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   2757篇
  免费   452篇
  国内免费   208篇
化学   1065篇
晶体学   17篇
力学   154篇
综合类   22篇
数学   540篇
物理学   1619篇
  2024年   2篇
  2023年   32篇
  2022年   65篇
  2021年   65篇
  2020年   77篇
  2019年   71篇
  2018年   71篇
  2017年   59篇
  2016年   112篇
  2015年   88篇
  2014年   165篇
  2013年   210篇
  2012年   156篇
  2011年   195篇
  2010年   146篇
  2009年   185篇
  2008年   192篇
  2007年   204篇
  2006年   180篇
  2005年   141篇
  2004年   108篇
  2003年   125篇
  2002年   93篇
  2001年   84篇
  2000年   71篇
  1999年   68篇
  1998年   108篇
  1997年   47篇
  1996年   51篇
  1995年   47篇
  1994年   36篇
  1993年   24篇
  1992年   27篇
  1991年   18篇
  1990年   12篇
  1989年   6篇
  1988年   14篇
  1987年   8篇
  1986年   6篇
  1985年   7篇
  1984年   10篇
  1983年   6篇
  1982年   8篇
  1981年   6篇
  1980年   3篇
  1979年   1篇
  1978年   2篇
  1977年   2篇
  1976年   2篇
  1973年   1篇
排序方式: 共有3417条查询结果,搜索用时 15 毫秒
91.
This paper analyzes the properties of reflected polarization dependent loss of apodized fiber Bragg gratings with temperature effective. We simulates reflected spectrum and polarization dependent loss of the apodized fiber Bragg gratings. The reflected spectrum and polarization dependent loss are measured under different temperatures. The analysis results show that reflected polarization dependent loss presents two peaks which shift an equal proportion to long wavelength with temperature increasing and have very good stability so we propose an idea that use polarization dependent loss as temperature sensor measurement parameters. The results show that the accuracy of measurement can be increased almost by one order of magnitude.  相似文献   
92.
93.
An efficient, ligand- and metal-oxidant-free, one-pot, cascade aerobic oxidative, palladium-catalyzed, multicomponent reaction has been developed through isocyanide insertion of less active secondary amide and aromatic amine, which leads to 2-amino-substituted-4(3H)-quinazolinones. This approach proves to be one of the simplest methods for the synthesis of this class of valuable bioactive heterocyclic scaffolds.  相似文献   
94.
A concise and efficient synthesis of (±)-clopidogrel, an antithrombotic agent, is achieved by inserting CO2 at the benzylic position as the key reaction without using any toxic transition metals. The overall yield of the synthetic process is 38% and the salient features include operationally simple process chemistry and fewer steps.  相似文献   
95.
96.
A semi-empirical method is proposed to calculate the ion ranges in energy region E?=?0.025–10?MeV/nucleon. The dependence of ion ranges on the projectile nuclear charge, mass and velocity is analysed. The calculations presented for ranges of ions with nuclear charges Z?=?2–10 in silicon are compared with SRIM results and experimental data.  相似文献   
97.
Some segregation results from the practices of organizations, some from specialized communication systems, some from correlation with a variable that is non‐random; and some results from the interplay of individual choices. This is an abstract study of the interactive dynamics of discriminatory individual choices. One model is a simulation in which individual members of two recognizable groups distribute themselves in neighborhoods defined by reference to their own locations. A second model is analytic and deals with compartmented space. A final section applies the analytics to ‘neighborhood tipping.’ The systemic effects are found to be overwhelming: there is no simple correspondence of individual incentive to collective results. Exaggerated separation and patterning result from the dynamics of movement. Inferences about individual motives can usually not be drawn from aggregate patterns. Some unexpected phenomena, like density and vacancy, are generated. A general theory of ‘tipping’ begins to emerge.  相似文献   
98.
In this paper, we consider the problem of determining the order of INAR(q) model on the basis of the Bayesian estimation theory. The Bayesian es-timator for the order is given with respect to a squared-error loss function. The consistency of the estimator is discussed. The results of a simulation study for the estimation method are presented.  相似文献   
99.
Although the reaction chemistry of transition metallacyclopropenes has been well-established in the last decades, the reactivity of rare-earth metallacyclopropenes remains elusive. Herein, we report the reaction of lutetacyclopropene 1 toward a series of unsaturated molecules. The reaction of 1 with one equiv. of PhCOMe, Ar1CHO (Ar1=2,6-Me2C6H3), W(CO)6, and PhCH=NPh provided oxalutetacyclopentenes, metallacyclic lutetoxycarbene, and azalutetacyclopentene via 1,2-insertion of C=O, C≡O, or C=N bonds into Lu−Csp2 bond, respectively. However, the reaction between 1 and Ar2N=C=NAr2 (Ar2=4-MeC6H4) gave an acyclic lutetium complex with a diamidinate ligand by the coupling of one molecule of 1 with two carbodiimides, irrespective of the amount of carbodiimide employed. More interestingly, when 1 was treated with two equiv. of Ar1CHO, the reductive coupling of two C=O bonds was discovered to give a lutetium pinacolate complex along with the release of tolan. Remarkably, the reactivity of 1 is significantly different from that of scandacyclopropenes; these metallacycles derived from 1 all represent the first cases in rare-earth organometallic chemistry.  相似文献   
100.
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