全文获取类型
收费全文 | 46411篇 |
免费 | 8325篇 |
国内免费 | 10957篇 |
专业分类
测绘学 | 3477篇 |
大气科学 | 9895篇 |
地球物理 | 11404篇 |
地质学 | 22717篇 |
海洋学 | 5396篇 |
天文学 | 1932篇 |
综合类 | 5069篇 |
自然地理 | 5803篇 |
出版年
2024年 | 119篇 |
2023年 | 641篇 |
2022年 | 1949篇 |
2021年 | 2142篇 |
2020年 | 1793篇 |
2019年 | 2068篇 |
2018年 | 2489篇 |
2017年 | 2289篇 |
2016年 | 2592篇 |
2015年 | 2253篇 |
2014年 | 2688篇 |
2013年 | 2707篇 |
2012年 | 2548篇 |
2011年 | 2626篇 |
2010年 | 2763篇 |
2009年 | 2696篇 |
2008年 | 2456篇 |
2007年 | 2344篇 |
2006年 | 1900篇 |
2005年 | 1775篇 |
2004年 | 1371篇 |
2003年 | 1431篇 |
2002年 | 1475篇 |
2001年 | 1493篇 |
2000年 | 1654篇 |
1999年 | 2219篇 |
1998年 | 1813篇 |
1997年 | 1756篇 |
1996年 | 1617篇 |
1995年 | 1409篇 |
1994年 | 1257篇 |
1993年 | 1070篇 |
1992年 | 882篇 |
1991年 | 687篇 |
1990年 | 527篇 |
1989年 | 475篇 |
1988年 | 422篇 |
1987年 | 248篇 |
1986年 | 219篇 |
1985年 | 157篇 |
1984年 | 139篇 |
1983年 | 119篇 |
1982年 | 90篇 |
1981年 | 88篇 |
1980年 | 64篇 |
1979年 | 48篇 |
1978年 | 22篇 |
1977年 | 19篇 |
1976年 | 9篇 |
1958年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
A wide-field time-series CCD photometric survey of variable stars in the field of the open cluster NGC 2168 was carried out using the BATC Schmidt telescope. In total 13 new variable stars are discovered with three W UMa systems, one EA type and two EB type eclipsing binaries (one of them could be a W UMa system), and seven pulsating stars including three candidates of δScuti stars. 相似文献
52.
本区至今还是综合科学考察的空白区。为查明基本情况,1989年夏以乘汽车路线考察方式穿行该区而作了预查,搜集了当地地质、地理、生物等基本资料。经考察发现,区内野生动物资源丰富,上新世以来火山活动普遍,湖泊众多,矿产资源可能有良好前景;所在区又为自然地域和地质构造的重要界线,对青藏高原研究具有重大科学价值。 相似文献
53.
Vertical drains are usually installed in subsoil consisting of several layers. Due to the complex nature of the problem, over the past decades, the consolidation properties of multi‐layered ground with vertical drains have been analysed mainly by numerical methods. An analytical solution for consolidation of double‐layered ground with vertical drains under quasi‐equal strain condition is presented in this paper. The main steps for the computation procedure are listed. The convergence of the series solution is discussed. The comparisons between the results obtained by the present analytical method and the existing numerical solutions are described by figures. The orthogonal relation for the system of double‐layered ground with vertical drains is proven. Finally, some consolidation properties of double‐layered ground with vertical drains are analysed. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
54.
55.
TANGYa XIEJiasui SUNHui 《山地科学学报》2004,1(1):38-45
Dry valleys are a striking geographic landscape in Hengduan Mountains Region and are characterized by low rainfall, desert type of vegetation and fragile environment. Past efforts and resources have been concentrated mainly on rehabilitation of degraded ecosystem and fragile environment, particularly reforestation, while socio-economic development has been largely overlooked. Despite successes in pocket areas, the overall trend of unsustainability and environmental deterioration are continuing. It is important to understand that uplift of the Tibetan Plateau is the root cause of development of dry valleys, and development and formation of dry valleys is a natural process. Human intervention has played a secondary role in development of dry valleys and degradation of dry valleys though human intervention in many cases has speeded up environmental degradation of the dry valleys. It is important to understand that dry valleys are climatic enclaves and an integrated approach that combines rehabilitation of degraded ecosystems and socio-economic development should be adopted if the overall goal of sustainable development of dry valleys is to be achieved. Promotion of niche-based cash crops, rural energy including hydropower, solar energy, biogas and fuelwood plantation is recommended as the priority activities. 相似文献
56.
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
57.
58.
59.
60.
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