首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   21798篇
  免费   3619篇
  国内免费   5950篇
测绘学   2814篇
大气科学   3723篇
地球物理   5213篇
地质学   9047篇
海洋学   3832篇
天文学   2081篇
综合类   1868篇
自然地理   2789篇
  2024年   78篇
  2023年   236篇
  2022年   671篇
  2021年   744篇
  2020年   983篇
  2019年   1187篇
  2018年   878篇
  2017年   1028篇
  2016年   1046篇
  2015年   1218篇
  2014年   1231篇
  2013年   1428篇
  2012年   1387篇
  2011年   1399篇
  2010年   1135篇
  2009年   1386篇
  2008年   1425篇
  2007年   1697篇
  2006年   1662篇
  2005年   1447篇
  2004年   1214篇
  2003年   1057篇
  2002年   915篇
  2001年   734篇
  2000年   838篇
  1999年   827篇
  1998年   694篇
  1997年   478篇
  1996年   417篇
  1995年   362篇
  1994年   309篇
  1993年   297篇
  1992年   225篇
  1991年   155篇
  1990年   103篇
  1989年   160篇
  1988年   80篇
  1987年   65篇
  1986年   57篇
  1985年   31篇
  1984年   25篇
  1983年   10篇
  1982年   13篇
  1981年   8篇
  1980年   6篇
  1979年   4篇
  1978年   3篇
  1977年   2篇
  1954年   7篇
  1877年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Galileo系统及其在中国的应用   总被引:9,自引:0,他引:9  
较详细地介绍了Galileo系统的全面体系结构、频率设计、服务内容及其广泛的应用范围,指出了它与GPS系统相比的优点,并对开发和增强Galileo系统在我国多层次、全方位的应用提出了建议。  相似文献   
82.
INTRODUCTIONItwasgenerallyconsideredpreviouslythatthecorrosionofsteelinseamudwasnotserious.However,itwaslaterfoundthatthecorrosionrateofsteelinsomeseamudwashigherthanthatinseawater(King,1980;Sehmide,1982;Guoetal.,1997;Hou,1998).Therearesomereportsthatthecorrosionr…  相似文献   
83.
饶长辉  姜文汉  凌宁 《天文学报》2001,42(2):134-139
分析以观测系统焦面上和探测器测到的太阳表面米粒结构的对比度与观测系统口径,大气湍流相干长度以及系统探测灵敏度的关系,给出了不同口径,不同大气湍流相干长度以及不同系统探测灵敏度时的米粒结构对比值计算结果,此外还给出了实验结果。  相似文献   
84.
The paper provides a new stereo‐analytical method, which is a combination of the stereographic method and analytical methods, to separate finite removable blocks from the infinite and tapered blocks in discontinuous rock masses. The methodology has applicability to both convex and concave blocks. Application of the methodology is illustrated through examples. Addition of this method to the existing block theory procedures available in the literature improves the capability of block theory in solving practical problems in rock engineering. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
85.
86.
87.
88.
Introducing a concept of equivalent mass depth of flow, this study describes the phenomenon of non‐point source pollutant (metal) transport for pavement (or overland) flow in analogy with wave propagation in wide open channels. Hysteretic and normal mass rating curves are developed for runoff rate and mass of 12 dissolved and particulate‐bound metal elements (pollutants) using the rainfall‐runoff and water quality data of the 15 × 20 m2 instrumented pavement in Cincinnati, USA. Normal mass rating curves developed for easy computation of pollutant load are found to be of a form similar to Manning's, which is valid for open channel flows. Based on the hysteresis analysis, wave types for dissolution and mixing of particulate‐bound metals are identified. The analysis finds that the second‐order partial‐differential equation normally used for metal transport does not have the efficacy to describe fully the strong non‐linear phenomena such as is described for various metal elements by dynamic waves. In addition, the proportionality concept of the popular SCS‐CN concept is extended for determining the potential maximum metal mass Mp of all the 12 elements transported by a rain storm and related to the antecedent dry period (ADP). For the primary metal zinc element, Mp is found to increase with the ADP and vice versa. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
89.
an extension of the Gylden-Meshcherskii problem when the mass depends both on time and distance between two bodies is considered. certain mass loss laws as well as the Meshcherskii position vector and time transformation are used to convert the problem into the cases with equations of motion arising from integrable potentials.  相似文献   
90.
In view of rapid developments in iterative solvers, it is timely to re‐examine the merits of using mixed formulation for incompressible problems. This paper presents extensive numerical studies to compare the accuracy of undrained solutions resulting from the standard displacement formulation with a penalty term and the two‐field mixed formulation. The standard displacement and two‐field mixed formulations are solved using both direct and iterative approaches to assess if it is cost‐effective to achieve more accurate solutions. Numerical studies of a simple footing problem show that the mixed formulation is able to solve the incompressible problem ‘exactly’, does not create pressure and stress instabilities, and obviate the need for an ad hoc penalty number. In addition, for large‐scale problems where it is not possible to perform direct solutions entirely within available random access memory, it turns out that the larger system of equations from mixed formulation also can be solved much more efficiently than the smaller system of equations arising from standard formulation by using the symmetric quasi‐minimal residual (SQMR) method with the generalized Jacobi (GJ) preconditioner. Iterative solution by SQMR with GJ preconditioning also is more elegant, faster, and more accurate than the popular Uzawa method. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号