全文获取类型
收费全文 | 3945篇 |
免费 | 175篇 |
国内免费 | 10篇 |
专业分类
化学 | 2618篇 |
晶体学 | 10篇 |
力学 | 128篇 |
数学 | 642篇 |
物理学 | 732篇 |
出版年
2023年 | 45篇 |
2022年 | 75篇 |
2021年 | 89篇 |
2020年 | 76篇 |
2019年 | 104篇 |
2018年 | 73篇 |
2017年 | 48篇 |
2016年 | 153篇 |
2015年 | 133篇 |
2014年 | 136篇 |
2013年 | 220篇 |
2012年 | 327篇 |
2011年 | 356篇 |
2010年 | 191篇 |
2009年 | 173篇 |
2008年 | 279篇 |
2007年 | 246篇 |
2006年 | 239篇 |
2005年 | 198篇 |
2004年 | 178篇 |
2003年 | 136篇 |
2002年 | 112篇 |
2001年 | 47篇 |
2000年 | 49篇 |
1999年 | 37篇 |
1998年 | 22篇 |
1997年 | 20篇 |
1996年 | 15篇 |
1995年 | 10篇 |
1994年 | 18篇 |
1993年 | 23篇 |
1992年 | 21篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 15篇 |
1988年 | 12篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 7篇 |
排序方式: 共有4130条查询结果,搜索用时 20 毫秒
21.
22.
A. L'Esprance J. Nicolas D.K. Wilson D.W. Thomson Y. Gabillet G. Daigle 《Applied Acoustics》1993,40(4)
This paper presents a set of acoustical and meteorological data from an outdoor sound propagation experiment. This experiment was done in a farm field near Rock Springs, Pennsylvania, on 7 July 1990. Meteorological and acoustical measurements were recorded simultaneously during six different times in the day. The meteorological measurements permitted determination of the sound speed profiles during each of the measurement sessions, using a method based on surface-layer similarity scaling. The acoustical measurements allowed precise determination of the relative sound pressure levels for a frequency range up to 3150 Hz at six different distances (66, 88, 125, 175, 250 and 350 m). The results show atmospheric conditions have an important effect on sound propagation. At medium and high frequencies, variations of the relative SPL have been measured at distances as short as 62 m. These effects increased with the distances so that variations as great as 30 dB have been measured during that day. Comparisons with the fast field program predictions are also presented, and amply demonstrate the accuracies of this model, especially for the downward refraction cases. 相似文献
23.
We study cemented granular media by introducing cohesive bonding (sliding or rolling friction and tensile strength) between grains in the framework of the contact dynamics method. We find that, for a wide range of bond parameters, the macroscopic angle of friction at the peak state can be split into three distinct terms of collisional, frictional and dilational origins. Remarkably, the macroscopic tensile strength depends only on the bond tensile strength, and the friction angle at the peak state is proportional to the dilatancy angle which varies linearly with sliding friction. 相似文献
24.
Pedro H.J. Nardelli Nicolas Rubido Chengwei Wang Murilo S. Baptista Carlos Pomalaza-Raez Paulo Cardieri Matti Latva-aho 《The European physical journal. Special topics》2014,223(12):2423-2437
This article reviews different kinds of models for the electric power grid that can be used to understand the modern power system, the smart grid. From the physical network to abstract energy markets, we identify in the literature different aspects that co-determine the spatio-temporal multilayer dynamics of power system. We start our review by showing how the generation, transmission and distribution characteristics of the traditional power grids are already subject to complex behaviour appearing as a result of the the interplay between dynamics of the nodes and topology, namely synchronisation and cascade effects. When dealing with smart grids, the system complexity increases even more: on top of the physical network of power lines and controllable sources of electricity, the modernisation brings information networks, renewable intermittent generation, market liberalisation, prosumers, among other aspects. In this case, we forecast a dynamical co-evolution of the smart grid and other kind of networked systems that cannot be understood isolated. This review compiles recent results that model electric power grids as complex systems, going beyond pure technological aspects. From this perspective, we then indicate possible ways to incorporate the diverse co-evolving systems into the smart grid model using, for example, network theory and multi-agent simulation. 相似文献
25.
We consider a model of spinless fermions on a lattice, interacting through a nearest neighbor repulsion. In the half-filled band case and for dimensionsd 2, we rigorously prove that there is long-range order in some domain of the parameters=(k
B
T)–1 andt/U, wheret is the hopping amplitude of the particles,U the strength of their repulsion, and the inverse temperature. Our technique is based on the usual Peierls argument of classical statistical mechanics but fails for the groundstate. We discuss the specific difficulties introduced by the Fermi statistics.Work supported in part by U.S. NSF grant PHY 90-19433-A02. 相似文献
26.
27.
28.
Parasteh Pirasteh Ali Soltani Lazhar Haji Nicolas Errien 《Applied Surface Science》2006,253(4):1999-2002
In order to understand the optical loss mechanisms in porous silicon based waveguides, structural and optical studies have been performed. Scanning and transmission electron microscopic observations of porous silicon layers are obtained before and after an oxidation process at high temperature in wet O2. Pore size and shape of heavily p-type doped Si wafers are estimated and correlated to the optical properties of the material before and after oxidation. The refractive index was measured and compared to that determined by the Bruggeman model. 相似文献
29.
Finite pulse durations in diverse pulse schemes lead to the reduction of the magnitude of the magnetization vector due to T1 and T2 effects during the radio-frequency pulses. This paper presents an analysis of the steady state signal in the presence of relaxation effects during radio-frequency pulses in MRI spoiled gradient echo sequences. It is shown that minor attenuations of the magnetization vector can have dramatic consequences on the measured signal, and may thus entail a loss in SNR benefits at high static magnetic fields if a careful analysis is not performed. It is emphasized that it is the time-integrated magnetization vector trajectory that matters for these effects and not only the pulse duration. Some experimental results obtained on a phantom at 3 T verify this analysis. 相似文献
30.
The curvature energy coefficient of the nuclear mass formulaa c is first calculated for the model case of a Fermi gas bounded by an external Woods-Saxon potential. The semiclassical theory of Wigner and Kirkwood is used anda c is found to be close to zero. It is, however, shown that this low value is due to the lack of selfconsistency of the potential. When available, the results of the model compare very well with quantal values and the extrapolation to the spherical cavity (billiard) checks with the value fora c known from the Balian-Bloch theory. Second, the selfconsistent case is generalised to finite range forces. No indication is found that this modifies the fact that all theoretical values for a c are larger than about 7 MeV which is an order of magnitude above the empirical value. 相似文献