Additive manufacturing,quasi-static and dynamic compressive behaviours of ceramic lattice structures

Ceramic lattice structures (CLSs) are used for construction in common and extreme environments because of the extraordinary properties of ceramics. In this study, we designed and additively manufactured CLSs with distinct structural parameters to explore their quasi-static and dynamic compressive behaviours in detail. It was demonstrated that both the relative density (?ρ) and inclination angle (ω) had a significant impact on the quasi-static and dynamic mechanical properties of the CLSs. Furthermore, the mathematical relationships between the quasi-static compressive properties, including quasi-static compressive strength (QS), quasi-static Young’s modulus (QY), and quasi-static energy absorption (QE), versus ?ρ and ω obeyed the Gibson–Ashby and Deshpande and Fleck models, respectively. It was revealed by experiment and simulation that as the stiffness increased, the quasi-static failure mode of the CLSs changed from a parallel-vertical-inclined mixed mode to a parallel-vertical mode. In addition, the relationship between the dynamic mechanical properties of the CLSs versus ?ρ and ω also followed the Gibson–Ashby and Deshpande and Fleck models. The exceptional dynamic increase factor indicated that CLSs are highly suitable for extreme environments. These findings will aid in the research and development of customised additively manufactured CLSs.     

剪切解络-超滤分离金属关键指标预测与验证

根据剪切解络-超滤过程中金属离子的动态浓度关系和质量守恒定律,采用CodeBlocks17.12软件编写程序,建立了剪切解络-超滤工艺关键指标预测模型,实现了金属离子浓度、回收率、体积稀释倍数等指标的预测,并用实验加以验证。依次在转速800 r/min、1 400 r/min、3 000 r/min下回收模拟废水中43 μg/L的Co²⁺、995 μg/L的Ni²⁺和88 μg/L的La³⁺,3种金属的回收率均达99.8%以上时,超滤过程体积稀释倍数的预测值分别为7.0、7.5、5.75,实验值分别为7.5、7.25、6.25,可见预测值与实验值基本一致。本模型对研究方案制定和调整有实际指导意义。     

A kinetic model involving oxidation and volatilization in SiC-B4C-xAl2O3 ceramics system

In this paper, a new kinetic model considering both oxidation and volatilization kinetics is established and applied to analyze the oxidation of SiC-B₄C-xAl₂O₃ ceramics and other systems in various oxidation conditions. The effects of diffusion area and volume changes during the oxidation process are considered in this model. The physical meaning of each parameter in this model is explicit and simple. According to this model, the diffusion coefficient of species and the corresponding diffusion activation energy are easily available. The practicability of this model is well verified by the experimental data of SiC-B₄C-xAl₂O₃ and other systems oxidized under different conditions. In addition, the practice shows that the model is applicable not only to the systems where oxidation and volatilization coexist, but also to the system where only oxidation plays a major role. We hope the model proposed in this work can be used in other materials with more complex environments.     

Improvement in the properties of low carbon MgO-C refractories through the addition of graphite-SiC micro-composite

Graphite–SiC micro-composites have been prepared in–house by carbothermal reduction process. Controlling the process parameters including the weight ratio of SiO₂ to graphite as well as carbothermal reduction temperature during the micro-composite preparation favors the homogeneous formation of SiC with preferred morphologies like ribbons and whiskers/fibers. The micro-composite modified low carbon MgO-C refractories have exhibited significantly improved bulk properties over the standard composition. To understand the beneficial role of SiC reinforcement on hot strength performance under air oxidizing conditions, we propose a scaling parameter known as strength factor (fs) based on the ratio of hot strength (HMOR) to cold strength (CCS). Correlating the strength factor data (fs) with oxidative damage provides new insights into the reinforcing effects of distinct SiC morphologies in this new class of micro-composite fortified refractory systems over the standard compositions.     

Tailoring the chemical heterogeneity of Mn-modified 0.75BiFeO3-0.25BaTiO3 ceramics for piezoelectric sensor applications

The Mn-modified 0.75BiFeO₃-0.25BaTiO₃ (75BFBTMn) piezoelectric ceramic possesses a high depolarization temperature of 500 °C and a large piezoelectric coefficient of 110 pC/N, showing the potential for high temperature piezoelectric sensors. However, 75BFBTMn ceramic usually suffers dielectric degradation and abrupt drop of piezoelectric coefficient in the range of 300 °C to 500 °C. Combined the high-energy synchrotron X-ray diffraction analysis with Backscatter-SEM results, it is demonstrated that the electrical thermal instability is owing to the existence of chemical inhomogeneity. The Air-annealing treatment is able to decrease the volume fraction of pseudo-cubic phase and the lattice distortion, removes the chemical inhomogeneity in the grain and free Bi₂O₃ at grain boundary, and then eliminates dielectric anomalies and piezoelectric degradation with temperature. These results indicate that air-annealing is a simple but effective method to eliminate the chemical inhomogeneity in 75BFBTMn ceramics, thereby improving the property thermal stability for high temperature piezoelectric sensor applications.     

Effect of temperature on fast forging process of Mg-Ni samples for fast formation of Mg2Ni for hydrogen storage

Fast-Forging was used as a Severe Plastic Deformation technique to process Mg/Ni fine powder mixtures at a ratio corresponding to the eutectic composition. The samples were processed at different temperature, increased successively from room temperature up to above 500 °C. The one shock forging operation led to a reduction rate comprised between 80 to more than 90% depending of the applied temperature. Interestingly, a threshold temperature was pointed out for which amounts of the binary Mg₂Ni alloy were directly synthesized in increasing proportions when increasing temperature. A maximum amount of Mg₂Ni was synthesized according the nominal proportions at the highest applied temperature. Besides, numerical simulations were developed to consider and integrate to the forging process, the heat arising from the mechanical energy at deformation. Interestingly the total temperature at shock – heat applied to and heat developed in – indicates that the threshold temperature correspond well with the eutectic temperature as reported in the phase diagram. Early hydrogenation cycles suggest that both mechanical defects in brittle Mg and presence of amounts of Mg₂Ni as catalyst should be combined to optimize the hydrogenation characteristics.     

Effect of strain path change on the through-thickness microstructure during tantalum rolling

High purity tantalum was respectively processed by unidirectional rolling (UR) and clock rolling (CR), and the through-thickness microstructures were investigated by multiple characterization techniques including electron backscatter diffraction (EBSD), Vickers hardness (HV) and X-ray diffraction (XRD). Results show that the through-thickness stored energy distribution in CR specimens is more homogeneous than in UR specimens due to uniform texture distribution. {111} grains possess larger Schmid factors and the corresponding Schmid factor difference ratio than {100} grains, indicating the activation of uniserial slipping in {111} grains, which leads to inhomogeneous deformation and higher stored energy. Besides, X-ray line profile analysis (XLPA) suggests that the stored energy of {111} grains increases successively from the surface to center layer, regardless of strain paths, due to the influence of redundant friction on the surface layers. While the occurrence of multiple slipping in {100} grains leads to homogeneous deformation and lower stored energy.     

Layered NiFe-LDH/MXene nanocomposite electrode for high-performance supercapacitor

Layered double hydroxide (LDH) is potentially excellent supercapacitor (SC) materials, but the low conductivity and easy agglomeration limit the further improvement of their electrochemical properties. Therefore, LDHs are requisite to grow on some conductive substrates to produce high-performance SC. In this paper, the conductive two-dimensional (2D) transition metal carbides, nitrides and carbonitrides (called MXene) were explored as the substrate to directly deposit NiFe-LDH nanosheets by a one-step hydrothermal method, then a three-dimensional (3D) porous NiFe-LDH/MXene electrode was obtained. The morphology and electrochemical performance of the composite electrodes were analyzed and investigated. The results show that the NiFe-LDH/MXene electrode has larger specific capacitance (720.2 F/g) than NiFe-LDH (465 F/g), and the capacitance of the composite electrode retained 86% after 1000 cycles (only 24% for NiFe-LDH), showing excellent cycle stability. The improved electrochemical performance of the composites is caused by the stable sheet-like structure of NiFe-LDH during charge-discharge time and the conductive network formed by the MXene, which can accelerates electron transport. In addition, the asymmetric SC based on NiFe-LDH/MXene positive electrode display a power density of 758.27 W/kg at an energy density of 42.4 Wh/Kg. These results indicate the NiFe-LDH/MXene composites can be applied as the novel candidate of high-performance SC electrodes.     

1250°C liquidus for the CaO–MgO–SiO2–Al2O3–TiO2 system in air

Ti-bearing blast furnace slags have been regarded as an important secondary material in modern society, and the efficient recycling of Ti oxides from it is of key interest. For this reason, more thermodynamic data is needed regarding the phase relations in different composition ranges and sections. Therefore, the equilibrium phase relations of CaO–MgO–SiO₂–Al₂O₃–TiO₂ system in a low w(CaO)/w(SiO₂) ratio of 0.6–0.8 at 1250 °C in air and fixed concentrations of MgO and Al₂O₃, were investigated experimentally using a high temperature equilibration and quenching method followed by SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer) analyses. The equilibrium solid phases of perovskite (CaO·TiO₂), a pseudo-brookite solid solution (MgO·2TiO₂, Al₂O₃·TiO₂)_ss, and anorthite (CaO·Al₂O₃·2SiO₂) were found to coexist with the liquid phase at 1250 °C. The calculated results of Factsage and MTDATA were used for comparisons, and significant discrepancies were found between predictions and the experimental results. The 1250 °C isotherm has been constructed and projected on the CaO–SiO₂–TiO₂-8 wt.% MgO-14 wt% Al₂O₃ quasi-ternary plane of the phase diagram. The obtained results provide new fundamental data for Ti-bearing slag recycling processes, and they add new experimental features for thermodynamic modeling of the high-order titanium oxide-containing systems.     

Enhanced strength and ductility of superhard boron carbide through injecting electrons

Injected electrons and holes play an important role in the mechanical properties of semiconductors because of the charged states in dislocation core. However, many superhard ceramics exhibit very limited plasticity at room temperature because of strong covalent and ionic bonding. To illustrate the effects of injected carriers on the mechanical properties of superhard ceramics, we selected boron carbide (B₄C) as the prototype system and employed density functional theory (DFT) to examine the deformation and failure mechanism under neutral, injected-electron and injected-hole states. We find that both strength and ductility of B₄C can be enhanced by injecting electrons, which arises from the modified failure mechanism due to the distribution of excess electrons. In contrast, injecting holes into B₄C does not affect the failure mechanism, but causes increased strength and decreased ductility. Our study suggesting that injecting electrons is a promising way to tune mechanical properties of superhard ceramics.