全文获取类型
收费全文 | 34122篇 |
免费 | 4592篇 |
国内免费 | 5864篇 |
专业分类
测绘学 | 6767篇 |
大气科学 | 4209篇 |
地球物理 | 3696篇 |
地质学 | 14236篇 |
海洋学 | 4051篇 |
天文学 | 619篇 |
综合类 | 3221篇 |
自然地理 | 7779篇 |
出版年
2024年 | 369篇 |
2023年 | 1218篇 |
2022年 | 1508篇 |
2021年 | 1789篇 |
2020年 | 1329篇 |
2019年 | 1676篇 |
2018年 | 1094篇 |
2017年 | 1236篇 |
2016年 | 1297篇 |
2015年 | 1522篇 |
2014年 | 2375篇 |
2013年 | 1771篇 |
2012年 | 2035篇 |
2011年 | 2039篇 |
2010年 | 1880篇 |
2009年 | 1950篇 |
2008年 | 2104篇 |
2007年 | 1830篇 |
2006年 | 1606篇 |
2005年 | 1533篇 |
2004年 | 1281篇 |
2003年 | 1328篇 |
2002年 | 1240篇 |
2001年 | 1064篇 |
2000年 | 887篇 |
1999年 | 731篇 |
1998年 | 726篇 |
1997年 | 684篇 |
1996年 | 693篇 |
1995年 | 591篇 |
1994年 | 563篇 |
1993年 | 461篇 |
1992年 | 491篇 |
1991年 | 441篇 |
1990年 | 418篇 |
1989年 | 317篇 |
1988年 | 96篇 |
1987年 | 84篇 |
1986年 | 54篇 |
1985年 | 57篇 |
1984年 | 35篇 |
1983年 | 30篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 21篇 |
1979年 | 7篇 |
1977年 | 7篇 |
1964年 | 10篇 |
1950年 | 5篇 |
1941年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 225 毫秒
41.
徐江 《地球信息科学学报》1996,(1):21-25
本文阐述了地理信息系统在环境地球化学中的应用,提出建立区域环境地球化学空间信息系统的方法、功能及组成,并以红枫湖区域环境地球化学空间信息系统为例,作了分析。 相似文献
42.
The effect of light intensity ( 1500 Ix and 5000 Ix) on the total lipid and fatty acid composition of six strains of marine diatoms Cylindrotheca fusiformis (B211 ), Phaeodactylum tricornutum (Bl14, Bl18 and B221) Nitzschia closterium (B222) and Chaetoceros gracilis (B13) was investigated.The total lipids of B13, Bl14, and B211 grown at 5000 Ix were lower than those grown at 1500 Ix. No evident changes were observed in Bl18, B221 and B222. Fatty acid composition changed considerably at different light intensity although no consistent correlation between the relative proportion of a single FA and light intensity. The major fatty acids of the 6 strains were 14:0, 16:0, 16:l(n- 7) and 20:5(n-3) . Cylindrothecafusiformis had high percentage of 20:4n - 6 (9.2 - 10.9% ) . The total polyunsaturated fatty acid /rl all 6 strains decreased with increasing light intensity. The percentage of the highly unsaturated fatty acid eicosapentaenoic acid (EPA) decreased with increasing light intensity in all strains except Chaetoceros gracilis. 相似文献
43.
吴泽忠 《成都信息工程学院学报》2004,19(2):258-262
在序线性拓扑空间里研究了含有集约束向量极值问题的最优性条件,并建立了充分性和必要性条件. 相似文献
44.
45.
Xiao-Hong Cui En-Wei Liang Rui-Jing Lu National Astronomical Observatories/Yunnan Observatory Chinese Academy of Sciences Kunming ciwei@.com Department of Physics Guangxi University Nanning Department of Astronomy Nanjing University Nanjing Graduate School of the Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(2):151-158
The narrowness of the distribution of the peak energy of the vFv spectrum of gamma-ray bursts (GRBs) and the unification of GRB populations are great puzzles yet to be solved. We investigate the two puzzles based on the global spectral behaviors of different GRB populations, the long GRBs, the short GRBs, and the X-ray flashes (XRFs), in the HR - Ep plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It is found that the long GRBs and the XRFs observed by HETE-2 seem to follow the same sequence in the HR-Ep plane, with the XRFs at the low end of this sequence. We fit the sequence by a universal Band function, and find that this sequence is mainly defined by the low energy index α, and is insensitive to the high energy index,β. With fixed β=-5, a best fit is given by α=-1.00 with X^2min/dof=2.2. The long and short GRBs observed by BATSE follow significantly different sequences in the HR-Ep plane, with most of the short GRBs having a larger hardness ratio than the long GRBs at a given Ep. For the long GRBs a best-fit yields α=-0.30 and β=-2.05. For the short GRBs, a best fit gives α=-0.60 with x^2min=1.1 (withβfixed at-2.0 because it is numerically unstable). The α value for the short GRBs is significantly greater than that for the long GRBs. These results indicate that the global spectral behaviors of the long GRB sample and the XRF sample are similar, while that of the short GRBs is different. The short GRBs seem to be a unique subclass of GRBs, and they are not the higher energy extension of the long GRBs. 相似文献
46.
47.
48.
49.
50.