Temperature dependence of indentation size effect,dislocation pile‐ups,and lattice friction in (001) strontium titanate

Nanoindentations with a Berkovich type indenter were performed on (001) strontium titanate (STO) single crystal at 25°C and 350°C, analyzing the influence of temperature on the indentation size effect (ISE) and dislocation structure around the residual impression. It is found that the STO exhibits an ISE, which is strongly reduced at 350°C compared to 25°C. The dislocation structure around the residual impression has been resolved using an etch‐pit technique. At 25°C, the extension of the dislocation pile‐ups were found to be shorter as compared to 350°C. This also correlates with the smaller size effects at 350°C. Peach‐Koehler forces and the elastic‐plastic indentation stress field were used to model the influence of the lattice frictional stress on the dislocation pile‐ups. Based on an equilibrium position of the outermost dislocations, the average lattice frictional stresses were calculated to be 89 MPa and 46 MPa at 25°C and 350°C, respectively.     

Conductivity of iron-doped strontium titanate in the quenched and degraded states

The electrical behavior of iron-doped strontium titanate (Fe:SrTiO₃) single crystals equilibrated at 900°C and quenched below 400°C at various oxygen partial pressures () was investigated via impedance spectroscopy and compared to defect chemistry models. Fe:SrTiO₃ annealed and quenched between 1.2 × 10⁻¹⁴ and 2.0 × 10⁻⁴ Pa exhibits a conduction activation energy (E_A) around 0.6 eV, consistent with ionic conduction of oxygen vacancies. However, sudden changes in E_A are found to either side of this range; a transition from 0.6 to 1 eV is found in more oxidizing conditions, while a sudden transition to 1.1 and then 0.23 eV is found in reducing These transitions, not described by the widely used canonical model, are consistent with predictions of transitions from ionic to electronic conductivity, based on first principles point defect chemistry simulations. These models demonstrate that activation energies in mixed conductors may not correlate to specific conduction mechanisms, but are determined by the cumulative response of all operative conduction processes and are very sensitive to impurities. A comparison to electrically degraded Fe:SrTiO₃ provides insight into the origins of the conductivity activation energies observed in those samples.     

Direct and indirect measurement of large electrocaloric effect in barium strontium titanate ceramics

Barium strontium titanate (Ba_1-xSr_xTiO₃-BST) ceramics, where x = 0.05, 0.15, 0.25, and 0.35, was prepared via a solid-state reaction method. The lattice structures and morphologies of the ceramic samples were analyzed using X-ray diffraction and scanning electron microscopy technologies. The dielectric and ferroelectric properties of the BST ceramics were characterized using a precision impedance analyzer and a ferroelectric polarization-electric field (P-E) hysteresis loop tester, respectively. The electrocaloric effect was indirectly calculated using the Maxwell relations and P-E loops as a function of temperature and electrical field, and also directly measured using a computer-controlled thermocouple and high-voltage power supply. An adiabatic temperature change of 1.82 K was procured, indicating a promising potential in the applications as cooling devices.     

Synthesis of Sr- and Ba titanate particles coated on Al2O3 substrates forming a core-shell structure

Sr- and Ba titanate particles doped with Nb, Mn, and Ce were produced via a wet-chemical precipitation process based on salt solutions and metal chlorides around Al₂O₃ flake-like particles. In a calcination process at high temperatures, a shell of doped oxides is formed that is responsible for the unusual electrical properties. The particles were investigated via scanning electron microscope (SEM) and transmission electron microscope which showed homogeneously covered Al₂O₃ particles. X-ray diffraction exposed secondary phases besides BaTiO₃ and SrTiO₃ because of the high solubility of Ba(OH)₂ and Sr(OH)₂ in water and a resulting shortfall of earth alkaline metal. The oxidation states of the doping elements were determined via X-ray photoelectron spectroscopy. Composite materials prepared from this filler and silicone rubber showed typical nonlinear voltage-current (U-I) characteristics, which are of special interest for high voltage engineering. The filler was evenly distributed within the matrix without settling while cross-linking of the polymer, which was shown in an SEM-backscattered electron image. The nonlinearity coefficient α, which is an important factor for field grading applications, was determined for different materials and reached a value of up to 7.     

Effect of poly(vinyl acetate) on structure and property of bismuth-doped strontium titanate thin films derived by sol–gel method

Bismuth-doped strontium titanate thin films with pure perovskite phase have been successfully deposited on Pt (1 1 1)/Ti/SiO₂/Si substrate by polymer-assisted sol–gel method. Poly(vinyl acetate) (PVAc) in precursor solution promoted the formation of perovskite phase during the heat treatment. SEM results revealed an increasing thickness from 40 to 80 nm every single layer and a porous structure with the addition of PVAc. The addition of polymer made the dielectric constant decrease from 140 to 40 and the tunability slightly increase compared with films without polymer in precursor.     

Stoichiometry control and structure evolution in hydrothermally derived (Ba,Sr)TiO3 films

(Ba,Sr)TiO₃ films were synthesized on the titanium metal substrates in solution of Ba(OH)₂ and Sr(OH)₂ by hydrothermal method. Crystallinity and microstructure of the films changed with time, concentration and temperature. Effects of the mole ratio of barium and strontium in solution on the composition of film have been studied. The barium contents in the BST films are fairly lower than those in the original solutions. This indicates that strontium is more readily incorporated into the BST films, relative to barium. The results of narrow-scan of XPS spectrum confirm that the valences of Ba, Sr, Ti and O elements of hydrothermally prepared BST films are +2, +2, +4, and −2, respectively. SEM photographs show that the BST films are dense and well-compact. AFM analyses show that the average surface roughness of the films is 40–50 nm. It is concluded that BST films of different mole ratio of barium and strontium with thickness of up to 2 μm have been prepared successively by the environmentally benign hydrothermal method.     

Protonation patterns in tetracycline:tet repressor recognition: simulations and experiments

Resistance to the antibiotic tetracycline (Tc) is regulated by its binding as a Tc:Mg2+ complex to the Tet Repressor protein (TetR). Tc:TetR recognition is a complex problem, with the protein and ligand each having several possible conformations and protonation states, which are difficult to elucidate by experiment alone. We used a combination of free-energy simulations and crystallographic analysis to investigate the electrostatic interactions between protein and ligand and the possible role of induced fit in Tc binding. Tc in solution was described quantum mechanically, while Tc:TetR interactions were described by a recent, high-quality molecular-mechanics model. The orientations of the amide and imidazole groups were determined experimentally by a careful analysis of Debye-Waller factors in alternate crystallographic models. The agreement with experiment for these orientations suggested that the simulations and their more detailed, thermodynamic predictions were reliable. We found that the ligand prefers an extended, zwitterionic state both in solution and in complexation with the protein. Tc is thus preorganized for binding, while the protein combines lock-and-key behavior for regions close to the ligand's amide, enolate, and ammonium groups, with an induced fit for regions close to the Mg2+ ion. These insights and the modeling techniques employed should be of interest for engineering improved TetR ligands and improved TetR proteins for gene regulation, as well as for drug design.     

Origin of low dielectric loss and giant dielectric response in (Nb+Al) co‐doped strontium titanate

(Nb+Al) co‐doped SrTiO₃ ceramics with a nominal composition of Sr(Nb_0.5Al_0.5)_xTi_1‐xO₃ (x = 0, 0.02, 0.04, and 0.06) were fabricated using the conventional solid‐state reaction method; giant permittivity (10500) and low dielectric loss (0.03) were obtained at x = 0.06. Dielectric and impedance spectroscopy, X‐ray photoelectron spectroscopy, and Raman spectroscopy, were employed to study why the dielectric property improved. The results indicate that the giant dielectric response occurs because of the combined effects of the off‐center Ti³⁺ reorientation and conduction of electrons with the polar ordering structure Ti³⁺/Ti⁴⁺. In contrast, the low dielectric loss can be attributed to electron localization that occurs because of the defect dipole . These fundamental understandings will benefit the design of doped SrTiO₃ ceramics with desired performance.     

Temperature‐Dependent Deformation and Dislocation Density in SrTiO3 (001) Single Crystals

This study evaluates the change of flow stress as related to dislocation density in SrTiO₃ single crystals in order to provide guidance for later electrical studies. The key parameters varied are temperature and loading rate during the deformation. It is found that in <100>‐oriented SrTiO₃ single crystals, the dislocation density is enhanced by plastic deformation, more so at higher temperature as compared to room temperature. The experimental approach of quantifying the dislocation density through a determination of ex situ X‐ray diffraction rocking curves was successfully applied over the upper temperatures region of the lower temperature ductility zone for strontium titanate, i.e., in the so‐called “A‐regime”. For 1.0% deformed samples deformed at 300°C, a fourfold increase in dislocation density to 1.4 × 10¹³ m^?1 was found as compared to the nondeformed state (3.7 × 10¹² m^?1). Cross‐section techniques confirmed that the observed dislocation densities measured at the surfaces were identical to those seen in the core of the crystals. The use of rapid changes in loading rate provided an estimate for activation volume of the dislocation core for both 25°C and 300°C.     

Thermally enhanced dislocation density improves both hardness and fracture toughness in single-crystal SrTiO3

Dislocation-tuned functionality in ceramic oxides for potential versatile applications gains increasing attention. As the widespread chemical doping suffers from poor temperature stability, dislocations in well-controlled mesoscopic structure may be an alternative to thermally stable intrinsic doping features. To this end, the dislocation density in plastic zones introduced by cyclic Brinell indentation is considered under thermal annealing conditions. The considerably enhanced dislocation density due to thermal treatment is found to impact both microhardness and fracture toughness, albeit only to a modest degree. The mechanistic understanding centers around enhanced mobility and multiplication of the pre-engineered dislocations at elevated temperatures driven by the residual indentation stress, as well as the strengthened interaction of point defects and dislocations at high temperature.     

Determination of the controlling parameters for dislocation nucleation in SrTiO3: An investigation by nanoindentation

We conduct nanoindentation to investigate dislocation nucleation in SrTiO₃ (STO) single crystals with surface orientations of (0 0 1), (0 1 1), and (1 1 1) with loading/unloading rates of 25, 250, and 2500 μN/s. Results reveal that the critical loads (P_c) at which “pop-in” event occurs depend strongly on surface orientations, but slightly related to loading rate. Based on P_c, the critical shear stress that triggers dislocation nucleation was determined by extracting the maximum resolved shear stress (τ_max) along the slip systems of STO using the Hertzian solution. The dislocation activation shear stress (τ_a) was determined by averaging τ_max. The determined τ_a is 9.0–12.0 GPa, close to the shear strength (∼G/2π) of STO, indicating that homogeneous dislocation nucleation dominates the pop-in events. The consistency of the determined τ_a demonstrates that the frameworks for nanoindentation pop-in analysis established for metals can be extended to ceramics, whereas the influence of the limited slip systems should be taken into consideration. Additionally, we estimated the activation volume and the activation energy via the statistical model proposed by Schuh et al. The small values of the determined activation volume (0.6–9.8 Å³) and the activation energy (0.13–0.70 eV) indicate that the dislocation nucleation possibly begins from a single-atom migration and local point defects may participate in the dislocation nucleation process. That is, heterogeneous nucleation may exist initially but the homogeneous dislocation nucleation dominates the pop-in events.     

A critical comparison of 3D experiments and simulations of tricalcium silicate hydration

Advances in nano‐computed X‐ray tomography (nCT), nano X‐ray fluorescence spectrometry (nXRF), and high‐performance computing have enabled the first direct comparison between observations of three‐dimensional nanoscale microstructure evolution during cement hydration and computer simulations of the same microstructure, using HydratiCA. nCT observations of a collection of triclinic tricalcium silicate () particles reacting in a calcium hydroxide solution are reported and compared to simulations that duplicate, as nearly as possible, the thermal and chemical conditions of those experiments. Particular points of comparison are the time dependence of the solid phase volume fractions, spatial distributions, and morphologies. Comparisons made at 7 hours of reaction indicate that the simulated and observed volumes of consumed by hydration agree to within the measurement uncertainty. The location of simulated hydration product is qualitatively consistent with the observations, but the outer envelope of hydration product observed by nCT encloses more than twice the volume of hydration product in the simulations at the same time. Simultaneous nXRF measurements of the same observation volume imply calcium and silicon concentrations within the observed hydration product envelope that are consistent with Ca(OH)₂ embedded in a sparse network of calcium silicate hydrate (C–S–H) that contains about 70% occluded porosity in addition to the amount usually accounted as gel porosity. An anomalously large volume of Ca(OH)₂ near the particles is observed both in the experiments and in the simulations, and can be explained as originating from the hydration of additional particles outside the field of view. Possible origins of the unusually large amount of observed occluded porosity are discussed.     

The structure and dynamics of adsorbed molecules in microporous solids; a comparison between experiments and computer simulations

A critical evaluation is made of the results of computer simulations of adsorbed molecules in microporous inorganic materials. Structural, thermochemical and dynamic data are compared with the corresponding experimental findings. The systems examined include inert gases and simple alkanes in zeolites Na-Y and silicalite, benzene and pyridine in Na-Y and K-L, respectively, andp-xylene in silicalite. Future developments in the area are also discussed.     

Free-energy simulations and experiments reveal long-range electrostatic interactions and substrate-assisted specificity in an aminoacyl-tRNA synthetase

Specific recognition of their cognate amino acid substrates by the aminoacyl-tRNA synthetase enzymes is essential for the correct translation of the genetic code. For aspartyl-tRNA synthetase (AspRS), electrostatic interactions are expected to play an important role, since its three substrates (aspartate, ATP, tRNA) are all electrically charged. We used molecular-dynamics free-energy simulations and experiments to compare the binding of the substrate Asp and its electrically neutral analogue Asn to AspRS. The preference for Asp is found to be very strong, with good agreement between simulations and experiment. The simulations reveal long-range interactions that electrostatically couple the amino acid ligand, ATP, and its associated Mg2+ cations, a histidine side chain (His448) next to the amino acid ligand and a flexible loop that closes over the active site in response to amino acid binding. Closing this loop brings a negatively charged glutamate into the active site; this causes His448 to recruit a labile proton, which interacts favorably with Asp and accounts for most of the Asp/Asn discrimination. Cobinding of the second substrate, ATP, increases specificity for Asp further and makes the system robust towards removal of His448, which is mutated to a neutral amino acid in many organisms. Thus, AspRS specificity is assisted by a labile proton and a cosubstrate, and ATP acts as a mobile discriminator for specific Asp binding to AspRS. In asparaginyl-tRNA synthetase, a close homologue of AspRS, a few binding-pocket differences modify the charge balance so that asparagine binding predominates.     

Grain boundary curvatures in polycrystalline SrTiO3: Dependence on grain size,topology, and crystallography

By mapping grain orientations on parallel serial sections of a SrTiO₃ ceramic, it was possible to reconstruct three-dimensional orientation maps containing more than 3000 grains. The grain boundaries were approximated by a continuous mesh of triangles and mean curvatures were determined for each triangle. The integral mean curvatures of grain faces were determined for all grains. Small grains with fewer than 16 neighbors mostly have positive mean curvatures while larger grains with more than 16 neighbors mostly have negative mean curvatures. It is also possible to correlate the mean curvature of individual triangles with the crystallographic characteristics of the grain boundary. The mean curvature is lowest for grain boundaries with (100) orientations and highest for grain boundaries with (111) orientations. This trend is inversely correlated to the relative areas of grain boundaries and directly correlated to the relative grain boundary energy. The direct correlation between the energy and curvature is consistent with the expected behavior of grain boundaries made up of singular orientations. Furthermore, because both the relative energy and curvature of grain boundaries with (100) orientations are minima in the distributions, these boundaries also have the lowest driving force for migration.     

MD simulations of the collision between a copper ion and a polyethylene surface: an application to the plasma-insulating material interaction

To allow a better understanding of the physical phenomena occurred between a plasma and an insulating material, we have developed a specific MD code to study this type of interaction. We report results of MD simulations of the interaction of an incoming copper ion with a polyethylene crystal surface. Three initial incoming velocities and four impact angle values are used to check the influence of both the incident energy and impact direction to the resulting surface damage. When the incoming ion velocity is sufficiently high, MD results show that the impact can cause bond breaking leading to uncoordinated carbon atoms and free hydrogen atoms. The values of local temperatures associated with the structural changes show a possible ablation of the polyethylene surface.     

Revealing the pyrolysis mechanism of polypropylene cable insulation: A combined study of TG-GC experiments and RMD/DFT simulations

Styrene grafted polypropylene (PP-g-St) is one of the insulating materials for high voltage power cables. However, the process of pyrolysis gas generation in polypropylene cable insulation remains poorly understood. To address this knowledge gap, this study employed a combination of thermogravimetric-gas chromatography experiments, density functional theory, and reactive molecular dynamics simulations. The experimental findings revealed that the pyrolysis gases primarily consisted of H₂, CO, C₂H₄, and CH₄. Higher temperatures were found to increase the yields of H₂ and CO. The simulation results indicated that H₂ and CO were generated through the rupture of less reactive olefins and radicals, while C₂H₄ was primarily produced by the rupture of C C bonds in the polypropylene chain. Additionally, CH₄ was formed when  CH₃ groups captured hydrogen from other molecules. By the chain reaction mechanism, we enable the calculation of the activation energy of PP-g-St. This study provides a theoretical foundation for understanding the pyrolysis gas generation.     

Quantitative relationships between intermolecular interaction and damping parameters of irganox‐1035/NBR hybrids: A combination of experiments,molecular dynamics simulations,and linear regression analyses

The damping mechanism of phenol(3,5‐bis(1,1‐dimethylethyl)‐4‐hydroxybenzenepropanoic acid thiodi‐2,1‐ethanediyl ester, abbreviated as Irganox‐1035)/nitrile‐butadiene rubber hybrids was studied by combining experiments, computer simulations, and linear regression analyses. Four important damping parameters [loss peak (tan δ_max), effective loss area (TA), glass transition temperature (T_g), and effective temperature region (ΔT)], were obtained by dynamic mechanical thermal analyses. Three intermolecular interaction parameters [the number of intermolecular hydrogen bonds (N_HBs), binding energy (E_binding), and fractional free volume (FFV)], were calculated by molecular dynamics simulations. Using linear regression analyses, the quantitative relationships between the intermolecular interaction and damping parameters were investigated. Linear and significant relationships between intermolecular interactions (N_HBs and E_binding) and damping parameters (tan δ_max and TA) (R² > 0.9; P < 0.001) were noted; FFV showed moderate linear correlations with damping parameters (R² < 0.9; P < 0.05); only E_binding showed strong correlations with T_g and ΔT (R² > 0.9; P < 0.001). Besides, after nondimensionalization, multivariate linear fitting equations based on intermolecular interaction parameters were developed to accurately predict damping parameters (R² > 0.98, P < 0.001). These studies were expected to provide the useful information in understanding the damping mechanism and to attempt a quantitative tool for designing high damping materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46202.     

在不同构型分子筛催化剂中吸附和扩散的分子模拟

基于巨正则蒙特卡洛和分子动力学,对NH₃-SCR反应体系中吸附质分子（NO与NH₃）在不同拓扑结构沸石分子筛（LTL、FER、LEV、BEA、MOR、FAU、CHA和MFI）上的吸附和扩散特性进行系统研究。结果表明,对于全硅分子筛而言,其分子筛的拓扑结构影响NO与NH3在分子筛上的吸附,综合吸附量及吸附作用能发现,MFI和LEV分子筛对NO具有较优的吸附特性;MFI和BEA分子筛对NH₃ 具有较优的吸附特性。研究了Si与Al物质的量比对BEA分子筛吸附性能影响,结果表明,随着Si与Al物质的量比降低,分子筛自由体积逐渐增加,进而有助于分子筛催化剂对NO和NH₃的吸附。采用分子动力学模拟计算NO与NH₃在不同构型全硅分子筛上的扩散系数,发现具有三维直通道且孔径较大的分子筛催化剂有利于NO和NH₃在其孔道内部的扩散,MFI虽然具备三维孔道结构,但由于存在Z型交叉通道,一定程度阻碍了反应物分子的扩散。