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   406123篇
  免费   4422篇
  国内免费   1277篇
化学   218286篇
晶体学   6145篇
力学   18399篇
综合类   16篇
数学   45631篇
物理学   123345篇
  2020年   3008篇
  2019年   3294篇
  2018年   4098篇
  2017年   4108篇
  2016年   6444篇
  2015年   4293篇
  2014年   6369篇
  2013年   17189篇
  2012年   13224篇
  2011年   16350篇
  2010年   11110篇
  2009年   11032篇
  2008年   15199篇
  2007年   15452篇
  2006年   14579篇
  2005年   13364篇
  2004年   12162篇
  2003年   10882篇
  2002年   10750篇
  2001年   12065篇
  2000年   9332篇
  1999年   7135篇
  1998年   5963篇
  1997年   5929篇
  1996年   5661篇
  1995年   5179篇
  1994年   5250篇
  1993年   4829篇
  1992年   5542篇
  1991年   5548篇
  1990年   5227篇
  1989年   5109篇
  1988年   5187篇
  1987年   5053篇
  1986年   4770篇
  1985年   6582篇
  1984年   6839篇
  1983年   5665篇
  1982年   5908篇
  1981年   5761篇
  1980年   5674篇
  1979年   5644篇
  1978年   5987篇
  1977年   5911篇
  1976年   5928篇
  1975年   5586篇
  1974年   5520篇
  1973年   5750篇
  1972年   3877篇
  1971年   3065篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
112.
113.
114.
115.
116.
An innovative volatolomic approach employs the detection of biomarkers present in cerumen (earwax) to identify cattle intoxication by Stryphnodendron rotundifolium Mart., Fabaceae (popularly known as barbatimão). S. rotundifolium is a poisonous plant with the toxic compound undefined and widely distributed throughout the Brazilian territory. Cerumen samples from cattle of two local Brazilian breeds (‘Curraleiro Pé-Duro’ and ‘Pantaneiro’) were collected during an experimental intoxication protocol and analyzed using headspace (HS)/GC–MS followed by multivariate analysis (genetic algorithm for a partial least squares, cluster analysis, and classification and regression trees). A total of 106 volatile organic metabolites were identified in the cerumen samples of bovines. The intoxication by S. rotundifolium influenced the cerumen volatolomic profile of the bovines throughout the intoxication protocol. In this way, it was possible to detect biomarkers for cattle intoxication. Among the biomarkers, 2-octyldecanol and 9-tetradecen-1-ol were able to discriminate all samples between intoxicated and nonintoxicated bovines. The cattle intoxication diagnosis by S. rotundifolium was accomplished by applying the cerumen analysis using HS/GC–MS, in an easy, accurate, and noninvasive way. Thus, the proposed bioanalytical chromatography protocol is a useful tool in veterinary applications to determine this kind of intoxication.  相似文献   
117.
Research on Chemical Intermediates - A series of quinoxaline derivatives were efficiently synthesized by convenient and simple procedure in excellent yields using 1 wt.% of titanium silicate (TS-1)...  相似文献   
118.
Metabolomics is a potential tool for the discovery of new biomarkers in the early diagnosis of diseases. An ultra-fast gas chromatography system equipped to an electronic nose detector (FGC eNose) was used to identify the metabolomic profile of Volatile Organic Compounds (VOCs) in type 2 diabetes (T2D) urine from Mexican population. A cross-sectional, comparative, and clinical study with translational approach was performed. We recruited twenty T2D patients and twenty-one healthy subjects. Urine samples were taken and analyzed by FGC eNose. Eighty-eight compounds were identified through Kovats's indexes. A natural variation of 30% between the metabolites, expressed by study groups, was observed in Principal Component 1 and 2 with a significant difference (p < 0.001). The model, performed through a Canonical Analysis of Principal coordinated (CAP), allowed a correct classification of 84.6% between healthy and T2D patients, with a 15.4% error. The metabolites 2-propenal, 2-propanol, butane- 2,3-dione and 2-methylpropanal, were increased in patients with T2D, and they were strongly correlated with discrimination between clinically healthy people and T2D patients. This study identified metabolites in urine through FGC eNose that can be used as biomarkers in the identification of T2D patients. However, more studies are needed for its implementation in clinical practice.  相似文献   
119.
120.
Galkina  E. G.  Zaspel  C. E.  Ivanov  B. A.  Kulagin  N. E.  Lerman  L. M. 《JETP Letters》2019,110(7):481-486
JETP Letters - The motion of domain walls in GdFeCo-type ferrimagnets near the point of compensation of sublattice spins s1 and s2, when the effects of the exchange increase in the limiting wall...  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