全文获取类型
收费全文 | 43762篇 |
免费 | 19603篇 |
国内免费 | 44604篇 |
专业分类
测绘学 | 3813篇 |
大气科学 | 22512篇 |
地球物理 | 14593篇 |
地质学 | 40014篇 |
海洋学 | 16875篇 |
天文学 | 1304篇 |
综合类 | 5314篇 |
自然地理 | 3544篇 |
出版年
2024年 | 220篇 |
2023年 | 527篇 |
2022年 | 1024篇 |
2021年 | 1356篇 |
2020年 | 2425篇 |
2019年 | 5881篇 |
2018年 | 6160篇 |
2017年 | 5800篇 |
2016年 | 5922篇 |
2015年 | 5196篇 |
2014年 | 4852篇 |
2013年 | 5382篇 |
2012年 | 5122篇 |
2011年 | 5009篇 |
2010年 | 4851篇 |
2009年 | 4187篇 |
2008年 | 3467篇 |
2007年 | 3515篇 |
2006年 | 3085篇 |
2005年 | 2846篇 |
2004年 | 3062篇 |
2003年 | 2805篇 |
2002年 | 2616篇 |
2001年 | 2356篇 |
2000年 | 1957篇 |
1999年 | 1973篇 |
1998年 | 1968篇 |
1997年 | 1994篇 |
1996年 | 1592篇 |
1995年 | 1507篇 |
1994年 | 1404篇 |
1993年 | 1342篇 |
1992年 | 1092篇 |
1991年 | 828篇 |
1990年 | 781篇 |
1989年 | 641篇 |
1988年 | 566篇 |
1987年 | 426篇 |
1986年 | 351篇 |
1985年 | 273篇 |
1984年 | 296篇 |
1983年 | 182篇 |
1982年 | 213篇 |
1981年 | 156篇 |
1980年 | 110篇 |
1979年 | 114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 0 毫秒
11.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
12.
Zhang Qing Wang Li-fan Hu Jing-yao P. A. Mazzali Wang Zhen-ru 《Chinese Astronomy and Astrophysics》1995,19(4):426-431
We present the modeling of the ultraviolet and optical spectra obtained simultaneously on 1993 April 15 with the HST and at Lick Observatory. A Monte Carlo code is employed in the modeling and a comparison is made between models reported by different groups. With an atmosphere similar to the Sun in chemical composition, the observed spectral lines are well reproduced by a power law density structure of index around 20 except the strong H and HeI λ5876 lines which have peculiar absorption profiles. The photospheric velocity is found to be 9500 km/s and the blackbody temperature of the spectrum is 7990 K. For H and HeI λ5876, we suggest a two-component density structure which has a smoother layer located immediately outside the steeply decreasing inner envelope. The power law indices are most probably 20 and 3, respectively, with the transition point at about 13 000 km/s. In addition, this outer smooth layer serves to flatten the far UV spectrum as observed. 相似文献
13.
The properties of salinity in the South China Sea (SCS), a significant marginal sea connecting the Pacific andIndian Oceans, are greatly influenced by the transport of fresh water flux between the two oceans. However, the long-termchanges in the intermediate water in the SCS have not been thoroughly studied due to limited data, particularly in relationto its thermodynamic variations. This study utilized reanalysis data products to identify a 60-year trend of freshening in theintermediate waters of the northern South China Sea (NSCS), accompanied by an expansion of low-salinity water. Thestudy also constructed salinity budget terms, including advection and entrainment processes, and conducted an analysis ofthe salinity budget to understand the impacts of external and internal dynamic processes on the freshening trend of theintermediate water in the NSCS. The analysis revealed that the freshening in the northwest Pacific Ocean and theintensification of intrusion through the Luzon Strait at intermediate levels are the primary drivers of the salinity changes inthe NSCS. Additionally, a weakened trend in the intensity of vertical entrainment also contributes to the freshening in theNSCS. This study offers new insights into the understanding of regional deep sea changes in response to variations in boththermodynamics and oceanic dynamic processes. 相似文献
14.
通过对准噶尔盆地中部Ш区块原油或油砂抽提物的饱和烃气相色谱-质谱分析,研究了原油或油砂抽提物中25-降藿烷的分布规律.结果表明,不同井区或同一口井不同深度(层位)的原油25-降藿烷的相对含量存在比较明显的差别,表明其所遭受的生物降解程度有所差异,沿构造带从南到北,同一油层中原油的生物降解程度增强,比如位于构造北部(构造部位相对较高)的永1井、永3井侏罗系原油25-降藿烷丰度较高,表明生物降解比较明显,而位于南部(构造低部位)的永6井白垩系和侏罗系油层中的原油均无明显的生物降解现象;同一口井随深度增大生物降解作用将弱,如永2井浅部白垩系油层的原油降解较严重,而深部西山窑组的原油降解作用则不明显.根据原油生物降解的特征,结合车-莫古隆起调整对研究区油气成藏的影响,讨论了原油生物降解差异分布的成因机制. 相似文献
15.
16.
17.
本文通过对辽西及邻区中生代盆地火山岩稀土元素配分特征的研究及模拟运算,探讨了其岩浆成因及过程。结果显示南北岩区岩浆成因不同,南岩区岩源为MORB地幔和长期富集型幔熔融混合而成,北岩区为MORB和近期亏损型地幔熔融混合而成。 相似文献
18.
1 INTRODUCTION Sitting in the central part of the Pearl River Delta, Guangzhou is one of the cities in China that are most polluted by acid rain. Broad attention has been brought to the problem. Since the early 1980s, a series of observation and studies h… 相似文献
19.
The characteristics of atmospheric heat source associated with the summer monsoon onset in the South China Sea (SCS) are studied using ECMWF reanalysis data from 1979 to 1993. A criterion of the SCS summer monsoon onset is defined by the atmospheric hea… 相似文献
20.
Problem on development control of marine source bed hold in Chinese petroleum industry progression. The Hongshuizhuang Formation,Tieling Formation and Xiamaling Formation in the Middle and Upper Proterozoic are important hydrocarbon source beds in northern North China, and investigation of their sedimentary environments and the controls has great significance for petroleum exploration in North China. Based on sedimentology (sequence stratigraphy), palaeoecology, sedimentary geochemistry, and sedimentary palaeogeography, their development pattern is discussed. All these studies indicate that the development controls of the hydrocarbon source beds include a favorite palaeogeographic location, exceeding propagation of biomes in low and middle latitudes, anoxic environments, enrichment of phosphorus element and the adsorption of clay minerals during the preservation of organic matter in the marine carbonates. 相似文献