Bulk Grain Boundary Research Articles

Polymorphism of Segmented Grain Boundaries in Two-Dimensional Transition Metal Dichalcogenides.

Grain boundaries (GBs) are vital to crystal materials and their applications. Although GBs in bulk and two-dimensional materials have been extensively studied, the segmented GBs observed in transition metal dichalcogenide monolayers by a sequence of folded segments remain a mystery. We visualize the large-area distribution of the segmented GBs in MoSe2 monolayers and unravel their structural origin using ab initio calculations combined with high-resolution atomic characterizations. Unlike normal GBs in two-dimensional materials with commonly one type of dislocation cores, the segmented GBs consist of two basic elements-4|8 and 4|4|8 cores, whose alloying results in structural diversity and distinctly high stability due to relieved stress fields nearby. The defective polygons can uniquely migrate along the segmented GBs via the movement of single molybdenum atoms, unobtrusively endowing a given GB with variable appearances. Furthermore, the segmented GBs can achieve useful functionalities such as intrinsic magnetism and highly active electrocatalysis.

An effective strategy to boost 3D perovskite light-emitting diode performance via solvent mixing strategy

In this study, we demonstrate a simple method to achieve highly efficient perovskite light-emitting diode (PeLED) via casting the neat methylammonium bromide (MABr) in mixed solvents with different solubility on top of the perovskite layer for bulk recrystallization and interfacial passivation. The perovskite crystals reacting with MABr can be reformed orderly and stereoscopically with a reduced domain size on surface and reduced grain boundaries in bulk, which is advantageous to obtain strong fluorescence intensity and long exciton lifetime. The remaining MABr molecule that does not react with perovskite can attach on the recrystallized perovskite surface to develop a passivation layer, which can effectively eliminate exciton interface recombination in PeLEDs. The optimized PeLED exhibits a maximum luminance of 27692.2 cd m−2, which is more than 11 times and 2 times higher than the control device without MABr treatment and the control device with high MABr molar ratio and thin perovskite film thickness, respectively. This work provides an effective strategy to boost PeLED performance by optimizing mixed solvent ratios and concentrations of organic ligand MABr without changing the thickness of the emitting layer.

Creep rupture strength of F12 steel welded joint after long-term service

Creep rupture strength of F12 steel welded joint which was used as the main steam pipe and serviced 165000 hours was investigated by using high temperature creep rupture testing. Microstructure analysis results show that the microstructure of F12 steel welded joint became embrittlement significantly after long-term service. The precipitates mainly consist of M23C6 carbide and Laves phase, and the amount of M23C6 phase is more, which precipitated along the grain boundaries in bulk or chainlike with larger size. At the same time, there are larger inclusions with directional distribution, in which these inclusions have poor bonding with substrate and easily become crack source during the process of high temperature creep. The high temperature enduring strength of welded joint of F12 steel decreased significantly, it is necessary to enhance the examination of the microstructure and life assessment of pipeline in the next overhaul period.

Nonequilibrium grain boundaries in bulk nanostructured metals and their recovery under the influences of heating and cyclic deformation. Review

Nonequilibrium grain boundaries in bulk nanostructured metals and their recovery under the influences of heating and cyclic deformation. Review

Nanoscale cracks at deformation twins stopped by grain boundaries in bulk and thin-film materials with nanocrystalline and ultrafine-grained structures

Nanoscale fracture processes initiated at intersections of deformation twins and grain boundaries (GBs) in nanocrystalline and ultrafine-grained materials are theoretically described. Within the suggested description scheme, nanocracks are formed at GBs at which deformation twins are stopped and thereby produce specific GB defect configurations that create high local stresses initiating nanocracks. The conditions for realization of this new fracture mode in nanocrystalline and ultrafine-grained materials with face-centred cubic and body-centred cubic crystal lattices are calculated. The free-surface effect on crack generation at deformation twins stopped by GBs in thin-film nanocrystalline specimens is theoretically described. The critical parameters for crack generation in nanocrystalline and ultrafine-grained bulk materials and thin films are revealed. It is shown that in bulk nanomaterials nanocracks are generated at twin thicknesses of several nanometres and grow very fast with increasing twin thickness. At the same time, in thin-film nanomaterials, the equilibrium nanocrack lengths significantly decrease if the distance between the twin lamella and the closest film surface does not exceed several twin thicknesses.

Solute concentrations and strains in nanograined materials

Taking advantage of both the Gibbs and McLean adsorption isotherms, we develop a Gibbs-approach-based adsorption isotherm for grain boundary (GB) segregation in nanograined (ng) polycrystals. An excess GB thickness is introduced to describe the excess of GB atomic volume in comparison with the atomic volume in lattice. The GB bulk modulus is determined with the excess GB thickness and a universal function. The newly developed adsorption isotherm is able to analyze simultaneously stresses, concentrations and their coupling behaviors in grains and GBs. Numerical calculations and plots are conducted to illustrate the theoretical analysis. The results show that the apparent solute concentration could be greatly enhanced in ng materials, due to a large grain boundary volume fraction and a considerable increase in the lattice concentration that is, in turn, boosted by the concentration-induced stresses.

Properties of grain boundaries in bulk, melt processed Y–Ba–Cu–O fabricated using bridge-shaped seeds

Single grain RE–Ba–Cu–O ((RE)BCO, where RE is a rare earth element or yttrium) bulk superconducting materials have significant potential for a variety of engineering applications due to their ability to trap high magnetic fields. However, it is well known that the presence of grain boundaries coupled with a high angle of misorientation (typically 5°) significantly reduces the critical current density, Jc, in all forms of high temperature superconducting materials. It is of considerable fundamental and technological interest, therefore, to investigate the grain boundary properties of bulk, film and tape (RE)BCO. We report a successful multi-seeding technique for the fabrication of fully aligned, artificial (0° misalignment) grain boundaries within large grain YBCO bulk superconductors using bridge-shaped seeds. The microstructure and critical current densities of the grain boundaries produced by this technique have been studied in detail.

Adsorption of alkali metals and their effect on electronic properties of grain boundaries in bulk of polycrystalline silicon

The adsorption of alkali metals and their effect on the electronic properties of grain boundaries in bulk of polycrystalline silicon has been studied experimentally. The results obtained show that the potential barrier grows during diffusion and adsorption of alkali metal atoms along grain boundaries.

Grain boundary transport properties in YBa/sub 2/Cu/sub 3/O/sub x/ coated conductors

Critical current data obtained as a function of magnetic field on an isolated grain boundary (GB) of a coated conductor and two other types of bicrystal GBs of YBa/sub 2/Cu/sub 3/O/sub x/ show a peak in the critical current and an unusual hysteresis. These results provide support for a new mechanism for enhanced GB critical currents, arising from interactions of GB vortices with pinned Abrikosov vortices in the banks of a GB, as suggested by Gurevich and Cooley (1994). A substantial fraction of this enhancement, which can exceed a factor of ten, also occurs upon surpassing the critical current of the grains after zero field cooling. A bulk GB and thin film GBs show qualitatively identical results.

Grain-boundary dissipation in high-T c superconductors

Thin-film and bulk [001] tilt bicrystal grain boundaries (GBs) in YBa 2Cu 3O 7 exhibit a strong dependence of critical current density, J c, on misorientation angle. What was initially difficult to understand was the 30x smaller J c in bulk GBs which are microscopically more perfect. We review an explanation of this zero-field data, which is based on the pinning of Josephson vortices by the meandering found in thin-film GBs. In addition, there is evidence that J c of GBs does not drop as quickly with applied magnetic field as expected by simple Josephson junction models. The long-wavelength pinning potential due to meandering is less effective at high fields, but Gurevich and Cooley (GC) proposed a new mechanism for an enhanced GB J c arising from pinned Abrikosov vortices in the banks of a GB which present a static, quasiperiodic pinning potential to pin GB vortices. We fine a peak in J c and an unusual hysteresis which give considerable support to the GC concept. In low fields, the GBs exhibit a larger J c for field cooling, which is opposite to the usual hysteresis but agrees with GC due to the larger Abrikosov vortex density in the banks. Magnetization data on the same sample are consistent including the identification of the irreversibility field.

Analyses of the Grain Boundary Misorientation and Oxygen Content of Bulk Processed YBa2Cu3O7-δ

ABSTRACTThe grain boundary misorientation distribution of 203 grain boundaries in bulk processed high Tc superconductor YBa2Cu3O7-δ with five processing conditions, was studied. Two complementary analytical approaches, Grain Boundary Misorientation Distribution (GBMD) from the random description, using a hypothesis test and X2 analysis, and Grain Boundary Character Distribution (GBCD), using the Coincidence Site Lattice (CSL) model, were applied. The GBMD and GBCD both showed grain boundary evolution departing from a random distribution above 935°C processing temperature. The GBCD analyses indicated an approximately linear increase in the population of CSL-related boundaries, among which the tetragonal CSL (c/a ≠ 3) boundaries grew in the same trend while orthorombic boundaries (c/a = 3) became stagnated. The results from comparing the corresponding GBCD and volume averaged Jc for each batch indicated that the tetragonal CSL boundaries were oxygen deficient and accounted for, among other current limiting factors, lower current carrying ability.

Determination of the local oxygen stoichiometry in YBa 2Cu 3O 7−δ by electron energy loss spectroscopy in the scanning transmission electron microscope

By using parallel detection electron energy loss spectroscopy (PEELS) in the scanning transmission electron microscope (STEM), the oxygen K-absorption edge spectrum is used to determine δ in YBa 2Cu 3O 7−δ to an accuracy of < 5%. The instability of YBa 2Cu 3O 7−δ to the electron beam enables oxygen reduction experiments to be performed in situ in the microscope. Results show a “two plateau” behaviour similar to that observed in transition temperature versus oxygen stoichiometry measurements, and can be interpreted in terms of oxygen vacancy ordering and charge distribution between the CuO chains and CuO 2 planes. In addition, the sub-nanometer probe of the STEM enables changes in oxygen deficiency to be observed with high spatial resolution. The high spatial resolution has been used to observe fluctuations in oxygen stoichiometry in the region of grain boundaries in bulk sintered samples.

Evidence for local composition variations within YBa2Cu3O7−δ grain boundaries

The composition variations associated with grain boundaries in bulk sintered YBa2Cu3O7−δ have been investigated by means of energy-dispersive x-ray microanalysis in a high spatial resolution scanning transmission electron microscope. A series of composition profiles indicates that considerable variations in composition occur on a very local scale both parallel and perpendicular to these grain boundaries. The regular, oscillatory (period ∼25 nm) nature of the composition variations observed along a grain boundary suggests that these heterogeneities may be an intrinsic feature of the boundary. Some possible implications of these observations for percolative conduction mechanisms are discussed briefly.

モリブデンに対するビーム振動を用いた電子ビーム溶接法の適用

The purpose of this study is to investigate the effects of macrostructure change of the weld joint on the ductility of electron-beam-welded molybdenum sheet. The material used was electron-beam-melted molybdenum. Beam oscillation was applied in order to change the unfavourable structure of the joint, particularly even flat and straight interfaces at the center of the weld metal or at the bond of weld. The ductility of the weldment was determined by tensile tests at -70°C and 20°C. Fracture mode and location of crack initiation and propagation were examined by a scanning electron microscopy.The results were summarized as follows:1) Fracture strength at -70°C of the weldment was increased by using beam oscillation.2) Strength and elongation at 20°C of the weldment were also increased by using beam oscillation. 3) Origin of fracture was located to be mainly at the grain boundaries on the bead surface at -70°C, whilst crack was initiated mainly from the grain boundaries in bulk at 20°C.4) Crack was propagated only along the center of the weld metal or the bond of weld without beam oscillation. On the other hand, the crack was propagated not only along these weak interfaces but also through other areas with beam oscillation.