首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   7670篇
  免费   397篇
  国内免费   3篇
  2022年   94篇
  2021年   170篇
  2020年   110篇
  2019年   116篇
  2018年   165篇
  2017年   133篇
  2016年   213篇
  2015年   273篇
  2014年   325篇
  2013年   440篇
  2012年   513篇
  2011年   441篇
  2010年   306篇
  2009年   230篇
  2008年   329篇
  2007年   320篇
  2006年   252篇
  2005年   280篇
  2004年   219篇
  2003年   234篇
  2002年   206篇
  2001年   192篇
  2000年   158篇
  1999年   145篇
  1998年   74篇
  1997年   56篇
  1996年   47篇
  1995年   50篇
  1994年   50篇
  1992年   119篇
  1991年   89篇
  1990年   130篇
  1989年   98篇
  1988年   105篇
  1987年   94篇
  1986年   78篇
  1985年   90篇
  1984年   82篇
  1983年   62篇
  1982年   65篇
  1981年   87篇
  1980年   64篇
  1979年   94篇
  1978年   63篇
  1977年   66篇
  1976年   47篇
  1975年   60篇
  1974年   57篇
  1973年   49篇
  1972年   47篇
排序方式: 共有8070条查询结果,搜索用时 93 毫秒
81.
The syntheses of (±) 2α,6β-diethyl-7α-ethynyl-3α-(p-hydroxyphenyl)-trans-bicyclo[4.3.0]nonan-7β-ol (8), (±)2β,6β-diethyl-7α-ethynyl-3β-(p-methoxyphenyl)-trans-bicyclo[4.3.0]nonan-7β-ol (12) and (±) 2α,6β-diethyl-7α-ethynyl-3β-(p-hydroxyphenyl)-trans-bicyclo[4.3.0]nonan-7β-ol (18) and their derivatives, which are essentially B-seco-steroids having cis-anti-trans, cis-syn-trans and trans-anti-trans geometries have been carried out. A study of their antiimplantation activities (AI) and receptor binding affinities (RBA) show that trans-anti-trans compounds are biologically most potent, followed by the corresponding cis-anti-trans and cis-syn-trans compounds. The most potent compound 18 is active at 1 mg/kg in rats. Introduction of 7α-ethynyl group increases their AI activity; however, no significant effect on their RBA is observed.  相似文献   
82.
From the uppermost parts of the Muth Quartzite Formation the following ammonoids are described which here are reponed from the Himalayas for the first time:Discoclymenia cucullata (v. Buch);Prionoceras (Imitoceras) intermedium Schindewolf;Pr. (I.) cf.rotundum Petter;Cyrtodymenia sp.; andDiscoclymenia striata (Münster). The levels containing ammonoids appear in the Famennian V(Clymenia zone). Their extension up to the earliest Tournaisian(Gattendorfia zone) is not likely although theoretically it cannot be excluded.  相似文献   
83.
Two new compounds, isolated from the rhizomes of Cryptocoryne spiralis, have been characterized as ethyl 14-oxotetracosanoate and 15-oxoeicosanyl 14-oxoheptadecanoate by spectral data and chemical studies. Hentriacontane and sitosterol have also been isolated and identified.  相似文献   
84.
85.
86.
A technique is described for determination of rubella hemagglutination-inhibiting antibody in only 50 to 100 muliters of whole blood.  相似文献   
87.
88.
The increased sensitivity and improved agglutination and settling patterns of formalinized sheep erythrocytes in a new buffer, HEPES (N-2-hydroxyethylpiperazine N'-2'-ethanesulfonic acid), make this system more suitable for use in the rubella hemagglutination-inhibition test.  相似文献   
89.
90.
Summary Five rates of limestone and 4 rates of fertilizers were used in a split-plot design to study their effects under field conditions on Mo, Cu, B, Mn, and Zn levels in mixed forage tissue and soil, and on the forage yield. An increase in soil pH resulted in an increase in Mo and Cu content of plant tissue while B, Mn, and Zn decreased. The micro-nutrient content of the tissue increased as the harvesting season progressed. Increasing rates of applied fertilizer did not affect the micro-nutrient content of the forage tissue or soil. Liming to a pH of 5.6 and above reduced the availability of Mn and Zn in the soil. In general, the available B was low at pH values greater than 6.1. Lime did not affect the quantities of Mo and Cu in the soil. Manganese is supplied in large quantities by limestone and is not apt to be deficient in limed soil. However, addition of B and Zn may be required on the high pH soils of Eastern Canada in future. Molybdenum was adequate where the soil was limed to a pH of 6.1 or greater. The dry-matter yield of forage increased significantly with successive increases in lime up to pH 6.6 and with each increment of fertilization. Contribution No.226, Research Branch, Research Station, P.O. Box 1210, Charlottetown, P.E.I. and No.166, Experimental Farm, Nappan, N.S.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号