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   434084篇
  免费   5585篇
  国内免费   1227篇
化学   232988篇
晶体学   5687篇
力学   18794篇
综合类   13篇
数学   50168篇
物理学   133246篇
  2021年   3750篇
  2020年   4206篇
  2019年   4550篇
  2018年   5886篇
  2017年   5780篇
  2016年   8899篇
  2015年   5614篇
  2014年   8621篇
  2013年   20315篇
  2012年   15735篇
  2011年   19349篇
  2010年   13568篇
  2009年   13424篇
  2008年   17707篇
  2007年   17591篇
  2006年   16290篇
  2005年   14574篇
  2004年   13548篇
  2003年   11923篇
  2002年   11752篇
  2001年   13456篇
  2000年   10103篇
  1999年   7934篇
  1998年   6582篇
  1997年   6327篇
  1996年   6095篇
  1995年   5495篇
  1994年   5401篇
  1993年   5204篇
  1992年   5833篇
  1991年   5796篇
  1990年   5558篇
  1989年   5363篇
  1988年   5382篇
  1987年   5292篇
  1986年   4977篇
  1985年   6496篇
  1984年   6624篇
  1983年   5237篇
  1982年   5331篇
  1981年   5310篇
  1980年   4881篇
  1979年   5309篇
  1978年   5448篇
  1977年   5356篇
  1976年   5305篇
  1975年   4883篇
  1974年   4847篇
  1973年   4854篇
  1972年   3434篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Fixed-frequency and threshold photoelectron spectra have been recorded for ionization from the S ls shells in SF6, CS2 and COS, the Si 1s shells in SiH4 and SiCl4 and the Cl 1s shell in SiCl4 using synchrotron radiation. Fixed-frequency spectra generally showed a single strong ionization feature with associated weak satellite structure due to excited ionized states. Threshold spectra closely resembled X-ray absorption spectra but with an additional feature due to direct ionization. In cases where resonant process enhanced the NEXAFS spectrum direct ionization was not observed.  相似文献   
992.
Methods of synthesizing previously unknown derivatives of coumarin — 2-(N-R-imino)-2H-1-benzopyrans, where R = Alk, Ar — are proposed. Possibilities of alternative synthetic schemes are discussed, and hypotheses concerning probable mechanisms of the reactions are formulated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 760–766, June, 1994. Original article submitted May 23, 1994.  相似文献   
993.
In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles.  相似文献   
994.
The influence of temperature in the 20–100°C range on processes of bimolecular interaction in the aerosil-phenanthrene-trinitrobenzene system has been studied. Temperature-stimulated CTC accumulation on the surface is found. Activation energies of complex formation are estimated. Quenching of phenanthrene fluorescence by acceptor has a mixed static and dynamic character.  相似文献   
995.
It has been established that CO oxidation with excess O2 on Ni/Al2O3 forms an inhomogeneous NiO contact surface that smoothes critical effects in the sharp reactivity change.  相似文献   
996.
One of the shortcomings of R&D evaluation is a lack of emphasis on analytical assessment of the value of an on-going R&D project. This paper addresses the problem concerning the distributive aspect of access to superior knowledge. Decision tree analysis and probability models appear to be appropriate tools for assessing the values of an intermediate result and patent reward of a firm's R&D decision in an environment of perfect information and oligopolistic competition. The assessed values are used as minimum prices acceptable to the firm when the knowledge is disseminated to the public. This paper will attempt to resolve this problem through the determination of the appropriate values of the reservation price of the first-stage invention in terms of the final reward and of a patent reward for which the inventor is willing to apply.  相似文献   
997.
This paper describes a method and the corresponding algorithms for simplification of large-scale linear programming models. It consists of the elimination of the balance constraints (i.e. constraints with zero RHS term). The idea is to apply some linear transformations to the original problem in order to nullify the balance constraints. These transformations are able to simultaneously eliminate more balance rows. The core of this contribution is the introduction of the reduction matrix and the associated theorems on the equivalent linear programs (original and reduced). The numerical experiments with this method of simplification proved this approach to be beneficial for a large class of LP problems.This research work was done while the first author was at Duisburg University, Mess-, Steuer und Regelungstechnik, Germany, under the greatly appreciated financial assistance given by the Alexander-von-Humboldt Foundation.  相似文献   
998.
We present a pole expansion for the propagators in the framework of normalized quantum electrodynamics and compare it with the more canonical results from S-matrix theory.  相似文献   
999.
1000.
Summary In order to improve the simulation of wind probability distributions at ground level, eight years of wind data of the multivariate analyses at 850, 700, 500 hPa levels over Italy, and eleven years of wind radiosoundings performed at 5 Italian Air Force stations at 950, 900, 800, 700, 600, and 500 hPa have been processed. The vertical profiles of the distribution parameters of wind speed present predictable behaviour, with the exception of Brindisi. The frequency distribution profiles of the wind directions are more complex. Temporarily seconded to the RISOE National Laboratory (DK) as part of the EEC-DGXII Programme ?Stimulation Action?.  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