Development Of Bands Research Articles

Synthesis and characterization of red-emitting Yb/Ho-CaSiO3 upconversion phosphors

Host material plays an important role in obtaining efficient photon upconversion and downshifting luminescence. Generally, fluoride and oxyfluoride glasses-based materials are used for high-efficiency photon upconversion. However, the poor thermal stability of fluoride glasses and the toxicity of fluorine ions limit their applications. In this report, Yb/Ho-doped CaSiO3 wollastonite phosphors have been demonstrated as efficient red-emitting upconversion phosphors. The phosphors have been synthesized by microwave hydrothermal process followed by heat treatment at 1050 ​°C in the air environment. 2M phase of the β-wollastonite has been confirmed by X-ray diffraction and Raman spectroscopy while the existence of the Yb and Ho dopants in the CaSiO3 lattice has been confirmed by X-ray photoelectron spectroscopy. The synthesized samples showed strong red upconversion emission centered at 662 ​nm and near-infrared downshifting emissions at 2016 ​nm upon 980 ​nm excitation. The emissions were found to depend significantly on the Ho concentration. Temporal evolution of the main emission bands was investigated to show that the energy transfer upconversion from Yb to Ho ions was responsible for the efficient upconversion and downshifting phenomenon.

Crystal plasticity finite element modeling of twin band formation and evolution together with the macroscale mechanical responses of hexagonal metals

Deformation twinning is one of the major plastic deformation modes of hexagonal metals. In this study, a new system of crystal plasticity finite element models is developed, with the cooperative interactions of dislocation slip and deformation twinning comprehensively involved. The obtained stress-strain curves for several differently-orientated Mg single crystals match well with experimental results, and the formation and thickening features of twin bands are captured. Simulation results reveal that (1) some thin twin bands are firstly formed in the shearing direction of the twinning plane, because of the much lower twinning resistance than that in the normal direction; (2) the softening and hardening of the twinning resistance play a critical role in the anisotropic mechanical response. By optimizing the twinning activation behaviors, the favorable interactions of twinning and dislocation slip can occur and the plastic deformation ability can be improved.

High frequency cutting force measurement during saw tooth chip formation of Al-7075-T6 for critical flow stress estimation

Shear banding is an unstable mode of deformation caused by material beginning to soften or weaken with additional strain, resulting in narrow bands of highly deformed material called shear bands. Shear bands occur in metal cutting when deforming material in the primary shear zone is subject to weakening by ductile damage and/or thermal softening by adiabatic heating. Recent work [1] has shown that the force supported by shear bands induced in hat-shaped test specimens subject to high-speed compression can be measured during the evolution of shear bands. This has given rise to the question whether very high band-width force measurement can be used to quantitatively measure the strain softening of material during high-speed machining. The challenge impeding measurement of cyclic changes in cutting and thrust force during shear banding cycles in orthogonal cutting is the rate at which these shear bands typically occur – from a few kHz to 100 kHz. This is well above the lowest natural frequency of even stiff cutting force dynamometer setups. This paper describes the development and validation of a high bandwidth dynamometer. This is used to measure both the cutting and thrust forces during orthogonal cutting of AA 7075-T6 with unprecedented fidelity. The critical flow stress at which flow softening initiates is calculated from the measured forces and chip morphology.

Defect engineered blue photoluminescence in ZnO:Al/TiO2 heterostructures

Tailoring the blue photoluminescence (PL) in Al-doped ZnO (AZO)/TiO2 heterostructures is demonstrated by a controlled induction of shallow defect centers by 50 keV Ar+-ions. This is established by a combination of temperature dependent PL and electron paramagnetic resonance spectroscopy. The dominant blue-violet PL in an as-grown sample comprises a near band-edge emission, along with a peak associated with a radiative recombination of the electrons in shallow donor levels (Zn interstitials) and the holes from the valence band. However, the evolution of an additional yellow-green PL band at a fluence of 1 × 1015 ions/cm2 is governed by deep donor levels, particularly ionized oxygen vacancies. Irradiation at 1 × 1016 ions/cm2 further leads to the formation of Zn vacancies (shallow acceptors) owing to the development of an O-rich surface. The structural modifications of these samples have been investigated by field-emission scanning electron microscopy , transmission electron microscopy, and Rutherford backscattering. While small micro-cracks are found at a fluence of 2 × 1016 ions/cm2, the formation of graded layers is obtained at the highest fluence of 5 × 1016 ions/cm2 owing to ballistic intermixing and diffusion of the constituents. Detailed investigation suggests that a significant amount of Ti atoms is diffused in AZO by a complete deterioration of the AZO/TiO2 matrix at the highest fluence.

Textile Bandwidth-Enhanced Coupled-Mode Substrate-Integrated Cavity Antenna with Slot

A textile bandwidth-enhanced coupled-mode substrate-integrated cavity antenna with a slot is presented. The original coupled-mode substrate-integrated cavity antenna is of two close resonances for the odd and even coupled modes, and a rectangular slot is added on the top layer to introduce a third resonance. Parameters are optimized to merge the bands of the three resonances to realize a widened −10 dB impedance band to cover the Medical Body Area Network band, 2.45 GHz Industrial Scientific Medical band and Long-Term Evolution Band7. The proposed antenna can operate in a −10 dB impedance band of 2.32–2.69 GHz with a 14.9% fractional bandwidth according to the measurements on a fabricated prototype. Simulation and measurement results illustrate the robustness of the proposed textile antenna in the vicinity of the human body and cylindrical bending conditions. In addition, the simulated specific absorption rate of the antenna radiation in the human body is lower than the IEEE and EN limits.

Shear band development under simple shear and the intrinsicality of strain softening of amorphous glassy polymers

The development of shear bands that induced by localized strain inhomogeneity is a characteristic deformation mechanism and determines ductility of amorphous glassy polymers. To investigate the essence of intrinsic strain softening and its influence on shear band evolution under shearing and reverse shearing, a full-network constitutive model based on the micro-mechanism is implemented into the material point method and finite element method respectively. The results show that the shear band nucleates and propagates in forward shearing, and gradually fades until disappears in following reverse shearing when the specimen regains its original geometry. As reverse shearing proceeds or there is a second forward shearing, the homogeneous plastic deformation occurs preferentially in the previously deformed band but it doesn't display any shear band like the initial forward shearing. It means the glassy polymers undergoing plastic deformation cannot be recovered by reverse straining. In further shearing, new shear bands tend to be initiated outside the existing band and then propagates to merge with the previous band. The initiation and propagation of shear band along the shearing direction is caused by the intrinsic strain softening, independent of loading histories and boundary conditions. The widening of shear band is related to the strain hardening. • Origin of intrinsic strain softening lies in the saturation of plastic shear strain. • Amount of intrinsic strain softening keeps fixed. • Glassy polymers undergoing plasticity cannot be recovered by reverse straining. • Full-network model can capture the shear band evolution under reverse shearing.

Evolution of geometric and electronic structures of oxygen-induced superstructures on Mo(110) surface: A LEED, ARPES, and DFT study

We have studied the evolution of atomic structure and electronic band dispersion of oxygen-induced superstructures on the Mo(110) surface by using low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory. Our experiments show that adsorption of oxygen at an elevated temperature (473 K) gets rid of the mixed-phase coexisting region observed at room temperature and exhibits three distinct superstructures: $p(2\ifmmode\times\else\texttimes\fi{}2$), $p(2\ifmmode\times\else\texttimes\fi{}1$), and $p(2\ifmmode\times\else\texttimes\fi{}6$) phases. Oxygenation of the Mo(110) surface leads to a confinement-induced gaplike opening at the zone center that can be tuned by the overlayer oxygen coverage. The ``hole'' pockets on the Fermi surface of the clean Mo(110) surface are found to persist upon oxygen adsorption with slightly changed volumes, while the ``electron'' pocket does not show any significant shift in momentum with no Fermi surface nesting behavior until saturation oxygen coverages. Apart from the significant modifications to the electronic states of the clean Mo(110) surface, we also observe the formation of new oxygen-induced bands. The evolution of the surface states near ${E}_{F}$ can be attributed to a change in the surface potential, while the evolution of oxygen-induced bands results from the hybridization between O-$2p\phantom{\rule{4pt}{0ex}}({m}_{j}=1/2$) and Mo-$4d$ (${m}_{j}=1/2$) orbitals at the interface. The interlayer separation, bond length, and lateral repulsive interactions between the adatoms are found to play a crucial role in the selective chemisorption of the Mo(110) surface.

Electronic and topological band evolution of VB-group transitionmetal monocarbides M2C (M=V, Nb, or Ta) bulk and monolayer

Two-dimensional transition metal monocarbides (2D MXenes) have attracted intense interest due to their versatility, predicted topological phase, and immense applications. In this study, we investigated the electronic and topological band evolutions of both the bulk and monolayer MXenes, M2C (M = V, Nb, or Ta), using first-principles calculations. Our study shows that all the three bulk M2C are semi-metallic and host topological phases. Interestingly, our study showed pristine monolayers V2C and Nb2C exhibit non-trivial topological phases, while Ta2C exhibits trivial phase. Unlike other MXenes, monolayer V2C and Nb2C host the quantum spin Hall effect without functionalizations. Furthermore, thickness-dependent calculations intriguingly show Lifshitz electronic transition from semi-metallic to topological insulating phase in V2C from bilayer to monolayer with a sufficiently large bandgap of 0.32 eV. Moreover, the topological phase transition between the TI state and the trivial state in V2C is driven by quantum size effects as the Z2 topological invariant notably oscillates between 1 and 0 with varying thickness. Finally, our study demonstrated that VB MXenes could be promising topological materials for spintronics applications.

Numerical analysis of bifurcation and shear band measurement in geomaterials

Study of strain localization in geomaterials generally consists of two aspects, the pre-localization regime and the post-localization regime. The former refers to the onset and the orientation of the shear band, which corresponds to the bifurcation theory. The latter deals with the width and the evolution of the shear band. The purpose of this paper is to make a full-range analysis of the shear band in structural level. By comparing the bifurcation conditions in classical continuum an analogical bifurcation condition in micropolar theory is defined. The factors which may affect the shear band orientation in the numerical modeling process are emphasized, and a basic principle of the geometric modeling is concluded. Furthermore, the existing shear band width measurement methods are summarized, and some other more effective methods considering the singularity of the discontinuous surface is suggested. At the last, the ratio between the node distance of the element and the internal length less than 5 is suggested as reasonable choice.

In vitro digestion of tofu with different textures using an artificial gastric digestive system

Two kinds of tofu with obvious differences in texture [“GDL” and “CaSO4”, standing for tofus made with the application of either glucono-δ-lactone (GDL) or calcium sulfate, with measured hardness 23.1 ± 3.3 g and 105.2 ± 25.1 g, respectively] were used as to investigate the in vitro progress and extent of tofu digestion, using an independently-developed artificial gastric digestion system (AGDS). The particle size distributions of both CaSO4 and GDL tofu shifted towards smaller particles as the digestion time increased, while the viscosity of the gastric digesta also increased. Tofu proteins were hydrolyzed in the simulated stomach, with GDL tofu showing a higher hydrolysis rate, based on the temporal evolution of SDS-PAGE bands, and had a higher amino acids accumulation than CaSO4 tofu at the end of gastric digestion. In the absence of peptic enzymes, the protein was acidically-hydrolyzed, but the degree of hydrolysis was much lower than in the presence of enzymes; these findings are in accord with the changes in microstructure observed by scanning electron microscopy. The results indicated that the in vitro extent of tofu digestion is related to its hardness, which is in turn related to its microstructure; they also indicated the potential of our developed in vitro dynamic stomach in studying semi-solid foods.

Geostationary Orbital Debris Collision Hazard after a Collision

Many space objects are densely distributed in the geostationary (GEO) band, and the long-term impact of the collision of GEO spacecraft and space debris on the GEO environment has attracted more and more attention. After summarizing the advantages and disadvantages of the long-term evolution model based on the “Cube” collision probability calculation model, the “Grid” model, a long-term evolution model especially suitable for GEO band, was established. For four types of collision and disintegration events, the “Grid” model was used to study the space environment in the GEO band after collisions between GEO spacecraft and space debris. Future collisions were simulated, and the number of space objects in the next 100 years was counted. Once space debris and massive spacecraft were completely disintegrated after collision, the number of space objects and the collision probability increased sharply, and this caused a collision cascading syndrome. Even if there was no initial disintegration event, collision and disintegration events occurred in the long-term evolution of the GEO band, which led to an increase in the number of space objects. However, the collision probability was much lower, and the number of space objects grew much more slowly without the initial collision.

Strain rate effects on the yielding strength and maximum temperature at shear bands in a Zr-based bulk metallic glass

The effects of strain rate on the yielding strength and maximum temperature at shear bands in a typical Zr41.2Ti13.8Ni10Cu12.5Be22.5 (Vit 1) bulk metallic glass are investigated under tension and compression over a wide range of strain rates at ambient temperature. Using the modified cooperative shear model incorporating the notable internal thermal effect at high strain rates, the transition of the strain rate effect of yielding strength from the sudden decrease to the subsequent slow change with increasing the strain rate is quantitatively characterized. The fracture surface temperature evolution under different shear band evolution times is captured by a hierarchical multi-scale model of heat conduction. Dynamic strain rates shorten the shear band evolution time, leading to an increase in the maximum temperature at shear bands compared to quasi-static loadings.

Microstructure evolution of twinning-induced shear bands and correlation with ‘RD-split’ texture during hot rolling in a Mg-1.1Zn-0.76Y-0.56Zr alloy

Unlike the recrystallization-induced shear band mechanism which has been well accepted for the RE-free Mg alloys during rolling, an understanding of deformed microstructure evolution (twin mode/variant selection and dislocations) at different strains is crucial for interpreting the shear band mechanism in the RE-containing Mg alloys upon hot rolling. In this study, a Mg-1.1Zn-0.76Y-0.56Zr alloy ingot was subjected to hot rolling at 440 °C with single-pass reductions of 10–50%, and the microstructure evolution and texture development of this hot-rolled plates at different reductions were investigated, with aim to disclose the roles of different types of twins/variants and dislocation mechanisms in the shear band (SB) formation and their correlation with the macro-texture characteristics. As the dominate twinning mode for most parent grains at lower reductions, and the primary {10−12} extension twins/variants together with basal <a > slip activated in twins contribute to formation of basal texture in the twinned grains, thus subsequently facilitating the operation of secondary {10−11} and {10−13} compression twins and following {10–12} twins. Afterwards cross-grain boundary (GB) paired twins for compression twin and their double twins were frequently observed at the nearby basal-oriented parent grains with low misorientation (≤34.5°), which should correspond to the predecessor of SBs. Further EBSD observation at higher reductions (R0.4 and R0.5) reveals that most of the banding structures are distributed homogenously and are likely transformed from the {10–11}-{10–12} twins, which are the predominant twin types under such state. The ‘RD-split’ textures start to be formed at lower reductions (R0.3) and keeps stable at higher reductions (R0.4-R0.5), which were explained by the correlations with some variant pairs of {10–12} twins and/or {10–11}-{10–12} twins predicted by Schimid factor analysis as well as shear bands. Nevertheless, the contributions of <c + a > slip to the ‘RD-split’ texture could be ignored on basis of corresponding EBSD analysis and reappearance of such texture after rolling at a low temperature (170 °C). • Primary {10-12} twins/variates and basal slip activated in twins contribute to basal texture formation in twinned grains. • Secondary compression twins and ternary {10-12} twins were mainly activated after basal orientation formation. • Paired twins for double twins at the nearby basal-oriented grains are likely the predecessor of shear bands. • The ‘RD-split’ texture was mainly correlated with {10-12} twins, double twins and shear bands with different activity.

A deep look into the diverse surface speciation of the mono-molybdate/lepidocrocite system by ATR-IR and polarized ATR-IR spectroscopy

The present study is focused on the adsorption of monomeric molybdate anions on the surface of lepidocrocite investigated by several infrared spectroscopy methods. To study the surface speciation of molybdates on lepidocrocite, the behavior of the anion at the interface upon increasing anion concentration (fixed pH) or decreasing pH (fixed concentration) was analyzed in situ. The obtained results expand the knowledge for surface speciation by revealing a second molybdate complex, which is more stable, arising from the interaction with defects on the (010) facets of lepidocrocite. A curve fitting procedure enabled us to confirm what is observed from the pristine spectra, and outer-sphere complexes were probed as well. The evolution of the different bands upon increasing adsorbate concentration and decreasing pH, as obtained by curve fitting, shows a complex evolution for the different surface species with protonation, with the formation of strongly bonded complexes at low pH. The 2D synchronous and asynchronous spectra support the interpretation of the data. Furthermore, insights into the orientation of the complexes with respect to the lepidocrocite surface are obtained by using linearly polarized radiation. The results confirm most of the conclusions ascertained on the basis of the unpolarized spectra. The combination of the different methodologies yields a consistent picture for the complexity of this system and paves the way for subsequent utilization of this approach to study other systems.

Topological band transition between hexagonal and triangular lattices with (p x , p y ) orbitals

By combining tight-binding modelling with density functional theory based first-principles calculations, we investigate the band evolution of two-dimensional (2D) hexagonal lattices with (p x , p y ) orbitals, focusing on the electronic structures and topological phase transitions. The (p x , p y )-orbital hexagonal lattice model possesses two flat bands encompassing two linearly dispersive Dirac bands. Breaking the A/B sublattice symmetry could transform the model into two triangular lattices, each featuring a flat band and a dispersive band. Inclusion of the spin–orbit coupling and magnetization may give rise to quantum spin Hall and quantum anomalous Hall (QAH) states. As a proof of concept, we demonstrate that half-hydrogenated stanene is encoded by a triangular lattice with (p x , p y ) orbitals, which exhibits ferromagnetism and QAH effect with a topological gap of ∼0.15 eV, feasible for experimental observation. These results provide insights into the structure-property relationships involving the orbital degree of freedom, which may shed light on future design and preparation of 2D topological materials for novel electronic/spintronic and quantum computing devices.

Erratum: Topological band evolution between Lieb and kagome lattices [Phys. Rev. B 99, 125131 (2019)

Received 18 March 2022DOI:https://doi.org/10.1103/PhysRevB.105.159901©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectronic structurePhase transitionsTopological phases of matterPhysical SystemsKagome latticeTopological materialsTechniquesTight-binding modelCondensed Matter, Materials & Applied Physics

Manipulation of Dirac band curvature and momentum-dependent g factor in a kagome magnet

The Zeeman effect describes the energy change of an atomic quantum state in a magnetic field. The magnitude and direction of this change depend on the dimensionless Landé g factor. In quantum solids, the response of the Bloch electron states to the magnetic field also exhibits the Zeeman effect, with an effective g factor that was theoretically predicted to depend on the momentum1,2,3, and which may be particularly strong in topological and magnetic systems. However, the momentum dependence of the g factor is difficult to extract and it has not been directly measured. Here we report the experimental discovery of a momentum-dependent g factor in the kagome magnet YMn6Sn6. We map the evolution of a massive Dirac band in the vicinity of the Fermi level as a function of the magnetic field. We find that electronic states at different lattice momenta exhibit different Zeeman energy shifts, giving rise to an anomalous g factor that peaks around the Dirac point. Our work provides a momentum-resolved visualization of Dirac band curvature manipulated by a magnetic field, which will be relevant to other topological kagome magnets.

Spatio-Temporal Statistical Characterization of Boundary Kinematic Phenomena of Triaxial Sand Specimens.

This paper follows up on a reference paper that inspired MDPI’s Topic “Stochastic Geomechanics: From Experimentation to Forward Modeling”, where global and local deformation effects on sand specimens are fully described from high resolution boundary displacement fields, and supported by its experimental database, which is open to the scientific community for further study. This paper introduces the use of spatio-temporal statistics from a subset of such an experimental database to characterize the specimens’ spatio-temporal displacement fields, populated by repeating a set of triaxial compression tests on drained, dry, vacuum-consolidated sand specimens, tested under similar experimentally controlled conditions. A three-dimensional digital image correlation (3D-DIC) technique was used to measure the specimens’ boundary displacement fields throughout the course of shearing under axial compression. Spatio-temporal first- and second-order statistics were computed for different data dimensionality conditions (0D, 0D-T, 1D-T, 3D-T) to identify and characterize the dominant failure mechanisms across different testing specimens. This allowed us to quantify localization phenomena’s spatio-temporal uncertainty. Results show that the uncertainty captured along the deformation process across different dimensionality conditions can be directly associated with different failure mechanisms, including localization patterns, such as the onset and evolution of shear, compression, and expansion bands. These spatio-temporal observations show the dependencies between locally distinctive displacement regions over a specimen’s surface, and across different times during a specimen’s shearing process. Results of this work provide boundary spatio-temporal statistics of experimental evidence in sands, which sets the basis for the development of research on the numerical simulation of sand’s constitutive behavior. Moreover, it allows to add a new understanding on the effect of uncertainty on the mechanistic interpretation of sands’ kinematic phenomena.

Influence of cellular subgrain feature on mechanical deformation and properties of directed energy deposited stainless steel 316 L

The fundamental deformation mechanism responsible for the exceptional combination of strength and ductility exhibited by SS 316 L material manufactured using directed energy deposition (DED) based additive manufacturing (AM) technique was studied. First, the stress-strain behaviors under uniaxial tensile testing of specimens containing different intensities of cellular subgrain feature were obtained. The stress-strain behaviors showed large differences with different intensities. Then, the microscopic deformation under in-situ tensile testing was characterized. At intragranular length scale, results revealed an extensive formation of a heterogeneous surface texture with a characteristic length scale and morphology consistent with the geometry of the cellular subgrain feature. The results also revealed that the high strain hardening rate regime of the macroscopic stress-strain behavior is primarily driven by deformation slipping, not deformation twinning. Finally, the evolution of the slip bands under uniaxial tensile testing was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results revealed a barrier effect of cellular subgrain feature on slip band evolution. For a given macroscopic strain the width, depth, and spacing of slip band formation varied with the intensity of the cellular subgrain feature. The heterogeneous texture formation and barrier effect on slip band evolution observed in this study were clear evidence of the extensive influence of cellular subgrain feature on microscopic deformation and mechanical properties. Additionally, these remarkable observations revealed that dynamic slip band refinement induced by cellular subgrain feature is the fundamental mechanism that facilitates the exceptional combination of strength and ductility of DED manufactured SS 316 L • Cellular subgrain feature of DED 316 L produces a textured deformation and disrupts slip band evolution substantially. • Causes dynamic slip band refinement resulting in high strain hardening rate. • High strain hardening rate results in excellent combination of strength and ductility. • Early-stage plastic deformation is dominated by deformation slipping, not deformation twinning.

Microstructural evolution of shear bands formation of metallic glasses under different loading conditions and strain rates

The shear bands (SBs) evolution of Cu50Zr50 metallic glasses (MGs) by the uniaxial loading under different strain rates are investigated on the temporal and spatial scales using the molecular dynamics (MD) simulation. The stress-strain curve shows a corresponding relation to the microstructure parameter curve in the form of four-stage, together with a synchronized evolution of multiple shear bands. During the propagation of the dominate shear band (DSB), the rearrangement of global microstructure is induced by the different transformation of three categories of clusters located in different zones. The synchronized evolutions of the microstructure, the deformation and the mechanical property are found to be less affected by loading condition or strain rate. However, with the increase of strain rate, the initiation and propagation of the DSB are observably retarded. As a consequence, the fracture mode of the MGs presents a change from a sliding along the DSB to a uniform deformation.