全文获取类型
收费全文 | 78885篇 |
免费 | 1399篇 |
国内免费 | 1875篇 |
专业分类
测绘学 | 2681篇 |
大气科学 | 5994篇 |
地球物理 | 15360篇 |
地质学 | 32562篇 |
海洋学 | 5908篇 |
天文学 | 13022篇 |
综合类 | 2382篇 |
自然地理 | 4250篇 |
出版年
2022年 | 290篇 |
2021年 | 493篇 |
2020年 | 479篇 |
2019年 | 538篇 |
2018年 | 7299篇 |
2017年 | 6588篇 |
2016年 | 4577篇 |
2015年 | 1082篇 |
2014年 | 1215篇 |
2013年 | 2090篇 |
2012年 | 3063篇 |
2011年 | 6168篇 |
2010年 | 5328篇 |
2009年 | 6010篇 |
2008年 | 4894篇 |
2007年 | 5880篇 |
2006年 | 1955篇 |
2005年 | 1986篇 |
2004年 | 1935篇 |
2003年 | 1880篇 |
2002年 | 1542篇 |
2001年 | 969篇 |
2000年 | 988篇 |
1999年 | 756篇 |
1998年 | 805篇 |
1997年 | 745篇 |
1996年 | 628篇 |
1995年 | 636篇 |
1994年 | 554篇 |
1993年 | 471篇 |
1992年 | 474篇 |
1991年 | 431篇 |
1990年 | 515篇 |
1989年 | 426篇 |
1988年 | 410篇 |
1987年 | 490篇 |
1986年 | 393篇 |
1985年 | 493篇 |
1984年 | 614篇 |
1983年 | 510篇 |
1982年 | 518篇 |
1981年 | 471篇 |
1980年 | 503篇 |
1979年 | 404篇 |
1978年 | 392篇 |
1977年 | 375篇 |
1976年 | 359篇 |
1975年 | 340篇 |
1974年 | 348篇 |
1973年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
James Etherington Witold Maciejewski 《Monthly notices of the Royal Astronomical Society》2006,367(3):1003-1010
If supermassive black holes in centres of galaxies form by merging of black hole remnants of massive Population III stars, then there should be a few black holes of mass one or two orders of magnitude smaller than that of the central ones, orbiting around the centre of a typical galaxy. These black holes constitute a weak perturbation in the gravitational potential, which can generate wave phenomena in gas within a disc close to the centre of the galaxy. Here, we show that a single orbiting black hole generates a three-arm spiral pattern in the central gaseous disc. The density excess in the spiral arms in the disc reaches values of 3–12 per cent when the orbiting black hole is about 10 times less massive than the central black hole. Therefore, the observed density pattern in gas can be used as a signature in detecting the most massive orbiting black holes. 相似文献
82.
Daniel P. Glavin Jason P. Dworkin Andrew Aubrey Oliver Botta James H. Doty Zita Martins Jeffrey L. Bada 《Meteoritics & planetary science》2006,41(6):889-902
Abstract— Amino acid analyses of the Antarctic CM2 chondrites Allan Hills (ALH) 83100 and Lewis Cliff (LEW) 90500 using liquid chromatography‐time of flight‐mass spectrometry (LC‐ToF‐MS) coupled with UV fluorescence detection revealed that these carbonaceous meteorites contain a suite of indigenous amino acids not present in Antarctic ice. Several amino acids were detected in ALH 83100, including glycine, alanine, β‐alanine, γ‐amino‐n‐butyric acid (γ‐ABA), and α‐aminoisobutyric acid (AIB) with concentrations ranging from 250 to 340 parts per billion (ppb). In contrast to ALH 83100, the CM2 meteorites LEW 90500 and Murchison had a much higher total abundance of these amino acids (440–3200 ppb). In addition, ALH 83100 was found to have lower abundances of the α‐dialkyl amino acids AIB and isovaline than LEW 90500 and Murchison. There are three possible explanations for the depleted amino acid content in ALH 83100: 1) amino acid leaching from ALH 83100 during exposure to Antarctic ice meltwater, 2) a higher degree of aqueous alteration on the ALH 83100 parent body, or 3) ALH 83100 originated on a chemically distinct parent body from the other two CM2 meteorites. The high relative abundance of ?‐amino‐n‐caproic acid (EACA) in the ALH 83100 meteorite as well as the Antarctic ice indicates that Nylon‐6 contamination from the Antarctic sample storage bags may have occurred during collection. 相似文献
83.
84.
85.
86.
We examine the potential contamination of cometary nuclei through impacts from asteroidal origin meteoroids. The paper uses
a simple model and has the goal of determining whether asteroidal contamination is potentially significant. We assume a meteoroid
power law mass distribution with index values in the range from s=1.83 to s=2.09. We used maximum and minimum models which we believe will bracket the true meteoroid mass distribution. We identify
those comets which are expected to be most significantly contaminated, and find values of up to 3.6 kg of asteroidal meteoroid
impact per square meter of the cometary surface per orbital revolution. This is less than the expected mass loss per perihelion
passage for most comets. Therefore any remnant effects of the contamination will depend on the penetration depth of the meteoroids
in the cometary nucleus, and possibly on the distribution of active and inactive areas on cometary nuclei. We present a simple
model which suggests that even small meteoroids will embed relatively deeply into a cometary nucleus. 相似文献
87.
88.
The experimental data obtained in transit observations of the extrasolar planet HD 209458b and their comparison with theoretical inferences have led to the conclusions that HD 209458b (and other similar hot jupiters) is of a (mainly) hydrogen nature and that these objects probably possess strong magnetic fields. The results of the studies of HD 209458b and prospects for searches for the transits of other extrasolar planets are considered in detail. 相似文献
89.
90.
ZHANGXue-qin GEQuan-sheng ZHENGJing-yun 《中国地理科学(英文版)》2003,13(3):207-215
How land-cover has been changed by human use over the last 300 years is one of the five overarching questions guiding the Land-use/Cover Change (LUCC) Science/Research Plan. China has variety of historical docu-ments providing unique data superiority. So the characteristics of farmland area in Shandong Province during the Qing Dynasty (1644-1911) are summarized firstly: 1) the rising trend of farmland area was striking; 2) farmland area had re-markable fluctuation; 3) farmland area per capita decreased dramatically; 4) wasteland reclamation index increased rapidly. Then, the driving forces of farmland area change are analyzed. It is concluded that natural and human factors are jointly influential. Among the driving forces, human dimensions are the main factors of farmland area change,which direct the general tendency of the changes mentioned above. And the natural factors influence the stability of farmland area as well. Variation of the natural factors would act as the major contributory factor to farmland area change during years or periods of abrupt climatic changes, or during the intensive occurring periods of natural hazards.Besides, the passive aspects of human factors, such as war chaos also influenced the fluctuation of the farmland area.This research indicates that it is feasible to study the land-use/cover change by Chinese historical literatures, which has huge potential to provide a comprehensive picture of the growing dominance of human land-use and land-cover pat-terns that can be used in many global change research oroiects. 相似文献