Computational study of the formation of aluminum-graphene nanocrystallites

The molecular dynamics method is used to study the formation of the Al/graphene nanocomposite in the structural grains of different size under the action of internal stresses. The behavior of graphene sheets inside an individual structural grain as well as in the process of two Al grains containing graphene are joined is investigated. The motion of graphene films, starting from the middle of the aluminum matrix, ends with their location at the crystallite boundaries. Graphene moves in the Al matrix along closely packed planes. In this case, graphene sheets acquire curvature. An intergrowth of graphene sheets is also observed. A contact between two Al-C nanocrystallites through a graphene interlayer is created. The self-diffusion coefficients of atoms and the partial potential energies increased with decreasing nanocrystallite size. The angular distribution of the nearest geometric neighbors and the distribution of distances to the nearest neighbors are determined using the construction of Voronoi polyhedra.     

Ordering and grain growth of FePt thin films by annealing

Microstructure variation of FePt thin film upon annealing at elevated temperatures was investigated by transmission electron microscopy (TEM). A special shape aperture was employed to observe the ordered L1₀ phase in the dark-field TEM images. With increasing the annealing temperature, crystal grains formed clusters with gathering of neighboring grains, and crystal grain growth proceeded within the cluster. L1₀ ordered crystal grains were preferentially formed near the grain boundaries, and their sizes grew with increasing the annealing temperature.     

Influence of copper crystal surface on the CVD growth of large area monolayer graphene

We study the influence of the surface structure of copper single crystals on the growth of large area monolayer graphene by chemical vapor deposition (CVD) in ultra-high vacuum (UHV). Using atomic-resolution scanning tunneling microscopy (STM), we find that graphene grows primarily in registry with the underlying copper lattice for both Cu(111) and Cu(100). The graphene has a hexagonal superstructure on Cu(111) with a significant electronic component,whereas it has a linear superstructure on Cu(100). Graphene on Cu(111) forms a microscopically uniform sheet, the quality of which is determined by the presence of grain boundaries where graphene grains with different orientations meet. Graphene grown on Cu(100) under similar conditions does not form a uniform sheet and instead displays exposed nanoscale edges. Our results indicate the importance of the copper crystal structure on the microstructure of graphene films produced by CVD.     

Molecular Dynamics Study of the Formation of Solid Al–C Nanocomposites

Migration of graphene fragments along the aluminum matrix in the solid phase was studied by molecular dynamics. The structure of the Al–C nanocomposite grain was studied by statistical geometry. The distributions of the topological and metric characteristics of truncated polyhedra were calculated for the Al subsystem; the distributions for the polyhedra constructed at the centers of mass of the hypothetical geometrical neighbors were calculated for the carbon subsystem. The graphene fragments are concentrated at the structural grain boundaries. The nanocomposite grains are preferably separated by single-layer graphene rather than by the two-layer graphene membrane.     

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH₄ flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.     

Grain boundaries in the refined solidification structure of undercooled Ni75Pd25

When a single-phase alloy solidifies in a low-undercooling range or above a critical undercooling, grain-refined structures are obtained. Taking Ni₇₅Pd₂₅ alloy as an example, the microscopic orientation of the refined grains was investigated by electron backscattered diffraction technology. It is revealed that the refined grains at low undercooling are completely randomly distributed. In the refined structure at high undercooling, certain grain boundaries with misorientation angles less than 5° can be observed, while most of the grain boundaries have large misorientation angles. The inverse pole figure indicates that the refined grains at high undercooling have a textured crystallographic orientation. The fact that twins exist in the refined structure at high undercooling and the dendritic substructure arms exhibit no misorientation supports such an argument that the grain refinement at high undercooling results from the recrystallization of the solidification dendrites.     

Grain growth and phason-strain field in quasicrystalline Al-Li-Cu

We report on grain growth and related structure change in single phased Al-Li-Cu quasicrystals. The icosahedral phase grains have been investigated using scanning ion microscopy and transmission electron microscopy. Regular boundaries between large grains have been observed both before and after high temperature annealing. The electron diffraction study shows that the grain growth is accompanied by a reduction of the phason-strains. The orientation relation between grains sets the 2-fold icosahedral axes parallel, and the coincidence of the planes depends on the phason strain-field. The effect of phason-strain field on these boundaries is discussed. It is proposed that the phason strain elimination can play a role in the grain growth. Received 1 February 1999 and Received in final form 12 May 1999     

The energy and structure of (0 0 1) twist grain boundary in noble metals

The relaxed energy and structure of (0 0 1) twist grain boundary (GB) in noble metals Au, Ag and Cu are simulated by the MAEAM. In-boundary translation between two adjacent grains results in a periodic energy variation and the period is a square with the side length L_Σ/Σ. The lowest energy appears when the two grains are translated relatively to either corner or center of the periodic square. The relaxed GB energy increases smoothly for low-angle boundaries and levels off for larger-angle boundaries except a cusp appeared at θ = 36.87° (Σ = 5). After relaxation, the symmetry of the GB structure is not changed but the displacement of the atoms parallel to the GB plane decreases with increasing the distance of the atoms from the GB plane.     

Atomic structure of grain boundaries in YBa2Cu3O7-δ as observed by Z-contrast imaging

Determination of the atomic structure of grain boundaries is the key to fundamental understanding of the critical current density in polycrystalline superconductors. High-resolution images with incoherent characteristics, obtained using a high-angle annular detector on an atomic resolution scanning transmission electron microscope, are used to study the atomic arrangements of these technologically important boundaries. The incoherent Z-contrast images do not experience contrast reversals with defocus or sample thickness and display no Fresnel Fringe effects at boundaries. Observed rigid shifts of atomic columns at grain boundaries are independent of sample thickness and objective lens defocus. These characteristics allow unambiguous and intuitive interpretations of the generated images. We find the atomic structures at grain boundaries in YBa₂Cu₃O_7-δ are strongly influenced by the strong tendency of this compound to exist only as complete unit cells terminated at {001} and {100} planes. The weak-link behavior associated with high-angle grain boundaries may follow from this structure in which there is no clear connection between the {100} facets of adjacent grains. Symmetric grain boundaries where adjacent grains share a common boundary plane have also been observed in YBa₂Cu₃O_7-δ. In these boundaries partial structural coupling of the grains is maintained. There is evidence that these two boundary forms produce junctions with very different superconducting properties.     

Band topology and the quantum spin Hall effect in bilayer graphene

We consider bilayer graphene in the presence of spin-orbit coupling, in order to assess its behavior as a topological insulator. The first Chern number n for the energy bands of single-layer graphene and that for the energy bands of bilayer graphene are computed and compared. It is shown that for a given valley and spin, n for a Bernal-stacked bilayer is doubled with respect to that for the monolayer. This implies that this form of bilayer graphene will have twice as many edge states as single-layer graphene, which we confirm with numerical calculations and analytically in the case of an armchair terminated surface. Bernal-stacked bilayer graphene is a weak topological insulator, whose surface spectrum is susceptible to gap opening under spin-mixing perturbations. We assess the stability of the associated topological bulk state of bilayer graphene under various perturbations. In contrast, we show that AA-stacked bilayer graphene is not a topological insulator unless the spin-orbit coupling is bigger than the interlayer hopping. Finally, we consider an intermediate situation in which only one of the two layers has spin-orbit coupling, and find that although individual valleys have non-trivial Chern numbers for the case of Bernal stacking, the spectrum as a whole is not gapped, so the system is not a topological insulator.     

Dominant influence of the compression effect of a magnetic flux in the intergranular medium of a granular high-temperature superconductor on dissipation processes in an external magnetic field

Experiments have been presented that demonstrate the effect of the compression of a magnetic flux in grain boundaries of a granular high-temperature superconductor in an external magnetic field on the dissipation processes. The compression of the magnetic flux is associated with the diamagnetic behavior of superconducting grains and the existence of a Josephson medium in grain boundaries. Under these conditions, grain boundaries are in an effective magnetic field that depends on the magnetic state (magnetization) of the superconducting grains. Based on the analysis of experimental data (dependences of the electrical resistance R and magnetization on the magnetic field H and temperature T, as well as current-voltage characteristics), the conclusion has been drawn that it is the temperature evolution of the effective magnetic field in the intergranular medium which primarily determines the behavior of the dependences R(T) in weak external magnetic fields of no more than ～10³ Oe. This should be taken into account in the interpretation of experiments on the magnetoresistance effect in granular high-temperature superconductors in terms of different theories. The conclusion drawn here also implies a significant correction of the previously obtained results.     

Multiscale model of deformed polycrystals. Hall-Petch problem

The paper puts forward a multiscale model of deformed polycrystals according to which the basis for self-consistent deformation of grains is rotational wave flows of planar structural transformations at their boundaries. Computer-aided engineering of grain boundaries reveals two types of rotational wave flows defined by the misorientation angle of adjacent grains. Grain boundary flows of the first type develop at low-angle boundaries and feature low curvature. These flows generate dislocations in the grain bulk and the Hall-Petch equation for them has the form σ=σ₀+kd^?1/2. Grain boundary flows of the second type develop at high-angle boundaries and feature high curvature. These flows generate curvature bands in near-boundary zones and inject them into the grain bulk, resulting in fragmentation of grains and breakdown of translation invariance. For such self-consistency of grains in a polycrystal, the Hall-Petch equation has the form σ=σ₀+kd^?1. Experimental data in support of the proposed multiscale model are presented.     

The microstructure of hydrogen- and deuterium-doped nanocrystalline palladium studied by small-angle neutron scattering

By means of small-angle neutron scattering the microstructure of two nanocrystalline Pd samples (prepared by inert gas condensation) has been studied at room temperature in a Q-range from [0pt] to [0pt] . An additional subsequent doping of the two samples with H as well as with D (concentrations < 4 at%) caused contrast variations that provided more detailed structural information. The measured scattering intensity was modeled by a Porod contribution from large heterogenities (e.g. pores) and a contribution from spherical grains with a log-normal distribution of their radii. To account for the presence of grain boundaries, the grains were considered to be surrounded by a shell with a reduced Pd density and a thickness half as large as the thickness of the grain boundaries. For the above model, the data of the H-doped, D-doped and undoped sample were simultaneously fitted with one single set of adjustable parameters. The fits yielded for the two samples volume-weighted mean grain radii of 10 nm and 13 nm. The values for the grain boundary thickness lie between 0.2 and 0.8 nm. Almost all of the H- and D-atoms are, at low hydrogen concentrations, located in the grain boundaries. Received 1 May 2000     

Magneto-transport and ferromagnetic resonance studies of polycrystalline La0.6Pb0.4MnO3 thin films

To understand the nature of grain boundaries in polycrystalline materials, magneto-transport and ferromagnetic resonance measurement have been performed in polycrystalline La_0.6Pb_0.4MnO₃ (LPMO) thin films prepared by pulsed laser deposition. Films are found to undergo a semiconductor to metal transition at 230 K and re-enter into the semiconducting state below 130 K. Microwave absorption measurements carried out as function of applied field show two components of resonant absorption signal. First component is in accordance with ferromagnetic transition of grains at Curie temperature and the second component shows antiferromagnetic transition of grain boundaries at 160 K. An additional non-resonant absorption signal centered at zero field has also been observed that supports transition from conducting to insulating grain boundaries at ∼160 K. Further, temperature dependence of resistance in semiconducting state at low temperatures is in accordance with coulomb blockade model indicating insulating nature of AFM grain boundaries.     

Finite temperature effect on mechanical properties of graphene sheets with various grain boundaries

The mechanical properties of graphene sheets with various grain boundaries are studied by molecular dynamics method at finite temperatures.The finite temperature reduces the ultimate strengths of the graphenes with different types of grain boundaries.More interestingly,at high temperatures,the ultimate strengths of the graphene with the zigzagorientation grain boundaries at low tilt angles exhibit different behaviors from those at lower temperatures,which is determined by inner initial stress in grain boundaries.The results indicate that the finite temperature,especially the high one,has a significant effect on the ultimate strength of graphene with grain boundaries,which gives a more in-depth understanding of their mechanical properties and could be useful for potential graphene applications.     

Dependence of grain boundary character distribution on the initial grain size of 304 austenitic stainless steel

Abstract

In order to study the dependence of the grain boundary character distributions (GBCD) on the grain size, annealing treatment was carried out on 304 austenitic stainless steel with different initial grain sizes. The evolution of the GBCD was analysed by electron backscatter diffraction. The experimental results showed that abnormal grain growth (AGG) occurred when grain size was small. With a smaller initial grain size, the number density of abnormally large grains and the fraction of low-Σ CSL boundaries increased but the size of abnormally large grains decreased and the random boundaries presented a continuous network. With a larger initial grain size, the fraction of low-Σ CSL boundaries also increased as well as the size of abnormally large grains but the number density of abnormally large grains decreased and the connectivity of random boundary network was disrupted by low-Σ CSL boundaries, especially Σ3ⁿ (n = 1, 2, 3) boundaries. However, with a very large initial grain size, normal grain growth (NGG) occurred, which had no effect on the fraction of low-Σ CSL boundaries and the connectivity of random boundary network.     

A study of Sm-substituted SrM magnets sintered using hydrothermally synthesised powders

Anisotropic SrM magnets with Sm substitution, which is observed to have the largest beneficial effect both on the coercivity and on the inhibition of grain growth at high temperature among the other elements such as La, Nd and Pr, were investigated. The average grain size of the samples decreases with increasing Sm/Sr ratio. All the magnets with Sm additions exhibit a bigger coercivity and remanence than those of the SrM magnet without Sm and the coercivity of the magnets increases with increasing Sm/Sr ratio. EDX quantitative analysis suggests that the solubility of Sm³⁺ in the SrM-type structure is very small and that the Sm³⁺ preferably goes into SrFeO_3−x, which is probably located around the SrM grain boundaries. The coercivity mechanism of the magnets is nucleation controlled. The formation and the distribution of the SrFeO_3−x phase around the SrM grain boundraies probably provides the inhibition of SrM grain growth, the reduction of the reverse domain nucleation at the grain surface and the isolation of the SrM grains. All these factors would contribute to the improvements of the coercivity of the magnets with Sm additions.     

Fracture of polycrystalline graphene membranes by in situ nanoindentation in a scanning electron microscope

Failure of polycrystalline graphene grown by chemical vapor deposition was investigated by nanoindentation in a scanning electron microscope. Circular graphene membranes were subject to central point loads using a nanomanipulator combined with an atomic force microscope cantilever as a force sensor. The grain boundaries of the polycrystalline graphene were visualized by Raman spectroscopy coupled with a carbon isotope labeling technique. Graphene membranes without any grain boundary had a failure strength of 45.4 ± 10.4 GPa, compared to 16.4 ± 5.1 GPa for those with grain boundaries when a Young's modulus was assumed to be 1 TPa. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Re-exchange of Fe and Cu at the interface in sintered Nd–Fe–B magnets: A method to eliminate Fe precipitation at grain boundaries

According to the decoupling hypothesis for magnetic grains, the coercivity in sintered Nd–Fe–B magnets is increased after Cu doping, which is due to the formation of non-magnetic grain boundaries. However, this method partially fails, and ferromagnetic Fe-segregation occurs at the grain boundary. We discovered both experimentally and through calculation that the Fe content at the grain boundaries can be tuned across a wide range by introducing another element of Ag. Segregated Fe at high temperature at the grain boundary re-dissolves into Nd₂Fe₁₄B grains during annealing at low temperature. Both configurable and magnetic entropies contribute a large driving force for the formation of nonmagnetic grain boundaries. Almost zero Fe content could be achieved at the grain boundaries of sintered Nd–Fe–B magnet.