全文获取类型
收费全文 | 15979篇 |
免费 | 519篇 |
国内免费 | 78篇 |
专业分类
测绘学 | 461篇 |
大气科学 | 788篇 |
地球物理 | 4135篇 |
地质学 | 4743篇 |
海洋学 | 1774篇 |
天文学 | 3740篇 |
综合类 | 44篇 |
自然地理 | 891篇 |
出版年
2023年 | 37篇 |
2021年 | 80篇 |
2020年 | 199篇 |
2019年 | 189篇 |
2018年 | 426篇 |
2017年 | 735篇 |
2016年 | 724篇 |
2015年 | 552篇 |
2014年 | 597篇 |
2013年 | 841篇 |
2012年 | 509篇 |
2011年 | 752篇 |
2010年 | 640篇 |
2009年 | 1006篇 |
2008年 | 851篇 |
2007年 | 863篇 |
2006年 | 738篇 |
2005年 | 711篇 |
2004年 | 586篇 |
2003年 | 592篇 |
2002年 | 561篇 |
2001年 | 462篇 |
2000年 | 430篇 |
1999年 | 524篇 |
1998年 | 313篇 |
1997年 | 198篇 |
1996年 | 166篇 |
1995年 | 183篇 |
1994年 | 131篇 |
1993年 | 122篇 |
1992年 | 124篇 |
1991年 | 78篇 |
1990年 | 98篇 |
1989年 | 88篇 |
1988年 | 98篇 |
1987年 | 166篇 |
1986年 | 56篇 |
1985年 | 71篇 |
1984年 | 123篇 |
1983年 | 57篇 |
1982年 | 103篇 |
1981年 | 87篇 |
1980年 | 99篇 |
1979年 | 67篇 |
1978年 | 87篇 |
1977年 | 65篇 |
1976年 | 54篇 |
1974年 | 36篇 |
1973年 | 43篇 |
1971年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
《New Astronomy》2021
The stellar halo contains some of the oldest stars in the Milky Way galaxy and in the universe. The detections of ‘Oumuamua, CNEOS 2014-01-08, and interstellar dust serve to calibrate the production rate of interstellar objects. We study the feasibility of a search for interstellar meteors with origins in the stellar halo. We find the mean heliocentric impact speed for halo meteors to be and the standard deviation is making the population kinematically distinct from all other meteors, which are an order-of-magnitude slower. We explore the expected abundance of halo meteors, finding that a network of all-sky cameras covering all land on Earth can take spectra and determine the orbits of a few hundred halo meteors larger than a few mm per year. The compositions of halo meteors would provide information on the characteristics of planetary system formation for the oldest stars. In addition, one could place tight constraints on baryonic dark matter objects of low masses. 相似文献
2.
Assessment of alternative adsorption models and global sensitivity analysis to characterize hexavalent chromium loss from soil to surface runoff
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We investigate our ability to assess transfer of hexavalent chromium, Cr(VI), from the soil to surface runoff by considering the effect of coupling diverse adsorption models with a two‐layer solute transfer model. Our analyses are grounded on a set of two experiments associated with soils characterized by diverse particle size distributions. Our study is motivated by the observation that Cr(VI) is receiving much attention for the assessment of environmental risks due to its high solubility, mobility, and toxicological significance. Adsorption of Cr(VI) is considered to be at equilibrium in the mixing layer under our experimental conditions. Four adsorption models, that is, the Langmuir, Freundlich, Temkin, and linear models, constitute our set of alternative (competing) mathematical formulations. Experimental results reveal that the soil samples characterized by the finest grain sizes are associated with the highest release of Cr(VI) to runoff. We compare the relative abilities of the four models to interpret experimental results through maximum likelihood model calibration and four model identification criteria (i.e., the Akaike information criteria [AIC and AICC] and the Bayesian and Kashyap information criteria). Our study results enable us to rank the tested models on the basis of a set of posterior weights assigned to each of them. A classical variance‐based global sensitivity analysis is then performed to assess the relative importance of the uncertain parameters associated with each of the models considered, within subregions of the parameter space. In this context, the modelling strategy resulting from coupling the Langmuir isotherm with a two‐layer solute transfer model is then evaluated as the most skilful for the overall interpretation of both sets of experiments. Our results document that (a) the depth of the mixing layer is the most influential factor for all models tested, with the exception of the Freundlich isotherm, and (b) the total sensitivity of the adsorption parameters varies in time, with a trend to increase as time progresses for all of the models. These results suggest that adsorption has a significant effect on the uncertainty associated with the release of Cr(VI) from the soil to the surface runoff component. 相似文献
3.
4.
5.
6.
7.
Roberto F. Viotti Lucio Angelo Antonelli Sonja Rebecchi Corinne Rossi 《Journal of Astrophysics and Astronomy》2002,23(1-2):19-22
We have investigated with BeppoSAX the long term behaviour of the harder X-ray component of the supposed supermassive binary
system η Car along its 5.52 year cycle. We have found that in March 1998 during egress from the last December 1997 eclipse,
this component was the same as outside eclipse, but for a large (×3.5) increase of NH
h
, that can be attributed to the presence or formation of opaque matter in front of the source near periastron. Unexpectedly,
at that time the iron 6.7 keV emission line was 40% stronger. BeppoSAX has for the first time found ahard X-ray tail extending to at least 50 keV, that cannot be adequately fitted with an additional hotter thermal component. The 2–100 keV
spectrum of η Car is instead well fitted with an absorbed powerlaw spectrum with photon index 2.53, suggesting non-thermal
emission as an alternative model for the core source. 相似文献
8.
9.
10.