全文获取类型
收费全文 | 3948篇 |
免费 | 826篇 |
国内免费 | 2091篇 |
专业分类
测绘学 | 191篇 |
大气科学 | 393篇 |
地球物理 | 908篇 |
地质学 | 3933篇 |
海洋学 | 668篇 |
天文学 | 242篇 |
综合类 | 226篇 |
自然地理 | 304篇 |
出版年
2024年 | 21篇 |
2023年 | 65篇 |
2022年 | 152篇 |
2021年 | 174篇 |
2020年 | 226篇 |
2019年 | 254篇 |
2018年 | 234篇 |
2017年 | 194篇 |
2016年 | 277篇 |
2015年 | 240篇 |
2014年 | 297篇 |
2013年 | 376篇 |
2012年 | 330篇 |
2011年 | 379篇 |
2010年 | 303篇 |
2009年 | 328篇 |
2008年 | 328篇 |
2007年 | 340篇 |
2006年 | 390篇 |
2005年 | 319篇 |
2004年 | 257篇 |
2003年 | 213篇 |
2002年 | 180篇 |
2001年 | 147篇 |
2000年 | 116篇 |
1999年 | 119篇 |
1998年 | 110篇 |
1997年 | 103篇 |
1996年 | 78篇 |
1995年 | 59篇 |
1994年 | 68篇 |
1993年 | 46篇 |
1992年 | 22篇 |
1991年 | 32篇 |
1990年 | 22篇 |
1989年 | 13篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 4篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有6865条查询结果,搜索用时 312 毫秒
1.
The impact of dropped anchor on submarine photoelectric composite cables may possibly cause electrical faults, i.e. electricity and optical signal transmission failure. In order to study the impact capacity and structural impact failure mechanism, a test setup is designed originally to examine the structural and functional integrity. A detailed finite element model (FEM) is created, considering material nonlinearity and component interaction. A parametric analysis has been performed to predict the deformation of components and impact forces, under different impact velocities and collision directions. Relationships between the armor layer indentation rate and that of internal power and optical units are achieved. The impact deformation of internal entities can be evaluated intuitively by armor layer indentation. The proposed experimental and numerical methods are well correlated, suitable to assess the impact capacity of subsea power cables and assist the protection design of subsea power cables in engineering. 相似文献
2.
3.
Mobile Agent技术是一门新兴的技术,具有移动性、智能性和异步计算的特点,已成为计算机网络和分布式系统最具活力的发展方向.现对Mobile Agent的概念、系统结构及其关键技术进行了分析和探讨. 相似文献
4.
5.
BIAN Jian-min LIN Nian-feng TANG Jie College of Environment Resources Jilin University Changchun Jilin China 《东北亚地学研究》2005,(Z1)
Desertification is an environmental issue in the world. The salt-alkalization desertification land area formed by both primary and secondary salt-alkalization has extended in a large scale, which has become a significant eco-environmental problem. Based on the characteristics of eco-environment and the situation of desertification in western Songnen plain, this paper reports the analyzes of its formation in background and cause. An early warning system on the salt-alkalization desertification is established and the GIS technology is used to abstract the information of desertification evaluation index. Supported by the integrated technology of the GIS and ANN, the orientation and quantitative result of desertification are gained, which is helpful to the eco-environment protection and resource development in western Songnen Plain. 相似文献
6.
2D numerical modelling of impact cratering has been utilized to quantify an important depth-diameter relationship for different crater morphologies, simple and complex. It is generally accepted that the final crater shape is the result of a gravity-driven collapse of the transient crater, which is formed immediately after the impact. Numerical models allow a quantification of the formation of simple craters, which are bowl-shaped depressions with a lens of rock debris inside, and complex craters, which are characterized by a structural uplift. The computation of the cratering process starts with the first contact of the impactor and the planetary surface and ends with the morphology of the final crater. Using different rheological models for the sub-crater rocks, we quantify the influence on crater mechanics. To explain the formation of complex craters in accordance to the threshold diameter between simple and complex craters, we utilize the Acoustic Fluidization model. We carried out a series of simulations over a broad parameter range with the goal to fit the observed depth/diameter relationships as well as the observed threshold diameters on the Moon, Earth and Venus. 相似文献
7.
A growing body of evidence implies that the concept of 'treeless tundra' in eastern and northern Europe fails to explain the rapidity of Lateglacial and postglacial tree population dynamics of the region, yet the knowledge of the geographic locations and shifting of tree populations is fragmentary. Pollen, stomata and plant macrofossil stratigraphies from Lake Kurjanovas in the poorly studied eastern Baltic region provide improved knowledge of ranges of north‐eastern European trees during the Lateglacial and subsequent plant population responses to the abrupt climatic changes of the Lateglacial/Holocene transition. The results prove the Lateglacial presence of tree populations (Betula, Pinus and Picea) in the eastern Baltic region. Particularly relevant is the stomatal and plant macrofossil evidence showing the local presence of reproductive Picea populations during the Younger Dryas stadial at 12 900–11 700 cal. a BP, occurring along with Dryas octopetala and arctic herbs, indicating semi‐open vegetation. The spread of Pinus–Betula forest at ca. 14 400 cal. a BP, the rise of Picea at ca. 12 800 cal. a BP and the re‐establishment of Pinus–Betula forest at ca. 11 700 cal. a BP within a span of centuries further suggest strikingly rapid, climate‐driven ecosystem changes rather than gradual plant succession on a newly deglaciated land. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
8.
基于统一强度理论的地基临界荷载公式 总被引:1,自引:0,他引:1
基于统一强度理论,按照临界荷载公式的推导方法,得到地基临界荷载的统一解公式。Mohr-Coulomb强度理论计算的结果为其特例。把计算结果与《建筑地基基础设计规范》中的结果进行了对比分析,认为当不同程度地考虑中间主应力σ2的影响时,可以有效地发挥地基的强度潜能。并通过实例计算了统一强度理论参数b取不同值时地基承载力特征值,并进行了对比分析。 相似文献
9.
S. Lepp 《Astrophysics and Space Science》2003,285(3-4):737-744
The evolution of the universe from times after the recombination of the first atoms is controlled by chemistry. Chemistry
can also help us to learn more about the first structures. In this paper we review the chemistry and atomic physics that are
important in the time shortly after recombination, including the formation of the first structures.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
网络安全是近年来研究的热点,各种新技术不断涌现,身份认证是安全应用中最重要的一个问题,也是PKI的核心服务之一。PKI作为信息安全的基石经过几年的发展不断走向成熟,在许多领域发挥着重要的作用。针对目前对PKI中的重要部件RA的研究很少的状况,着重对RA的实现模型进行了探讨。 相似文献