全文获取类型
收费全文 | 70507篇 |
免费 | 1025篇 |
国内免费 | 1455篇 |
专业分类
测绘学 | 2578篇 |
大气科学 | 5351篇 |
地球物理 | 14086篇 |
地质学 | 27603篇 |
海洋学 | 5067篇 |
天文学 | 12065篇 |
综合类 | 2232篇 |
自然地理 | 4005篇 |
出版年
2021年 | 299篇 |
2020年 | 365篇 |
2019年 | 410篇 |
2018年 | 5432篇 |
2017年 | 4695篇 |
2016年 | 3575篇 |
2015年 | 892篇 |
2014年 | 1139篇 |
2013年 | 2373篇 |
2012年 | 2165篇 |
2011年 | 4252篇 |
2010年 | 3396篇 |
2009年 | 4169篇 |
2008年 | 3618篇 |
2007年 | 4034篇 |
2006年 | 1866篇 |
2005年 | 1653篇 |
2004年 | 1874篇 |
2003年 | 1761篇 |
2002年 | 1578篇 |
2001年 | 1161篇 |
2000年 | 1207篇 |
1999年 | 1062篇 |
1998年 | 1041篇 |
1997年 | 1003篇 |
1996年 | 800篇 |
1995年 | 810篇 |
1994年 | 792篇 |
1993年 | 678篇 |
1992年 | 637篇 |
1991年 | 558篇 |
1990年 | 659篇 |
1989年 | 538篇 |
1988年 | 539篇 |
1987年 | 606篇 |
1986年 | 511篇 |
1985年 | 739篇 |
1984年 | 805篇 |
1983年 | 782篇 |
1982年 | 682篇 |
1981年 | 647篇 |
1980年 | 664篇 |
1979年 | 557篇 |
1978年 | 556篇 |
1977年 | 520篇 |
1976年 | 528篇 |
1975年 | 512篇 |
1974年 | 526篇 |
1973年 | 507篇 |
1972年 | 329篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Hector Socas-Navarro David Elmore Anna Pietarila Anthony Darnell Bruce W. Lites Steven Tomczyk Steven Hegwer 《Solar physics》2006,235(1-2):55-73
The Spectro-Polarimeter for Infrared and Optical Regions (SPINOR) is a new spectro-polarimeter that will serve as a facility
instrument for the Dunn Solar Telescope at the National Solar Observatory. This instrument is capable of achromatic polarimetry
over a very broad range of wavelengths, from 430 to 1600 nm, allowing for the simultaneous observation of several visible
and infrared spectral regions with full Stokes polarimetry. Another key feature of the design is its flexibility to observe
virtually any combination of spectral lines, limited only by practical considerations (e.g., the number of detectors available, space on the optical bench, etc.).
Visiting Astronomers, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc.
(AURA), under cooperative agreement with the National Science Foundation. 相似文献
92.
The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 105 to 1.6 × 106 M. The expansion velocity of the Ring is estimated to be 30–40 km s−1, which gives an expansion time scale of 1.5 × 105 year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN− from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring. 相似文献
93.
94.
95.
ZHANGXue-qin GEQuan-sheng ZHENGJing-yun 《中国地理科学(英文版)》2003,13(3):207-215
How land-cover has been changed by human use over the last 300 years is one of the five overarching questions guiding the Land-use/Cover Change (LUCC) Science/Research Plan. China has variety of historical docu-ments providing unique data superiority. So the characteristics of farmland area in Shandong Province during the Qing Dynasty (1644-1911) are summarized firstly: 1) the rising trend of farmland area was striking; 2) farmland area had re-markable fluctuation; 3) farmland area per capita decreased dramatically; 4) wasteland reclamation index increased rapidly. Then, the driving forces of farmland area change are analyzed. It is concluded that natural and human factors are jointly influential. Among the driving forces, human dimensions are the main factors of farmland area change,which direct the general tendency of the changes mentioned above. And the natural factors influence the stability of farmland area as well. Variation of the natural factors would act as the major contributory factor to farmland area change during years or periods of abrupt climatic changes, or during the intensive occurring periods of natural hazards.Besides, the passive aspects of human factors, such as war chaos also influenced the fluctuation of the farmland area.This research indicates that it is feasible to study the land-use/cover change by Chinese historical literatures, which has huge potential to provide a comprehensive picture of the growing dominance of human land-use and land-cover pat-terns that can be used in many global change research oroiects. 相似文献
96.
WANGXie-kang HUANGEr CUIPeng 《中国地理科学(英文版)》2003,13(3):262-266
Debris flow is one of the most destructive phenomena of natural hazards. Recently, major natural haz-ard, claiming human lives and assets, is due to debris flow in the world. Several practical methods for forecasting de-bris flow have been proposed, however, the accuracy of these methods is not high enough for practical use because of the stochastic and non-linear characteristics of debris flow. Artificial neural network has proven to be feasible and use-fill in developing models for nonlinear systems. On the other hand, predicting the future behavior based on a time se-ries of collected historical data is also an important tool in many scientific applications. In this study we present a three-layer feed-forward neural network model to forecast surge of debris flow according to the time series data collect-ed in the Jiangjia Ravine, situated in north part of Yunnan Province of China. The simulation and prediction of debris flow using the proposed approach shows this model is feasible, however, further studies are needed. 相似文献
97.
98.
This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
99.
100.