Geometric Phase Analysis Research Articles

Lowering Sodium‐Storage Lattice Strains of Layered Oxide Cathodes by Pushing Charge Transfer on Anions

Due to a high energy density, layered transition‐metal oxides have gained much attention as the promising sodium‐ion batteries cathodes. However, they readily suffer from multiple phase transitions during the Na extraction process, resulting in large lattice strains which are the origin of cycled‐structure degradations. Here, we demonstrate that the Na‐storage lattice strains of layered oxides can be reduced by pushing charge transfer on anions (O2−). Specifically, the designed O3‐type Ru‐based model compound, which shows an increased charge transfer on anions, displays retarded O3–P3–O1 multiple phase transitions and obviously reduced lattice strains upon cycling as directly revealed by a combination of ex situ X‐ray absorption spectroscopy, in situ X‐ray diffraction and geometric phase analysis. Meanwhile, the stable Na‐storage lattice structure leads to a superior cycling stability with an excellent capacity retention of 84% and ultralow voltage decay of 0.2 mV/cycle after 300 cycles. More broadly, our work highlights an intrinsically structure‐regulation strategy to enable a stable cycling structure of layered oxides meanwhile increasing the materials' redox activity and Na‐diffusion kinetics.

Revealing corrosion behavior of B4C/pure Al composite at different interfaces between B4C particles and Al matrix in NaCl electrolyte

In the present work, the corrosion behavior of the B4C/pure Al composite in a Cl− containing environment is investigated. Thereby, the B4C/pure Al composites have been subjected to the different heat treatments to highlight the impact of the corrosion behavior at the different interfaces. The results of current sensing atomic force microscope (CSAFM) and scanning Kelvin probe microscopy (SKPFM) show that the dissolution of the Al matrix is relevant to the micro-galvanic corrosion between anodic Al matrix and cathodic B4C particles. The as quenched (AQ) composites with a higher volta potential difference (VPD) show inferior anticorrosion property than that of the as produced (AP) composites and the as annealed (AA) composites. By combining X-Ray diffraction (XRD) and geometric phase analysis (GPA), the higher residual stress and obvious interfacial strain between the B4C particles and the Al matrix of the AP samples is detected, which result in increasing the driving force of micro-galvanic corrosion.

Initiation of high-temperature fatigue cracks at the most tip of fractured grain boundaries

Two 2.2Cr heat resistant steel samples that had been fatigue tested respectively at 550 °C and 650 °C are analyzed on the atomic scale to reveal the different crack initiation mechanisms. Cracks formed along prior austenite grain boundaries (PAGBs) are selected and manufactured into foil specimens with the aid of focused ion beam (FIB). Hence the interior crack progressing routes are illustrated and the atomic structure particularly at the most tip of cracks are characterized by using the high resolution TEM. It is revealed that the cracks mainly progress within a spinel phase (Cr2MnO4) while not its interface with neighbouring martensitic matrix. Moreover, geometric phase analysis (GPA) and lattice misfit at the most tip of cracks further manifest a elastic dominated and a plastic dominated mechanism corresponding to the two specimens, respectively. Cracking of the 650 °C specimen is mainly attributes to the accumulation of plastic strain that caused by the viscoelastic transformation of PAGBs, which causes intriguing periodic grain boundary sliding (GBS) waves ahead of the crack tip.

Synergistic effects of multi-strengthening mechanisms in exceptionally high-strength W-Ni-Fe composites

Tungsten heavy alloys (WHAs) are typical tungsten-based composites and are considered as important materials in kinetic energy penetrators that currently use depleted uranium munitions. However, the incongruity between strength and plasticity is the “Achilles’ heel”, which restricts further application in national defense and military industries. Herein, a novel fine-grained La2O3 strengthened W-Mo-Ni-Fe composites were fabricated successfully via using nanoscale solid-solution composite powder and the optimized two-stage sintering as well as vacuum heat-treatment techniques. In the model W-xMo-4.9Ni-2.1Fe-1La2O3 composites (x = 2.5, 5, 10 and 20 wt%), the heat-treated alloys had a highly homogeneous microstructure, consisting of equiaxed W grains, matrix phase and dispersed La2O3 reinforced particles. Increasing Mo content promoted microstructure refinement and caused the formation of MoNi3 intermetallic compound (IMC). As a result, benefiting from the multiple strengthening mechanisms containing fine-grained, solid-solution and oxide dispersion strengthening effects, the 88 W-5Mo-4.9Ni-2.1Fe-1La2O3 composite exhibited a superb synergy of tensile strength and ductility (1116 MPa and ∼ 15%). Moreover, the analysis of the lattice strain fluctuations and dislocation behaviors of WHAs were conducted by means of the geometric phase analysis (GPA) approach for the first time. The plastic deformation mechanism was studied to elucidate the fracture characteristics and slip systems with Schmid factor (SF) and Kernel Average Misorientation (KAM) analysis. Our findings not only shed light on the understanding of the deformation mechanisms of WHAs, but also provide a new avenue for the design of WHAs with outstanding mechanical properties.

Mitigating Heavy Ion Irradiation‐Induced Degradation in p‐type SnO Thin‐Film Transistors at Room Temperature

The study investigates the mitigation of radiation damage on p‐type SnO thin‐film transistors (TFTs) with a fast, room‐temperature annealing process. Atomic layer deposition is utilized to fabricate bottom‐gate TFTs of high‐quality p‐type SnO layers. After 2.8 MeV Au4+ irradiation at a fluence level of 5.2 × 1012 ions cm−2, the output drain current and on/off current ratio (Ion/Ioff) decrease by more than one order of magnitude, field‐effect mobility (μFE) reduces more than four times, and subthreshold swing (SS) increases more than four times along with a negative shift in threshold voltage. The observed degradation is attributed to increased surface roughness and defect density, as confirmed by scanning electron microscopy (SEM), high‐resolution micro‐Raman, and transmission electron microscopy (TEM) with geometric phase analysis (GPA). A technique is demonstrated to recover the device performance at room temperature and in less than a minute, using the electron wind force (EWF) obtained from low‐duty‐cycle high‐density pulsed current. At a pulsed current density of 4.0 × 105 A cm−2, approximately four times increase in Ion/Ioff is observed, 41% increase in μFE, and 20% decrease in the SS of the irradiated TFTs, suggesting effectiveness of the new annealing technique.

Relation between sampling, sensitivity and precision in strain mapping using the Geometric Phase Analysis method in Scanning Transmission Electron Microscopy

The sensitivity and the precision of the Geometric Phase Analysis (GPA) method for strain characterization is a topic widely discussed in the literature and is usually difficult to quantify. Indeed, the GPA precision is intricately linked to the resolution of the strain maps defined when masking the periodic reflections in Fourier space. In this study an additional parameter, sampling, is proposed to be analyzed regarding the precision of GPA by developing the concept of a phase noise in the GPA equations. Both experimentally and theoretically, the following article demonstrates how the precision, and the sensitivity of the GPA method is improved when using a larger pixel spacing to record an electron micrograph in Scanning Transmission Electron Microscopy (STEM). The counterintuitive concept of increasing the field of view to improve the GPA precision results is an extension of the application of strain characterization methods in STEM towards low deformation levels.

Direct observation of the ledge-typed twin boundary in a martensitic steel

The structure at the {112} twin boundaries in martensitic steels has been extensively debated recently. On the one hand, it was claimed to be the omega phase, similar to Ti alloys. On the other hand, it was considered artifacts induced by the double diffraction effect from the overlaps of twin-matrix crystals at their boundaries. In this study, we clarify the twin structure in a martensitic steel providing a detailed investigation of the α′ martensite twin boundaries using experiment and simulation. The fundamental for rejecting the existence of the omega phase in steels is based on the atomic resolution imaging from the <111> direction of twin structure, which was so far challenged to be acquired for such nano-sized twinned systems. The obtained result confirms the ledge-typed twin boundary that causes the visually omega-like forms when observed from other zone axes. The ledges occurred on {110} planes, making them secondary twin planes with coherent characteristics. Furthermore, these ledges were found to transit in a gradual manner from the primary (112) twin plane to (011) and then to (1¯10) secondary twin planes; a perpendicular transition from (112) to (1¯10) was not observed. Such a transitional behavior was explained in view of lowering total twin boundary energy, evaluated by the theoretical prediction of system energy and the geometric phase analysis of the twin planes. Double diffraction and rotational Moiré effects clarified the extra spots in the diffraction pattern obtained at the twin-matrix overlapped regions. This study gives light of understanding twinning in the bcc crystals.

Geometric phase analysis for characterization of 3D morphology of carbon fiber reinforced composites

In carbon fiber-reinforced resin matrix composites, differences in the mechanical properties of carbon fibers and matrix resins can lead to differences in height, causing step faults at the interphase during grinding and polishing. Accurately and efficiently characterizing the step height at the interphase is crucial for analyzing the mechanical properties and residual stress distribution of the composite materials. In this paper, an orthogonal grating, with a frequency of 10,000 lines/mm, was prepared on the surface of the composite material using the SEM electron beam grating technique. The surface morphology of the T700 carbon fiber-reinforced epoxy resin matrix composite was analyzed using the geometric phase analysis method. The average step height between the fiber and matrix was found to be 128.6 nm. Moreover, the geometric phase analysis method can provide a morphology distribution map within the range of 155 × 100 μm2. This research will provide an effective way to quantitative characterization of three-dimensional surface morphology of composite materials.

Addressing the hydrogen entrapment in Al2024 alloys using scanning transmission electron microscopy and density functional theory

The effect of precipitate coherency in an aged aluminum (Al) 2024 alloy is investigated using scanning transmission electron microscopy (STEM) and density functional theory (DFT) methods. The focus of the study is on understanding the effects of hydrogen (H2) entrapment in the alloy, which leads to the degradation of its corrosion resistance. We employed STEM to analyze the structure and strain properties of the alloy at high spatial resolution. This has been achieved by applying the geometrical phase analysis (GPA) to the acquired HRSTEM images. The determination of strain field around the precipitates was utilized to gauge the degree of coherency at the interface of the precipitates with Al matrix. We also applied density functional theory (DFT) simulations to investigate the interfacial energy at the interfaces of the precipitates with the Al matrix. The obtained results of the DFT simulations showed that the interfacial energy at the semi-coherent precipitates was determined to be 0.06 eV per atom, which was higher compared to the coherent precipitates. Our findings suggest that the semi-coherent precipitates have a less stable interface with the Al matrix. Furthermore, the study revealed that the H2 entrapped at the semi-coherent precipitate regions exhibits a binding energy which is 3.2 eV lower than that of the coherent precipitates. This implies that hydrogen binds more strongly at the semi-coherent precipitate cites, which enhances the process of the alloy embrittlement.

Dimensional Engineering of Hierarchical Nanopagodas for Customizing Cross‐Scale Magnetic Coupling Networks to Enhance Electromagnetic Wave Absorption

AbstractDimensional engineering of appropriate structure is an effective approach to achieve high‐performance electromagnetic (EM) wave absorption for magnetic materials. However, controllable modulation of the material configuration and a comprehensive understanding of the relationship between structure and loss mechanism remain challenging. Herein, magnetic CoNi pentagonal nanopagodas (PNPs) are ingeniously tailored using a competition orientation strategy, in which high‐density PNPs with sharp corners/edges and hierarchical structure are rooted in situ on the surface of CoNi alloy microsphere. This unique configuration of PNPs originates from the orientation growth of CoNi fivefold twins, which can be effectively regulated by manipulating metal ratio and evolves into flake‐, serrated thorn‐, prismoid‐, and blocks‐like morphologies. Optimized CoNi microspheres with high‐density PNPs exhibit a broad absorption bandwidth of 6.82 GHz at only 1.8 mm thickness. Cross‐scale magnetic coupling networks strengthen magnetic loss ability, magnetocrystalline defects induce dielectric polarization enhancement, which are intuitively confirmed by Lorentz off‐axis electron holography and geometric phase analysis. The underlying mechanism of enhanced magnetic interaction in sharp structure is clearly deciphered by micromagnetic simulation. This study provides methodological guidance for dimensional engineering in fabricating hierarchical magnetic structure and significant insights into the morphology‐dependent loss mechanism for magnetic EM absorption materials.

Quantitative mapping of strain and displacement fields over HR-TEM and HR-STEM images of crystals with reference to a virtual lattice

A method for the reciprocal space treatment of high-resolution transmission electron microscopy (HR-TEM) and high-resolution scanning transmission electron microscopy (HR-STEM) images has been developed. Named “Absolute strain” (AbStrain), it allows for quantification and mapping of interplanar distances and angles, displacement fields and strain tensor components with reference to a user-defined Bravais lattice and with their corrections from the image distortions specific to HR-TEM and HR-STEM imaging. We provide the corresponding mathematical formalism. AbStrain goes beyond the restriction of the existing method known as geometric phase analysis by enabling direct analysis of the area of interest without the need for reference lattice fringes of a similar crystal structure on the same field of view. In addition, for the case of a crystal composed of two or more types of atoms, each with its own sub-structure constraint, we developed a method named “Relative displacement” for extracting sub-lattice fringes associated to one type of atom and measuring atomic columns displacements associated to each sub-structure with reference to a Bravais lattice or to another sub-structure. The successful application of AbStrain and Relative displacement to HR-STEM images of functional oxide ferroelectric heterostructures is demonstrated.

Ultrafast, room temperature rejuvenation of SiC Schottky diodes from forward current-induced degradation

In this work, we demonstrate the rejuvenation of Ti/4H-SiC Schottky barrier diodes after forward current-induced degradation, at room temperature and in a few seconds, by exploiting the physics of high-energy electron interactions with defects. The diodes were intentionally degraded to a 42% decrease in forward current and a 9% increase in leakage current through accelerated electrical stressing. The key feature of our proposed rejuvenation process is very high current density electrical pulsing with low frequency and duty cycle to suppress any temperature rise. The primary stimulus is, therefore, the electron wind force, which is derived from the loss of the momentum of the high energy electrons upon collision with the defects. Such defect-specific or “just in location” mobilization of atoms allows a significant decrease in defect concentration, which is not possible with conventional thermal annealing that requires higher temperatures and longer times. We show evidence of rejuvenation with additional improvement in leakage current (16%) and forward current (38%) beyond the pristine condition. Transmission electron microscopy, geometric phase analysis, Raman spectroscopy, and energy dispersive x-ray-spectroscopy reveal the enhancement of defects and interfaces. The ultrafast and room temperature process has the potential for rejuvenating electronic devices operating in high power and harsh environmental conditions.

Quantitative analysis of atomic-scale local lattice distortions in high-entropy fluorite oxide (Zr0·2Ce0.2Hf0.2Y0·2Al0.2)O2-δ

Novel high-entropy oxides have attracted considerable attention as promising candidates for high-temperature structural applications. The lattice distortion effect in such materials is closely related to their microstructural evolution and thermophysical performance. In this study, the local lattice distortions in the high-entropy fluorite oxide (Zr0·2Ce0.2Hf0.2Y0·2Al0.2)O2-δ were observed at the atomic scale using aberration-corrected scanning transmission electron microscopy and studied quantitatively using geometric phase analysis. The results showed that the radius and brightness of the atomic column projections exhibit slight variations owing to the random distribution of the cations, resulting in differences in the relative contents of the five elements in each atomic column. The anions were influenced by both the random distribution of the cations and the large number of oxygen vacancies, leading to M-O-M (M = Zr, Ce, Hf, Y, and Al) bond angles of 80–105°. The distributions of strains εyy and εxx (along the lattice spacing direction and vertical direction, respectively) within the crystal structure were mainly centred around ±4%, and there was a tendency for alternating positive and negative values in strains maps without significant aggregations. In addition, strains εxx and εyy increased significantly to 26.4% and 9.2%, respectively, within the dislocation areas.

Role of dislocation climb on twin boundary and antiphase boundary formations in inverse-spinel MnAl2O4

The defects, e.g., stacking faults (SFs), antiphase boundaries (APBs), and twin, have been considered the critical components commonly introduced during the synthesis process of spinel oxides. In this study, employing advanced transmission electron microscopy, the formation mechanisms of twin and APBs in MnAl2O4 spinel oxide were clarified. Atomic resolution scanning transmission electron microscopy image and image simulation clarify that MnAl2O4 holds the inverse-spinel-typed structure. Detailed analysis of the twin structure reveals that the asymmetric twin structure in MnAl2O4 forms through an (Mn, Al)-containing plane missing by dislocation climb-up and then continuous shearing on {111} plane. APB forms by dislocation climb-down on an (Mn, Al)-containing {111} plane. The detailed lattice distortion induced by dislocation climb-down was analyzed by geometric phase analysis. The burgers vector for APB formation is identified as a/61̅1̅1+a/411̅0. Frank dislocation of a/61̅1̅1 climb-down and the lattice strain from introducing the extra plane brings a displacement of a/411̅0, which results in terrace-ledge-like morphology of APBs. This study provides a detailed understanding of twin and APB formation mechanisms in inverse-spinel-typed MnAl2O4.

Heavy ion irradiation induced failure of gallium nitride high electron mobility transistors: effects of in-situ biasing

While radiation is known to degrade AlGaN/GaN high-electron-mobility transistors (HEMTs), the question remains on the extent of damage governed by the presence of an electrical field in the device. In this study, we induced displacement damage in HEMTs in both ON and OFF states by irradiating with 2.8 MeV Au4+ ion to fluence levels ranging from to ions cm−2, or 0.001–2 displacement per atom (dpa). Electrical measurement is done in situ, and high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray (EDX), geometrical phase analysis (GPA), and micro-Raman are performed on the highest fluence of Au4+ irradiated devices. The selected heavy ion irradiation causes cascade damage in the passivation, AlGaN, and GaN layers and at all associated interfaces. After just 0.1 dpa, the current density in the ON-mode device deteriorates by two orders of magnitude, whereas the OFF-mode device totally ceases to operate. Moreover, six orders of magnitude increase in leakage current and loss of gate control over the 2-dimensional electron gas channel are observed. GPA and Raman analysis reveal strain relaxation after a 2 dpa damage level in devices. Significant defects and intermixing of atoms near AlGaN/GaN interfaces and GaN layer are found from HRTEM and EDX analyses, which can substantially alter device characteristics and result in complete failure.

Anomalous Local Lattice Softening around Kink Boundaries in a Mille-Feuille Structured Dilute Mg–Zn–Y Alloy

We have evaluated local elastic properties at kink boundaries in dilute Mg-Zn-Y alloys based on scanning transmission electron microscopy (STEM), first-principles calculations, and geometrical phase analysis (GPA) by measuring atomic-scale strain fields around dislocation cores. STEM observations showed that dislocations constituting kink boundaries are extended into Shockley partial dislocations by accompanying solute-enriched stacking faults (SESF). Using GPA analysis, we found that strain distributions around the partial dislocation cores are asymmetric across the Mg matrix and the SESF, showing local elastic heterogeneity. Comparison with the calculated strain field model, it is indicated that the observed asymmetric strain profiles are essentially caused by a softening of the Mg matrix. This anomalous elastic softening can be interpreted as a change in strain energy of dislocations at the kink boundary, which may contribute to the unique deformation mechanism of kink in dilute Mg alloys.

Reverse transition of ball milling induced m-Y2O3 nano-crystallites during in-situ annealing

Structural transition during in-situ annealing of monoclinic-Yttria (m-Y2O3) is studied using X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The m-Y2O3 has been prepared by high energy ball milling of cubic-Y2O3 (c-Y2O3). Monoclinic to cubic (m → c) phase transition with detailed crystallite size analysis from the Rietveld fit on XRD profiles and strain analysis from Geometric Phase Analysis (GPA) on High-Resolution phase contrast-TEM micrographs has been discussed. During annealing, the m- and c-phases of Y2O3 coexist up to ∼850 °C. The data has been compared with those reported earlier for the milling induced c → m transition. At 1000 °C, the phase is completely c-Y2O3. HRTEM time-lapse images have been employed to examine the growth mechanism involved as they develop into cubic crystallites. Grain rotation and defect-assisted grain alignment govern the sluggish growth of crystallites.

Oxide-confined GaAs-based vertical-cavity surface-emitting laser: Measurement and modeling of the strain field

The strain field of the emitters of one unstressed and two electrically stressed oxide-confined GaAs-based vertical-cavity surface-emitting lasers has been analyzed. The components of the strain tensor in the optical aperture, in the quantum wells, and at the oxide edge of the lamellas have been measured by nano-beam precession electron diffraction and geometrical phase analysis. The measurements have been used to validate the simulations based on the finite element method (FEM) of the mechanical behavior of the emitters. The FEM model is used to quantify the strain caused by the lattice mismatch of the GaAs-based epitaxial layers and the volume variation of the Al 98Ga 2As layer due to selective oxidation. The first is calculated as a function of the misfit strain and corresponds very well to the measured values. The latter is calibrated assuming that the thickness of the oxide is expanded by 4 % in both the unstressed sample and the stressed sample with dislocations. Instead, it is contracted by 8 % for the sample where the stress test has caused further oxidation.

Influence of the hierarchical architecture of multi-core iron oxide nanoflowers on their magnetic properties

Magnetic properties of superparamagnetic iron oxide nanoparticles are controlled mainly by their particle size and by their particle size distribution. Magnetic properties of multi-core iron oxide nanoparticles, often called iron oxide nanoflowers (IONFs), are additionally affected by the interaction of magnetic moments between neighboring cores. The knowledge about the hierarchical structure of IONFs is therefore essential for understanding the magnetic properties of IONFs. In this contribution, the architecture of multi-core IONFs was investigated using correlative multiscale transmission electron microscopy (TEM), X-ray diffraction and dynamic light scattering. The multiscale TEM measurements comprised low-resolution and high-resolution imaging as well as geometric phase analysis. The IONFs contained maghemite with the average chemical composition gamma-Fe_{2.72pm 0.02}O_4. The metallic vacancies located on the octahedral lattice sites of the spinel ferrite structure were partially ordered. Individual IONFs consisted of several cores showing frequently a specific crystallographic orientation relationship between direct neighbors. This oriented attachment may facilitate the magnetic alignment within the cores. Individual cores were composed of partially coherent nanocrystals having almost the same crystallographic orientation. The sizes of individual constituents revealed by the microstructure analysis were correlated with the magnetic particle sizes that were obtained from fitting the measured magnetization curve by the Langevin function.

Influence of Ti elements on the evolution of microstructure, mechanical properties and thermal stability of Al-Cu alloy

This paper compares the evolution patterns of microstructure, mechanical properties and heat exposure response behavior of alloys with low (0.15 wt%) and high Ti (0.75 wt%) content prepared by conventional casting processes. The results show that the alloy with high Ti content has slightly finer grains and no improvement in the thermal stability of the θ' precipitates compared to the low Ti content alloy. Moreover, after exposing the as-aged alloy to 300 ℃ for 100 h, the high Ti content alloy exhibits higher tensile strength at room temperature, attributing to the reinforcement effect of Al3Ti nanoparticles. The results of geometric phase analysis and transmission electron microscope images reveal that the compressive strain field occurring on the Al3Ti/matrix interface can validly block the movement of dislocations, thus hardening the alloy. Finally, the classical nucleation theory was referred to explain the difference on the thermal exposure response behavior of two alloys with various Ti contents at 300 ℃. With increasing thermal exposure time, the stiffening role of the Al3Ti nanoparticles gradually overcomes the softening behavior caused by the destabilization of coarsened θ' and becomes the dominant factor governing the mechanical properties of the alloy. The theoretical analysis is consistent with the microstructure and the thermal exposure hardness response behavior.