Atomic Structure Of Grain Boundaries Research Articles

Stabilization and self-passivation of symmetrical grain boundaries by mirror symmetry breaking

While computational methods for the study of point defects in materials have received significant attention, methods for the investigation of grain boundaries require further development. In this study, we applied a genetic algorithm to find the atomic structure of grain boundaries in semiconductor materials with given Miller indices and investigated their electronic structure. Our study of the {\Sigma}3 (112) grain boundary in CdTe shows that the stability of grain boundaries can be greatly enhanced by breaking mirror symmetry, locally or globally. The resulting grain boundary can be electrically less harmful because the origin of the defect levels are removed from the middle of the band gap and the grain boundaries can serve as a channel for electron extraction.

Exploring fractality of microcrystalline diamond films

Diamond is renowned as a material with superlative physical qualities, most of which originate from the strong covalent bonding between its atoms. The possibility to deposit polycrystalline diamond films via chemical vapor deposition (CVD) methods on large areas and on a large variety of substrates is posing numerous scientific challenges but also enables relevant industrial applications. Especially for small grain sizes, the grain’s misorientation and consequently the atomic structure of grain boundaries plays a significant role on transport properties and mechanical properties. Hence, the size and shape of the crystallites of poly- and nanocrystalline diamond films are one important key to optimize film properties for their specific applications. Fractal theory helps to find and define order in systems where disorder seems to prevail. Therefore, we apply fractal geometry analysis to characterize the grain morphology and surface topology of CVD grown diamond films.

Extracting knowledge from molecular mechanics simulations of grain boundaries using machine learning

In this paper, we demonstrate that the “process-structure-property” (PSP) paradigm of materials science can be extended to atomistic grain boundary (GB) simulations through the development of a novel framework that addresses the objective identification of the atoms in the grain boundary regions using the centro-symmetry parameter and local regression, and the quantification of the resulting structure by a pair correlation function (PCF) derived from kernel density estimation (KDE). For asymmetric tilt GBs (ATGBs) in aluminum, models were successfully established connecting the GB macro degrees of freedom (treated as process parameters) and energy (treated as property) to a low-rank GB atomic structure approximation derived from principal component analysis (PCA) of the full ensemble of PCFs aggregated for this study. More specifically, it has been shown that the models produced in this study resulted in average prediction errors less than 13 mJ/m2, which is less than the error associated with the underlying simulations when compared with experiments. This demonstration raises the potential for the development and application of PSP linkages from atomistic simulation datasets, and offers a powerful route for extracting high value actionable and transferrable knowledge from such computations.

Continuous description of the atomic structure of grain boundaries using dislocation and generalized-disclination density fields

An atomistic-to-continuum method is developed to derive dislocation, generalized-disclination density fields and the associated elastic strain, rotation, curvature and second-distortion fields from the atomic structure of grain boundaries. From the relaxed and un-relaxed atomic positions, calculation of the transformation gradient feeds a mechanical framework, where discontinuities of the lattice elastic displacement and distortion (rotation and strain) are captured by smooth incompatible strain and second-distortion fields associated with the dislocation and generalized-disclination density fields, respectively. The method is applied to a copper symmetrical tilt boundary as obtained from molecular dynamics simulations. The core structure of the boundary is found to contain edge dislocations and dipoles of generalized-disclinations, including standard wedge-disclination dipoles. The latter reflect in particular localized shear and stretch discontinuities across the interface, in addition to the overall rotation discontinuity.

An energetic and kinetic investigation of the role of different atomic grain boundaries in healing radiation damage in nickel

In this paper, we investigate the role that the atomic structure of grain boundaries (GBs) in Ni has in the interaction with point defects produced during the collision cascades. Using molecular dynamics and statics, we study the influence of different GB structures on the production and evolution of defects, through calculation of defect formation energy, vacancy diffusion barrier, interstitial-vacancy annihilation barrier, and the corresponding energetic and kinetic influence ranges near pristine and damaged GBs. We observe that, as previously reported, GBs preferentially absorb interstitials over vacancies, which result in accumulation of vacancies in the bulk grain and interstitials to highly localize at the GBs. Also, we find that, in several layers near the GBs, the formation energy of defects is substantially reduced, indicating that the GBs serve as a sink for the radiation-produced defects. Further investigation also reveals that the sink strength of the GBs is significantly stronger for interstitials than vacancies. Also in terms of kinetic behavior, we find that the energy barrier of vacancy diffusion reduces as the defect migrates to the close vicinity of the GBs, implying that in the presence of thermally assisted events, vacancies diffuse toward the GBs via low-barrier processes, which result in healing of the damaged boundaries at high temperatures. Moreover, we observe that in the neighboring region of the defect-loaded GBs, the annihilation of a close vacancy–interstitial pair is spontaneous. Finally, via continuous ion bombardment simulation, we study the stability of the GBs in severe irradiation condition. We find that the GBs lose their stability as the number of trapped interstitials increases at the GB plane. Besides, we conclude that the GBs have high efficiency in defect removal on picosecond time scale and act as efficient defect sinks, despite the instability and saturation observed in repeated ion bombardments.

Effects of atomic grain boundary structures on primary radiation damage in α-Fe

In this paper, we used five different grain boundary (GB) structures including two twists, two symmetric tilts and one asymmetric tilt for α-Fe, in order to study the influence of different GB structures on the production and time evolution of defects. Energetic behavior of point defects near GBs is investigated and analyzed using Molecular Static (MS) method to calculate and compare the “defect absorbency” of each structure. The primary radiation damage state near each GB structure is simulated using Molecular Dynamic (MD) method for 3keV and 6keV primary-knocked on atom (PKA) with velocity vector perpendicular to the GB plane at various distances in 300K. We found that all five GB structures can decrease the defect number in bulk region, if cascade center locates on the GB plane (prefect overlap) and increase the vacancy number, if the overlap is imperfect. This depends on the energy of PKA and its distance from GB plane. Also, the results proved that the magnitude of the observed variations depends on the atomic structure of GB. Furthermore, the GBs that have stronger “interstitial absorbency” produce an excess concentration of vacancies in the bulk region, while the edge of the cascades overlaps with GB plane. This is the result of bigger “interstitial absorbency” of GBs in comparison with vacancies.

Large-Area Synthesis of Monolayer and Few-Layer MoSe2Films on SiO2Substrates

We present successful synthesis of large area atomically thin MoSe2 films by selenization of MoO3 in a vapor transport chemical vapor deposition (CVD) system. The homogeneous thin film can reach an area of 1 × 1 cm(2) consisting primarily of monolayer and bilayer MoSe2 film. Scanning transmission electron microscopy (STEM) images reveal the highly crystalline nature of the thin film and the atomic structure of grain boundaries in monolayers. Raman and photoluminescence spectroscopy confirm the high quality of as-grown MoSe2 in optics, and electronic transport measurements highlight the potential applications of the sample in nanoelectronics.

Atomic structure of grain boundaries in iron modeled using the atomic density function

A model based on the continuous atomic density function (ADF) approach is applied to predict the atomic structure of grain boundaries (GBs) in iron. Symmetrical [100] and [110] tilt GBs in bcc iron are modeled with the ADF method and relaxed afterwards in molecular dynamics (MD) simulations. The shape of the GB energy curve obtained in the ADF model reproduces well the peculiarities of the angles of 70.53 deg. [$\Sigma$ 3(112)] and 129.52 deg. [$\Sigma$ 11(332)] for [110] tilt GBs. The results of MD relaxation with an embedded-atom method potential for iron confirm that the atomic GB configurations obtained in ADF modeling are very close to equilibrium ones. The developed model provides well-localized atomic positions for GBs of various geometries.

Electronic States at Low-Angle Grain Boundaries in Polycrystalline Naphthalene

We investigated the influence of grain boundaries on electronic properties of polycrystalline organic semiconductor naphthalene. The atomic structure of grain boundaries was found using a Monte Carlo method, whereas electronic structure calculations were performed using the charge patching method. We found that grain boundaries introduce trap states within the band gap of the material. Our results show that spatial positions and energies of trap states can be predicted solely from geometrical arrangement of molecules near the boundary. Wave functions of these states are localized on closely spaced pairs of molecules from opposite sides of the boundary. The energies of trap states are strongly correlated with the distances between the molecules in the pair. These findings were used to calculate the electronic density of trap states, which was found to exhibit a qualitatively different behavior for grain boundaries perpendicular to the a and b directions of the crystal unit cell.

MULTI-SCALE SIMULATION OF GRAIN BOUNDARY STRUCTURE EFFECTS IN POLY-Si THIN FILM SOLAR CELL

The atomic structures of grain boundary (GB) and their effect on the performance of poly- Si thin film solar cell are studied by multi-scale simulations. First, the atomic structures of various GBs are calculated using molecular dynamics. Subsequently, the energy band diagram are obtained by ab-initio calculations. Then, finite difference method is performed to obtain solar cell performance. The results show that the Σ5 (twist) GB can greatly enhance the carriers recombination and results in small short-circuit current density (J SC ) and open-circuit voltage (V OC ). However, the Σ17 (twist and tilt) GBs have little influence on the cell performance. Also revealed in the simulations is that the GB near the p–n junction leads to very small J SC and V OC . When the distance between GB and p–n junction increases from about 1.10 μm to 3.65 μm, the conversion efficiency increases by about 29%. The thickness effect of solar cell containing the Σ5 (twist) GB on the cell performance is also studied. The results show that the conversion efficiency and J SC increase rapidly as the thickness increases from about 5.2 μm to 40 μm. When the thickness ranges from about 40 μm to 70 μm, the efficiency and the J SC both increase gradually and reach their own peak values at about 70 μm. When the thickness exceeds 70 μm, the efficiency and J SC both decrease gradually. However, the V OC keeps increasing with increase in thickness. The effects of GB on the carrier transport and recombination processes are discussed to understand the above results.

Atomic structures and energies of grain boundaries in Mg2SiO4 forsterite from atomistic modeling

Grain boundaries influence many physical and chemical properties of crystalline materials. Here, we perform molecular dynamics simulations to study the structure of a series of [100] symmetric tilt grain boundaries in Mg2SiO4 forsterite. The present results show that grain boundary energies depend significantly on misorientation angle. For small misorientation angles (up to 22°), grain boundary structures consist of an array of partial edge dislocations with Burgers vector \(\frac{1}{2}[001]\) associated with stacking faults and their energies can be readily fit with a model which adds the Peach-Koehler equation to the Read-Shockley dislocation model for grain boundaries. The core radius of partial dislocations and the spacing between the partials derived from grain boundary energies show that the transition from low- to high-angle grain boundaries occurs for a misorientation angle between 22° and 32°. For high misorientation angles (32.1° and 60.8°), the cores of dislocations overlap and form repeated structural units. Finally, we use a low energy atomic configuration obtained by molecular dynamics for the misorientation of 12.18° as input to simulate a high-resolution transmission electron microscopy (HRTEM) image. The simulated image is in good agreement with an observed HRTEM image, which indicates the power of the present approach to predict realistic atomic structures of grain boundaries in complex silicates.

Role of atomic structure on grain boundary-defect interactions in Cu

We investigate the role that the atomic structure of grain boundaries in Cu has in the interaction with point defects produced during irradiation. We focus on three aspects of boundary-defect interaction: how defects interact with pristine boundaries, how boundaries modify defect production during collision cascades, and how defects interact with damaged boundaries. We find that there are generic features common to most boundaries, including biased absorption of interstitials over vacancies during collision cascades and strong interactions with vacancies for interstitial-loaded boundaries. However, we find that the magnitude of these behaviors depends strongly on the atomic structure of the boundary. In particular, the biased absorption is much stronger for a high-angle twist boundary and smallest for a more general twist-asymmetric tilt boundary. Further, the strength of boundary-defect interactions is also sensitive to the boundary structure. We conclude that the sink strength of grain boundaries for interacting with point defects is not an intrinsic property of the boundary but rather depends on the irradiation conditions through the absorbed defect content at the boundary.

Grain Boundaries in Uranium Dioxide: Scanning Electron Microscopy Experiments and Atomistic Simulations

The distribution and atomic structure of grain boundaries has been investigated in UO 2 . Our scanning electron microscopic/ electron backscatter diffraction experiments on a depleted UO 2 sample showed real nuclear fuels contain a combination of special coincident site lattice (CSL) and general boundaries. The experimental data indicated that ∼ 16% of the boundaries were CSL boundaries and the CSL distribution was dominated by low Σ boundaries; namely E9, E3, and Σ5 Based on our experimental observations, the structures of select low E (E5 tilt, Σ5 twist, Σ3 tilt) and a random boundary were analyzed in greater detail using empirical potential atomic-scale calculations. Our calculations indicate that the boundaries have very different structures and each CSL boundary had multiple minima on the γ-surface. The presence of a significant fraction of CSL boundaries and the differences in their structures are expected to have important consequences on fuel properties.

Effects of segregated Cu on an Fe grain boundary by first-principles tensile tests

Cu is known as one of the harmful tramp elements in recycled Fe. In the present work, theeffects of Cu on Fe grain boundary (GB) embrittlement have been investigated byfirst-principles tensile tests. Because the Fe–Cu bonds are rather isotropic and the effectsdue to their difference in atomic size are negligibly small, the GB atomic structure prior tostraining is little changed by Cu segregation. However, the Fe–Fe bond around the Cu atomis weakened due to charge transfer from the Fe atom to the Cu atom, and premature bondbreaking occurs at the weakened Fe–Fe bond, resulting in an enhancement of GBembrittlement by Cu segregation. The s and p electrons play a vital role in the chargetransfer.

Dimensionality of interstitial He migration in 〈1 1 0〉 tilt grain boundaries in α-Fe

In this work we study the migration of an interstitial helium atom in different 〈1 1 0〉 tilt grain boundaries ( Σ19{3 3 1}, Σ9{2 2 1}, Σ3{1 1 1}, Σ3{1 1 2}, Σ11{1 1 3}, Σ9{1 1 4}) in α-Fe, with the misorientation angle varying in the range 26°–141°. Systematic molecular statics and molecular dynamics simulations were performed to characterize the interaction of He with the core of the grain boundaries (GB) and to estimate the diffusion coefficient, migration mechanism and effective migration energy. The simulations were performed in the temperature range 300–1000 K, applying a recently proposed set of interatomic potentials for Fe–He system by Juslin et al., specially fitted to the properties of He in bulk Fe. It was found that the migration of an interstitial He near the core of Σ3 〈1 1 0〉{1 1 1} GB is essentially three-dimensional in the investigated temperature range. Two-dimensional diffusion prevails in the case of Σ3〈1 1 0〉{112} and Σ9〈1 1 0〉{1 1 4} GBs and it changes to the three-dimensional one above ∼900 K. A completely one-dimensional diffusion along the tilt axis was observed in the core of Σ19〈1 1 0〉{3 3 1} and Σ9〈1 1 0〉{2 2 1} GBs. No core diffusion was detected at Σ11〈1 1 0〉{1 1 3} GB. Thus, only the release from and re-absorption at the GB core was seen in this GB. These results clearly demonstrate that the accommodation, migration mechanism and diffusivity of He is extremely sensitive to variations in the atomic structure of grain boundaries. Some of the results obtained are compared with previously published data, generated using early developed interatomic potentials. In general, the results obtained support and extend most of conclusions on He properties near grain boundaries drawn already, yet several important discrepancies are also found.

Grain Boundary Structure of Ultrafine Grained Pure Copper Fabricated by Accumulative Roll Bonding

Grain boundary structures of ultrafine grained pure copper (Cu) fabricated by the accumulative roll bonding (ARB) have been studied. The atomic structures of grain boundaries in the ARB processed Cu (ARB-Cu) were observed by high resolution electron microscopy. In order to compare the grain boundaries in the ARB-Cu with equilibrium grain boundaries, the grain boundary energy and structure of symmetric tilt boundaries with 〈110〉 common axis in pure Cu were computed by molecular dynamics simulation (MD). The low angle boundaries in the ARB-Cu were basically described by conventional dislocation model and simultaneously there were some local structures having certainly high energy configurations. The grain boundaries with large misorientation in the ARB-Cu are basically described by the structural units predicted for the normal grain boundaries by MD. The present results indicate that the atomic structures of the boundaries in the Cu severely deformed by the ARB are rather similar to those of the equilibrium grain boundaries, except for the local distortions.

Dopant-segregation-controlled ZnO single-grain-boundary varistors

A ZnO single-grain-boundary varistor was designed using a bicrystal. Pr and Co dopant cosegregation at the boundary, key to obtain high varistic property, is optimized by controlling grain-boundary misorientation and, hence, grain-boundary atomic structure. Thus obtained single grain boundary exhibited sufficiently high varistic property. The present result opens up the possibility of single-grain-boundary varistors required for future nanoscale electronic devices.

Computer simulation of the interaction of vacancies with the special tilt grain boundaries

In this paper, a computer simulation of [1 0 0] tilt grain boundaries in Al is described. The atomic interaction was approximated by Morse’s empirical potential. The reconstruction of the atomic structure of grain boundaries (6Bs) the absorption and emission of vacancies was studied. The energetical spectrums of GB states were calculated. The possibility of grain boundary slip was analysed by the construction of the γ-surface. The mechanism of grain boundary slip along special GBs was suggested. Burgers vectors of the grain boundary dislocations were determined.

Molecular dynamics study on interfaces generated between atomic clusters

The atomic structure of grain boundaries generated in the process of coalescence between copper atom clusters is investigated using the molecular dynamics method. The effective medium theory is adopted as interatomic interaction for precise estimation of surface and defect energies. The effect of cluster size, velocity and crystalline orientation on the atomic behaviour are studied. When cluster size is relatively small, dislocations (detected as stacking faults) generated in the contact region develop throughout the cluster body. On the other hand, in larger-size clusters, stacking faults are multiple and they interact with each other inside clusters. The local atomic structure is effectively assessed by a topological method, by which atoms are recognized as a face-centred cubic (fcc) structure or a hexagonal close-packed (hcp) structure. Large approaching velocity results in a flattened shape of clusters and induces complicated propagation of stacking faults, where the critical speed for this transition is estimated at 600–700m/s approximately. When a slight orientation angle is applied to clusters, the difference in angle is cancelled by the spontaneous spin of clusters. However, when a large orientation mismatch is given to clusters, nucleation and growth of stacking faults are obstructed according to the Schmid factor of slip.

Atomic Resolution Transmission Electron Microscopy of the Intergranular Structure of a Y2O3‐Containing Silicon Nitride Ceramic

High‐resolution transmission electron microscopy (HRTEM) employing focus‐variation phase‐reconstruction methods is used to image the atomic structure of grain boundaries in a silicon nitride ceramic at subangstrom resolution. Complementary energy‐dispersive X‐ray emission spectroscopy experiments revealed the presence of yttrium ions segregated to the 0.5–0.7‐nm thin amorphous boundary layers that separate individual grains. Our objective here is probing if yttrium ions attach to the prismatic planes of the Si3N4 at the interface toward the amorphous layer, using Scherzer and phase‐reconstruction imaging, as well as image simulation. Crystal structure images of grain boundaries in thin sample (<100 Å) areas do not reveal the attachment of yttrium at these positions, although lattice images from thicker areas do suggest the presence of yttrium at these sites. It is concluded that most of the yttrium atoms are located in the amorphous phase and only a few atoms may attach to the terminating prism plane. In this case, the line concentrations of such yttrium in the latter location are estimated to be at most one yttrium atom every 17 Å.