全文获取类型
收费全文 | 502787篇 |
免费 | 5244篇 |
国内免费 | 1657篇 |
专业分类
化学 | 252826篇 |
晶体学 | 8241篇 |
力学 | 23959篇 |
综合类 | 8篇 |
数学 | 60656篇 |
物理学 | 163998篇 |
出版年
2021年 | 4182篇 |
2020年 | 4552篇 |
2019年 | 5160篇 |
2018年 | 6960篇 |
2017年 | 7113篇 |
2016年 | 9962篇 |
2015年 | 5838篇 |
2014年 | 9728篇 |
2013年 | 23396篇 |
2012年 | 17620篇 |
2011年 | 21505篇 |
2010年 | 15661篇 |
2009年 | 15563篇 |
2008年 | 19455篇 |
2007年 | 19402篇 |
2006年 | 18075篇 |
2005年 | 15975篇 |
2004年 | 14864篇 |
2003年 | 13146篇 |
2002年 | 13107篇 |
2001年 | 15338篇 |
2000年 | 11585篇 |
1999年 | 9099篇 |
1998年 | 7615篇 |
1997年 | 7381篇 |
1996年 | 7010篇 |
1995年 | 6224篇 |
1994年 | 6127篇 |
1993年 | 6086篇 |
1992年 | 6561篇 |
1991年 | 6850篇 |
1990年 | 6515篇 |
1989年 | 6370篇 |
1988年 | 6192篇 |
1987年 | 6215篇 |
1986年 | 5817篇 |
1985年 | 7670篇 |
1984年 | 7902篇 |
1983年 | 6427篇 |
1982年 | 6678篇 |
1981年 | 6397篇 |
1980年 | 6203篇 |
1979年 | 6569篇 |
1978年 | 6609篇 |
1977年 | 6586篇 |
1976年 | 6667篇 |
1975年 | 6193篇 |
1974年 | 6166篇 |
1973年 | 6256篇 |
1972年 | 4422篇 |
排序方式: 共有10000条查询结果,搜索用时 150 毫秒
851.
852.
Gowravaram Sabitha G.S.Kiran Kumar ReddyK.Bhaskar Reddy J.S. Yadav 《Tetrahedron letters》2003,44(34):6497-6499
The three component condensation of an aldehyde, a β-keto ester and urea (thiourea) in the presence of a catalytic amount of VCl3 is disclosed for the solution phase synthesis of dihydropyrimidinones. The ease of synthesis and work-up allowed the parallel synthesis of a 48-membered library of dihydropyrimidinones quickly and efficiently in good yields. 相似文献
853.
We examine several interesting relationships and expressions involving Fourier-Feynman transform, convolution product and
first variation for functionals in the Fresnel class F(B) of an abstract Wiener space B. We also prove a translation theorem and Parseval's identity for the analytic Feynman integral.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
854.
The effect of viscous dissipation on the development of the boundary layer flow from a cold vertical surface embedded in a Darcian porous medium is investigated. It is found that the flow evolves gradually from the classical Cheng–Minkowycz form to the recently discovered asymptotic dissipation profile which is a parallel flow. 相似文献
855.
856.
A room temperature ionic liquid, 1-pentyl-3-methylimidazolium bromide, [pmIm]Br efficiently catalyzes Michael addition of thiols and diethyl dithiophosphate to a variety of conjugated alkenes such as α,β-unsaturated carbonyl compounds, carboxylic esters, nitriles and chalcones without requiring any other organic solvent and catalyst. The ionic liquid can be recycled for subsequent reactions without any appreciable loss of efficiency. 相似文献
857.
858.
Mechatronic subsystems have been extensively developed in automotive applications as they increase performance compared to passive components. The Hydractive CRONE suspension based on fractional differentiation is presented as a hybrid system undergoing an infinite sequence of autonomous switchings. The modelling and operation of Hydractive CRONE suspension as a hybrid system are presented. The stability of Hydractive CRONE suspension is demonstrated using the maximum principle extended to hybrid systems. Finally, comfort mode synthesis is proposed using the third generation CRONE control system design. 相似文献
859.
The interaction between multiple incompressible air jets has been studied numerically and experimentally. The numerical predictions have been first validated using experimental data for a single jet configuration. The spreading features of five unequal jets in the configuration of one larger central jet surrounded by four smaller equi‐distant jets, have been studied, for different lateral spacing ratios of 1.5, 2.0 and 2.5 and a central jet Reynolds number of 1.24×105 (corresponding to a Mach number of 0.16). Flow of five equal jets has also been simulated, for the sake of comparison. The jet interactions commence at an axial distance of about 3–4 diameters and complete by an axial distance of about 10 diameters for the lowest spacing ratio of 1.5. For larger spacing ratios, the length required for the start and completion of jet interaction increase. Peripheral jets bend more towards the central jet and merge at a smaller distance, when their sizes are smaller than that of the central jet. The entrainment ratio for multiple jets is higher than that for a single jet. Excellent agreement is observed between the experimental data and theoretical predictions for both mean flow field and turbulent quantities, at regions away from the jet inlet. The potential core length and initial jet development, however, are not predicted very accurately due to differences in the assumed and actual velocity profiles at the jet inlet. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
860.
A new approach to the robust handling of non‐linear constraints for GAs (genetic algorithms) optimization is proposed. A specific feature of the approach consists of the change in the conventional search strategy by employing search paths which pass through both feasible and infeasible points (contrary to the traditional approach where only feasible points may be included in a path). The method (driven by full Navier–Stokes computations) was applied to the problem of multiobjective optimization of aerodynamic shapes subject to various geometrical and aerodynamic constraints. The results demonstrated that the method retains high robustness of conventional GAs while keeping CFD computational volume to an acceptable level, which allowed the algorithm to be used in a demanding engineering environment. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献