全文获取类型
收费全文 | 71166篇 |
免费 | 1347篇 |
国内免费 | 521篇 |
专业分类
测绘学 | 1779篇 |
大气科学 | 5801篇 |
地球物理 | 15070篇 |
地质学 | 23407篇 |
海洋学 | 5986篇 |
天文学 | 15916篇 |
综合类 | 159篇 |
自然地理 | 4916篇 |
出版年
2021年 | 446篇 |
2020年 | 597篇 |
2019年 | 590篇 |
2018年 | 1172篇 |
2017年 | 1173篇 |
2016年 | 1690篇 |
2015年 | 1244篇 |
2014年 | 1675篇 |
2013年 | 3642篇 |
2012年 | 1801篇 |
2011年 | 2558篇 |
2010年 | 2211篇 |
2009年 | 3187篇 |
2008年 | 2846篇 |
2007年 | 2545篇 |
2006年 | 2607篇 |
2005年 | 2250篇 |
2004年 | 2364篇 |
2003年 | 2149篇 |
2002年 | 2060篇 |
2001年 | 1849篇 |
2000年 | 1811篇 |
1999年 | 1555篇 |
1998年 | 1536篇 |
1997年 | 1541篇 |
1996年 | 1312篇 |
1995年 | 1254篇 |
1994年 | 1129篇 |
1993年 | 1022篇 |
1992年 | 965篇 |
1991年 | 826篇 |
1990年 | 1043篇 |
1989年 | 870篇 |
1988年 | 765篇 |
1987年 | 959篇 |
1986年 | 838篇 |
1985年 | 1044篇 |
1984年 | 1209篇 |
1983年 | 1145篇 |
1982年 | 1043篇 |
1981年 | 1000篇 |
1980年 | 849篇 |
1979年 | 835篇 |
1978年 | 884篇 |
1977年 | 809篇 |
1976年 | 757篇 |
1975年 | 717篇 |
1974年 | 713篇 |
1973年 | 726篇 |
1972年 | 453篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
461.
462.
J. M. Charnock C. M. B. Henderson J. F. W. Mosselmans R. A. D. Pattrick 《Physics and Chemistry of Minerals》1996,23(7):403-408
The X-ray absorption spectra of the transition metal L3,2-edges of the dichalcogenides FeS2 (pyrite), FeSe2, FeTe2, CoS2, CoSe2, CoTe2, NiS2, NiSe2 and NiTe2 have been studied. These spectra are compared with theoretical simulations make using the atomic multiplet approach assuming on octahedral site for the metal atoms. The Ni spectra can be closely simulated using this approach but the Fe and Co spectra show less good fits to the experimental data. The reasons for this and its implications for the use of L-edge spectra in the study of such minerals are discussed in terms of the covalency and the ground states of the compounds; these results indicate the mixing of the ground states increases in the order Ni<Co<Fe. 相似文献
463.
A.?Pulido-BoschEmail author J.?M.?Calaforra P.?Pulido-Leboeuf S.?Torres-García 《Environmental Geology》2004,46(5):583-590
The Messinian gypsum of Sorbas is intensively karstified, with hundreds of dolines and numerous, highly unusual exo- and endokarstic forms. Amongst these the tumuli are especially spectacular, as are the large caverns, adorned with numerous speleothems, some of which have not been described anywhere else in the World. The extraction of the gypsum has impacted practically all the elements of the environment (landscape, water, soil, subsoil, flora, fauna, etc.) and, in particular, all of the karstic forms. In addition to endangering the caves and the natural treasures that they contain, it affects the surface waters, markedly modifying the hydrographic network. Groundwaters, which now occur in the bottom of a quarry, are also affected. 相似文献
464.
Andrew D. Czaja Anatoliy B. Kudryavtsev George D. Cody J. William Schopf 《Organic Geochemistry》2009,40(3):353-364
The processes of organic maturation that occur during the permineralization of fossils and the detailed chemistry of the resulting products are incompletely understood. Primary among such processes is the geochemical alteration of biological matter to produce kerogen, such as that which comprises the cell walls of the fossils studied here: essentially unmetamorphosed, Eocene plant axes (specimens of the fossil fern Dennstaedtiopsis aerenchymata cellularly permineralized in cherts of the Clarno Formation of Oregon and the Allenby Formation of British Columbia). The composition and molecular structure of the kerogen that comprises the cell walls of such axes were analyzed using ultraviolet Raman spectroscopy (UV–Raman), solid state 13C nuclear magnetic resonance spectroscopy (13C NMR) and pyrolysis–gas chromatography–mass spectrometry (py–GC–MS).Cellularly well-preserved fern axes from both geologic units exhibit similar overall molecular structure, being composed primarily of networks of aromatic rings and polyene chains that, unlike more mature kerogens, lack large polycyclic aromatic hydrocarbon (PAH) constituents. The cell walls of the Allenby Formation specimens are, however, less altered than those of the Clarno chert, exhibiting more prevalent oxygen-containing and alkyl functional groups and comprising a greater fraction of rock mass.The study represents the first demonstration of the effectiveness (and limitations) of the combined use of UV–Raman, 13C NMR and py–GC–MS for the analysis of the kerogenous cell walls of chert-permineralized vascular plants. 相似文献
465.
466.
467.
Measurement of the coronal magnetic field is a crucial ingredient in understanding the nature of solar coronal phenomena at all scales. We employed STEREO/COR1 data obtained during a deep minimum of solar activity in February 2008 (Carrington Rotation CR 2066) to retrieve and analyze the three-dimensional (3D) coronal electron density in the range of heights from 1.5 to 4 R⊙ using a tomography method. With this, we qualitatively deduced structures of the coronal magnetic field. The 3D electron-density analysis is complemented by the 3D STEREO/EUVI emissivity in the 195 Å band obtained by tomography for the same CR. A global 3D MHD model of the solar corona was used to relate the reconstructed 3D density and emissivity to open/closed magnetic-field structures. We show that the density-maximum locations can serve as an indicator of current-sheet position, while the locations of the density-gradient maximum can be a reliable indicator of coronal-hole boundaries. We find that the magnetic-field configuration during CR 2066 has a tendency to become radially open at heliocentric distances greater than 2.5 R⊙. We also find that the potential-field model with a fixed source surface is inconsistent with the boundaries between the regions with open and closed magnetic-field structures. This indicates that the assumption of the potential nature of the coronal global magnetic field is not satisfied even during the deep solar minimum. Results of our 3D density reconstruction will help to constrain solar coronal-field models and test the accuracy of the magnetic-field approximations for coronal modeling. 相似文献
468.
Dynamic spectra of a Jovian non-Io-A storm recorded simultaneously by the Voyager 1 spacecraft and by the Kiiminki radio spectrograph are compared. It seems that the emission beam of the storm co-rotates with the planet and has a sloped leading edge, in accordance with the result of Maeda and Carr (1984). 相似文献
469.
470.
The eclipsing binary AV Hya has been observed photoelectrically inU, B andV filters on 15 nights. The primary eclipse appeared to be total with a totality of 23 min duration. A modified period of 0d.6 834 062 has been obtained. Geometrical elements could only be determined for partial transit case. 相似文献