全文获取类型
收费全文 | 40881篇 |
免费 | 19328篇 |
国内免费 | 44526篇 |
专业分类
测绘学 | 4325篇 |
大气科学 | 21932篇 |
地球物理 | 13415篇 |
地质学 | 38284篇 |
海洋学 | 17650篇 |
天文学 | 865篇 |
综合类 | 4907篇 |
自然地理 | 3357篇 |
出版年
2024年 | 259篇 |
2023年 | 604篇 |
2022年 | 1086篇 |
2021年 | 1472篇 |
2020年 | 2517篇 |
2019年 | 5793篇 |
2018年 | 5910篇 |
2017年 | 5609篇 |
2016年 | 5683篇 |
2015年 | 5141篇 |
2014年 | 4620篇 |
2013年 | 5271篇 |
2012年 | 5093篇 |
2011年 | 4926篇 |
2010年 | 4855篇 |
2009年 | 4246篇 |
2008年 | 3610篇 |
2007年 | 3548篇 |
2006年 | 3128篇 |
2005年 | 2847篇 |
2004年 | 3043篇 |
2003年 | 2701篇 |
2002年 | 2454篇 |
2001年 | 2197篇 |
2000年 | 1785篇 |
1999年 | 1673篇 |
1998年 | 1730篇 |
1997年 | 1782篇 |
1996年 | 1393篇 |
1995年 | 1340篇 |
1994年 | 1204篇 |
1993年 | 1167篇 |
1992年 | 995篇 |
1991年 | 747篇 |
1990年 | 702篇 |
1989年 | 579篇 |
1988年 | 498篇 |
1987年 | 389篇 |
1986年 | 319篇 |
1985年 | 249篇 |
1984年 | 285篇 |
1983年 | 163篇 |
1982年 | 206篇 |
1981年 | 144篇 |
1980年 | 108篇 |
1979年 | 111篇 |
1978年 | 49篇 |
1977年 | 44篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
231.
我国是世界上著名的季风气候区.但过去相当多的研究是从气候概念来理解季风的.实际上,各年季风的起迄日期有早有晚,而强度也不同.正是由于这种差异,造成了我国各地的旱涝灾害.因此从个别年来研究季风,并探讨形成季风年际变化可能的原因是十分必要的.本文尝试通过1971年与1972年季风活动的对比分析,一方面了解环流特征季节变化的异同,另一方面希望能对这两年旱涝产生的原因有一个更明确的认识. 相似文献
232.
Photoinduced toxicity single and binary mixtures of four polycyclic aromatic hydrocarbons to the marine diatom Skeletonema costatum 总被引:1,自引:0,他引:1
Diatom Skeletonema costatum Cleve is one of the main predominant phytoplankton species in the Changjiang Estuary in China. In order to provide some basic information for future assessment of the potential risk on phytoplankton communities in this estuary caused by polycyclic aromatic hydrocarbons(PAHs), this alga was selected as a representative to investigate the photoinduced toxicity of PAHs, in single and mixture. Four PAHs including three-ring phenanthrene and anthracene, four-ring fluoranthene and pyrene were tested in the laboratory. The single toxicity of each PAH on this microalga was compared with and without the simulated solar UV radiation. The results showed that this microalga was sensitive to PAH's photoinduced toxicity. Ratios of the 72 h median effect concentration obtained for fluorescent and UV light tests were about 8.4 for phenanthrene, 13.0 for anthracene, 6.5 for fluoranthene, and 5.7 for pyrene, indicating that UV light enhanced the PAH toxicity to this alga significantly. Under the fluorescent radiation (lacking UV), the dose-response curves based on chemical concentrations revealed that the order of toxic strength was fluoranthene greater than pyrene greater than anthracene greater than phenanthrene; while under the UV radiation (476 μW/cm2 for UVA, 6.5 μW/cm2 for UVB) it became fluoranthene approximately equaling anthracene greater than pyrene greater than phenanthrene, indicating that the UV light also changed its relative toxicity to this alga. The photoinduced toxicity of PAHs to the marine diatom S. costatum might be a synergistic effect of photosensitization reactions (e.g., generation of single-state oxygen) and photomodification (photooxidation and/or photolysis).The combined effects of six binary mixtures on the marine diatom S. costatum were investigated using the additive-index method. Four binary-mixtures (phenanthrene plus anthracene; phenanthrene plus pyrene; anthracene plus fluoranthene; anthracene plus pyrene) were found to be synergistic joint action mode, while two binary-mixtures (phenanthrene plus fluoranthene; fluoranthene plus pyrene) displayed antagonist, revealing a complex pattern of possible interactions. The combined action mode of PAHs might be related to various factors such as the mixture compounds, mixture ratios and test conditions, etc. 相似文献
233.
Application of LICOM to the numerical study of the water exchangebetween the South China Sea and its adjacent oceans 总被引:8,自引:1,他引:8
1 IntroductionThe South China Sea (SCS) is the largestmarginal sea in the western Pacific (see Fig. 1). It con-nects with the SCS through the Taiwan Strait, with thePacific through the Luzon Strait, with the Sulu Seathrough the Mindoro and Balabac Straits and with theJava Sea and Andaman Sea through the Sunda Shelf(For convenience, here we refer to the section at 1.5°N,Fig. 2). It is shown that the seasonal SCS circulation ismostly affected by the summer/winter monsoon, andthe no… 相似文献
234.
Weng Keqin
Associate Professor Hydraulic Hydro-Power Engineering Dept. Tsinghua University Beijing 《中国海洋工程》1995,(2)
This paper reveals that the long-period statistic distribution of the characteristic heights of deep-water waves assumes the lognormal distribution. Thereafter, the largest wave-height which may occur in the service life of coastal structures is derived in this paper. 相似文献
235.
罗氏沼虾育苗用水中Mg2+与Ca2+含量及Mg2+/Ca2+对出苗率的影响 总被引:13,自引:0,他引:13
1991年3-6月和1992年3-6月在南汇县东海水产养殖公司对罗氏沼虾育苗用水调配原则与不同基础水的调配方法进行试验研究。结果表明,育苗用水中Mg^2+与Ca^2+含量及Mg^2+/Ca^2+必须达到一定的范围,这是调配的基本原则。以鱼塘水作基础水时,需添加6种化学药品,其中Mg^2+与Ca^2+含量可适范围分别为300.0-440.0mg/L与170.0-244.0mg/L,Mg^2+/Ca^ 相似文献
236.
Thesecondorderspectrumoftwo-dimensionalrandomwaves¥DingPingxing;SunFuandYuZhouwen(ReceivedJanuary15,1993;acceptedMay31,1993)A... 相似文献
237.
Full-rangenonlinearanalysisoffatiguebehaviorsofreinforcedconcretestructuresbyfiniteelementmethod¥SongYupu;ZhaoShunbo;WangRuim... 相似文献
238.
239.
240.
A method combining boundary and finite element approaches as well as modal analysis to solve the acoustical scattering from elastic obstacle in infinite fluid is described. An elastic cylinder with finite length immersed in water is chosen as an example for numerical demonstration. The computed frequency response and direction pattern of the backscattering target strength of the cylinder are conformable to those measured. 相似文献