The formed surface characteristics of SiCf/SiC composite in the nanosecond pulsed laser ablation

For the advantages of high-temperature resistance, corrosion resistance and ultra-high hardness, SiC_f/SiC composite is becoming a preferred material for manufacturing aero-engine parts. However, the anisotropy and heterogeneity bring great challenges to the processing technology. In this study, a nanosecond pulsed laser is applied to process SiC_f/SiC composite, where the influence of the scanning speed and laser scanning direction to the SiC fibers on the morphology of ablated grooves is investigated. The surface characteristics after ablation and the involved chemical reaction of SiC_f/SiC are explored. The results show that the increased laser scanning speed, accompanied by the decreasing spot overlap rate, leads to the less accumulation of energy on the material surface, so the ablation effect drops. In addition, for the anisotropy of the SiC_f/SiC material, the obtained surface characteristics are closely dependent on the laser scanning direction to the SiC fibers, resulting in different groove morphology. The element composition and phase analysis of the machined surface indicate that the main deposited product is SiO₂ and the carbon substance. The results can provide preliminary technical support for controlling the machining quality of ceramic matrix composites.     

热处理对挤压态Mg-6Gd-5Y-1Zn合金组织与性能的影响

通过对Mg-6Gd-5Y-1Zn(质量分数,%)合金在固溶和时效处理状态下显微组织和力学性能的研究发现,α-Mg基体、沿挤压方向分布的条状18R-LPSO相、少量的Mg₂₄(GdYZn)₅ 相以及细层片状的14H-LPSO相构成了挤压态合金的组成相。挤压态合金经固溶(T4)处理后,一部分18R-LPSO相溶入基体,并且基体中的14H-LPSO相伸长同时粗化。挤压态合金经过固溶加时效(T6)处理后,大量β′相从α-Mg基体中析出。T6态合金的室温力学性能最好,其屈服强度、抗拉强度及伸长率分别为272 MPa、406 MPa和6.1%。β′相沉淀也发生在挤压态合金的直接人工时效(T5)处理过程,但相比于T6处理,14H-LPSO相和β′相在基体中的体积分数均偏低。     

基于改进交叉熵算法的电力系统随机生产模拟

蒙特卡洛模拟的计算效率与系统的可靠性密切相关，在其用于高可靠性系统的随机模拟时存在计算效率偏低的问题。为此，提出了一种基于多层交叉熵与对偶变数抽样技术相结合的随机模拟算法。首先使用多层交叉熵构造零方差概率密度函数的近似函数，提高小概率失效事件的抽取概率；其次基于已构造的近似概率函数，采用对偶变数抽样法进行抽样，进一步提高抽样的收敛速度。以IEEE RTS修改系统为例进行了算例分析，算例结果验证了所提出的基于改进交叉熵的电力系统随机生产模拟算法的有效性。     

Influence of Y2O3 addition on the mechanical and oxidation behaviour of carbon fibre reinforced ZrB2/SiC composites

The influence of Y₂O₃ addition on the microstructure, thermo-mechanical properties and oxidation resistance of carbon fibre reinforced ZrB₂/SiC composites was investigated. Y₂O₃ reacted with oxide impurities present on the surface of ZrB₂ and SiC grains and formed a liquid phase, effectively lowering the sintering temperature and allowing to reach full density at 1900 °C. The presence of a carbon source (fibres) led to additional reactions which resulted in the formation of new secondary phases such as yttrium boro-carbides. Mechanical properties were significantly enhanced compared to the un-doped composite. Further tests at high temperatures resulted in strength increase up to 700 MPa at 1500 °C which was attributed to stress relaxation. Oxidation tests carried out at 1500 °C and 1650 °C in air showed that the presence of the Y-based secondary phases enhanced the growth of ZrO₂ grains, but offered limited protection to oxygen due to the lower availability of surficial SiO₂ formed from SiC.     

Biogas dry reforming over Ni–Ce catalyst supported on nanofibered alumina

Four catalysts based on Ni and Ni–Ce supported on two γ-aluminas with different morphology (nanofibers and nanograins) have been prepared and studied in the dry reforming of simulated biogas. Catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), inductively coupled plasma with optical emission spectrometer (ICP-OES), chemisorption of H₂ and elemental analysis (EA) to determine their most relevant physicochemical properties. Characterization results show that metallic Ni particles supported on nanofibered alumina (NFA) presents a higher dispersion and smaller size than that supported on the nanograiny alumina (NGA) probably due to the higher mesoporosity presented by the NFA support. On the other hand, the incorporation of Ce has a similar effect than the fibrous morphology, decreasing also the size of the Ni metallic particles and increasing their dispersion. In the dry reforming of synthetic biogas (CH₄/CO₂ = 1.5) the nanofibered alumina containing 5 wt% Ni and 1.5 wt% Ce (NiCe/NFA) showed the highest catalytic activity at 750 °C (98% CO₂ conversion) and stability (7.7% nickel sinterization level and 2.9 wt% carbon deposition). The stability of this catalyst was also demonstrated at 750 °C during 55 h of reaction time with a loss of activity at the steady state under 2%. In addition, the catalyst was regenerated at 600 °C in oxygen flow, recovering completely its initial catalytic performance. The excellent catalytic behavior of NiCe/NFA material has been related to the fibrous morphology of the alumina support, which promotes a better dispersion of the supported Ni metal particles, decreasing their size and increasing the number of actives sites where dry reforming reaction can take place. In addition, the incorporation of Ce seems to have also an important role by increasing the Ni-support interactions, decreasing sinterization of the metallic Ni particles and coke deposition. The contribution of both effects (morphology and Ce), separately and in combination, have been proved to enhance significantly the catalytic activity and stability of the synthesized catalysts in the dry reforming of simulated biogas.     

热冲压工艺参数对22MnB5马氏体钢汽车B柱性能影响的有限元模拟

通过有限元仿真软件Autoform分析了热冲压过程中工艺参数的变化对22MnB5马氏体钢B柱起皱、回弹、减薄、马氏体量以及强度的影响。结果表明:22MnB5马氏体钢B柱热冲压最优化工艺参数为加热温度930 ℃,冷却速率80 ℃/s。该工艺参数下,热冲压过程各处均完成马氏体转变,硬度分布均匀,材料减薄率较低,热冲压成形效果好,尺寸精度高,冲压件强度均大于1400 MPa。     

基于功频下垂控制的光伏并网发电系统惯量阻尼机理研究

针对光伏以低惯量、弱阻尼的特征大规模接入电网，给电网稳定性带来不利影响的问题，基于功频下垂控制的光伏并网发电系统，借鉴经典电气转矩分析法，从物理机制层面上分析影响系统惯量、阻尼以及同步能力的作用规律。研究结果表明对于控制参数的影响规律而言，惯量特性主要受功率环的比例系数Kp影响，且随Kp的增大而增强；阻尼特性受频率下垂系数Dp影响比较明显，且随Dp的增大而减弱；同步特性只受功率环的比例积分系数Ki影响，且随Ki的增大而增强。通过仿真验证了理论分析的正确性。     

基于模糊推理的碟式太阳能跟踪系统改进研究

在碟式太阳能系统的长期实验数据中发现,太阳辐照度较低时系统集热效率非常低。为使碟式太阳能系统始终运行在高集热效率状态,提出模糊推理方法改进跟踪系统的启停判定条件,并根据集热器出口气流温度的影响因素建立模糊推理系统。仿真结果表明模糊系统在已知太阳辐照度、辐照变化度和空气流量的前提下,可智能控制系统在集热效率高时运行,集热效率低时待机,证明了该方法的可行性和有效性。     

Structural performance of ballastless track slabs reinforced with BFRP and SFCB

Because of the inductive impedance caused by steel meshes in traditional reinforced ballastless track slabs, the electrical properties, primarily the rail resistance and inductance, of jointless track circuits are affected by electromagnetic induction between the slabs and the electric current in the rail. This problem results in poor transmission performance throughout the track circuit. Insulating sleeves or cards between the steel meshes have been used to improve the insulation capability of steel meshes in slabs; however, they reduce the bonding performance between the steel bars and concrete. Because of the good insulation properties of fiber-reinforced polymer composite bars (FRPs) and steel-fiber reinforced polymer composite bars (SFCBs), these composite materials have shown potential to overcome this insulation problem. However, the structural performance of the ballastless track slabs reinforced by basalt fiber reinforced polymer composite bars (BFRPs) and SFCBs, which play a key role in the structure and transportation safety, needs to be investigated. In this paper, six ballastless track slabs reinforced with BFRPs, SFCBs, and steel bars were constructed and tested. The following results were obtained. (1) Shear failures were observed for all slabs, both the BFRP and SFCB slabs meet the load level requirements, and SFCBs reinforcements have higher strength utilization compared with BFRPs reinforcements. (2) The bond-quality of SFCBs and BFRPs reinforcements proved slightly poorer than that of the steel bars. Because of the good corrosion resistance of the FRP, the maximum crack width limits can be slightly larger than that of the RC slabs. (3) Bischoff’s equation was initially used to calculate the deflection of partially prestressed concrete slabs under service loads. The results demonstrated a good agreement between the theoretical and experimental analysis. (4) Considering the tensile stiffness, the modified ACI equation was used to calculate the slabs’ crack width and the theoretical and experimental results showed a good agreement.