Ultra-high temperature ceramic(UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal,...Ultra-high temperature ceramic(UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal, mechanical and chemical loading. Therefore, it is critical to evaluate the mechanical properties of UHTCs subject to an environment with elevated temperature, mechanical stress and oxygen. In this paper, an experimental investigation of the uniaxial tensile properties of a ZrB_2-SiC-graphite subject to an environment with a simultaneously elevated temperature, mechanical stress and oxygen is conducted based on a high-temperature mechanical testing system. To improve efficiency, an orthogonal experimental design is used. It is suggested that the temperature has the most important effect on the properties, and the oxidation time and stress have an almost equal effect. Finally, the fracture morphology is characterized using scanning electron microscopy(SEM), and the mechanism is investigated. It was concluded that the main fracture mode involved graphite flakes pulling out of the matrix and crystalline fracture, which indicates the presence of a weak interface in the composites.展开更多
从土壤物理、化学、生物等方面系统地研究了油菜、豆科禾本科牧草混播人工草地土壤环境效应。干旱年型(生长季降水167.5 m m )混播草地至开花盛期土壤贮水量高于天然草地,之后呈降低趋势;丰水年型(生长季降水355.9 m m )土壤贮水量均高...从土壤物理、化学、生物等方面系统地研究了油菜、豆科禾本科牧草混播人工草地土壤环境效应。干旱年型(生长季降水167.5 m m )混播草地至开花盛期土壤贮水量高于天然草地,之后呈降低趋势;丰水年型(生长季降水355.9 m m )土壤贮水量均高于天然草地,1 m 土体贮水量增加5.7～67.6 m m 。土壤盐分表聚性特征明显,混播草地改善土壤盐分状况显著,耕层和1 m 土体含盐量分别较天然草地降低0.31～6.00g/kg和0.12～1.21 g/kg。混播草地肥力状况改善,速磷和有机质含量增加,而速氮、全氮、全磷含量则呈降低趋势,磷酸酶和H2O2 酶活性增强,脲酶活性下降;> 0.25m m 水稳性团聚体含量提高6.03% 。,结构系数、团聚度提高,改善了土壤结构状况,增大了土壤持水能力。混播草地地上部干物质和粗蛋白产量明显增大,较天然草地分别提高17.8% ～319.1% 和27.9% ～316.1% ,其生物产量和水分利用效率分别较天然草地提高477.1% 和15.2 kg/(m m ·hm 2),建植混播人工草地,显著提高生物产量和水分利用效率。展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.11472092,11672088,11502058)the National Basic Research Program of China(Grant No.2015CB655200)
文摘Ultra-high temperature ceramic(UHTC) composites are widely used in high-temperature environments in aerospace applications. They experience extremely complex environmental conditions during service, including thermal, mechanical and chemical loading. Therefore, it is critical to evaluate the mechanical properties of UHTCs subject to an environment with elevated temperature, mechanical stress and oxygen. In this paper, an experimental investigation of the uniaxial tensile properties of a ZrB_2-SiC-graphite subject to an environment with a simultaneously elevated temperature, mechanical stress and oxygen is conducted based on a high-temperature mechanical testing system. To improve efficiency, an orthogonal experimental design is used. It is suggested that the temperature has the most important effect on the properties, and the oxidation time and stress have an almost equal effect. Finally, the fracture morphology is characterized using scanning electron microscopy(SEM), and the mechanism is investigated. It was concluded that the main fracture mode involved graphite flakes pulling out of the matrix and crystalline fracture, which indicates the presence of a weak interface in the composites.
文摘从土壤物理、化学、生物等方面系统地研究了油菜、豆科禾本科牧草混播人工草地土壤环境效应。干旱年型(生长季降水167.5 m m )混播草地至开花盛期土壤贮水量高于天然草地,之后呈降低趋势;丰水年型(生长季降水355.9 m m )土壤贮水量均高于天然草地,1 m 土体贮水量增加5.7～67.6 m m 。土壤盐分表聚性特征明显,混播草地改善土壤盐分状况显著,耕层和1 m 土体含盐量分别较天然草地降低0.31～6.00g/kg和0.12～1.21 g/kg。混播草地肥力状况改善,速磷和有机质含量增加,而速氮、全氮、全磷含量则呈降低趋势,磷酸酶和H2O2 酶活性增强,脲酶活性下降;> 0.25m m 水稳性团聚体含量提高6.03% 。,结构系数、团聚度提高,改善了土壤结构状况,增大了土壤持水能力。混播草地地上部干物质和粗蛋白产量明显增大,较天然草地分别提高17.8% ～319.1% 和27.9% ～316.1% ,其生物产量和水分利用效率分别较天然草地提高477.1% 和15.2 kg/(m m ·hm 2),建植混播人工草地,显著提高生物产量和水分利用效率。
