mn grain-boundary segregation in the Fe–Mn system is studied. 3D simulation results are compared against atom probe tomography measurements conducted for three alloy compositions. We show that a continuous increase in the alloy composition results in a discontinuous jump in the segregation isotherm. The jump corresponds to a spinodal phase separation at grain boundary. For alloy compositions above the jump, we reveal an interfacial transient spinodal phase separation. The transient spinodal phenomenon opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries. The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase changes at grain boundaries.

Ab initio study of oxygen segregation in silicon grain boundaries: The role of strain and vacancies

Multi-crystalline silicon is widely used for producing low-cost and high-efficiency solar cells. During crystal growth and device fabrication, silicon solar cells contain grain boundaries (GBs) which are preferential segregation sites for atomic impurities such as oxygen atoms. GBs can induce charge carriers recombination significantly reducing carrier lifetimes and therefore they can be detrimental for Si device performance. We studied the correlation between structural, energetic and electronic properties of Σ3{111} Si GB in the presence of vacancies, strain and multiple O segregation. The study of the structural and energetic properties of GBs in the presence of strain and vacancies gives an accurate description of the complex mechanisms that control the segregation of oxygen atoms. We analysed tensile and compressive strain and we obtained that local tensile strain around O impurities is very effective for segregation. We also studied the role of multiple O impurities in the presence of Si vacancies finding that the segregation is favorite for those structures which have restored tetrahedral covalent bonds. The presence of vacancies attract atomic impurities in order to restore the electronic stability: the interstitial impurity becomes substitutional. This analysis was the starting point to correlate the change of the electronic properties in Σ3{111} Si GBs with O impurities in the presence of strain and vacancies. For each structure we analysed the density of states and its projection on atoms and states, the band gaps, the segregation energy and their correlation in order to characterise the nature of new energy levels. Actually, knowing the origin of defined electronic states would allow the optimization of materials in order to reduce non radiative electron-hole recombination avoiding charge and energy losses and therefore improving solar cell efficiency.

Impact of Aliovalent Alkaline-Earth metal solutes on Ceria Grain Boundaries: A density functional theory study

Ceria has proven to be an excellent ion-transport and ion-exchange material when used in polycrystalline form and with a high-concentration of aliovalent doped cations. Despite its widespread application, the impact of atomic-scale defects in this material are scarcely studied and poorly understood. In this article, using first-principles simulations, we provide a fundamental understanding of the atomic-structure, thermodynamic and electronic properties of undoped grain-boundaries (GBs) and alkaline-earth metal (AEM) doped GBs in ceria. Using density-functional theory simulations, with a GGA+U functional, we find the Σ3 (111)/[1¯01] GB is energetically more stable than the Σ3 (121)/[1¯01] GB due to the larger atomic coherency in the Σ3 (111)/[1¯01] GB plane. We dope the GBs with ∼20% [M]GB (M=Be, Mg, Ca, Sr, and Ba) and find that the GB energies have a parabolic dependence on the size of solutes, the interfacial strain and the packing density of the GB. We see a stabilization of the GBs upon Ca, Sr and Ba doping whereas Be and Mg render them energetically unstable. The electronic density of states reveal that no defect states are present in or above the band gap of the AEM doped ceria, which is highly conducive to maintain low electronic mobility in this ionic conductor. The electronic properties, unlike the energetic properties, exhibit complex inter-dependence on the structure and chemistry of the host and the solutes. This work makes advances in the atomic-scale understanding of aliovalent cation doped ceria GBs serving as an anchor to future studies that can focus on understanding and improving ionic-transport.

Segregation behavior of alloying elements at Ni Σ5 [001](210) symmetrical tilt grain boundary in nickel-based superalloys and their stabilization and strengthening mechanisms for the grain boundary

To reduce or even eliminate the negative effects of grain boundaries (GBs) in nickel-based superalloys, the GB-segregation behaviors of 14 main alloying elements X of superalloys and their effects on the stability and fracture strength of Ni Σ5 [001](210) GB were systematically investigated based on the concept of GB segregation engineering (GBSE), using first-principles calculations. We found that 10 alloying elements can segregate towards the GB. Their GB-segregation tendencies increase in the order of W < Mo < Al < Mn < Cu < Ti < Ru < Ta < Hf < Zr, due to the increase in atomic radius and electronegativity differences between X and host Ni atoms. The GB energies of X-segregated GBs depend linearly on the GB segregation energies of solutes X, the stronger segregation tendency of solute X could lower the GB energy and make the GB more stable. Except Cu, the segregations of the other 9 solutes increase the GB fracture strength. The segregation-induced GB stabilizing and strengthening mechanisms were further revealed on the electronic level. This work could be beneficial for manipulating the composition, structure and properties of Ni GBs and designing Ni-based superalloys with desired performances based on the GBSE.

Atomic and electronic band structures of Ti-doped Al2O3 grain boundaries

Doping is one of the most popular strategies to modify the properties of polycrystalline materials, because the dopants prefer to segregate at the grain boundaries (GBs) and influence the structural and electronic properties of the materials. Understanding how dopants segregate at GBs and how they affect the resultant GB properties are essential. Here, we experimentally characterized the atomic structures and the electronic band structures of Ti-doped Σ7{45¯10} and Σ7{23¯10} GBs in α-Al2O3 by atomic resolution scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDXS) and valence electron energy-loss spectroscopy (EELS). It was found that Ti preferentially segregated at specific atom sites driven by ionic size mismatch between Ti3+ and Al3+, which leads to structural transformations in both GBs. Direct valence EELS measurement revealed the segregation of Ti3+ ions introduces impurity band within the bandgap in Al2O3 GBs. These results provide an in-depth understanding of the local atomic and electronic band structures for Ti-doped GBs.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Effects of Grain Boundary Faceting, Grain Boundary Anisotropy and Interface Natures on the Maximum Pinning Force of a Differently Shaped and Oriented Particle

Zener pinning between a curved Cu grain boundary (GB) and a differently shaped and oriented Ag particle has been simulated via molecular dynamics. The computed magnitudes of the maximum pinning force agreed with theoretical predictions only when the force was small. As the force increased, discrepancy became obvious. Through carefully inspecting structures of the Cu-Ag interfaces, detailed interaction processes and variation of the Cu GB during the interaction, the discrepancy is found to mainly result from GB faceting, which can reduce the maximum pinning force and facilitate boundary passage. GB anisotropy and/or interface natures are also found to slightly contribute to the discrepancy. These findings suggest that the assumption of an isotropic GB with constant energy utilized in previous theoretical works for deriving the maximum pinning force is inappropriate and a correct maximum pinning force could not be predicted without knowing the effects of GB evolution together with detailed properties of both GBs and interfaces.