全文获取类型
收费全文 | 51961篇 |
免费 | 779篇 |
国内免费 | 1150篇 |
专业分类
测绘学 | 2013篇 |
大气科学 | 4057篇 |
地球物理 | 10162篇 |
地质学 | 21137篇 |
海洋学 | 3304篇 |
天文学 | 7940篇 |
综合类 | 2212篇 |
自然地理 | 3065篇 |
出版年
2021年 | 224篇 |
2020年 | 280篇 |
2019年 | 282篇 |
2018年 | 5215篇 |
2017年 | 4502篇 |
2016年 | 3216篇 |
2015年 | 751篇 |
2014年 | 790篇 |
2013年 | 1389篇 |
2012年 | 1811篇 |
2011年 | 3806篇 |
2010年 | 2960篇 |
2009年 | 3574篇 |
2008年 | 2998篇 |
2007年 | 3421篇 |
2006年 | 1119篇 |
2005年 | 1094篇 |
2004年 | 1300篇 |
2003年 | 1271篇 |
2002年 | 1038篇 |
2001年 | 778篇 |
2000年 | 729篇 |
1999年 | 616篇 |
1998年 | 621篇 |
1997年 | 607篇 |
1996年 | 469篇 |
1995年 | 444篇 |
1994年 | 394篇 |
1993年 | 351篇 |
1992年 | 331篇 |
1991年 | 287篇 |
1990年 | 324篇 |
1989年 | 291篇 |
1988年 | 248篇 |
1987年 | 327篇 |
1986年 | 265篇 |
1985年 | 367篇 |
1984年 | 408篇 |
1983年 | 389篇 |
1982年 | 353篇 |
1981年 | 351篇 |
1980年 | 342篇 |
1979年 | 287篇 |
1978年 | 320篇 |
1977年 | 275篇 |
1976年 | 283篇 |
1975年 | 283篇 |
1974年 | 250篇 |
1973年 | 240篇 |
1972年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
Donald D. BOGARD Daniel H. GARRISON Jozef MASARIK 《Meteoritics & planetary science》2001,36(1):107-122
Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The 39Ar‐40Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of 40Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic 3He, 21Ne, and 38Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic 21Ne and 38Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of 36Ar in the halite were produced during regolith exposure by neutron capture on 35Cl, followed by decay to 36Ar. The thermal neutron fluence seen by the halite was (2–4) × 1014 n/cm2. The thermal neutron flux during regolith exposure was ~0.4‐0.7 n/cm2/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the 36Arn/21Necos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded. 相似文献
43.
44.
45.
APPLICATION OF ANALYTIC HIERARCHY PROCESS TO ASSESSING THE ECOLOGICAL VULNERABILITY OF WETLANDS IN THE JIANGHAN PLAIN 总被引:5,自引:1,他引:4
WANGXue-lei NINGLong-mei HUWang-bin 《中国地理科学(英文版)》2003,13(3):272-276
Wetlands in the Jianghan Plain are important components of wetland types in lake area in the middle and lower reaches the Changjiang (Yangtze) River and they fulfill many uses and functions related to hydrology,waste assimilation, ecosystem productivity and biodiversity. Owing to natural factors and human activities, especially excessive reclamation from lakes, the shrinking process of the lakes has been accelerated. Wetland ecosystem has shown the characteristics of vulnerability. According to the analysis of wetland ecological function in the Jianghan Plain, this paper presented an index system related to productivity, stability and environmental capacity. By using the method of Analytic Hierarchy Process, we computed the values of the relative weights of the indexes, and evaluated the vulnerability level of the wetland ecosystem by the method of multi-indexes. The case study showed that the fragile extent of wetland ecosystem in the Jianghan Plain is 5.6. This means that the wetland ecosystem in the Jiang-han Plain is laid to the state of middle vulnerability. Therefore, the wetland conservation and eco-rehabilitation in the JiangJaan Plain should be paid attention to. 相似文献
46.
We have derived the galaxy luminosity function in various regions of the cluster of galaxies Abell 496 from a wide field image
in the I band. A Schechter function fit in the 17≤ IAB ≤22(–19.5≤ MIAB ≤ –14.5) magnitude interval gives a steep power law index, which is somewhat steeper in the outer regions than in the inner
zones. This result agrees with previous findings obtained by several teams on the Coma cluster and can be interpreted as due
to the fact that faint galaxies are accreted by large ones in the central regions.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
47.
48.
N. Ivanova K. Belczynski J. M. Fregeau F. A. Rasio 《Monthly notices of the Royal Astronomical Society》2005,358(2):572-584
We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code ( startrack ) and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing three- and four-body interactions ( fewbody ). We find that the combination of stellar evolution and dynamical interactions (binary–single and binary–binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster such as 47 Tuc, assuming an initial binary fraction of 100 per cent, is only ∼ 5–10 per cent. We show that this is in good agreement with recent Hubble Space Telescope observations of close binaries in the core of 47 Tuc, provided that a realistic distribution of binary periods is used to interpret the results. Our findings also have important consequences for the dynamical modelling of globular clusters, suggesting that 'realistic models' should incorporate much larger initial binary fractions than has usually been the case in the past. 相似文献
49.
50.
B. Ahmed G. J. Alner H. Araujo J. C. Barton A. Bewick M. J. Carson D. Davidge J. V. Dawson T. Gamble S. P. Hart R. Hollingworth A. S. Howard W. G. Jones M. K. Joshi V. A. Kudryavtsev T. B. Lawson V. Lebedenko M. J. Lehner J. D. Lewin P. K. Lightfoot I. Liubarsky R. Lüscher J. E. McMillan B. Morgan G. Nicklin S. M. Paling R. M. Preece J. J. Quenby J. W. Roberts M. Robinson N. J. T. Smith P. F. Smith N. J. C. Spooner T. J. Sumner D. R. Tovey 《Astroparticle Physics》2003,19(6):691-702
The NAIAD experiment (NaI Advanced Detector) for weakly interacting massive particle (WIMP) dark matter searches at Boulby mine (UK) is described. The detector consists of an array of encapsulated and unencapsulated NaI(Tl) crystals with high light yield. Six crystals are collecting data at present. Data accumulated by four of them (10.6 kg × year exposure) have been used to set upper limits on the WIMP–nucleon spin-independent and WIMP–proton spin-dependent cross-sections. Pulse shape analysis has been applied to discriminate between nuclear recoils, as may be caused by WIMP interactions, and electron recoils due to gamma background. Various calibrations of crystals are presented. 相似文献