全文获取类型
收费全文 | 132798篇 |
免费 | 6876篇 |
国内免费 | 7285篇 |
专业分类
测绘学 | 3985篇 |
大气科学 | 13809篇 |
地球物理 | 28362篇 |
地质学 | 49927篇 |
海洋学 | 12381篇 |
天文学 | 25470篇 |
综合类 | 3335篇 |
自然地理 | 9690篇 |
出版年
2022年 | 1622篇 |
2021年 | 2219篇 |
2020年 | 2136篇 |
2019年 | 2289篇 |
2018年 | 3756篇 |
2017年 | 3525篇 |
2016年 | 4478篇 |
2015年 | 3007篇 |
2014年 | 4451篇 |
2013年 | 6993篇 |
2012年 | 4314篇 |
2011年 | 5441篇 |
2010年 | 5051篇 |
2009年 | 6285篇 |
2008年 | 5610篇 |
2007年 | 5378篇 |
2006年 | 5048篇 |
2005年 | 4069篇 |
2004年 | 4078篇 |
2003年 | 3904篇 |
2002年 | 3689篇 |
2001年 | 3320篇 |
2000年 | 3484篇 |
1999年 | 3551篇 |
1998年 | 3317篇 |
1997年 | 3221篇 |
1996年 | 2879篇 |
1995年 | 2629篇 |
1994年 | 2401篇 |
1993年 | 2144篇 |
1992年 | 1930篇 |
1991年 | 1709篇 |
1990年 | 1781篇 |
1989年 | 1532篇 |
1988年 | 1423篇 |
1987年 | 1517篇 |
1986年 | 1341篇 |
1985年 | 1597篇 |
1984年 | 1766篇 |
1983年 | 1607篇 |
1982年 | 1522篇 |
1981年 | 1416篇 |
1980年 | 1240篇 |
1979年 | 1217篇 |
1978年 | 1244篇 |
1977年 | 1080篇 |
1976年 | 1038篇 |
1975年 | 988篇 |
1974年 | 944篇 |
1973年 | 982篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
1.
2.
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
3.
文中所作的主要工作是为云南天文台十米抛物面设计、制造一付好的对数周期偶极子天线(LPDA)作馈源。工作有:(1)设计一对交叉的LP-DA;(2)分析LPDA特性;(3)估算抛物面的电特性;(4)讨论线极化向园极化转换的问题;(5)天线测量。LPDA的测量结果比较满意。该天线的方向图、平均输入阻抗、三分贝和十分贝波瓣宽度都在从0.5-1.5GHz的频率范围内比较一致和稳定,变化很小。天线理论上的许多成就都是与数子计算机有关的。在文中,我们充分利用了计算机的功能来设计天线,分析天线特性,并从测量所得的所有数据文件中计算天线的参数。所编FORTRAN程序使这一切工作变得容易。最后,我们还讨论了用功率合成器实现线极化向园极化转换的各种情况 相似文献
4.
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
5.
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
6.
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
7.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
8.
9.
J. Raitala 《Earth, Moon, and Planets》1996,74(3):191-214
The intrablock deformation of Meshkenet Tessera on Venus is mostly due to responses of the uppermost surface bedrock to tensional stresses. It is found that complex deformation structures within the highland blocks resemble those of formed in chocolate tablet boudinaging which has taken place after original parallel faulting and bar-like crustal block formation. The high-angle tessera structures with varying cross-cutting relations define styles and locations of multiphase deformation most evidently related to local relaxation of tessera topography. Series of progressive or superposed fracturing events with alternating fault directions took place at high angles during this relaxational deformation. Compressional ridges often surround these tesserae. 相似文献
10.