全文获取类型
收费全文 | 48693篇 |
免费 | 7430篇 |
国内免费 | 11258篇 |
专业分类
测绘学 | 4569篇 |
大气科学 | 5430篇 |
地球物理 | 11275篇 |
地质学 | 25859篇 |
海洋学 | 5349篇 |
天文学 | 6328篇 |
综合类 | 3097篇 |
自然地理 | 5474篇 |
出版年
2024年 | 146篇 |
2023年 | 509篇 |
2022年 | 1317篇 |
2021年 | 1656篇 |
2020年 | 1822篇 |
2019年 | 2345篇 |
2018年 | 1721篇 |
2017年 | 2062篇 |
2016年 | 2104篇 |
2015年 | 2280篇 |
2014年 | 3013篇 |
2013年 | 3232篇 |
2012年 | 3125篇 |
2011年 | 3363篇 |
2010年 | 2834篇 |
2009年 | 3552篇 |
2008年 | 3437篇 |
2007年 | 3588篇 |
2006年 | 3503篇 |
2005年 | 2890篇 |
2004年 | 2664篇 |
2003年 | 2283篇 |
2002年 | 1787篇 |
2001年 | 1680篇 |
2000年 | 1497篇 |
1999年 | 1408篇 |
1998年 | 1368篇 |
1997年 | 1050篇 |
1996年 | 897篇 |
1995年 | 792篇 |
1994年 | 677篇 |
1993年 | 618篇 |
1992年 | 455篇 |
1991年 | 324篇 |
1990年 | 246篇 |
1989年 | 264篇 |
1988年 | 197篇 |
1987年 | 114篇 |
1986年 | 88篇 |
1985年 | 93篇 |
1984年 | 61篇 |
1983年 | 28篇 |
1982年 | 45篇 |
1981年 | 29篇 |
1980年 | 40篇 |
1979年 | 17篇 |
1978年 | 22篇 |
1977年 | 36篇 |
1976年 | 27篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
为了便于大型软件系统的后续开发和用户软件系统的升级,介绍了软件系统在Internet/Intranet上更新功能或数据的原理和方法,并用C#.Net进行了编程实现。 相似文献
33.
34.
MohammedHazaea HUKe MohammedMattash CHENBing M.O.AI-JabaliAref MohammedAbdussalam 《东北亚地学研究》2004,7(2):181-184
The chemical composition of several thermal springs around Changbaishan area has been investigated. Cenozoic basaltic rocks are widely distributed in Northeast China and geothermal characteristics have been described. About one hundred hot springs exist around Changhaishan Volcano at the border between China and D.P.R.Korea with high temperature about 82~C. The pH values of the spring water range from 6.9 to 7.1 and the total flow rate is about 4.8 L/sec. The chemical composition of the thermal springs is sodium carbonate; the high-mineral contents of thermal water are believed to have medicinal properties. Bathhouses are already built along the hot springs to take the advantage of the supposed healing properties. The high quality of those hot springs is believed to be utilitized for mineral water. The chemical equilibrium temperatures were estimated at about 160℃ based on the Na-K-Ca geochemical thermometer. 相似文献
35.
薛建康 《成都信息工程学院学报》1990,(1)
本文针对冬半年高原低涡几个不同源地的低涡生成背景,着重分析500hpa环流场,指出:地形对低涡生成是非常重要的条件,但一定范围内有利的环境流场也是低涡生成不可缺少的外部条件。并给出了不同涡源区有利于低涡发生,发展的500hpa环流形势。 相似文献
36.
Majid T. Manzari 《国际地质力学数值与分析法杂志》2004,28(10):1011-1032
A micropolar elastoplastic model for soils is formulated and a series of finite element analyses are employed to demonstrate the use of a micropolar continuum in overcoming the numerical difficulties encountered in application of finite element method in standard Cauchy–Boltzmann continuum. Three examples of failure analysis involving a deep excavation, shallow foundation, and a retaining wall are presented. In all these cases, it is observed that the length scale introduced in the polar continuum regularizes the incremental boundary value problem and allows the numerical simulation to be continued until a clear collapse mechanism is achieved. The issue of grain size effect is also discussed. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
37.
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
38.
Optimal deflection of NEOs en route of collision with the Earth 总被引:1,自引:0,他引:1
Recently, a method for the n-body analysis of the velocity change required to deflect a hazardous near-Earth object (NEO) was presented by Carusi et al. [Carusi, A., Valsecchi, G.B., D'Abramo, G., Boattini A., 2002. Icarus 159, 417-422]. We extent this method in order to optimize the velocity change vector instead of its along-track magnitude. From an application of both methods to a fictitious NEO we find Carusi's parallel approach to be reasonable for phases of unperturbed two-body motion. But, for orbit phases inhering third-body perturbations, i.e., for planetary close approaches or prior to a collision, the results obtained from the new method show the radial component of deflection impulse to play a major role. We show that a fivefold greater efficiency can be achieved by a deflection impulse being non-parallel to orbital velocity. The new method is applied to two possible 99942 Apophis impact trajectories in order to provide constraints for future Apophis deflection mission analysis. 相似文献
39.
40.