全文获取类型
收费全文 | 139741篇 |
免费 | 2230篇 |
国内免费 | 1861篇 |
专业分类
测绘学 | 4247篇 |
大气科学 | 10487篇 |
地球物理 | 27571篇 |
地质学 | 51912篇 |
海洋学 | 11473篇 |
天文学 | 27737篇 |
综合类 | 2427篇 |
自然地理 | 7978篇 |
出版年
2021年 | 914篇 |
2020年 | 1058篇 |
2019年 | 1126篇 |
2018年 | 7803篇 |
2017年 | 7051篇 |
2016年 | 5934篇 |
2015年 | 2028篇 |
2014年 | 2966篇 |
2013年 | 5612篇 |
2012年 | 4383篇 |
2011年 | 7725篇 |
2010年 | 6724篇 |
2009年 | 8184篇 |
2008年 | 6965篇 |
2007年 | 7584篇 |
2006年 | 4183篇 |
2005年 | 3760篇 |
2004年 | 3954篇 |
2003年 | 3748篇 |
2002年 | 3345篇 |
2001年 | 2717篇 |
2000年 | 2674篇 |
1999年 | 2220篇 |
1998年 | 2256篇 |
1997年 | 2127篇 |
1996年 | 1901篇 |
1995年 | 1770篇 |
1994年 | 1587篇 |
1993年 | 1464篇 |
1992年 | 1400篇 |
1991年 | 1283篇 |
1990年 | 1474篇 |
1989年 | 1238篇 |
1988年 | 1162篇 |
1987年 | 1354篇 |
1986年 | 1197篇 |
1985年 | 1490篇 |
1984年 | 1705篇 |
1983年 | 1564篇 |
1982年 | 1459篇 |
1981年 | 1377篇 |
1980年 | 1233篇 |
1979年 | 1191篇 |
1978年 | 1239篇 |
1977年 | 1080篇 |
1976年 | 1043篇 |
1975年 | 988篇 |
1974年 | 948篇 |
1973年 | 986篇 |
1971年 | 610篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
J. C. Eichelberger 《Bulletin of Volcanology》1989,51(1):72-75
This work was sponsored at Sandia National Laboratories (contract DE-AC04-76DP00789) by the US Department of Energy, Office of Basic Energy Sciences, as part of the Continental Scientific Drilling Program 相似文献
22.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
23.
24.
J. Raitala 《Earth, Moon, and Planets》1996,74(3):191-214
The intrablock deformation of Meshkenet Tessera on Venus is mostly due to responses of the uppermost surface bedrock to tensional stresses. It is found that complex deformation structures within the highland blocks resemble those of formed in chocolate tablet boudinaging which has taken place after original parallel faulting and bar-like crustal block formation. The high-angle tessera structures with varying cross-cutting relations define styles and locations of multiphase deformation most evidently related to local relaxation of tessera topography. Series of progressive or superposed fracturing events with alternating fault directions took place at high angles during this relaxational deformation. Compressional ridges often surround these tesserae. 相似文献
25.
26.
27.
The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray 相似文献
28.
The observed times of minimum light derived from the photometry of the Wolf-Rayet eclipsing binary stars CQ Cep and V444 Cyg are used to estimate the mass-loss rate of the Wolf-Rayet components in several modes of mass-loss and mass-exchange. 相似文献
29.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
30.