Total Grain Boundary Area Research Articles

A synergistic effect of the ion beam sputtered NiO x hole transport layer and MXene doping on inverted perovskite solar cells

The synergistic effect of high-quality NiO x hole transport layers (HTLs) deposited by ion beam sputtering on ITO substrates and the Ti3C2T x MXene doping of CH3NH3PbI3 (MAPI) perovskite layers is investigated in order to improve the power conversion efficiency (PCE) of p-i-n perovskite solar cells (PSCs). The 18 nm thick NiO x layers are pinhole-free and exhibit large-scale homogeneous surface morphology as revealed by the atomic force microscopy (AFM). The grazing-incidence x-ray diffraction showed a 0.75% expansion of the face-centered cubic lattice, suggesting an excess of oxygen as is typical for non-stoichiometric NiO x . The HTLs were used to fabricate the PSCs with MXene-doped MAPI layers. A PSC with undoped MAPI layer served as a control. The size of MAPI polycrystalline grains increased from 430 ± 80 nm to 620 ± 190 nm on the doping, as revealed by AFM. The 0.15 wt% MXene doping showed a 14.3% enhancement in PCE as compared to the PSC with undoped MAPI. The energy-resolved electrochemical impedance spectroscopy revealed one order of magnitude higher density of defect states in the band gap of MXene-doped MAPI layer, which eliminated beneficial effect of reduced total area of larger MAPI grain boundaries, decreasing short-circuit current. The PCE improvement is attributed to a decrease of the work function from −5.26 eV to −5.32 eV on the MXene doping, which increased open-circuit voltage and fill factor.

Relation of embrittlement to phosphorus grain-boundary segregation for an advanced Ni–Cr–Mo RPV steel

Embrittlement is an essential issue for reactor pressure vessel (RPV) steels. Grain-boundary segregation (GBS) of impurity phosphorous is one of the major reasons for non-hardening embrittlement of an RPV steel. The main aim of the present work is to establish a relation of embrittlement to phosphorous GBS for an advanced SA508-4N Ni–Cr–Mo RPV steel so that one can forecast its embrittling tendency. To obtain this type of relation, the steel samples doped with phosphorous are quenched from 890 to 1100 °C to obtain different prior austenite grain sizes (PAGSs) and then tempered at 650 °C, followed by ageing at different lower temperatures so as to achieve different phosphorous segregation levels at prior austenite grain boundaries. The characterization work is done with the use of mini-Charpy impact tests (specimen: 2.5 mm × 2.5 mm in cross section) together with Auger electron spectroscopy, scanning electron microscopy, and optical microscopy. The results indicate that there is a linear relationship between ductile-to-brittle transition temperature (DBTT/oC) and phosphorous GBS concentration (Cp/at.%) for both cases (quenching from 890 °C: PAGS = 34 μm; quenching from 1100 °C: PAGS = 112 μm). The relations can be expressed as DBTT=13.13Cp−335.70 (PAGS = 34 μm) and DBTT=6.69Cp−223.87 (PAGS = 112 μm). The rate of material embrittlement, characterized by the slope of the line, is associated with both the phosphorous segregation-induced grain-boundary (GB) embrittlement and the total GB area per unit volume. Interestingly, there is a critical phosphorous GBS concentration, below which the DBTT of the coarse-grained specimen is higher due to its higher intrinsic brittleness and above which that of the fine-grained specimen is higher at the same phosphorous GBS level due to the considerable phosphorous segregation-induced GB embrittlement along with a much larger total GB area per unit volume.

Effect of preheating on the microstructure and properties of fiber laser welded girth joint of thin-walled nanostructured Mo alloy

Fiber laser girth welding of a thin-walled nanostructured Mo (NS-Mo) alloy tube was conducted. The microstructures, properties, and residual stresses of the welded girth joints achieved at different preheating temperatures were compared. Combining finite element simulation with experimental data, it was found that as the preheating temperature increased, the maximum welding residual tensile stress monotonically decreased while the tensile strength of the joints increased at first and then declined. At the preheating temperature of 673 K, the tensile strength reached a maximum, which was approximately 50% that of the base metal. The results showed that oxygen content was segregated at the grain boundary of fusion zone during welding and further produced Mo oxides. As the preheating temperature increased, vaporized Mo oxides might escaped from the molten pool, so the oxygen concentration at the grain boundary first decreased. However, when the preheating temperature increased over 773 K, the oxygen concentration rose due to the increased oxidation tendency of Mo at high temperatures and grains in the fusion zone coarsened greatly, while the total area of grain boundaries reduced.

Optical Properties of Multilayered Sol–Gel Zinc-Oxide Films

Study of structural, optical and photocatalytic properties of multilayered (1–8 layers) zinc oxide films deposited on glass substrates by sol-gel method showed, that after thermal treatment at 500°C they consist of random oriented hexagonal crystalline grains with size of 34–40 nm, forming larger particles with sizes of 100–150 nm, which do not depend on number of layers. With an increase in the number of layers, the intensity of exciton photoluminescence decreases by a factor of 10, the absorption of light in the visible and near IR ranges increases, and the efficiency of photocatalytic decomposition of the test organic dye rhodamine B increases by 10–12%. The observed changes are related to the increase in the total area of grain boundaries and to the change in the integral amount of oxygen vacancies and interstitial atoms as the number of layers increases, which makes it possible to control the properties of zinc oxide films for applications in optoelectronics, photovoltaics and photocatalysis.

Оптические свойства многослойных золь-гель пленок оксида цинка

AbstractStudy of structural, optical and photocatalytic properties of multilayered (1–8 layers) zinc oxide films deposited on glass substrates by sol-gel method showed, that after thermal treatment at 500°C they consist of random oriented hexagonal crystalline grains with size of 34–40 nm, forming larger particles with sizes of 100–150 nm, which do not depend on number of layers. With an increase in the number of layers, the intensity of exciton photoluminescence decreases by a factor of 10, the absorption of light in the visible and near IR ranges increases, and the efficiency of photocatalytic decomposition of the test organic dye rhodamine B increases by 10–12%. The observed changes are related to the increase in the total area of grain boundaries and to the change in the integral amount of oxygen vacancies and interstitial atoms as the number of layers increases, which makes it possible to control the properties of zinc oxide films for applications in optoelectronics, photovoltaics and photocatalysis.

Hydrogen Susceptibility of Nanostructured Bainitic Steels

Abstract Nanostructured steels with an ultimate tensile strength of 1.6 GPa were produced with austenite content varying from 0 to 35 vol pct. The effect on the mechanical properties was assessed after saturating the steel with hydrogen. Elongation was reduced to 2 to 5 pct and UTS to 65 to 70 pct of prior value. Thermal desorption measurements confirmed the higher solubility of hydrogen in the steel with higher austenite content. The level of hydrogen saturation was found to correlate to the total area of grain boundaries rather than to the volume fraction of retained austenite.

Prediction of the Grain-Microstructure Evolution Within a Friction Stir Welding (FSW) Joint via the Use of the Monte Carlo Simulation Method

A thermo-mechanical finite element analysis of the friction stir welding (FSW) process is carried out and the evolution of the material state (e.g., temperature, the extent of plastic deformation, etc.) monitored. Subsequently, the finite-element results are used as input to a Monte-Carlo simulation algorithm in order to predict the evolution of the grain microstructure within different weld zones, during the FSW process and the subsequent cooling of the material within the weld to room temperature. To help delineate different weld zones, (a) temperature and deformation fields during the welding process, and during the subsequent cooling, are monitored; and (b) competition between the grain growth (driven by the reduction in the total grain-boundary surface area) and dynamic-recrystallization grain refinement (driven by the replacement of highly deformed material with an effectively “dislocation-free” material) is simulated. The results obtained clearly revealed that different weld zones form as a result of different outcomes of the competition between the grain growth and grain refinement processes.

Structural Features of Nano-Scale Damascene Copper Lines after Annealing in Wide Temperature Range

Grain structure and texture evolved in damascene nano-scale copper lines after annealing in a wide temperature interval of 200oC-500oC were analyzed. Electron backscatter diffraction (EBSD) technique was used for rigorous investigation of the overburden layer, upper and bottom parts of the lines. In all cases, the microstructures were found to stabilize after achieving the same level of total grain-boundary area per unit volume. The grain growth behavior was supposed to be governed by the pinning effect of second-phase particles entrapped during the electrodeposition process.

Nucleation of recrystallization in fine-grained materials: an extension of the Bailey–Hirsch criterion

A new criterion for nucleation in the case of dynamic recrystallization is proposed in order to include the contribution of the grain boundary energy stored in the microstructure in the energy balance. Due to the nucleation events, the total surface area of pre-existing grain boundaries decreases, leading to a nucleus size smaller than expected by conventional nucleation criteria. The new model provides a better prediction of the nucleus size during recrystallization of pure copper compared with the conventional nucleation criterion.

An in situ experimental study of grain growth in a nanocrystalline Fe91Ni8Zr1 alloy

Grain growth and microstructural evolution of thermally stabilized Fe91Ni8Zr1 were investigated by in situ and ex situ studies. Our investigations suggest that the microstructural evolution is fairly slow and the microstructure shows stabilization up to about 700 °C. Above this temperature, a certain fraction of grains grow abnormally into the nanocrystalline matrix, resulting in a bimodal microstructure and causing the complete loss of thermal stability. The reason for abnormal grain growth and the loss of thermal stability is identified as the appearance of the fcc γ-phase and consequent reduction in the total area of grain boundaries and the overall stored energy.

Atom-by-Atom Observation of Grain Boundary Migration in Graphene

Grain boundary (GB) migration in polycrystalline solids is a materials science manifestation of survival of the fittest, with adjacent grains competing to add atoms to their outer surfaces at each other's expense. This process is thermodynamically favored when it lowers the total GB area in the sample, thereby reducing the excess free energy contributed by the boundaries. In this picture, a curved boundary is expected to migrate toward its center of curvature with a velocity proportional to the local radius of boundary curvature (R). Investigating the underlying mechanism of boundary migration in a 3D material, however, has been reserved for computer simulation or analytical theory, as capturing the dynamics of individual atoms in the core region of a GB is well beyond the spatial and temporal resolution limits of current characterization techniques. Here, we similarly overcome the conventional experimental limits by investigating a 2D material, polycrystalline graphene, in an aberration-corrected transmission electron microscope, exploiting the energy of the imaging electrons to stimulate individual bond rotations in the GB core region. The resulting morphological changes are followed in situ, atom-by-atom, revealing configurational fluctuations that take on a time-averaged preferential direction only in the presence of significant boundary curvature, as confirmed by Monte Carlo simulations. Remarkably, in the extreme case of a small graphene grain enclosed within a larger one, we follow its shrinkage to the point of complete disappearance.

SPH Analysis of ECAP Process by Using Grain Refinement Model

Plastic working of metals using severe plastic deformation (SPD) recently attracts interest of researchers, because it is a method that will improve the toughness as well as the strength just by applying enormously large strain on the material. In this study, deformation, stress and strain of aluminum in ECAP (equal channel angular pressing) are analyzed computationally by using smoothed particle hydrodynamics (SPH) method which is one of particle methods. In addition to elastic-plastic constitutive relation, a newly developed theoretical framework in which grain refinement process is involved is proposed here. We try to implement the framework into the SPH computation. The main idea to conduct grain refinement in the material is that applied energy by plastic working causes continuous change in the total area of grain boundaries and averaged diameter in the material becomes smaller and smaller. The reasonable change can be observed by using such simple but theoretical framework. The present paper is focused on implication, formulation and possibility of the grain refinement model.

Inhibition of void dissolution by vacancies

Dissolution of voids is a classical problem in the theory of sintering. The foundations of the theory were established by Frenkel, Pines and Geguzin (see Refs. 1 and 2 for a review). However, an important aspect of the problem appears to have been overlooked by previous workers. In a recent paper, the present authors identified a mechanism which inhibits processes associated with vacancy generation during grain growth, especially in fine grain sized materials [3]. The essence of the mechanism is the rise of the free energy of the system due to vacancy production in the course of defect ‘dissolution’ (decrease of the total area of grain boundaries) which under certain conditions may inhibit the rate of the primary process. A similar effect may also be operative in the case of void dissolution which is accompanied by a massive release of vacancies into the surrounding bulk of the material. In this paper we consider the conditions for intermittent temporary arrest of the void dissolution process leading to an effective decrease of the rate of dissolution.

Modelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials

The large relative change in total grain-boundary area that accompanies grain growth in a nanocrystalline material has a potentially strong influence on the kinetics of grain growth whenever grain-boundary migration is controlled by solute (impurity) drag. As the grain-boundary area decreases, the concentration of solute or impurity atoms segregated to the boundaries is expected to increase rapidly, introducing a grain-size dependence to the retarding force on boundary migration. We have modified the Burke equation—which assumes the drag force to be independent of the average grain size—to take into account a linear dependence of grain-boundary pinning on grain size. The form of the resulting grain-growth curve is surprisingly similar to Burke's solution; in fact, a constant rescaling of the boundary mobility parameter is sufficient to map one solution approximately onto the other. The activation energies for grain-boundary motion calculated from the temperature dependence of the mobility parameter are therefore identical for both models. This fact provides an explanation for the success of Burke's solution in fitting grain-growth data obtained in systems, such as nanocrystalline materials, for which the assumption of grain-size-independent solute drag is incorrect.

Effects of high temperature rapid thermal annealing using a flat gas flame on the electrical properties of phosphorus-doped polycrystalline silicon films

The effects of high temperature rapid thermal annealing (HT-RTA) using a flat gas flame on the electrical properties of phosphorus-doped polycrystalline silicon (poly-Si) films in association with their microscopic structure were studied. Samples with a phosphorus concentration of 3.1×1017–6.0×1020 cm−3 were prepared and annealed by HT-RTA ranging from 1150 to 1350 °C. During HT-RTA, the sample surface was laterally swept by gas flames. The resistivity of the samples decreased with increasing annealing temperature, and the lowest resistivity was 4.8×10−4 Ω cm for the sample doped with P of 6.0×1020 cm−3 when annealed at 1350 °C. Hall mobility, on the other hand, increased first and then decreased with increasing P concentration. The highest Hall mobility was 71.3 cm2/V s for the sample annealed at 1350 °C of which the P concentration was 3.5×1019 cm−3. The results suggest that the grain boundary potential barriers for carriers decreased with increasing doping concentration and annealing temperature, and that the total area of grain boundaries in the films decreased with increasing annealing temperature because of secondary grain growth and the shrinkage of boundary width.

Fe-7∼11 mass%Cr鋼粉末圧粉体の焼結緻密化特性

High chromium steel powder compacts are sintered in single-phase region of austenite (γ) or ferrite (δ) and two-phase region of ferrite/austenite (δ/γ) and the sintered compacts are subjected to the measurement of dimensional changes, in order to clarify the relationship between sintering shrinkage and matrix phase at sintering temperature.The specimens sintered in δ-phase shrink much larger than that sintered in γ-phase, owing to high self-diffusibility of δ-phase. For the specimens sintered in a two-phase region of (δ/γ), the shrinkage rate increases with increase in amount of δ-phase. The shrinkage rate of the two-phase region sintered specimens is, at the same time, calculated by the law of mixture using the data for the specimens sintered in the single-phase regions of γ and δ. The comparison of the calculated and experimental values suggests an additional effect of (δ/γ) two-phase sintering on enhancement of the sintering shrinkage. This additional effect is associated with the preferential precipitation of δ-grains from the matrix/pore interface and the change in total area and type of grain boundaries.

Effects of CdCl2 on the electrical properties of sintered CdS films

CdS films on glass substrates with various amounts of CdCl2 have been prepared by coating and sintering, and their microstructure, electrical resistivity, carrier concentration and mobility have been investigated. The sintering rate is strongly increased near the melting temperature of CdCl2 and the enhancement in the sintering is via a liquid-phase sintering mechanism. Doping of chlorine also occurs during the sintering and the solubility of chlorine in CdS is of the order of 1018 cm−3 at 650° C. The electron concentration of the sintered CdS films which contained CdCl2 before sintering increases with increasing grain size. The electron mobility increases sharply with increasing average grain size due to a sharp decrease in the total trap density, which is proportional to the total grain-boundary area.

The Effect of Grain Boundaries on the Low-Temperature Diffusion of Hydrogen in Decarburized Mild Steel

AbstractDiffusion coefficients, D, pertaining to hydrogen evolution from decarburized mild-steel specimens in two conditions of large and small grain size are reported. These were measured to investigate the possibility that grain boundaries alone are responsible for the anomalously low D values found below ∼ 200° C in non-steady-state diffusion work. However, there were no significant differences in the coefficients obtained over the temperature range 25–200° C for the two specimens. All the results lay close to values given in the literature for α-iron,which are all significantly below the values predicted by extrapolation from higher temperatures. Thus, the so-called ‘trapping phenomenon’ was present to the same extent in all the measurements, irrespective of a change in the total grain-boundary area of between one and two orders of magnitude.