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931.
L. S. Dubrovinsky P. Lazor S. K. Saxena P. Häggkvist H.-P. Weber T. Le Bihan D. Hausermann 《Physics and Chemistry of Minerals》1999,26(7):539-545
Iron pressurized to 60 gigapascal (GPa) was heated with laser up to temperatures of over 2200 K. The structural changes were
determined in-situ using third generation synchrotron X-ray source; the changes were recorded on an imaging plate with a monochromatic beam.
The results strongly support the existence of a phase transformation of the hexagonal close-packed (hcp) structure to the
new polymorph (β-phase of iron) at high pressure and temperature. We interpret the X-ray data as belonging to the double hexagonal
close-packed (dhcp) structure distorted by stress due to laser heating.
Received: 2 February 1998 / Accepted: 23 August 1998 相似文献
932.
南极威德尔海区域海冰异常对初夏东亚大气环流和天气影响的数值研究 总被引:5,自引:2,他引:3
利用经过改变用于长期数值预报的CCM1(R15L7)模式以1975年1月16日00Z模式适应场为初始场积分5个月,研究南极威德尔海附近(60°W~30°E)海冰的面积异常对东亚初夏环流转换季节的影响.发现当南极海冰偏多时,在亚洲北部冷空气活动在初夏仍然很多,势力还很强,东亚南北两支急流分支仍很明显,各种环流特征更偏向于冬季型,不利于东亚初夏的环流季节转换.海冰异常偏少时则相反,亚洲北部的冷空气活动明显减弱,南方暖气流势力明显加强北移,东亚的两支急流也趋于合并北抬,环流形势更接近于夏季型,海冰的减少促进了东亚初夏的环流季节转换过程. 相似文献
933.
During the past 50 years forests have recolonized extensive areas of Puerto Rico. Between 1950 and 1990 forest cover increased from 9% to 37% of the island's land area. In proportional terms more land has reverted to forest in Puerto Rico than anywhere else on earth during the second half of the twentieth century. This paper explores the geography of this process by matching changes in land cover with the characteristics of the land and communities in Puerto Rico. The reversion of agricultural lands to forest occurred most frequently in humid, upland, coffee‐growing regions characterized by heavy out‐migration and populations of smallholders who earned some of their income from off‐farm sources. These findings suggest that changes in non‐farm labor markets, as well as changes in the political economy of agriculture, have important impacts on the prospects for converting agricultural lands into forests. 相似文献
934.
气候变化对历史上农牧过渡带影响的个例研究 总被引:31,自引:6,他引:25
从北魏平城迁都、元朝中叶岭北地区移民、12世纪初科尔沁沙地演变、明初兀良哈三卫南迁等四个历史实例出发,讨论了气候变冷变干时,农牧过渡带变化以及相应的社会变化现象。可以肯定,气候变化对历史上农牧过渡带变迁的影响是存在的。同时气候变化对农牧过渡带的影响是通过人类社会系统起作用的,不同的社会状态和组合会产生不同的农牧过渡带实况和相应的社会问题。当农耕民族与游牧民族在农牧过渡带附近对峙时,气候向寒冷方向的变化常常成为社会动荡的触发因素,极端情况下可以产生很严重的后果。 相似文献
935.
936.
Branislav S. Djurdjev 《GeoJournal》2000,50(2-3):133-138
The paper considers regional differences in population growth in Serbia and highlights the contrast that has emerged between
Kosovo and Metohia and the other regions of the country since World War Two. Due to continuing high fertility coupled with
declining mortality, growth in Kosovo and Metohia has been three times greater than in Serbia Proper and five times greater
than in Voivodina, regions which have been following closely the European demographic transition. Since the population in
Kosovo and Metohia is overwhelmingly Albanian these divergent demographic trends are sharpening ethnic tension as the demographic
weight of the Serbs decreases in the country as a whole. At the same time the dominance of the Albanians within the province
may well contribute to the independence struggle. Since the maintenance of high fertility, with a relatively slow decline
- even in comparison with Albania - could be linked with externalities, it is suggested that a solution might be found in
more autonomous development for the province which might bring an increase in local responsibility for sustainable development
and a decrease in the currently high level of demographic investment.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
937.
938.
Xie Zuo-heng Wang Yi-ren Zhang Suo-chun Wang Wei-zhong 《Chinese Astronomy and Astrophysics》2000,24(4):462-466
We have found that the introduction of strange quark phase transition in the simulation of the evolution of supernovae results in a stronger shock wave. This is probably due to an increased convective instability in the core. For our range of calculation we found that a first-order strange phase transition can induce explosion in a WW(88) model with a 1.28 M iron core (the upper bound for prompt explosion was about 1.1 M). Our result supports DAI Zi-gao et al.'s thesis that strange quark phase transition can raise the probability of supernova explosion. 相似文献
939.
We study via numerical experiments the localisation property of an acoustic wave in a viscoelastic soft medium containing randomly-distributed air bubbles. The behaviours of the oscillation phases of bubbles are particularly investigated in various cases for distinguishing efficiently the acoustic localisation from the effects of acoustic absorption caused by the viscosity of medium. The numerical results reveal the phenomenon of `phase transition' characterized by an unusual collective oscillation of bubbles, which is an effective criterion to unambiguously identify the acoustic localisation in the presence of viscosity. Within the localisation region, the phenomenon of phase transition persists, and a remarkable decrease in the fluctuation of the oscillation phases of bubbles is observed. The localisation phenomenon will be impaired by the enhancement of the viscosity factors, and the extent to which the acoustic wave is localised may be determined by appropriately analyzing the values of the oscillation phases or the amount of reduction of the phase fluctuation. The results are particularly significant for the practical experiments in an attempt to observe the acoustic localisation in such a medium, which is in general subjected to the interference of the great ambiguity resulting from the effect of acoustic absorption. 相似文献
940.
MIAO Chang-hong 《中国地理科学(英文版)》2000,10(2):131-137
lINTRoDUCnONSincethelatel98os,rural-urbantransitionhasbeenbecomingoneofthemostimportantaspectsinChina'sfasttransformationfromtraditionaltomodes(ZHENG,l996).Becausethistransitionhastakenplaceurderthespecialbackgnundofthesimultaneoustransformationsofthesocial-economicstructuresandtheeconomicsystem,thereexistmanyparticularitiesinthetransitionalprocessesandfactors.Amongthem,ruralindustrializaionwithraPidgrowthhasbeenplayingaleadingrole.Inthispaper,wewanttodiscusstheim-pactsofrUralindustrial… 相似文献