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   493080篇
  免费   5616篇
  国内免费   1847篇
化学   267272篇
晶体学   7231篇
力学   20547篇
综合类   9篇
数学   59443篇
物理学   146041篇
  2020年   3450篇
  2019年   3669篇
  2018年   4126篇
  2017年   3969篇
  2016年   7089篇
  2015年   5312篇
  2014年   7576篇
  2013年   22691篇
  2012年   16737篇
  2011年   20874篇
  2010年   13261篇
  2009年   13203篇
  2008年   18856篇
  2007年   19172篇
  2006年   18199篇
  2005年   16726篇
  2004年   14988篇
  2003年   13304篇
  2002年   13036篇
  2001年   14630篇
  2000年   11250篇
  1999年   8991篇
  1998年   7383篇
  1997年   7222篇
  1996年   7067篇
  1995年   6516篇
  1994年   6193篇
  1993年   6067篇
  1992年   6712篇
  1991年   6648篇
  1990年   6253篇
  1989年   6082篇
  1988年   6309篇
  1987年   5976篇
  1986年   5776篇
  1985年   8182篇
  1984年   8373篇
  1983年   6852篇
  1982年   7470篇
  1981年   7366篇
  1980年   7121篇
  1979年   7204篇
  1978年   7449篇
  1977年   7292篇
  1976年   7356篇
  1975年   6954篇
  1974年   6917篇
  1973年   7264篇
  1972年   4377篇
  1971年   3300篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
This paper analyzes the requirements that CASE tools should meet for effective database reverse engineering (DBRE), and proposes a general architecture for data-centered applications reverse engineering CASE environments. First, the paper describes a generic DBMS-independent DBRE methodology, then it analyzes the main characteristics of DBRE activities in order to collect a set of desirable requirements. Finally, it describes DB-MAIN, an operational CASE tool developed according to these requirements. The main features of this tool that are described in this paper are its unique generic specification model, its repository, its transformation toolkit, its user interface, the text processors, the assistants, the methodological control and its functional extensibility. Finally, the paper describes five real-world projects in which the methodology and the CASE tool were applied. This is a heavily revised and extended version of “Requirements for Information System Reverse Engineering Support” by J.-L. Hainaut, V. Englebert, J. Henrard, J.-M. Hick, D. Roland, which first appeared in the Proceedings of the Second Working Conference on Reverse Engineering, IEEE Computer Society Press, pp. 136–145, July 1995. This paper presents some results of the DB-MAIN project. This project is partially supported by the Région Wallonne, the European Union, and by a consortium comprising ACEC-OSI (Be), ARIANE-II (Be), Banque UCL (Lux), BBL (Be), Centre de recherche public H. Tudor (Lux), CGER (Be), Cockerill-Sambre (Be), CONCIS (Fr), D'Ieteren (Be), DIGITAL, EDF (Fr), EPFL (CH), Groupe S (Be), IBM, OBLOG Software (Port), ORIGIN (Be), Ville de Namur (Be), Winterthur (Be), 3 Suisses (Be). The DB-Process subproject is supported by the Communauté Fran?aise de Belgique.  相似文献   
102.
103.
Chemical kinetics of benzonitrile nitration with mixed acid is investigated in the temperature range 283–299 K. Pseudo-first-order rate constants are evaluated by means of rate experiments on homogeneous reacting mixtures having large stoichiometric excesses of nitric acid. The second-order kinetic constants for nitronium ion attack to the aromatic substrate are derived on the basis of the assessed nitration mechanism. An activation energy of 604 ± 37 kJ mol?1 is calculated for this reaction step. © 1993 John Wiley & Sons, Inc.  相似文献   
104.
This research extends previous work with dynamic models to manage groundwater quality by using the consumptive nitrate use rate instead of the nitrate application rate. The analysis indicates that misspecification results in overestimation of economic benefits, and supra-optimum nitrogen fertilizer application rates and groundwater nitrate stocks at a steady state.  相似文献   
105.
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   
106.
Taking advantage of the long 13C T1 values generally encountered in solids, selective saturation and inversion of more than one resonance in 13C CP/MAS experiments can be achieved by sequentially applying several DANTE pulse sequences centered at different transmitter frequency offsets. A new selective saturation pulse sequence is introduced composed of a series of 90 degrees DANTE sequences separated by interrupted decoupling periods during which the selected resonance is destroyed. Applications of this method, including the simplification of the measurement of the principal values of the 13C chemical shift tensor under slow MAS conditions, are described. The determination of the aromaticity of coal using a relatively slow MAS rate is also described.  相似文献   
107.
Comparative measurements were conducted for the backscattered intensities of light from uniform random and fractal aggregated media. Different features are found for the backscattered intensity peak shapes. A crossover between the θ1-D and θ-2 dependences of the backscattered intensity occurs in the case of fractal aggregated medium, where D indicates the fractal dimension.  相似文献   
108.
In this paper we construct a primitive, non-symmetric 3-class association scheme with parameters v = 36, v1 = 7, p111 = 0 and p211 = 4 and show that such a scheme is determined by its parameters.  相似文献   
109.
The authors study a one-dimensional model for optical tunnellingwith a refractive index in the shape of a square well. The relevanceof the model and its limiting cases are discussed. The mainresult is the leading behaviour of the exponentially small imaginarypart of the eigenvalue which determines the radiation loss.The leading behaviour of the imaginary part is calculated usingBerry's formula which controls the asymptotic expansion of theAiry function Bi(z) to better-than-exponential accuracy.  相似文献   
110.
Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by 1H, 2H, and 13C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented.  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