Effect of focused ion beam sample preparation on the phase composition of zirconia

The investigation of phase transformations in metastable ceramic systems such as zirconia often requires local phase analysis within the areas of interest. Electron backscatter diffraction is a suitable method in combination with focused ion beam sample preparation. The interaction between ion beam and sample has to be carefully considered. In case of metastable Y-PSZ and Mg-PSZ, phase transformations were observed after FIB preparation with 30?kV, 30?nA and 5° incidence angle. Damage was the dominating effect for angles of 72°. The expected local temperature increase due to the ion bombardment with 30?kV and 30?nA is 700?K for ZrO₂. Thus, the observed phase transformations can be explained on the basis of the temperature increase in the corresponding Y-PSZ phase diagram. In case of Mg-PSZ, the transition temperature is 1083?°C. The local temperature increase was obviously lower. The excitation energy for the observed phase transformation was smaller than expected from the phase diagrams of the thermodynamic equilibrium. Using 5?kV, 4.8?nA and 5° incidence angle, no phase transformations and no damage were observed. Thus, these conditions are well suited for the FIB preparation of metastable zirconia.     

基于儒可夫斯基变换的涡轮钻具叶片设计及性能分析

为提高涡轮钻具叶片使用性能和创新叶片设计理论,提出了基于儒可夫斯基保角变换法与经典水力翼型相结合的涡轮钻具叶片设计新方法,以?127涡轮钻具叶片为研究对象,运用搭建的涡轮钻具叶型参数化设计平台和自主设计的涡轮钻具性能测试台架,完成了5种翼型的造型设计,研究了5种叶片流场性能,开展了设计叶片的性能测试实验,对比分析了设计叶片与塔里木油田某型在役?127涡轮叶片的实际效能。实验及仿真结果表明,基于新方法设计的NACA-0012翼型加厚叶片在600 r/min转速及15 L/s流量的设计工况下,其单级扭矩达5.83 N·m,较相同条件下某在役?127涡轮的单级扭矩提高约6.4%,级效率提高约1.16%,整体性能有了实质提升。     

如何实施社会转型期的民办高校品牌战略

在社会转型期，面对日益激烈的市场竞争环境，如何制定民办高效发战战略，引导民办高效的可持续发展，是关系到民办高校兴衰的关键。本文主要从民办高校品牌战略入手，对社会转型及高等教育转型进行论述，并对如何加强民办高校品牌建设进行探讨。     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.     

Formation and properties of high entropy oxides in Co-Cr-Fe-Mg-Mn-Ni-O system: Novel (Cr,Fe,Mg,Mn,Ni)3O4 and (Co,Cr,Fe,Mg,Mn)3O4 high entropy spinels

Possibility of formation of quinary and senary equimolar high entropy oxides from the Co-Cr-Fe-Mg-Mn-Ni-O system is presented. Different proposed compositions are synthesized using the solid-state reaction route at high temperatures (900−1100 °C) and quenched to room temperature. Phase composition of the samples is studied, showing tendency toward formation of two main phases: rock salt-structured Fm-3 m and spinel-structured Fd-3 m. It is documented that the annealing temperature has a profound effect on stability of both structures, and at 1100 °C usually the highest content of Fm-3 m phase is usually observed. Three different oxides, namely, (Co,Cr,Fe,Mn,Ni)₃O₄, (Co,Cr,Fe,Mg,Mn)₃O₄ and (Cr,Fe,Mg,Mn,Ni)₃O₄ are obtained as single-phase materials, which structure can be described as the high entropy Fd-3 m spinel one. The latter two compounds have not been previously reported in the literature. Activated character of the electrical conductivity dependence on temperature is observed, with relatively high total conductivity at high temperatures and corresponding high absolute values of Seebeck coefficient.     

单相硼化物的制备及摩擦磨损性能研究

为了深入研究渗硼层中硼化物的性能，采用真空感应熔炼法制备单相硼化物材料。观察分析制备的硼化物微观组织，测试其力学性能。采用MMU-5G型销-盘式摩擦磨损试验机，在干摩擦条件下，研究了不同载荷下单相硼化物的摩擦学性能，观察其磨损表面形貌特征，探讨其磨损方式。结果表明：制备的硼化物为单一相，试样纯度高，试样的平均显微硬度为HV2065，平均断裂韧性值为1.68 MPa·m1/2；硼化物的断口处没有宏观的塑性变形，断口齐平光亮，表现为脆性断裂特征；干摩擦条件下随着载荷从10 N增加到30 N，硼化物的摩擦因数先降低后增加，20 N载荷时达到最小值，而其磨损量随着载荷的增加不断上升；随着载荷从10 N增加到30 N，磨损表面的粗糙度先逐渐上升后急剧上升； 10~20 N载荷下，硼化物的磨损以磨粒磨损为主，而25~30 N载荷下，硼化物的主要磨损方式从磨粒磨损转变为脆性破损。     

One-step synthesis of high purity ZnO micro/nanostructures from pure Zn and pre-alloyed brass powders by vapor phase transport

In this research, vapor phase transport (VPT) was introduced as a facile, inexpensive method to produce ZnO micro/nanostructures from various Zn sources such as pure Zn and alpha brass pre-alloyed powders (Cu–20Zn and Cu–28Zn) at different processing temperatures of 930 °C–1050 °C. Simultaneous thermal analysis (STA) was carried out to investigate Zn evaporation and ZnO micro/nanostructure formation. STA results showed an exothermic peck at 711 °C and 728 °C for Cu–20Zn and Cu–28Zn, respectively, due to oxidation of the evaporated Zn element and formation of ZnO micro/nanostructures. X-ray diffraction results showed that high purity ZnO micro/nanostructures were successfully synthesized via VPT process and the crystallite size was increased from ~60 nm to ~100 nm with increasing processing temperature. Field emission scanning electron microscopy observations showed morphology (e.g. rods, column, tetrapods, and combs) and size of the synthesized micro/nanostructures were dependent on the Zn sources and processing temperature, in which average diameter of the synthesized ZnO structures was increased with increasing the processing temperature. The smallest (98 nm) and largest (603 nm) average diameters of synthesized ZnO micro/nanostructures were attained from the pure Zn and Cu–28Zn brass powders at 930 °C and 1050 °C, respectively.     

地勘单位转制后税务风险识别与控制

结合“减税降费”的政策背景,以陕西省某转制地勘单位为例,引入风险控制相关理论并应用层次分析法,构建了税务风险评估指标体系,对转制地勘单位面临的税务风险进行了识别、分析、评估。研究结果表明:针对不同的税收类型,应着重关注不同方面的风险,排序前6位的风险依次为企业所得税扣除类风险、增值税决策环节风险、增值税销售环节风险、房产税计税依据风险、企业所得税税收优惠类风险、增值税采购环节风险;并对上述风险的控制机制进行了分析,从企业管控、政策优化两方面分别提出了相关对策和建议。     

A first-principles investigation on mechanical and metallic properties of titanium carbides under pressure

The titanium carbides are potential candidates to achieve both high hardness and refractory property. We carried out a structural search for titanium carbides at three pressures of 0 GPa, 30 GPa and 50 GPa. A phase diagram of the Ti-C system at 0 K was obtained by elucidating formation enthalpies as a function of compositions, and their mechanical and metallic properties of titanium carbides were investigated systematically. We also discussed the relation of titanium concentration to the both mechanical and metallic properties of titanium carbides. It has been found that the average valence electron density and tractility improved at higher concentrations of titanium, while the degree of covalent bonding directionality decreased. To this effect, the hardness of titanium carbide decreases as the content of titanium increases. Our results indicated that the titanium content significantly affected the metallic properties of the Ti-C system.     

数字储频及其性能分析

文章介绍了3种宽带数字储频的基本结构，分析了宽带数字储频的一个重要指标——量化噪声，根据输出信号频谱的杂散电平比较了三种结构的优劣。