全文获取类型
收费全文 | 72032篇 |
免费 | 11802篇 |
国内免费 | 18378篇 |
专业分类
测绘学 | 7248篇 |
大气科学 | 8894篇 |
地球物理 | 13973篇 |
地质学 | 42071篇 |
海洋学 | 9965篇 |
天文学 | 3580篇 |
综合类 | 5282篇 |
自然地理 | 11199篇 |
出版年
2024年 | 433篇 |
2023年 | 1117篇 |
2022年 | 2228篇 |
2021年 | 2555篇 |
2020年 | 2811篇 |
2019年 | 3187篇 |
2018年 | 2566篇 |
2017年 | 2959篇 |
2016年 | 3201篇 |
2015年 | 3502篇 |
2014年 | 4013篇 |
2013年 | 4014篇 |
2012年 | 4451篇 |
2011年 | 4528篇 |
2010年 | 3911篇 |
2009年 | 4547篇 |
2008年 | 4526篇 |
2007年 | 5108篇 |
2006年 | 4983篇 |
2005年 | 4400篇 |
2004年 | 4018篇 |
2003年 | 3751篇 |
2002年 | 3369篇 |
2001年 | 2847篇 |
2000年 | 2775篇 |
1999年 | 2573篇 |
1998年 | 2188篇 |
1997年 | 1878篇 |
1996年 | 1712篇 |
1995年 | 1424篇 |
1994年 | 1424篇 |
1993年 | 1230篇 |
1992年 | 980篇 |
1991年 | 705篇 |
1990年 | 569篇 |
1989年 | 532篇 |
1988年 | 357篇 |
1987年 | 244篇 |
1986年 | 159篇 |
1985年 | 111篇 |
1984年 | 49篇 |
1983年 | 40篇 |
1982年 | 37篇 |
1981年 | 27篇 |
1980年 | 27篇 |
1979年 | 35篇 |
1978年 | 41篇 |
1977年 | 27篇 |
1975年 | 5篇 |
1954年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20
km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and
release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust
grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to
match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance
pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO2 fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below
the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission
of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined
by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically
towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
52.
LAST GLACIATION AND MAXIMUM GLACIATION IN THE QINGHAI-XIZANG (TIBET) PLATEAU: A CONTROVERSY TO M. KUHLE,S ICE SHEET HYPOTHESIS 总被引:1,自引:0,他引:1
Since the late 1950's, many Chinese scientists have explored the remains of the Quaternary glaciation in the Qinghai-Xizang (Tibet) Plateau and its surrounding mountains. In the main, 3-4 glaciations have been recognized. The largest one occurred in the Late Middle Pleistocene with piedmont glaciers, ice caps and trellis valley glaciers in many high peak regions. But here is no evidence of a unified ice sheet covering the whole plateau as described by M. Kuhle. Due to the further uplifting of the Himalayas and Qinghai-Xizang Plateau the climate became progressively drier, diminishing the extension of glaciers during the Late Pleistocene. The elevation of the snow line during the Last Glaciation was about 4,000 m on the south, east and northeast edges of the plateau and ascended to 5500 m on the hinder northwest of the plateau. The thermal effect of the big plateau massif, the sharp increase of aridity from the southeast rim to the northwest inland area and the abrupt decrease of precipitation during the 相似文献
53.
T. H. BELL S. E. JOHNSON B. DAVIS A. FORDE N. HAYWARD C. WILKINS 《Journal of Metamorphic Geology》1992,10(3):295-307
Extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to their growth. This has been demonstrated for all tectonic environments so far investigated. The work also has provided new insights and data on metamorphic, structural and tectonic processes including: (1) the intimate control of deformation partitioning on metamorphic reactions; (2) solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones, and relative plate motion; and (3) a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens. 相似文献
54.
55.
讨论了应用曲面拟合技术和利用离散的重力观测点的重力变化对特定研究区的重力场变化进行数值模拟以获取该区重力变化空间梯度场图像的具体方法;并应用该方法获得了2004年3月福建安溪3.6级地震前的重力变化空间梯度场图像,发现空间梯度场图像能清晰地反映出地震前重力场群体性变化的差异性;利用此方法对2005年6月~2006年3月间台湾海峡西岸重力场变化进行数值模拟,获得其空间梯度场图像。 相似文献
56.
APPLICATION OF ANALYTIC HIERARCHY PROCESS TO ASSESSING THE ECOLOGICAL VULNERABILITY OF WETLANDS IN THE JIANGHAN PLAIN 总被引:5,自引:1,他引:4
WANGXue-lei NINGLong-mei HUWang-bin 《中国地理科学(英文版)》2003,13(3):272-276
Wetlands in the Jianghan Plain are important components of wetland types in lake area in the middle and lower reaches the Changjiang (Yangtze) River and they fulfill many uses and functions related to hydrology,waste assimilation, ecosystem productivity and biodiversity. Owing to natural factors and human activities, especially excessive reclamation from lakes, the shrinking process of the lakes has been accelerated. Wetland ecosystem has shown the characteristics of vulnerability. According to the analysis of wetland ecological function in the Jianghan Plain, this paper presented an index system related to productivity, stability and environmental capacity. By using the method of Analytic Hierarchy Process, we computed the values of the relative weights of the indexes, and evaluated the vulnerability level of the wetland ecosystem by the method of multi-indexes. The case study showed that the fragile extent of wetland ecosystem in the Jianghan Plain is 5.6. This means that the wetland ecosystem in the Jiang-han Plain is laid to the state of middle vulnerability. Therefore, the wetland conservation and eco-rehabilitation in the JiangJaan Plain should be paid attention to. 相似文献
57.
58.
59.
N. Ivanova K. Belczynski J. M. Fregeau F. A. Rasio 《Monthly notices of the Royal Astronomical Society》2005,358(2):572-584
We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code ( startrack ) and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing three- and four-body interactions ( fewbody ). We find that the combination of stellar evolution and dynamical interactions (binary–single and binary–binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster such as 47 Tuc, assuming an initial binary fraction of 100 per cent, is only ∼ 5–10 per cent. We show that this is in good agreement with recent Hubble Space Telescope observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modelling of globular clusters, suggesting that 'realistic models' should incorporate much larger initial binary fractions than has usually been the case in the past. 相似文献
60.