全文获取类型
收费全文 | 130402篇 |
免费 | 2111篇 |
国内免费 | 1050篇 |
专业分类
测绘学 | 3042篇 |
大气科学 | 8826篇 |
地球物理 | 25817篇 |
地质学 | 46564篇 |
海洋学 | 11826篇 |
天文学 | 29446篇 |
综合类 | 375篇 |
自然地理 | 7667篇 |
出版年
2022年 | 789篇 |
2021年 | 1408篇 |
2020年 | 1540篇 |
2019年 | 1666篇 |
2018年 | 3522篇 |
2017年 | 3297篇 |
2016年 | 4019篇 |
2015年 | 2194篇 |
2014年 | 3891篇 |
2013年 | 6923篇 |
2012年 | 4164篇 |
2011年 | 5543篇 |
2010年 | 4881篇 |
2009年 | 6288篇 |
2008年 | 5664篇 |
2007年 | 5627篇 |
2006年 | 5193篇 |
2005年 | 3954篇 |
2004年 | 3838篇 |
2003年 | 3615篇 |
2002年 | 3447篇 |
2001年 | 3117篇 |
2000年 | 3017篇 |
1999年 | 2430篇 |
1998年 | 2550篇 |
1997年 | 2353篇 |
1996年 | 2026篇 |
1995年 | 2022篇 |
1994年 | 1725篇 |
1993年 | 1612篇 |
1992年 | 1537篇 |
1991年 | 1484篇 |
1990年 | 1563篇 |
1989年 | 1348篇 |
1988年 | 1227篇 |
1987年 | 1478篇 |
1986年 | 1274篇 |
1985年 | 1570篇 |
1984年 | 1757篇 |
1983年 | 1682篇 |
1982年 | 1576篇 |
1981年 | 1418篇 |
1980年 | 1305篇 |
1979年 | 1238篇 |
1978年 | 1201篇 |
1977年 | 1050篇 |
1976年 | 1033篇 |
1975年 | 985篇 |
1974年 | 989篇 |
1973年 | 1030篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Abstract— Active capture is a new process for the incorporation of large quantities of heavy noble gases into growing surfaces. Adsorption in the conventional sense involves surface bonding by polarization (Van der Waals forces). What is referred to as “anomalous adsorption” of heavy noble gases involves chemical bonds and can occur when other (more chemically active) species are not available to preempt sites with unfilled bonds. Anomalous adsorption has been observed under conditions of fracture, vacuum deposition and ionizing radiation. Active capture depends upon anomalous adsorption to retain noble gases on a surface long enough to be captured in a growing surface film as it is deposited. The fundamental principle may be the impingement onto the growing film with sufficient energy to liberate surface electrons (work function energy of a few electronvolts) so that they are retained by anomalous adsorption long enough to be entrapped in the growing surface. Trapping efficiencies of ?1% have been observed for Kr and Xe in laboratory experiments, implying a fundamentally new mechanism for the incorporation of heavy noble gases onto surfaces. It may play a role in explaining the large concentrations of planetary noble gases contained in phase‐Q. 相似文献
2.
We present our observations of the galaxy UGS 5600 with a long-slit spectrograph (UAGS) and a multipupil field spectrograph (MPFS) attached to the 6-m Special Astrophysical Observatory telescope. Radial-velocity fields of the stellar and gaseous components were constructed for the central region and inner ring of the galaxy. We proved the existence of two nearly orthogonal kinematic subsystems and conclude that UGC 5600 is a galaxy with an inner polar ring. In the circumnuclear region, we detected noncircular stellar motions and suspected the existence of a minibar. The emission lines are shown to originate in H II regions. We estimated the metallicity from the intensity ratio of the [N II]λ6583 and Hα lines to be nearly solar, which rules out the possibility that the polar ring was produced by the accretion of gas from a dwarf companion. 相似文献
3.
4.
5.
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
6.
7.
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
8.
9.
10.
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