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Variable-resolution fluctuation electron microscopy (VR-FEM) data from measurements on amorphous silicon and PdNiP have been obtained at varying experimental conditions. Measurements have been conducted at identical total electron dose and with an identical electron dose normalized to the respective probe size. STEM probes of different sizes have been created by variation of the semi-convergence angle or by defocus. The results show that defocus yields a reduced normalized variance compared to data from probes created by convergence angle variation. Moreover, the trend of the normalized variance upon probe size variation differs between the two methods. Beam coherence, which affects FEM data, has been analyzed theoretically using geometrical optics on a multi-lens setup and linked to the illumination conditions. Fits to several experimental beam profiles support our geometrical optics theory regarding probe coherence. The normalized variance can be further optimized if one determines the optimal exposure time for the nanobeam diffraction patterns.
Li4Ti5O12 (LTO) and its doped analogues Li4Ti4.95M0.05O12 (M = Al3+, Co3+, Ni2+, and Mg2+) were synthesized and characterized using in situ PXRD to monitor the phase transitions during the sol–gel synthesis of the spinel material. These results are complimented by thermogravimetric analysis, which illustrates the decomposition of the materials synthesized, where the final LTO products are seen to form at approximately 550 °C. The material has an amorphous structure from room temperature, coupled with a crystalline phase which is speculated to be H2Ti2O5·H2O. This crystalline phase disappears at 250 °C, with the material still in the amorphous state. The crystalline LTO phase starts at approximately 550 °C, with anatase co-crystallizing with the spinel phase. Rutile appears at 600 °C and co-crystallizes with the final product at 850 °C, where anatase is no longer seen. The rutile impurity remains present after cooling the material to room temperature, and results indicate that prolonged heating at 850 °C is required to reduce the rutile content. Rietveld refinement of diffraction patterns show that the unit-cell parameter increases with increasing temperature, coupled with a decrease when cooling the sample. The crystallite sizes follow the same trend, with a significant increase above temperatures of 750 °C.
In this research, the mechanical alloying (MA) technique was used to study solid solubility in the immiscible Zr–Cr alloy system. At first, Zr and Cr powders were milled, and then, the phase evolution, alloying mechanism, microstructural change, and mechanical properties of the milled powders were investigated by X-ray diffraction technique, scanning electron microscopy along with energy dispersive spectroscopy, transition electron microscopy, and microhardness measurements. Moreover, the solubility limit of Zr in Cr lattice was obtained by Vegard's law. The results showed that the MA was significantly enhanced the solubility of Zr in Cr up to about 21.6 wt% after an optimum milling time of 32 h and led to form an amorphous/nanocrystalline composite of Zr-reach and Cr-reach supersaturated solid solutions with the microhardness value of 503 Hv approximately. Also, the thermodynamic analysis indicated that the Gibbs free energy changes for the amorphous and solid solution were positive, which were provided by the MA process.
High-temperature (1500 °C) interactions of promising environmental-barrier coating (EBC) ceramics in the rare-earth (RE) pyrosilicate system, Yb(2-x)YxSi2O7 (x = 0, 0.2, 1, or 2), with three different calcia–magnesia–aluminosiliate (CMAS) glass compositions, are explored. Only the Ca/Si ratio is varied in the CMAS: 0.76, 0.44, or 0.10. Interaction between the highest Ca/Si CMAS and the EBC ceramic with the lowest x (=0, Yb2Si2O7) promotes no reaction but the formation of “blister” cracks. In contrast, the highest x (=2, Y2Si2O7) promotes the formation of an apatite reaction product, but no “blister” cracks. Observationally, it is found that a decrease in the CMAS Ca/Si ratio (0.76–0.10) and a decrease in Y-content decreases the propensity for reaction crystallization (apatite formation) and “blister” cracks. These results are rationalized based on the relative affinities between Ca2+ in the CMAS and Y3+ or Yb3+ in the EBC ceramics, suggesting a way to tune the CMAS interactions in RE pyrosilicate solid solutions.
Ab initio design of polymer nanocomposite materials for high breakdown strength requires prediction of localized trap states at the polymer–filler interface. Systematic first-principles calculations of realistic interfaces can be challenging, particularly for amorphous polymers and fillers that necessitate the calculation of ensembles of large unit cells with hundreds of atoms. We present a computational approach for automatically generating reasonable structures for amorphous polymer–filler interfaces, combining classical molecular dynamics and Monte Carlo simulations. We identify trap states by analyzing the localization of electronic eigenstates calculated using density functional theory on ensembles of interface structures, clearly distinguishing shallow trap states from delocalized band-edge states. Applying this approach to silica–polyethylene interfaces as an initial example, we find under-coordination and distorted coordination structures at amorphous silica surfaces contribute a combination of deep and shallow traps at these interfaces, whereas polyethylene does not generate localized interfacial states.
The study report on Vanadium dioxide thin films of about 100nm thickness deposited using pulsed laser deposition on Si (100). The novel phase change reported is attributed to the post-treatment of the films via ion implantation with 25 KeV C+ ion beam at varying particle fluence (1E15, 1E16, and 1E17 /cm2). At the initial fluence, the preferred phase is retained while amorphization and recrystallization of the film is observed as the fluence increase to 1E16 ions/cm2and 1E17 ions/cm2, respectively. The phase transition of the samples is observed to occur at a temperature below 320 K while stabilization of the low phase structure is observed for the middle fluence. Further increase restores the SMT behaviour/trend that occurred at elevated temperatures.
The Ti-Zr-Ni quasicrystal alloys have prospected to be one of the promising materials for hydrogen storage. This is because this type of quasicrystal contains 140 interstitial sites (T-sites) constituted in the Bergman Cluster that could accommodate hydrogen. The number of available sites is far greater than the number found in regular crystals, therefore the improvement of hydrogen storage capacity could be expected. For this study, we focus on the effect of substitution of Cr, in place of Ni in Ti-Zr-Ni amorphous and quasicrystal alloys. The studied samples are synthesized by the combination of mechanical alloying and sintering process. The subsequent measurements of electrochemical hydrogenation and dehydrogenation are carried out by a three-electrode cell at room temperature. The studied samples are structurally characterized by X-ray diffraction and their morphology is analyzed by scanning electron microscope and transmission electron microscope. The influence of the 4th substituted element on the possibility of a new-formed Cr quasicrystalline phase and the potential improvement of hydrogenation and dehydrogenation kinetics for both amorphous and quasicrystalline phase is evaluated. Our measurements showed the maximum discharge capacity achieved by Ti45Zr38Ni7Cr10 amorphous and Ti45Zr38Ni12Cr5 i-phase electrodes at a current density of 15 mA·g-1 to be 9.8 mAh·g-1and 55.2 mAh·g-1 respectively. The maximum estimated H/M value for the Ti45Zr38Ni12Cr5 i-phase electrode reached 1.36. These results are encouraging and show the merit of the usage of quasicrystals as hydrogen storage materials.
The fatigue behavior of a low-cost Zr52.1Ti5Cu17.9Ni14.6Al10Y0.4 (at%) (ZrCuNi-based) bulk-metallic glass (BMG) prepared by industrial-grade material was investigated under three-point bending loading modes. In order to obtain the fatigue stress-life (S-N) data, stress-controlled experiments were conducted using a computer-controlled material test system electrohydraulic testing machine at 60 Hz with a 0.1 R ratio in the air at room temperature. The fatigue limit (~174 MPa) in stress amplitude and fatigue ratio (~0.14) of this BMG is comparative to the similar BMG (Vit-105) prepared by high pure raw materials. The crack initiated from inclusions near the rectangular corners at the outer surface of the rectangular beam due to stress concentration. The striations and vein-like patterns were observed in the crack propagation region and fast fracture region, respectively.
YBO3:Eu3+ crystals with flower-like hierarchitecture are readily synthesized through a folic acid assisted hydrothermal process using polyborate precursors in the aqueous solution. It was found that the pH value , borate/yittrium ratio and the mass of folic acid take effects on the morphology and photoluminescence emission intensity of YBO3:Eu3+ crystals. The product with the small flower-like hierarchitecture was obtained under the conditions of pH value at 9, borate/yittrium ratio at 2 and the mass of folic acid at 0.44 g, showing the strongest photoluminescence intensity. The growth process of the YBO3:Eu3+ flowers and microflowers was invesitgated based on the time-dependent experiments, which showed that the growth mechanism of the flower-like hierarchitecture follows an in situ growth rather than self-assembly process as reported previously. Such a hydrothermal route using folic acid as a capping agent may provide a green and effective method for fabricating useful and complex 3D architectures of LEDs phosphors.
In the recent years, there has been high interest in renewable energy and highly efficient devices, promoted by the need to stop changing weather patterns. One of the most interesting methods for this is using thermoelectric materials, which are low cost and highly durable. However, the need for higher efficiency values and a higher resistance to oxidation leads to a technological problem in the field of coating. Due to its diverse properties, glass coating has been proposed as a solution to both sublimation of the thermoelectric materials and oxidation. Lead silicate glasses with 30% PbO were doped with 0–5% of Na2O and B2O3 to produce glasses with different properties. Differential scanning calorimetry and dilatometry measurements showed that the glass temperature can vary between 428 and 505 °C. The softening temperature is varied between 493 and 560 °C. Below Tg, the coefficient of thermal expansion is varied between 5.9 and 9 ppm/K and above Tg it varied between 17 and 58 ppm/K. This allows the tuning of the glass composition for each thermoelectric material, such as 0.5% B and 1% Na doped PbO -SiO2 glass for skutterudites and 1% doped B and 1% Na doped for Mg2Si, PbTe, and GeTe.
High-current switching performance of ovonic threshold switching (OTS) selectors have successfully enabled the commercialization of high-density three-dimensional (3D) stackable phase-change memory in Intel’s 3D Xpoint technology. This bridges the huge performance gap between dynamic random access memory (DRAM) and Flash. Similar to phase-change memory, OTS uses chalcogenide-based materials, but whereas phase-change memory reversibly switches between a high-resistance amorphous phase and a low-resistance crystalline phase, OTS freezes in the amorphous phase. In this article, we review recent developments in OTS materials and their performance in devices, especially current density and selectivity. Advantages and challenges of OTS devices in the integration with the phase-change memory are discussed. We introduce the evolution of theoretical models for explaining the OTS behavior, including thermal runaway, field-induced nucleation, and generation/recombination of charge carriers.
The exploitation of phase-change materials (PCMs) in diverse technological applications can be greatly aided by a better understanding of the microscopic origins of their functional properties. Over the last decade, simulations based on electronic-structure calculations within density functional theory (DFT) have provided useful insights into the properties of PCMs. However, large simulation cells and long simulation times beyond the reach of DFT simulations are needed to address several key issues of relevance for the performance of devices. One way to overcome the limitations of DFT methods is to use machine learning (ML) techniques to build interatomic potentials for fast molecular dynamics simulations that still retain a quasi-ab initio accuracy. Here, we review the insights gained on the functional properties of the prototypical PCM GeTe by harnessing such interatomic potentials. Applications and future challenges of the ML techniques in the study of PCMs are also outlined.
Glasses were prepared in systems based on two stoichiometric sulfides that were selected from Ga2S3, GeS2, and Sb2S3, with the incorporation of excess sulfur and CsCl. We investigated the fundamental properties, including glass transition, density, and optical absorption, and their variations with the incorporation of excess sulfur and CsCl into the pseudo two-component sulfide glasses. The incorporation of CsCl into the GeS2–Sb2S3 glasses shifted the absorption edge at the short-wavelength side to the long-wavelength direction, particularly for glasses with more amount of GeS2 than SbS3/2. In both cases of CsCl incorporation into the Ga2S3–GeS2 glass and Ga2S3–Sb2S3 glass systems, the absorption edges shifted to the short-wavelength direction regardless of the compositions. Ag photodoping behaviors were investigated for the bulk sulfide glasses with excess sulfur and CsCl. The results are discussed based on the diffusion of silver in the glass network that is modified by the incorporation.
Siliceous-sulphate rock coatings were observed at Zhenzhu Spring, an acid sulphate hot spring in the Tengchong volcanic field, China. These rock coatings are mainly formed of gypsum and amorphous silica. Some alum-(K), voltaite, α-quartz and muscovite were also found. Four different laminae are developed in the rock coatings: gypsum layer, tight siliceous layer, tabular siliceous layer and siliceous debris layer. The gypsum layer is located at the top of the rock coatings, while other siliceous layers appear below the gypsum layer. Geochemical modelling of the fluids was performed to identify the mechanisms responsible for the formation of gypsum and amorphous silica. The results indicated that the occurrence of gypsum is related to the acid-fog deposition and amorphous silica mainly originates from spring water. Fog deposition provided the rock coatings with abundant SO42− and Ca, and the subsequent complete evaporation of the condensed fluids produced gypsum. Seasonal climate change (especially variation in rainfall) determines the fluctuations of capillary action and dissolution. Rainfall events in the wet season led to periods of non-precipitating gypsum and promoted the capillary rise of the spring water. Slightly diluted capillary water (a small amount of rainwater) covered the rock coatings, formed a tight siliceous layer on the rock-coating surface and/or filled the pores among the gypsum crystals forming many tabular siliceous aggregates. Heavy rainfall (high dilution), however, resulted in non-precipitating amorphous silica and accelerated the gypsum dissolution, leaving tabular pores around tabular siliceous aggregates and forming a tabular siliceous layer.
The influence of repeated thermal cycling on mechanical properties, structural relaxation, and evolution of the potential energy in binary glasses is investigated using molecular dynamics simulations. The authors consider a binary mixture annealed with different cooling rates and then exposed to one thousand thermal cycles at constant pressure. It is found that during the first few hundred transient cycles, the potential energy minima after each cycle gradually decrease and the structural relaxation proceeds through collective, irreversible displacements of atoms. With increasing cycle number, the amplitudes of the volume and potential energy oscillations are significantly reduced, and the potential energy minima saturate to a constant value that depends on the thermal cycling amplitude and the initial cooling rate. In the steady state, the glasses thermally expand and contract but most of the atoms return to their cages after each cycle, similar to limit cycles found in periodically driven amorphous materials. The results of tensile tests demonstrate that the elastic modulus and the yielding peak, evaluated after the thermal treatment, acquire maximum values at a particular thermal cycling amplitude, which coincides with the minimum of the potential energy.
A collection of 65 formulated tablets and capsules were analyzed for phase composition by full pattern matching powder diffraction methods. The collection contained 32 of the top 200 prescription drugs sold in 2016 as well as many high-volume prescriptions and over the counter drugs from prior years. The study was used to evaluate new methods of analysis as well as the efficacy of programs designed to collect references on high volume excipients and pharmaceuticals for inclusion in the Powder Diffraction File™. The use of full pattern matching methods as well as reference pattern additions of many common excipients enabled major phase excipient identification in all formulations. This included identification of crystalline, nanocrystalline, and amorphous ingredients because full pattern matching involved the use of characteristic coherent and incoherent scatter. Oftentimes identification of the major excipients significantly aided the clean identification of the active pharmaceutical ingredients (APIs) and their polymorphic form, even at low concentrations (1–10 wt. %). Overall 93% of the APIs were identified, most through a PDF® material reference, but also through patent cross-referencing and similarity analysis comparisons.
Intrinsic size effects in nanoglass plasticity have been connected to the structural length scales imposed by the interfacial network, and control over this behavior is critical to designing amorphous alloys with improved mechanical response. In this paper, atomistic simulations are employed to probe strain delocalization in nanoglasses with explicit correlation to the interfacial characteristics and length scales of the amorphous grain structure. We show that strength is independent of grain size under certain conditions, but scales with the excess free volume and degree of short-range ordering in the interfaces. Structural homogenization upon annealing of the nanoglasses increases their strength toward the value of the bulk metallic glass; however, continued partitioning of strain to the interfacial regions inhibits the formation of a primary shear band. Intrinsic size effects in nanoglass plasticity thus originate from biased plastic strain accumulation within the interfacial regions, which will ultimately govern strain delocalization and homogenous flow in nanoglasses.
Amorphous/crystalline (A/C) nanolayers provide an effective model system to study the mechanical behavior and size effects of metallic glasses and crystalline metals in confined geometries. In this work, we experimentally investigated the structure–property relationship in A/C nanolayers containing HCP crystalline layers. CuTi/Ti and CuZr/Zr nanolayers were prepared by magnetron sputtering with layer thicknesses in the range 10–100 nm. The hardness values of the CuTi/Ti and CuZr/Zr nanolayers were close to those of the monolithic CuTi and CuZr, respectively. The hardness remained virtually the same for different layer thicknesses as opposed to CuTi/Cu amorphous/FCC crystalline nanolayers, which exhibit increasing strength with decreasing layer thickness. Confined layer slip model predicts that the effective flow stress of HCP crystalline layers is higher than that of the amorphous layers. As a result, the strength and size effects are governed by the mechanical behavior of the softer amorphous layer.
A kind of novel Ni–P gradient coating/stannate conversion film was deposited on AZ91D magnesium alloy (AZ91D alloy) by an integrative method involved stannate conversion and electroless plating. The results indicated that using sodium hypophosphite concentrations varied as 5, 10, 22, 46, and 60 g/L in the bath, the electroless Ni–P gradient coating with typical cell morphologies was successfully prepared, and the structures transited from crystalline → microcrystalline → amorphous were obtained as increasing P content from 3.31 to 12.58 wt%. Furthermore, the corrosion morphologies, polarization curves, and the electrochemical impedance spectroscopy result indicated that the corrosion resistance of AZ91D alloy substrate was significantly improved and the corrosion resistance of Ni–P gradient coating was superior than that of stannate conversion film, which might be attributed to the gradient structure and rising P content with unique function.
We present an information-based total-energy optimization method to produce nearly defect-free structural models of amorphous silicon. Using geometrical, structural, and topological information from disordered tetrahedral networks, we have shown that it is possible to generate structural configurations of amorphous silicon, which are superior than the models obtained from conventional reverse Monte Carlo and molecular dynamics simulations. The new data-driven hybrid approach presented here is capable of producing atomistic models with structural and electronic properties which are on a par with those obtained from the modified Wooten-Winer-Weaire (WWW) models of amorphous silicon. Structural, electronic, and thermodynamic properties of the hybrid models are compared with the best dynamical models obtained from using machine-intelligence-based algorithms and efficient classical molecular dynamics simulations, reported in the recent literature. We have shown that, together with the WWW models, our hybrid models represent one of the best structural models so far produced by total-energy-based Monte Carlo methods in conjunction with experimental diffraction data.