全文获取类型
收费全文 | 407379篇 |
免费 | 4934篇 |
国内免费 | 1193篇 |
专业分类
化学 | 221443篇 |
晶体学 | 5687篇 |
力学 | 16541篇 |
综合类 | 8篇 |
数学 | 48644篇 |
物理学 | 121183篇 |
出版年
2021年 | 2733篇 |
2020年 | 3255篇 |
2019年 | 3239篇 |
2018年 | 3812篇 |
2017年 | 3820篇 |
2016年 | 6572篇 |
2015年 | 4734篇 |
2014年 | 6630篇 |
2013年 | 18352篇 |
2012年 | 14212篇 |
2011年 | 17392篇 |
2010年 | 11285篇 |
2009年 | 11115篇 |
2008年 | 16110篇 |
2007年 | 16176篇 |
2006年 | 15523篇 |
2005年 | 14077篇 |
2004年 | 12630篇 |
2003年 | 11226篇 |
2002年 | 11072篇 |
2001年 | 12337篇 |
2000年 | 9445篇 |
1999年 | 7439篇 |
1998年 | 6082篇 |
1997年 | 5836篇 |
1996年 | 5904篇 |
1995年 | 5381篇 |
1994年 | 5141篇 |
1993年 | 5082篇 |
1992年 | 5500篇 |
1991年 | 5308篇 |
1990年 | 5041篇 |
1989年 | 4798篇 |
1988年 | 5030篇 |
1987年 | 4694篇 |
1986年 | 4511篇 |
1985年 | 6392篇 |
1984年 | 6515篇 |
1983年 | 5383篇 |
1982年 | 5760篇 |
1981年 | 5701篇 |
1980年 | 5440篇 |
1979年 | 5594篇 |
1978年 | 5716篇 |
1977年 | 5684篇 |
1976年 | 5535篇 |
1975年 | 5386篇 |
1974年 | 5262篇 |
1973年 | 5396篇 |
1972年 | 3170篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
O. N. Kozakov 《Optics and Spectroscopy》2002,93(4):598-602
A method of calculating the partial characteristics of radiation absorption by the components of light-scattering disperse layers is proposed. This method is based on statistical modeling (the Monte Carlo method). The absorptivities of photographic gelatin and silver bromide microcrystals and the corresponding distributions of the absorbed energy over the layer thickness are calculated using the example of an interaction between actinic radiation and silver halide photographic layers in the wavelength range λ=200–440 nm. The following structural parameters of the photographic layer are used in the calculation: the mean size of emulsion crystals d=0.5 μm; the polydispersity C V =25%; the volume concentrations C V =10, 20, and 30%; and the thickness of the emulsion layer H=10 μm. 相似文献
43.
A comprehensive study of the long pulse Nd:YAG laser drilling of multi-layer carbon fibre composites 总被引:2,自引:0,他引:2
W. S. O. Rodden S. S. Kudesia D. P. Hand J. D. C. Jones 《Optics Communications》2002,210(3-6):319-328
The results of an extensive experimental study of the free running Nd:YAG laser drilling of a multi-layer carbon fibre composite, where adjacent layers have differently orientated fibres, are reported. For holes drilled with the laser operating in fixed-Q mode at 1064 nm, parallel sections of blind holes illustrating discontinuities in the hole size along a given section direction will be shown to occur at the interface between adjacent layers. An explanation for this effect is proposed. Detailed single pulse drilling characteristics will be presented illustrating the exit hole diameter as a function of pulse energy and material thickness. These characteristics illustrate a ‘stable' drilling regime in which the exit hole diameters are least sensitive to changes in pulse energy or material thickness and a less ‘stable' regime in which they are more strongly dependent on these parameters. Drilling characteristics will be given for two different beam qualities, illustrating the greater drilling depth and reduced hole size achievable with an improved beam quality. Finally holes drilled through a 2 mm thick sample of material with multiple pulses are considered. Size distribution curves for entrance and exit holes will be presented. The total energy required (number of pulses × pulse energy) to drill through 2 mm thick material will be reported as a function of pulse energy in stationary air and argon atmospheres and in a partial vacuum, illustrating a threshold energy which is dependent upon the drilling atmosphere. The threshold energies will be discussed with reference to plasma formation and the reactivity of the drilling atmosphere. 相似文献
44.
The adsorption of hydrogen on a clean Cu10%/Ni90% (110) alloy single crystal was studied using flash desorption spectroscopy (FDS), Auger electron spectroscopy (AES), and work function measurements. Surface compositions were varied from 100% Ni to 35% Ni. The hydrogen chemisorption on a-surface of 100% nickel revealed strong attractive interactions between the hydrogen atoms in accordance with previous work on Ni(100). Three desorption states (β1, β2 and α) appeared in the desorption spectra. The highest temperature (α) state was occupied only after the initial population of the β2-state. As the amount of copper was increased in the nickel substrate, desorption from the higher energy binding α-state was reduced, indicating a decrease in the attractive interactions among hydrogen atoms. The hydrogen coverage at saturation was not affected by the addition of copper to the nickel substrate until the copper concentration was greater than 25% at which a sharp reduction in saturation coverage occurred. This phenomenon was apparently due to the adsorption of hydrogen on Ni atoms followed by occupation of NiNi and CuNi bridged adsorption sites, while occupation of CuCu sites was restricted due to an energy barrier to migration. 相似文献
45.
A. P. Prudnikov Yu. A. Brychkov O. I. Marichev 《Journal of Mathematical Sciences》1991,54(6):1239-1341
Translated from Itogi Nauki i Tekhniki, Seriya Matematicheskii Analiz, Vol. 27, pp. 3–146, 1989. 相似文献
46.
Summary Considered here are model equations for weakly nonlinear and dispersive long waves, which feature general forms of dispersion and pure power nonlinearity. Two variants of such equations are introduced, one of Korteweg-de Vries type and one of regularized long-wave type. It is proven that solutions of the pure initial-value problem for these two types of model equations are the same, to within the order of accuracy attributable to either, on the long time scale during which nonlinear and dispersive effects may accumulate to make an order-one relative difference to the wave profiles.This research was supported in part by the National Science Foundation. A considerable portion of the project was completed while the first author was resident at the Institute for Mathematics and Its Applications, University of Minnesota. 相似文献
47.
The determination of the past and the future of a physical system are complementary aims of measurements. An optimal determination of the past of a system can be achieved by an informationally complete set of physical quantities. Such a set is always strongly noncommutative. An optimal determination of the future of a physical system can be obtained by a Boolean complete set of quantities. The two aims can be reconciled to a reasonable degree with using unsharp measurements.This work was partly supported by the Bundesministerium für Forschung und Technologie, Bonn, the Research Institute for Theoretical Physics, Helsinki, and the University of Turku Foundation, Turku. 相似文献
48.
William J. Reed 《Natural Resource Modeling》1989,3(4):463-480
It is assumed that the probability of destruction of a biological asset by natural hazards can be reduced through investment in protection. Specifically a model, in which the hazard rate depends on both the age of the asset and the accumulated invested protection capital, is assumed. The protection capital depreciates through time and its effectiveness in reducing the hazard rate is subject to diminishing returns. It is shown how the investment schedule to maximize the expected net present value of the asset can be determined using the methods of deterministic optimal control, with the survival probability regarded as a state variable. The optimal investment pattern involves “bang-bang-singular” control. A numerical scheme for determining jointly the optimal investment policy and the optimal harvest (or replacement) age is outlined and a numerical example involving forest fire protection is given. 相似文献
49.
50.
We study matter-wave solitons in Bose-Einstein condensates of ultracold gaseous atoms with spin degrees of freedom and present a class of exact solutions based on the inverse scattering method. The one-soliton solutions are classified with respect to the spin states. We analyze collisional effects between solitons in the same or different spin state(s), which reveals a very interesting possibility: we can manipulate the spin dynamics by controlling the parameters of colliding solitons. 相似文献