Stacking Periodicity Research Articles

Nucleation and evolution mechanism of precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy

The evolution process and nucleation mechanism of the precipitates during the initial aging stage in Al-Zn-Mg-Cu alloy were studied using the combination of the Cs-corrected high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and first-principles calculations. A new nucleation mode of η' phase with the thickness of 7 at. layers was found, which is parallel to {111}Al planes and consists of 5-layer transition structures and 2 transition layers at the interface. The precipitate is the initial aged η' phase with the simplest structure. Its interfaces are coherent with the Al matrix and meet the stacking periodicity, which is more stable than ordinary η' phases. Besides, the interfaces have a tendency to segregate heavy elements. The presence of segregating atoms enhances the interface interaction and makes it more stable. These two aspects improve the stability of the nano-scale 7-layer particle and maintain the aging strengthening effect of Al alloy.

First principles study of stacking periodicity effects on structural, electronic and magnetic properties of [formula omitted] superlattice

In this article, it is proven that the stacking periodicity has an impact on the structural, electrical, and also magnetic properties of the (MnAs)n/(AlAs)n Superlattice using the full potential linearized augmented plane waves (FP-LAPW) method, which is built on the theory of density functional (DFT). The exchange-correlation potential was determined using the GGA-PBE approximation under ambient conditions. Among these effects, the lattice constant and total magnetic moment of the SLs rise linearly as the number of layers increases. The (MnAs)1/(AlAs)1 SL has a half-metallic behavior; however, the (MnAs)2/(AlAs)2 and (MnAs)3/(AlAs)3 has a semi-metallic behavior, and the origin of the (DOS) and the magnetic moment of the SLs are the d states of the Mn atom. These results demonstrate that controlling the number of MnAs layers may be used to engineer the magnetic properties of the MnAs/AlAs superlattice, leading to a novel spintronic device.

Effect of interlayer K ordering on water intercalation behavior in δ-type layered manganese dioxide

Layered manganese dioxide (birnessite) that contains K cations and water molecules in between the MnO2 layers exhibits reversible heat storage properties via a water-intercalation mechanism (0.5 mol H2O per K0.33MnO2). However, a certain amount of an irreversible capacity of water intercalation is observed after the initial thermal cycle, which limits the reversibly available energy density of the material for the subsequent cycles. In this study, we analyzed crystal structures of the K-containing birnessite, K0.33MnO2, before and after heat treatment, by electron diffraction and atomic-resolution scanning transmission electron microscopy, to elucidate the origin of the initial irreversible capacity (0.83 mol per K0.33MnO2). The K-containing birnessite synthesized by thermal decomposition of KMnO4 was found to basically possess a superstructure of tripled periodicity along the in-plane direction due to an ordered arrangement of the interlayer K cations, while the stacking periodicity of the interlayer K cations along the out-of-plane direction was partially lost. The K cations preferentially occupy the 2c site (space group P63/mmc), which is the most stable position as confirmed by ab initio calculations. The K cations occupy every third 2c site in almost all the interlayers after heat treatment, although the K cations in the as-synthesized state are randomly arranged in some of the interlayers because excess water molecules prevent the interlayer K cations from occupying the most stable 2c site. Thus, the ordering of the K cations once occurring without the water molecules during heat treatment hinders the excess water initially included to re-intercalate in the subsequent cycle process.

Supramolecular Networks Obtained by Block Copolymer Self-Assembly in a Polymer Matrix: Crystallization Behavior and Its Effect on the Mechanical Response

In recent years, there has been growing interest in the study of supramolecular networks obtained by self-assembly of amphiphilic molecules due to their responsive behavior to different external stimuli. The possibility of embedding supramolecular networks into polymer matrices opens access to a new generation of functional polymers with great potential for various applications. However, very little is known about how the dynamics of the supramolecular network is affected by diffusional and topological limitations imposed by the polymer matrix. In this work, we investigate the behavior of supramolecular networks embedded into a rubbery polymer. Crystallization-driven self-assembly of a poly(ethylene-block-ethylene oxide) (PE-b-PEO) diblock copolymer was used to generate supramolecular networks in dimethacrylate monomers, which were then photopolymerized at room temperature. PE-b-PEO self-assembles into nanoribbons with a semicrystalline PE core bordered by coronal chains of PEO, and the nanoribbons, in turn, bundle into lamellar aggregates with an average stacking period of around 45 nm. The nanoribbons are interconnected through crystalline nodes in a 3D network structure. Small-angle X-ray scattering experiments show that the polymer matrix preserves the structure of the supramolecular network and avoids its disintegration when the material is heated above the melting temperature of PE cores. Successive self-nucleation and annealing studies reveal that the polymer matrix does not influence the crystallization–melting processes of PE, which take place through the interconnected cores of the supramolecular network. In contrast, the matrix imposes strong effects of topological confinement on the crystallization of PEO, limiting the dimensions of the crystalline lamellae that can be formed. Mechanical tests show that the deformation capacity of these materials can be precisely tuned by programming the temperature within the melting range of the supramolecular network. This behavior was also characterized by shape memory cyclic tests.

Collective Diffraction Effects in Perovskite Nanocrystal Superlattices.

ConspectusFor almost a decade now, lead halide perovskite nanocrystals have been the subject of a steadily growing number of publications, most of them regarding CsPbBr3 nanocubes. Many of these works report X-ray diffraction patterns where the first Bragg peak has an unusual shape, as if it was composed of two or more overlapping peaks. However, these peaks are too narrow to stem from a nanoparticle, and the perovskite crystal structure does not account for their formation. What is the origin of such an unusual profile, and why has it been overlooked so far? Our attempts to answer these questions led us to revisit an intriguing collective diffraction phenomenon, known for multilayer epitaxial thin films but not reported for colloidal nanocrystals before. By analogy, we call it the multilayer diffraction effect.Multilayer diffraction can be observed when a diffraction experiment is performed on nanocrystals packed with a periodic arrangement. Owing to the periodicity of the packing, the X-rays scattered by each particle interfere with those diffracted by its neighbors, creating fringes of constructive interference. Since the interfering radiation comes from nanoparticles, fringes are visible only where the particles themselves produce a signal in their diffraction pattern: for nanocrystals, this means at their Bragg peaks. Being a collective interference phenomenon, multilayer diffraction is strongly affected by the degree of order in the nanocrystal aggregate. For it to be observed, the majority of nanocrystals within the sample must abide to the stacking periodicity with minimal misplacements, a condition that is typically satisfied in self-assembled nanocrystal superlattices or stacks of colloidal nanoplatelets.A qualitative understanding of multilayer diffraction might explain why the first Bragg peak of CsPbBr3 nanocubes sometimes appears split, but leaves many other questions unanswered. For example, why is the split observed only at the first Bragg peak but not at the second? Why is it observed routinely in a variety of CsPbBr3 nanocrystals samples and not just in highly ordered superlattices? How does the morphology of particles (i.e., nanocrystals vs nanoplatelets) affect the appearance of multilayer diffraction effects? Finally, why is multilayer diffraction not observed in other popular nanocrystals such as Au and CdSe, despite the extensive investigations of their superlattices?Answering these questions requires a deeper understanding of multilayer diffraction. In what follows, we summarize our progress in rationalizing the origin of this phenomenon, at first through empirical observation and then by adapting the diffraction theory developed in the past for multilayer thin films, until we achieved a quantitative fitting of experimental diffraction patterns over extended angular ranges. By introducing the reader to the key advancements in our research, we provide answers to the questions above, we discuss what information can be extracted from patterns exhibiting collective interference effects, and we show how multilayer diffraction can provide insights into colloidal nanomaterials where other techniques struggle. Finally, with the help of literature patterns showing multilayer diffraction and simulations performed by us, we demonstrate that this collective diffraction effect is within reach for many appealing nanomaterials other than halide perovskites.

Effect of different stacking orders of Ta2O5 and SiO2 films on the reflective properties of a blue distributed Bragg reflector

The Ta2O5 and SiO2 single-layer films were deposited on the glass substrates to measure their refractive properties under different optical wavelengths. After that, the Ta2O5 and SiO2 films were used as high and low refractive index materials to design the blue distributed Bragg reflectors (DBRs) with different periods. The approximate formula proposed by Sheppard was used to calculate the maximum reflectance ratio of the bi-layer SiO–Ta2O5 DBRs with two, four, six, and eight periods. The COMSOL Multiphysics software was used to simulate the reflective spectra of the SiO2–Ta2O5 DBRs with different stacking orders and periods by incorporating the refractive indexes of the Ta2O5 and SiO2 films at a wavelength of 550 nm. The SiO2–Ta2O5 DBRs with different periods were fabricated by depositing the Ta2O5 and SiO2 films on glass substrates, and their reflective properties were measured. We compared the measured reflective spectra of the fabricated SiO2–Ta2O5 DBRs with the simulated reflective spectra to show the effect of the different stacking orders.

Constructing clean cookstove stack in Ghana

BACKGROUND AND AIM: Transition to cleaner cooking technologies has the ability to increase economic and environmental benefits, studies have shown that adoption of improved cooking stoves is very low and those who adopt clean cooking stove/ fuels are likely to stack with biomass fuel. The aim of this study was to identify promising “clean stacks” of stoves and fuels that are relevant to household energy needs in middle belt of Ghana. METHODS: A mixed method approach was used in data collection. Ten households who were exclusive users of biomass fuel were randomly selected from the Kintampo Health Demography Surveillance Survey. We used questionnaires to collect information on sociodemographic characteristics, household energy needs, stove use, and acceptability of stoves. The stoves that were given to participants were Mimi motos (a forced draft pellet biomass stove), Electric induction, Ethanol stoves and LPG stoves. Fuel was provided at no cost to participants. Each stove was assigned to the households for a period of two weeks, one after the other. After the single technology assessment, two most preferred stoves were given back to participants to use simultaneously for a period of one month. During this one-month period, participants exposure to PM2.5 were assessed. RESULTS:LPG and Ethanol stoves were the two most preferred stoves. The participants noted inconveniences and numerous mechanical and operational difficulties with the mimi moto and the electric stove. Induction electric stove recorded a mean reading of 0.10 volts Forty-eight-hour personal PM2.5 assessed during the clean stack period was slightly higher with mean concentration of 10.82 μg/m3 (0.54) compared to WHO guideline (10 μg/m3), but was substantially lower than baseline. CONCLUSIONS:Of the four stoves tested, participants expressed satisfaction with LPG stove and Ethanol stove as it met their daily energy needs. Adoption and acceptability of the improved stoves varied with different dishes or cooking tasks. KEYWORDS: clean fuel stack, exposure assessment, Ghana

Relative stabilities of Si polytypes under biaxial stress: A first-principles study

We explore the possibility of the production of Si polytypes via the total energies and free energies obtained in the density-functional theory and the density-functional perturbation theory. We first calculate the total energies of three polytypes of the tetrahedrally coordinated $s{p}^{3}$ Si atoms having the two-layer, three-layer, and four-layer stacking periods in the hexagonal unit cell (``$2H$'', ``$3H$'', and ``$4H$'') as a function of the in-plane lattice constant $a$ using the local-density approximation (LDA) and generalized gradient approximation (GGA) comparatively. In the LDA calculation, the $4H$ phase is energetically the most favorable among three phases in the range of 3.55 \AA{} $\ensuremath{\le}$ $a$ $\ensuremath{\le}$ 3.73 \AA{}. While the $4H$ phase is found to be more stable than the $2H$ phase at a relatively wide range of the $a$ value we studied, the $2H$ phase is found to be more stable than the $3H$ ($3C$) phase in the range of 3.57 \AA{} $\ensuremath{\le}a\ensuremath{\le}$ 3.61 \AA{}. In the GGA calculation, on the other hand, the $4H$ phase becomes the most favorable phase for almost the same range as the LDA case, 3.57 \AA{} $\ensuremath{\le}a\ensuremath{\le}$ 3.73 \AA{}. The $2H$ phase also becomes as stable as the $3H$ phase as in the case of the LDA around the small $a$ value of 3.62 \AA{}, although the $3H$ phase energy is always lower than that of the $2H$ phase in the GGA calculation. Considering the chemical vapor deposition (CVD) growth process, we calculate the biaxial stress values to reduce the lattice constant of the substrate Si crystal to realize the small $a$ values, which can lead to the homoepitaxial growth of the $4H$ phase. The values obtained are 4.69 GPa and 8.14 GPa for the LDA and GGA, respectively. Next, we study the thermal effect on the relative stabilities of the 3 phases. We calculate the phonon dispersion, bulk modulus and thermal expansion including the vibrational and thermal effect. From the comparison with experimental values of the $3C$ phase, it is found that the LDA results show much better agreement with experiment than the GGA, indicating that the LDA calculation is reliable to predict thermal properties of the real systems. Finally, we derive the free energy as a function of $a$ under several designated temperatures. The relative stabilities of the $2H$ and $4H$ phases with respect to the $3H$ phase are found to be enhanced. At around $a=3.65$ \AA{}, stabilities of $2H$ and $4H$ phases relative to the $3H$ phase become most prominent. Although stabilities of the $2H$ and $4H$ phases relative to the $3H$ phase are reduced with increasing temperature at around $a$ = 3.60 \AA{}, $2H$ phase is found to be as stable as $3H$ and $4H$ phases at higher temperatures, indicating the possibility of the production of the $2H$ phase via high-temperature CVD process with the biaxial stress.

Stacking disordered structures in Mg–Co–Y alloys

Stacking disordered (SD) structures in Mg–Co–Y as-cast alloys have been examined using atomic-scale high-angle annular dark-field scanning transmission electron microscopy. All these SD structures mainly consist of F-blocks, T-blocks, 2-Mg, 3-Mg, and 4-Mg, which are the structural components of previously observed 15R, 12H, 21R, and 18R LPSO structures. These components can further form complex structures with similar stacking characters, which are disordered or locally ordered. The stacking sequence, periodicity, and symmetry of these SD structures were characterized and illustrated by our previously introduced notation. The structural similarity and difference between the SD structures and LPSO structures featuring various stacking layers were analyzed by comparing their stacking sequences, the arrangement of clusters in the basal plane and along the stacking directions.

The LPSO Structure with an Extra Order beyond Stacking Periodicity

The LPSO Structure with an Extra Order beyond Stacking Periodicity

Electronic Structure and Polarization of NaMgF3/NaCaF3 Superlattices: Insight from First-Principles

Employing First-principles calculations, we have studied the electronic structure and polarization properties of the fluoroperovskite NaMgF3/NaCaF3 superlattices. The in-plane lattice constants of the superlattices are found to lie between those of the bulks. The Berry phase calculations show that the polarization of the NaCaF3 bulk is much larger than that of the NaMgF3, and the polarization of the NaMgF3/NaCaF3 superlattices can be tuned by altering the stacking periodicity. The band structure dependence of the stacking period is also investigated.

Significantly enhanced ferroelectric and dielectric properties in BaTiO3/LaNiO3 superlattices

BaTiO3/LaNiO3 (BTO/LNO) ferroelectric superlattices with different stacking periods have been prepared by pulsed laser deposition (PLD). Compared to the pure BTO films, the BTO/LNO superlattices have significantly enhanced ferroelectric and dielectric properties. This is because, in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current in the superlattices. The results indicate that the use of ultra-thin metallic oxide layers as the constituent material of the ferroelectric superlattices is a feasible and effective way to improve the properties of superlattices.

Research Methods of Different Levels of Reservoirs Based on Seismic Data

The lack of drilling data is a difficult problem in the study of reservoir configuration under the condition of sparse well pattern. In this paper, the seismic data are fully excavated, and the methods of frequency RGB fusion, small scale river sand attribute fusion and horizontal well data combination are used to study the river level, the level point bar level, and the internal configuration of point bar. The frequency division RGB fusion method has a good effect on the characterization of large-scale channel reservoirs, especially the channel stacking period and channel boundary response. The integrated attributes of river channel and sand body distribution method has a good effect on depicting the small-scale end river channel and abandoned river channel, and on this basis, the research on point bar sand body is consistent with the actual drilling. The high-precision edge detection section and RGB fusion section are superimposed to display the development of lateral volume, and verify with the actual horizontal well. River facies reservoirs of different scales have been verified by drilling with targeted research techniques. The results show that under the condition of sparse well pattern, well-seismic combination is the key to the division of small-scale sedimentary units, multi-attribute fusion under high-resolution seismic data is an important means to identify and depict point bar and abandoned river channels, and the application of horizontal well data is a supplementary means to test results and depict lateral accumulation. The research results have achieved good application results in guiding the actual production of oil field.

Significantly Enhanced Ferroelectric and Dielectric Properties in BaTiO <sub>3</sub>/LaNiO <sub>3</sub> Superlattices

BaTiO3/LaNiO3 (BTO/LNO) ferroelectric superlattices with different stacking periods have been prepared by pulsed laser deposition (PLD). Compared to the pure BTO films, the BTO/LNO superlattices have significantly enhanced ferroelectric and dielectric properties. This is because, in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current in the superlattices. Therefore, these results indicate that the use of ultra-thin metallic oxide layers as the constituent material of the ferroelectric superlattices is a feasible and effective way to improve the properties of superlattices.

Formation of long period stacking ordered phases in Mg-10Gd-1Zn-0.5Zr (wt.%) alloy

The formation of long period stacking ordered (LPSO) phases in Mg-10Gd-1Zn-0.5Zr (wt.%) alloy is investigated in this study. The LPSO phases observed in the as-cast, solution-treated and aged conditions are examined systematically by using scanning electron microscopy and transmission electron microscopy. Various types of LPSO phases including 10H, 14H, 18R and 24R are observed. Besides the known LPSO phases, the LPSO phase with the stacking periodicity of the close-packed atomic planes along the [101¯1] direction of the α-Mg matrix is observed after solution treatment at 500 °C for 12 h. In addition, the results show that the second phase, solute segregation and heat treatment temperature have significant influences on the formation of LPSO phases.

8H-10H Stacking Periodicity Control in Twinned Hexagonal Perovskite Dielectrics.

Isovalent substitution of Zr4+ for smaller Ti4+ was performed in the 8-layer twinned hexagonal perovskite (referred to as 8H) tantalate Ba8Ti3Ta4O24, which stabilizes a 10-layer twinned hexagonal perovskite (referred to as 10H). The formation of the 10H phase occurs at low substitution concentration ( x = 0.1) in Ba8Zr xTi3- xTa4O24 at 1300 °C and reverts back to the 8H phase upon heating at elevated temperatures. Such a 10H-to-8H phase transformation is suppressed at higher Zr-substitution contents ( x > 0.1). The approach combining simulated annealing and Rietveld refinement with compositional constrain indicates that the 10H Ba8Zr0.4Ti2.6Ta4O24 ( x = 0.4) composition adopts a simply P63/ mmc disordered structure with Zr cations preferably located in corner-sharing octahedral (CSO) sites compared to face-sharing octahedral (FSO) sites. This 8H-10H phase competition, dependent on the substitution of Zr4+ for Ti4+ and firing temperature, is discussed in terms of the FSO B-B repulsion controlled by the cationic size, as well as the stacking periodicity which affects the thermodynamic stability. Both 8H- and 10H-phase pellets of Ba8Zr xTi3- xTa4O24 exhibit comparable and poorer microwave dielectric properties than the parent 8H Ba8Ti3Ta4O24, which is characterized by cationic disorder and FSO B-B repulsion. The 8H and 10H Ba8Zr xTi3- xTa4O24 ceramics display electrical insulator behavior but with electrically heterogeneous microstructure on the bulk grains. This study demonstrates the opportunity to control the stacking periodicity for the twinned hexagonal perovskites via tuning the B-cationic size and the firing temperature.

Thermal escape and carrier dynamics in multilayer CdTe/ZnTe quantum dots

We present an optical spectroscopy study of multilayer CdTe/ZnTe quantum dots by photoluminescence (PL) and time-resolved PL. We demonstrate that the PL blueshift and enhanced PL intensity were due to an increase in the material intermixing and the density of the emitting states. An integrated PL as a function of temperature revealed that the separated energy of two different states was 4.5–6.2 meV and was nearly independent of the stack period. The main thermal escape was assisted by multi-longitudinal optical phonons at high temperature with activation energies in the range 48–54 meV.

Formation Mechanism of Boron-Based Nanosheet through the Reaction of MgB2 with Water

A recent experiment demonstrated that ultrasonication of MgB2 in water yields Mg-deficient hydroxyl-functionalized boron nanosheets at room temperature. Herein, we examined the mechanism of nanosheet formation. Analysis of the reaction products and temporal variation in pH and H2 production shows that the reaction between MgB2 and water comprises two steps: (i) an ion-exchange process between protons and a part of Mg cations of MgB2 with its exfoliation and (ii) the hydrolysis reaction between Mg-deficient boron hydride and water to produce H2 and Mg-deficient hydroxyl-functionalized boron sheets. The sheets with a stacking periodicity of 0.70 nm were obtained as the supernatant of the reaction product of water with MgB2. The stacking sheets can be further exfoliated if the reaction is conducted under ultrasonication. The derived nanosheets are composed of sp2-bonded boron framework and possess a disordered structure containing hydroxyl species and oxidized magnesium.

Internally Oxidized Ru–Zr Multilayer Coatings

In this study, equiatomic Ru–Zr coatings were deposited on Si wafers at 400 °C by using direct current magnetron cosputtering. The plasma focused on the circular track of the substrate holder and the substrate holder rotated at speeds within 1–30 rpm, resulting in cyclical gradient concentration in the growth direction. The nanoindentation hardness levels of the as-deposited Ru–Zr coatings increased as the stacking periods of the cyclical gradient concentration decreased. After the coatings were annealed in a 1% O2–99% Ar atmosphere at 600 °C for 30 min, the internally oxidized coatings shifted their respective structures to a laminated structure, misaligned laminated structure, and nanocomposite, depending on their stacking periods. The effects of the stacking period of the cyclical gradient concentration on the mechanical properties and structural evolution of the annealed Ru–Zr coatings were investigated in this study.

Precipitation of Long‐Period Stacking Ordered Structure in Mg–Gd–Zn–Mn Alloy

Precipitation of long‐period stacking ordered (LPSO) structure in Mg–15Gd–0.8Zn–0.8Mn (wt%) alloy after solution treatment at 520 °C for 1 000 h has been investigated using high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM). The stacking periodicity of the LPSO phase is lengthened 14‐fold along the c‐axis and the LPSO phase consists of Gd‐ and Zn‐enriched solute clustering. Detailed HAADF‐STEM imaging shows a clear evidence of a co‐segregation of Gd and Zn within a LPSO phase. This paper provides a better understanding of the interaction between Gd and Zn, which can, therefore, provide useful guidelines to develop high performance Mg alloys.