全文获取类型
收费全文 | 57460篇 |
免费 | 590篇 |
国内免费 | 327篇 |
专业分类
测绘学 | 1411篇 |
大气科学 | 4120篇 |
地球物理 | 10467篇 |
地质学 | 23558篇 |
海洋学 | 4495篇 |
天文学 | 11430篇 |
综合类 | 240篇 |
自然地理 | 2656篇 |
出版年
2022年 | 225篇 |
2021年 | 340篇 |
2020年 | 374篇 |
2019年 | 423篇 |
2018年 | 3737篇 |
2017年 | 3440篇 |
2016年 | 2487篇 |
2015年 | 719篇 |
2014年 | 1012篇 |
2013年 | 1766篇 |
2012年 | 2157篇 |
2011年 | 4044篇 |
2010年 | 3687篇 |
2009年 | 4165篇 |
2008年 | 3387篇 |
2007年 | 4013篇 |
2006年 | 1459篇 |
2005年 | 1551篇 |
2004年 | 1458篇 |
2003年 | 1463篇 |
2002年 | 1222篇 |
2001年 | 930篇 |
2000年 | 894篇 |
1999年 | 711篇 |
1998年 | 745篇 |
1997年 | 731篇 |
1996年 | 542篇 |
1995年 | 537篇 |
1994年 | 485篇 |
1993年 | 428篇 |
1992年 | 374篇 |
1991年 | 363篇 |
1990年 | 391篇 |
1989年 | 342篇 |
1988年 | 303篇 |
1987年 | 402篇 |
1986年 | 322篇 |
1985年 | 446篇 |
1984年 | 468篇 |
1983年 | 469篇 |
1982年 | 414篇 |
1981年 | 397篇 |
1980年 | 416篇 |
1979年 | 360篇 |
1978年 | 388篇 |
1977年 | 336篇 |
1976年 | 326篇 |
1975年 | 331篇 |
1974年 | 304篇 |
1973年 | 298篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
71.
72.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
73.
74.
75.
J. BERGER O. FEMENIAS J. C. C. MERCIER D. DEMAIFFE 《Journal of Metamorphic Geology》2005,23(9):795-812
The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western French Massif Central. This ophiolitic section comprises mantle‐harzburgite, mantle‐dunite, wehrlites, troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite‐bearing chloritites. With various thermobarometric methods, it is possible to show that the different varieties of amphibole have registered low‐P (c. 0.2 GPa) conditions with temperature ranging from high‐T, late‐magmatic conditions to greenschist–zeolite metamorphic facies. The abundance of undeformed metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca–Al (–Mg) metasomatism illustrated by the growth of Ca–Al silicates in veins or replacing the primary magmatic minerals, the P–T conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean‐floor hydrothermal metamorphism in the vicinity of a palaeo‐ridge. Among the West‐European Variscan ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in any HP (subduction‐related) or MP (orogenic) metamorphism as observed in other ophiolite occurrences (i.e. France, Spain and Germany). 相似文献
76.
Longitudinal and transverse dispersion in estuaries may be determined in principle by measuring the change in time of the concentration field of a water soluble dye. In practice, measurements may take the form of several sampling runs crossing the dye patch. The coefficients of dispersion are usually estimated from the second moments of the concentration field; the use of second moments, in particular, introduces errors because measurements at long times or distance are unduly emphasized.A self-consistent method, free of the disadvantages inherent in the use of higher moments, is described in which the distribution of concentration in space is related to the zeroth spatial moment of concentration. In this formulation, based upon sampling runs across the dye patch, the concentration field in the estuary may be directly and simply related to the dispersion coefficients in both the intercept and gradient of a linear graph.The methods are illustrated by the characterization of a series of experiments in the Inner Estuary of Milford Haven. 相似文献
77.
78.
Non-Newtonian topographic relaxation on Europa 总被引:1,自引:0,他引:1
Models of topographic support on Europa by lateral shell thickness variations have previously assumed a Newtonian ice viscosity. Here I show that using a more realistic stress-dependent viscosity gives relaxation times which can be significantly different. Topography of wavelength 100 km cannot be supported by lateral shell thickness variations for ∼50 Myr, unless the shell thickness is <10 km or the ice grain size >10 mm. Shorter wavelength topography would require even thinner shells, but may be supported elastically. Global-scale variations in shell thickness, however, can be supported for geological timescales if the shell thickness is O(10 km). 相似文献
79.
Long-term measurements from Advanced Very High Resolution Radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration satellites were evaluated to assess variability in cloud-top temperatures over central and eastern Europe that saw radical infrastructural changes after the fall of the East Bloc in 1989 that has affected the pollution levels and hence cloud albedo. Four years in the late 1980s (1985-1988) and in the late 1990s (1997-2000) were chosen, as these are distinctively marked as episodes of very high and lower air pollution (sulphates and particulate matter). During the late 1980s, low- and medium-level clouds were colder by more than 2 K and convective clouds even by 4 K. Cloud-tops over and around polluted regions are higher, and their temperatures showed stronger variability, suggesting an indirect aerosol effect in the thermal spectral range as well. 相似文献
80.