Fabrication and properties of Si3N4 based ceramics using combustion synthesized powders

Silicon nitride (Si₃N₄) based ceramics were fabricated with β-SiAlON and Si₃N₄ powders synthesized by combustion synthesis method via power injection molding (PIM). In the PIM process, the solids loading for each material was first determined from the results of the torque rheometer experiment. The mixing process was repeated to produce the homogeneous feedstock, and homogeneity of feedstocks was evaluated by observing the shear viscosity with time at a constant shear rate. The rheological behavior of feedstocks was investigated using capillary rheometer. It found that both feedstocks have no problem in injection molding. The binder decomposition behavior was also investigated, and a wax-polymer binder system was nearly removed by the optimized solvent and thermal debinding processes. Thereafter, the debound samples were sintered at 1750 and 1800 °C for 4 h in nitrogen atmosphere. Regardless of sintering temperature, the relative density of higher than about 96% was achieved. When comparing mechanical properties including bending strength, Vickers hardness and fracture toughness, Si₃N₄ with 2 wt% Y₂O₃ and 5 wt% Al₂O₃ (Si₃N₄+2Y5A) had higher values than β-SiAlON with 4 wt% Y₂O₃ (β-SiAlON+4Y) regardless of sintering temperature. It was supported by observing the microstructures of the plasma-etched samples.     

Experimental and computational analysis for thermo-erosive stability assessment of ZrB2-SiC based multiphase composites

High temperature erosion tests were conducted on spark plasma sintered ZrB₂-SiC based multiphase ceramic composites at 1073 K in thermo-erosive environment for 1200 s with a net energy deposition per unit area of 50.5 MJ/m². The thermo-erosive mechanisms were qualitatively discussed using XRD and SEM-EDS analyses. Efforts were made to assess feasibility of identified reactions at the computed temperatures to support reaction mechanism for oxide formation in eroded region. Finite element (FE) analysis with high-quality structural elements was used to determine the spatial temperature and stress distribution in the eroded region. Taken together, the present study highlights the significance of combined approach of computational and experimental analysis in understanding the thermo-erosive-structural stability in applications where erosion can limit the performance of ceramic composites.     

Robust proton exchange membrane for vanadium redox flow batteries reinforced by silica-encapsulated nanocellulose

High ion selectivity and mechanical strength are critical properties for proton exchange membranes in vanadium redox flow batteries. In this work, a novel sulfonated poly(ether sulfone) hybrid membrane reinforced by core-shell structured nanocellulose (CNC-SPES) is prepared to obtain a robust and high-performance proton exchange membrane for vanadium redox flow batteries. Membrane morphology, proton conductivity, vanadium permeability and tensile strength are investigated. Single cell tests at a range of 40–140 mA cm⁻² are carried out. The performance of the sulfonated poly(ether sulfone) membrane reinforced by pristine nanocellulose (NC-SPES) and Nafion® 212 membranes are also studied for comparison. The results show that, with the incorporation of silica-encapsulated nanocellulose, the membrane exhibits outstanding mechanical strength of 54.5 MPa and high energy efficiency above 82% at 100 mA cm⁻², which is stable during 200 charge-discharge cycles.     

共轭凸轮轮廓线的反求设计方法

针对纺织机械中凸轮修复设计时手工测量绘制效率低且测量精度不高、曲线的光滑程度和再修改上也存在很大缺陷等问题,提出了一种按逆向工程原理,利用三坐标测量仪及AutoCAD、Pro/E等分析绘图软件进行凸轮轮廓曲线的逆向分析、修复和绘制的方法.工厂实际运行结果表明:由该方法加工出的凸轮满足了生产工艺要求,振动及噪音明显降低,该方法可为类似凸轮设计提供借鉴.     

Process parameters design of a three-dimensional and five-directional braided composite joint based on finite element analysis

This paper presents an analytical method for designing the configuration of composite joint with three-dimensional (3D) five-directional braided composites. Based on the analysis of 3D braided structure characteristics, the elastic properties of the 3D five-directional braided composites were determined by the volume averaging method. The effects of the braiding angle and fiber volume fraction on the elastic constants of the braided composites were also discussed. Finite element analysis on the load capacity of the 3D five-directional braided composite joint was implemented using the software ANSYS Workbench 14.0. The influence of braiding angle on the stress, strain and deformation of the composite joint under tensile loading were calculated. The results show that when the fiber volume fraction of the 3D five-directional braided preform is given, the equivalent stress of the composite joint decreases monotonically as the braiding angle increases, while the normal stress, maximum principal stress and total deformation firstly decreases and then increases. Based on the finite element analysis, we found that at the fiber volume fraction of 60%, the braiding angle within the range of 30–35° are the optimum processing parameters for the 3D five-directional braided composite joint structure that used in the tensile load 320 N condition.     

Effects of high temperature treatment on microstructure and mechanical properties of laser-clad NiCrBSi/WC coatings on titanium alloy substrate

Laser-clad composite coatings on the Ti6Al4V substrate were heat-treated at 700, 800, and 900 °C for 1 h. The effects of post-heat treatment on the microstructure, microhardness, and fracture toughness of the coatings were investigated by scanning electron microscopy, X-ray diffractometry, energy dispersive spectroscopy, and optical microscopy. The wear resistance of the coatings was evaluated under dry reciprocating sliding friction at room temperature. The coatings mainly comprised some coarse gray blocky (W,Ti)C particles accompanied by the fine white WC particles, a large number of black TiC cellular/dendrites, and the matrix composed of NiTi and Ni₃Ti; some unknown rich Ni- and Ti-rich particles with sizes ranging from 10 nm to 50 nm were precipitated and uniformly distributed in the Ni₃Ti phase to form a thin granular layer after heat treatment at 700 °C. The granular layer spread from the edge toward the center of the Ni₃Ti phase with increasing temperature. A large number of fine equiaxed Cr₂₃C₆ particles with 0.2–0.5 μm sizes were observed around the edges of the NiTi supersaturated solid solution when the temperature was further increased to 900 °C. The microhardness and fracture toughness of the coatings were improved with increased temperature due to the dispersion-strengthening effect of the precipitates. Dominant wear mechanisms for all the coatings included abrasive and delamination wear. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900 °C presented the most excellent wear resistance.     

具有震颤抑制的微创机器人主从控制系统设计

详述了一种用于抑制微创手术机器人震颤现象的主从控制系统，提出了针对人手生理震颤的新型零相位滤波方法及针对从操作臂关节“粘滑行为”的前馈补偿PD伺服算法。新型零相位滤波避免了传统低通滤波器容易造成延时和传统零相位滤波无法在线使用的缺点，前馈补偿PD伺服算法通过摩擦补偿克服了非线性摩擦对从操作臂运动造成的影响。最后，对系统进行仿真及实验，结果表明该方法能有效地抑制机器人手术工具末端的震颤现象。     

Structural evolution in graphitization of nanofibers and mats from electrospun polyimide–mesophase pitch blends

The evolution of structure in multi-step thermal treatment of polyimide–mesophase pitch (PI–pitch) blend nanofiber mats obtained by an electrospinning process is described. The mats were thermally treated at a series of stages up to 3000 °C. The structural transformation of the nanofiber mats consisted of three regimes. First regime corresponds to the removal of the majority of non-carbon elements and the formation of initial residual carbon. Second regime involves slow growth of the graphitic layers and slow improvement of their stacking order. Progressive graphitization occurs in regime three when the fibers become highly graphitic. The addition of pitch was found to give rise to overall enhanced graphitic order in the PI–pitch blend nanofibers as reflected in the smaller inter-layer spacing d₀₀₂ approaching that of the perfect graphite crystal, and the larger crystal sizes, L_c and L_a, confirmed by XRD analysis, as well as the higher ratio of graphitic structure revealed by Raman spectroscopy. Development of highly localized oriented domains in these nanofibers were observed by dark field TEM. The addition of pitch led to enhancement of both electrical and thermal conductivity.     

Evaluation of gluing of CFRP onto concrete structures by infrared thermography coupled with thermal impedance

Carbon Fiber Reinforced Polymers (CFRPs) have been increasingly employed for structural strengthening, and are attached to structures using bonding adhesives. The aim of this work is to characterize defects in the bond between CFRP and concrete (after they are located by pulse infrared thermography), and assign the defects a “numerical value” (ranging from 0 for a complete air–gap to 1 for a fully glued bond). Quantitative characterization is performed by measuring the thermal impedance, and then identifying the thermophysical parameters of the system through fitting the measured impedance to a theoretical model. An inversion procedure is carried out to estimate the unknown parameters, without prior knowledge of sample properties. In particular, it is possible to estimate more accurately both the amount of glue within a defect and the thermal contact resistance.     

Magneto-optical imaging deviation model of micro-gap weld joint

Seam tracking is important for butt joint laser welding. A magneto-optical imaging approach is proposed to detect the micro-gap weld whose width is less than 0.2 mm. The symmetry of the magnetic field above the weld joint is an important precondition to ensure the detection accuracy of the magneto-optical imaging method. However, in actual complex industrial scene, it is difficult to guarantee complete symmetry of the magnetic field. This paper proposes an effective method for solving the problem of magneto-optical imaging deviation under an asymmetric magnetic field. Two possible factors causing the asymmetry of magnetic field above weld joint are firstly investigated using finite element analysis. By analyzing the characteristics of the magneto-optic medium used in the sensor and measuring the magnetic field distribution of weld joint at different lift-off height and different excitation voltage, the prediction model of deviation between the weld position detected by magneto-optical imaging and the actual weld position is built by back propagation (BP) neural network. The experimental result of weld seam tracking based on magneto-optical imaging shows that the change of the lift-off height will affect the detection accuracy of the weld position, and sufficient accuracy can be ensured after correcting the deviation according to the prediction model of magneto-optical imaging deviation.