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   31401篇
  免费   184篇
  国内免费   289篇
化学   12131篇
晶体学   285篇
力学   1450篇
综合类   12篇
数学   9488篇
物理学   8508篇
  2021年   111篇
  2020年   78篇
  2019年   73篇
  2018年   1243篇
  2017年   1486篇
  2016年   799篇
  2015年   625篇
  2014年   557篇
  2013年   802篇
  2012年   3257篇
  2011年   2505篇
  2010年   1932篇
  2009年   1634篇
  2008年   696篇
  2007年   757篇
  2006年   720篇
  2005年   4534篇
  2004年   4018篇
  2003年   2369篇
  2002年   553篇
  2001年   327篇
  2000年   124篇
  1999年   205篇
  1998年   136篇
  1997年   103篇
  1996年   90篇
  1995年   78篇
  1994年   81篇
  1993年   61篇
  1992年   201篇
  1991年   185篇
  1990年   161篇
  1989年   132篇
  1988年   115篇
  1987年   71篇
  1986年   52篇
  1985年   57篇
  1984年   48篇
  1983年   43篇
  1982年   41篇
  1981年   35篇
  1979年   37篇
  1976年   85篇
  1975年   42篇
  1974年   44篇
  1973年   52篇
  1972年   40篇
  1969年   37篇
  1968年   35篇
  1966年   37篇
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
11.
When an organization solves a portfolio problem with public projects evaluated by multiple criteria, in which the economic dimension is not essential or not well characterized, the classical methods are not useful. We propose a non-linear preference model developed from normative Value Theory and using fuzzy sets to model some sources of imprecision. This model can be considered as a generalization of the classical approaches. However, the optimization problem is very complex in order to be solved with non-linear programming techniques. Therefore, the model is exploited by an evolutionary algorithm, able to achieve a strong improvement of the quality of solution.  相似文献   
12.
We consider the one-dimensional steady-state semiconductor deviceequations modelling a pnpn device. There are two relevant scalingsof the equations corresponding to small and large applied voltages.In both scalings, the semiconductor equations can be consideredas singularly perturbed. It turns out that the small-voltagescaling breaks down for current values between two saturationcurrents. In that interval, the large-voltage scaling has tobe employed. For both scalings, we derive the first-order termsof an asymptotic expansion and show that the reduced problemhas a solution. An example verifies that the current-voltagecurves obtained have the expected qualitative structure.  相似文献   
13.
Could simple intraannular-arm macrocyclic systems exist in enantiopure stable forms? The effective synthesis of two representative compounds of such a class, their resolution into enantiomers, and experiments justifying their stability toward racemization are presented.  相似文献   
14.
Comparison of the anion binding properties of a series of uncharged macrocyclic tetraamides reveal significant effects of the receptor's size on the strength of its anion complexes. This study allowed for estimation of the optimal size of a macroring for complexation of common anions.  相似文献   
15.
The development during the last 15 years and the state-of-the-art in the analysis of bulk steroid hormone drugs and hormone-like structures and pharmaceutical formulations made thereof are summarized. Other steroids (sterols, bile acids, cardiac glycosides, vitamins D) as well as biological-clinical aspects and pharmacokinetic and metabolic studies are excluded from this review. The state-of-the-art is summarized based on comparisons of monographs in the latest editions of the European Pharmacopoeia, United States Pharmacopoeia and the Japanese Pharmacopoeia. This is followed by sections dealing with new developments in the methodology for the fields of spectroscopic and spectrophotometric, chromatographic, electrophoretic and hyphenated techniques as well electroanalytical methods. The review is terminated by two problem-oriented sections: examples on impurity and degradation profiling as well as enantiomeric analysis.  相似文献   
16.
The electrochemical reduction of 1-([(4-halophenyl)imino]methyl)-2-naphthols on graphite electrodes was studied using cyclic voltammetry, chronoamperometry, constant-potential coulometry and preparative constant-potential electrolysis techniques. The data revealed that the reduction on graphite was irreversible and followed an EC mechanism. The diffusion coefficients and the number of electrons transferred were determined using the chronoamperometric Cottrell slope and the ultramicro disc Pt-electrode steady-state current. The number of electrons was also determined by bulk electrolysis. The compounds were subjected to constant-potential preparative electrolysis and the electrolysis products were purified and identified by spectroscopic methods. Based on these findings, a mechanism for the electro-reduction process is proposed.  相似文献   
17.
Two novel polymerizable dyes with anthraquinone residue have been prepared, and their structures were characterized by FTIR, ^1HNMR and EA.  相似文献   
18.
19.
20.
Paramagnetic pyrroline and 1,2,3,6-tetrahydropyridine derivatives of BODIPY and their diamagnetic analogs have been synthesized and characterized as novel redox double sensor and cation sensitive reagents.  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