全文获取类型
收费全文 | 10060篇 |
免费 | 2208篇 |
国内免费 | 3304篇 |
专业分类
测绘学 | 1327篇 |
大气科学 | 1600篇 |
地球物理 | 2024篇 |
地质学 | 6301篇 |
海洋学 | 1685篇 |
天文学 | 296篇 |
综合类 | 1053篇 |
自然地理 | 1286篇 |
出版年
2024年 | 116篇 |
2023年 | 308篇 |
2022年 | 645篇 |
2021年 | 719篇 |
2020年 | 577篇 |
2019年 | 714篇 |
2018年 | 623篇 |
2017年 | 573篇 |
2016年 | 606篇 |
2015年 | 580篇 |
2014年 | 629篇 |
2013年 | 699篇 |
2012年 | 902篇 |
2011年 | 770篇 |
2010年 | 816篇 |
2009年 | 755篇 |
2008年 | 638篇 |
2007年 | 712篇 |
2006年 | 622篇 |
2005年 | 513篇 |
2004年 | 403篇 |
2003年 | 327篇 |
2002年 | 304篇 |
2001年 | 286篇 |
2000年 | 300篇 |
1999年 | 279篇 |
1998年 | 164篇 |
1997年 | 186篇 |
1996年 | 128篇 |
1995年 | 111篇 |
1994年 | 108篇 |
1993年 | 101篇 |
1992年 | 74篇 |
1991年 | 58篇 |
1990年 | 46篇 |
1989年 | 35篇 |
1988年 | 23篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 8篇 |
1984年 | 12篇 |
1983年 | 15篇 |
1982年 | 7篇 |
1981年 | 10篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1958年 | 2篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
卫星电视授时系统及精度估计 总被引:1,自引:0,他引:1
我国已建成短波、长波授时台,进行毫秒级、微秒级的时间服务.随着现代科技和经济的发展,上述方法已不能满足国家及国际交流的需要.本文提出卫星电视授时系统,同步卫星定位精度小于110m,卫星授时精度为亚微秒。两原子钟利用卫星进行时刻同步,其精度与两原子钟的距离有关,相距3千多公里的两钟同步精度小于35ns,相距数百公里的两原子钟同步精度小于20ns 相似文献
13.
14.
15.
Biogenic silicate accumulation in sediments, Jiaozhou Bay 总被引:1,自引:0,他引:1
1 INTRODUCTION Silicate, or silicic acid (H4SiO4), is a very im- portant nutrient in the ocean. Unlike other major nu- trients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical req… 相似文献
16.
On September 14–18, 2000, a medium-small solar active region was observed at Ganyu Station of Purple Mountain Observatory. Its spots were not large, but it had a peculiar active filament. On Sep.16, a flare of importance IIIb with rather intense geophysical effects was produced. Our computation of the magnetic structure of the active region reveals that the rope-shaped filament was concerned with a low magnetic arc close to magnetic neutral line. An intense shear of magnetic field occurred near magnetic rope. The QSL analysis shows that a 3-D magnetic reconnection might appear in the vicinity of filament, and this can be used to interpret the formation of a large flare. 相似文献
17.
Study is made on a 45 km-long artificial ecosystem without irrigation in Tengger desert on the basis of long-term ecological monitoring and ecohydrological fundamentals. Changes in water allocation, utilization, cycle and balance patterns in more than 40-year evolution of the soil-plant system are analyzed. The formation of a drought horizon in shrub rhizosphere and its effect, ecohydrological function of the crust and its effect on the soil-plant system change are discussed. Driven by water self-regulation and water stress, the soil-plant system is going to develop towards the steppe desert to ensure more effective use and optimum collocation of water resource.
相似文献18.
介绍了一种新颖的静态传感器网络实现方法,它利用可移动的中继节点来收集静态传感器网络的数据,有效的节约各传感器节点的能源,延长整个传感器网络的有效工作时间。也可以对该方法进行适当的改进,应用到某些实时性要求较高的场合中去。然后介绍了应用该原理实现的一个通讯协议。 相似文献
19.
Three experiments on the initially slightly flattened spheroids (=0.45) have been made. The first one is run without perturbation and the others with perturbation. The results have shown that the spheroid without perturbation will evolve to nearly sphere and the other two will almost keep the originally projected shape after perturbation and relaxation.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
20.
Xie-Yan Song Hua-Wen Qi Paul T. Robinson Mei-Fu Zhou Zhi-Min Cao Lie-Meng Chen 《Lithos》2008,100(1-4):93-111
The Emeishan continental flood basalt (ECFB) sequence in Dongchuan, SW China comprises a basal tephrite unit overlain by an upper tholeiitic basalt unit. The upper basalts have high TiO2 contents (3.2–5.2 wt.%), relatively high rare-earth element (REE) concentrations (40 to 60 ppm La, 12.5 to 16.5 ppm Sm, and 3 to 4 ppm Yb), moderate Zr/Nb and Nb/La ratios (9.3–10.2 and 0.6–0.9, respectively) and relatively high Nd (t) values, ranging from − 0.94 to 2.3, and are comparable to the high-Ti ECFB elsewhere. The tephrites have relatively high P2O5 (1.3–2.0 wt.%), low REE concentrations (e.g., 17 to 23 ppm La, 4 to 5.3 ppm Sm, and 2 to 3 ppm Yb), high Nb/La (2.0–3.9) ratios, low Zr/Nb ratios (2.3–4.2), and extremely low Nd (t) values (mostly ranging from − 10.6 to − 11.1). The distinct compositional differences between the tephrites and the overlying tholeiitic basalts cannot be explained by either fractional crystallization or crustal contamination of a common parental magma. The tholeiitic basalts formed by partial melting of the Emeishan plume head at a depth where garnet was stable, perhaps > 80 km. We propose that the tephrites were derived from magmas formed when the base of the previously metasomatized, volatile-mineral bearing subcontinental lithospheric mantle was heated by the upwelling mantle plume. 相似文献