全文获取类型
收费全文 | 502579篇 |
免费 | 7025篇 |
国内免费 | 1622篇 |
专业分类
化学 | 275616篇 |
晶体学 | 7587篇 |
力学 | 20602篇 |
综合类 | 23篇 |
数学 | 58120篇 |
物理学 | 149278篇 |
出版年
2020年 | 3845篇 |
2019年 | 3919篇 |
2018年 | 4381篇 |
2017年 | 4387篇 |
2016年 | 7697篇 |
2015年 | 5611篇 |
2014年 | 8077篇 |
2013年 | 22878篇 |
2012年 | 17087篇 |
2011年 | 20972篇 |
2010年 | 13768篇 |
2009年 | 13599篇 |
2008年 | 19077篇 |
2007年 | 19082篇 |
2006年 | 18360篇 |
2005年 | 16698篇 |
2004年 | 15015篇 |
2003年 | 13410篇 |
2002年 | 13031篇 |
2001年 | 14900篇 |
2000年 | 11528篇 |
1999年 | 9044篇 |
1998年 | 7279篇 |
1997年 | 7065篇 |
1996年 | 7121篇 |
1995年 | 6565篇 |
1994年 | 6224篇 |
1993年 | 5965篇 |
1992年 | 7000篇 |
1991年 | 6697篇 |
1990年 | 6379篇 |
1989年 | 6243篇 |
1988年 | 6489篇 |
1987年 | 6248篇 |
1986年 | 5962篇 |
1985年 | 8296篇 |
1984年 | 8354篇 |
1983年 | 6748篇 |
1982年 | 7194篇 |
1981年 | 7270篇 |
1980年 | 6849篇 |
1979年 | 7219篇 |
1978年 | 7268篇 |
1977年 | 7378篇 |
1976年 | 7124篇 |
1975年 | 6804篇 |
1974年 | 6666篇 |
1973年 | 6673篇 |
1972年 | 4115篇 |
1968年 | 3431篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
S. K. Godovikov 《JETP Letters》1998,68(8):629-633
In research on the production of beams of coherent γ rays (γ-ray lasers), conditions under which a substantial change Δλ/λ=?(0.114 ±0.027) in the radioactive decay constant λ (the isomeric level 89.53 keV 119m Sn, T 1/2=293 days) can occur have been found experimentally for the first time. This is made possible by coherent Mo ssbauer (23.87 keV) backscattering from a resonant screen located nearby. An interpretation of the effect observed is proposed on the basis of the idea of dynamic synchronization of oscillations between a nuclear level and a standing wave of Mössbauer radiation. Possibilities for further increasing Δλ/λ up to 0.5 are found. 相似文献
952.
We consider an optimal growth (multi-sector) model with nonconvex technology. Using the Clarke results on generalized gradients, we prove that the value function has left and right derivatives with respect to the initial capital stock, without requiring supermodularity assumptions. 相似文献
953.
R. Grac M. Pugnet J.H. Collet B. Lambert C. De Matos H. L'Haridon A. Le Corre J.O. White 《Superlattices and Microstructures》1997,22(4):505-509
We report new results on the diffraction properties of photoinduced gratings in InGaAs/InGaAsP MQW structures. The original feature of this device is that the QWs are enclosed in an asymmetric Fabry–Perot microcavity in order to increase the diffraction efficiency. We observe oscillations in the diffraction efficiency due to resonant effects in the microcavity. The experimental spectra are compared with theory. Diffraction efficiency at 1.55 μm attains a maximum value of 2.7% at a write beam fluence of 260 μ J cm−2, and then decreases at higher fluences. We explain this phenomenon by an absorption saturation at high excitation. 相似文献
954.
The Modeling of Velocity Enhancement in Polymer Flooding 总被引:1,自引:0,他引:1
In single-phase polymer flooding experiments it has repeatedly been observed that the average velocity of the polymer molecules is higher than the average velocity of the water molecules. This effect is incorporated in many conventional Enhanced Oil Recovery (EOR) simulators by the introduction of a constant velocity enhancement factor. In this paper we show that, in absence of dispersion, a constant enhancement factor in the mathematical model for two-phase polymer flow (Buckley--Leverett displacement) leads to ill-posedness of the model equations. We propose a saturation dependent enhancement factor, derived from a model based on percolation concepts, for which this problem does not occur. 相似文献
955.
Fully and partially observed stochastic control of systems with nonlinear dynamics and terminal and running costs are considered.
Measure changes are introduced which allow both state and observation dynamics to be thought of as linear. In the case when
the terms of the cost have a special form the measure change transformation “cancels out” the nonlinearities and changes the
original nonlinear problem into a classical LQG one and standard results can be applied. We also consider unnormalized conditional
densities of the whole path as state variables and obtain dynamic programming and verification results.
R. J. Elliott wishes to acknowledge support of the Natural Sciences and Engineering Research Council of Canada, Grant A7964. 相似文献
956.
957.
Simon G. Bott Alan P. Marchand Dongxia Xing Rajesh Shukla Stephen James Obrey K. Venkatesan J. Narasimha Moorthy 《Journal of chemical crystallography》1997,27(11):661-665
NaBH4 reduction of a cage dione proceeds in a stereospecific fashion to give theendo,endo-diol. This reactivity is related to the crystal structure. 相似文献
958.
Summary It is shown that the outer automorphism group of a Coxeter groupW of finite rank is finite if the Coxeter graph contains no infinite bonds. A key step in the proof is to show that if the
group is irreducible andΠ
1 andΠ
2 any two bases of the root system ofW, thenΠ
2 = ±ωΠ
1 for some ω εW. The proof of this latter fact employs some properties of the dominance order on the root system introduced by Brink and
Howlett.
This article was processed by the author using the Springer-Verlag TEX PJour1g macro package 1991. 相似文献
959.
960.