全文获取类型
收费全文 | 65321篇 |
免费 | 1024篇 |
国内免费 | 603篇 |
专业分类
测绘学 | 1443篇 |
大气科学 | 5027篇 |
地球物理 | 13216篇 |
地质学 | 23409篇 |
海洋学 | 5605篇 |
天文学 | 14128篇 |
综合类 | 152篇 |
自然地理 | 3968篇 |
出版年
2021年 | 556篇 |
2020年 | 652篇 |
2019年 | 744篇 |
2018年 | 1438篇 |
2017年 | 1432篇 |
2016年 | 1676篇 |
2015年 | 961篇 |
2014年 | 1615篇 |
2013年 | 3064篇 |
2012年 | 1821篇 |
2011年 | 2480篇 |
2010年 | 2172篇 |
2009年 | 2843篇 |
2008年 | 2513篇 |
2007年 | 2472篇 |
2006年 | 2412篇 |
2005年 | 1802篇 |
2004年 | 1793篇 |
2003年 | 1836篇 |
2002年 | 1720篇 |
2001年 | 1478篇 |
2000年 | 1469篇 |
1999年 | 1260篇 |
1998年 | 1248篇 |
1997年 | 1267篇 |
1996年 | 1081篇 |
1995年 | 1042篇 |
1994年 | 918篇 |
1993年 | 834篇 |
1992年 | 769篇 |
1991年 | 794篇 |
1990年 | 811篇 |
1989年 | 737篇 |
1988年 | 695篇 |
1987年 | 811篇 |
1986年 | 801篇 |
1985年 | 905篇 |
1984年 | 1039篇 |
1983年 | 1030篇 |
1982年 | 940篇 |
1981年 | 885篇 |
1980年 | 821篇 |
1979年 | 786篇 |
1978年 | 777篇 |
1977年 | 726篇 |
1976年 | 657篇 |
1975年 | 647篇 |
1974年 | 696篇 |
1973年 | 715篇 |
1972年 | 441篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
3.
The elm decline of 5000 14C yr ago has been the most widely discussed phenomenon in post‐glacial vegetation history. This pan‐European reduction of elm populations, echoed in the decimation of elmwoods in Europe during the twentieth century, has attracted a series of interrelated hypotheses involving climate change, human activity, disease and soil deterioration. The elm bark beetle (Scolytus scolytus L.) is an essential component of disease explanations. We present evidence for the presence of the beetle over a prolonged period (ca. 7950–4910 yr BP [8800–5660 cal. yr BP]) from a lowland raised mire deposit in northeast Scotland, with its final appearance at this site, and the first and only appearance in another mire of a single scolytid find, around the time of the elm decline. The subfossil S. scolytus finds are not only the first from Scotland, but they also represent the most comprehensive sequence of finds anywhere. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
4.
The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems. 相似文献
5.
6.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
7.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators. 相似文献
8.
Y.-B. WU S. GAO H.-J. GONG H. XIANG W.-F. JIAO S.-H. YANG Y.-S. LIU H.-L. YUAN 《Journal of Metamorphic Geology》2009,27(6):461-477
U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and 176 Lu/177 Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean 207 Pb/206 Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ LuN ≥2213; 14.7 ≤ LuN /SmN ≤ 354) and 176 Lu/177 Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean 207 Pb/206 Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T DM2 ) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust. 相似文献
9.
10.
We report intermediate resolution H spectroscopy of the black hole candidate Nova Muscae 1991 during quiescence. We classify the companion star as a K3-K4V which contributes 85±6 percent to the total flux from the binary. The photospheric absorption lines are broadened by 106±13 kms−1 with respect to template field stars, leading to a system mass ratio of q =M1/M2 = 7.8−2.0+3.4. Doppler imaging of the H line shows strong emission coming from the secondary star (EW=3.1±0.6Å) which we associate with chromospheric activity. However, the hot-spot is not detected and this may indicate a lower mass transfer rate than in other X-ray transients of comparable orbital periods. The surface brightness distribution of the accretion disk in H follows a relation I∝R−1.1, less steep than typically observed in cataclysmic variables. Updated system parameters are also presented. 相似文献