全文获取类型
收费全文 | 53556篇 |
免费 | 2162篇 |
国内免费 | 423篇 |
专业分类
测绘学 | 1742篇 |
大气科学 | 3895篇 |
地球物理 | 12599篇 |
地质学 | 18409篇 |
海洋学 | 4385篇 |
天文学 | 12096篇 |
综合类 | 158篇 |
自然地理 | 2857篇 |
出版年
2021年 | 531篇 |
2020年 | 623篇 |
2019年 | 789篇 |
2018年 | 1540篇 |
2017年 | 1540篇 |
2016年 | 1969篇 |
2015年 | 1342篇 |
2014年 | 1904篇 |
2013年 | 3039篇 |
2012年 | 1993篇 |
2011年 | 2413篇 |
2010年 | 2174篇 |
2009年 | 2717篇 |
2008年 | 2384篇 |
2007年 | 2262篇 |
2006年 | 2123篇 |
2005年 | 1654篇 |
2004年 | 1671篇 |
2003年 | 1512篇 |
2002年 | 1532篇 |
2001年 | 1318篇 |
2000年 | 1235篇 |
1999年 | 1032篇 |
1998年 | 1015篇 |
1997年 | 1011篇 |
1996年 | 787篇 |
1995年 | 755篇 |
1994年 | 708篇 |
1993年 | 592篇 |
1992年 | 538篇 |
1991年 | 528篇 |
1990年 | 536篇 |
1989年 | 528篇 |
1988年 | 465篇 |
1987年 | 548篇 |
1986年 | 491篇 |
1985年 | 587篇 |
1984年 | 704篇 |
1983年 | 623篇 |
1982年 | 601篇 |
1981年 | 543篇 |
1980年 | 472篇 |
1979年 | 464篇 |
1978年 | 469篇 |
1977年 | 408篇 |
1976年 | 354篇 |
1975年 | 374篇 |
1974年 | 344篇 |
1973年 | 383篇 |
1972年 | 281篇 |
排序方式: 共有10000条查询结果,搜索用时 319 毫秒
151.
This paper presents a method that incorporates a non‐associated flow rule into the limit analysis to investigate the influence of the dilatancy angle on the factor of safety for the slope stability analysis. The proposed method retain's the advantage of the upper bound method, which is simple and has no stress involvement in the calculation of the energy dissipation and the factor of safety. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
152.
Historical Changes in the Flood Hazard at Khartoum, Sudan: Lessons and Warnings for the Future 总被引:1,自引:0,他引:1
The 1988 floods at Khartoum were frequently described as "unprecedented". However, an examination of evidence for floods caused by the Nile and rain storms during the nineteenth and twentieth centuries leads to the conclusion that neither the Nile flood nor the rain storms nor their coincidence in one season were unprecedented and that similar situations will recur.We argue that most problems were created by complacency, lack of planning and mismanagement. We conclude with some simple suggestions for flood amelioration in the future and warn that complacency is the single most important phenomenon likely to recur to the detriment of Greater Khartoum in the future. 相似文献
153.
We have studied the influence of the magnetic helicity on solar particle propagation using the IMF data observed by the HELIOS spacecraft in the range 0.31–0.95 AU, during eight solar proton events. For this, we have derived power and helicity spectra of the turbulence of the magnetic field during the time of the events. These are used to compute the particle pitch-angle scattering coefficients according to the quasi-linear theory (QLT) treatment of particle propagation in turbulent magnetic fields. The results show that in all the cases the helicity effects are negligible and the particle's mean free paths deduced from the pitch-angle diffusion coefficients are the same regardless of whether or not helicity effects are included in the calculations. The computed mean free paths are quite different in each case.Deceased 10 April, 1995. 相似文献
154.
Pittichovâ J. Sekenina Z. Birkle K. Boehnhardt J. Engels D. Keller P. 《Earth, Moon, and Planets》1997,78(1-3):329-338
The Sekanina-Farrell particle fragmentation model for the striated tails of dust comets is successfully applied to two images
of comet Hale-Bopp to study the motions of 12 striae in a time span of March 12–15, 1997. There is evidence for recurring
outbursts with a periodicity of 11h21m, consistent with results based on analysis of dust jets. The ejecta in all the striae appear to have been released from one
source on the nucleus between the end of January and the second half of February 1997, some 60 to 40 days before perihelion.
The parent particles were subjected to a radiation pressure acceleration of βp ≃ 0.55 and their fragmentation lifetimes in 11 of the 12 striae were practically constant and equal to 13–15 days, when normalized
to 1 AU from the Sun. Brief analysis of Watanabe et al.'s measurements of striae on their images from March 5–9, 1997 shows
even shorter fragmentation lifetimes for the parent particles, mostly about 7–11 days at1 AU.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
155.
R. Vílchez-Gómex G. A. Bruzual R. Pelló R. Domínguez 《Astrophysics and Space Science》2001,276(2-4):757-764
We have developed model predictions for the morphological distribution of cluster galaxies as a function of the cluster-centric
distance and the local galaxy density, using a semi-analytical code. This code allows us to obtain magnitudes and colours
for cluster galaxies at different redshifts, and thus to study in detail the evolution of the colour–magnitude relation of
specific distant clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
156.
157.
Theoretical line ratios involving 2s
2
S - 3p
2
P, 2p
2
P - 3s
2
S, and 2p
2
S - 3d
2
D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed. 相似文献
158.
D.W. Crawford D.A. Purdie A.P.M. Lockwood P. Weissman 《Estuarine, Coastal and Shelf Science》1997,45(6):799-812
In the Southampton Water estuary (southern England, U.K.), red-tides caused by the planktonic, phototrophic ciliateMesodinium rubrum(=Myrionecta rubra) occur during most summers and sometimes in autumn. These events were investigated in detail between 1985 and 1987 and were characterized by levels of chlorophylla(chl a) of over 100 μg l−1, cell numbers ofM. rubrumof over 1×103 ml−1, oxygen saturations of around 150%, and depleted numbers of macrozooplankton. Initiation of red-water did not appear to be triggered by irradiance or nutrients, but coincided with an increase in temperature and water column stability. This enhanced stability was promoted by increased surface to bottom gradients of both temperature and salinity, and by reduced mixing during neap tides. Development of red-water was accompanied by removal of most of the dissolved NH+4from the water column, whereas some NO−3persisted, presumably maintained by freshwater input. NO−3and NH+4gradually returned to pre-bloom concentrations as the red-water declined in late summer. Maximal biomass ofM. rubrumappeared to be limited by irradiance, and self-shading probably imposed an upper limit of around 300 mg chl a m−2within the water column. At the observed levels of chl a, irradiance values within the population maximum between 1 and 3 m depth were only just of the order (≈15 μmol photons m−2 s−1) required to balance estimated respiratory demands. Oxygen concentration became undersaturated during the late bloom phase, with minimal values of 20–30% saturation recorded in deeper waters; however, despite this and reduced numbers of macrozooplankton, direct deleterious effects on other organisms were not observed. 相似文献
159.
A. Bhatnagar R. M. Jain J. T. Burkepile I. M. Chertok A. Magun H. Urbarz P. Zlobec 《Astrophysics and Space Science》1996,243(1):209-213
The powerful cosmic ray flare of Sept. 29, 1989 occurred behind the limb and was observed over a wide spectral range. The analysis of optical, radio, and other relevant data suggest two phases of energy release. After an impulsive phase a prolonged post eruption energy release occurred in an extended region of the corona following the eruption of a large coronal mass ejection (CME). This phase is responsible for numerous coronal and interplanetary phenomena including the ground-level increase of cosmic rays. 相似文献
160.
P. Cox 《Astrophysics and Space Science》1997,255(1-2):489-497
This review summarizes spectroscopic and imaging results obtained on board the Infrared Space Observatory (ISO) on well-known
planetary nebulae: the young nebula NGC 7027, and two fully evolved nearby nebulae, the Dumbbell and the Helix.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献