全文获取类型
收费全文 | 20525篇 |
免费 | 3141篇 |
国内免费 | 5970篇 |
专业分类
测绘学 | 516篇 |
大气科学 | 1861篇 |
地球物理 | 2761篇 |
地质学 | 14956篇 |
海洋学 | 1915篇 |
天文学 | 5278篇 |
综合类 | 955篇 |
自然地理 | 1394篇 |
出版年
2024年 | 81篇 |
2023年 | 220篇 |
2022年 | 510篇 |
2021年 | 614篇 |
2020年 | 665篇 |
2019年 | 925篇 |
2018年 | 679篇 |
2017年 | 653篇 |
2016年 | 804篇 |
2015年 | 812篇 |
2014年 | 1211篇 |
2013年 | 1277篇 |
2012年 | 1284篇 |
2011年 | 1456篇 |
2010年 | 1379篇 |
2009年 | 1818篇 |
2008年 | 1684篇 |
2007年 | 1721篇 |
2006年 | 1588篇 |
2005年 | 1379篇 |
2004年 | 1222篇 |
2003年 | 1059篇 |
2002年 | 920篇 |
2001年 | 812篇 |
2000年 | 758篇 |
1999年 | 719篇 |
1998年 | 642篇 |
1997年 | 450篇 |
1996年 | 403篇 |
1995年 | 298篇 |
1994年 | 297篇 |
1993年 | 267篇 |
1992年 | 202篇 |
1991年 | 178篇 |
1990年 | 120篇 |
1989年 | 107篇 |
1988年 | 105篇 |
1987年 | 64篇 |
1986年 | 48篇 |
1985年 | 49篇 |
1984年 | 38篇 |
1983年 | 20篇 |
1982年 | 22篇 |
1981年 | 8篇 |
1980年 | 16篇 |
1979年 | 4篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1973年 | 3篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
141.
We present predictions for the numbers of ultracool dwarfs in the Galactic disc population that could be detected by the WFCAM/UKIDSS Large Area Survey and Ultra Deep Survey. Simulated samples of objects are created with masses and ages drawn from different mass functions and birthrates. Each object is then given absolute magnitudes in different passbands based on empirically derived bolometric correction versus effective temperature relationships (or model predictions for Y dwarfs). These are then combined with simulated space positions, velocities and photometric errors to yield observables such as apparent magnitudes and proper motions. Such observables are then passed through the survey selection mechanism to yield histograms in colour. This technique also produces predictions for the proper motion histograms for ultracool dwarfs and estimated numbers for the as yet undetected Y dwarfs. Finally, it is shown that these techniques could be used to constrain the ultra-low-mass mass function and birthrate of the Galactic disc population. 相似文献
142.
143.
144.
145.
146.
We present a 2D general circulation model of Titan's atmosphere, coupling axisymmetric dynamics with haze microphysics, a simplified photochemistry and eddy mixing. We develop a parameterization of latitudinal eddy mixing by barotropic waves based on a shallow-water, longitude-latitude model. The parameterization acts locally and in real time both on passive tracers and momentum. The mixing coefficient varies exponentially with a measure of the barotropic instability of the mean zonal flow. The coupled GCM approximately reproduces the Voyager temperature measurements and the latitudinal contrasts in the distributions of HCN and C2H2, as well as the main features of the zonal wind retrieved from the 1989 stellar occultation. Wind velocities are consistent with the observed reversal time of the North-South albedo asymmetry of 5 terrestrial years. Model results support the hypothesis of a non-uniform distribution of infrared opacity as the cause of the Voyager temperature asymmetry. Transport by the mean meridional circulation, combined with polar vortex isolation may be at the origin of the latitudinal contrasts of trace species, with eddy mixing remaining restricted to low latitudes most of the Titan year. We interpret the contrasts as a signature of non-axisymmetric motions. 相似文献
147.
黑色高岭石粘土岩,是铜川王石凹煤矿石炭纪煤系地层顶部,五号煤层的上部一层稳定的煤矸石。经偏光显微镜及费氏台测试光性特征、化学试验及分析鉴定表明,这是一种由高岭石组成的,成分很纯、颗粒大、结晶良好的高岭石粘土岩。据微量元素含量分析,此高岭石系淡水沉积物。 相似文献
148.
J. P. Bernstein † Suketu P. Bhavsar ‡ 《Monthly notices of the Royal Astronomical Society》2001,322(3):625-630
The brightest, or first-ranked, galaxies (BCGs) in rich clusters show a very small dispersion in luminosity, making them excellent standard candles. This small dispersion raises questions about the nature of BCGs. Are they simply the extremes of normal galaxies formed via a stochastic process, or do they belong to a special class of atypical objects? If they do, are all BCGs special, or do normal galaxies compete for the first rank? To answer these questions, we undertake a statistical study of BCG magnitudes using results from extreme value theory. Two-population models do better than one-population models. A simple scenario where a random boost in the magnitude of a fraction of bright normal galaxies forms a class of atypical galaxies best describes the observed distribution of BCG magnitudes. 相似文献
149.
150.
N. Kameswara Rao V. R. Venugopal A. Patnaik 《Journal of Astrophysics and Astronomy》1985,6(3):153-157
VLA observations at 2 and 6 cm have been obtained for six hydrogen-deficient stars R CrB, HD 160641, BD — 9°4395, V348 Sgr,
MV Sgr and Sgrv Upper limits to the massloss rates have been estimated for some of these using the upper limits to the radio flux density.
National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National
Science Foundation, USA 相似文献