全文获取类型
收费全文 | 56172篇 |
免费 | 10020篇 |
国内免费 | 13401篇 |
专业分类
测绘学 | 3672篇 |
大气科学 | 12176篇 |
地球物理 | 14012篇 |
地质学 | 27619篇 |
海洋学 | 6643篇 |
天文学 | 2576篇 |
综合类 | 6172篇 |
自然地理 | 6723篇 |
出版年
2024年 | 252篇 |
2023年 | 939篇 |
2022年 | 2301篇 |
2021年 | 2714篇 |
2020年 | 2304篇 |
2019年 | 2563篇 |
2018年 | 2960篇 |
2017年 | 2702篇 |
2016年 | 3137篇 |
2015年 | 2616篇 |
2014年 | 3344篇 |
2013年 | 3107篇 |
2012年 | 2981篇 |
2011年 | 3165篇 |
2010年 | 3342篇 |
2009年 | 3247篇 |
2008年 | 2910篇 |
2007年 | 2794篇 |
2006年 | 2252篇 |
2005年 | 2116篇 |
2004年 | 1660篇 |
2003年 | 1648篇 |
2002年 | 1633篇 |
2001年 | 1625篇 |
2000年 | 1913篇 |
1999年 | 2694篇 |
1998年 | 2229篇 |
1997年 | 2234篇 |
1996年 | 1981篇 |
1995年 | 1777篇 |
1994年 | 1575篇 |
1993年 | 1403篇 |
1992年 | 1143篇 |
1991年 | 866篇 |
1990年 | 617篇 |
1989年 | 621篇 |
1988年 | 531篇 |
1987年 | 340篇 |
1986年 | 292篇 |
1985年 | 207篇 |
1984年 | 186篇 |
1983年 | 144篇 |
1982年 | 129篇 |
1981年 | 107篇 |
1980年 | 88篇 |
1979年 | 62篇 |
1978年 | 27篇 |
1977年 | 18篇 |
1976年 | 8篇 |
1958年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
22.
23.
本文论述了珠穆朗玛峯地区土壤中稀土元素的含量,以及它们在土壤中的分布模式,并发现土壤中稀土元素的含量水平在很大程度上受土壤母质的制约。 相似文献
24.
在回转椭球状降水粒子的假设下求出了降水粒子的散射截面,并且考虑了粒子轴线铅垂和随机取向两种重要的典型情况;推导了双极化雷达的回波方程以及降雨量公式。该方程除包含降水粒子的信息外,关于雷达与降水粒子间的几何关系都以雷达天线仰角及雷达与降水粒子间的距离表示,同时还考虑了地球的球面效应和大气折射效应。由于采用了并矢方法,使推导简捷明了 相似文献
25.
Brine shrimp (Artemia parthenogenetica) which had ingested three water-insoluble antibacterial drugs i.e. sulfadiazine(SD), oxytetracycline (OTC) and erythromycin
estolate (ERY-Es) were fed toTilapia and Mysis IIIof Penaeus orientalis K. The drug contents in the predators were then determined. After administration of drugs toTilapia and Mysis III, through the bio-encapsulation of the brine shrimp, efficacious therapeutical concentration of OTC and ERY-Es
(but not SD) in the predators could be reached and maintained for more than 8 hours. 相似文献
26.
27.
28.
Maarten A. Prins Hongbo Zheng Kay Beets Simon Troelstra Patrick Bacon Ilse Kamerling Wouter Wester Martin Konert Xiangtong Huang Wang Ke Jef Vandenberghe 《第四纪科学杂志》2009,24(1):75-84
The Mangshan Plateau is located on the south bank of the Huang He (Yellow River) just west of the city of Zhengzhou, well outside the Loess Plateau in central China. Mixing models of the grain‐size data indicate that the loess deposits are mixtures of three loess components. Comparison of the mixing model with existing models established for a series of loess–palaeosol sequences from the Loess Plateau indicates that the Mangshan loess has been supplied from a proximal dust source, the Huang He floodplain, during major dust outbreaks. The high accumulation rates, the composition of the loess components, and especially the high proportions of a sandy loess component support this. Owing to the exceptionally high accumulation rates, the Mangshan grain size, magnetic susceptibility and carbonate records provide a high‐resolution archive of environmental and climate change. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
29.
30.
Starfish oocytes with intact germinal vesicles (GVs) were cut along desired planes with glass needles or ligated using silk
thread loops into two parts and allowed to mature in vitro, and inseminated. The experimental results showed that (1) only
the parts with GVs or partial GV contents (PGVCs) cleaved, those without any GV materials did not; but nucleated and non-nucleated
fragments cut from mature eggs were able to divide; (2) the development of animal parts of oocytes containing GVs or PGVCs
was like that of animal fragments of matured oocytes with female pronuclei; most of them gave rise to permanent blastulae,
and just a few formed ectodermal vesicles with a little primary mesenchyme; (3) a large part of vegetal fragments with GVs
or PGVCs, and the vegetal parts of mature eggs without female pronuclei developed into small but normal embryos; (4) the fragments
containing GVs or PGVCs obtained from the oocytes along a plane parallel to the animal-vegetal (A-V) axis developed as normally
as the halves (with or without female pronuclei) severed from mature eggs along the same axis. Based on the data above, it
was concluded that (1) the non-chromatin materials in the oocyte GVs are indispensable for successful fertilization and cleavage
of starfish eggs; (2) some factor (s) located asymmetrically in the vegetal hemispheres of starfish oocytes is (are) responsible
for formation of the archenteron and primary mesenchyme. It is evident from the above findings that the oocyte cytoplasm of
the starfish had already regionalized before the GV break-down.
Contribution No. 1722 from the Institute of Oceanology, Academia Sinica 相似文献