全文获取类型
收费全文 | 3460篇 |
免费 | 184篇 |
国内免费 | 1篇 |
专业分类
林业 | 126篇 |
农学 | 129篇 |
基础科学 | 8篇 |
581篇 | |
综合类 | 370篇 |
农作物 | 480篇 |
水产渔业 | 248篇 |
畜牧兽医 | 1502篇 |
园艺 | 40篇 |
植物保护 | 161篇 |
出版年
2022年 | 56篇 |
2021年 | 88篇 |
2020年 | 50篇 |
2019年 | 56篇 |
2018年 | 90篇 |
2017年 | 86篇 |
2016年 | 90篇 |
2015年 | 95篇 |
2014年 | 131篇 |
2013年 | 184篇 |
2012年 | 199篇 |
2011年 | 245篇 |
2010年 | 131篇 |
2009年 | 116篇 |
2008年 | 207篇 |
2007年 | 208篇 |
2006年 | 141篇 |
2005年 | 151篇 |
2004年 | 149篇 |
2003年 | 133篇 |
2002年 | 114篇 |
2001年 | 77篇 |
2000年 | 69篇 |
1999年 | 48篇 |
1998年 | 17篇 |
1997年 | 26篇 |
1996年 | 18篇 |
1994年 | 14篇 |
1993年 | 12篇 |
1992年 | 37篇 |
1991年 | 40篇 |
1990年 | 37篇 |
1989年 | 31篇 |
1988年 | 22篇 |
1987年 | 34篇 |
1986年 | 30篇 |
1985年 | 35篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1979年 | 20篇 |
1978年 | 12篇 |
1977年 | 21篇 |
1976年 | 12篇 |
1974年 | 17篇 |
1973年 | 23篇 |
1972年 | 18篇 |
1971年 | 14篇 |
1970年 | 18篇 |
1969年 | 20篇 |
1968年 | 13篇 |
排序方式: 共有3645条查询结果,搜索用时 15 毫秒
991.
992.
Pimentel D Moran MA Fast S Weber G Bukantis R Balliett L Boveng P Cleveland C Hindman S Young M 《Science (New York, N.Y.)》1981,212(4499):1110-1115
Residues remaining after the harvest of crop and forestry products are being proposed as a substantial energy source for the nation. An estimated 22 percent of the residues might be utilized, providing a renewable source of high-grade energy with the potential of supplying 1 percent of the current U.S. gasoline consumption as ethanol or 4 percent of the total electrical energy used. These net energy benefits are limited by high energy costs to collect, transport, and process the residues. Environmental threats include soil erosion, water runoff, and nutrient loss. 相似文献
993.
Lohi H Young EJ Fitzmaurice SN Rusbridge C Chan EM Vervoort M Turnbull J Zhao XC Ianzano L Paterson AD Sutter NB Ostrander EA André C Shelton GD Ackerley CA Scherer SW Minassian BA 《Science (New York, N.Y.)》2005,307(5706):81
Epilepsy afflicts 1% of humans and 5% of dogs. We report a canine epilepsy mutation and evidence for the existence of repeat-expansion disease outside humans. A canid-specific unstable dodecamer repeat in the Epm2b (Nhlrc1) gene recurrently expands, causing a fatal epilepsy and contributing to the high incidence of canine epilepsy. Tracing the repeat origins revealed two successive events, starting 50 million years ago, unique to canid evolution. A genetic test, presented here, will allow carrier and presymptomatic diagnosis and disease eradication. Clinicopathologic characterization establishes affected animals as a model for Lafora disease, the most severe teenage-onset human epilepsy. 相似文献
994.
995.
996.
997.
Foroughi J Spinks GM Wallace GG Oh J Kozlov ME Fang S Mirfakhrai T Madden JD Shin MK Kim SJ Baughman RH 《Science (New York, N.Y.)》2011,334(6055):494-497
Rotary motors of conventional design can be rather complex and are therefore difficult to miniaturize; previous carbon nanotube artificial muscles provide contraction and bending, but not rotation. We show that an electrolyte-filled twist-spun carbon nanotube yarn, much thinner than a human hair, functions as a torsional artificial muscle in a simple three-electrode electrochemical system, providing a reversible 15,000° rotation and 590 revolutions per minute. A hydrostatic actuation mechanism, as seen in muscular hydrostats in nature, explains the simultaneous occurrence of lengthwise contraction and torsional rotation during the yarn volume increase caused by electrochemical double-layer charge injection. The use of a torsional yarn muscle as a mixer for a fluidic chip is demonstrated. 相似文献
998.
Smith AD Brown CJ Bulman CM Fulton EA Johnson P Kaplan IC Lozano-Montes H Mackinson S Marzloff M Shannon LJ Shin YJ Tam J 《Science (New York, N.Y.)》2011,333(6046):1147-1150
Low-trophic level species account for more than 30% of global fisheries production and contribute substantially to global food security. We used a range of ecosystem models to explore the effects of fishing low-trophic level species on marine ecosystems, including marine mammals and seabirds, and on other commercially important species. In five well-studied ecosystems, we found that fishing these species at conventional maximum sustainable yield (MSY) levels can have large impacts on other parts of the ecosystem, particularly when they constitute a high proportion of the biomass in the ecosystem or are highly connected in the food web. Halving exploitation rates would result in much lower impacts on marine ecosystems while still achieving 80% of MSY. 相似文献
999.
Wiltshire MC Pendry JB Young IR Larkman DJ Gilderdale DJ Hajnal JV 《Science (New York, N.Y.)》2001,291(5505):849-851
Magnetic resonance imaging and spectroscopy systems use coils, either singly or as arrays, to intercept radio-frequency (RF) magnetic flux from regions of interest, often deep within the body. Here, we show that a new magnetic material offers novel possibilities for guiding RF flux to the receiver coil, permitting a clear image to be obtained where none might otherwise be detectable. The new material contains microstructure designed according to concepts taken from the field of photonic band gap materials. In the RF range, it has a magnetic permeability that can be produced to specification while exhibiting negligible direct-current magnetism. The latter property is vital to avoid perturbing the static and audio-frequency magnetic fields needed to obtain image and spectral data. The concept offers a new paradigm for the manipulation of RF flux in all nuclear magnetic resonance systems. 相似文献
1000.
The existence of a monoclinic phase of hydroxyapatite, Ca(2)(PO(4))(4)OH, has been confirmed, by single-crystal structure analysis (weighted "reliability" factor = 3.9 percent on |F|(2)). The structure has space group P21/b, a = 9.4214(8) angstroms, b = 2a, c = 6.8814(7) angstroms, and gamma = 120 degrees , and is analogous to that of chlorapatite. The distortions from the hexagonal structure with which the monoclinic structure is pseudosymmetric are similar to those in chlorapatite, including enlargement of that triangular array of oxygen atoms in which the chlorine ion or, in hydroxyapatite, the hydroxyl hydrogen ion is approximately centered. The hydroxyapatite specimen was prepared by the conversion of a single crystal of chlorapatite in steam at 1200 degrees C, was mimetically twinned, and was approximately 37 percent monoclinic. 相似文献