全文获取类型
收费全文 | 3652篇 |
免费 | 981篇 |
国内免费 | 1416篇 |
专业分类
测绘学 | 574篇 |
大气科学 | 598篇 |
地球物理 | 878篇 |
地质学 | 2555篇 |
海洋学 | 611篇 |
天文学 | 48篇 |
综合类 | 279篇 |
自然地理 | 506篇 |
出版年
2024年 | 46篇 |
2023年 | 134篇 |
2022年 | 219篇 |
2021年 | 268篇 |
2020年 | 239篇 |
2019年 | 246篇 |
2018年 | 215篇 |
2017年 | 228篇 |
2016年 | 231篇 |
2015年 | 276篇 |
2014年 | 269篇 |
2013年 | 267篇 |
2012年 | 338篇 |
2011年 | 330篇 |
2010年 | 331篇 |
2009年 | 320篇 |
2008年 | 285篇 |
2007年 | 319篇 |
2006年 | 294篇 |
2005年 | 239篇 |
2004年 | 190篇 |
2003年 | 129篇 |
2002年 | 165篇 |
2001年 | 126篇 |
2000年 | 99篇 |
1999年 | 57篇 |
1998年 | 25篇 |
1997年 | 16篇 |
1996年 | 7篇 |
1995年 | 11篇 |
1994年 | 15篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1954年 | 6篇 |
排序方式: 共有6049条查询结果,搜索用时 15 毫秒
151.
为合理评价孙疃煤矿10煤层带压开采的安全可靠性,建立了10煤层隔水底板稳定性可靠度分析模型。考虑底板隔水层厚度、底板岩石容重、抗拉强度等参数为随机变量,采用斯列萨列夫安全隔水层厚度计算公式建立极限状态方程。在研究随机变量概型的基础上,采用JC法、最优化法以及基于最优化的蒙特卡罗法计算隔水底板的可靠度以及失效概率,并探讨了该矿10煤层安全开采的可靠指标,避免了单纯依据定值作为突水判据的某些不合理性。 相似文献
152.
153.
煤及其燃烧产物飞灰和底灰中稀土元素地球化学特征及集散规律 总被引:2,自引:4,他引:2
采用ICP—MS法测试了褐煤、肥煤和无烟煤以及在不同燃烧条件下获取的飞灰、底灰等29个样品的稀土元素含量;分析了稀土元素地球化学特征。结果表明,不同煤种的稀土元素含量不同,相同煤种在不同燃烧条件下获取的飞灰、底灰中的稀土元素含量也不同;褐煤、肥煤、无烟煤及其燃煤产物飞灰、底灰的稀土元素分布模式呈左高右低的宽缓的“V”型曲线;Eu存在明显负异常。研究了燃煤过程中稀土元素的分布及集散规律,稀土元素在飞灰、底灰中的含量比原煤有明显提高,其增加幅度为几倍至20多倍不等,表明煤炭燃烧后稀土元素在飞灰、底灰中进一步聚集;飞灰和底灰中稀土元素含量、飞灰和底灰对煤的稀土元素含量比和富集因子以及飞灰对底灰的稀土元素含量比和富集因子等,不仅与原煤中稀土元素有直接关系,而且还受锅炉燃烧方式、燃烧温度(炉温)等人为因素的影响。 相似文献
154.
YANG Xiangning JIN Xiaochi JI Zhansheng WANG Yizhao YAO Jianxin and YANG Hailin Department of Earth Sciences Nanjing University Nanjing Jiangsu Institute of Geology Chinese Academy of Geological Sciences Baiwanzhuang Ro Beijing Geological Survey of Yunnan Yujiang Ro Yuxi Yunnan 《《地质学报》英文版》2004,78(1):15-21
Fossil materials of Shanita and Hemigordius from a section northeast of the Woniusi (the Woniu Temple) of Baoshan, western Yunnan are described and figured in this paper. Field investigation indicates that fossils of these two foraminifer genera are extremely abundant and form a typical Shanita-Hemigordius assemblage in the so-called "Cracked Limestone" in the Baoshan area. Preliminary study has revealed that the features of this assemblage, which includes Shanita amosi, S. chagouensis, Hemigordius renzi and H. biconcavus, are similar not only to the Shanita fauna previously reported from the Shazipo Formation in the Zhengkang area, west Yunnan, but also to those from the Permian of Burma, Thailand, Iran and Turkey. 相似文献
155.
156.
157.
South China as an amalgamation of the Yangtze and Cathaysia blocks is composed of Archean to Mesoproterozoic basement overlain by Neoproterozoic and younger cover. Both the constituent Yangtze and Cathaysia blocks contain well-preserved Neoproterozoic rocks that have been extensively studied in terms of the age and tectonic nature, but less is known about their earlier crustal history due to the incomplete rock record. Recent efforts in investigating the yet survived crustal nature based on isotopic and elemental signatures preserved in igneous and sedimentary rocks have steadily improved our knowledge about the pre-Neoproterozoic continental crustal evolution in South China. In this paper, we summarize the up-to-date pre-Neoproterozoic records, including petrological, geochronological, geochemical and geophysical data, across South China, and discuss its spatiotemporal patterns of the pre-Neoproterozoic crust and the relevant tectonic events. While the xenocrystic/inherited and detrital zircon records suggest widespread Archean (mainly ca. 2.5 Ga) crustal components within both the Yangtze and Cathaysia blocks, exposed Archean rocks are only limited to isolated crustal provinces in the Yangtze Block. These Archean rocks are dominated by TTGs (tonalite-trondhjemite-granodiorite) with varied ages (3.3–2.5 Ga) and zircon Hf isotopes, indicating a compositionally heterogeneous nature of the Archean Yangtze Block and, by inference, the development of multiple ancient terranes. The early Paleoproterozoic (2.4–2.2 Ga) tectonomagmatic events characterize the western Yangtze Block and are supportive of an east-west subdivision of the Yangtze basement, whereas the late Paleoproterozoic (2.1–1.7 Ga) orogeneses may have affected a larger area covering both the western and eastern parts of the Yangtze Block, and also the Cathaysia Block. The eastern Yangtze Block with generally northeastward-younging late Paleoproterozoic magmatism and metamorphism likely experienced a prolonged 2.05–1.75 Ga orogenic process welding the various Archean proto-continents, consistent with the documentation of a buried late Paleoproterozoic orogenic belt imaged by deep seismic profiling from its central part and of a slightly older ophiolitic mélange in the northern part. The Cathaysia Block was probably involved in a short-lived 1.9–1.8 Ga orogenic event. The two orogeneses overlapped in time and may have contributed to the cratonization of a possible unified South China, and are referred to be linked with the assembly of the Nuna Supercontinent. The subsequent late Paleoproterozoic to early Mesoproterozoic rift successions and intrusions (1.7–1.5 Ga) in the southwestern Yangtze Block, and the ca. 1.43 Ga rifting in Hainan Island of the Cathaysia Block could be responses to the Nuna break-up. Late Mesoproterozoic (1.2–1.0 Ga) magmatism of varied age and nature in different localities of the Yangtze Block is reflective of a complex tectonic process in the context of the assembly of the Rodinia Supercontinent. Similar-aged metamorphism (1.3–1.0 Ga) is recorded in Hainan Island, reflecting the Grenvillian continental collision during the Rodinia assembly, but further studies are necessary to better constrain the late Mesoproterozoic tectonic framework of South China. 相似文献
158.
青藏高原唐古拉山北坡夏季风降水特征的初步分析 总被引:9,自引:1,他引:9
利用GAME-Tibet加强观测期间(1998年5 ̄9月)D105站的降水资料,初步分析了青藏高原唐古拉山北坡夏季风降水的特征。表明从6月7日到9月18日,这104d中有81d产生降水,总结水量为279.2mm,每天平均降水量达3.45mm。可以认为6月7日左右为唐古拉山北坡D105站处夏季风爆发日。8月份降水量最大。为121.6mm,占观测期间总降水量的43.6%。在夏季风期间,季风降水存在着明 相似文献
159.
为了系统研究和比较我国产区的一些海水和淡水贝壳珍珠层的物相和元素成分,我们采集了2种淡水育珠三角帆蚌壳和8种海水贝壳,其中4种是海水育珠马氏珠母贝壳,另4种是其它海水贝壳,并进行预处理以制备其贝壳珍珠层。然后使用X射线衍射、氨基酸分析、原子吸收光谱、原子荧光光谱法对贝壳珍珠层进行分析。结果表明这10种贝壳珍珠层的矿物物相都十分简单,主要由文石组成,仅含少量的方解石,并且它们的物相成分无明显的差别,难以通过X射线衍射(XRD)分析技术加以鉴定。与其它4种海水贝壳珍珠层相比,马氏珠母贝和三角帆蚌壳珍珠层的氨基酸含量明显大些。各种贝壳珍珠层的微量元素Na、Pb、Hg、As、Cu、Se、Cr、Mn的含量,与其品种基本无关,但可能与其生长的水环境有关。 相似文献
160.
正We studied the solid-liquid transformation of low-grade solid potash deposit in Dalangtan Basin and simplified the liquid phase system.We did experiments to optimize conditions of the solid-liquid transformation.The Suitable 相似文献