全文获取类型
收费全文 | 493080篇 |
免费 | 5616篇 |
国内免费 | 1847篇 |
专业分类
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 |
出版年
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.
J.-L. Hainaut V. Englebert J. Henrard J.-M. Hick D. Roland 《Applied Categorical Structures》1996,3(1-2):9-45
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.
M. Trunk J. Popp I. Hartmann M. Lankers E. Urlaub W. Kiefer 《Fresenius' Journal of Analytical Chemistry》1996,355(3-4):354-356
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.
J Z Hu R J Pugmire A M Orendt D M Grant C Ye 《Solid state nuclear magnetic resonance》1992,1(4):185-195
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