Periodic Stacking Research Articles

Heterodimensional superlattice with in-planeanomalous Hall effect.

Superlattices-a periodic stacking of two-dimensional layers of two or more materials-provide a versatile scheme for engineering materials with tailored properties1,2. Here we report an intrinsic heterodimensional superlattice consisting of alternating layers of two-dimensional vanadium disulfide (VS2) and a one-dimensional vanadium sulfide (VS) chain array, deposited directly by chemical vapour deposition. This unique superlattice features an unconventional 1T stacking with a monoclinic unit cell of VS2/VS layers identified by scanning transmission electron microscopy. An unexpected Hall effect, persisting up to 380 kelvin, is observed when the magnetic field is in-plane, a condition under which the Hall effect usually vanishes. The observation of this effect is supported by theoretical calculations, and can be attributed to an unconventional anomalous Hall effect owing to an out-of-plane Berry curvature induced by an in-plane magnetic field, which is related to the one-dimensional VS chain. Our work expands the conventional understanding of superlattices and will stimulate the synthesis of more extraordinary superstructures.

On the cyclic hardening/softening behavior of a rare earth-containing magnesium alloy during strain-controlled fatigue

The origin of cyclic hardening under low strain amplitude (with N = 25284 cycles to failure) followed by pseudo-plateau response under high strain amplitude (with N = 2671 cycles to failure) during fully reversal (R = −1) strain-controlled pull–push fatigue of Mg-8.2Gd-3.6Y-1.6Zn-0.5Zr (wt.%) at room temperature condition was explored. The (i) dislocations trapping between LPSO (long period stacking order) lamellas, (ii) substructure development, and (iii) deformation twinning and tangling/rearrangement of partials within twin band were known as the main factors contributing to the room temperature cyclic hardening response. The pseudo-plateau behavior and cyclic stabilization were also attributed to the co-formation of the veins structure in the vicinity of 18R LPSO and reversion of 14H LPSO at higher strain amplitudes.

Structural Change of MgNiY Long Period Stacking Order Investigated with Atom Probe

Structural Change of MgNiY Long Period Stacking Order Investigated with Atom Probe

Laser welding Al–Si coated hot stamping steel in conduction mode: weld formation and Al-rich microstructure

The non-uniform mixture microstructure is always revealed in laser welds of Al–Si coated hot stamping steel. It is very necessary to understand aluminum-rich segregation on phase morphology evolution. At same time the uniform and high aluminum welds were tried to obtain by replacing keyhole mode with conduction mode. The uniform fusion zones contained 3–6 wt.% Al, higher than normal keyhole welds. The weld microstructure was mixed of delta ferrite, lath martensite and few discussed needle phases with different morphology. Lower Al region revealed high fraction of island martensite, when aluminum content w(Al) > 5 wt.%, the needle phase was precipitated from the matrix of delta ferrite. High resolution transmission electron microscopy confirmed it long periodical stacking ordered structure in body-cubic crystal matrix.

Excitation of Hybrid Waveguide-Bloch Surface States with Bi2Se3 Plasmonic Material in the Near-Infrared Range.

Bloch surface waves (BSWs) with Bi2Se3 in a composite structure consisting of a coupling prism, distributed Bragg reflector (DBR) and cavity layer have been demonstrated. The design relies on the confinement of surface waves that originates from the coupling between the defective layer of plasmonic material (Bi2Se3) and DBR. The presence of the cavity layer modifies the local effective refractive index, enabling direct manipulation of the BSWs. The transfer matrix method (TMM) is used to evaluate the reflectance and absorptance responses in the spectral domain for various angles of incidence, demonstrating the presence of sharp resonances associated with the BSW. With an optimal thickness of DBR bilayers, the energy of an evanescent wave can be transferred into the periodic stack resulting in the excitation of waveguide modes (WGMs). It is believed that the proposed design possesses the advantage in terms of easy fabrication to develop integrated photonic systems, especially for biological and chemical sensing.

Study on broad flexural wave bandgaps of piezoelectric phononic crystal plates for the vibration and noise attenuation

This paper explores a new approach to reducing the interior noise of one vehicle body through the flexural wave bandgap. The phononic crystal is designed by attaching the periodic piezoelectric stacks with the shunting circuits on the host plate. To better implement the frequency-dependent properties, the theoretical model of the piezoelectric stack is improved through capacitance identification. On this basis, the plane wave expansion method is employed to calculate the bandgap. An ultra-wide bandgap is achieved by tuning the shunting circuits and confirmed via the finite element method. The influence of electrical parameters on flexural wave attenuation is investigated. The applicability of the presented resonators for different host plates under the presented design guideline is examined. Subsequently, the concept of phononic crystal with the unique bandgap is exploited to enhance the interior sound performance of one vehicle body. Combining the sound response analysis with the panel acoustic contribution analysis, the numerical study is performed to pick out the critical panels for the arrangement of resonators. Then a circuit control strategy is proposed to activate the wide bandgap. The results demonstrate that the present piezoelectric phononic crystal plate has a significant advantage in hindering the flexural wave propagation and suppressing structure-borne noise over a broadband low-frequency range.

Towards high strength cast Mg-RE based alloys: Phase diagrams and strengthening mechanisms

Mg-rare earth (RE) based systems provide several important commercial alloys and many alloy development opportunities for high strength applications, especially in aerospace and defense industries. The phase diagrams, microstructure, and strengthening mechanisms of these multicomponent systems are very complex and often not well understood in literature. We have calculated phase diagrams of important binary, ternary, and multicomponent RE-containing alloy systems, using CALPHAD (CALculation of PHAse Diagrams). Based on these phase diagrams, this paper offers a critical overview on phase equilibria and strengthening mechanisms in these alloy systems, including precipitation, long period stacking order (LPSO), and other intermetallic phases. This review also summarized several promising Mg-RE based cast alloys in comparison with commercial WE54 and WE43 alloys; and explored new strategies for future alloy development for high strength applications. It is pointed out that the combination of precipitation and LPSO phases can lead to superior strength and ductility in Mg-RE based cast alloys. The precipitates and LPSO phases can form a complex three-dimensional network that effectively impedes dislocation motion on the basal and non-basal planes. The LPSO phases can also prevent the coarsening of precipitates when they interact, thus providing good thermal stability at elevated temperatures. Future research is needed to determine how the combination of these two types of phases can be used in alloy design and industrial scale applications.

Modeling hydrolysis kinetics of dual phase α-Mg/LPSO alloys

This work proposes a new modeling of hydrolysis reaction in simulated seawater solution (35 g/L NaCl) with alloys containing α-Mg and Long Period Stacking Ordered (LPSO) phases. Alloys with different chemical compositions or thermal treatments were synthesized, leading to different (i) microstructures, (ii) α-Mg over LPSO phase fractions or (iii) LPSO type (18R or 14H). Classical nucleation and growth equation is used as an indicator of the reaction mechanisms but a new model is proposed to describe the complex kinetics curves (Vol (H2) = f(t)) obtained. This new model has several advantages: (i) it allows to discriminate the contribution of each phase, (ii) it could be applied to any alloy with phases reacting and (iii) it applies to the whole kinetics curves (i.e. from 0 to 100% yield). It is supported by the comparisons between the different alloys and SEM observations of their microstructure before and after exposure to the seawater solution.

Multilayer transducer for highly efficient initiation of time-resolved Brillouin scattering

Structures made of a metallic film deposited on a substrate are conventionally used as opto-acoustic transducers for picosecond ultrasonic experiments where detection in the time domain of the Brillouin scattering in a transparent sample is sought. In this paper, we substitute the metallic film for a periodic stack of nanometric layers made of gold and lithium fluoride to increase the amplitude, at the Brillouin frequency shift, of the strain generated by the photo-thermal effect. A model is used to analyze the generated strain amplification with the volume fraction and with the total thickness of this structure and to evaluate the gain in terms of sample dynamic reflectivity changes. Amplification by a factor of 20 is measured when using the composite structure with respect to signals detected with a transducer made of a single gold layer.

Effects of Laser Surface Remelting on Microstructure and Corrosion Properties of Mg-12Dy-1.1Ni Alloy

Effects of laser surface remelting (LSR) on microstructure and corrosion properties of as-cast Mg-12Dy-1.1Ni (wt.%) alloy were investigated. The as-cast alloy was mainly composed of α-Mg, lamellar Mg12DyNi phase with an 18R-long period stacking order (LPSO) structure and Mg24Dy5. After LSR treatment, fine and compact 18R-LPSO phase reprecipitated on dendrite boundaries as a continuous network. In addition, the volume fraction of the LPSO phase remarkably increased to 34.9% and the grain size of as-cast alloy was refined to ca. 4 μm. Electrochemical and immersion tests indicated that the LSR-treated alloy exhibited a lower weight loss rate (2.8 ± 0.2 mg/cm2/h) and corrosion current density (160.1 ± 20.7 μA/cm2, and of 37% reduction after LSR) than the as-cast alloy in 0.1 M NaCl solution. The improved corrosion resistance of the LSR alloy was mainly ascribed to the grain refinement and continuous distribution of 18R-LPSO phase on dendrite boundaries.

Substrate orientation dependent characteristics of half-metallic and metallic superlattices [La0.7Sr0.3MnO3/LaNiO3]10

We report a detailed study on the orientation dependent growth characteristics, electronic structure, transport, magnetic, and vibrational excitations in atomically flat interfaces of [La0.7Sr0.3MnO3/LaNiO3]10 superlattices (SLs) coherently grown on (001/011/111)-SrTiO3 substrates by the pulsed laser deposition technique. X-ray reflectometry confirms the periodic superlattice stacks from the Kiessig interference fringes and well-defined even interfaces between the nickelate and manganite layers. A complex local atomic environment across the interfaces was noticed, yet trivalent La, divalent Sr, and mixed valent Ni2+/3+ and Mn3+/4+ electronic states prevail at the core level with enhanced relative intensity ratio of the Mn ions in the superlattices grown on (111) oriented SrTiO3 substrates as compared to those grown on (001) and (011) oriented SrTiO3. The temperature (5≤T≤300K) dependence of electrical resistivity ρ(T) analysis reveals 3D variable range hopping model [ρ(T)=ρ0exp⁡(T0/T)(1/4)] with large magnitude of hopping energies (≥40 meV) for the SL-111 system associated with the high energy gap developed by the accumulation of disorderness in the individual constituents of polar layers. Moreover, all SL systems exhibit reduced ferromagnetic ordering temperatures (67≤TC≤110K) with a low-temperature anomaly (11.4≤T∗≤22K) and a substantial enhancement in the effective exchange interaction (Jeff∼3.52meV) having altered ground state-spin configuration S∼1/2 different from S=3/2 of La0.75Sr0.25MnO3. Nevertheless, the SL-011 system exhibits large anisotropy field HK∼18kOe and cubic anisotropy constant K1∼9.3×103J/m3 in comparison to the other two orientations. The second order two-phonon interaction driven by the local polaronic distortion causes significant changes in the vibrational excitations of the investigated system. Nonetheless, most of the Raman modes follow the substrate-induced, highly oriented epitaxial growth pattern except for two modes ν4 (326cm−1) and ν8 (728cm−1), which slightly differ in the case of SL-111 superlattices.

Phase equilibria and microstructure investigation of Mg-Gd-Y-Zn alloy system

• Confirmation of W-phase (Mg 3 Y 2 Zn 3 ) instead of Mg 3 RE in Mg-Gd-Y-Zn alloy system. • Accurate prediction (within 5 mol%) of LPSO 14H phase due to improved thermodynamic model. • Critical phase equilibria understanding of Mg-Gd-Y-Zn alloy system for alloy design. In order to develop high strength Mg-Gd-Y-Zn alloys, key experiments coupled with CALPHAD (CALculation of PHAse Diagrams) calculations were carried out in the current work to provide critical understanding of this important alloy system. Three Mg-10Gd-xY-yZn ( x = 4 or 5, y = 3 or 5, wt.%) were mapped on Mg-Gd-Y-Zn phase diagrams for phase equilibria and microstructure investigation. Electron microscopy was performed for phase identification and phase fraction determination in as-cast and solution treated conditions. In all three alloys, the major phases were Mg-matrix and long period stacking order (LPSO) 14H phase. With ST at 400 and 500 °C, the phase fraction of LPSO 14H increased, particularly the fine lamellar morphology in the Mg matrix. The as-cast and 400 °C Mg10Gd5Y3Zn samples had Mg 5 (Gd,Y) present. At 500 °C, Mg 5 (Gd,Y) is not stable and transforms into LPSO 14H. The Mg10Gd5Y5Zn alloy included the W-Phase, which showed a reduction in phase fraction with solution treatment. These experimental results were used to validate and improve the thermodynamic database of the Mg-Gd-Y-Zn system. Thermodynamic calculations using the improved database can well describe the available experimental results and make accurate predictions to guide the development of promising high-strength Mg-Gd-Y-Zn alloys.

Atypical pathways for lamellar and twinning transformations in rapidly solidified TiAl alloy

Additively manufactured alloys undergo a rapid solidification process followed by thermal cycles, which promotes the microstructures to return to equilibrium. The microstructural behavior therein is especially complicated and inconceivable at times, thereby greatly restricting the tailorability of microstructures for additive manufacturing. To improve the understanding of microstructure formation in rapidly solidified TiAl alloy, Ti–47Al–2Cr–2Nb powders were aging-treated to reveal the microstructural evolution behavior on multiple scales, and the microstructural transformation mechanisms were comprehensively analyzed from the viewpoints of thermodynamics and kinetics. The results show that the rapidly solidified TiAl alloy almost retains the α2 phase, which then transforms into nanolamellar structures or equiaxed γ microstructures depending on the temperature. The completely different microstructure selections were determined by the competitive growth of lamellar structures and equiaxed γ grains. More importantly, atypical pathways for lamellar and twinning transformations were first discovered in these microstructural transformation processes. A 6H-type long periodic stacking ordered (LPSO) phase was observed to be in the form of straight laths in the retained α2 phase, and 9R- and 18R-type LPSO phases were discovered in the equiaxed γ grains. The 6H-type LPSO phase transformed into nano γ laths, whose formation driving force was deduced to result from the extremely high elastic energy stored in the retained α2 phase. The 9R- and 18R-type LPSO phases transformed into a more stable γtwin phase to reduce the entire energy of the system. This research may potentially enhance the understanding of the microstructure of additively manufactured TiAl alloy and consequently, the tunability of such microstructure as required.

Quasiperiodic Van der Waals Heterostructures of Graphene and Hexagonal Boron Nitride

The tunability of properties of 2D materials stacked in heterostructures is largely explored nowadays. In this work (see article number 2100423 by Biplab Sanyal and co-workers), quasiperiodic van der Waals heterostructures of 2D graphene and h-BN sheets were studied in a Fibonacci sequence by density functional theory. Graphene and h-BN being semimetal and insulator respectively, suitable combinations can produce interesting changes in electronic properties. The authors show that these quasiperiodic heterostructures are more stable than normal periodic stacking of monolayers of graphene and h-BN. Moreover, for certain arrangements of atomic layers, sizeable bandgaps can be obtained at the Dirac point.

Microstructure and yield phenomenon of an extruded Mg-Y-Cu alloy with LPSO phase

A yield phenomenon was firstly reported in an extruded Mg-6.8Y-2.5Cu alloy and the corresponding microstructure was also investigated in this work. The cast alloy is mainly composed of α-Mg, 18R long period stacking order (LPSO) phase, eutectic phase (Mg20Cu4Y1), and Mg2Cu phase. The 18R LPSO phase at the dendritic grain boundary transforms into the 14H LPSO phase in the grain interior during homogenization. After extrusion, the grain size of the homogenized alloy is remarkably refined to –3.69 μm and the second phase is significantly broken and distributed in the extrusion direction. Tensile testing curves of the extrude alloy at room temperature indicate that the yield strength and ultimate tensile strength increase while the elongation of the alloy decreases with increasing strain rate. Interestingly, a yield plateau forms and gradually decreases with increasing strain rate. The yield phenomenon is related to the dislocation multiplication and the interaction between the movable dislocations and solute atoms.

Corrosion of Mg Alloys with Long Period Stacking Ordered Structure

Corrosion of Mg Alloys with Long Period Stacking Ordered Structure

The Effect of Polymethylsilsesquioxane Concentration on the Liquid Foundation: Characterization of Nanostructure and Biophysical Properties

Aims: Polymethylsilsesquioxane (PMQS) is a silicone derivative that serves as a skin conditioning agent and moisturizer in cosmetics. The objective of this study is to identify the effect of different concentrations PMSQ (0.12, 0.20 and 0.35 wt %) on liquid foundation (LF) and find the optimized composition in vitro.
 Study Design: Structural and morphological features of the examined nano/micro-sized samples were investigated by different methods.
 Place and Duration of Study: Hacettepe University, Department of Physics Engineering, X-Rays Laboratory, between September 2017 and September 2019.
 Methodology: Micro/nano scale structural and morphological properties of samples were investigated by means of a multi-methodological approach based on Small‐ and/or Wide‐Angle X‐ray Scattering (SWAXS), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDX), Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) Spectroscopy and UV-vis Spectroscopy. The LF (control) and PMSQ-LF films at one-week intervals for 4 weeks, and bovine leather coated samples with LF and PMSQ-LF were also investigated by SWAXS. The effect of PMSQ on biofilm formation activity in E. coli and S. aureus were also examined.
 Results: The most significant finding of this study is that even the low concentration of polymer in LF showed differences in nano structure and it was found that a decrease in biofilm formation. Nano formation becomes more pronounced, and the number of agglomerations decreases after 4 weeks. The reason for the study over a 4-week period was the desire to take into account the independent effect of each LF use, as well as the periodic stacking and diffusing effect that will occur in the depths of the skin over time. In the UVA range (320-400 nm) lower transmittance values were found for LF and PMSQ - LF films. 
 Conclusion: The results suggest that optimum polymer concentration in liquid foundation should be considered as a main step. The smart and systematic use of polymer additives is directly manifested in the nanoscopic structure and can improve the harmful UV rays prevention properties, which are important for health as well as cosmetic effects for beauty purposes.

Gap solitons in one-dimensional χ(2) hetero-structures induced by the thermo-optic effect

The existence of stable optical spatial solitons in a one-dimensional periodic stack whose unit cell is composed of two quadratic nonlinear materials is demonstrated. The stack is subjected to an array of micro-heaters, arranged in such a way that a periodic temperature gradient imposed upon the original structure, induces a variation of the refractive index due to the thermo-optic effect. Within a multiple scale approach the governing wave equations are obtained. A linear stability analysis together with the numerical evolution of these equations are carried out, evidencing distinctive soliton dynamics. By varying parameters like the period and the depth of the temperature modulation one is able to steer these solitons, to change their shape and, move them independently.

Quasiperiodic Van der Waals Heterostructures of Graphene and Hexagonal Boron Nitride

Advancement toward opening a bandgap at the Dirac point induced by symmetry breaking paved the way to realize 2D heterostructures with graphene and hexagonal boron nitride (h‐BN). An alternate arrangement of graphene and h‐BN layers in a 3D stacking can tune the bandgaps of these composites depending on the position of B and N atoms with respect to C atoms of graphene. Herein, a unique possibility of arranging graphene and h‐BN atomic layers in a quasiperiodic Fibonacci sequence to study the possibilities of controlling the electronic properties of these heterostructures is explored. Density functional theory calculations combined with van der Waals corrections reveal that these quasiperiodic heterostructures are more stable than normal periodic stacking of monolayers of graphene and h‐BN. Moreover, for certain arrangements of atomic layers, sizeable bandgaps can be obtained.

On the microstructure and RE-texture evolution during hot tensile deformation of Mg-Gd-Y-Zn-Zr alloy

This study was conducted to investigate the correlation between the microstructure/texture evolutions and the high temperature flow behavior of Mg-8.3Gd-3.6Y-1.6Zn-0.5Zr (wt. %) alloy containing long period stacking order (LPSO) phases. Toward this end, the hot tensile tests were conducted at 350 and 400 °C on two different starting microstructures: (i) as-extruded containing blocky LPSO, and (ii) as-annealed, which contained both blocky and lamellar LPSO phases. The peak stresses of the annealed microstructures at 350 and 400 °C, i.e. 200 and 80 MPa, were almost twice that of extruded ones at the same temperatures, i.e. 117 and 39 MPa, respectively. The annealing treatment effectively strengthened the alloy and decreased the ductility value. This was justified relying on the grain growth, deformation twinning and texture evolution, but a special emphasize was paid on the effect of lamellar and blocky LPSO phases. It was found that the blocky LPSO could assist the dynamic recrystallization through activation of particle stimulated nucleation (PSN) mechanism and effectively contributed in the formation of rare earth (RE)-texture component. This provided high Schmid factors for operation of basal slip and improved the ductility values. On the contrary, the fine lamellar LPSO effectively increased the strength of the alloy via texture hardening but reduced the ductility. The lamellar LPSO played a key role in the formation and strengthening of the <10-10> fiber texture, retarded the recrystallization process and suppressed the formation of RE-texture component.