首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   8205篇
  免费   2421篇
  国内免费   3240篇
测绘学   1399篇
大气科学   1332篇
地球物理   1751篇
地质学   5427篇
海洋学   1861篇
天文学   110篇
综合类   707篇
自然地理   1279篇
  2024年   60篇
  2023年   207篇
  2022年   588篇
  2021年   636篇
  2020年   570篇
  2019年   601篇
  2018年   580篇
  2017年   516篇
  2016年   535篇
  2015年   623篇
  2014年   585篇
  2013年   726篇
  2012年   832篇
  2011年   778篇
  2010年   856篇
  2009年   746篇
  2008年   723篇
  2007年   685篇
  2006年   718篇
  2005年   561篇
  2004年   401篇
  2003年   305篇
  2002年   271篇
  2001年   274篇
  2000年   229篇
  1999年   84篇
  1998年   33篇
  1997年   18篇
  1996年   18篇
  1995年   5篇
  1994年   12篇
  1993年   7篇
  1992年   6篇
  1991年   8篇
  1990年   12篇
  1989年   2篇
  1988年   3篇
  1987年   7篇
  1986年   3篇
  1985年   4篇
  1984年   5篇
  1983年   2篇
  1982年   3篇
  1979年   9篇
  1978年   1篇
  1977年   1篇
  1976年   3篇
  1973年   1篇
  1957年   7篇
  1954年   6篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
991.
To study arsenic(As) content and distribution patterns as well as the genesis of different kinds of water, especially the different sources of drinking water in Guanzhong Basin, Shaanxi province, China, 139 water samples were collected at 62 sampling points from wells of different depths, from hot springs, and rivers. The As content of these samples was measured by the intermittent flowhydride generation atomic fluorescence spectrometry method(HG-AFS). The As concentrations in the drinking water in Guanzhong Basin vary greatly(0.00–68.08 μg/L), and the As concentration of groundwater in southern Guanzhong Basin is different from that in the northern Guanzhong Basin. Even within the same location in southern Guanzhong Basin, the As concentrations at different depths vary greatly. As concentration of groundwater from the shallow wells(50 m deep, 0.56–3.87 μg/L) is much lower than from deep wells(110–360 m deep, 19.34–62.91 μg/L), whereas As concentration in water of any depth in northern Guanzhong Basin is 10 μg/L. Southern Guanzhong Basin is a newly discovered high-As groundwater area in China. The high-As groundwater is mainly distributed in areas between the Qinling Mountains and Weihe River; it has only been found at depths ranging from 110 to 360 m in confined aquifers, which store water in the Lishi and Wucheng Loess(Lower and Middle Pleistocene) in the southern Guanzhong Basin. As concentration of hot spring water is 6.47–11.94 μg/L; that of geothermal water between 1000 and 1500 m deep is 43.68–68.08 μg/L. The high-As well water at depths from 110 to 360 m in southern Guanzhong Basin has a very low fluorine(F) value, which is generally 0.10 mg/L. Otherwise, the hot springs of Lintong and Tangyu and the geothermal water in southern Guanzhong Basin have very high F values(8.07–14.96 mg/L). The results indicate that highAs groundwater in depths from 110 to 360 m is unlikely to have a direct relationship with the geothermal water in the same area. As concentration of all reservoirs and rivers(both contaminated and uncontaminated) in the Guanzhong Basin is 10 μg/L. This shows that pollution in the surface water is not the source of the high-As in the southern Guanzhong Basin. The partition boundaries of the high- and low-As groundwater area corresponds to the partition boundaries of the tectonic units in the Guanzhong Basin. This probably indicates that the high-As groundwater areas can be correlated to their geological underpinning and structural framework. In southern Guanzhong Basin, the main sources of drinking water for villages and small towns today are wells between 110–360 m deep. All of their As contents exceed the limit of the Chinese National Standard and the International Standard(10 μg/L) and so local residents should use other sources of clean water that are 50 m deep, instead of deep groundwater(110 to 360 m) for their drinking water supply.  相似文献   
992.
正1 Introduction As the lightest metal with the unique properties of energy production and storage,lithium is regarded as the new century energy metal.Lithium and its compounds were widely used in various industrial fields,especially in  相似文献   
993.
正Iodine and iodine compounds are widely applied in medicaments,dyes,energy materials,food additives etc.The development and utilization of iodine resources have attracted much attention.A large amount of iodide is  相似文献   
994.
正1 Introduction The brines with high concentrations of magnesium and boron resources are widely distributed in the Qaidam Basin of the Qinghai-Tibet plateau,China(ZhengTang,1988).Although some works on the ternary system  相似文献   
995.
正1 Introduction With the industrial development of lithium battery,nuclear and aerospace industry,the demands of metal lithium and its compounds are increasing significantly.Lithium is called as the energy of the metal in the new century(Zhang et al.2001).The total reserve of lithium resources around the world7  相似文献   
996.
正The Mengyejing potash salt deposit(MPSD)is the only pre-Quaternary potash salt deposit in China.The MPSD is located in the southern Simao Basin,southeastern Tibetan Plateau.The MPSD,along with rock salts and clastic rocks,  相似文献   
997.
正Amounts of lithium-containing salt lake brine resources are widely distributed in the four provinces named Qinghai,Tibet,Inner Mongolia and Xinjiang province,especially the salt lakes in Qinghai-Tibet Plateau are abundant of  相似文献   
998.
正With the rapid increasing demand of metal lithium and its compounds in science and technology,studies on lithium recovery though lithium ion extraction from salt lake brines are blooming bacause more than 60 percent of  相似文献   
999.
正1 Introduction Mengla Basin is a sub-basin in southern evaporitebearing Lanping-Simao Basin.There are many salt springs in the basin.In 2012,11 spring samples were collected for analyses of chemistry and boron,hydrogen and oxygen  相似文献   
1000.
青藏高原南部中新世下地壳流动是当前大陆动力学研究的热点,关键科学问题是下地壳的流动方向。LA-ICP-MS定年结果表明冲江斑状黑云母二长花岗岩的形成时代为14.9~14.8 Ma,朱诺斑状花岗岩的形成时代为15.3~14.9 Ma。含矿埃达克质斑岩的特征如下:Si O2含量67.72%~74.49%,K2O含量2.85%~5.98%,Sr含量93~804μg/g,高Sr/Y(16~139)、(La/Yb)N(21~43)比值,Eu/Eu*值为0.6~0.91。冲江岩体锆石εHf(t)值为1.2~5.1,朱诺岩体锆石εHf(t)值为–6.9~–0.1,他们与徐旺春等报道的镁铁质麻粒岩的锆石Hf值(–2.5~4.8)具有很好的叠合性,暗示镁铁质麻粒岩(印度镁铁质下地壳)可以作为他们的岩浆源区。此外,Sr-Nd同位素表现出雅鲁藏布江蛇绿岩和拉萨地块两个端元混合的特征,Pb同位素表现出雅鲁藏布江蛇绿岩和喜马拉雅富集陆壳端元的特征。上述同位素地球化学特征表明,冈底斯中新世埃达克质斑岩的岩浆源区物质组成包括:拉萨陆壳、印度陆壳、雅鲁藏布江蛇绿岩(地幔成分),表现出加厚下地壳部分熔融特征,暗示青藏高原南部由南向北的下地壳流动方向。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号