全文获取类型
收费全文 | 55957篇 |
免费 | 9613篇 |
国内免费 | 14115篇 |
专业分类
测绘学 | 4239篇 |
大气科学 | 7271篇 |
地球物理 | 10750篇 |
地质学 | 33022篇 |
海洋学 | 7739篇 |
天文学 | 2185篇 |
综合类 | 4021篇 |
自然地理 | 10458篇 |
出版年
2024年 | 216篇 |
2023年 | 560篇 |
2022年 | 1618篇 |
2021年 | 1798篇 |
2020年 | 1927篇 |
2019年 | 2372篇 |
2018年 | 1898篇 |
2017年 | 2072篇 |
2016年 | 2235篇 |
2015年 | 2451篇 |
2014年 | 3092篇 |
2013年 | 3013篇 |
2012年 | 3385篇 |
2011年 | 3619篇 |
2010年 | 2970篇 |
2009年 | 3652篇 |
2008年 | 3639篇 |
2007年 | 4004篇 |
2006年 | 3927篇 |
2005年 | 3473篇 |
2004年 | 3298篇 |
2003年 | 3133篇 |
2002年 | 2719篇 |
2001年 | 2409篇 |
2000年 | 2255篇 |
1999年 | 2101篇 |
1998年 | 1791篇 |
1997年 | 1633篇 |
1996年 | 1470篇 |
1995年 | 1234篇 |
1994年 | 1267篇 |
1993年 | 1045篇 |
1992年 | 832篇 |
1991年 | 581篇 |
1990年 | 498篇 |
1989年 | 405篇 |
1988年 | 292篇 |
1987年 | 202篇 |
1986年 | 129篇 |
1985年 | 95篇 |
1984年 | 51篇 |
1983年 | 60篇 |
1982年 | 62篇 |
1981年 | 55篇 |
1980年 | 24篇 |
1979年 | 29篇 |
1978年 | 35篇 |
1977年 | 24篇 |
1975年 | 4篇 |
1954年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 329 毫秒
81.
82.
应用显微光度术、显微傅立叶红外光谱(Micro-FT.IR)和飞行时间二次离子质谱(TOF-SIMS)等原位微分析技术并结合均一温度测量对胜利油气区下第三系沙河街组沙三段中有机包裹体进行了研究。区分出两类有机包裹体,即原生有机包裹体和次生有机包裹体。结果表明两类有机包裹体特征不同,二者物质组成、有机质成分及化学结构、热演化程度等差别也较大。结合地质分析表明原生有机包裹体是沙三段烃源岩生成烃类运移产物,具“自生自储”特点,而次生有机包裹体是沙四段烃源岩生成的烃类二次运移的产物。沙三段是沙河街组油气运移和聚集的主要层位,因而是寻找油气资源的主要目标层。研究表明,有机包裹体是研究油气生成、运移、聚集和演化等成藏系统最有效的手段之一,在油气勘探中有重要的应用意义。 相似文献
83.
D.W. Crawford D.A. Purdie A.P.M. Lockwood P. Weissman 《Estuarine, Coastal and Shelf Science》1997,45(6):799-812
In the Southampton Water estuary (southern England, U.K.), red-tides caused by the planktonic, phototrophic ciliateMesodinium rubrum(=Myrionecta rubra) occur during most summers and sometimes in autumn. These events were investigated in detail between 1985 and 1987 and were characterized by levels of chlorophylla(chl a) of over 100 μg l−1, cell numbers ofM. rubrumof over 1×103 ml−1, oxygen saturations of around 150%, and depleted numbers of macrozooplankton. Initiation of red-water did not appear to be triggered by irradiance or nutrients, but coincided with an increase in temperature and water column stability. This enhanced stability was promoted by increased surface to bottom gradients of both temperature and salinity, and by reduced mixing during neap tides. Development of red-water was accompanied by removal of most of the dissolved NH+4from the water column, whereas some NO−3persisted, presumably maintained by freshwater input. NO−3and NH+4gradually returned to pre-bloom concentrations as the red-water declined in late summer. Maximal biomass ofM. rubrumappeared to be limited by irradiance, and self-shading probably imposed an upper limit of around 300 mg chl a m−2within the water column. At the observed levels of chl a, irradiance values within the population maximum between 1 and 3 m depth were only just of the order (≈15 μmol photons m−2 s−1) required to balance estimated respiratory demands. Oxygen concentration became undersaturated during the late bloom phase, with minimal values of 20–30% saturation recorded in deeper waters; however, despite this and reduced numbers of macrozooplankton, direct deleterious effects on other organisms were not observed. 相似文献
84.
矿床谱系是对成矿多样性的理论概括,而成矿多样性又是由不同级别、不同性质的致矿地质异常决定的.本文论述了山东省内生金矿矿床谱系及其致矿地质异常. 相似文献
85.
本文分析了国外用于测定度盘分划改正的几种方法,并分析了固定角距法的优点和不足之处;详细叙述了组合固定角距法测定对径改正的原理和计算公式;并且讨论了两对显微镜之间夹角变化以及显微镜比例尺变化对对径改正的影响,给出了相应的修正方法。用固定角距法所测的对径改正精度可达±0″.009。 相似文献
86.
87.
88.
G. M. Karataeva V. A. Yakovleva V. A. Hagen-Thorn O. V. Mikolaichuk 《Astronomy Letters》2001,27(2):74-80
We analyze our BVR c photometry for UGC 5600, a candidate polar ring galaxy, obtained with the 6-m telescope. We have confirmed the existence of an inner polar ring and show that the outer ring-shaped structure represents spiral arms; i.e., UGC 5600 belongs to the rare class of gas-rich spiral galaxies with inner polar rings. 相似文献
89.
Donald C. Gordon Peter J. Cranford Con Desplanque 《Estuarine, Coastal and Shelf Science》1985,20(2):205-227
The Cumberland Basin, a 118 km2 estuary at the head of the Bay of Fundy which has an average tidal range of about 11m, contains large tracts of salt marsh (15% of the area below highest high water). Low marsh (below about 0·9 m above mean high water) is composed almost exclusively of Spartina alterniflora while the vegetation on high marsh is more diverse but dominated by Spartina patens. Because of its higher elevation, high marsh is flooded infrequently for short periods by only extreme high tides. Low marsh is inundated much more frequently by water as much as 4m deep for periods as long as 4 h per tide. Temporal variability in the occurrence of extreme tides influences the flooding frequency of high marsh for any given month and year. Using a modification of Smalley's method, the mean annual net aerial primary production (NAPP) of low and high marsh is estimated to be 272 and 172 g C m?2, respectively. Vegetation turnover times average 1·0 and 2·0 y for low and high marsh, respectively. Because of abundant tidal energy, much of the low marsh production appears to be exported and distributed widely about the estuary. Since high levels of turbidity suppress phytoplankton production, salt marshes produce approximately half of the carbon fixed photosynthetically in the Cumberland Basin. It is concluded that salt marshes play a major ecological role in the Cumberland Basin. 相似文献
90.