全文获取类型
收费全文 | 188074篇 |
免费 | 10379篇 |
国内免费 | 163篇 |
专业分类
林业 | 7942篇 |
农学 | 5919篇 |
基础科学 | 1193篇 |
23221篇 | |
综合类 | 30842篇 |
农作物 | 7223篇 |
水产渔业 | 9276篇 |
畜牧兽医 | 98682篇 |
园艺 | 2409篇 |
植物保护 | 11909篇 |
出版年
2019年 | 1858篇 |
2018年 | 2805篇 |
2017年 | 3130篇 |
2016年 | 2865篇 |
2015年 | 2474篇 |
2014年 | 3125篇 |
2013年 | 7394篇 |
2012年 | 5552篇 |
2011年 | 6677篇 |
2010年 | 4480篇 |
2009年 | 4559篇 |
2008年 | 6632篇 |
2007年 | 6347篇 |
2006年 | 5977篇 |
2005年 | 5496篇 |
2004年 | 5318篇 |
2003年 | 5437篇 |
2002年 | 4980篇 |
2001年 | 6090篇 |
2000年 | 5997篇 |
1999年 | 4782篇 |
1998年 | 1923篇 |
1997年 | 1897篇 |
1996年 | 1794篇 |
1995年 | 2110篇 |
1994年 | 1771篇 |
1993年 | 1852篇 |
1992年 | 3784篇 |
1991年 | 4023篇 |
1990年 | 3934篇 |
1989年 | 3995篇 |
1988年 | 3605篇 |
1987年 | 3635篇 |
1986年 | 3624篇 |
1985年 | 3456篇 |
1984年 | 2767篇 |
1983年 | 2486篇 |
1982年 | 1688篇 |
1979年 | 2506篇 |
1978年 | 1964篇 |
1977年 | 1702篇 |
1976年 | 1603篇 |
1975年 | 1748篇 |
1974年 | 2135篇 |
1973年 | 2210篇 |
1972年 | 2153篇 |
1971年 | 1971篇 |
1970年 | 2002篇 |
1969年 | 1874篇 |
1967年 | 1635篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The concentration of electrical energy usage in alternating-current motor drives presents an opportunity for substantial conservation. Emerging advances in power semiconductor transistor systems will support a major commerical effort to this end. An alternating-current synthesizer for this purpose may soon be available. The synthesizer produces electricial power of variable and programmable frequency, voltage, and wave form so that performance can be optimized. This technology provides the additional opportunity for fundamental improvements in electrical distribution and usage systems in the longer term. Power processing with semiconductor a-c motor controls is expected to become widespread in the near future. 相似文献
992.
993.
Both laboratory and in situ studies indicate that the nitrogen-fixing blue-green nuisance algae Anabaena spp. have developed adaptive means of dominating surface lake waters. During the dramatic diurnal shifts in surface light intensity and oxygen saturation accompanying blooms, Anabaena can overcome oxygen toxicity by sequential optimization of carbon dioxide and nitrogen fixation and by pigment alteration. These mechanisms allow optimal utilization of the radiant energy while minimizing competition for photoreductant between two main energy-demanding processes. 相似文献
994.
995.
996.
Marshall E 《Science (New York, N.Y.)》1979,203(4375):30-31
997.
Magnitude of shear stress on the san andreas fault: implications of a stress measurement profile at shallow depth 总被引:1,自引:0,他引:1
A profile of measurements of shear stress perpendicular to the San Andreas fault near Palmdale, California, shows a marked increase in stress with distance from the fault. The pattern suggests that shear stress on the fault increases slowly with depth and reaches a value on the order of the average stress released during earthquakes. This result has important implications for both long- and shortterm prediction of large earthquakes. 相似文献
998.
Marshall E 《Science (New York, N.Y.)》1979,205(4410):982-983
999.
In 1977, heating, cooling, lighting, and other operations in residential and commercial buildings used 27 quads (1 quad = 10(15) British thermal units) of energy. This is more than one-third of the nation's total energy budget. Future trends in energy use in buildings are likely to depend strongly on fuel prices and government policies designed to save energy. Three scenarios are examined: (i) a base line in which fuel prices rise as projected by the Department of Energy; (ii) a conservation case that includes higher gas and oil prices plus the regulatory, financial incentive, and information programs authorized by the 94th Congress and proposed in the April 1977 National Energy Plan; and (iii) another conservation case that also includes new technologies (more efficient equipment, appliances, and structures). These scenarios are analyzed for changes in energy use, costs, and employment by means of detailed engineering-economic models of energy use in residential and commercial buildings developed at the Oak Ridge National Laboratory and input-output analyses developed at the University of Illinois. 相似文献
1000.
Although micrometeorites of cometary origin are thought to be the dominant component of interplanetary dust, it has never been possible to positively identify such micrometer-sized particles. Two such particles have been identified as definitely micrometeorites since their abundances of volatile and nonvolatile trace elements closely match those of primitive solar system material. 相似文献