全文获取类型
收费全文 | 21273篇 |
免费 | 4857篇 |
国内免费 | 7275篇 |
专业分类
测绘学 | 3787篇 |
大气科学 | 3363篇 |
地球物理 | 4237篇 |
地质学 | 12883篇 |
海洋学 | 3832篇 |
天文学 | 506篇 |
综合类 | 1777篇 |
自然地理 | 3020篇 |
出版年
2024年 | 184篇 |
2023年 | 436篇 |
2022年 | 1204篇 |
2021年 | 1526篇 |
2020年 | 1204篇 |
2019年 | 1347篇 |
2018年 | 1235篇 |
2017年 | 1228篇 |
2016年 | 1267篇 |
2015年 | 1390篇 |
2014年 | 1375篇 |
2013年 | 1680篇 |
2012年 | 1789篇 |
2011年 | 1818篇 |
2010年 | 1698篇 |
2009年 | 1577篇 |
2008年 | 1737篇 |
2007年 | 1600篇 |
2006年 | 1620篇 |
2005年 | 1317篇 |
2004年 | 1041篇 |
2003年 | 747篇 |
2002年 | 758篇 |
2001年 | 701篇 |
2000年 | 598篇 |
1999年 | 462篇 |
1998年 | 269篇 |
1997年 | 252篇 |
1996年 | 204篇 |
1995年 | 201篇 |
1994年 | 167篇 |
1993年 | 135篇 |
1992年 | 121篇 |
1991年 | 91篇 |
1990年 | 85篇 |
1989年 | 64篇 |
1988年 | 43篇 |
1987年 | 43篇 |
1986年 | 40篇 |
1985年 | 26篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 17篇 |
1980年 | 7篇 |
1979年 | 14篇 |
1978年 | 5篇 |
1958年 | 4篇 |
1957年 | 5篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
32.
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
33.
低频扩频中伪随机序列产生方法的研究 总被引:2,自引:0,他引:2
伪随机序列的产生是实现BPC低频时码发播台扩展频谱技术升级的一个重要环节。针对BPC低频时码发播台的现有系统,提出了伪随机序列的产生方案,并对其性能作了分析。 相似文献
34.
35.
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
36.
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
37.
为了给公路设计部门提供选线依据,运用瞬变电磁法在山西境内的某两条高速公路隧道工程设计线路段上进行探测,以查明下覆不良地质构造的发育情况.通过对瞬变电磁法的探测能力进行理论上的讨论,对不同地电断面情况下的异常体电磁响应的定性分析,得出用瞬变电磁法探测不良地质体的可行性结论.经过资料处理,查出不良地质构造赋存的位置,解释结果与钻探资料完全吻合. 相似文献
38.
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
39.
1 INTRODUCTION One of the most intriguing phenomena in the late Neoproterozoic (~750 to 543 Ma) is the globa occurrence of thin carbonates that directly overlie glacial deposits in almost every continent (Kennedy 1996; Hoffman et al., 1998; Hoffman and Schrag 2002; Brasier and Shields, 2000; James et al., 2001 Jiang et al., 2003; Nogueira et al., 2003). These “cap carbonates”, commonly several to tens of meters thick, have attracted enormous interests because o their unusually negati… 相似文献
40.
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