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   533202篇
  免费   59749篇
  国内免费   338篇
  2018年   5227篇
  2017年   5089篇
  2016年   6956篇
  2015年   8785篇
  2014年   10489篇
  2013年   15300篇
  2012年   16979篇
  2011年   17375篇
  2010年   11722篇
  2009年   10757篇
  2008年   15182篇
  2007年   15742篇
  2006年   14695篇
  2005年   14085篇
  2004年   13945篇
  2003年   13297篇
  2002年   12833篇
  2001年   28377篇
  2000年   28229篇
  1999年   21987篇
  1998年   6730篇
  1997年   7312篇
  1996年   6737篇
  1995年   6216篇
  1994年   5986篇
  1993年   5964篇
  1992年   17052篇
  1991年   16299篇
  1990年   15705篇
  1989年   15215篇
  1988年   13929篇
  1987年   12939篇
  1986年   12048篇
  1985年   11818篇
  1984年   9674篇
  1983年   8090篇
  1982年   5994篇
  1981年   5373篇
  1980年   5099篇
  1979年   8946篇
  1978年   6814篇
  1977年   6273篇
  1976年   5647篇
  1975年   6240篇
  1974年   6758篇
  1973年   6546篇
  1972年   5980篇
  1971年   5431篇
  1970年   4683篇
  1969年   4402篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A total of 600 isolates of Escherichia coli were isolated, over a 9 month period during 1984, from healthy human adults, raw sewage and the sewage-polluted River Tigris in Nineva. Over 90% of these organisms were E. coli type 1, but only 8.3% could be serogrouped as enteropathogenic E. coli . Resistance of these organisms to 11 antimicrobial drugs was assessed. Over 40% were antibiotic-resistant and of these 77.1% were resistant to more than one antibiotic. The minimal inhibitory concentration of ampicillin for 193 selected strains from the various sources was determined and ranged from <0.625- > 160 μg/ml. The high incidence of antibiotic-resistant E. coli in this locality and the possible implications to human health are discussed.  相似文献   
992.
The microbial degradation of morpholine   总被引:5,自引:1,他引:4  
Morpholine can be completely degraded microbiologically, and two organisms have been isolated, each capable of growth in a simple mineral salts medium with morpholine as the sole source of carbon, nitrogen and energy. Excess nitrogen is liberated as ammonia. The enzymes responsible for the oxidation of morpholine are inducible and, in organism Mor G, will also oxidize piperidine, piperazine and pyrrolidine, which are not growth substrates. Ethanolamine is a likely intermediate, though the metabolic steps in morpholine degradation do not give rise solely to acetyl-CoA. After a period of acclimation, a laboratory scale activated sludge plant effectively removed morpholine over the long period it was operated; the sludge was also capable of nitrification. The possible effects of other chemicals in trade wastes containing morpholine on nitrification and morpholine oxidation are described.  相似文献   
993.
994.
995.
996.
Highly viscous polysaccharide (250–350 kDa) of an alginate nature with a predominance of α-L-guluronic acid (M/G = 0.22) was obtained from Azotobacter vinelandi. The yield of polysaccharide was 20.5 ± 0.5 g/l when cultured in a medium containing molasses at a viscosity of the cultural liquid of over 30000 cSt. The biopolymer is stable at pH 4.0–9.0 in a wide temperature range and well soluble in highly mineralized water; it retains a high viscosity level and can be used in the petroleum industry for enhanced oil recovery.  相似文献   
997.
The synthesis of some monocyclic beta-lactams (monobactams) by the Staudinger reaction using D-glucosamine propanedithioacetal as chiral auxiliary is reported. The influence of several radicals at C3, C4, and C1' (sugar moiety) as well as other structural aspects are considered in relation to the antielastase activity.  相似文献   
998.
999.
This study evaluated the relative ability of various combinations of copper sulfate with either boric acid or calcium-precipitating agent, N′-N-(1, 8-naphthalyl) hydroxylamine (NHA-Na), to inhibit fungal degradation and attack by Formosan subterranean termites (Coptotermes formosanus Shiraki). Wood specimens were treated with either 1%, 0.5%, or 0.1% concentrations of copper sulfate, boric acid, NHA-Na, copper sulfate + boric acid, or copper sulfate + NHA-Na mixtures. Treated specimens were subjected to laboratory decay-resistance tests by using petri dishes inoculated with the Basidiomycetes fungi Tyromyces palustris and Trametes versicolor for 12 weeks. Treated wood specimens were also subjected to termite-resistance tests under laboratory conditions. Increased efficacy of copper sulfate against the brown-rot fungus T. palustris was observed when either boric acid or NHA-Na was added. The most effective treatments against the fungi tested were NHA-Na only treatments at 1% and 0.5% concentration levels. Boric acid treatments were not able to protect wood against decay after leaching because of excessive leaching of boron. Similar results were obtained in termite-resistance tests in comparison with decay-resistance tests. These results indicate that the efficacy of the treatments in preventing fungal and termite attack is a function of the type of preservative.  相似文献   
1000.
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