全文获取类型
收费全文 | 13204篇 |
免费 | 3317篇 |
国内免费 | 2845篇 |
专业分类
测绘学 | 1611篇 |
大气科学 | 2011篇 |
地球物理 | 3929篇 |
地质学 | 6704篇 |
海洋学 | 1808篇 |
天文学 | 864篇 |
综合类 | 924篇 |
自然地理 | 1515篇 |
出版年
2024年 | 67篇 |
2023年 | 166篇 |
2022年 | 556篇 |
2021年 | 707篇 |
2020年 | 583篇 |
2019年 | 809篇 |
2018年 | 815篇 |
2017年 | 828篇 |
2016年 | 930篇 |
2015年 | 932篇 |
2014年 | 1048篇 |
2013年 | 1176篇 |
2012年 | 1075篇 |
2011年 | 991篇 |
2010年 | 980篇 |
2009年 | 832篇 |
2008年 | 861篇 |
2007年 | 760篇 |
2006年 | 689篇 |
2005年 | 618篇 |
2004年 | 489篇 |
2003年 | 468篇 |
2002年 | 539篇 |
2001年 | 468篇 |
2000年 | 353篇 |
1999年 | 309篇 |
1998年 | 174篇 |
1997年 | 209篇 |
1996年 | 156篇 |
1995年 | 133篇 |
1994年 | 118篇 |
1993年 | 99篇 |
1992年 | 94篇 |
1991年 | 69篇 |
1990年 | 53篇 |
1989年 | 61篇 |
1988年 | 27篇 |
1987年 | 29篇 |
1986年 | 21篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 4篇 |
1981年 | 11篇 |
1980年 | 2篇 |
1979年 | 7篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1958年 | 6篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
There are 4 species of Littorinidae on the intertidal hard bed in the Jiulong River Estu-ary. They are Littorina scabra, L. brevicula, Nodilittorina pyramidalis pyramidalis and N.millegrana. Of these species, L. scabra is a dominant species. According to the results of in-vestigation, we find distribution patterns of the Littorinidae on intertidal hard bed in theestuary as follows: (1)There is no Littorinid species at Haicheng Station. (2)There is only Lit- 相似文献
122.
123.
124.
INTRODUCTIONBystudyingchlorophyllandprimaryproductivityinocean ,eitherinthebig scaleorinthesmallscale ,theproductivitydistributionandvariationofmarineorganicmaterialinthetempo ralandspatialcanbeunderstood .Theseaareas ,situatinginthewesterntropicalPacific… 相似文献
125.
S. M. Gong 《Astrophysics and Space Science》1989,158(1):1-7
The maximum volume of the closed Friedmann universe is further investigated and is shown to be 22
R
3
(t), instead of 2
R
3
(t) as found previously. This discrepancy comes from the incomplete use of the volume formula of 3-dimensional spherical space in the astronomical literature. Mathematically, there exists the maximum volume at any cosmic timet in a 3-dimensional spherical case. However, the Friedmann closed universe in expansion reaches its maximum volume only at the timet
m
of the maximum scale factorR(t
m
). The particle horizon has no limitation for the farthest objects in the closed Friedmann universe if the proper distance of objects is compared with the particle horizon as it should be. It will lead to absurdity if the luminosity distance of objects is compared with the proper distance of the particle horizon. 相似文献
126.
本文根据滇西地区18个大地电磁测深点资料的数据处理和分析结果,对测区深部导电率在纵、横向上的变化特征进行了研究。结果表明:滇西地区深部电性为多层结构,大致可分四至五个电性结构层;深部电性结构横向变化大,明显受区域构造控制;该区上部地壳内普遍存在低阻层;上地幔高导层明显存在两个隆起区,一个以剑川—鹤庆为中心呈北北西向展布的隆起区,另一个以腾冲—潞西为轴呈南北向展布的隆起区。 本文还讨论了地壳上地幔电性结构与大地构造的关系,滇西北裂陷区盆地的形成,以及该区地震活动与深部构造的关系 相似文献
127.
128.
129.
130.
Calculating Pollution Indices by Heavy Metals in Ecological Geochemistry Assessment and a Case Study in Parks of Beijing 总被引:4,自引:0,他引:4
Pollution index is a powerful tool for ecological geochemistry assessment. The commonly used pollution indices by heavy metals in soils and sediments were classified as two types of single index and integrated index in an algorithm point of view. Four single indices of contamination factor (or concentration factor), ecological risk factor, enrichment factor, and index of geo-accumulation were illustrated, and the reference values for calculating single indices were distinguished into background levels and threshold pollution values. Eight integrated indices were divided into two groups. One group is suitable for the normal distribution single indices including the sum, average, weighted average,vector modulus, and Nemerow pollution indices, and the other for log-normal distribution including the product, root of product, and weighted power product pollution indices. Using background levels as reference values, five contamination classes were divided, and the terminologies are suggested for the single and integrated indices to unify the assessment results. Software of EGAPI was developed in a single document interface to calculate the four single and eight integrated indices by heavy metals to assess the quality of soil and sediment ecological geochemistry. Pollution indices by heavy metals of Cu,Pb, and Zn in soils in parks of Beijing were calculated using EGAPI software, and these five contamination classes and terminologies suggested in this study were evaluated and used. Results ofintegrated indices of Cu, Pb, and Zn in soils indicated that the soil qualities are unpolluted as a whole and varied from low polluted to unpolluted status from the center to the outskirts of Beijing City. 相似文献