全文获取类型
收费全文 | 412119篇 |
免费 | 3051篇 |
国内免费 | 1375篇 |
专业分类
化学 | 207161篇 |
晶体学 | 5997篇 |
力学 | 20378篇 |
综合类 | 9篇 |
数学 | 66401篇 |
物理学 | 116599篇 |
出版年
2021年 | 2599篇 |
2020年 | 2925篇 |
2019年 | 3294篇 |
2018年 | 14484篇 |
2017年 | 14293篇 |
2016年 | 12130篇 |
2015年 | 4483篇 |
2014年 | 6019篇 |
2013年 | 14663篇 |
2012年 | 14466篇 |
2011年 | 23327篇 |
2010年 | 14870篇 |
2009年 | 15191篇 |
2008年 | 19038篇 |
2007年 | 21018篇 |
2006年 | 12256篇 |
2005年 | 12166篇 |
2004年 | 11247篇 |
2003年 | 10867篇 |
2002年 | 9687篇 |
2001年 | 10790篇 |
2000年 | 8262篇 |
1999年 | 6217篇 |
1998年 | 4978篇 |
1997年 | 5047篇 |
1996年 | 4757篇 |
1995年 | 4076篇 |
1994年 | 4122篇 |
1993年 | 3968篇 |
1992年 | 4545篇 |
1991年 | 4658篇 |
1990年 | 4458篇 |
1989年 | 4361篇 |
1988年 | 4284篇 |
1987年 | 4295篇 |
1986年 | 4025篇 |
1985年 | 5136篇 |
1984年 | 5293篇 |
1983年 | 4242篇 |
1982年 | 4480篇 |
1981年 | 4212篇 |
1980年 | 4005篇 |
1979年 | 4453篇 |
1978年 | 4653篇 |
1977年 | 4589篇 |
1976年 | 4544篇 |
1975年 | 4288篇 |
1974年 | 4260篇 |
1973年 | 4408篇 |
1972年 | 2964篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
991.
992.
Deformed classical mechanics and gravity is discussed. Infinitesimally deformed quantum field theory is reviewed. Infinitesimally deformed Bose-Einstein distribution is derived. Infinitesimally deformed thermofield dynamics is constructed and applied to a scalar field and string theories.On leave of absence from: Mathematics Department, Faculty of Science, Mansoura University, Mansoura, Egypt 相似文献
993.
Daniel T. Valentine 《国际流体数值方法杂志》1995,21(2):155-180
The disarrangement of a perturbed lattice of vortices was studied numerically. The basic state is an exponentially decaying, exact solution of the Navier-Stokes equations. Square arrays of vortices with even numbers of vortex cells along each side were perturbed and their evolution was investigated. Whether the energy in the perturbation grows somewhat before it decays or decays monotonically depends on the initial strength of the vortices of the basic state, the extent of lateral confinement and the structure of the perturbation. The critical condition for temporally local instability, i.e. the critical amplitude of the basic state that must be exceeded to allow energy transfer from the basic state to the perturbation, is discussed. In the strongly confined case of a square lattice of four vortices the appearance of enchancement of global rotation is the result of energy transfer from the basic state to a temporally local unstable mode. Energy is transferred from the basic state to larger-scaled structures (inverse cascade) only if the scales of the larger structures are inherently contained in the initial structure of the perturbation. The initial structure of the double array of vortices is not maintained except for a very special form of perturbation. The facts that large scales decay more slowly than small scales and that, when non-linearities are sufficiently strong, energy is transferred from one scale to another explain the differences in the disarrangement process for different initial strengths of the vortices of the basic state. The stronger vortices, i.e. the vortices perturbed in a manner that increases their strength, tend to dominate the weaker vortices. The pairing and subsequent merging (or capture) of vortices of like sense into larger-scale vortices are described in terms of peaks in the evolution of the square root of the palinstrophy divided by the enstrophy. 相似文献
994.
There has been considerable recent attention given to the stressed and buckled states of items with complicated configuration made of different nonlinearly elastic materials joined by complete adhesion. However, effective analytical solutions for such problems have been hindered by mathematical difficulties. Approximate methods have thus been developed for such problems. A variational combined principle has been formulated in this communication. A nonlinear geometrical approach has been used for formulating a mixed-type functional with physical relationships given by Euler equations, nonlinear equilibrium equations, and nonlinear boundary conditions for a piecewise-nonuniform nonlinearly elastic body composed of finite elements (particles). As an example, buckling along the nonuniform thickness of nonlinearly elastic rings was analyzed hypothetically assuming plane cross-sections. Options for two-, three-, four-, five-, and six-layered rings in a periodical structure have been reviewed. The critical buckling forces for an even number of layers have been found to be equal to each other. The ratios of the critical forces, elasticity moduli, and proportionality levels were determined for all five variants by the Runge-Kutta method.Presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October, 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 2, pp. 262–268, March–April, 1995. 相似文献
995.
Granular films prepared from well-defined In clusters embedded in Kr (? 10-40 at % Kr) show sharp resistance anomalies very close to the superconducting transition temperature. The observed resistance peak(s), which go(es) above the normal state resistance RN and which can be as high as ? 1.5 RN, can be explained by a “mesoscopic” effect: Superconducting percolation aggregates of size ξρ, larger than the superconducting coherence length ξS, but smaller than the quasiparticle relaxation length λQ*, have a superconducting gap, but are not yet able to carry a supercurrent. 相似文献
996.
We study the critical behavior of certain two-parameter families of correlated percolation models related to the Ising model on the triangular and square lattices, respectively. These percolation models can be considered as interpolating between the percolation model given by the + and – clusters and the Fortuin-Kasteleyn correlated percolation model associated to the Ising model. We find numerically on both lattices a two-dimensional critical region in which the expected cluster size diverges, yet there is no percolation. 相似文献
997.
Summary A Modified-Melt-Powder-Melt-Growth (MMPMG) technique has been developed for the preparation of melt-textured Y-Ba−Cu−O pellets.
To test their performances, the samples were characterized with respect to their microstructure, pinning strength and interaction
force with permanent magnets. A superconducting magnetic bearing was built and integrated in a levitating flywheel system
for energy storage.
Paper presented at the “VII Congresso SATT?, Torino, 4–7 October 1994.
Under a Fondazione Confalonieri grant. 相似文献
998.
Sulfide cluster-derived ensembles are promising models of the active sites in commercial hydrotreatment catalysts. A series of sulfide clusters were adsorbed intact onto high-surface-area γ-alumina, magnesium oxide and activated carbon supports, then pretreated to produce highly dispersed catalytic ensembles with sizes similar to those of their precursor clusters. The activities of the bimetallic cluster-derived catalysts were significantly higher than those of the monometallic catalysts. We took this as evidence that direct interactions between molybdenum and the promoter element cause the promotional effect observed in commercial hydrotreatment catalysts. The hydrodesulfurization and hydrodenitrogenation activities correlated with the extent of molybdenum reduction. Our results suggested that the active sites in promoted hydrotreatment catalysts are centered on molecular-scale ensembles containing molybdenum, sulfur and the promoter element. 相似文献
999.
We make a thorough non-covariant analysis of the derivation of the equation of motion for a charged particle, including radiation reaction, by means of a simple model for the charge: a dumbbell. This model and our method to expand retarded quantities, based on complex-variable theory, allow us to avoid some of the usual approximations, so we can show several features of the radiation reaction problem. 相似文献
1000.