西太平洋副热带高压演变物理机制的研究

针对前人利用低谱模式研究大气运动多平衡态存在的不足,应用四因子最优子集反演方法和相应的资料分别反演获得了描述西太平洋副热带高压位置偏北年和偏南年500hPa高度场和相应OLR场低谱截断基函数具体形式.应用多平衡态理论研究了西太平洋副热带高压演变的物理机制.结果表明,造成西太平洋副热带高压位置异常的原因是前期外部的热力强迫的差异,导致了大气产生不同的响应波型.较强的经向和纬向外部热力差异致使大气所产生的响应波型间存在波流和波波相互作用,从而使得西太平洋副热带高压发生北跳后而位置偏北;相反,较弱的经向和纬向外部热力差异致使大气所产生的响应波型间不存在波流相互作用,使得西太平洋副热带高压不发生北跳,只随外源强迫的振荡而振荡,造成西太平洋副热带高压位置偏南.

Mechanical responses, texture and microstructural evolution of high purity aluminum deformed by equal channel angular pressing

Ultrafine-grained(UFG) high purity aluminum exhibits a variety of attractive mechanical properties and special deformation behavior. Equal channel angular pressing(ECAP) process can be used to easily and effectively refine metals. The microstructure and microtexture evolutions and grain boundary characteristics of the high purity aluminum(99.998%) processed by ECAP at room temperature are investigated by means of TEM and EBSD. The results indicate that the shear deformation resistance increases with repeated EACP passes, and equiaxed grains with an average size of 0.9 μm in diameter are formed after five passes. Although the orientations distribution of grains tends to evolve toward random orientations, and microtextures(80°, 35°, 0°),(40°, 75°, 45°) and(0°, 85°, 45°) peak in the sample after five passes. The grain boundaries in UFG aluminum are high-angle geometrically necessary boundaries. It is suggested that the continuous dynamic recrystallization is responsible for the formation of ultrafine grains in high purity aluminum. Microstructure evolution in the high purity aluminum during ECAP is proposed.

Deformation behavior and microstructural evolution of Al-Zn-Cu-Mg-Sc-Zr alloy during high temperature compression

The deformation behavior of a new Al-Zn-Cu-Mg-Sc-Zr alloy was investigated with compression tests in temperature range of 380?470 ℃ and strain rate range of 0.001-10 s-1 using Gleeble 1500 system, and the associated microstructural evolutions were studied by metallographic microscopy and transmission electron microscopy. The results show that true stress—strain curves exhibit a peak stress, followed by a dynamic flow softening at low strains (ε<0.05). The stress decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener-Hollomon exponential equation with the activation energy for deformation of 157.9 kJ/mol. The substructure in the deformed specimens consists of few fine precipitates with equaixed polygonized subgrains in the elongated grains and developed serrations at the grain boundaries. The dynamic flow softening is attributed mainly to dynamic recovery and dynamic recrystallization.

Hydraulic operating mechanisms for high voltage circuit breakers: Progress evolution and future trends

High voltage circuit breakers are the most important protection and control apparatus in power system. As a core part of circuit breakers, the operating mechanisms have a trend to be hydraulic-style in high voltage power grid. Compared with other hydranlic systems, the hydraulic operating mechanisms have the characteristics of high hydraulic pressure, high speed, high power and long-term waiting etc., and it is because of the characteristics that the hydraulic operating mechanisms become dif- ficult to be developed and have been arousing significant study interest from more and more researchers as well as their promising applications. Therefore, it is significant to summarize the hydraulic operating mechanisms on their development, characteristics, and key technologies etc. In this review, the evolution process and recent studies of hydraulic operating mechanisms at home and abroad are viewed. The review then focuses on the characteristics and key technologies of hydraulic operating mechanisms, especially on time and velocity characteristics, high-speed cylinder cushioning, fast response and great flow rate control valve, temperature compensation, system monitoring and fault diagnosis, intelligent operation, energy storage module, etc. In the end, the future trends of this field are presented.

Microstructural evolution and mechanism of grain growth in magnesia ceramics prepared by high pressure and temperature with ultra-high heating rate

The fast densification method of combustion reaction plus quick pressing was adopted to prepare nanocrystalline ceramics.The densification process of magnesia compact with a particle size of 100 nm was investigated,under the applied pressure of up to 170 MPa,and the temperature range of 1740–2080 K with ultra-high heating rate(above 1700 K/min).High-purity magnesia ceramics with a relative density of 98.8%and an average grain size of 120 nm was obtained at 1740 K,and the grain growth during the densification process was effectively restrained.The characteristic morphology of evaporation-condensation was observed in the compact prepared at 2080 K,which revealed the actual process of mass transfer by gas diffusion.Moreover,the investigation on the microstructure evolution and mechanism of grain growth was carried out,on the basis of as-preserved nanocrystalline ceramics.The result indicated that the grain growth of the nanocrystalline MgO was controlled by the mechanism of evaporation-condensation rather than surface diffusion.Furthermore,the pressure had an influence of restraining the grain growth based on solid diffusion and strengthening the effect of gas diffusion with the increasing temperature.Under the particular conditions,there existed an appropriate temperature for the densification of nanocrystalline magnesia,while the excessive temperature would exaggerate grain growth and impede densification.