全文获取类型
收费全文 | 255634篇 |
免费 | 5177篇 |
国内免费 | 3433篇 |
专业分类
测绘学 | 6954篇 |
大气科学 | 18916篇 |
地球物理 | 53543篇 |
地质学 | 88771篇 |
海洋学 | 21213篇 |
天文学 | 56043篇 |
综合类 | 1001篇 |
自然地理 | 17803篇 |
出版年
2021年 | 2238篇 |
2020年 | 2584篇 |
2019年 | 2843篇 |
2018年 | 3333篇 |
2017年 | 3023篇 |
2016年 | 5600篇 |
2015年 | 4208篇 |
2014年 | 6908篇 |
2013年 | 14212篇 |
2012年 | 6391篇 |
2011年 | 7660篇 |
2010年 | 6717篇 |
2009年 | 9369篇 |
2008年 | 8187篇 |
2007年 | 7590篇 |
2006年 | 9590篇 |
2005年 | 7649篇 |
2004年 | 7560篇 |
2003年 | 7034篇 |
2002年 | 6677篇 |
2001年 | 5967篇 |
2000年 | 5911篇 |
1999年 | 5203篇 |
1998年 | 5223篇 |
1997年 | 5015篇 |
1996年 | 4675篇 |
1995年 | 4416篇 |
1994年 | 4094篇 |
1993年 | 3846篇 |
1992年 | 3615篇 |
1991年 | 3586篇 |
1990年 | 3759篇 |
1989年 | 3515篇 |
1988年 | 3296篇 |
1987年 | 3842篇 |
1986年 | 3401篇 |
1985年 | 4216篇 |
1984年 | 4733篇 |
1983年 | 4399篇 |
1982年 | 4314篇 |
1981年 | 3911篇 |
1980年 | 3641篇 |
1979年 | 3506篇 |
1978年 | 3478篇 |
1977年 | 3278篇 |
1976年 | 3041篇 |
1975年 | 2963篇 |
1974年 | 2920篇 |
1973年 | 3077篇 |
1972年 | 2025篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Ulrich Hopp Regina E. Schulte-Ladbeck Jürgen Kerp 《Astrophysics and Space Science》2003,284(2):647-650
We are investigating the hypothesis that Compact High-Velocity Clouds (CHVCs) are the left-over building blocks of Local Group
galaxies. To this end, we are searching for their embedded stellar populations using FORS at the VLT. The search is done with
single-star photometry in V and I bands, which is sensitive to both, young and old, stellar populations. Five CHVCs of our
sample have been observed so far down to I=24. We pointed the VLT towards the highest HI column density regions, as determined
in Effelsberg radio data. In an alternate approach, we searched 2MASS public data towards those 5 CHVCs down to K=16. While
the VLT data probe the central regions out to distance moduli of about 27, the 2MASS data are sensitive to a population of
red giant stars to distance moduli of about 20. The 2MASS data, on the other hand, cover a much wider field of view than the
VLT data (radius of 1 degree versus FORS field of 6.8 arcmin). We did not find a stellar population intrinsic to the CHVCs
in either data. In this paper, we illustrate our search methods.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
972.
973.
Darwin is a cornerstone mission of the Horizons 2000+ program of the European Space Agency. It has the express purpose of
carrying out the first direct search for terrestrial exoplanets, and to achieve unprecedented spatial resolution in the infrared wavelength region. The
detection and study of terrestrial exoplanets promises to usher in a new era in science and will affect a broad spectrum of
disciplines. Further, the time line for implementation of such an instrument is now likely to be of the close order of 10
years, leading to possible answers to one of mankind's most fundamental questions in the second decade of the 21st century.
It has been found that in order to realize am interferometer in space in the next 10–15 years, it is necessary to prepare
the way through a number of intermediary steps – both on the ground and in space. In this context, we here describe GENIE
– a ground based nulling interferometry experiment to be implemented at the VLTI in a partnership between ESA and ESO.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
974.
There is a general belief that hydrous minerals cannot exist on Venus under current surface conditions. This view was challenged when Johnson and Fegley (2000, Icarus 146, 301-306) showed that tremolite (Ca2Mg5Si8O22(OH)2), a hydrous mineral, is stable against thermal decomposition at current Venus surface temperatures, e.g., 50% decomposition in 4 Ga at 740 K. To further explore hydrous mineral thermal stability on Venus, we experimentally determined the thermal decomposition kinetics of fluorine-bearing tremolite. Fluor-tremolite is thermodynamically more stable than OH-tremolite and should decompose more slowly. However how much slower was unknown. We measured the decomposition rate of fluorine-bearing tremolite and show that its decomposition is several times to greater than ten times slower than that of OH-tremolite. We also show that F-bearing tremolite is depleted in fluorine after decomposition and that fluorine is lost as a volatile species such as HF gas. If tremolite ever formed on Venus, it would probably also contain fluorine. The exceptional stability of F-bearing tremolite strengthens our conclusions that if hydrous minerals ever formed on Venus, they could still be there today. 相似文献
975.
976.
977.
978.
979.
Velocities inside avalanches have been calculated for many years by calculatingthe cross-correlation between opto-electronic sensors using a method pioneered by inlinecite Nishimura et al. and Dent et al. Their approachhas been widely adopted but there has been little discussion of the optimal designof such instruments and the best analysis techniques. This paper discusses some ofthe different sources of error that arise and how these can be mitigated. A statisticalframework that describes such instruments is developed and used to quantify the errors. 相似文献
980.
C. S. M. Turney K. Van Den Burg S. Wastegrd S. M. Davies N. J. Whitehouse J. R. Pilcher C. Callaghan 《第四纪科学杂志》2006,21(4):335-345
High‐precision correlation of palaeoclimatic and palaeoenvironmental records is crucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional method for dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacial transition (LGIT; 15–9 ka), there are inherent problems with the method, particularly during periods of climate change which are often accompanied by major perturbations in atmospheric radiocarbon content. An alternative method is the use of tephras that act as time‐parallel marker horizons. Within Europe, numerous volcanic centres are known to have erupted during the LGIT, providing considerable potential for high‐precision correlation independent of past radiocarbon fluctuations. Here we report the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending the known provenance of these events. We have also identified two new horizons (the Roddans Port Tephras A and B) and tentatively recognise an additional horizon from Vallensgård Mose (Denmark) that provide crucial additional chronological control for the LGIT. Two phases of the Laacher See Tephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of the Middle Laacher See Tephra (MLST‐C2) indicating a more northeasterly distribution of this fan than reported previously. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献