全文获取类型
收费全文 | 11466篇 |
免费 | 3682篇 |
国内免费 | 4篇 |
专业分类
林业 | 571篇 |
农学 | 596篇 |
基础科学 | 35篇 |
2330篇 | |
综合类 | 877篇 |
农作物 | 305篇 |
水产渔业 | 2791篇 |
畜牧兽医 | 5843篇 |
园艺 | 176篇 |
植物保护 | 1628篇 |
出版年
2023年 | 33篇 |
2022年 | 51篇 |
2021年 | 202篇 |
2020年 | 581篇 |
2019年 | 1120篇 |
2018年 | 1012篇 |
2017年 | 1040篇 |
2016年 | 1033篇 |
2015年 | 911篇 |
2014年 | 912篇 |
2013年 | 1197篇 |
2012年 | 734篇 |
2011年 | 825篇 |
2010年 | 796篇 |
2009年 | 399篇 |
2008年 | 516篇 |
2007年 | 373篇 |
2006年 | 377篇 |
2005年 | 365篇 |
2004年 | 348篇 |
2003年 | 351篇 |
2002年 | 352篇 |
2001年 | 130篇 |
2000年 | 169篇 |
1999年 | 88篇 |
1998年 | 56篇 |
1997年 | 64篇 |
1996年 | 58篇 |
1995年 | 43篇 |
1994年 | 52篇 |
1993年 | 55篇 |
1992年 | 39篇 |
1991年 | 47篇 |
1990年 | 50篇 |
1989年 | 44篇 |
1988年 | 38篇 |
1987年 | 39篇 |
1986年 | 56篇 |
1985年 | 33篇 |
1984年 | 44篇 |
1983年 | 37篇 |
1982年 | 39篇 |
1981年 | 28篇 |
1980年 | 21篇 |
1979年 | 24篇 |
1978年 | 33篇 |
1977年 | 31篇 |
1975年 | 23篇 |
1973年 | 25篇 |
1972年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Effects of basal debris on glacier flow 总被引:1,自引:0,他引:1
Iverson NR Cohen D Hooyer TS Fischer UH Jackson M Moore PL Lappegard G Kohler J 《Science (New York, N.Y.)》2003,301(5629):81-84
Glacier movement is resisted partially by debris, either within glaciers or under glaciers in water-saturated layers. In experiments beneath a thick, sliding glacier, ice containing 2 to 11% debris exerted shear traction of 60 to 200 kilopascals on a smooth rock bed, comparable to the total shear traction beneath glaciers and contrary to the usual assumption that debris-bed friction is negligible. Imposed pore-water pressure that was 60 to 100% of the normal stress in a subglacial debris layer reduced shear traction on the debris sufficiently to halt its deformation and cause slip of ice over the debris. Slip resistance was thus less than debris shearing resistance. 相似文献
994.
Transition metal speciation in the cell: insights from the chemistry of metal ion receptors 总被引:1,自引:0,他引:1
The essential transition metal ions are avidly accumulated by cells, yet they have two faces: They are put to use as required cofactors, but they also can catalyze cytotoxic reactions. Several families of proteins are emerging that control the activity of intracellular metal ions and help confine them to vital roles. These include integral transmembrane transporters, metalloregulatory sensors, and diffusible cytoplasmic metallochaperone proteins that protect and guide metal ions to targets. It is becoming clear that many of these proteins use atypical coordination chemistry to accomplish their unique goals. The different coordination numbers, types of coordinating residues, and solvent accessibilities of these sites are providing insight into the inorganic chemistry of the cytoplasm. 相似文献
995.
996.
997.
Johnson TC Brown ET McManus J Barry S Barker P Gasse F 《Science (New York, N.Y.)》2002,296(5565):113-132
High-resolution profiles of the mass accumulation rate of biogenic silica and other geochemical proxies in two piston cores from northern Lake Malawi provide a climate signal for this part of tropical Africa spanning the past 25,000 years. The biogenic silica mass accumulation rate was low during the relatively dry late Pleistocene, when the river flux of silica to the lake was suppressed. Millennial-scale fluctuations, due to upwelling intensity, in the late Pleistocene climate of the Lake Malawi basin appear to have been closely linked to the Northern Hemisphere climate until 11 thousand years ago. Relatively cold conditions in the Northern Hemisphere coincided with more frequent north winds over the Malawi basin, perhaps resulting from a more southward migration of the Intertropical Convergence Zone. 相似文献
998.
We present three-dimensional reconstructions of coronal mass ejections (CMEs), which were obtained through polarization analysis of single-view images recorded with the use of the Large Angle and Spectrometric Coronagraph (LASCO) C2 coronagraph on board the Solar and Heliospheric Observatory (SOHO) spacecraft. Analysis of a loop-like CME shows a complex three-dimensional structure centered at 40 degrees from the plane of the sky, moving radially at 250 kilometers/second. Reconstruction of two halo CMEs suggests that these events are expanding loop arcades. 相似文献
999.
1000.
Radar soundings of the subsurface of Mars 总被引:2,自引:0,他引:2
Picardi G Plaut JJ Biccari D Bombaci O Calabrese D Cartacci M Cicchetti A Clifford SM Edenhofer P Farrell WM Federico C Frigeri A Gurnett DA Hagfors T Heggy E Herique A Huff RL Ivanov AB Johnson WT Jordan RL Kirchner DL Kofman W Leuschen CJ Nielsen E Orosei R Pettinelli E Phillips RJ Plettemeier D Safaeinili A Seu R Stofan ER Vannaroni G Watters TR Zampolini E 《Science (New York, N.Y.)》2005,310(5756):1925-1928
The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick. 相似文献