全文获取类型
收费全文 | 96708篇 |
免费 | 1961篇 |
国内免费 | 772篇 |
专业分类
测绘学 | 2459篇 |
大气科学 | 7411篇 |
地球物理 | 19843篇 |
地质学 | 32449篇 |
海洋学 | 8707篇 |
天文学 | 21659篇 |
综合类 | 199篇 |
自然地理 | 6714篇 |
出版年
2021年 | 710篇 |
2020年 | 841篇 |
2019年 | 895篇 |
2018年 | 1747篇 |
2017年 | 1651篇 |
2016年 | 2267篇 |
2015年 | 1485篇 |
2014年 | 2232篇 |
2013年 | 4831篇 |
2012年 | 2423篇 |
2011年 | 3524篇 |
2010年 | 3127篇 |
2009年 | 4379篇 |
2008年 | 3873篇 |
2007年 | 3692篇 |
2006年 | 3577篇 |
2005年 | 3044篇 |
2004年 | 3096篇 |
2003年 | 2927篇 |
2002年 | 2827篇 |
2001年 | 2479篇 |
2000年 | 2401篇 |
1999年 | 2048篇 |
1998年 | 2075篇 |
1997年 | 2038篇 |
1996年 | 1766篇 |
1995年 | 1663篇 |
1994年 | 1513篇 |
1993年 | 1389篇 |
1992年 | 1320篇 |
1991年 | 1180篇 |
1990年 | 1362篇 |
1989年 | 1208篇 |
1988年 | 1105篇 |
1987年 | 1304篇 |
1986年 | 1187篇 |
1985年 | 1449篇 |
1984年 | 1643篇 |
1983年 | 1532篇 |
1982年 | 1413篇 |
1981年 | 1401篇 |
1980年 | 1181篇 |
1979年 | 1157篇 |
1978年 | 1169篇 |
1977年 | 1109篇 |
1976年 | 1016篇 |
1975年 | 971篇 |
1974年 | 966篇 |
1973年 | 982篇 |
1972年 | 605篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Zsolt Sándor Bálint Érdi Carl D. Murray 《Celestial Mechanics and Dynamical Astronomy》2002,84(4):355-368
The dynamics of co-orbital motion in the restricted three-body problem are investigated by symplectic mappings. Analytical and semi-numerical mappings have been developed and studied in detail. The mappings have been tested by numerical integration of the equations of motion. These mappings have been proved to be useful for a quick determination of the phase space structure reflecting the main characteristics of the dynamics of the co-orbital problem. 相似文献
23.
Andrew J. Westphal Christopher Snead Janet Borg Eric Quirico Pierre‐Ivan Raynal Michael E. Zolensky Gianluca Ferrini Luigi Colangeli Pasquale Palumbo 《Meteoritics & planetary science》2002,37(6):855-865
Abstract— It has now been about a decade since the first demonstrations that hypervelocity particles could be captured, partially intact, in aerogel collectors. But the initial promise of a bonanza of partially‐intact extraterrestrial particles, collected in space, has yet to materialize. One of the difficulties that investigators have encountered is that the location, extraction, handling and analysis of very small (10 μm and less) grains, which constitute the vast majority of the captured particles, is challenging and burdensome. Furthermore, current extraction techniques tend to be destructive over large areas of the collectors. Here we describe our efforts to alleviate some of these difficulties. We have learned how to rapidly and efficiently locate captured particles in aerogel collectors, using an automated microscopic scanning system originally developed for experimental nuclear astrophysics. We have learned how to precisely excavate small access tunnels and trenches using an automated micromanipulator and glass microneedles as tools. These excavations are only destructive to the collector in a very small area—this feature may be particularly important for excavations in the precious Stardust collectors. Using actuatable silicon microtweezers, we have learned how to extract and store “naked” particles—essentially free of aerogel—as small as 3 μm in size. We have also developed a technique for extracting particles, along with their terminal tracks, still embedded in small cubical aerogel blocks. We have developed a novel method for storing very small particles in etched nuclear tracks. We have applied these techniques to the extraction and storage of grains captured in aerogel collectors (Particle Impact Experiment, Orbital Debris Collector Experiment, Comet‐99) in low Earth orbit. 相似文献
24.
25.
The surface digital photometry for NGC 5253 and NGC 2915 is given. The luminosity profiles and colour gradients are discussed and compared. It is concluded that NGC 2915 differs from NGC 5253, both in the extension and intensity of the star burst event because of the kinematical behaviour of the captured gas.CONICET, Buenos Aires, Visiting Astronomer, Cerro Tololo Inter-American Observatory, La Serena, Chile, operated by NSF 相似文献
26.
27.
JOKOB J. MØLLER TOR K. DANIELSEN ARNE FJALSTAD 《Boreas: An International Journal of Quaternary Research》1992,21(1):1-13
Morphological, seismic and lithostratigraphic investigations of a moraine deposit at Bleik (the Bleik moraine), northern Andøya, show short-distance transported till overlying long-distance transported predominantly glaciofluvial ice-marginal deposits. Consolidated glaciomarine sediments from a core at present sea-level, c . 400 m distally to the moraine complex, contain 31 species of foraminifera, among which Cassidulina reniforme, Islandiella helenae and Trifarina fluens dominate, and fragments of the molluscs Mya truncata and Astarte sp. and the echinoid Strongylocentrotus sp. Amino acid analyses of the foraminifera Cibicides lobatulus and the mollusc Mya truncata indicate ages between 22,000 and 16,000 BP. Radiocarbon dating of fragments of Mya truncata from the upper part of the core gave an age of 17,940 ± 245 BP, while a dating of unidentified shell fragments from the lower part gave an infinite age (>40,000 BP). The sediment was probably disturbed by icebergs beyond the end moraine zone, and the radiocarbon and amino acid dating of Mya truncata therefore represent a maximum age for this process. This new evidence indicates two phases with a higher relative sea-level than at present at Bleik, c . 18,000 and >40,000 BP. The Bleik moraine probably represents the early Late Weichselian glacial maximum ( c . 22,000 BP), while the underlying deglaciation deposit and associated beach-ridge (Bruvollen) is of pre-Late Weichselian age. Moraine ridges 3–4 km to the south of Bleik probably indicate advances of local glaciers between 22,000 and 18,000 BP. 相似文献
28.
Isolation of resonance in acoustic backscatter from elastic targetsusing adaptive estimation schemes
It has been shown that at certain frequencies the acoustic backscatter from elastic targets exhibits certain resonance behavior which closely relates to the physical properties of the target such as dimension, thickness, and composition. The purpose of this paper is to develop an automated approach for identifying the presence of resonance in the acoustic backscatter from an unknown underwater target by isolating the resonance part from the specular contribution. An adaptive transversal filter structure is used to estimate the specular part of the backscatter and consequently the error signal would provide an estimate of the resonance part. An important aspect of this scheme lies in the fact that it does not require an underlying model for the elastic return. The adaptation rule is based upon fast recursive least squares (RLS) learning. The approach taken in this paper is general in the sense that it can be applied to targets of unknown geometry and thickness and, further, does not require any a priori information about the target and/or the environment. Test results on acoustic data are presented which indicate the effectiveness of the proposed approach 相似文献
29.
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
30.
Measurement of variations in the radial velocities of stars due to the reflex orbital motion of the star around the planetary-system barycenter constitutes a powerful method of searching for substellar or planetary mass companions. After several years of patient data acquisition, radial-velocity searches for planetary systems around other stars are now beginning to bear fruit. In late 1995 and early 1996, three candidate systems were announced with Jovian-mass planets around solar-type stars. The current paradigm for low-mass star formation suggests that planetary systems should be able to form in the circumstellar disks surrounding young stellar objects. These newly discovered systems, and other discoveries which will soon follow them, will test critically our understanding of the processes of star- and planet-formation. We review the techniques used in these radial-velocity searches and their results to date. We then discuss planned improvements in the surveys, and the prospects for the next 20 years. 相似文献