Plastic deformation of single crystals of TiSi2 with the C54 structure

The deformation behavior of single crystals of TiSi₂ with the orthorhombic C54 structure has been investigated as a function of crystal orientation in the temperature range from room temperature to 1400 °C in compression. Plastic flow is possible only when slip along <110> on (001) is operative and no other slip systems are observed in the whole temperature range investigated. While plastic flow is observed above 300 °C for as-grown crystals, the onset temperature for plastic flow is lowered considerably down to room temperature when the crystal is prestrained at 1300 °C and then re-deformed at low temperatures. The critical resolved shear stress (CRSS) for (001)<110> slip decreases with increasing temperature, exhibiting a moderate peak (or plateau) in the temperature range from 800 to 1100 °C. The deformation mechanism of TiSi₂ is discussed in comparison with those reported for other transition-metal disilicides with the C11_b and C40 structures, which are closely related to the C54 structure of TiSi₂.     

Sm2Ce2O7-Gd2Zr2O7复合材料制备及热导率

目的探讨了Gd_2Zr_2O_7的颗粒度和含量对(Sm_2Ce_2O_7)1-x(Gd_2Zr_2O_7)x复合材料热导率的影响。方法用纳米级和微米级粉体制备了两个系列的(Sm_2Ce_2O_7)1-x(Gd_2Zr_2O_7)x复合材料。用X射线衍射技术分析了材料的相组成,用扫描电镜观察了复合材料的显微形貌,用纽曼科普定律计算了复合材料的比热,用激光脉冲法测试了材料的热扩散系数。根据比热、密度和热扩散系数计算了复合材料的热导率,并根据最终热导率结果,分析了Gd_2Zr_2O_7颗粒度和含量对复合材料热导率的影响。结果所合成的粉体均具有单一的萤石晶体结构,纳米级Gd_2Zr_2O_7粉体最大比表面积为15.413 m~2/g,微米级Sm_2Ce_2O_7粉体最小比表面积为0.226 m~2/g。所制备的两个系列的(Sm_2Ce_2O_7)1-x(Gd_2Zr_2O_7)x复合材料也表现出单一的萤石晶体结构,但晶粒大小不均匀。结论当x=0.5时,纳米粉体制备的复合材料存在明显的纳米晶。微米级Gd_2Zr_2O_7对复合材料声子热导率抑制不明显,但可以抑制高温光子热导率;纳米级Gd_2Zr_2O_7的引入可明显降低复合材料的声子热导率,但对高温光子热导率抑制不明显。两个系列复合材料的热导率均低于YSZ。     

Heat treatment effect on coating shock resistance of thermal barrier coating system with different types of bond coat

The effect of heat treatment, growth of the TGO layer, oxidation of bond coat, and the impact of the presence of two bond coats on the TBC's thermal shock resistance has been investigated experimentally. TGO oxide layers were created with two-time heat treatment of 12 and 24 h at 1000. Then the thermal shock test was performed on the APS/APS and HVOF/APS/APS samples. The results show that the use of two BCs and the presence of a thin TGO layer has a good effect on TBC performance. The presence of two BC layers increased the shock resistance by an average of 37.2%. 12 h heat treatment caused a 14.0% and 17.4% shock resistance increase in samples with the HVOF/APS/APS layer and APS/APS layer, respectively. 24 h heat treatment decreased the samples' performance by 6.7% and 10.2% for samples with two BC and one BC, respectively.     

Role of Mg(OH)2 in pore evolution and properties of lightweight brine magnesia aggregates

Lightweight magnesia aggregates were fabricated using high-purity MgO agglomerates with the addition of Mg(OH)₂ as a pore former. The pore evolution and its relationship to the resulting properties were investigated. Mg(OH)₂ decomposition increased the number of inter-agglomerate pores, which subsequently affected the porosity and pore structure. When Mg(OH)₂ was 0–20 wt%, the inter-agglomerate pores were converted to both open and closed small pores, which effectively reduced the thermal conductivity and improved the thermal shock resistance (TSR) by accommodating thermal stress and inducing crack deflection. Small pores also favored the formation of a dense (Mg, Fe)O corrosion layer, preventing further slag penetration. However, large open pores occurred with further increasing Mg(OH)₂ content, which dramatically deteriorated the TSR and slag resistance. The specimen with 20 wt% Mg(OH)₂ exhibited the best overall performance, with a thermal conductivity of 16.6 W/(m·K) at 500 °C, and a residual flexural strength ratio of 32.3%; its slag resistance was comparable with that of dense magnesia.     

PyC interphase growth mechanism and control models of C/SiC composites

To better understand the pyrocarbon (PyC) interphase growth mechanism, a series of experiments was conducted on the PyC deposited on T-300™ and T-700™ carbon fibers by the chemical vapor infiltration (CVI) method. Nine groups of fabrication parameters were used to analyze the effects of deposition temperature, pressure, and residence time on the PyC interphase growth mechanism. Atomic force microscopy (AFM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), Raman spectroscopy, and nanoindentation tests were performed to characterize the microstructures of carbon fibers and PyC interphase. The PyC interphase growth mechanism was discussed, and the relationships between the fabrication parameters, R (C₂/C₆) value, texture type, and interphase thickness were established through numerical simulations. The hardness and modulus of PyC for T-300™ and T-700™ carbon fibers were measured. The tensile behaviors of C/SiC minicomposites with medium and high textures PyC interphases were analyzed. The C/SiC composite with the medium texture PyC interphase possessed the higher fracture strength and failure strain with a longer fiber pullout length at the fracture surface.     

A novel method for fabricating of high-performance multi-seeding GdBCO bulk superconductors

A novel multi-seeding (NMS) method is proposed for improving the performances of GdBCO samples by optimizing the shapes and the space configurations of the single-domain GdBCO grains. Three multi-seeding GdBCO bulk superconductors with dimensions of 60 × 20 × 14 mm³ were fabricated by a Gd + 011 infiltration growth method with different seeding techniques: samples S1 and S2 by the traditional multi-seeding method (TMS) with two and three seeds, respectively; and sample S3 by the NMS method with three seeds. The results show that among the three, sample S3 has both of the largest levitation force, 111 N, and the largest magnetic field coupling ratio, 68.57%, which are clearly higher than the levitation force of 100 N and 90 N, and the magnetic field coupling ratios of 34.25% and 11.33% for the samples S1 and S2, respectively. These results indicate that the NSM method is a useful way to improve the mechanical strength, homogeneity, and performances of multi-seeding GdBCO samples.     

YB2C2: The first damage tolerant ceramic with melting point over 2500 oC

The ultra-high temperature thermal stability of YB₂C₂ between 2300 ^oC and 2550 ^oC in argon was systematically investigated by induction heating method. The phase composition and microstructure of the samples after heat treatment at different temperatures were characterized with X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. The results show that YB₂C₂ has excellent ultra-high temperature thermal stability, and it can maintain the structure and phase stability after heat treatment at 2500 ^oC for 20 min. It would melt rather than decompose with the rising of the temperature. The melting point of YB₂C₂ is between 2500 ^oC and 2550 ^oC, and it has great potential for future ultra-high temperature applications.     

Toughening and ablation mechanism of Si3N4 short fiber toughened ZrB2-based ceramics

ZrB₂-based ceramics with Si₃N₄ short fiber (ZSN) were prepared by wet-spinning extrusion and hot pressing. The toughness of ZSN was 5.6 MPa·m^1/2, which was 20% higher than that of monolithic ceramic (4.7 MPa·m^1/2). The ablation performance of ZSN was evaluated by air discharge plasma ablation platform with a heat flux of 8.04 MW/m² for 120 s. The mass and linear ablation rates of ZSN were − 0.19 mg/s and − 0.25 µm/s, respectively. The specimens of ZSN remained intact while monolithic ceramics exhibited destructive fracture. The better ablation performance of ZSN is attributed to the addition of Si₃N₄ short fiber which increased the fracture toughness, reduced the elastic modulus, and improved the thermal conductivity at high temperature.     

开阀喷射模式对进气道喷射式汽油机性能的影响

采用台架试验和数值计算的方法研究了开阀喷射模式对发动机性能的影响规律。研究结果表明:在20%节气门开度下,较早的喷油时刻与较晚的喷油时刻相比,功率升高了0.2kW,HC排放降低了30×10~(-6);当节气门开度达到100%时,由于进气气流和机体温度的影响使得喷油时刻对动力性和排放性能的影响可以忽略;处于20%节气门开度时未挥发燃油最大比例大约占总喷油量的50%,而在100%节气门开度下未挥发燃油仅占15%左右,可见燃油未完全挥发是造成开阀喷射模式时发动机性能下降的主要原因,提高开阀喷射模式下发动机燃油挥发性能是提升发动机性能的主要途径。     

Novel low pressure drop and easy regeneration ceramic whisker membrane for submicron particulate matter filtration

Aiming to efficiently remove submicron particulate matter (PM), a low pressure drop ceramic whisker membrane was prepared via precursor solution dip coating followed by high temperature firing. Aluminum nitrate 9-Hydrate and boric acid were used as the precursors for the formation of aluminum borate. A highly porous film comprising interconnected single phase 9Al₂O₃·2B₂O₃ whiskers of high aspect ratio, was successfully formed on an alumina support. Flue gas filtration tests indicated that as-fabricated ceramic whisker membrane exhibited outstanding filtration performance. More than 95 % PM in 0.3–1 µm was removed by using the as-prepared ceramic whisker membrane, and the pressure drop after three rounds of back blow regeneration remained stably at approximate only 1.3 times of the air pressure drop. This work provided a feasible approach to the preparation of a novel ceramic membrane with low pressure drop, and a novel strategy for high efficiency removal of submicron PM.