Boundary Core Structure Research Articles

Electrostatic fields control grain boundary structure in SrTiO3

Functional properties of oxide ceramics are often controlled by the addition of dopant elements and the resulting alteration of oxygen vacancy concentrations within grain boundary core structures. A challenge in designing nanoscale ceramic microstructures is forming stable grain boundary networks, while minimizing unwanted impurity concentrations. In this study, it was discovered that the application of electrostatic fields during diffusion bonding of undoped SrTiO3 bicrystals leads to modifications of grain boundary core structures while misorientation angles remained unchanged. The applied electric field not only changes atomic and electronic interface structures, but also causes modifications of ensuing dielectric properties by altering local oxygen vacancy concentrations. The observations for this model system demonstrate the potential to control and modify the microscopic degrees of freedom of grain boundaries in the absence of dopant elements. Field-assisted modifications of grain boundary networks may become a disruptive technology in designing oxide microstructures for a wide range of applications.

An applied electrostatic field changed grain boundary structures in a ceramic

By forming ceramic bicrystals, researchers eliminated other parameters to show how an applied electrostatic field changed grain boundary core structures in SrTiO3.

Understanding luminescence properties of grain boundaries in GaN thin films and their atomistic origin

We report our findings on the optical properties of grain boundaries in GaN films grown on graphene layers and discuss their atomistic origin. We combine electron backscatter diffraction with cathodoluminescence to directly correlate the structural defects with their optical properties, enabling the high-precision local luminescence measurement of the grain boundaries in GaN films. To further understand the atomistic origin of the luminescence properties, we carefully probed atomic core structures of the grain boundaries by exploiting aberration-corrected scanning transmission electron microscopy. The atomic core structures of grain boundaries show different ordering behaviors compared with those observed previously in threading dislocations. Energetics of the grain boundary core structures and their correlation with electronic structures were studied by first principles calculation.

Effect of grain boundary character on segregation-induced structural transitions

Segregation-induced structural transitions in metallic grain boundaries are studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system, with a specific emphasis on understanding the effect of grain boundary character. With increasing global composition, the six grain boundary types chosen for this study first form ordered complexions, with the local segregation pattern depending on the grain boundary core structure, then transform into disordered complexions when the grain boundary composition reaches a critical value that is temperature dependent. The tendency for this transition to a disordered interfacial structure consistently depends on the relative solute excess, instead of the grain boundary energy or misorientation angle. Grain boundaries with high relative solute excess go through gradual disordering transitions, whereas those with low relative solute excess remain ordered until high global Zr concentrations but then abruptly transform into thick disordered films. The results presented here provide a clear picture of the effect of interface character on both dopant segregation patterns and disordered intergranular film formation, showing that all grain boundaries are not equal when discussing complexion transitions.

Direct observation of Σ7 domain boundary core structure in magnetic skyrmion lattice.

Skyrmions are topologically protected nanoscale magnetic spin entities in helical magnets. They behave like particles and tend to form hexagonal close-packed lattices, like atoms, as their stable structure. Domain boundaries in skyrmion lattices are considered to be important as they affect the dynamic properties of magnetic skyrmions. However, little is known about the fine structure of such skyrmion domain boundaries. We use differential phase contrast scanning transmission electron microscopy to directly visualize skyrmion domain boundaries in FeGe1-x Si x induced by the influence of an "edge" of a crystal grain. Similar to hexagonal close-packed atomic lattices, we find the formation of skyrmion "Σ7" domain boundary, whose orientation relationship is predicted by the coincidence site lattice theory to be geometrically stable. On the contrary, the skyrmion domain boundary core structure shows a very different structure relaxation mode. Individual skyrmions can flexibly change their size and shape to accommodate local coordination changes and free volumes formed at the domain boundary cores. Although atomic rearrangement is a common structural relaxation mode in crystalline grain boundaries, skyrmions show very unique and thus different responses to such local lattice disorders.

Self-diffusion near symmetrical tilt grain boundaries in UO 2 matrix: A molecular dynamics simulation study

Molecular dynamics simulations have been carried out to study the influence of grain boundaries in stoichiometric UO 2 on uranium and oxygen self-diffusions over a large range of temperature varying from 300 K to 2100 K. The study was carried out on two symmetrical tilt grain boundaries, Σ5 and Σ41, which have respectively two different atomic structures. Firstly, the study of the temperature effect on the grain boundary core structure is presented. With the raise of temperature, the grain boundary core grows with an increase of disorder. Secondly, self-diffusion near both grain boundaries is studied. It has been found that grain boundaries accelerate the uranium and oxygen self-diffusion rates over several nanometres from the grain boundary interface. Uranium and oxygen self-diffusion are anisotropic, with a high acceleration along the grain boundary interface. Using the self-Van Hove correlation functions, hopping mechanisms were identified for Σ41 in all directions while for Σ5 hopping mechanism takes place along the grain boundary interface and random diffusion appears in the perpendicular direction of the grain boundary plane.

High Temperature Deformation Behavior of [0001] Symmetrical Tilt Σ7 and Σ21 Grain Boundaries in Alumina Bicrystals

High temperature deformation behavior of [0001] symmetrical tilt grain boundaries of Al2O3 was investigated by using bicrystals. Four kinds of the grain boundaries Σ7/{4510}, Σ21/{4510}, Σ7/{2310} and Σ21/{2310} were selected in the present study, and compressive mechanical tests were performed at 1450°C in air to investigate the sliding behavior of the respective boundaries. It was found that, stresses readily increased with increasing strains for all specimens during compression tests, but the bicrystals showed abrupt sliding along the grain boundary planes to fracture. Among the boundaries studied, Σ7/{4510} exhibited the highest resistance to the grain boundary sliding. The other boundaries showed similar sliding resistance, and yet the stress and strain values at their failure were much smaller than those of Σ7/{4510}. In order to understand the mechanism of the sliding behavior of the respective boundaries, the grain boundary core structures and their atomic densities were examined, based on the structure models obtained in our previous studies. It was found that Σ7/{4510} having the highest sliding resistance showed a larger atomic density at the core, as compared to the others. Therefore, the observed sliding resistance of the boundaries is closely related to the detailed atomic structures at the grain boundary cores.

Non-Stoichiometry at Tilt Grain Boundaries in SrTiO3

Abstract The origin of electrically active grain boundaries in perovskite oxides and related materials remains controversial. The stoichiometry of the grain boundary core structure and the role of oxygen vacancies are the key issues involved. Previous results have given no indications of any non-stoichiometry at SrTiO3 grain boundaries, despite the fact that SrTiO3 is the system where atomic column EELS resolution has been demonstrated [1], and where atomic resolution images of the core structures have been obtained by Z-contrast imaging [2,3]. Here we show EELS spectra from individual dislocation cores in an 8° [100] tilt grain boundary in SrTiO3 that show significant oxygen depletion. The low angle grain boundary with well-separated dislocation cores makes it possible to study individual cores with high energy resolution in EELS. Previous work either averaged over all structural units in the grain boundary, sacrificing spatial resolution [2,3], or, used a very low probe current to obtain the highest spatial resolution [1].

Electronic Properties of the Grain Boundary Core in Polycrystals of Cubic Metals

Local densities of conduction s-electrons in the grain boundary (GB) core of transition and noble metals were determined using two methods. The difference between isomer shifts of components of emission nuclear gamma resonance (NGR) spectra due to 57 Co atomic probes occupying states in the lattice sites and the grain boundary core was corrected for the change in the volume of these 57 Co atomic probes in the given states. The corrected values of the difference between isomer shifts pointed to a low density of s-electrons at the substitutional and the interstitials sites of the grain boundary core structure. The magnetic ordering temperature of nearly ferromagnetic Pd and Pt matrices was proportional to the density of conduction electrons. Different Curie temperatures in the lattice and the grain boundary core of these matrices also suggested a lower (compared to the corresponding lattice sites) density of electrons at interstitial sites of the grain boundary core structure in Pd and Pt.

Equilibrium Sites Occupied in the Grain Boundary Core During Intercrystalline Diffusion of <sup>57</sup>Co in Metal Polycrystals

Only two types of states are occupied in metal polycrystals during intercrystalline diffusion of atomic probes. The states-1 are localized in the grain boundary core of metal polycrystals. The states-2 are localized in lattice regions adjacent to grain boundaries. In 4d and 5d metals 57 Co atomic probes represent small-radius impurities and occupy interstitial sites in the grain boundary core of these metals. In 3d metals 57 Co atomic probes are no longer small-radius impurities and therefore occupy only the substitutional sites in the grain boundary core structure.

Atomic-Scale Grain Boundary Relaxation Modes in Metals and Ceramics

Abstract: The atomic-scale structure of tilt grain boundaries has been studied by high-resolution electron microscopy (HREM) in several ceramics and metals such as NiO, yttria stabilized zirconia, Au, and Al. It is found that when grain boundaries are formed between two crystals a considerable variety of modes of relaxations, i.e., the rearrangements of atoms in and near the grain boundary core are possible, depending on grain boundary geometry and the interatomic interactions. The atomic relaxations within and near the grain boundary core often include relaxations that involve the formation of stacking faults in low-stacking-fault energy fcc materials. Grain boundary dissociations are also observed for several grain boundary geometries. Computer simulations of grain boundary structures in metals have in general been able to reproduce the relaxation features at least on a qualitative level. In contrast, ceramic grain boundaries typically are more complex and not always tractable by simple computer relaxation procedures, since the formation of point defects and partial occupancy of atomic columns may be involved in the relaxation processes. It is found that when a boundary is not allowed to relax to its lowest energy state due to geometrical constraints, a multitude of grain boundary core structures can exist. Such conditions are expected to be characteristic for nanocrystalline materials. Because of its close connection to grain boundary properties, quantification of the volume expansion by HREM techniques is an important goal of grain boundary research. Image simulations of grain boundaries suggest that for most materials of interest the 3-fold astigmatism must be corrected to better than 100 nm to achieve the desired accuracies.

Conductivity enhancement of caf 2 by grain boundary activation with lewis acids

CaF 2 fine powder and films were treated with the Lewis acids SbF 5 and BF 3 to create a strong driving force to draw F − ions out of the boundary regions according to L (surface) + F F (boundary) → LF − (surface) + V F (boundary) ( L = SbF 5 or BF 3). The conductivity results of polycrystalline ceramics treated in this way indicate a significant enhancement of the ionic conductivity with a slope that is characteristic for F − vacancy migration. The impedance spectra of the CaF 2 films showed two semi-circles of the impedance spectra after the acid adsorption. Judging from the microstructure, the high frequency arc is attributed to bulk conductivity plus interfacial contributions being in parallel with the bulk pathway and the low frequency semi-circle is characteristic of the blocking effect due the grain boundary core structure and the gas adsorbates therein being perpendicular to the current flow. A quantitative estimation of the enhanced conductivity assuming space-charge effects is compatible with the enhanced conductivity results.

Finite temperature structure and properties of ∑ = 5 (310) tilt grain boundaries in nacl a molecular dynamics study

Abstract Molecular dynamics simulations at constant temperature and stress were employed to study structure and excess properties of [Sgrave] = 5 (310) tilt grain boundaries in NaCl from low temperatures up to bulk melting. Several metastable grain boundary core structures were found. The differences in free energies of these structures, especially at high temperatures, were found to be small. Indeed two of these different core structures related by small displacement shift complete vectors were observed to coexist at high temperatures due to their small free energy differences. The temperature at which the boundary becomes mobile can be very different for different core structures. In all cases, grain boundary migration is coupled with grain boundary sliding. Melting of the system always starts from the grain boundaries at a temperature of approximately 0·98 Tm, where Tm is the bulk melting temperature. Excess enthalpies and volumes are almost constant as a function of temperature and diverge close to the bulk melting temperature. Grain boundary stress was also calculated and its anisotropy was found to decrease as a function of temperature. Differences between the calculated structures and those in NiO deduced from high-resolution electron microscopy studies are discussed.

A study of grain boundary translational states in a Σ3[ [formula omitted]10]/( [formula omitted]1) bicrystal

High-resolution electron microscopy images of a Σ3[ 1 10]/( 11 1) bicrystal produced by heavy cold rolling and annealing show a multiplicity of grain boundary core structures. Two grain boundary states are separated by a partial grain boundary dislocation with Burgers vector b = 1 2 b 1 (where b 1 = 1 6 [112] is the Burgers vector of a secondary dislocation). This implies a change in translational state. All other structures are related through continuous transport of atoms from one grain to the other resulting in [ 11 1] grain boundary steps. Calculations of the energy associated with the observed structures indicate that the two perfect twin states t = 0 and 2 b 1 are stable, while all other grain boundary translational states are metastable.

Comments on ‘A model of diffusion-induced grain boundary migration’ by T. K. Chaki

Abstract Chaki (1988) has recently proposed a model for diffusion-induced grain boundary migration (DIGM). In his Letter he concluded that the interface energy of a grain boundary and the free energy of mixing are responsible for DIGM and, from the equations he derived, it was claimed that they can explain many DIGM experimental results. However, on examining his model closely there appear to be some fundamental difficulties. The following are comments on his Letter: (1) In Chaki's calculation of ΔGs he assumed that, after interface 1 had moved a distance δx, the radius of curvature of this interface increased from R to R + δx, which gives a surface energy drop of 2γVm δx/R2. However from many DIGM observations (for example Balluffi and Cahn (1981)) the curvature of a migrating boundary is increased rather then decreased, that is, the surface free- energy term actually prevents a grain boundary from migrating rather than helping it! (2) Chaki considered ΔGcryst during the migration of interface 2. It is also necessary to consider AGcrys, during the migration of interface 1, since within the migration distance δx the structure is changed from a crystalline structure to a grain boundary core structure. ΔGm should also be considered during the migration of interface 2 since the concentration of the area swept by interface 2 will not be the same as that of a grain boundary core.