全文获取类型
收费全文 | 10653篇 |
免费 | 2302篇 |
国内免费 | 3295篇 |
专业分类
测绘学 | 1248篇 |
大气科学 | 2002篇 |
地球物理 | 2526篇 |
地质学 | 6188篇 |
海洋学 | 1770篇 |
天文学 | 312篇 |
综合类 | 1023篇 |
自然地理 | 1181篇 |
出版年
2024年 | 55篇 |
2023年 | 195篇 |
2022年 | 552篇 |
2021年 | 655篇 |
2020年 | 566篇 |
2019年 | 632篇 |
2018年 | 625篇 |
2017年 | 626篇 |
2016年 | 673篇 |
2015年 | 663篇 |
2014年 | 738篇 |
2013年 | 742篇 |
2012年 | 810篇 |
2011年 | 815篇 |
2010年 | 761篇 |
2009年 | 706篇 |
2008年 | 722篇 |
2007年 | 692篇 |
2006年 | 618篇 |
2005年 | 522篇 |
2004年 | 415篇 |
2003年 | 350篇 |
2002年 | 308篇 |
2001年 | 284篇 |
2000年 | 310篇 |
1999年 | 337篇 |
1998年 | 263篇 |
1997年 | 280篇 |
1996年 | 210篇 |
1995年 | 224篇 |
1994年 | 172篇 |
1993年 | 149篇 |
1992年 | 111篇 |
1991年 | 106篇 |
1990年 | 71篇 |
1989年 | 66篇 |
1988年 | 48篇 |
1987年 | 37篇 |
1986年 | 37篇 |
1985年 | 20篇 |
1984年 | 11篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 15篇 |
1980年 | 6篇 |
1979年 | 11篇 |
1978年 | 3篇 |
1958年 | 5篇 |
1957年 | 2篇 |
1954年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
21.
Zheng Xing-wu 《Chinese Astronomy and Astrophysics》1994,18(4):443-449
This paper presents observations of OH maser lines of W 33A for the transitions 2Π3/2, J = 3/2, F = 1 → 1 and F = 2 → 2. Two models, a thin tube and a sphere, were used for modelling the masing region and a molecular hydrogen density of about 107 cm−3 was obtained. To give a maser photon emission of the order of 1046 s−1, both models require a pump rate of 1 OH cm−3s−1, while the sphere model requires a higher pump efficiency. 相似文献
22.
23.
介绍了近年来河外H2O超脉泽的主要观测结果。H2O超脉泽通常起源于活动星系核中央的拱核盘。它们主要寄生在 Serfert 2星系或低电离核区。至今为止,已有20个星系探测到H2O超脉泽。脉泽辐射的各向同性光度为10~6000L⊙。所有超脉泽星系显示出核的活动,显然,脉泽是由核活动所产生的射电和X射线光子或激波来抽运的。H2O超脉泽倾向存在于高倾斜度的星系,这使得沿视线上的分子柱密度增高,产生足够大的放大光深。最有可能产生H2O超脉泽辐射的星系应有一个包含着射电源的侧向的分子盘以及一个适当的抽运机制。 相似文献
24.
25.
CORROSION AND ADHERENCE AT SEA OIL-PLATFORM 总被引:1,自引:0,他引:1
Measurements of the adhesive corrosion and protective potentials of 3 Bohai Sea oilplatforms under different cathodic protection showed corrosion still occurred on these platforms undertheir designed protective potentials. Preliminary conclusions from indoor experiments on the relation betweenadhesive corrosion and cathodic protection showed adhering organisms caused the corrosion onthese platforms. 相似文献
26.
Four bromophenols were isolated from the extract of marine red alga,Rhodomela confervoides by column chromatography and HPLC methods. By means of spectroscopic methods inclding IR, MS, 1D, and 2D-NMR techniques,
their structures were elucidated as (1) 3-bromo-4,5-dihydroxy benzoic acid methyl ester; (2) bis (2,3-dibromo-4,5-dihydroxybenzyl)
ether; (3) 3,4-dibromo-5-(methoxymethyl)-1,2-benzenediol and (4) 3-bromo-4,5-dihydroxy-benzaldehyde. Compound 1 was first
isolated from the algae in nature, and 1, 4 were found to have selective cytotoxic activities against KB, Bel 7402 and A549
cells, 2 showed powerful antibacterial activities againstStaphylococcus aurens andPseudomonas aeruginosa.
Supported by National “863” Program (No. 2004AA625030, 2001AA620503), Quingdao marine sciences project (No. 04-2-NN-26) and
Key Knowledge Innovative Project of the Chinese Academy of Sciences (KZCX3-SW-215). 相似文献
27.
Zheng Jianyuan 《中国海洋湖沼学报》1986,4(3):304-312
The present paper is an introduction of a universal model of cluster and discrimination in the field of fisheries science.
The mathematical principle and algorithm are described and the application results analysed.
This model is called “weighted generalized distance” method, based on the principle of “minimum distance”. For different types
of data, different methods are put forward. The quantitative attributes are standarized by “norm” method and the qualitative
attributes are quantified by indicative value based on Shannon's information theory to locate the specimen in the coordinates.
An approach by progressive readjustment of pivotal points is employed for the cluster rational. Finally, any new specimen
can be discriminated by the principle of “minimum distance”, too. 相似文献
28.
Univariate and multivariate stress release models are fitted to historical earthquake data from North China. It is shown that a better fit is obtained by treating separately the Eastern part of the region, including the North China Plain and Bohai Sea, and the Western part of the region, including the Ordos Plateau and its Eastern boundary. Further improvement is obtained by fitting the large events (M7.6) and smaller events in the Western region by different stress release models. The comparisons are made by computing the likelihoods of the fitted models and discounting the number of parameters used by Akaike's AIC criterion. The models are used to develop long-term risk scenarios for the East and West regions. 相似文献
29.
遗迹化石在层序地层学研究中的应用 总被引:2,自引:0,他引:2
以济阳坳陷古近系沙河街组和塔里木盆地志留系为例,探讨了遗迹化石与层序地层学的关系,表现在3个方面:1)层序界面的识别层序界面位置发育有特征性的受基底控制的遗迹相,且界面上下遗迹组合类型发生变化;2)准层序界面的识别较深水环境中,界面上下生物扰动强度明显差异,界面之下扰动强度较大,扰动指数可达到3,而界面之上几乎没有生物扰动,且在同一准层序内部,由下至上,扰动强度逐渐增大;3)旋回层序的识别:在潮控滨线环境中,单个准层序内部,由下至上,高能环境的Skolithos遗迹组合逐渐变为中低能环境的Planolites-Palaeophycus遗迹组合和低能环境的Cochlichnus-Planolites遗迹组合,由下至上呈现旋回性变化特征。 相似文献
30.