Environmental-barrier coating ceramics for resistance against attack by molten calcia-magnesia-aluminosilicate (CMAS) glass: Part I,YAlO3 and γ-Y2Si2O7

The high-temperature (1500?°C) interactions of two promising dense, polycrystalline EBC ceramics, YAlO₃ (YAP) and γ-Y₂Si₂O₇, with a calcia-magnesia-aluminosilicate (CMAS) glass have been explored as part of a model study. Despite the fact that the optical basicities of both the EBC ceramics and the CMAS are similar, they both react with the CMAS. In the case of the Si-free YAlO₃, the reaction zone is small and it comprises three regions of reaction-crystallization products, including Y-Ca-Si apatite solid-solution (ss) and Y₃Al₅O₁₂ (YAG)(ss). In contrast, only Y-Ca-Si apatite(ss) forms in the case of Si-containing γ-Y₂Si₂O₇, and the reaction zone is an order-of-magnitude thicker. These CMAS interactions are analyzed in detail, and are found to be strikingly different than those observed in Y-free EBC ceramics (β-Yb₂Si₂O₇ and β-Sc₂Si₂O₇) in the accompanying Part II paper. This is attributed to the presence of the Y in the YAlO₃ and γ-Y₂Si₂O₇ EBC ceramics.     

Highly porous nano-SiC with very low thermal conductivity and excellent high temperature behavior

Highly porous nano-SiC is fabricated by partial sintering and decarburizing process using SiC nano-powders as starting materials and graphite flakes as pore forming agents. The prepared porous nano-SiC ceramics possess multiple pore structures, including well-distributed meso-pores in the skeleton and interconnected flakelike micro-pores. The samples prepared at 1800 °C have relatively low thermal conductivities of 5.61 ～ 0.25 W m^?1 K^?1 with porosities of 55.5–76.1%. While the samples sintered at 1500 °C with porosities between 54.0% to 76.3% show very low thermal conductivities of 0.74 ～ 0.14 W m^?1 K^?1, which is attributed to the integrated nano-scale phonon-scattering mechanisms and duplex pore structures. Porous nano-SiC ceramics also show good retention of elastic stiffness up to 1350 °C and low thermal conductivity at 1400 °C. Our results shed light on porous nano-SiC as a promising thermal insulator used in extreme thermal and chemical environments.     

Effects of deformation rate on properties of Nd,Y-codoped CaF2 transparent ceramics

Different deformation rates of Nd,Y-codoped CaF₂ transparent ceramics were prepared by ceramization of single crystals. The deformation rate effects on the crystallization behaviors, microstructures, mechanical properties, and optical performances were investigated for the first time. The results indicate that the comprehensive performances of Nd,Y-codoped CaF₂ ceramic (△a?=?62%) are the most optimal compared with other ceramics having different deformation rates (△a?=?34%, 40%, 50%, and 75%). In further investigations of the optical properties, the Nd,Y-codoped CaF₂ ceramic (△a?=?62%) sample exhibited a high transparency (T_a?>?91%, 3-mm thick，250?～?1200?nm), low light scattering, superior fracture toughness (K_1c?～?0.71?MPa·m^1/2), strong fluorescence emission, long lifetime (τ?=?348.72?μs), and broad FWHM (29.2?nm), promising a good candidate for high-power laser material.     

钢轨交流闪光焊接过程过梁爆破特征分析

以高速摄影技术作为主要研究手段，研究钢轨交流闪光焊接过程过梁爆破特征，并结合热成像系统采集焊接过程温度场，以此为数据基础定量分析了钢轨交流闪光焊接过梁爆破特征. 结果表明,钢轨交流脉动闪光焊接工艺模式下，加速闪光阶段具有几个阶段中最高的闪光频次46.93次/s，脉动闪光阶段具有最低的闪光频次11.07次/s；连续闪光阶段闪光烈度为0.009 2 mm/次，是几个阶段中最强的，脉动阶段的闪光烈度最弱，为0.003 8 mm/次；从闪光加热因子来看，脉动阶段最高，符合了该阶段积累热量的主要目的；加速闪光阶段最低，是由于该阶段为了制造保护气氛产生的气化过梁带走了较多的热量；从闪光均匀度看，闪光最不均匀的阶段是闪平阶段，最均匀的阶段是脉动阶段，这恰恰说明了两待焊端面的微观平行情况在闪平阶段最差、脉动阶段最佳.     

An approach to measure infill matric suction of irregular infilled rock joints under constant normal stiffness shearing

Rock joints infilled with sediments can strongly influence the strength of rock mass. As infilled joints often exist under unsaturated condition, this study investigated the influence of matric suction of infill on the overall joint shear strength. A novel technique that allows direct measurement of matric suction of infill using high capacity tensiometers (HCTs) during direct shear of infilled joints under constant normal stiffness (CNS) is described. The CNS apparatus was modified to accommodate the HCT and the procedure is explained in detail. Joint specimens were simulated by gypsum plaster using three-dimensional (3D) printed surface moulds, and filled with kaolin and sand mixture prepared at different water contents. Shear behaviours of both planar infilled joints and rough joints having joint roughness coefficients (JRCs) of 8–10 and 18–20 with the ratios of infill thickness to asperity height (t/a) equal to 0.5 were investigated. Matric suction shows predominantly unimodal behaviour during shearing of both planar and rough joints, which is closely associated with the variation of unloading rate and volumetric changes of the infill material. As expected, two-peak behaviour was observed for the rough joints and both peaks increased with the increase of infill matric suction. The results suggest that the contribution of matric suction of infill on the joint peak normalised shear stress is relatively independent of the joint roughness.     

港里矿软破围岩采矿的分析与研究

港里矿围岩软弱破碎，随开采深度延伸，地压显现越来越剧烈，严重影响港里矿正常回采。为研究港里矿软破围岩采矿，分析了影响采矿正常进行的因素和机理，调整采矿的顺序，使港里矿软破围岩采矿的困难得到根本扭转，保证了采矿生产的正常进行，选择围岩稳固及充分卸压部位布置斜坡道和联巷、优先回采围岩破碎的高应力区是地压管理的关键问题。     

Tribological behavior of graphene reinforced chemically bonded ceramic coatings

To investigate tribological behavior of graphene reinforced chemically bonded ceramic coatings at different temperatures, tribological tests at room temperature, 200 °C and 500 °C were carried out. Results show that the fracture toughness and the hardness of the coating are improved with the introduction of graphene. Besides, the friction coefficient of the coating decreases with the addition of graphene at the room temperature and 200 °C. The coating without graphene achieves the similar friction coefficient at all temperatures. However, the coating with graphene achieves the lowest friction coefficient at 200 °C, and achieves the highest at 500 °C. In addition, the wear rate of the coating decreases with the increase of graphene. Besides, the wear rate at 200 °C is almost similar with that at room temperature. In contrast, the wear rate at 500 °C is much larger than those at room temperature and 200 °C. The mechanisms for graphene to decrease the friction coefficient and improve the wear resistance of chemically bonded ceramic coatings at evaluated temperatures are clarified.     

Study on the hydrogen storage properties of a TiZrNbTa high entropy alloy

High-entropy alloys (HEAs), as a new class of metallic materials, have received more and more attention due to its excellent mechanical properties. In this study, the hydrogen absorption properties, such as hydrogen absorption capacity, thermodynamics, kinetics and cyclic properties, as well as the hydride structure of a newly designed TiZrNbTa HEA were investigated. The results showed that multiple hydrides including ε-ZrH₂, ε-TiH₂ and β-(Nb,Ta)H were found in the TiZrNbTa HEA after hydrogenation. With the increase of temperature from 293 K to 493 K, the maximum hydrogen absorption capacity decreased from 1.67 wt% to 1.25 wt% and the plateau pressure related with β-(Nb,Ta)H hydrides increased from 1.6 kPa to 14.8 kPa. The formation enthalpy of β-(Nb,Ta)H hydride was determined to be −6.4 kJ/mol, which was less stable than that of NbH and TaH hydrides. The results also showed that the TiZrNbTa HEA exhibited a rapid hydrogen absorption kinetic even at the room temperature with a short incubation time, and the hydrogen absorption mechanism was determined to be the nucleation and growth mechanism. Moreover, the hydrogen absorption capacity at 293 K decreased slowly with the cycle numbers, and remained 86% capacity after 10 cycles. Cracking occurred after hydrogen absorption and became worse with cycles.     

高精度大跨距同轴孔的精密研磨加工

对高精度大跨距同轴孔的加工现状进行了分析,确认采用现有加工方法无法满足要求。在此基础上,提出针对高精度大跨距同轴孔采用精密研磨加工,并改进了研磨工具。介绍了高精度大跨距同轴孔精密研磨加工的注意事项。这一方法可应用于加工孔距1 m以内的高精度大跨距同轴孔,加工效果良好。