全文获取类型
收费全文 | 15606篇 |
免费 | 557篇 |
国内免费 | 45篇 |
专业分类
化学 | 10446篇 |
晶体学 | 254篇 |
力学 | 496篇 |
综合类 | 1篇 |
数学 | 1200篇 |
物理学 | 3811篇 |
出版年
2024年 | 65篇 |
2023年 | 171篇 |
2022年 | 452篇 |
2021年 | 437篇 |
2020年 | 473篇 |
2019年 | 529篇 |
2018年 | 498篇 |
2017年 | 476篇 |
2016年 | 693篇 |
2015年 | 484篇 |
2014年 | 774篇 |
2013年 | 1308篇 |
2012年 | 1147篇 |
2011年 | 1200篇 |
2010年 | 795篇 |
2009年 | 615篇 |
2008年 | 774篇 |
2007年 | 763篇 |
2006年 | 608篇 |
2005年 | 524篇 |
2004年 | 402篇 |
2003年 | 334篇 |
2002年 | 270篇 |
2001年 | 172篇 |
2000年 | 150篇 |
1999年 | 111篇 |
1998年 | 80篇 |
1997年 | 112篇 |
1996年 | 107篇 |
1995年 | 92篇 |
1994年 | 88篇 |
1993年 | 121篇 |
1992年 | 119篇 |
1991年 | 82篇 |
1990年 | 75篇 |
1989年 | 83篇 |
1988年 | 58篇 |
1987年 | 55篇 |
1986年 | 47篇 |
1985年 | 73篇 |
1984年 | 75篇 |
1983年 | 61篇 |
1982年 | 54篇 |
1981年 | 46篇 |
1980年 | 56篇 |
1979年 | 72篇 |
1978年 | 57篇 |
1977年 | 58篇 |
1976年 | 48篇 |
1975年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
32.
“Optimization of aircraft maintenance/support infrastructure using genetic algorithms—level of repair analysis” 总被引:4,自引:0,他引:4
Level of repair analysis (LORA) is an approach used during the design stage of complex equipment for analysis of the cost
effectiveness of competing maintenance strategies. LORA is carried as a part of the life cycle cost and cost of ownership
analysis and plays a significant role in minimizing the life cycle cost and cost of ownership of the capital equipment. Since
many purchasing decisions of complex equipment are based on cost of ownership, it has become essential to carry out LORA to
compete in the market. In this paper, we develop a mathematical model for LORA and propose a solution methodology based on
genetic algorithms. The concept is illustrated using a hypothetical aircraft engine. 相似文献
33.
The paper presents an analytical study of blood flow through a stenosed artery using a suitable mathematical model. The artery is modelled as an anisotropic viscoelastic cylindrical tube containing a non-Newtonian viscous incompressible fluid representing blood. The blood flow is assumed to be characterized by the Herschel–Bulkley model. The effect of the surrounding connective tissues on the motion of the arterial wall has been incorporated. Initially, the relevant solutions of the boundary value problem are obtained in the Laplace transform space, through the use of a suitable finite difference technique. Laplace inversion is carried out by employing suitable numerical techniques. Finally, the variations of the vascular wall displacements, the velocity distribution of the blood flow, the flux, the resistance to flow and the wall shear stress in the stenotic region are quantified through numerical computations and presented graphically. 相似文献
34.
A four-noded rectangular element with seven degrees of freedom at each node is developed for buckling analysis of laminated plate structures having any number of layers with a constant thickness of individual layers. The displacement model is so chosen that it can explain adequately the parabolic distribution of transverse shear stresses and the non-linearity of in-plane displacements across the thickness. A geometrical stiffness matrix is developed using in-plane stresses. A wide range of plates from thick to thin are examined under uniaxial loading conditions. The results are compared with the existing analytical and numerical solutions. The present formulations confirm its applicability for buckling analysis of a wide range of plates. 相似文献
35.
We investigate the change in the calculated value of asymptotic normalization constant (ANC) by the hyperspherical harmonics
expansion method with the inclusion of three nucleon force (3BF) in addition to two nucleon force. We see that ANC does not
change very much with the inclusion of 3BF indicating that the 3BF does not alter the asymptotic behaviours of HHE wavefunction
significantly. 相似文献
36.
Studies of breakdown threshold intensity for air at various pressures in the range of 24–760 torr using 0.355, 0.532 and 1.06
μm radiation are reported. We observep
−0.8 scaling ofI
th at 1.06 μm and a weak scaling ofp
−0.4 at 0.532 and 0.355 μm radiation. Strong dependence of breakdown spot size on laser power but weak dependence on air pressure
is observed. 相似文献
37.
Unsteady flow dynamics in doubly constricted 3D vessels have been investigated under pulsatile flow conditions for a full cycle of period T. The coupled non‐linear partial differential equations governing the mass and momentum of a viscous incompressible fluid has been numerically analyzed by a time accurate Finite Volume Scheme in an implicit Euler time marching setting. Roe's flux difference splitting of non‐linear terms and the pseudo‐compressibility technique employed in the current numerical scheme makes it robust both in space and time. Computational experiments are carried out to assess the influence of Reynolds' number and the spacing between two mild constrictions on the pressure drop across the constrictions. The study reveals that the pressure drop across a series of mild constrictions can get physiologically critical and is also found to be sensitive both to the spacing between the constrictions and the oscillatory nature of the inflow profile. The flow separation zone on the downstream constriction is seen to detach from the diverging wall of the constriction leading to vortex shedding with 3D features earlier than that on the wall in the spacing between the two constrictions. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
38.
Vineet Kumar Rai 《Applied physics. B, Lasers and optics》2007,88(2):297-303
The dependence of temperature on the fluorescence lifetime and fluorescence intensity ratio using Stark sublevels and thermally
coupled (close lying) levels in triply ionized rare earth ions, doped into a variety of glasses and fibers, have been reviewed.
Also, it is claimed that not only for the two closely lying levels of triply ionized rare earth ion of the same kind, but
also for two different triply ionized rare earth ions, having their excited levels very close to each other, may be used to
monitor the temperature.
PACS 42.70.Ce 相似文献
39.
Justina Grabowska Karuna Kar Nanda R.T. Rajendra Kumar J.P. Mosnier M.O. Henry Simon B. Newcomb Patrick McNally Lisa OReilly Xu Lu Enda McGlynn 《Superlattices and Microstructures》2007,42(1-6):327
Self-organized ZnAl2O4 nanostructures with the appearance (in SEM) of high aspect ratio horizontal nanowires are grown on uncatalysed c-sapphire by vapour phase transport. The nanostructures grow as three equivalent crystallographic variants on c-sapphire. Raman and cathodoluminescence spectroscopy confirm that the nanostructures are not ZnO and TEM shows that they are the cubic spinel, zinc aluminate, ZnAl2O4, formed by the reaction of Zn and O with the sapphire substrate. 相似文献
40.
Two binary mixtures of polar liquid crystal materials were previously reported to exhibit three incommensurate smectic A phases predicted for such materials on the basis of phenomenological theory. Results of our recent high-resolution X-ray scattering experiments show that no incommensurate phases exist in the two systems. Wide coexistence regions are found at first order transitions between various frustrated smectic phases of these mixtures. These regions were previously identified as the incommensurate smectic A phases. The phase diagrams of the two systems determined with high-resolution X-ray technique are shown to be in excellent agreement with Baroisa-Prost-Lubensky theory. 相似文献