全文获取类型
收费全文 | 46713篇 |
免费 | 11417篇 |
国内免费 | 16787篇 |
专业分类
测绘学 | 8148篇 |
大气科学 | 6560篇 |
地球物理 | 8911篇 |
地质学 | 30430篇 |
海洋学 | 8922篇 |
天文学 | 758篇 |
综合类 | 4110篇 |
自然地理 | 7078篇 |
出版年
2024年 | 397篇 |
2023年 | 1025篇 |
2022年 | 2798篇 |
2021年 | 3508篇 |
2020年 | 2708篇 |
2019年 | 3427篇 |
2018年 | 2906篇 |
2017年 | 2738篇 |
2016年 | 2750篇 |
2015年 | 3328篇 |
2014年 | 3321篇 |
2013年 | 3994篇 |
2012年 | 4280篇 |
2011年 | 4299篇 |
2010年 | 4149篇 |
2009年 | 3914篇 |
2008年 | 3976篇 |
2007年 | 3740篇 |
2006年 | 3572篇 |
2005年 | 3030篇 |
2004年 | 2287篇 |
2003年 | 1616篇 |
2002年 | 1732篇 |
2001年 | 1521篇 |
2000年 | 1223篇 |
1999年 | 660篇 |
1998年 | 353篇 |
1997年 | 286篇 |
1996年 | 250篇 |
1995年 | 198篇 |
1994年 | 141篇 |
1993年 | 128篇 |
1992年 | 124篇 |
1991年 | 62篇 |
1990年 | 81篇 |
1989年 | 64篇 |
1988年 | 34篇 |
1987年 | 27篇 |
1986年 | 33篇 |
1985年 | 29篇 |
1984年 | 23篇 |
1983年 | 22篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1979年 | 25篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1958年 | 6篇 |
1957年 | 18篇 |
1954年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
Z. Q. Qu C. L. Xu X. Y. Zhang X. L. Yan C. L. Jin 《Monthly notices of the Royal Astronomical Society》2006,370(4):1790-1796
The influence of magnetic fields on the energy level populations of atoms is firstly studied by analysing the Stokes profiles of Fe i 6302.5 forming in the solar magnetized atmosphere, with the aid of a departure factor defined to evaluate the deviation from the normal Boltzmann distribution without a magnetic field. This factor is directly related to the ratio of line-source function to the continuum one. The relationship between the departure and the magnetic field reveals an effect that the magnetic field induces an exponential increase in the level population of the lower level of Fe i 6302.5 (Landé factor g = 2.5 ) with the field strength. This indicates that the magnetic field can cause the redistribution of populations of those levels whose Landé factors are non-zero. Therefore, this effect should be included in the calculation of the statistical equilibrium. Secondly, an experiment utilizing the Hg5461 line in the laboratory on the Earth is carried out to reveal that the exponential relation is independent of variations in temperature, and the excitation is completely magneto-induced. Finally, the exponential relation is explained by taking account of the magnetic energy in the Boltzmann distribution. 相似文献
53.
抗旱保水剂膨润土-聚丙烯酸盐吸水树脂研究 总被引:2,自引:0,他引:2
利用丙烯酸盐在蒙脱石层间进行原位聚合,研究开发一种适用于农林业的抗旱保水剂。实验结果表明:聚合产物的综合性能较为理想,吸水树脂的保水作用主要体现在能够吸收大量的水,并且在压力下不脱水。 相似文献
54.
为了明确折扣方式下应税金额的确定方法,分别从计税销售额、营业额和所得额等方面进行分析,其结果将使纳税人的财务会计处理和具体纳税操作更为规范和准确。 相似文献
55.
56.
利用降水平均递增率求山地最大降水高度 总被引:6,自引:0,他引:6
本文引入降水平均递增率的概念,改进了文献中的计算方法,可以一次求出山地降水量与海拔的经验关系式中全部参数的最佳值,其中包括山地最大降水高度。并用实例说明了本法的步骤及特点。 相似文献
57.
Uranium-series dating of oxygen and carbon isotope records for stalagmite SJ3 collected in Songjia Cave, central China, shows significant variation in past climate and environment during the period 20-10 ka. Stalagmite SJ3 is located more than 1000 km inland of the coastal Hulu Cave in East China and more than 700 km north of the Dongge Cave in Southwest China and, despite minor differences, displays a clear first-order similarity with the Hulu and Dongge records. The coldest climatic phase since the Last Glacial Maximum, which is associated with the Heinrich Event 1 in the North Atlantic region, was clearly recorded in SJ3 between 17.6 and 14.5 ka, in good agreement in timing, duration and extent with the records from Hulu and Dongge caves and the Greenland ice core. The results indicate that there have been synchronous and significant climatic changes across monsoonal China and strong teleconnections between the North Atlantic and East Asia regions during the period 20-10 ka. This is much different from the Holocene Optimum which shows a time shift of more than several thousands years from southeast coastal to inland China. It is likely that temperature change at northern high latitudes during glacial periods exerts stronger influence on the Asian summer monsoon relative to insolation and appears to be capable of perturbing large-scale atmospheric/oceanic circulation patterns in the Northern Hemisphere and thus monsoonal rainfall and paleovegetation in East Asia. Climatic signals in the North Atlantic region propagate rapidly to East Asia during glacial periods by influencing the winter land-sea temperature contrast in the East Asian monsoon region. 相似文献
58.
59.
60.