全文获取类型
收费全文 | 65507篇 |
免费 | 11854篇 |
国内免费 | 16012篇 |
专业分类
测绘学 | 4912篇 |
大气科学 | 8556篇 |
地球物理 | 14477篇 |
地质学 | 39322篇 |
海洋学 | 7681篇 |
天文学 | 2223篇 |
综合类 | 4513篇 |
自然地理 | 11689篇 |
出版年
2024年 | 263篇 |
2023年 | 691篇 |
2022年 | 1824篇 |
2021年 | 2286篇 |
2020年 | 2364篇 |
2019年 | 2848篇 |
2018年 | 2296篇 |
2017年 | 2466篇 |
2016年 | 2717篇 |
2015年 | 2958篇 |
2014年 | 3767篇 |
2013年 | 4072篇 |
2012年 | 3964篇 |
2011年 | 4407篇 |
2010年 | 3749篇 |
2009年 | 4329篇 |
2008年 | 4329篇 |
2007年 | 4746篇 |
2006年 | 4686篇 |
2005年 | 4036篇 |
2004年 | 3756篇 |
2003年 | 3516篇 |
2002年 | 3123篇 |
2001年 | 2770篇 |
2000年 | 2524篇 |
1999年 | 2280篇 |
1998年 | 1962篇 |
1997年 | 1799篇 |
1996年 | 1581篇 |
1995年 | 1305篇 |
1994年 | 1318篇 |
1993年 | 1105篇 |
1992年 | 868篇 |
1991年 | 621篇 |
1990年 | 522篇 |
1989年 | 434篇 |
1988年 | 312篇 |
1987年 | 203篇 |
1986年 | 140篇 |
1985年 | 105篇 |
1984年 | 52篇 |
1983年 | 47篇 |
1982年 | 34篇 |
1981年 | 34篇 |
1980年 | 29篇 |
1979年 | 30篇 |
1978年 | 41篇 |
1977年 | 24篇 |
1973年 | 5篇 |
1954年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
941.
942.
黑色页岩与土壤重金属污染 总被引:6,自引:1,他引:5
本文利用ICP—MS等技术分析了湘中地区黑色页岩及其相应土壤的重金属含量,在对分析结果进行统计分析的基础上,探讨了黑色页岩与土壤重金属污染的关系。研究表明,黑色页岩是富集多种重金属元素的特殊岩石。以黑色页岩岩系为母岩的土壤,不仅明显富集Cu、Cd、Cr、Co、Pb、Zn、Mo、Ni、V、U、Sn、Sb、T1、Th等多种重金属元素,而且受到Mo、Sb、Cd、U、Tl、Cu、V、Sn、Th等重金属的污染,其中以Mo、Cd、Sb、U、Tl等的污染尤为严重。黑色页岩土壤重金属污染在一些地方已产生明显的负面环境效应,值得关注。 相似文献
943.
944.
945.
946.
某基坑工程土钉支护工作性状试验研究 总被引:1,自引:0,他引:1
通过对广州天河荟雅苑基坑工程土钉支护进行试验和研究,分析了土钉支护体系的工作性状,并为同类工程的设计提供了一定的依据。 相似文献
947.
成都洛带上沙溪庙组气藏地层水分布特征研究 总被引:2,自引:1,他引:1
成都洛带沙溪庙组气藏普遍含水,气藏不同位置地层水含量对于天然气的产量及产出时间有很大的影响。地层水矿化度较高,属CaCl2型,具混合水的水化学特征。地层水与天然气共同赋存同一储层中,属于同一压力系统,与天然气伴生产出,无边、底水存在,缺乏有效补给。含水层往往分布于有利的沉积微相一分流河道及河口坝中,厚度大、物性好的砂体含水特征更为明显。通过对该气藏水水化学特征及气水关系的研究,初步明确了该地层水的分布特征。 相似文献
948.
949.
950.
Cambrian explosion: Birth of tree of animals 总被引:5,自引:5,他引:0
Excluding the sponges the Kingdom Animalia is usually divided into three subkingdoms: Diploblasta, Protostomia and Deuterostomia. The Cambrian Explosion consists of three major episodes, two of which were in the early Early Cambrian (one represented by the small skeletal fossils “SSFs” at the base of the Cambrian and the other represented by the succeeding Chengjiang faunas “CFs”), and the other episode as their prelude took place in the “Eocambrian” (i.e. the latest Precambrian), represented by the Ediacaran faunas. This unique Big Bang of life has been recognized as giving birth to the entire morphological Tree Of Animals (or metazoans), in short the TOA. Its “seed” in the deep Precambrian, represented by some sort of protist from which the complete TOA must have grown, remains unknown paleontologically. However, the fossil evidence suggests that the three major episodes of the Cambrian Explosion are responsible for the earliest radiations of the three subkingdoms of animals respectively. While the observed Ediacaran fauna might constitute only a small part of the whole Ediacaran biota, our evidence supports that it was dominated by diploblasts (the “trunk” of the TOA) with only a few possible stem-group triploblasts. The Early Cambrian in turn in two phases explosively yielded almost all the major triploblastic crown-branches (Bilateria: the huge “crown” of the TOA), which include the other two subkingdoms: first the extremely diverse protostomes in the Meishucunian Age and then followed by a nearly entire lineage of early deuterostomes from the Chengjiang, including even its most derived member – the earliest true vertebrates. Among the four most significant milestones of morphological origins and radiations in animal history, the first one (i.e. appearance of metazoans) took place in the Ediacaran Period or earlier times, and the other three can be seen in the windows available from the Chengjiang and the Meishucunian fossil assemblages. The newly discovered extinct Phylum Vetulicolia, which has primitively segmented body with simple gill slits in its anterior division, most probably represents one of the roots of the deuterostome subkingdom. Showing a mosaic of basic features possessed in both the bilateral vetulicolians and some primitive echinoderms, the soft-bodied vetulocystids are best regarded as one of the roots of the extant pentamerous echinoderms. Standing on the “top” of the deuterostome super-branch in the early Cambrian TOA are the “the first fish” Myllokunmingia and Haikouichthys, which bear paired eyes and salient proto-vertebrae. These animals represent the real root of the remainder of the vertebrates or craniates. On the contrary, yunnanozoans, including Yunnanozoon and Haikouella, possess neither eyes nor unequivocal vertebrae, and may have nothing to do with the craniates, let alone the vertebrates. Those enigmatic creatures share a similar body-plan with vetulicolians and should be treated as a side-branch within the lower deuterostomes. 相似文献