Mobility Of Junctions Research Articles

In Situ Crystal Growth and Fusing-Confined Engineering of Quasi-Monocrystalline Perovskite Thick Junctions for X-ray Detection and Imaging.

Metal halide perovskites exhibit great promise for utilization in X-ray detection owing to their excellent optoelectronic properties and high X-ray attenuation capabilities. However, fabricating large-area thick films for high-performance perovskite X-ray detection remains challenging. This study develops an in situ crystal growth and fusing-confined approach to prepare high-quality, large-scale perovskite quasi-monocrystalline thick junctions. The perovskite crystals are grown in situ using a highly concentrated perovskite colloidal solution in 2-methoxyethanol. Introducing methylammonium chloride enhances grain reorganization during in situ growth and fusing-confined processes, effectively reducing grain boundaries and surface defects. This allows for the preparation of quasi-monocrystalline thick junctions of large grains (>100 μm) with high crystallinity, uniform orientation, and vertical penetration across the film thickness. Additionally, the carrier mobility and lifetime of the thick junctions are significantly enhanced. The optimized MAPbI3 detectors demonstrate an X-ray sensitivity of 2.6 × 104 μC Gyair-1 cm-2 and an exceptionally low detection limit of 1 nGyair s-1. Furthermore, inspired by a honeycomb structure, these detectors realize X-ray imaging in 64 × 64 pixels through a pixelated separation design, effectively reducing the charge-sharing effect. This study offers valuable insights into the preparation of large-scale perovskite quasi-monocrystalline thick junctions for highly sensitive X-ray detection and imaging applications.

A stochastic model of grain boundary dynamics: A Fokker–Planck perspective

Many technologically useful materials are polycrystals composed of small monocrystalline grains that are separated by grain boundaries of crystallites with different lattice orientations. The energetics and connectivities of the grain boundaries play an essential role in defining the effective properties of materials across multiple scales. In this paper we derive a Fokker–Planck model for the evolution of the planar grain boundary network. The proposed model considers anisotropic grain boundary energy which depends on lattice misorientation and takes into account mobility of the triple junctions, as well as independent dynamics of the misorientations. We establish long time asymptotics of the Fokker–Planck solution, namely the joint probability density function of misorientations and triple junctions, and closely related the marginal probability density of misorientations. Moreover, for an equilibrium configuration of a boundary network, we derive explicit local algebraic relations, a generalized Herring Condition formula, as well as formula that connects grain boundary energy density with the geometry of the grain boundaries that share a triple junction. Although the stochastic model neglects the explicit interactions and correlations among triple junctions, the considered specific form of the noise, under the fluctuation–dissipation assumption, provides partial information about evolution of a grain boundary network, and is consistent with presented results of extensive grain growth simulations.

Dual phase patterning during a congruent grain boundary phase transition in elemental copper

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemental copper by atomic resolution imaging. The same grain boundary phases are found by computational grain boundary structure search indicating a first-order transformation. Finite temperature atomistic simulations reveal a congruent, diffusionless transition between these phases under ambient pressure. The patterning of the grain boundary at room temperature is dominated by the grain boundary phase junctions separating the phase segments. Our analysis suggests that the reduced mobility of the phase junctions at low temperatures kinetically limits the transformation, but repulsive elastic interactions between them and disconnections could additionally stabilize the pattern formation.

Influence of Finite Mobilities of Triple Junctions on the Grain Morphology and Kinetics of Grain Growth

Grain boundary networks composed of equal microstructural elements were investigated in a recent paper. In this work a more complicated artificial grain topology consisting of one four-sided, two six-sided and one eight-sided grain is designed to further investigate the influence of grain boundary and triple junction mobilities on the kinetics of the system in more detail. Depending on the value of the equal mobility of all triple junctions, the initially square-shaped four-sided grain changes its shape to become more or less rectangular. This indicates that the grain morphology is influenced by the value of the mobility of the triple junctions. It is also demonstrated that a grain arrangement with low mobility triple junctions controlling the kinetics of grain growth enhances growth of the large eight-sided grains. In addition, grain growth is investigated for different values of mobilities of triple junctions and grain boundaries. A strong elongation of several grains is predicted by the modeling results for reduced mobilities of the microstructural grain boundary elements. The two-dimensional modeling results are compared to micrographs of a heat-treated titanium niobium microalloyed steel. This feature, namely the evolution of elongated grains, is observed in the micrograph due to the pinning effect of (Ti, Nb)C precipitates at elevated soaking temperatures of around 1100 °C. Furthermore, the experiments show that a broader distribution of the grain sizes occur at 1100 °C compared to soaking temperatures, where pinning due to precipitates plays a less prominent role. A widening of the distribution of the grain sizes for small triple junction mobilities is also predicted by the unit cell model.

Исследование влияния избыточного свободного объема на подвижность тройных стыков границ зерен

Исследование влияния избыточного свободного объема на подвижность тройных стыков границ зерен

Supporting data for the characterization of PNA-DNA four-way junctions.

Holliday or DNA four-way junctions (4WJs) are cruciform/bent structures composed of four DNA duplexes. 4WJs are key intermediates in homologous genetic recombination and double-strand break repair. To investigate 4WJs in vitro, junctions are assembled using four asymmetric DNA strands. The presence of four asymmetric strands about the junction branch point eliminates branch migration, and effectively immobilizes the resulting 4WJ. The purpose of these experiments is to show that immobile 4WJs composed of DNA and peptide nucleic acids (PNAs) can be distinguished from contaminating labile nucleic acid structures. These data compare the electrophoretic mobility of hybrid PNA–DNA junctions vs. i) a classic immobile DNA 4WJ, J1 and ii) contaminating nucleic acid structures.

Control of microstructure in the manufacturing of aluminum foil

Experimental results from studying of grain growth in two-dimensional aluminum foils are correlated with results from the generalized theory of two-dimensional grain growth, which considers the limited mobility of triple junctions.

Local mobility of micelles of new cationic surfactants and their interactions with hydrophobically modified polyacrylamides

The local dynamics and organization of micelles of new long-chain cationic surfactants with saturated hydrocarbon fragments (from C16 to C22) are investigated via the EPR spin-probe technique. The local mobility of spin probes in the hydrocarbon core of a micelle changes insignificantly, while the order parameter noticeably increases with lengthening of the hydrocarbon fragment of the surfactant molecule. The specific features of the interaction of the surfactants with network junctions of the gels formed by two types of hydrophobically modified polyacrylamides—either containing charged groups (sodium acrylate) in the backbone or lacking these groups—are studied. In both cases, the local mobility of network junctions of the gel increases after the introduction of the surfactant (C18). Moreover, for surfactant with a long alkyl group (C2), the microscopic viscosity of the gel based on the uncharged polymer decreases, although the local mobility of the network junctions increases. Possible causes of the observed specific features are discussed.

Effect of a Finite Boundary Junction Mobility on the Growth Rate of Two and Three Dimensional Grains

t has been shown by computer simulations that the MacPherson-Srolovitz relation predicts accurately the growth rate of a grain undergoing ideal grain growth. However, since a finite mobility of the boundary junctions (triple lines and quadruple junctions) affects the evolution of a granular system, it is necessary to modify this equation in order to take into account their effect. In the present contribution, an equation which allows considering these factors is presented and used to modify the von Neumann-Mullins and MacPherson-Srolovitz equations. In order to corroborate these equations two and three dimensional network model simulations were performed. The results showed a very good agreement with the theoretical approaches for both dimensions and all topological classes except those near the classes of zero growth rate in 3D. The reason is that the proposed function is very sensitive to small changes of the finite mobility of the junctions.

Understanding Grain Boundary Junctions: Effect of the Grain Size on Microstructure Evolution

In a previous work [ we introduced the geometry of a granular system that allowed the study of the effect of a finite mobility of the quadruple and triple junctions on grain boundary migration. One of the most important conclusions of this work was that the triple junctions drag more effectively the motion of the grain boundaries than the quadruple junctions. Nevertheless, this conclusion was drawn without consideration of the grain size. For this reason, this conclusion might be contradictory with our understanding of the grain boundary junctions because while the effect of the triple lines is inverse linear with the grain size that of the quadruple junctions is proportional to the inverse square of the grain size and thus, quadruple junctions are expected to drag more effectively, at least, for very small grain sizes. In the present investigation, we studied comprehensively the effect of grain size on the evolution of the granular system under the assumption of a finite mobility of the boundary junctions. For this purpose, several network model simulations were carried out for different grain sizes ranging from nanoto micrometers using a fully periodic grain arrangement. The results seem to corroborate that the triple junctions drag more effectively the motion of the grain boundaries, however, for very low junction mobility and grain sizes the effect appears to be indistinguishable. It was also observed that for very low quadruple junction mobility the geometry of the granular system undergoes a severe transformation which results in the unfulfillment of the equation derived in [.

Effect of finite boundary junction mobility on the growth rate of grains in 3D polycrystals

With decreasing grain size, grain boundary junctions become increasingly important for microstructure evolution. We show that the effect of a limited mobility of triple junctions on the growth rate of polycrystals can be implemented in theories of three-dimensional (3D) grain growth. Respective analytical relations are derived on the basis of the average n-hedra approach introduced by Glicksman to describe the volume rate of change of 3D grains in a polycrystalline aggregate under the impact of a limited triple junction mobility. The theoretical predictions were compared to network-model computer simulations, and good agreement was obtained.

2D vertex modeling for the simulation of grain growth and related phenomena

A vertex model for the simulation of grain growth and grain boundary migration is developed and its implementation is explained in detail. The utilization of the model is also exemplified with different setups. In particular, the model was used to study the evolution of magnetically affected grain growth in samples with randomly oriented grains and the effect of the finite mobility of the boundary junctions on boundary migration. These examples emphasize the versatility of the model and illustrate its ample scope. The results of the simulations showed that a magnetic field can effectively affect the evolution of the grain growth kinetics and texture independently of the initial texture since the magnetic field will cause grains, with particular orientations, to have an advantage for their growth. As consequence, the sample will become strongly textured during grain growth. Theoretical considerations for the grain boundary migration with a finite mobility of the triple junctions were also reproduced and confirmed by means of computer simulation. The simulation results showed a very good agreement with the theoretical expectations.

Local mobility and rheological characteristics of gels based on hydrophobically modified poly(acrylamides)

For hydrophobically modified poly(acryl amide), we analyze the effect of various parameters of the macromolecular structure (number and length of side hydrophobic groups, content of charged groups, type of bonding between side chains and the polymer backbone, and the degree of blocking of hydrophobic groups distributed along the chain) on the local mobility of physical network junctions and rheological characteristics of gels. We have found that the local mobility measured by the method of spin-probe EPR spectroscopy is either independent or it slightly depends on the above parameters. At the same time, these parameters exert a strong effect on the rheological characteristics of gels. This disagreement can be explained by the fact that local mobility of junctions is primarily controlled by the intermolecular interactions of hydrophobic groups and by the covalent bonding between these groups and a macromolecule. However, the rheological characteristics depend on the number of junctions, their dimensions, and other parameters.

Impact of grain boundary junctions on grain growth in polycrystals with different grain sizes

Effect of finite mobility of triple and quadruple grain boundary junctions on grain growth is studied by numerical simulation. They retard the growth process and transform its kinetics from parabolic one into a sequence consisting of exponential, linear and parabolic steps. This relates even to polycrystals with large initial grain sizes. Such junctions can contribute to microstructure stabilization in nanomaterials.

EPR spin probe study of polymer associative systems

Molecular dynamics of polyacrilamide gels, polymeric micelles and hydrogel of polyacrylic acid and macrodiisocyanate was investigated by the ESR spectroscopy of spin probes. The local mobility in network junction of polyacrylamide gels is found to be essentially slower than that in the micelles created by the low molecular weight detergents and does not depend on the amount and length of hydrophobic groups (C9 or C12) in the polymer chain. The immersion of 10–30 mol.% of ionic monomers into the polymer chain (sodium acrylate) influences insufficiently on the local mobility of network junctions. In aqueous solutions, polystyrene-block-poly-( N-ethyl-4-vinylpyridinium bromide) block copolymers create polymeric micelles. The local mobility in the polystyrene core of the micelles is about twice as much as that in the solid polystyrene. Partially swellable polymer network in aqueous solutions was synthesized from polyacrylic acid and macrodiisocyanate. The local mobility in hydrophobic regions of the gel is substantially lower than that in the hydrophilic regions. It was concluded that the hydrophobic and hydrophilic regions and the local dynamics of them dictate practical application of the polymer associative systems.

Relation between grain growth and grain-boundary diffusion in a pure material by molecular dynamics simulations

Traditionally, due to the impurities inevitably present in real materials that limit the grain-boundary (GB) mobility, the processes of grain growth and GB diffusion are thought to involve similar activation barriers. However, recent molecular dynamics (MD) simulations of GB migration in bicrystals have suggested that, in pure materials, GB diffusion and GB migration involve distinct mechanisms and, hence, different activation barriers. Here we report MD simulations of grain growth in an impurity-free model nanocrystalline palladium microstructure containing only high-energy GBs. By contrast with the bicrystal simulations, we observe virtually identical activation energies for grain growth and GB diffusion. We discuss several mechanisms that might be responsible for this difference between the bicrystal and polycrystal results, including accommodation of the curvature-driven GB migration by the elimination of GB area and GB excess free volume and a possible finite mobility of the triple junctions. The qualitative agreement of our observations with the general experimental findings is remarkable given the absence of any impurities in our model system. This suggests that a fundamental, intrinsic process may be responsible for the similar activation energies for GB migration and GB diffusion in polycrystals. In our model system this process seems to involve the accommodation of the curvature-driven GB migration by GB diffusion to eliminate the GB area and related GB free volume at the triple junctions.

Grain boundary junction engineering

Abstract Under certain circumstances grain growth in polycrystals is controlled by the mobility of grain boundary junctions. The resulting microstructure is distinctly different from the granular assembly in the course of normal grain growth. Also, the structure established under junction control is rather stable even under the conditions characteristic for grain growth governed by grain boundary motion. This provides a means of controlling the grain microstructure evolution, in particular of ultrafine grained and nanocrystalline materials. It is demonstrated that such an effect can be expected not only for 2D arrangements but for 3D microstructures with quadruple junctions as well. The latter statement is supported by an assessment of the mobility of quadruple junctions. We propose to introduce a new branch of grain boundary engineering, namely grain boundary junction engineering.

Grain microstructure evolution and grain boundary junction engineering

The grain microstructure evolution in the course of two dimensional (2D) grain growth is considered in greater detail, taking into account the influence of grain boundary triple junctions. It is shown that there are two limiting regimes of grain growth in polycrystals: the first one is associated with the situation when the kinetics of grain growth are controlled by the motion of grain boundaries, while the second one is defined by the motion of grain boundary triple junctions, i.e. when the mobility of triple junctions determines the kinetics of grain growth. A generalised theory of 2D grain growth including a limited triple junction mobility is presented. The theoretical predictions are compared with results of computer simulations by a virtual vertex model. We introduce a new branch of grain boundary engineering, namely, grain boundary junction engineering that utilises junction properties for microstructure control.

A novel concept to determine the mobility of grain boundary quadruple junctions

Abstract We put forward a new concept which makes it possible to study experimentally the mobility of a grain boundary quadruple point.

Triple junction motion and grain microstructure evolution

The classical concepts of grain growth in polycrystals are based on the dominant role of grain boundaries. This is reflected by the well known von Neumann–Mullins relation. According to this approach triple junctions do not affect grain boundary motion, and their role in grain growth is reduced to maintaining the thermodynamically prescribed equilibrium angles at the lines where boundaries meet. In the current study the experimental data of triple junction mobility are considered with respect to the process of grain growth in 2D systems, in particular with regard to the controlling kinetics. When boundary kinetics prevails grain growth in a polycrystal complies with the von Neumann–Mullins relation. When grain growth is governed by the mobility of triple junctions the kinetics change, and the von Neumann–Mullins relation does not hold anymore. This is the more pronounced the smaller the triple junction mobility. We present a generalized theory of 2D grain growth including a limited triple junction mobility. In this concept the criterion Λ plays a central role. It reflects the ratio of boundary to triple junction mobility but is proportional to the grain size as well. The generalized von Neumann–Mullins relation can be expressed in terms of Λ. For small values of Λ, conspicuous changes of microstructure evolution during grain growth and of microstructural stability are predicted. The theoretical predictions are compared to results of computer simulations by a virtual vertex model.