Formation Of Grain Boundaries Research Articles

Understanding the transport properties of perovskite compounds as a function of temperature, frequency and DC-bias voltage using experimental measurements and appropriate theoretical models.

The present paper provides interesting measurements and methodologies to investigate the key factors controlling the electrical response of perovskite systems. The studied system was successfully prepared using a solid-state route. The chemical analysis and the X-ray diffraction results confirm the formation of the desired perovskite phase. As a result, the DC-resistivity analysis shows that the transport properties are governed by hopping mechanisms above the transition temperature. In this case, thermal agitation allows the charge-carriers to hop across the insulating barrier in the form of grain boundaries. Then, the dominance of the grain boundary contribution is proved. AC-resistivity spectra are investigated in terms of numerous power laws (UDR, SPL and NCL). Accordingly, the decrease in the resistivity values at high frequencies is explained through hopping and tunneling processes. Indeed, it is proved that the electrical transport phenomena are governed by QMT and CBH models. The coexistence of direct (C-C) and indirect (C-A-C) interactions explains the multi-behavior of the AC-resistivity. The scaling representation displays a single muster curve describing the universal dynamic response (UDR). Thus, it reveals the universality of the electrical resistivity of the system. However, the double Schottky barrier model is used to explain the grain boundary effects. A critical frequency "νc" is detected under temperature and DC-bias voltage effects. These observations disclose, in a different way, the separate contributions of the electro-active regions of the sample in the conduction phenomenon.

Exploring high-temperature deformation and damage behaviour in high-performance ferritic (HiperFerSCR) steel with Laves phase particles

The mechanical behaviour at high temperatures and the corresponding microstructural characteristics of a newly developed high-performance ferritic, salt-corrosion-resistant (HiperFerSCR) steel containing Laves phase particles were studied to investigate the active deformation mechanisms and the evolution of damage. A series of tensile tests were conducted at both room temperature and elevated temperatures (550, 600, and 650 °C), along with detailed microstructural analyses using scanning electron microscopy and electron backscatter diffraction. The tensile flow characteristics of HiperFerSCR at room temperature exhibit significant strain hardening, which is due to the Laves phase particles that encourage planar slip, demonstrated by the formation of numerous slip bands and low-angle grain boundaries. The intersections of these slip bands with grain boundaries appear to be the initiation sites for cracks, which then propagate intergranularly as well as within grain interiors along the slip bands. At high temperatures, the flow characteristics show strain softening, which is attributed to a dynamic recovery (DRV) mechanism, evident in the formation of subgrain boundaries decorated with Laves phase particles. During high-temperature deformation, damage initiates in the particle-free zone (PFZ), which expands at higher temperatures, leading to strain localisation, void formation along the grains, and eventually, grain boundary decohesion.

Interface engineering of composite systems: Focusing on the compatibility of reduced graphene oxide and Bi0.8La0.1Ba0.1Fe0.9Ti0.1O3 hybrid systems

The composite of reduced graphene oxide and perovskite-based ferroelectric materials offers exciting opportunities to enhance the optical performance of optoelectronic devices. The present work reporting rGO modified x(Bi0.8La0.1Ba0.1Fe0.9Ti0.1O3)-1-x(rGO) (La-BBFT/rGO) composite to investigate their structural, morphological, electrical, magnetic and optical properties by using robust solid-state and spun calcination method. X-Ray diffraction patterns confirm the rhombohedral crystal structure with space group (R3c) of both samples with the presence of rGO as second phase. The functional groups present in reduced graphene oxide (rGO) can act as nucleation sites for crystallization, leading to the formation of more grains and Grain boundaries. The ferroelectric phase transition was observed in La-BBFT/rGO composites at comparatively low temperature near 250 °C. The impedance spectroscopy confirms the non-Debye behaviour of relaxation process. The electric and magnetic remanence shows the compatibility of the switching state. Additionally, the optical band gap energy of both samples was evaluated and found to be in the range of 2.0 to 1.9eV. Incorporation of rGO in La-BBFT/rGO, were marginally improved, the dialectic, impedance, and optical properties, which could be beneficial for electronic and optical components in the opto-electronic industries.

Manipulating nanostructure during wire arc additive manufacturing defeats hydrogen embrittlement

Wire arc additive manufacturing (WAAM) leads to a unique nano/microstructure. The combination of multiscale experimental and numerical analyses firstly shows that the intrinsic heat treatment during WAAM leads to an unusual depletion of Mg near the grain boundaries. During the WAAM process, a high dislocation density leads to formation of sub-grains and high-angle grain boundaries (HAGBs). It is revealed that the uniformly distributed micropores formed during the WAAM are strong hydrogen traps. The reduction in Mg content at the HAGBs, an increase in total length of HAGBs and hydrogen trapping by micropores help to defeat intergranular hydrogen embrittlement by WAAM.

Mechanisms of grain boundary α precipitation in the metastable β-titanium Ti-5Al-5Mo-5V-3Cr

In this study, we investigated the early nucleation and growth of grain boundary α during the aging heat treatment of Ti-5553. Through a correlative approach using scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, transmission electron microscopy, and atom probe tomography, high-angle β-grain boundaries were investigated in detail to understand the evolution of the precipitation of grain boundary α through nucleation and subsequent growth. It is found that at grain boundaries there is a pronounced nucleation of α precipitates with similar orientations. Growth of these similarly orientated α precipitates, in the later stages, leads to their agglomeration and the formation of grain boundary α. This investigation provides insights into the formation of grain boundary α in its early stages of nucleation and growth.

Grain boundary reconstruction for high-performance regeneration of Ce-abundance sintered Nd-Ce-Fe-B magnet scraps

In this study, we focus on the short-process regeneration of high-Ce content sintered Nd-Ce-Fe-B magnet scraps, aiming to enhance the coercivity and overall magnetic properties through the reconstruction of grain boundaries (GBs) based on the analysis of rare earth (RE) loss and proportions at each stage of the regeneration process. CeHx, (Nd0.75Pr0.25)Hx, and their mixtures were added as intergranular additives to produce sintered magnets. When 2.31 wt% of (Nd0.75Pr0.25)Hx is added, a high-performance magnet is obtained with the remanence of 11.72 kG, coercivity of 11.56 kOe, and maximum energy product of 32.23 MGOe, reaching performance recovery rates of 98.8%, 103.5%, and 98%, respectively, of the original magnets. It is indicated that the intergranular additives were beneficial to the formation of continuous smooth RE-rich GBs. In addition, a portion of Nd/Pr diffused into the 2:14:1 phase as a partial replacement for Ce, resulting in the migration of Ce from the interior of the grains to the outer region. This led to the formation of new RE-rich triple junction phases on the exterior, thereby reducing the magnetic dilution effect of Ce in the primary phase and enhancing the magnetic crystal anisotropy. These findings offer new insights into the regeneration of high-Ce sintered Nd-Ce-Fe-B scraps.

Microstructure evolution and mechanical properties of refractory high-entropy alloy nitride film

The (TiZrNbMo)Nx refractory high-entropy alloys (RHEAs) nitride films were fabricated using reactive magnetron sputtering by modifying the N2 flow rate ratio in the mixed N2 + Ar atmosphere. The incorporation of the N element induced a phase transition in the (TiZrNbMo)Nx films from body-centered cubic (BCC) to face-centered cubic (FCC) structure, accompanied by increasing in the amorphous phase. With the introduction of N2, the hardness of (TiZrNbMo)Nx films firstly increased significantly to 25.2 GPa when the N2 flow rate ratio was at 50 % and then decreased slightly to 18.5 GPa when the N2 flow rate ratio was at 75 %. Theoretical analysis shows that the doping of N element enhances the solid strengthening effect, with the formation of nitride phases and grain boundary (GB) strengthening effect significantly contributing to the enhanced hardness. Furthermore, the addition of N element can also inhibit grain rotation and GB sliding during deformation. This work provides a new insight into understanding the structure and properties of refractory high-entropy alloy films.

On the mechanism of piezoresistance in nanocrystalline graphite.

Strain sensors are sensitive to mechanical deformations and enable the detection of strain also within integrated electronics. For flexible displays, the use of a seamlessly integrated strain sensor would be beneficial, and graphene is already in use as a transparent and flexible conductor. However, graphene intrinsically lacks a strong response, and only by engineering defects, such as grain boundaries, one can induce piezoresistivity. Nanocrystalline graphene (NCG), a derivative form of graphene, exhibits a high density of defects in the form of grain boundaries. It holds an advantage over graphene in easily achieving wafer-scale growth with controlled thickness. In this study, we explore the piezoresistivity in thin films of nanocrystalline graphite. Simultaneous measurements of sheet resistance and externally applied strain on NCG placed on polyethylene terephthalate (PET) substrates provide intriguing insights into the underlying mechanism. Raman measurements, in conjunction with strain applied to NCG grown on flexible glass, indicate that the strain is concentrated at the grain boundaries for smaller strain values. For larger strains, mechanisms such as grain rotation and the formation of nanocracks might contribute to the piezoresistive behavior in nanocrystalline graphene.

Unveiling the Synergistic Role of Frustrated Lewis Pairs in Carbon-Encapsulated Ni/NiOx Photothermal Cocatalyst for Enhanced Photocatalytic Hydrogen Production.

The development of high-density and closely spaced frustrated Lewis pairs (FLPs) is crucial for enhancing catalyst activity and accelerating reaction rates. However, constructing efficient FLPs by breaking classical Lewis bonds poses a significant challenge. Here, this work has made a pivotal discovery regarding the Jahn-Teller effect during the formation of grain boundaries in carbon-encapsulated Ni/NiOx (Ni/NiOx@C). This effect facilitates the formation of high-density O (VO) and Ni (VNi) vacancy sites with different charge polarities, specifically FLP-VO-C basic sites and FLP-VNi-C acidic sites. The synergistic interaction between FLP-VO-C and FLP-VNi-C sites not only reduces energy barriers for water adsorption and splitting, but also induces a strong photothermal effect. This mutually reinforcing effect contributes to the exceptional performance of Ni/NiOx@C as a cocatalyst in photothermal-assisted photocatalytic hydrogen production. Notably, the Ni/NiOx@C/g-C3N4 (NOCC) composite photocatalyst exhibits remarkable hydrogen production activity with a rate of 10.7 mmol g-1 h-1, surpassing that of the Pt cocatalyst by 1.76 times. Moreover, the NOCC achieves an impressive apparent quantum yield of 40.78% at a wavelength of 380nm. This work paves the way for designing novel defect-state multiphase cocatalysts with high-density and adjacent FLP sites, which hold promise for enhancing various catalytic reactions.

One-Dimensional orthorhombic CsPbI3 polycrystalline thick film for efficient and highly stable direct X-ray detection and imaging

An ideal direct X-ray detector should convert a low dose of X-ray photons into large quantity of electrical signals, coupled with sustained long-term stability. Nevertheless, a notable conflict arises between X-ray absorption efficiency and carrier transport in thick polycrystalline perovskite films. Moreover, conventional perovskite materials exhibit inherent challenges regarding the stability of their crystal phase and the consistency of the photocurrent. To circumvent these limitations, a polycrystalline perovskite thick film is proposed using orthorhombic CsPbI3 (δ-CsPbI3) as a highly stable active material. Combined with the liquid epitaxy process, δ-CsPbI3 prefers one-dimensional growth along the carrier transportation direction, which suppresses the formation of grain boundaries, enabling high carrier mobility while maintaining X-ray absorption for a thick polycrystalline film. Consequently, the polycrystalline δ-CsPbI3 based detector exhibits a highest sensitivity of 1768.46 μC Gyair-1 cm−2 at an electric field of 432 V mm−1 which is 88.42-fold higher than α-Se based detectors and 190-fold higher than the single crystalline δ-CsPbI3 based detector, respectively. The sensitivity maintains 91.34 % of its initial state after 7 days exposure in the air. Combing with a thin-film transistor array, the detector achieves 8-bit imaging within a 64 × 64 matrix. This work provides a feasible method for commercialized production of high-performance direct X-ray detectors.

Grain growth competition and formation of grain boundaries during solidification of hcp alloys

Grain growth competition during directional solidification of a polycrystal with hexagonal (hcp) symmetry (Mg-1wt%Gd alloy) is studied by phase-field modeling, exploring the effect of the temperature gradient G on the resulting grain boundary (GB) orientation selection. Results show that selection mechanisms and scaling laws derived for cubic (fcc, bcc) crystals also apply to hcp materials (within their basal plane), provided a re-estimation of fitting parameters and re-scaling to account for the sixfold symmetry. While grain growth competition remains stochastic with rare events of unexpected elimination or side-branching along the developing GBs, we also confirm an overall transition from a geometrical limit to a favorably oriented grain limit behavior with an increase of thermal gradient within the dendritic regime, and the progressive alignment of dendrites and GBs toward the temperature gradient direction with an increase of G during the dendritic-to-cellular morphological transition. Comparisons with original thin-sample directional solidification experiments show a qualitative agreement with PF results, yet with notable discrepancies, which nonetheless can be explained based on the stochastic variability of selected GB orientations, and the statistically limited experimental sample size. Overall, our results extend the understanding of GB formation and grain growth competition during solidification of hcp materials, and the effect of thermal conditions, nonetheless concluding on the challenges of extending the current studies to three dimensions, and the need for much broader (statistically significant) data sets of GB orientation selected under well-identified solidification conditions.

Modification of structural, morphological, and dielectric properties of barium calcium titanate ceramics with yttrium addition

Barium calcium titanate (BCT) ceramics with varying yttrium doping concentrations were fabricated using the solid-state compaction process to explore the attributes of dopants. (Ba0.75Ca0.25) TiO3 and (Ba0.75Ca0.25) (YyTi(1-y)) O3 where, y = 0.00, 0.10, 0.15, and 0.20 ceramics were synthesized by pressing isostatically in pellet press apparatus, then sintered at 1250 °C with consequent cooling in furnace ambient. The structural, morphological, and dielectric properties were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and impedance spectroscopy interpretations, respectively. The XRD analysis revealed that the cubic BCT lattice was transformed into a tetragonal structure with Yttrium doping. Scanning electron microscopy (SEM) disclosed that yttrium doping countered the liquid phase formation of BCT as well as influenced grain development and microstructure, leading to the formation of distinct grain boundaries and improved densification. The average grain size (18–29 nm) of the Y-BCT increases as the doping level rises. At 60 Hz, it was reported that the dielectric constant obtained a maximum value of 70000 with a resistivity of 5 × 108 Ώ-cm for y = 0.15. The manifestation of the secondary phase confirmed from XRD, allocating an easy path for oxygen migration, might be responsible for the rise in oxygen vacancy, higher leakage current, and dielectric loss for y = 0.01. Co-doping of calcium and yttrium in BCT ceramics has modified the basic structure and ameliorated composites' structural stability and dielectric characteristics. The optimized sample, upon demonstrating outstanding efficiency, ought to be employed for specific uses such as energy storage devices and capacitors.

Unraveling effect of Nb addition on the microstructure, phase transformation behaviour and shape memory effect of CuAlMn alloy fabricated by laser powder bed fusion

In this work, the various amount of Nb powder was mechanically mixed into the Cu–Al–Mn shape memory alloys (SMAs) to prepare quaternary SMAs by laser powder bed fusion (LPBF). Altered microstructure, transformation behaviour of alloys and its underlying mechanisms were analyzed. Corresponding shape memory effect (SME) was also studied. Result shows that Nb content has significant effect on the microstructure, phase transition temperatures (TTs) and SME. The microstructure of the alloy was refined by addition of appropriate Nb content. The formation of precipitates and massive grain boundaries after the addition of Nb caused a reduction of TTs and wide thermal hysteresis. Sample with 1 at%Nb containing fine and small volume fraction precipitate shows the best available strain.

Understanding the formation and evolution of low-angle grain boundaries in Nickel-Based single crystal superalloys

Grain boundaries have long been of utmost importance in the research and application of single-crystal superalloys. However, there are still many unsolved mysteries regarding the understanding of grain boundaries throughout the solidification process. The work focuses on investigating the formation, the evolution, and the disappearance of low-angle grain boundaries, including subgrain boundaries. The findings show that the low-angle grain boundaries and subgrain boundaries in various dendritic arrays exhibit morphological variances and different development processes. The subgrain boundaries in the aligned array appear to be straight, where irregularly fine-grained γ' precipitates have formed. In contrast, low-angle grain boundaries show a curved and sawtooth-like structure in misaligned arrays. The γ' precipitates locating in the region are regularly long strips. The generation of grain boundaries is a result of the adjustment in dendritic spacing, where the influence of solute leads to dendrite deflection. The misorientation of grain boundary is constantly changing in response to dynamic changes in dendrite spacing. The low-angle grain boundaries eventually dissolve as the dendritic array evolves. This study enriches understanding of the interplay between dendrite arrays and grain boundaries, offering theoretical insights into the evolutionary patterns of grain boundaries within single-crystal superalloys.

Dielectric Property, AC Conductivity, and Electric Modulus Studies of Pristine and Green‐Synthesized ZnO Nanoparticles

Frequency‐dependent dielectric constant and dielectric loss of pristine and green‐synthesized (using Tabernaemontana divaricata flower extract) ZnO nanoparticles (NPs) are studied at different temperatures. Below 1 kHz frequency and at temperatures above 100 °C, ZnO NPs have giant dielectric constant values (≈3 × 104). At lower frequencies and at/below 100 °C, the nature of temperature dependence of dielectric constant for pristine and green‐synthesized ZnO NPs are reversed, and this anomalous temperature dependence is correlated with the in‐plane and out‐of‐plane thermal expansions of ZnO. A temperature‐dependent poly‐dispersive relaxation mechanism in ZnO has been observed. The electrical conduction mechanism is found to be significantly modulated by the use of flower extract. The electric modulus study suggests similar mechanism of electrical conduction and dielectric polarization followed in these materials. A plausible growth mechanism of the ZnO NPs in presence of the flower extract is explored. Variations in the dielectric constant, dielectric loss, conductivity, and polarization mechanisms of the ZnO NPs are understood as signatures of the capping effect of the phytochemicals present in the flower extract on the crystal growth and formation of grain boundaries.

Realization of future neuro-biological architecture in power efficient memristors of Fe3O4/WS2 hybrid nanocomposites

The future generation of digital technology will heavily rely on power efficient non-volatile resistive memory systems as a potential alternative to flash memory due to its limitations in scalability and endurance. To attain the commercial benchmark, memristors have still lacked performance. This study reports a novel and cost-effective solution processable method for growing surface-modified hybrid nanocomposites (Nc) on a large scale, as an active layer. The solution-processed synthesis approach used for Ag/Fe50W50/Pt hybrid nanocomposite memristor device results in the formation of heterophase grain boundaries, which create residual filaments along these boundaries. The device Fe3O4-WS2(Nc) shows excellent performance, having ultra-low energy consumption (0.1fJ), high reproducibility (10 devices), scalability, excellent endurance (106), and excellent environment stability. Density functional theory (DFT) simulations reveal that structural symmetry distortion and interfacial interaction of hybrid nanocomposite at the interface plays a vital role in the switching mechanism. As high-performance electronic synapses, the optimal pulse scheme enables a steady interaction of short- and long-term plasticity principles, such as spike -time dependent plasticity (STDP) and pulse pair facilitation (PPF), essential for learning and neuromorphic computing analogous to human brain. Moreover, by using Modified National Institute of Standards and Technology (MINST), the memristor device attained a high learning accuracy of 95.4% under convolution neural network (CNN) simulations. The present study revealed that the performance of hybrid-nanocomposite memristors could lead to efficient future neuromorphic architecture.

Formation of grain boundaries in nanocrystalline SiC ceramics examined by powder diffraction supported by MD simulations

Atomic structure of nanocrystalline SiC powder sintered under high-pressure high-temperature conditions was examined with use of powder diffraction technique supported by molecular dynamics simulation. Shape and atomic structure of grain surfaces was identified based on real space PDF analysis of experimental diffraction data and theoretical calculations performed for atomic models of nanograins relaxed by MD simulation. It is shown that in presence of about 5 % of excess carbon in the initial 11 nm size SiC powder HPHT sintering leads to formation of (100) and (110) grain boundaries formed by single C planes and (111) boundaries formed by double C planes. Under pressure 3 GPa and 1600°C sphere-like grains tend to transform to dodecahedrons terminated by single C atomic planes. Under 8 GPa the grains adopt the shape of cube-octahedrons terminated by (100) single C planes and (111) planes terminated by single and double C planes.

Analytical energy formalism and kinetic effects of grain boundaries: A case study of graphene

Grain boundaries (GBs), inherent in polycrystalline materials, manifest a diverse array of features that substantially affect material properties. However, the incomplete knowledge of the relevance between structures and energetics of GBs impedes the understanding of their effects. Here, taking graphene as an example, we propose an analytical energy formula for GBs in grain-boundary angle space. Our study reveals that any given GB can be characterized by a geometric combination of symmetric GBs, adhering to the principle of uniformly distributing their dislocation cores along straight trajectories. The formation probability of GBs, as predicted by our theoretical derivation, aligns well with both high-throughput calculations and experimental statistics. Furthermore, we unveil the elusive kinetic effects on GBs by contrasting experimental statistics with energy-dependent thermodynamic effects. This study not only presents a robust model to describe energetically favorable GBs in graphene, offering insight into the formation of GBs in two-dimensional materials, but also reveals the kinetic effects of GBs in material synthesizing process.

Research on Flexible Transparent Conductive Electrodes Based on Ultra-Thin Ag in the Form of Grain Boundary with IZO Layer

In this paper, we investigated a transparent conductive electrode (TCE) that satisfies electrical, optical, and mechanical properties, formed by depositing ultra-thin Ag metal in the form of a random grain boundary with an indium zinc oxide (IZO) layer on a PET substrate. Commonly used ITO electrodes are brittle and difficult to apply to flexible devices. In contrast, IZO-based electrodes are mechanically flexible and can be used as flexible TCE, and have high electrical and optical properties. A 90 nm thick IZO electrode has a transmittance of 90.2% at a wavelength of 460 nm and a sheet resistance of 29.5 ohm/sq. In particular, Ag metal was deposited in the form of an atypical metal island using an RF magnetron sputtering system. At 3 nm there were few metal clusters in the form of islands, and many void channels were formed, resulting in high sheet resistance as well as a decrease in optical transmittance. However, about 5 nm thickness, the number of void channels decreased and the optical path changed, improving the electrical and optical properties. Results showed that the sheet resistance was reduced to 19.8 ohm/sq, and the transmittance was also increased to 91.1%. The mechanical properties were also found to be the same for conventional IZO and Ag/IZO TCE.

Effect of Single Crystal Growth and Solidification Grain Boundaries on Weld Solidification Cracking Behavior of CMSX-4 Superalloy

Single-crystal superalloys have been popularly employed in high-temperature parts of gas turbines, such as blades. However, the welds of such alloys are highly susceptible to solidification cracking, which limits their applicability to high-temperature turbine blades. In this study, the effects of characteristics of weld solidification on solidification cracking susceptibilities (solidification brittle temperature range, BTR) were fundamentally investigated for the CMSX-4 single-crystal superalloy. We applied a transverse-Varestraint test procedure for both the linear and oscillated arc welds by changing the weld solidification characteristics, such as the degree of single crystal growth and formation of solidification grain boundaries. The BTR for the CMSX-4 alloy is 336 K for linear welding condition, whereas the values are 434 K and 342 K for 0.6 and 1.5 Hz oscillated welds. Interestingly, the BTR continuously increases with the weld oscillation frequency. By contrast, almost no changes in the weld mushy-zone temperature range are theoretically calculated for each welding condition via the diffusion-controlled Scheil model. The mechanism underlying the increase in BTR under oscillation welding is clarified based on the relationship between the achievement ratio of the weld single crystal growth and fraction of high-angle (>15<sup>o</sup>) solidification boundaries, which affect severe dendrite coalescence undercooling. The lower fraction of the high-angle weld solidification grain boundaries attributed to the superior achievement ratio of weld single crystal growth, which reduces the dendrite coalescence undercooling and BTR. Consequently, it enhances the solidification crack propagation resistance.