Orientation Dependence Research Articles

Experimental aspects of 14N overtone RESPDOR solid-state NMR spectroscopy under MAS beyond 60 kHz

Nitrogen-14 (14N) overtone (OT) spectroscopy under fast magic angle spinning (MAS) conditions (>60 kHz) has emerged as a powerful technique for observing correlations and distances between 14N and 1H, owing to the absence of the first-order quadrupolar broadenings. In addition, 14NOT allows selective manipulation of 14N nuclei for each site. Despite extensive theoretical and experimental studies, the spin dynamics of 14NOT remains under debate. In this study, we conducted experimental investigations to assess the spin dynamics of 14NOT using the rotational-echo saturation-pulse double-resonance (RESPDOR) sequence, which monitors population transfer induced by a14NOT pulse. The 14NOT spin dynamics is well represented by a model of a two-energy-level system. Unlike spin-1/2, the maximum excitation efficiency of 14NOT coherences of powdered solids, denoted by p, depends on the radiofrequency field (rf-field) strength due to orientation dependence of effective nutation fields even when pulse lengths are optimized. It is also found that the p factor, contributing to the 14NOT spin dynamics, is nearly independent of the B0 field. Consequently, the filtering efficiency of RESPDOR experiments exhibits negligible dependence on B0 when the 14NOT pulse length is optimized. The study also identifies the optimal experimental conditions for 14NOT/1H RESPDOR correlation experiments.

Fabrication of Textured Ti<sub>2</sub>AlC-MAX Phase Ceramic by Slip Casting under Strong Magnetic Field and Spark Plasma Sintering, and Its Crystal Orientation Dependence of Room Temperature Deformation Behavior

Fabrication of Textured Ti<sub>2</sub>AlC-MAX Phase Ceramic by Slip Casting under Strong Magnetic Field and Spark Plasma Sintering, and Its Crystal Orientation Dependence of Room Temperature Deformation Behavior

Symmetry constraints on the orientation dependence of high-order elastic constants for the hexagonal boron nitride monolayer.

Group theory is a powerful tool to explore fundamental symmetry constraints for the physical properties of crystal structures, e.g. it is well-known that only a few components of the elastic constants are independent due to the symmetry constraint. This work further applies group theory to derive constraint relationships for high-order elastic constants with respect to the orientation angle, where the constraint relationships are more explicit than the traditional tensor transformation law. These analytic symmetry constraints are adopted to explain the molecular dynamics simulation results, which disclose that the high-order elastic constants are highly anisotropic with an anisotropy percentage of up to 25% for the hexagonal boron nitride monolayer. The elastic constant is a basic quantity in the mechanics field, so its high anisotropy shall cause strong anisotropy for other mechanical properties. Based on the anisotropic high-order elastic constants, we demonstrate that Poisson's ratio is highly anisotropic for the hexagonal boron nitride at large strains. These findings provide fundamental insights into the symmetry dependence of high-order elastic constants and other mechanical properties.

Temperature dependent stereodynamics in surface scattering measured through subtle changes in the molecular wave function.

A magnetically manipulated molecular beam technique is used to change the rotational orientation of H2 molecules which collide with a stepped Cu(511) surface and explore how the polarisation dependence of molecules scattering into the specular channel changes as a function of surface temperature. At all temperatures, H2 molecules that are rotating like cartwheels are more likely to be scattered into the specular channel than those that are rotating like helicopters. Furthermore, the scattered molecules are more likely to be rotating like cartwheels, regardless of their state before the collision. Increasing the temperature of the Cu(511) surface causes the polarisation effects to become stronger, with the scattering becoming more selective for H2 with cartwheel like rotation. Therefore, scattering a molecular beam of H2 from a Cu(511) surface and taking the molecules scattered into the specular channel provides a method to create a rotationally polarised beam of H2, where the polarisation can be tuned by changing the surface temperature. In contrast, the rotational orientation dependence observed for specular scattering from a flat Cu(111) surface is independent of surface temperature within the same temperature range.

Interrelationships of stress-induced martensitic phase transformation and pitting corrosion in iron-based shape memory alloys

Orientation dependence of stress-induced martensitic transformation under compression and the influence of a corrosion attack on superelastic properties were investigated for Fe42.7Mn34.7Al13.4Ni7.7Cr1.5 (at.−%) single crystals. The results of incremental strain tests show that the crystallographic orientation has a considerable impact on the superelastic performance, eventually resulting from the formation of twinned or detwinned martensite to accommodate strain as well martensite variant interaction. In order to investigate the effect of a corrosive environment on the mechanical performance and martensitic transformation, compression specimens were immersed in a 5.0 wt.−% NaCl solution for 24 h before tested in incremental strain tests. The immersion of the compression specimens revealed a partial surface corrosion attack including localized pitting corrosion. The localized corrosion attack increased the number of active martensite plates, most probably due to an induced multiaxial stress state. Further investigations on specimens subjected to −6% compressive strain revealed that areas with retransformed martensite serve as nucleation zones for corrosion damage. Stress-induced corrosion cracks developed, which eventually deteriorate functional response.

Resolving the Dependence of Aluminum Alloy Corrosion on Grain Orientation by Scanning Electrochemical Cell Microscopy

The study of grain-dependent corrosion behaviors of practical polycrystalline metals remains challenging due to the influences of other microstructural features, such as intermetallic particles and grain boundaries. In this work, we employed the oil-immersed scanning electrochemical cell microscopy (SECCM) to investigate the effect of grain orientations of a practical aluminum alloy AA7075-T73 on the surface electrochemical behaviors. Thousands of spatially resolved microscopic potentiodynamic polarization (PDP) measurements were performed, allowing the extraction of information only from grain interior areas excluding intermetallic particles and grain boundaries. The differences in the corrosion behaviors between grains were revealed. Cathodic currents exhibited a strong grain orientation dependence with a decreasing order of {101} > {001} > {111}, agreeing with the prediction from the order of atomic planar density. By contrast, the dependence of anodic currents on grain orientation was weak, and pitting was independent of grain orientation, which could be due to the limited mass transport of ions within the surface oxide film. This work highlights the capability of oil-immersed scanning electrochemical cell microscopy in resolving small electrochemical differences, which will greatly promote the study of grain-dependent behaviors of practical polycrystalline samples.

Long-period stacking ordered phase kinking, twinning behavior and dynamic recrystallization of Mg-Gd-Y-Zn-Zr alloy under severe shear deformation

A novel severe shear specimen with a maximum shear strain of 3 was designed to further clarify kinking and fragment behavior of long period stacking ordered (LPSO) phases, as well as their interaction with twinning behavior and dynamic recrystallization (DRX) behavior under severe shear deformation (SSD). The results indicate that promoted kinking and fragment behavior for LPSO phases is obseved for hihger activation of basal <a> dislocations induced by shear deformation. Besides the known crack causd by LPSO kinking, a new fragment process without kink bands formation is found for block-shaped LPSO phases under SSD, which follows three stages of micro-crack at the interface of LPSO/α-Mg matrix, crack and crush. Moreover, the actvition of tension twinning detected at kinked boundaries is reveled to be controlled by the combined effect of LPSO kinking and shear stress state. Specifically, crystal rotation related to LPSO kinking reorient grains to be possbile for twinning, and then high effective Schmid factor (ESF) under shear stress furter prioritise the twining acivaiton. Additionally, orientation dependence of DRX behaviors was weakened due to the particle-stimulated nucleation (PSN) mechanism caused by interdendritic block-shaped LPSO phases, which can accelerate the formation of low angle grain boundaries and DRX grains by activated pyramidal <c+a> slip systems even at basal grains.

Real-space tight-binding model for twisted bilayer graphene based on mapped Wannier functions

Twisted multi-layer heterostructures have been considered a platform for studying highly correlated many-particle systems, hosting the emerging phenomena of correlation physics. Electronic structure calculations of such materials which mainly focused on Twisted Bilayer Graphene (TBG) in the framework of the independent-electron approximation, despite the complexity, accurately match the experimental results around the Fermi level. Here, we present a convenient real space π-bands tight-binding model to calculate the band structure of TBG based on the Wannier interlayer hopping parameters obtained from non-twisted bilayer graphene. Our approach is based on mapping hopping parameters onto the real space TBG interlayer couplings, using Radial Basis Function (RBF) interpolation method. This accurate mapping, naturally, transfers all the symmetries and orbital shape features of the bilayer graphene lattice to the TBG lattice and preserves coupling orientation dependencies in addition to distance dependencies. The importance of this fact is explained by showing the effect of the threefold rotation symmetry of AB′ interlayer coupling on the flat band of the TBG band structure. In addition, due to real space study, the model gives us a comprehensive and intuitive view of the role of each interlayer orbital coupling in the TBG band structure and the structural variation relative to twisting.

Orientation-dependent hardness of Mo3Si studied by cube-corner nanoindentation

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Investigation on the planar Poisson’s ratio of -oriented Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics

Textured perovskite ferroelectric ceramics have attracted considerable attention from both academia and industry. In a recent study, an interesting phenomenon was observed in textured perovskite ferroelectric ceramics, i.e. the planar Poisson’s ratios of these ceramics ranged from − 0.05 to 0.1, much lower than those of their non-textured counterparts (0.3–0.4). In this study, we fabricated a < 001 > -textured Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramic using 5 vol% BaTiO3 templates. By studying the orientation dependence of Poisson’s ratio, we found that the < 001 > -oriented grain exhibits a large anisotropy of planar Poisson’s ratio, where some orientations even show negative Poisson’s ratio, and the lowest Poisson’s ratio is present as the x-axis is along < 110 > directions. Based on these results, the near-zero Poisson’s ratio was attributed to the intrinsic microstructural feature of < 001 > -textured perovskite ceramic, i.e. the out-of-plane orientation of most grains is along the < 001 > crystallographic direction and the in-plane orientations of the grains are randomly distributed.

Emergent zero-field anomalous Hall effect in a reconstructed rutile antiferromagnetic metal

The anomalous Hall effect (AHE) that emerges in antiferromagnetic metals shows intriguing physics and offers numerous potential applications. Magnets with a rutile crystal structure have recently received attention as a possible platform for a collinear-antiferromagnetism-induced AHE. RuO2 is a prototypical candidate material, however the AHE is prohibited at zero field by symmetry because of the high-symmetry [001] direction of the Néel vector at the ground state. Here, we show AHE at zero field in Cr-doped rutile, Ru0.8Cr0.2O2. The magnetization, transport and density functional theory calculations indicate that appropriate doping of Cr at Ru sites reconstructs the collinear antiferromagnetism in RuO2, resulting in a rotation of the Néel vector from [001] to [110] while maintaining a collinear antiferromagnetic state. The AHE with vanishing net moment in the Ru0.8Cr0.2O2 exhibits an orientation dependence consistent with the [110]-oriented Hall vector. These results demonstrate that material engineering by doping is a useful approach to manipulate AHE in antiferromagnetic metals.

Combined effect of piezoelectric, elastic, dielectric, and electro-optical properties on single-domain potassium tantalate niobate single crystals

The complete matrix parameters of the elastic, dielectric, and piezoelectric constants of high-quality Fe-doped KTa1-xNbxO3 (KTN) single crystals poled along [001]PC were successfully confirmed. The piezoelectric coefficients d33 and k33, which are of interest for various applications, are 137 pC/N and 50.4 %, respectively. The orientation dependence of these properties for the single-domain Fe:KTN samples was calculated, and it was found that the [001]PC direction is the optimal orientation for the highest performance with a single-domain state, at which the sample exhibits good transparency. Further, the single-domain sample exhibits a large effective electro-optical coefficient (γc = 126.6 p.m./V), and the combined piezoelectric and electro-optical effects provide a 0.4 % electro-optical coefficient. This is beneficial for applying high-response speed modulators with an effective electro-optical working mode and a low contribution of the inverse piezoelectric effect, owing to slower frequency response characteristics.

Nonreciprocal Antisymmetric Magnetoresistance and Unconventional Hall Effect in a Two-Dimensional Ferromagnet.

Two-dimensional (2D) ferromagnets with high Curie temperatures provide a rich platform for exploring the exotic phenomena of 2D magnetism and the potential of spintronic devices. As a prototypical 2D ferromagnet, Fe5-xGeTe2 has recently been reported to possess a high Curie temperature with Tc ∼ 310 K, making it a promising candidate for advancing 2D nanoelectromechanical systems. However, due to its intricate magnetic ground state and magnetic domains, a thorough study of the transport behavior related to its lattice and domain structures is still lacking. Here, we report a nonreciprocal antisymmetric magnetoresistance in Fe5-xGeTe2 nanoflakes observed under an external magnetic field between 85-120 K. Through a detailed examination of its temperature, field orientation, and sample thickness dependence, we trace its origin to an additional electric field induced by the domain structure. This differs from the previously reported antisymmetric magnetoresistance due to thickness inhomogeneity. Notably, at lower temperatures, we observed an unconventional Hall effect (UHE), which can be attributed to the Dzyaloshinskii-Moriya interaction (DMI) resulting from the non-coplanar magnetic moment structure. The pronounced influence of sample thickness on magneto-transport properties underscores the competition between magnetic anisotropy and DMI in Fe5-xGeTe2 flakes with varying thicknesses. Our findings provide a deeper understanding of the magneto-transport behavior of the exotic magnetic structure in 2D ferromagnetic materials, which may benefit future spintronic device applications.

RETRACTED: Influence of At%Ce on the electronic structure and mechanical properties of Al–Ce compounds from first-principles calculations

Ce as an additive in aluminum alloys can significantly improve the microstructure and properties of aluminum alloys. However, the structure and the associated overall properties of the Ce-Al binary system are not known. Here, we applied a first-principles approach to investigate the effects of At%Ce on the electronic structure, mechanical and thermodynamic properties of the Ce-Al compounds. The results show that CeAl2 exhibits the best stability at T = 0 K. The electronic structure analysis shows that they are both metallic. The ionic bonding of the Ce-Al compounds gradually weakened with increasing At%Ce. In addition, the Poisson's ratio of the Ce-Al compounds varies in the range of 0.2 and 0.32, and most of the Ce-Al compounds are brittle. The anisotropy results show that all the Ce-Al compounds exhibit more or less elastic anisotropy, where Young's modulus of the Ce-Al compounds has a strong orientation dependence. The obtained Debye temperatures indicate that Ce3Al11 has the strongest covalent bonds and good thermal conductivity.

Expanding the mechanical and mass-transport combination for bone scaffolds: Through stretched structure

Optimizing the mechanical and mass-transport properties of porous biomaterials simultaneously can be challenging, as they often correspond one-to-one through structure. For instance, increasing permeability by reducing relative density frequently leads to lower mechanical properties. In this study, we introduced a stretched structure to optimize the two properties simultaneously. Initially, the stretched scaffolds were evaluated at the same porosity, revealing that the elastic modulus and permeability of the stretched scaffold are approximately twice that of the original type. Subsequently, the orientation dependence of these properties was investigated. The results suggest that the stiffness of the stretched structure in the width direction is compromised, aligning with the mechanical properties of cortical bone in shaft bones. The permeability of the stretched structure exhibits significant anisotropy, with values much higher than those of original structure. Moreover, our biological experiments demonstrate that extremely stretched structures exhibit low and uneven curvature, potentially impeding cell growth. Hence, striking a balance between curvature and the stretching method employed is crucial. The stretching method can also be applied to other strut-based structures, enabling greater design flexibility in achieving a desirable mechanical and mass-transport combination, providing a foundation for high-performance artificial bone prostheses.

Effect of deformation temperature on strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C low-density steel

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect.

Influence of magnetic field on a novel scintillation dosimeter in a 1.5 T MR-linac.

For commissioning and quality assurance for adaptive workflows on the MR-linac, a dosimeter which can measure time-resolved dose during MR image acquisition is desired. The Blue Physics model 10 scintillation dosimeter is potentially an ideal detector for such measurements. However, some detectors can be influenced by the magnetic field of the MR-linac. To assess the calibration methods and magnetic field dependency of the Blue Physics scintillator in the 1.5 T MR-linac. Several calibration methods were assessed for robustness. Detector characteristics and the influence of the calibration methods were assessed based on dose reproducibility, dose linearity, dose rate dependency, relative output factor (ROF), percentage depth dose profile, axial rotation and the radial detector orientation with respect to the magnetic field. The potential application of time-resolved dynamic dose measurements during MRI acquisition was assessed. A variation of calibration factors was observed for different calibration methods. Dose reproducibility, dose linearity and dose rate stability were all found to be within tolerance and were not significantly affected by different calibration methods. Measurements with the detector showed good correspondence with reference chambers. The ROF and radial orientation dependence measurements were influenced by the calibration method used. Axial detector dependence was assessed and relative readout differences of up to 2.5% were observed. A maximum readout difference of 10.8% was obtained when rotating the detector with respect to the magnetic field. Importantly, measurements with and without MR image acquisition were consistent for both static and dynamic situations. The Blue Physics scintillation detector is suitable for relative dosimetry in the 1.5 T MR-linac when measurements are within or close to calibration conditions.

Orientation dependence of the nanocrystallization and magnetic behavior during annealing of the FINEMET alloy

The microscopic strain evolution and microstructural of FeCuNbSiB amorphous alloy samples were studied under both free and tensile stress annealing conditions using in-situ synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM). An amorphous-nanocrystalline structure was developed in both samples after annealing at 813 K, as the microstrains increased and decreased along the parallel and perpendicular stress directions respectively. The applied stress during annealing imposed limitations on the size of nanocrystals along the tensile and perpendicular stress directions, resulting in smaller sizes compared to those obtained under free annealing conditions, due to the reduction in Gibbs free energy by the applied stress and the subsequent enhancement of nucleation rates. The nanocrystalline size perpendicular to the stress direction was slightly larger than that along the parallel stress direction because the suppression of atomic diffusion perpendicular to the stress direction during crystallization could be favorable for local atomic rearrangement.

Enhanced piezoelectricity of Cu-doped K(Ta,Nb)O3 single crystals

As a class of solid solution material, K(Ta,Nb)O3 (KTN) single crystals have attracted significant interest due to their excellent piezoelectric and electro-optic performance. In this study, the piezoelectric properties of KTN were improved through Cu doping. Utilizing the advantages of composition-regulating phase transition, a large, high-quality Cu:KTN crystal measuring 30 mm × 25 mm × 20 mm in size was grown using the improved top seeded solution growth method. Cu-doped KTN exhibited better piezoelectric properties than pure KTN, and the full matrix parameters were investigated. Excellent dielectric, piezoelectric, and electromechanical coupling responses (ε11T∼ 2,136, d33∼303 pC/N, kt∼0.515, and k33∼0.672) were successfully obtained. To realize the optimized orientation of the piezoelectric properties, the orientation dependence of the single domain properties was investigated, and the mechanism of Cu doping to improve piezoelectric properties was explored. We found that 0.36% (in mass) Cu doped crystal consisted of A-site substitution, following ferroelectric and domain analysis. The small ferroelectric domain sizes led to a large domain wall mobility rate and high domain wall density, which contributed to high piezoelectric properties. This work revealed the effect of A-position doping on piezoelectric properties and provided a theoretical and experimental foundation for the performance optimization of KTN-based materials.

Hydrogen embrittlement of iron nanowires: investigating size and orientation dependence on loading behaviour

ABSTRACT With the ever-increasing use and applications of nanowires it has never been more imperative to understand how environmental interactions modify their unique mechanical properties and loading behaviour. While experimental research has shown that atomic hydrogen degrades mechanical properties through hydrogen embrittlement, results are limited and often do not directly quantify the hydrogen concentration or consider small diameter nanowires. In this study, we have used molecular dynamics simulations to the study the effect of atomic hydrogen on iron nanowires with various orientations and diameters. Results demonstrate that with increasing hydrogen concentration there is a clear reduction in the elastic modulus and yield stress as compared to the hydrogen free case for all diameters and orientations considered. In addition, this reduction in mechanical properties appears to exhibit a size dependence, with larger reductions being found in nanowires with larger cross-sectional diameters. We suggest that smaller diameter nanowires, with a higher ratio of surface to bulk atoms, are more influenced by free surface atoms than lattice distortions from atomic hydrogen. As this ratio of surface to bulk atoms is decreased, the larger diameter nanowires become less affected by free surfaces and more susceptible to the effect of atomic hydrogen.