全文获取类型
收费全文 | 31817篇 |
免费 | 5809篇 |
国内免费 | 8190篇 |
专业分类
测绘学 | 2337篇 |
大气科学 | 6543篇 |
地球物理 | 7671篇 |
地质学 | 16433篇 |
海洋学 | 4378篇 |
天文学 | 1266篇 |
综合类 | 3194篇 |
自然地理 | 3994篇 |
出版年
2024年 | 154篇 |
2023年 | 555篇 |
2022年 | 1585篇 |
2021年 | 1811篇 |
2020年 | 1527篇 |
2019年 | 1752篇 |
2018年 | 1961篇 |
2017年 | 1689篇 |
2016年 | 1917篇 |
2015年 | 1659篇 |
2014年 | 1969篇 |
2013年 | 1973篇 |
2012年 | 1842篇 |
2011年 | 1945篇 |
2010年 | 1871篇 |
2009年 | 1811篇 |
2008年 | 1585篇 |
2007年 | 1476篇 |
2006年 | 1249篇 |
2005年 | 1170篇 |
2004年 | 929篇 |
2003年 | 916篇 |
2002年 | 892篇 |
2001年 | 954篇 |
2000年 | 1012篇 |
1999年 | 1394篇 |
1998年 | 1080篇 |
1997年 | 1072篇 |
1996年 | 1029篇 |
1995年 | 847篇 |
1994年 | 764篇 |
1993年 | 674篇 |
1992年 | 566篇 |
1991年 | 420篇 |
1990年 | 340篇 |
1989年 | 304篇 |
1988年 | 254篇 |
1987年 | 155篇 |
1986年 | 157篇 |
1985年 | 111篇 |
1984年 | 91篇 |
1983年 | 71篇 |
1982年 | 58篇 |
1981年 | 49篇 |
1980年 | 36篇 |
1979年 | 30篇 |
1978年 | 22篇 |
1976年 | 12篇 |
1975年 | 10篇 |
1958年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
This paper presents an on-line trained neural net work controller for ship track-keeping problems. Following a brief review of the ship track-keeping control development since the 1980's, an analysis of various existing backpropagation-based neural controllers is provided. We then propose a single-input multioutput (SIMO) neural control strategy for situations where the exact mathematical dynamics of the ship are not available. The aim of this study is to build an autonomous neural controller which uses rudder to regulate both the tracking error and heading error. During the whole control process, the proposed SIMO neural controller adapts itself on-line from a direct evaluation of the control accuracy, and hence the need for a “teacher” or an off-line training process can be removed. With a relatively modest amount of quantitative knowledge of the ship behavior, the design philosophy enables real time control of a nonlinear ship model under random wind disturbances and measurement noise. Three different track-keeping tasks have been simulated to demonstrate the effectiveness of the training method and the robust performance of the proposed neural control strategy 相似文献
42.
通过对辽北法库地区构造岩的观察和研究,确定了本区存在一大型推覆韧性剪切带。根据同构造侵入岩的同位素年龄,确定该韧性剪切带形成时期为海西-印支期,这与天山-兴安褶皱系最终形成时间相一致 相似文献
43.
Tidal effects on temperature front in the Yellow Sea 总被引:5,自引:0,他引:5
Temperature front (TF) is one of the important features in the Yellow Sea, which forms in spring, thrives in summer, and fades
in autumn as thermocline declines. TF intensity ⋎S
T
⋎ is defined to describe the distribution of TF. Based on the MASNUM wave-tide-circulation coupled model, temperature distribution
in the Yellow Sea was simulated with and without tidal effects. Along 36°N, distribution of TF from the simulated results
are compared with the observations, and a quantitative analysis is introduced to evaluate the tidal effects on the forming
and maintaining processes of the TF. Tidal mixing and the circulation structure adapting to it are the main causes of the
TF.
Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No.
49736190). 相似文献
44.
采用PCR技术对2种亚洲龙鱼的mtDNAD_Loop全序列进行扩增和测序,序列结构分析和序列同源性比对结果表明,2种亚洲龙鱼的mtDNAD_Loop在靠近5’端有3个终止相关序列TAS(Ⅰ、Ⅱ、Ⅲ),靠近D_Loop的3’端有4个保守区域CSB1、CSB2、CSB3、CSB-D。在终止相关序列和保守区域之间是连续重复区域。经DNASP4.0软件分析,全序列中检测出多态位点数(S)为26,其中有17个转换,核苷酸多样性(Pi)为0.013,平均核苷酸差异数(K)为17.333。 相似文献
45.
CHEN Bing-lu ZHANG Yun-ni CHEN Xin-geng WANG Zhi-gang YANG Guang-xing 《中国地理科学(英文版)》2001,11(2):155-162
1 HYDROLOGIC FEATURES Lingdingyang Estuary, located at the middle south of Guangdong Province, is a bell-shaped estuary with a north-south direction. Its area is about 2100km2. The north of Qi′ao Island and Inner-Lingding Island, and the south of Humen are grouped as Neilingdingyang Estuary, having an area of 1041km2. Affected by topography, runoff and tide, its dynamic condition is very complicated. Different water areas have different hydrologic features. The topography under … 相似文献
46.
Dynamic analysis of three-dimensional marine cables 总被引:1,自引:0,他引:1
Shan Huang 《Ocean Engineering》1994,21(6)
Marine cables are widely used in a variety of offshore activities. In this paper, a numerical approach is presented which is capable of predicting the dynamic behaviour of three-dimensional marine cables. The solution of this problem is of practical importance in the estimation of dybamic loading and motion, and thus has direct application to the enhancement of safety and the effectiveness of the offshore activities. The approach is based upon the lump-mass-and-spring model and the finite difference method. Mathematical analysis is provided on the basic characteristics of the model and the numerical stability. A numerical example is given to demonstrate the validity of the approach. 相似文献
47.
IntroductionThe area of eastern Liaoning is an importantmetal and nonmetal metallogenetic district in China,and the Liaohe group is one of the most importantstrata that hosts Pb, Zn, Au, B and Mg etcstratabound deposits. Up to now many geo1ogistssuch as Z… 相似文献
48.
With a new theory on the 1PN celestial mechanics recently developed by Damour, Soffel and Xu (1991,1992,1993,1994), definitions
and expressions of the 1PN spin angular momentum are investigated and analysed. The total spin angular momentum of a system
of extended bodies such as the solar system is calculated and expressed as the function of local parameters and observables
under reasonable assumptions, which would find its application in the evolution and dynamics of systems of celestial bodies.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
49.
50.
基于拖带坐标法建立了岩层大变形的一维非线性微分方程,然后用解析与数值方法研究了变刚度、可伸缩岩层在侧向压力及自重作用下的分叉特性,初步揭示了褶皱岩层初始挠曲的形成机理。 相似文献