Polycrystalline Grain Boundaries Research Articles

Investigation of the effects of graphene on Al0.3CoCrFeNi high entropy alloy/3graphene composite materials based on different crystal structures

The influence of graphene layers and crystal structure types on the nanoindentation reinforcement mechanism of Al0.3CoCrFeNi high-entropy alloy/graphene (HEA/Gp) composite materials was studied using molecular dynamics (MD) simulations. The results indicate that the influence of graphene layers on composite materials is related to the position of the indenter. Before the indenter contacts the graphene layer, some dislocations are absorbed by the graphene, unable to form dislocation cells and dislocation locks to hinder the deformation of the matrix, resulting in the hardness of the composite material being lower than that of the HEA matrix. After the indentation contacts the graphene layer, dislocations can penetrate the graphene to form a high-density dislocation cell. Due to graphene's excellent deformation resistance, the composite material's load-bearing capacity is much greater than that of the HEA matrix. In addition, stress propagates along the grain boundaries in polycrystalline and twinned polycrystalline structures. Grain boundaries decrease the material strength, while twinned boundaries can refine grain size and enhance grain stability. Additionally, composite materials' elastic recovery increases first and then decreases with the increase in graphene layers and twinning. The hardness of the material shows an anomalous phenomenon about grain size following a Hall-Petch relationship. When the angle between graphene layers is 0, the stress distribution inside the material is most uniform.

Reduced thermal conductivity and improved ZT of Cu-doped SnS-based bulk thermoelectric materials via compositing SnS nano-fiber strategy

Tin sulfide (SnS), as a low-cost and eco-friendly thermoelectric (TE) material, exhibits relatively poor phonon and charge transport capacities when the long-range ordered layered crystal structure is broken by the grain boundaries of polycrystalline. Nevertheless, doping and compositing are effective strategies for improving the electrical and thermal conductivity. Herein, utilizing the composite powders synthesized via Cu-doped SnS nanosheets grown in-situ on the surface of Cu-doped SnS nanofibers, we design a homogeneous nanofibers composited Cu-doped SnS polycrystalline bulk to boost its TE performance. Comparing with pure phase SnS, Cu doping can effectively increase the carrier concentration and mobility of SnS, realizing an increase of electrical conductivity from 4.11 S·cm−1 to 31.57 S·cm−1. Simultaneously, quite a number of fiber/matrix composite interfaces for phonon scattering are formed due to incorporating of composite nanofibers, which significantly reduce the lattice thermal conductivity of Cu-doped SnS polycrystalline bulk. The results show that thermal conductivity reduced as low as 0.64 W·m−1 K−1 at 324 K when the additive amount of nanofibers was fix at 2 wt%. Ultimately, the power factor and ZT value are markedly enhanced at low and medium temperature ranges (300–500 K) compared with SnS polycrystalline bulk without fiber incorporation. This provides an effective strategy for optimizing the thermoelectric performance of SnS-based polycrystalline.

Analysis of grain-boundary segregation of hydrogen in bcc-Fe polycrystals via a nano-polycrystalline grain-boundary model

Hydrogen embrittlement caused by hydrogen segregation at grain boundaries (GBs) is the most serious issue in the development of high-strength steels, but the mechanisms behind this process are still not well understood. The GB segregation behavior of hydrogen in body-centered cubic (bcc)-Fe polycrystals was comprehensively analyzed based on the interatomic potentials derived from first-principles calculations. Considering that the atomic structure of GBs is almost independent of the grain size, the GBs in polycrystals were modeled as nano-polycrystalline GBs with random orientations. The segregation energies of hydrogen for ∼17 million interstitial sites in this GB model were calculated. From these segregation energies, the effective segregation energy for the polycrystalline GB at thermal equilibrium under various temperature and hydrogen content conditions were determined, and the validity of the calculation method was verified by comparing the results with experimental data. The relationship between the segregation energy of hydrogen at each segregation site and the surrounding local atomic environment was used to identify the major hydrogen segregation sites at the atomic level, and the changes in the crystal structure near the GB that dominated segregation were clarified. The effective segregation energies at the polycrystalline GBs were in the range of −0.48 to −0.42 eV, which are in good agreement with the experimentally reported binding energy of hydrogen at GBs of bcc-Fe polycrystals (−0.52 eV). The major hydrogen segregation sites were octahedral sites with Voronoi volumes larger than 7.0 Å3, and the segregation energy was mainly due to the uniaxially distorted crystal structure in the short-axis direction of octahedral sites. Our findings and the developed calculation method contribute to the understanding of the hydrogen segregation behavior and hydrogen embrittlement mechanism in polycrystalline metallic materials.

Small grains as recombination hot spots in perovskite solar cells

Summary Non-radiative recombination in the perovskite bulk and at its interfaces prohibits the photovoltaic performance from reaching the Shockley-Queisser limit. While interfacial recombination has been widely discussed and demonstrated, bulk recombination and especially the influence of grain boundaries remain under debate. Most studies explore the role of grain boundaries on perovskite films rather than devices, making it difficult to link the film properties with those of the devices. Here, we systematically investigate the effects of grain boundaries on the performance of perovskite solar cells by two different methods. By combining experimental characterization with theoretical device simulations, we find that the recombination at grain boundaries is diffusion limited and hence is inversely proportional to the grain area to the power of 3/2. Consequently, the prevalence of small grains—which act as recombination hot spots—across the perovskite active layer dictates the photovoltaic performance of the perovskite solar cells.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Electric Field‐Enhanced Solid‐State Conversion of Ceramic Sr5(PO4)3F to Crystals

The single crystal solid‐state conversion of fluorapatite‐type Sr5(PO4)3F (Sr‐FAP) has been achieved by spark plasma sintering with the assistance of NaF additive. NaF was determined to act as both a sintering aid and impurity solute along the grain boundaries (GBs), controlling both the space charge and GB migration rate. Postsintering isothermal annealing was performed to examine the effect of DC electric field on grain growth. From the space charge potential determined from impedance spectra measurements, in combination with the theoretical contribution of space charge to grain‐boundary energy reduction, it was concluded that the magnitude of the space charge in Sr‐FAP is temperature dependent. As such, a moderate decrease in polycrystalline GB driving force is the main cause for the increased single crystal migration length that was observed in this study.

Effect of the synthesis conditions on the properties of polycrystalline films of lead zirconate titanate of nonstoichiometric composition

The model representations of the formation of the internal fields at the grain boundaries in polycrystalline lead zirconate titanate (PZT) films are discussed. According to the model proposed, the local distortion of the stoichiometric composition of the PZT films caused by the segregation of the oxygen and lead ions from the bulk PZT grains towards their boundaries during high-temperature annealing gives rise to electrical double layers near the grain boundaries and fixes the polarization in these areas. As a result, the ferroelectric polarization that can be switched by the electric field in polycrystalline PZT films decreases.

Cation ratio fluctuations in Cu2ZnSnS4 at the 20 nm length scale investigated by analytical electron microscopy

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a sustainable material for thin‐film photovoltaics with device efficiencies greater than 12% have been demonstrated. Despite similar crystal structure and polycrystalline film microstructures, there is widespread evidence for larger‐amplitude potential and bandgap fluctuations in CZTS than in the analogous Cu(In,Ga)Se2 (CIGSe) chalcopyrite material. This disorder is believed to account for a sizable part of the larger open‐circuit voltage (VOC) deficit in CZTS devices, yet the detailed origins and length scales of these fluctuations have not been fully elucidated. Herein, we present a transmission electron microscopy study focusing on composition variation within bulk multicrystals of CZTS grown by the travelling heater method (THM). In these slow‐cooled, solution grown crystals we find direct evidence for spatial composition fluctuations of amplitude <1 at.% (∼5 × 1020 cm−3) and thus, explainable by point defects. However, rather than being homogeneously‐distributed we find a characteristic 20 nm length scale for these fluctuations, which sets a definite length scale for band gap and potential fluctuations. At Σ3 grain boundaries, we find no evidence of composition variation compared to the bulk. The finding highlights such variations reported at grain boundaries in polycrystalline thin‐films are direct consequences of processing methods and not intrinsic properties of CZTS itself.

Investigation of the formation kinetics of grain boundary inclusions of lead oxide in lead zirconate–titanate films

The formation kinetics of disperse inclusions of lead oxide at the grain boundaries in polycrystalline lead zirconate–titanate (LZT) films is analyzed based on the combined solutio of the continuity and material balance equations. Functions of the inclusion distribution over sizes are constructed for various process times. It is shown that lead oxide inclusions in LZT films with a lead excess are aggregated at the grain boundaries with an increase of the temperature and duration of annealing when thermal processing has still not resulted in a substantial lead loss.

Structural, Magnetic and Mechanical Properties of Oligocrystalline Ni-Mn-Ga Shape Memory Microwires

In single-crystalline Ni-Mn-Ga alloys, high reversible strain (∼10%) induced by an external magnetic field due to twin variants reorientation can be achieved. However, twin boundary motion is hindered by grain boundaries in polycrystalline Ni-Mn-Ga alloys, resulting in a low strain output. In this work, oligocrystalline Ni-Mn-Ga microwires with near bamboo-like grain structure (1-3 grains across the wire diameter of 30-80μm) were prepared by melt-extraction technique and subsequent annealing treatment. Single twin variant, with twin lamella thickness up to 6μm, spans across the entire diameter of the microwire. The oligocrystalline microwires are of ferromagnetic nature at ambient temperature. Twining boundary motion was demonstrated by tensile test performed on a Q800 DMA where different twining stresses corresponding to various grain orientations in a single microwire were detected. A maximum strain of ∼1.57% was obtained and related to the orientation of a subset grains. The martensite twin variant orientations after tensile test were observed using a SEM.

137 多結晶薄膜の結晶粒界品質支配因子の解明

137 多結晶薄膜の結晶粒界品質支配因子の解明

The features of impurity thermal-Photovoltaic and thermal-Voltaic effect of polycrystalline Structures

A model of a p-n junction in the area of the grain boundaries in polycrystalline p-n structures is proposed. Shown mechanism of formation of impurity thermal-voltaic and impurity thermal-photovoltaic effects in IGB.A concept of creating highly effective comparatively inexpensive solar energy converters will be justified by the results we obtained during the study of the ITPV and ITV effects. Proposed theoretical model and experimentally obtained results can be useful in study of poly-Si n+-p structures, for example during external treatment. They allow explaining why in certain temperature ranges, an additional local heating, or applied potential, or a local illumination of pre-heated surface of poly-Si n+-p structure containing deep level impurities, can dramatically change the flow of current. In other words, such external treatment become trigger mechanism for discharge of charges accumulated in IGB due to impurity related thermal voltaic or thermal and photo-voltaic effects.

Determination of the parameters of grain-boundary diffusion and segregation of Co in W using an improved model of grain-boundary diffusion

Based on the recently proposed model of grain-boundary diffusion, the available research data on grain-boundary diffusion of Co in W, and on emission Mossbauer studies of grain boundaries in polycrystalline W have been analyzed in detail. Using joint processing of primary data for tracer analysis and Mossbauer studies, all the parameters of grain-boundary diffusion of Co in W have been determined: the coefficient of grain-boundary segregation, coefficient of grain-boundary diffusion, and the diffusion width of the grain boundary.

Model of the grain boundary in p-n structures based on polycrystalline semiconductors

A model of a p-n junction in the area of the grain boundaries in polycrystalline p-n structures is proposed.

Investigation of Charge Trapping at Grain Boundaries in Polycrystalline and Multicrystalline Silicon Solar Cells

AbstractScanning capacitance microscopy (SCM) often shows a change in contrast at grain boundaries [1-3]. The origins of this contrast and the efficacy of SCM as a tool to identify band bending at grain boundaries in pc-Si and mc-Si are discussed. Contrast at these grain boundaries could be influenced by different oxide growth rates or by defect states at the oxide interface. In order to determine the influence of such mechanisms on the SCM signal, such effects must be modeled; we show that a simple one-dimensional model agrees well with more detailed models of SCM signal strength and indicates, for example, that very small changes in oxide thickness measurably affect the SCM signal. In our experimental data, the uniformity and quality of the oxide layer are confirmed, and increased contrast consistent with depletion regions is still observed at higher order grain boundaries as identified by electron backscattering diffraction, including 9 and 27a. Scans of the SCM signal as a function of dc probe voltage allow such regions to be more quantitatively investigated.

Emission Mössbauer Spectroscopy of Grain Boundaries in Poly- and Nanocrystalline Metals

Capabilities of the emission nuclear gamma resonance (NGR) spectroscopy in studying grain boundaries in metals are considered. It is demonstrated that this method enables to evaluate grain-boundary segregation factors, to determine grain-boundary diffusion mechanisms, to estimate an effective diffusion coefficient in near-boundary areas, etc. Besides, this method permits to evaluate quantitatively differences in the properties of grain boundaries of polycrystalline and nanostructured materials.

Grain boundary diffusion and segregation of Cr in α-Zr

51Cr diffusion along grain boundaries in polycrystalline α-Zr was measured by means of the radiotracer technique in the temperature range 449–680 K. The use of Harrison´s C and B kinetics provided direct data about grain boundary diffusivity ( D gb) and the apparent grain boundary diffusivity ( P gb) in the temperature range of power reactors service. The grain boundaries segregation factor s of Cr in α-Zr was determined at the limit of very dilute solute concentration.

Physical properties and inversion of conductivity type in nanocrystalline PbTe films

This work is concerned with the effect of the nanostructure on the electrical conductivity of n-type In-doped PbTe thin films. Nanostructured thin films were prepared by varying the rate of nucleation as a function of the nature and temperature of the substrates. On low-temperature substrates, the increased rate of nucleation and the limited mobility of the atoms give rise to a nanostructured deposit with a 50 nm grain size. The broken bonds at the grain boundaries generate acceptor states in n-type films, capture electrons from the interior of the grains and give rise to p-type inversion layers between adjacent grains. In the nanostructured n-type films with the high density of grain boundaries, thermally activated hole conductivity becomes dominant and leads to its inversion from n to p type. The experimental results confirm the physical model put forward by Neustroev and Osipov [L. N. Neustroev and V. V. Osipov, Sov. Phys. Semicond. 20, 34 (1986)] regarding the creation of inversion channels (p-type conductivity), generated by the diffusion of oxygen along the grain boundaries of polycrystalline lead chalcogenide films. In the present case, these inversion channels appear on account of the acceptor states that have been generated at the grain boundaries of the nanostructured thin layers.

Anisotropic Grain Boundary Segregation in Y-Doped TiO2

A variety of properties of polycrystalline TiO2, such as conductivity, creep and grain boundary diffusion, are strongly dependent on the atomic-level structure and chemistry of grain boundaries. TiO2 has been studied as a model dielectric system because of its relatively simple structure and well-understood point defect chemistry. Defect segregation in grain boundaries of polycrystalline and bi-crystal TiO2 have been studied by several groups and significant variations in solute segregation levels from boundary to boundary were observed. in this paper, we address this issue of anisotropic grain boundary segregation. We have measured solute segregation as a function of grain boundary misorientation to determine any correlation between segregation and misorientation.Yttria-doped TiO2 polycrystalline samples were prepared by mixing 99.999% pure TiO2 powder with 0.1% mole percent 99.99% purity Y(NO3)3 (both powders are commercially available from Aldrich Co. or Alfa Co.), followed by uni-axial pressed to 200-400Pa and sintering at 1300 °C for about 5-7 hours.

Effect of CdCl2 Treatment on the Interior of CdTe Crystals

ABSTRACTAn essential processing step in CdTe/CdS polycrystalline solar cells is heat treatment in CdCl2. We present photoluminescence results from single crystals of CdTe that have been exposed to CdCl2 treatments at 387 C similar to those used in actual cell fabrication. Using sub band gap excitation from a tunable diode laser, we probe states in the interior of the crystal. We show that high-purity (99.998 percent) CdCl2 treatment results in the appearance of a 1.45 eV donor-acceptor transition that is likely due to a Cl-Cu center. Low purity (99.7 percent) CdCl2treatment results in the appearance of the 1.45 eV line and a 1.555 eV Cu-related emission. These results indicate that the CdCl2 treatment has an effect on the interior of CdTe grains, in addition to its already well established effect on grain boundaries in polycrystalline CdS/CdTe devices. They also imply that CdCl2 treatment may result in the incorporation of Cu into the CdTe grains. The results will be related to the effects of CdCl2 on polycrystalline CdS/CdTe devices that have been observed by other groups.This work is supported by NREL