High-angle Tilt Boundaries Research Articles

Improved Crystallinity of Graphene Grown on Cu/Ni (111) through Sequential Mobile Hot-Wire Heat Treatment

Over the past few years, many efforts have been devoted to growing single-crystal graphene due to its great potential in future applications. However, a number of issues remain for single-crystal graphene growth, such as control of nanoscale defects and the substrate-dependent nonuniformity of graphene quality. In this work, we demonstrate a possible route toward single-crystal graphene by combining aligned nucleation of graphene nanograins on Cu/Ni (111) and sequential heat treatment over pregrown graphene grains. By use of a mobile hot-wire CVD system, prealigned grains were stitched into one continuous film with up to ∼97% single-crystal domains, compared to graphene grown on polycrystalline Cu, which was predominantly high-angle tilt boundary (HATB) domains. The single-crystal-like graphene showed remarkably high thermal conductivity and carrier mobility of ∼1349 W/mK at 350 K and ∼33 600 (38 400) cm2 V-1 s-1 for electrons (holes), respectively, which indicates that the crystallinity is high due to suppression of HATB domains.

Significantly Enhanced Thermoelectric Performance of Graphene through Atomic-Scale Defect Engineering via Mobile Hot-Wire Chemical Vapor Deposition Systems.

Over the years, numerous studies have attempted to develop two-dimensional (2D) materials for improving both the applicability and performance of thermoelectric devices. Among the 2D materials, graphene is one of the promising candidates for thermoelectric materials owing to its extraordinary electrical properties, flexibility, and nontoxicity. However, graphene synthesized through traditional methods suffers from a low Seebeck coefficient and high thermal conductivity, resulting in an extremely low thermoelectric figure of merit (ZT). Here, we present an atomic-scale defect engineering strategy to improve the thermoelectric properties of graphene using embedded high-angle tilt boundary (HATB) domains in graphene films. These HATB domains serve as both energy filtering sites to filter out lower-energy charge carriers and scattering sites for phonons. Compared to the conventionally grown chemical vapor deposited graphene, the graphene with HATB domains shows an improved Seebeck coefficient (50.1 vs 21.1 μV K-1) and reduced thermal conductivity (382 vs 952 W m-1K-1), resulting in a ZT value that is ∼7 times greater at 350 K. This defect engineering strategy is promising not only for graphene-based materials but also for 2D materials, in general, where further research and optimization could overcome the limitations of conventional bulk thermoelectric materials in energy-harvesting systems.

Молекулярно-динамическое исследование влияния концентрации вакансий на скорость миграции границ наклона в никеле

The influence of vacancy concentration on the migration rate of high-angle tilt boundaries with misorientation axes <111> and <100> in nickel was studied by the method of molecular dynamics. It is shown that the dependence of the migration rate on the concentration of vacancies introduced at the initial stage of modeling has a maximum near 1%. The decrease in the migration rate with a further increase in the free volume is mainly due to the deceleration of the boundary by low-mobile vacancy clusters, which at high vacancy concentrations the boundary is no longer capable of sorbing. The second reason for the decrease in the migration rate with an increase in the concentration of vacancies above 1% is a decrease in the surface tension of grain boundaries and, accordingly, the driving force of their migration due to the finite sorption capacity of the boundaries with respect to the excess free volume.

Влияние концентрации вакансий на скорость миграции границ наклона в никеле: молекулярно-динамическое моделирование

The effect of vacancy concentration on the migration rate of high-angle tilt boundaries with misorientation axes <111> and <100> in nickel was studied by the method of molecular dynamics. It is shown that the dependence of the migration rate of grain boundaries on the vacancy concentration is nonmonotonic and has a maximum at a concentration of vacancies introduced at the initial stage of about 1%. With a further increase in concentration, especially above 4%, the migration rate of the considered boundaries decreased as a result of deceleration of boundaries by low-mobility vacancy clusters attached to it, which the boundary could no longer sorb, as in the case of relatively small clusters.

Determination of the Radius of Triple Junctions of Tilt Boundaries: A Molecular Dynamics Simulation

Using the method of molecular dynamics, an investigation of the free volume and energy distribution is performed in the region of triple junctions of high-angle tilt boundaries with the and misorientation axes in nickel formed as a result of recrystallization. Crystallization is simulated from three crystalline nuclei for the varied free volume value. It is shown that during recrystallization the free volume is mainly concentrated in the region of grain boundaries and its fraction increases on approaching the grain-boundary junction. From the effective energy distribution in the region of triple junctions, their effective radius is determined. In the tilt boundaries and their junctions, the free volume during recrystallization is dispersed more effectively that it is in the junctions formed by the boundaries. For this reason, the energy radius of the triple junction of the boundaries hardly varies with an introduction of a free volume: from 5 to 7 A, while that of the junction of the boundaries upon introduction of 2% vacancies into the initial computational cell becomes as large 14 A.

Effect of nucleation density on the crystallinity of graphene grown from mobile hot-wire-assisted CVD

Chemical vapor deposition (CVD) is widely used for producing high-quality graphene in large area. However, grain boundaries (GBs) are inevitably generated when isolated graphene domains, grown from numerous graphitic nuclei, unify, resulting in severe degradation of intrinsic material properties of graphene. Using mobile hot-wire-assisted CVD (MHW-CVD), here, we successfully demonstrated the progress toward single-crystal graphene; graphene grown from MHW-CVD on pure copper substrate generally has large density of high angle tilt boundary. The main strategy of the improvement is the composition control of catalytic substrate to copper-nickel alloy (4.25 at% of Ni), via both increasing the initial density of graphitic nuclei and facilitating the rotations of isolated graphene domains during the recrystallization process of 2D graphene. Compared to graphene grown from the pure copper substrate, the areal density of the single-crystal graphene from the alloy increases from 22% to 88%. The improvement of electrical property of graphene is also confirmed by the measurement of sheet resistance and mobility whose value is enhanced to 40% and 17%, respectively. Our findings will likely benefit facile CVD synthesis of large-area, high-quality graphene for future thermal, electronic or mechanical devices.

Dislocation-based description of the sliding of a free-surface emerging grain boundary

The sliding phenomenon of a high-angle tilt boundary containing a disclination dipole emerging at the free-surface of a thin film deposited on a semi-infinite substrate is studied theoretically from a static point of view in the framework of a dislocation-based description. The formation from the surface of a dislocation in the boundary is investigated, and its different equilibrium positions are determined in the stress field of the disclination dipole. It is found that the formation of the dislocation in the stress field of the disclination is only possible for an inclined dipole with respect to the normal of the free-surface. Finally, the effects of the inclination angle and strength of the grain boundary disclination dipole on the stable and metastable equilibrium positions of the dislocation are analyzed.

Grain boundary phases in bcc metals.

We report a computational discovery of novel grain boundary structures and multiple grain boundary phases in elemental body-centered cubic (bcc) metals represented by tungsten, tantalum and molybdenum. While grain boundary structures created by the γ-surface method as a union of two perfect half crystals have been studied extensively, it is known that the method has limitations and does not always predict the correct ground states. Herein, we use a newly developed computational tool, based on evolutionary algorithms, to perform a grand-canonical search of high-angle symmetric tilt and twist boundaries, and we find new ground states and multiple phases that cannot be described using the conventional structural unit model. We use molecular dynamics (MD) simulations to demonstrate that the new structures can coexist at finite temperature in a closed system, confirming that these are examples of different grain boundary phases. The new ground state is confirmed by first-principles calculations.

Mechanical properties and failure behavior of phosphorene with grain boundaries

Using the density-functional tight-binding method, we studied the effect of grain boundaries on the mechanical properties and failure behavior of phosphorene. We found that the high-angle tilt boundaries with a higher density of (5∣7) defect pairs (oriented along the armchair direction) are stronger than the low-angle tilt boundaries with a lower defect density, and similarly the high-angle boundaries with a higher density of (4∣8) defect pairs (oriented along the zigzag direction) are stronger than the low-angle boundaries with a lower defect density. The failure is due to the rupture of the most pre-strained bonds in the heptagons of the (5∣7) defect pair or octagons of the (4∣8) pairs. The high-angle grain boundaries are better at accommodating the pre-strained bonds in heptagon and octagon defects, leading to a higher failure stress and strain. The results cannot be described by a Griffith-type fracture mechanics criterion, since this does not take into account the bond pre-stretching. Interestingly, these anomalous mechanical and failure characteristics of tilt grain boundaries in phosphorene are also shared by graphene and hexagonal boron nitride, signifying that they may be universal for 2D materials. The findings revealed here may be useful in tuning the mechanical properties of phosphorene via defect engineering for specific applications.

Continuous description of a grain boundary in forsterite from atomic scale simulations: the role of disclinations

We present continuous modelling at inter-atomic scale of a high-angle symmetric tilt boundary in forsterite. The model is grounded in periodic arrays of dislocation and disclination dipoles built on information gathered from discrete atomistic configurations generated by molecular dynamics simulations. The displacement, distortion (strain and rotation), curvature, dislocation and disclination density fields are determined in the boundary area using finite difference and interpolation techniques between atomic sites. The distortion fields of the O, Si and Mg sub-lattices are detailed to compare their roles in the accommodation of lattice incompatibility along the boundary. It is shown that the strain and curvature fields associated with the dislocation and disclination fields in the ‘skeleton’ O and Si sub-lattices accommodate the tilt incompatibility, whereas the elastic strain and rotation fields of the Mg sub-lattice are essentially compatible and induce stresses balancing the incompatibility stresses in the overall equilibrium.

The roles of crystallographic orientation, high-angle grain boundary, and indenter diameter during nano-indentation

Molecular dynamics simulation of nano-indentation of single-crystal and bicrystal FCC aluminum is performed. The role of crystallographic orientation during nano-indentation of single-crystal aluminum is assessed. Then, the influence of the presence of a grain boundary is analyzed by adding high-angle symmetric tilt boundaries of Σ5 /(100) and Σ5 /(100) parallel to the surface on which the indentation is performed. Furthermore, in both cases, the size of the indenter is changed to investigate how the surface curvature of the indenter affects the nano-indentation process. The results suggest that in each crystallographic orientation, the presence of a grain boundary increases the required force for indentation, while the distance of a grain boundary from the indentation surface could affect the increase in the required force. Simulations prove that the grain boundary acts as a source of generation and emission of dislocations and restricts the penetration of the indenter by limiting the slip band formation and plastic deformation. The dislocation emission from the grain boundary restricts the penetration of the indenter and limits the formation of the octahedral slip systems of type {111} and consequently increases the required force for indentation in bicrystals.

MOLECULAR-DYNAMICS RESEARCH OF STRUCTURE OF TRIPLE JUNCTIONS OF TILT BOUNDARIES AND MIXED-TYPE GRAIN BOUNDARIES IN NICKEL

Atomic structure of triple junctions of high-angle tilt boundaries and and mixed-type grain boundaries in nickel were studied by method of molecular dynamics. It is shown that equilibrium triple junction has not got any structural features, which able to single it out as a special defect among its constituent grain boundaries. The effective radius of considered triple junctions and the width of the grain boundaries were determined.

Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle

Mantle flow involves large strains of polymineral aggregates. The strongly anisotropic plastic response of each individual grain in the aggregate results from the interactions between neighbouring grains and the continuity of material displacement across the grain boundaries. Orthorhombic olivine, which is the dominant mineral phase of the Earth's upper mantle, does not exhibit enough slip systems to accommodate a general deformation state by intracrystalline slip without inducing damage. Here we show that a more general description of the deformation process that includes the motion of rotational defects referred to as disclinations can solve the olivine deformation paradox. We use high-resolution electron backscattering diffraction (EBSD) maps of deformed olivine aggregates to resolve the disclinations. The disclinations are found to decorate grain boundaries in olivine samples deformed experimentally and in nature. We present a disclination-based model of a high-angle tilt boundary in olivine, which demonstrates that an applied shear induces grain-boundary migration through disclination motion. This new approach clarifies grain-boundary-mediated plasticity in polycrystalline aggregates. By providing the missing mechanism for describing plastic flow in olivine, this work will permit multiscale modelling of the rheology of the upper mantle, from the atomic scale to the scale of the flow.

Molecular dynamics investigation of the diffusion permeability of triple junctions of tilt and mixed-type boundaries in nickel

The diffusion permeability of triple junctions of high-angle tilt boundaries 〈111〉 and 〈100〉 and mixed-type boundaries in nickel has been investigated using the molecular dynamics method. It has been shown that the diffusion permeability of equilibrium triple junctions does not exceed the permeability of the grain boundaries forming them. The effective diffusion radius of the considered triple junctions and the width of the grain boundaries are determined.

Исследование влияния точечных дефектов на диффузионную проницаемость тройных стыков границ зерен в никеле

The effect of point defects on diffusion permeability of triple junctions of high-angle tilt boundaries <111> and <100> and mixed-type grain boundaries in nickel were studied by the method of molecular dynamics. It was shown that intensity of self-diffusion and bond energy of point defects with the triple junction practically independent of the number of defects introduced in the junction area. This is explained, in particular, that the part of the introduced defects migrates to grain boundaries which form the junction.

An elasto-plastic theory of dislocation and disclination fields

A linear theory of the elasto-plasticity of crystalline solids based on a continuous representation of crystal defects – dislocations and disclinations – is presented. The model accounts for the translational and rotational aspects of lattice incompatibility, respectively associated with the presence of dislocations and disclinations. The defects content relates to the incompatible plastic strain and curvature tensors. The stress state is described by using the conjugate variables to strain and curvature, i.e., the stress and couple-stress tensors. Defect motion is described by two transport equations. A dynamic interplay between dislocations and disclinations results from a disclination-induced source term in the transport of dislocations. Thermodynamic guidance provides the driving forces conjugate to dislocation and disclination velocity in a continuous context, as well as admissible constitutive relations for the latter. When dislocation and disclination velocity vanish, the model reduces to deWit’s elasto-static theory of crystal defects. It also reduces to Acharya’s linear elasto-plastic theory for dislocation fields when the disclination density is ignored. The theory is intended for use in instances where rotational defects matter, such as grain boundaries. To illustrate its applicability, a finite high-angle tilt boundary is modeled using a disclination dipole and its behavior under tensile loading normal to the boundary is shown.

Charles Crussard's early contributions: Recrystallization in situ and a Grain Boundary study with J. Friedel and B. Cullity

Abstract Charles Crussard's discovery of what he called Recrystallization in situ in 1944 is compared with what Egon Orowan and Robert Cahn named Polygonization. The context of the previous research is recalled in order to show the novelty of these contributions. Progress in accurate deformation and observation techniques such as tensile machines and X-ray methods were important, as were theoretical models about plausible microscopic mechanisms. With Jacques Friedel and Barney Cullity, Ch. Crussard also wrote a pioneering article on grain boundaries, which contains an experimental study of the relative energies of both symmetrical and asymmetrical grain boundaries observed in aluminium together with the first atomistic calculation of energies of symmetrical low and high angle tilt boundaries in several metals. This less well-known contribution is also recalled.

Faceting and critical current densities of [001] high-angle tilt boundaries in YBCO films

The [001] high-angle tilt boundaries in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films deposited on SrTiO/sub 3/ (STO) bicrystal substrates have been studied by Scanning Squid Microscopy (SSM) and transmission electron microscopy (TEM). Generally critical current density across a boundary decreases with increasing misorientation. However, misorientation angle is found to be insufficient to predict the J/sub c/ behavior of a YBCO grain boundary. The two inclination angles of the adjacent grains are found to be important parameters. In two separate cases of same misorientation but different inclinations, the boundaries exhibit drastically different superconducting behaviors and this has been observed by SSM imaging of Josephson vortices. The YBCO film boundary is not an exact copy of the design boundary of the substrate bicrystal template. YBCO boundaries meander due to island formation during film growth. These microscopically wavy boundaries contain straight segments of atomistic facets. Three energetic criteria that affect faceting have been identified. The particular criterion that a facet plane cannot be too deviated from the design boundary plane (/spl phi/<28/spl deg/) explains the strong effect of the design inclination angles on J/sub c/ despite extensive faceting. The original scattered values of J/sub c/ each calculated from individual fluxes of the same design boundary are likely to be a bona fide effect corresponding to individual facet having its own J/sub c/ and /spl lambda//sub j/ values.

Transport critical current density of Ag-sheathed Pr-doped (Tl,Cr)Sr2CaCu2O7 superconductor tapes in applied magnetic field

Powder-in-tube Tl-1212 (Tl,Cr)-Sr-(Ca,Pr)-Cu-O/Ag tapes were prepared and processed with different thermomechanical treatments. The processing conditions caused the variation of the transport critical current density (Jc) in the tapes. Tapes subjected to rolling showed lower Jc compared to the pressed tapes. Uniaxial pressing is more effective in densifying the tape cores by forcing the grains into closer contact and enhances the connectivity between grains. All the tapes showed a majority phase of 1212. The existence of 1201 phase considerably reduced Jc in the rolled tapes. Intermediate rolling is not favorable in improving Jc for the irregular grains structure, in contrast to Bi-2223 superconductors with plate-like grains. The in-field behavior of the tapes indicates that they are dominated by weak links as Jc falls rapidly in low applied field (H<0.06 T). No anisotropic transport properties were observed for all the tapes. SEM images showed randomly oriented irregular grains making texturing difficult to achieve. Neither the grain size nor the morphology was changed significantly by the thermomechanical treatments. The observed irregular grains imply that high-angle tilt boundaries were formed with intergranular weak links. Grain morphology is suggested to be the key factor that limits the Jc in these tapes.

Self-diffusion in high-angle fcc metal grain boundaries by molecular dynamics simulation

Abstract Recent molecular dynamics simulations of high-energy high-angle twist grain boundaries (GBs) in Si revealed a universal liquid-like high-temperature structure which, at lower temperatures, undergoes a reversible structural and dynamical transition from a confined liquid to a solid; low-energy boundaries, by contrast, were found to remain solid all the way up to the melting point. Here we demonstrate for the case of palladium that fcc metal GBs behave in much the same manner. Remarkably, at high temperatures the few representative high-energy high-angle (tilt or twist) boundaries examined here exhibit the same, rather low self-diffusion activation energy and an isotropic liquid-like diffusion mechanism that is independent of the boundary misorientation. These observations are in qualitative agreement with recent GB self- and impurity-diffusion experiments by Budke et al. on Cu. Our simulations demonstrate that the decrease in the activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure at low temperatures to a liquid-like structure at high temperatures. Consistent with the experiments, the transition temperature decreases with increasing GB energy, that is with increasing degree of short-range GB structural disorder. By contrast, the degree of long-range structural disorder in the zero-temperature GB appears to play no role in whether or not the GB undergoes such a transition at elevated temperatures.