High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi₂–Al₂O₃ composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ～2 μm thick Mo₅Si₃ diffusion layer, a ～65 μm thick MoSi₂ inner layer and a ～36 μm thick outer layer of mixture of MoSi₂ and Al₂O₃. After oxidation at 1600 °C for 5 h, all MoSi₂ phases were completely converted to intermediate silicide Mo₅Si₃ by solid-state diffusion, and the formed Mo₅Si₃ phase would be transformed into Mo₃Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO₂-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi₂–Al₂O₃ coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi₂–Al₂O₃ coating is due to the formation of multi-layer structure containing a SiO₂-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.     

Bi2O3–Nd2O3–WO3 system: Phase formation,polymorphism, and conductivity

Cubic, tetragonal, and monoclinic (Bi₂O₃)_x (Nd₂O₃)_y (WO₃)_z (x + y + z = 1) solid solutions based on the Bi₂O₃ oxygen ion conductor have been prepared by solid-state reactions in the ternary system Bi₂O₃–Nd₂O₃–WO₃. The field of monoclinic compounds with a Bi_3·24La₂W_0·76O_10.14-type structure has been shown to account for most of the ternary system. Compounds with a cubic fluorite structure exist at the boundary of the monoclinic phase field in two small regions at high (83–91 mol% Bi₂O₃, δ-phase) and low (20–55 mol% Bi₂O₃, δ′-phase) Bi concentrations. The cubic samples of the δ-phase retain their structure only during rapid heating and cooling, but annealing in the range of 300–700 °C results in structure degradation to lower symmetry phases. The monoclinic compounds and Bi-poor cubic compounds (δ′-phase) have good thermal stability. The cubic samples of the δ′-phase are hygroscopic. Their bulk conductivity noticeably increases with atmospheric humidity, suggesting that these materials are potential proton conductors.     

纳米抛光碳化硅压力对相变影响的分子动力学模拟

为了研究纳米抛光碳化硅时压力变化对表面的影响规律,建立了金刚石磨粒纳米抛光碳化硅的分子动力学模型,数值模拟了纳米尺度下的碳化硅抛光过程,具体分析了抛光压力线性增大过程中的配位数为1至6的原子数量的变化规律,揭示了线性改变抛光压力对被加工表面相变的影响规律,仿真结果表明:压力是诱导碳化硅相变的主要因素,当抛光压力增大时,发生相变的原子数增多,碳化硅的相变深度增加,其中配位数为1、2和4的原子数减少,配位数为3、5和6的原子数增多。     

Interplay of the phase and the chemical composition of the powder feedstock on the properties of porous 8YSZ thermal barrier coatings

Some chemical impurities enhance sintering kinetics of ceramic Thermal Barrier Coatings (TBCs) which can cause their premature failure during operation in gas turbine engine by causing reduction in coating’s strain compliance as well as faster bond-coat oxidation due to increased thermal conductivity. Certain chemical impurities are also believed to suppress resistance to tetragonal to monoclinic phase transformation in 8YSZ, which can also be an important factor regarding TBC’s performance. Most of the impurities and some of the monoclinic phase present in the powder feedstocks can survive into the as-sprayed coating. Therefore, there is a general trend towards OEMs requiring the lowest amounts of chemical impurities and the lowest amounts of monoclinic phase in the powder feedstocks. This paper presents a comprehensive investigation aimed at understanding the role and the relative importance of the chemical and phase purities of the powder feedstock for the properties and performance of thick 8YSZ TBCs.     

The thermochromic characteristics of Zn-doped VO2 that were prepared by the hydrothermal and post-annealing process and their polyurethane composite films

In this paper, Zn-doped VO₂ nanoparticles have been successfully fabricated by a two-step hydrothermal-annealing process, and the thermally induced visible light transmittance enhancement of Zn-doped VO₂ has been studied for the first time. It is found that Zn-doped VO₂ not only exhibits excellent solar modulation ability (ΔT_sol = 15.27%) but also can reduce the phase transition temperature and increase the visible light transmittance after the heat-induced phase transition (ΔT_lum=+5.78%). Moreover, with the increase of Zn doping concentration, the phase transition temperature (T_c) and phase transition hysteresis (ΔT) both decrease. It is shown that the Zn-doped VO₂-PU films not only have good solar light modulation ability and properties of improving visible light transmission after phase transition, but also have good durability. The research result is of great significance for improving the visible light transmittance after phase transition and realizing the practical application of VO₂ in the field of smart windows.     

Ferroelectric domain structure and atomic-scale phase distribution in a Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal

Lead-based relaxor ferroelectric crystals, such as Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, are promising for functional applications due to their outstanding ferroelectric properties. Revealing ferroelectric domain structure and phase distribution at the nanoscale are important to understand the basic theory of ferroelectricity and promote their practical applications. In this study, the ferroelectric domain structure and atomic-scale phase distribution in a Pb(Mg_1/3Nb_2/3)O₃ - 35 at.% PbTiO₃ single crystal in the region of morphotropic phase boundary are systematically investigated by a combination of optical microscopy, scanning electron microscopy, and aberration-corrected transmission electron microscopy. It is found that 90° and 180° coarse domains are formed simultaneously in the ferroelectric single crystal. Moreover, there are tweed-like nano-domains formed inside each coarse single domain. Through careful investigations of distribution of stress and tetragonality, it is revealed that the tweed-like nano-domains are composed of nano-sized rhombohedral (R), monoclinic (M), and tetragonal (T) phases. The nano-scale T, M, R phases have the best, moderate, and lowest ferroelectricity, respectively.     

Microstructure and Mechanical Properties of Mg–RE–TM Cast Alloys Containing Long Period Stacking Ordered Phases: A Review

Casting magnesium alloys hold the greatest share of magnesium application products due to their short processing period, low cost and near net shape forming. Compared with conventional commercial magnesium alloys or other Mg–RE-based alloys, the novel Mg–RE–TM cast alloys with long period stacking ordered(LPSO) phases usually possess a higher strength and are promising candidates for aluminum alloy applications. Up to now, two ways: alloying design and casting process control(including subsequent heat treatments), have been predominantly employed to further improve the mechanical properties of these alloys. Alloying with other elements or ceramic particles could alter the solidifi cation pattern of alloys, change the morphology of LPSO phases and refi ne the microstructures. Diff erent casting techniques(conventional casting, rapidly solidifi cation, directional solidifi cation, etc.) introduce various microstructure characteristics, such as dendritic structure, nanocrystalline, metastable phase, anisotropy. Further heat treatments could activate the transformation of various LPSO structures and precipitation of diverse precipitates. All these evolutions exert great impacts on the mechanical properties of the LPSO-containing alloys. However, the underlying mechanisms still remain a subject of debate. Therefore, this review mainly provides the state of the art of the casting magnesium alloys research and the accompanying challenges and summarizes some topics that merit future investigation for developing high-performance Mg–RE–TM cast alloys.     

Digital hysteresis sliding mode control for interleaved DC–DC converters

This paper presents a digital constant frequency sliding mode control (SMC) law for interleaved DC–DC converters. Constant switching frequency and interleaving are achieved by dynamically adjusting the hysteresis of the control signals generating comparators. The interleaving method neither imposes constraints on the number of required phases to obtain a specific output voltage nor uses quasi-SMC. Hence, the control scheme achieves high flexibility, robustness, and performance. Furthermore, a sliding mode observer (SMO) for reconstructing the inductor currents is proposed. Experimental results for a two-phase buck converter are reported. The control concept accomplishes an improved dynamic performance in comparison with quasi-SMC.     

Numerical investigation of gelled crude oil particle melting in water for hydrogen production

This paper describes a numerical analysis of the melting process of gelled crude oil in water for hydrogen production. The melting characteristics of crude oil particles are analyzed, and the influence of flow velocity, particle radius, and hot water temperature on the melting process is discussed. The results indicate that the melting rate on the surface of the crude oil is different, the oil particles do not melt concentrically, and the monitoring points and rate of increase in temperature are not substantially affected by the phase change interval. The flow rate and particle radius are found to have a significant influence on the phase distribution of the oil droplets, but changes in water temperature have little effect. After validating a model of the melting process, we investigate the influence of oil particle size, water temperature, flow rate, and oil temperature. It is found that increases in water temperature and flow rate, or a decrease in oil particle size, tend to reduce the melting time.