全文获取类型
收费全文 | 3901篇 |
免费 | 642篇 |
国内免费 | 591篇 |
专业分类
测绘学 | 43篇 |
大气科学 | 293篇 |
地球物理 | 1587篇 |
地质学 | 1195篇 |
海洋学 | 646篇 |
天文学 | 583篇 |
综合类 | 321篇 |
自然地理 | 466篇 |
出版年
2024年 | 35篇 |
2023年 | 59篇 |
2022年 | 116篇 |
2021年 | 141篇 |
2020年 | 138篇 |
2019年 | 134篇 |
2018年 | 139篇 |
2017年 | 143篇 |
2016年 | 133篇 |
2015年 | 139篇 |
2014年 | 186篇 |
2013年 | 202篇 |
2012年 | 189篇 |
2011年 | 153篇 |
2010年 | 196篇 |
2009年 | 287篇 |
2008年 | 244篇 |
2007年 | 269篇 |
2006年 | 221篇 |
2005年 | 196篇 |
2004年 | 200篇 |
2003年 | 187篇 |
2002年 | 148篇 |
2001年 | 136篇 |
2000年 | 181篇 |
1999年 | 152篇 |
1998年 | 146篇 |
1997年 | 120篇 |
1996年 | 107篇 |
1995年 | 91篇 |
1994年 | 75篇 |
1993年 | 55篇 |
1992年 | 61篇 |
1991年 | 41篇 |
1990年 | 18篇 |
1989年 | 24篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 10篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1979年 | 7篇 |
1978年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有5134条查询结果,搜索用时 15 毫秒
971.
A. Reiners 《Astronomische Nachrichten》2007,328(10):1034-1036
Differential rotation can be detected in single line profiles of stars rotating more rapidly than about v sin i = 10km s-1 with the Fourier transform technique. This allows to search for differential rotation in large samples to look for correlations between differential rotation and other stellar parameters. I analyze the fraction of differentially rotating stars as a function of color, rotation, and activity in a large sample of F-type stars. Color and rotation exhibit a correlation with differential rotation in the sense that more stars are rotating differentially in the cooler, less rapidly rotating stars. Effects of rotation and color, however, cannot be disentangled in the underlying sample. No trend with activity is found. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
972.
Seasonal variability regarding the nature of precipitation and the activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River Basin(Jianghuai)has been investigated by calculating apparent heat source/apparent moisture sink and analyzing TBB(cloud-top blackody radiation temperature)data.It is found that three periods of strong ascending motion during the Meiyu season lead to three episodes of heavy rain,and the latent heat due to the precipitation is of the sole heat source of the atmosphere.The nature of precipitation shows distinct seasonal variability,from frontal precipitation of the first episode to the extremely strong convective precipitation of the third episode.TBB field of East Asia may well reflect not only the intensity of convection and rainfall,but also the movement of rain belt and convection belt.In the whole Meiyu season.convection belt mainly stays in Jianghuai.but may shift within the domain of East Asia.Its locating in Jianghuai or not determines the maintenance or break of Meiyu.In the third episode,the narrow convection belt over Jianghuai is mainly caused by southwest monsoon which takes moist and convective atmosphere from tropical ocean. 相似文献
973.
利用平板涂布法从南海海绵生物样品中分离海绵共栖细菌,以枯草芽孢杆菌、金黄色葡萄球菌、大肠杆菌及酿酒酵母作为实验用指示菌对海绵共栖细菌进行了抗菌活性筛选。利用气相质谱联用仪(GC-MS)建立了群体感应信号分子酰基高丝氨酸内酯类化合物(AHLs)的检测方法,分析了活性菌株粗提液中信号分子的产生及种类。同时采用16S rDNA同源性和系统发育分析对活性菌株进行种属鉴定和系统发生学研究。结果表明,在分离到的94株海绵共栖细菌中18株(占19.1%)细菌具有抗菌活性。通过GC-MS检测确定大多数活性菌株都含有AHLs,因此提出了海绵共栖细菌抗菌活性与群体感应之间可能存在必然联系的推测。通过细菌分类鉴定,结果显示具有抗菌活性的细菌大部分属于芽孢杆菌属(Bacillus)。 相似文献
974.
975.
976.
研究南海礁栖腔节藻Coelarthrum sp.共附生真菌Pestalotiopsis neglecta SCSIO41403的次级代谢产物。采用中压正相柱层析色谱法和半制备高效液相色谱法, 对真菌发酵产物进行分离纯化, 制备获得6个次级代谢产物单体化合物, 使用核磁共振、质谱等波谱学方法, 结合其理化性质以及文献数据对比, 分别鉴定为: gamahorin (1)、microsporaline C (2)、7-hydroxy-5-methoxy-4,6-dimethylphthalidc (3)、demethyl nectriapyrone A (4)、4-hydroxybenzaldehyde (5)和4-hydroxyphenethyl acetate (6)。这6个化合物均为首次从该菌株发现, 其中化合物1、2、5和6是首次从海洋动植物共附生真菌中发现。抗菌活性筛选显示其中化合物3对白色念珠菌有明显抑制活性。 相似文献
977.
人类活动胁迫下岩溶含水层脆弱性分析 总被引:3,自引:0,他引:3
从水文地质本身的内部要素分析岩溶含水层高度脆弱的原因,详细论述了农业活动、工业和城市化以及矿山开采等对岩溶含水层脆弱性的影响,认为人类活动的叠加无疑加剧了岩溶含水层的脆弱性,并在很大程度上影响着地下水的水质、水量.因此,合理规范人类活动,减少人类活动对环境的污染,是岩溶地下水资源可持续利用的保证. 相似文献
978.
The aim of this study was to determine the effect of drought on grazing patterns of livestock in a communal rangeland in Namaqualand. Water points were the foci of almost all grazing routes even after the drought, and the areas away from the water points were less heavily utilized. The mountains could be regarded as key resource areas for livestock since the grazing intensity was higher in the elevated areas of the rangeland than on the surrounding low lying areas during and immediately after drought. Spatial patterns of resource use by livestock on the commons did not differ during the drought and post-drought period. The lengths of the daily grazing routes were similar for both periods. This may be a consequence of the weakness of animals during the drought when forage was scarce which prevented them from walking further. The size of home ranges of the herds did not change during the two monitoring periods. We attribute this to social reasons because herders try to avoid the mixing of herds. In the commons of Namaqualand, herders are also spatially constrained and this restricts their options to adapt to drought, and hence many animals died. 相似文献
979.
980.