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   490759篇
  免费   5999篇
  国内免费   1773篇
化学   259876篇
晶体学   7496篇
力学   21790篇
综合类   8篇
数学   58522篇
物理学   150839篇
  2020年   3488篇
  2019年   3727篇
  2018年   4410篇
  2017年   4233篇
  2016年   7193篇
  2015年   5180篇
  2014年   7759篇
  2013年   21846篇
  2012年   16499篇
  2011年   20419篇
  2010年   13532篇
  2009年   13439篇
  2008年   18792篇
  2007年   19029篇
  2006年   18253篇
  2005年   16540篇
  2004年   15041篇
  2003年   13336篇
  2002年   13168篇
  2001年   14833篇
  2000年   11377篇
  1999年   9123篇
  1998年   7491篇
  1997年   7346篇
  1996年   7224篇
  1995年   6581篇
  1994年   6330篇
  1993年   6251篇
  1992年   6740篇
  1991年   6740篇
  1990年   6300篇
  1989年   6097篇
  1988年   6443篇
  1987年   6013篇
  1986年   5746篇
  1985年   8135篇
  1984年   8320篇
  1983年   6856篇
  1982年   7268篇
  1981年   7227篇
  1980年   6914篇
  1979年   7196篇
  1978年   7240篇
  1977年   7205篇
  1976年   7189篇
  1975年   6864篇
  1974年   6783篇
  1973年   6956篇
  1972年   4446篇
  1971年   3488篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
A new strategy for the synthesis of tetrahydroisoquinolines based on the Pd0‐catalyzed intramolecular α‐arylation of sulfones is reported. The combination of this Pd‐catalyzed reaction with intermolecular Michael and aza‐Michael reactions allows the development of two‐ and three‐step domino processes to synthesize diversely functionalized scaffolds from readily available starting materials.  相似文献   
52.
53.
54.
55.
56.
This study was aimed at the development of a conductometric biosensor based on acetylcholinesterase considering the feasibility of its application for the inhibitory analysis of various toxicants. In this paper, the optimum conditions for enzyme immobilization on the transducer surface are selected as well as the optimum concentration of substrate for inhibitory analysis. Sensitivity of the developed biosensor to different classes of toxic compounds (organophosphorus pesticides, heavy metal ions, surfactants, aflatoxin, glycoalkaloids) was tested. It is shown that the developed biosensor can be successfully used for the analysis of pesticides and mycotoxins, as well as for determination of total toxicity of the samples. A new method of biosensor analysis of toxic substances of different classes in complex multicomponent aqueous samples is proposed.  相似文献   
57.
58.
Profiling engineered data with robust mining methods continues attracting attention in knowledge engineering systems. The purpose of this article is to propose a simple technique that deals with non-linear multi-factorial multi-characteristic screening suitable for knowledge discovery studies. The method is designed to proactively seek and quantify significant information content in engineered mini-datasets. This is achieved by deploying replicated fractional-factorial sampling schemes. Compiled multi-response data are converted to a single master-response effectuated by a series of distribution-free transformations and multi-compressed data fusions. The resulting amalgamated master response is deciphered by non-linear multi-factorial stealth stochastics intended for saturated schemes. The stealth properties of our method target processing datasets which might be overwhelmed by a lack of knowledge about the nature of reference distributions at play. Stealth features are triggered to overcome restrictions regarding the data normality conformance, the effect sparsity assumption and the inherent collapse of the ‘unexplainable error’ connotation in saturated arrays. The technique is showcased by profiling four ordinary controlling factors that influence webpage content performance by collecting data from a commercial browser monitoring service on a large scale web host. The examined effects are: (1) the number of Cascading Style Sheets files, (2) the number of JavaScript files, (3) the number of Image files, and (4) the Domain Name System Aliasing. The webpage performance level was screened against three popular characteristics: (1) the time to first visual, (2) the total loading time, and (3) the customer satisfaction. Our robust multi-response data mining technique is elucidated for a ten-replicate run study dictated by an L9(34) orthogonal array scheme where any uncontrolled noise embedded contribution has not been necessarily excluded.  相似文献   
59.
60.
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