全文获取类型
收费全文 | 168646篇 |
免费 | 5916篇 |
国内免费 | 4736篇 |
专业分类
化学 | 92880篇 |
晶体学 | 2428篇 |
力学 | 8628篇 |
综合类 | 323篇 |
数学 | 19233篇 |
物理学 | 55806篇 |
出版年
2022年 | 2243篇 |
2021年 | 2857篇 |
2020年 | 3124篇 |
2019年 | 3406篇 |
2018年 | 4208篇 |
2017年 | 4162篇 |
2016年 | 5294篇 |
2015年 | 3282篇 |
2014年 | 4946篇 |
2013年 | 7878篇 |
2012年 | 7668篇 |
2011年 | 8264篇 |
2010年 | 6516篇 |
2009年 | 6552篇 |
2008年 | 7139篇 |
2007年 | 6722篇 |
2006年 | 6304篇 |
2005年 | 5523篇 |
2004年 | 5036篇 |
2003年 | 4564篇 |
2002年 | 4702篇 |
2001年 | 4564篇 |
2000年 | 3454篇 |
1999年 | 2735篇 |
1998年 | 2562篇 |
1997年 | 2267篇 |
1996年 | 2099篇 |
1995年 | 1821篇 |
1994年 | 1881篇 |
1993年 | 1825篇 |
1992年 | 1850篇 |
1991年 | 1959篇 |
1990年 | 1898篇 |
1989年 | 1838篇 |
1988年 | 1662篇 |
1987年 | 1607篇 |
1986年 | 1567篇 |
1985年 | 1707篇 |
1984年 | 1806篇 |
1983年 | 1567篇 |
1982年 | 1586篇 |
1981年 | 1392篇 |
1980年 | 1310篇 |
1979年 | 1501篇 |
1978年 | 1646篇 |
1977年 | 1650篇 |
1976年 | 1673篇 |
1975年 | 1534篇 |
1974年 | 1574篇 |
1973年 | 1547篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Analytical expressions are obtained that describe the changes in the degree of coherence and in the thickness of the coherence layers occurring upon propagation of a dispersed broadband laser beam. It is found that the greater the tilting of the coherence layers with respect to the phase fronts, the more rapidly the spatial coherence is violated with increasing distance. A comparison with the case of an undispersed beam is performed. It is shown that, as the beam propagates, the decrease in the degree of coherence is accompanied by the appearance of spatial fluctuations of this parameter. The degree of mutual coherence of intersecting dispersed beams with parallel correlated coherence layers, which determines the efficiency of their coherent interaction, is investigated. The existence of spatial fluctuations of the degree of mutual coherence is established. 相似文献
52.
O. I. Berngardt 《Radiophysics and Quantum Electronics》2006,49(6):415-431
We propose a model of coherent-echo signals during the monostatic sounding of the ionosphere. The model is based on the previously
obtained radar equation for separate samples of the scattered signal. The dielectric-permittivity perturbation described by
a discrete set of spatial harmonics modulated in space and time is used as the scattering irregularities. The model was tested
using Irkutsk incoherent-scatter radar data obtained during the coherent-echo observations in July 15 and 16, 2000. The test
shows that the model is suitable for describing the observed characteristics of separate sample spectra of the coherent-echo
signals.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 6, pp. 459–477, June 2006. 相似文献
53.
Yu. V. Brezhnev 《Theoretical and Mathematical Physics》2002,133(3):1657-1662
We construct Dubrovin equations and trace formulas for finite-band operators with trigonal curves and present some examples. 相似文献
54.
55.
Guofen Yu Yuanci Gao Shenggang Liu Wenxiang Wang Jin Wu 《International Journal of Infrared and Millimeter Waves》2002,23(8):1261-1269
Analysis of the field distributions in a single biological cell under electromagnetic wave is given. With Debye approximation, the dielectric relaxation of each part of the cell, including the extracellular and cellular media, the cell membrane and the nuclear membrane, was taken into account. Making use of some typical parameters for a cell, the voltage across nuclear and cytoplasma membranes under electromagnetic waves are calculated up to millimeter wave frequency range. The calculated result indicates that it is unlikely to generate electroporation by present available millimeter wave sources. 相似文献
56.
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
57.
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
58.
A. I. Bril' V. P. Kabashnikov N. V. Kuz'mina Yu. V. Khodyko 《Journal of Applied Spectroscopy》1991,55(3):934-939
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 3, pp. 478–484, September, 1991. 相似文献
59.
In this paper, we consider a class of functional equations and prove three theorems which give the approximation properties
and error estimates of the optimal polynomial approximation of slution of those functional equations. 相似文献
60.
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