全文获取类型
收费全文 | 16037篇 |
免费 | 2913篇 |
国内免费 | 3644篇 |
专业分类
测绘学 | 768篇 |
大气科学 | 1108篇 |
地球物理 | 4119篇 |
地质学 | 9089篇 |
海洋学 | 2183篇 |
天文学 | 2075篇 |
综合类 | 822篇 |
自然地理 | 2430篇 |
出版年
2024年 | 57篇 |
2023年 | 200篇 |
2022年 | 492篇 |
2021年 | 548篇 |
2020年 | 575篇 |
2019年 | 735篇 |
2018年 | 588篇 |
2017年 | 619篇 |
2016年 | 634篇 |
2015年 | 688篇 |
2014年 | 840篇 |
2013年 | 821篇 |
2012年 | 893篇 |
2011年 | 988篇 |
2010年 | 804篇 |
2009年 | 1139篇 |
2008年 | 1082篇 |
2007年 | 1143篇 |
2006年 | 1128篇 |
2005年 | 964篇 |
2004年 | 955篇 |
2003年 | 941篇 |
2002年 | 749篇 |
2001年 | 713篇 |
2000年 | 654篇 |
1999年 | 588篇 |
1998年 | 636篇 |
1997年 | 379篇 |
1996年 | 341篇 |
1995年 | 328篇 |
1994年 | 289篇 |
1993年 | 256篇 |
1992年 | 209篇 |
1991年 | 130篇 |
1990年 | 89篇 |
1989年 | 103篇 |
1988年 | 66篇 |
1987年 | 71篇 |
1986年 | 44篇 |
1985年 | 25篇 |
1984年 | 11篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1954年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
51.
52.
53.
54.
The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique
geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of
uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found.
Most of them are mined out. The reactor zone of Bangombé, is only 10–11 m below the surface. This site has been influenced
by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombé during the
course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns,
the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced
by the anomalous isotope ratios of REE because of the fission process. The 149Sm/147Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of 149Sm and subsequent transmutation, whereas 147Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios
of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few
decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE.
Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering
model.
Received: 26 November 1999 · Accepted: 2 May 2000 相似文献
55.
P. A. Patsis 《Monthly notices of the Royal Astronomical Society》2005,358(2):305-315
We investigate the morphological relation between the orbits of the central family of periodic orbits ( x 1 family) and the bar itself using models of test particles moving in a barred potential. We show that different bar morphologies may have as a backbone the same set of x 1 periodic orbits. We point out that by populating initially axisymmetric stellar discs exponentially with test particles in circular, or almost circular motion, we may end up with a response bar which reveals a shape different in crucial details from that of the individual stable x 1 orbits. For example, a bar model in which the x 1 orbits are pure ellipses may have a much more complicated response morphology. This depends on the particular invariant curves around x 1 , which are populated in each model. 相似文献
56.
The accuracy of the computed stress distribution near the free surface of vertical slopes was evaluated in this study as a function of the element size, including aspect ratio. To accomplish this objective, a parametric study was carried out comparing stresses computed using the finite element method (FEM) to those obtained from a physical model composed of photoelastic material. The results of the study indicate a reasonable agreement between a gelatin model and the FEM model for shear stresses, and an overall good agreement between the two models for the principal stresses. For stresses along the top of the slope, the height of the element tends to be more important than width or aspect ratio, at least for aspect ratios up to 4. In all cases, the greatest difference between the two models occurs in the vicinity of the slope. Specifically, if H is defined as the slope height, an element height of H/10 appears to be adequate for the study of stresses deep within the slope, such as for typical embankment analyses. However, for cases where tensile stresses in the vicinity of the slope face which are critical, such as for the stability analysis of steep slopes, element heights as small as H/32, or higher‐order elements, are necessary. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
57.
N. Yoshida J. Colberg S.D.M. White A.E. Evrard T.J. MacFarland † H.M.P. Couchman A. Jenkins C.S. Frenk F.R. Pearce G. Efstathiou J.A. Peacock P.A. Thomas 《Monthly notices of the Royal Astronomical Society》2001,325(2):803-816
We create mock pencil-beam redshift surveys from very large cosmological N -body simulations of two cold dark matter (CDM) cosmogonies, an Einstein–de Sitter model ( τ CDM) and a flat model with Ω0 =0.3 and a cosmological constant (ΛCDM). We use these to assess the significance of the apparent periodicity discovered by Broadhurst et al. Simulation particles are tagged as 'galaxies' so as to reproduce observed present-day correlations. They are then identified along the past light-cones of hypothetical observers to create mock catalogues with the geometry and the distance distribution of the Broadhurst et al. data. We produce 1936 (2625) quasi-independent catalogues from our τ CDM (ΛCDM) simulation. A couple of large clumps in a catalogue can produce a high peak at low wavenumbers in the corresponding one-dimensional power spectrum, without any apparent large-scale periodicity in the original redshift histogram. Although the simulated redshift histograms frequently display regularly spaced clumps, the spacing of these clumps varies between catalogues and there is no 'preferred' period over our many realizations. We find only a 0.72 (0.49) per cent chance that the highest peak in the power spectrum of a τ CDM (ΛCDM) catalogue has a peak-to-noise ratio higher than that in the Broadhurst et al. data. None of the simulated catalogues with such high peaks shows coherently spaced clumps with a significance as high as that of the real data. We conclude that in CDM universes, the regularity on a scale of ∼130 h −1 Mpc observed by Broadhurst et al. has a priori probability well below 10−3 . 相似文献
58.
59.
Tao Kiang Yong-Feng Wu Xing-Fen Zhu Dunsink Observatory Dublin Institute for Advanced Studies Dublin Ireland Center for Astrophysics University of Science Technology of China Hefei 《中国天文和天体物理学报》2004,4(3):209-220
Rhombic cell analysis as outlined in the first paper of the present seriesis applied to samples of varying depths and liming luminosities of the IRAS/PSCzCatalogue. Numerical indices are introduced to summarize essential information.Because of the discrete nature of the analysis and of the space distribution of galax-ies, the indices for a given sample must be regarded as each having an irreduciblescatter. Despite the scatter, the mean indices show remarkable variations acrossthe samples. The underlying factor for the variations is shown to be the limitingluminosity rather than the sampling depth. As samples of more and more luminousgalaxies are considered over a range of some 2.5 magnitudes (a factor of some 75in space density), the morphology of the filled and empty regions defined by thegalaxies degrades steadily towards insignificance, and the degrading is faster for thefilled than the empty region. 相似文献
60.