Microstructural Control of a 70% Silicon Nitride– 30% Barium Aluminum Silicate Self-Reinforced Composite

The processing response of a 70% silicon nitride–30% barium aluminum silicate (70%-Si₃N₄–30%-BAS) ceramic-matrix composite was studied using pressureless sintering, at temperatures ranging from 1740°C, which is below the melting point of BAS, to 1950°C. The relationship between the processing parameters and the microstructural constituents, such as morphology of the β-Si₃N₄ whisker and crystallization of the BAS matrix, was evaluated. The mechanical response of this array of microstructures was characterized for flexural strength, as well as fracture behavior, at test temperatures up to 1300°C. The indentation method was used to estimate the fracture resistance, and R -curves were obtained from modified compact-tension samples of selected microstructures at room temperature.     

Equilibrium Amorphous Silicon–Calcium–Oxygen Films at Interfaces in Copper–Alumina Composites Prepared by Melt Infiltration

The microstructure of copper–alumina (Cu-Al₂O₃) composites that have been prepared via the melt infiltration of liquid copper into porous alumina preforms was studied in detail, using various transmission electron microscopy (TEM) techniques. Two different samples—with open pore diameters of 0.2 and 0.8 μm—were investigated. For both specimens, a single crystalline copper network that extended throughout the open porosity of the alumina preform was observed. An amorphous glass phase that contained silicon and calcium was observed at the Al₂O₃/Cu/Al₂O₃ triple junctions. The diameters of these amorphous pockets, which were strongly faceted along the Al₂O₃ grains, were up to 20 and 100 nm for the initial pore sizes of 0.2 and 0.8 μm, respectively. A glass phase that contained silicon and calcium also was present at the Cu/Al₂O₃ interfaces, whereas the Al₂O₃ boundaries remained dry. Detailed high-resolution transmission electron microscopy investigations have shown that the interfacial glass phase at the Cu/Al₂O₃ interfaces exhibited a uniform equilibrium film thickness along the interface region. However, the interfacial film thickness was dependent on the orientation of the Al₂O₃ grain, and its value varied from 0.4 nm for Al₂O₃ rhombohedral-plane termination ((1¯012)) up to 1 nm for Al₂O₃ basal-plane termination ((0001)).     

Relationship between Microstructure and Mechanical Performance of a 70% Silicon Nitride–30% Barium Aluminum Silicate Self-Reinforced Ceramic Composite

The abnormal grain growth of β-Si₃N₄ was observed in a 70% Si₃N₄–30% barium aluminum silicate (70%-Si₃N₄–30%-BAS) self-reinforced composite that was pressureless-sintered at 1930°C; Si₃N₄ starting powders with a wide particle-size distribution were used. The addition of coarse Si₃N₄ powder encouraged the abnormal growth of β-Si₃N₄ grains, which allowed microstructural modification through control of the content and size distribution of β-Si₃N₄ nuclei. The mechanical response of different microstructures was characterized in terms of flexural strength, as well as indentation fracture resistance, at room temperature. The presence of even a small amount of abnormally grown β-Si₃N₄ grains improved the fracture toughness and minimized the variability in flexural strength.     

X-ray Photoelectron Spectroscopy Study on the Process of Apatite Formation on a Sodium Silicate Glass in Simulated Body Fluid

The process of apatite formation on the surface of Na₂O–SiO₂ glass in a body environment was investigated, mainly by X-ray photoelectron spectroscopy, as a function of soaking time in a simulated body fluid (SBF). The glass was found to release Na⁺ ions via exchange with H₃O⁺ ions in the SBF to form Si—OH groups on its surface. These Si—OH groups induced apatite formation indirectly, by forming calcium silicate and amorphous calcium phosphate. The formation of the calcium silicate and amorphous calcium phosphate is attributed to electrostatic interactions between the Si—OH groups on the glass surface and the calcium and phosphate ions in the SBF.     

Stiffening of Polycarbonate by Addition of a Highly Dispersed and Fibrillated Amorphous Polyamide‐Based Nanocomposite

Ternary systems consisting of blends of polycarbonate (PC) from bisphenol A and minority amounts of an amorphous polyamide reinforced with organically modified nanoclay (naPA), were obtained in the melt state. The nanoclay was widely exfoliated inside the dispersed naPA phase. The dispersed phase exhibited a very fine size (up to 0.36 µm), indicating compatibilization. Compatibilization was attributed to interactions between the aPA and the PC. The nanocomposite showed a lower compatibility than their corresponding blends. This lower compatibility of the nanocomposite was attributed to a hindrance of the interaction by the migrated surfactant of the organoclay. The presence of fibrillation in conjunction with a dispersed nanoclay resulted in additive enhancing effects on the modulus and yield stress. This led to modulus increases up to 46% with respect to that of the neat matrix upon the addition of 25% naPA‐10. Besides exhibiting these remarkable modulus values, these systems show an elongation at break similar to that of the neat PC matrix.

     

碱性溶液中非晶/纳米晶Ni-Mo合金的形成机制研究

针对碱性溶液中Ni-Mo合金的形成机制,采用极谱法和循环伏安法进行研究。实验发现:Ni,Mo原子为多步氧化还原反应,Ni-Mo合金的形成过程为异常共沉积与诱导共沉积的复合沉积过程,进而形成致密光滑的结构镀层。非晶/纳米晶Ni-Mo合金具有一定的储氢能力,其化学活性较高,是较好的电极材料。     

NMR Studies of Fluorine in Aluminosilicate–Lanthanum Fluoride Glasses and Glass-Ceramics

NMR determinations of fluorine environments in transparent oxyfluoride glass-ceramics were made to learn about the crystallization of LaF₃, as well as to ascertain the structural role of fluorine in the surrounding glassy matrix. The fraction of fluorine in LaF₃ was measured as a function of heat treatments, demonstrating significant differences between glasses modified with barium and those containing sodium. The results of these measurements showed that not all of the fluorine formed LaF₃ in these glass-ceramics, with resolution of additional fluoride sites at −135 and −185 ppm, due primarily to Si–F and Al–F bonding, respectively. Not only is the evidence of Si–F bonding unexpected, given the presence of aluminum, but the amount of Si–F bonding is sensitive to the type of modifier in the glass. Samples containing barium oxide as the modifier showed a higher fraction of Si–F bonding than those modified with sodium oxide.     

Some Aspects of Stability and Nanophase Formation in Quasicrystals during Mechanical Milling

Non-equilibrium processing and other related techniques can activate quasicrystalline structures to a higher energy level and enable them to attain various stable/metastable structures. During mechanical milling of quasicrystalline alloys, the stability of the complex structures is of immense importance from a scientific and technological point of view. The evolution of nanoquasicrystalline, nanocrystalline, and amorphous phases and their composites has been observed in the course of milling. The experimental results for mechanical alloying of elemental powders (required for the synthesis of quasicrystals) and mechanical milling of quasicrystals are discussed. The salient features observed in quasicrystals during other non-equilibrium processing are highlighted. Nanospinel formation from the Al-based quasicrystalline precursor is mentioned.     

Metastable Tantalum Oxide Formation During the Devitrification of Amorphous Tantalum Thin Films

Microstructural evolution during the devitrification of amorphous tantalum thin films synthesized via pulsed laser deposition was investigated using in situ transmission electron microscopy (TEM) combined with ex situ isothermal annealing, bright‐field imaging, and electron‐diffraction analysis. The phases formed during crystallization and their stability were characterized as a function of the chamber pressure during deposition, devitrification temperature, and annealing time. A range of metastable nanocrystalline tantalum oxides were identified following devitrification including multiple orthorhombic oxide phases, which often were present with, or evolved to, the tetragonal TaO₂ phase. While the appearance of these phases indicated the films were evolving to the stable form of tantalum oxide—monoclinic tantalum pentoxide—it was likely not achieved for the conditions considered due to an insufficient amount of oxygen present in the films following deposition. Nevertheless, the collective in situ and ex situ TEM analysis applied to thin film samples enabled the isolation of a number of metastable tantalum oxides. New insights were gained into the transformation sequence and stability of these nanocrystalline phases, which presents opportunities for the development of advanced tantalum oxide‐based dielectric materials for novel memristor designs.     

Effect of P2O5 on the Nonisothermal Sinter‐Crystallization Process of a Lithium Aluminum Silicate Glass

Dense (~98.5%), lithium aluminum silicate glass‐ceramics were obtained via the sinter‐crystallization of glass particle compacts at relatively low temperatures, that is, 790–875°C. The effect of P₂O₅ on the glass‐ceramics' sinter‐crystallization behavior was evaluated. We found that P₂O₅ does not modify the surface crystallization mechanism but instead delays the crystallization kinetics, which facilitates viscous flow sintering. Our glass‐ceramics had virgilite (Li_xAl_xSi_3‐xO₆; 0.5 < x < 1), a crystal size <1 μm, and a linear thermal expansion coefficient of 2.1 × 10^?6°C^?1 in the temperature range 40–500°C. The overall heat treatment to obtain these GCs was quite short, at ~25 min.     

Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature

Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi_1/2Na_1/2TiO₃–0.08Bi_1/2K_1/2TiO₃–0.04BaTiO₃ (BNKBT88) with copper. A combination of Bi₂O₃, CuO, ZnO, Li₂CO₃, and B₂O₃ are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10^?8 atm, which is necessary for the coexistence of Cu metal and Bi₂O₃. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.     

Protective Properties of An Inhibitor Layer Formed on Copper in Neutral Chloride Solution

The aim of this work was to investigate the efficiency of 1-phenyl-4-methylimidazole for corrosion inhibition of copper in 3% NaCl solution. The formation of a thick layer on the copper surface was observed in the presence of this compound. The protective properties of this layer were characterized by means of cyclic voltammetry and through AFM and SEM/EDX measurements. Voltammetric measurements were performed in stirred and quiescent solutions as well as at different immersion times. Voltammetric measurements showed that the protective properties of the layer were enhanced by stirring and with an increase in immersion time. The time dependence of the protective layer formation was characterized by AFM measurements. From SEM/EDX measurements it was concluded that this protective layer has a complex structure consisting of the inhibitor and corrosion products.     

Formation of a Protective Layer During the Hydration of Cement

The mechanism for the slow rate of reaction between portland cement and water during the early stage is not well understood, but it probably is controlled by either the rate that the reactants diffuse through a barrier that surrounds the unreacted cement grains or by the rate that nuclei of the stable product form and grow or by both rates. New evidence using environmental scanning electron microscopy is presented about the structure of a layer that forms around the particles of cement. Preliminary observations that relate mixing to the structure of the layer are also presented.     

不同吸收层结构的非晶硅/锗太阳能电池转换效率的模拟计算(英文)

设计了2种太阳能电池结构：结构A为a-Si：H/a-Si0.65Ge0.35：H多吸收层结构;结构B为a-Si0.65Ge0.35：H单吸收层结构。采用AMPS-ID程序分析了2种电池结构的光电性质。模拟计算中光学吸收系数和缺陷浓度均采用实验数据以便确保模拟的可靠性。分析了2种电池结构的短路电流密度、断路电压、填充因子和转换效率。结果表明：对于结构A,当吸收层厚度达180nm时,转换效率达到饱和值6.88%;对于结构B,当吸收层厚度达45nm时,转换效率达到最大值3.44%;利用载流子产生和复合机制分析了采用多吸收层结构更有利于提高太阳能电池的转换效率。     

Molecular dynamics study on the co-doping effect of Al2O3 and fluorine to reduce Rayleigh scattering of silica glass

Further attenuation of Rayleigh scattering of silica glass is required to extend the capacity of long-distance optical fiber communication. To theoretically examine the effect of aluminum and fluorine co-doping on the density fluctuations of silica glass, which is related to Rayleigh scattering, a set of force-matching potentials (FMP) for simulating F-doped aluminosilicates was developed using Bayesian optimization based on density functional theory calculations. Molecular dynamics (MD) simulations with the new FMP could evaluate the densities of silica glasses to which a small amount of fluorine was doped and those of aluminosilicate glasses with a wide range of aluminum contents within reasonable accuracy. The FMP successfully represents the changing role of aluminum from a network former without a compensating cation (threefold coordination) to that with a compensating cation and a charge compensator in the aluminosilicates. Indeed, relative concentrations of four, five, and sixfold-coordinated aluminum observed by NMR measurements were reproduced better than the original Teter potential at a high aluminum content. At an aluminum content lower than 1 mol%, threefold-coordinated aluminum was observed, which is consistent with ESR measurements. After careful validations of the FMP, the effect of the co-doping of alumina and fluorine on the density fluctuations of silica glass was computationally examined. Consequently, it was expected that the co-doping might not sufficiently attenuate the Rayleigh scattering, even though 1 wt% fluorine would be able to reduce the density fluctuations of aluminosilicate glasses for some extent. This is because more alumina-doping increases density fluctuations of silica glass if the drawing temperature and procedure are the same with those for silica glass fiber. Thereby, a possible fabrication process to sufficiently attenuate the Rayleigh scattering of the F-doped aluminosilicate glass was proposed, according to the density fluctuation analysis at high temperature.     

Thermodynamic Behavior of Copper-Coated Silicon Carbide Particles during Conventional Heating and Spark Plasma Sintering

75Cu·25SiC (vol%) compacts were prepared using copper-coated SiC particles and spark plasma sintering (SPS). The preliminary thermal performance of the coated particles was determined using simultaneous DSC-TG-MS measurement. Characterization of compacts using XRD and SEM techniques was conducted to investigate the physical and chemical changes during the SPS operation. It was found that CuO decomposed at 850° and 500°C during conventional heating and SPS, respectively. Cu₂O facilitated the densification of Cu/SiC composites. The optimized sintering temperature of Cu/SiC composites using SPS was ∼730°C. The compacts showed improved hardness because of the SiC reinforcement.     

Two-Layer Crystallization of Amorphous YMnO3 Thin Films on Si (100) Substrates

During a rapid thermal annealing process at 850°C in a N₂ ambient, an as-deposited amorphous YMnO₃ thin film on Si (100) substrates was crystallized with two distinct layers. High-resolution transmission electron microscopy showed a top layer of c -axis-oriented YMnO₃ and a bottom layer of polycrystalline YMnO₃ in the 100-nm-thick YMnO₃ thin film. The abrupt change of the crystalline orientation from the c -axis-preferred orientation to the random orientation is caused primarily by high stress induced by the c -axis-oriented YMnO₃ layer. High-resolution X-ray diffraction showed that the c -axis-oriented YMnO₃/polycrystalline YMnO₃ structure effectively relieved the stress.     

Formation of Zinc–Antimony-Based Spinel Phases

Formation of spinel phases in ZnO–Sb₂O₃and ZnO–Sb₂O₃–Bi₂O₃systems is studied by the use of X-ray diffraction. The formation of nonstoichiometric Zn_2.33Sb_0.67O₄phase is observed in both the systems at ∼900°C. However, in these systems, at higher temperatures ( T ≥ 1100°C), formation of the inverse spinel phase Zn₇Sb₂O₁₂is observed. The study has been extended to understand the effect of CrO₃doping on the stability of the different spinel phases in the previously mentioned systems. Interestingly, in both the systems, samples doped with CrO₃, displayed the presence of Zn_2.33Sb_0.67O₄phase <1200°C, indicating the stabilization of the spinel phase by CrO₃.     

Effect of Oxidation State of Iron on Phase Separation in Sodium Silicate Glasses

The effect of the oxidation state of iron on the phase separation of x Na₂O·(100 – x )SiO₂ glasses, x = 18.56 and 13, containing 0.5 mol% Fe₂O₃ was studied. The oxidation state of iron in the glasses was varied by changing the melting conditions, such as melting temperature and melting atmosphere. The oxidation states of the iron ion were determined using colorimetric and UV–VIS–NIR spectrophotometric methods, and a comparison was made between the results obtained using these two methods. Immiscibility temperatures of the glasses were determined using opalescence and clearing methods. The immiscibility temperature of the sodium silicate binary glasses decreased ∼25°C with the addition of 0.5 mol% Fe₂O₃. The immiscibility temperature of the doped glasses increased slightly with increased concentration of Fe²⁺ ion in the glass. The prediction of immiscibility tendency on the addition of a minor amount of third component was made using models proposed by Tomozawa and Obara and Nakagawa and Izumitani. The Tomozawa and Obara model showed good agreement with measured immiscibility values.     

Formation of Convoluted Silica Precipitates during Amorphous Phase Separation in the Ca3(PO4)2–SiO2–MgO System

Dispersed aggregates of peculiar morphology have been obtained in phase-separated glasses of the system Ca₃(PO₄)₂–SiO₂–MgO, where the separated phase is amorphous silica. The formation of such convoluted aggregates is tentatively explained in terms of a fast coalescence process of initial isolated quasi-spherical droplets which behave as a dispersed phase in an emulsion-like system.