全文获取类型
收费全文 | 359491篇 |
免费 | 2798篇 |
国内免费 | 1014篇 |
专业分类
化学 | 174957篇 |
晶体学 | 5738篇 |
力学 | 18154篇 |
综合类 | 23篇 |
数学 | 60102篇 |
物理学 | 104329篇 |
出版年
2021年 | 2448篇 |
2020年 | 2823篇 |
2019年 | 3215篇 |
2018年 | 14291篇 |
2017年 | 14305篇 |
2016年 | 12052篇 |
2015年 | 4252篇 |
2014年 | 6063篇 |
2013年 | 13367篇 |
2012年 | 13589篇 |
2011年 | 21829篇 |
2010年 | 14511篇 |
2009年 | 14830篇 |
2008年 | 17122篇 |
2007年 | 18934篇 |
2006年 | 10243篇 |
2005年 | 9895篇 |
2004年 | 9285篇 |
2003年 | 8948篇 |
2002年 | 7866篇 |
2001年 | 8511篇 |
2000年 | 6742篇 |
1999年 | 4982篇 |
1998年 | 4222篇 |
1997年 | 4093篇 |
1996年 | 3632篇 |
1995年 | 3278篇 |
1994年 | 3143篇 |
1993年 | 2983篇 |
1992年 | 3640篇 |
1991年 | 3581篇 |
1990年 | 3494篇 |
1989年 | 3529篇 |
1988年 | 3480篇 |
1987年 | 3431篇 |
1986年 | 3274篇 |
1985年 | 4106篇 |
1984年 | 4186篇 |
1983年 | 3357篇 |
1982年 | 3483篇 |
1981年 | 3454篇 |
1980年 | 3199篇 |
1979年 | 3571篇 |
1978年 | 3628篇 |
1977年 | 3763篇 |
1976年 | 3501篇 |
1975年 | 3215篇 |
1974年 | 3202篇 |
1973年 | 3148篇 |
1972年 | 2207篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
For three‐dimensional flows with one inhomogeneous spatial coordinate and two periodic directions, the Karhunen–Loeve procedure is typically formulated as a spatial eigenvalue problem. This is normally referred to as the direct method (DM). Here we derive an equivalent formulation in which the eigenvalue problem is formulated in the temporal coordinate. It is shown that this so‐called method of snapshots (MOS) has some numerical advantages when compared to the DM. In particular, the MOS can be formulated purely as a matrix composed of scalars, thus avoiding the need to construct a matrix of matrices as in the DM. In addition, the MOS avoids the need for so‐called weight functions, which emerge in the DM as a result of the non‐uniform grid typically employed in the inhomogeneous direction. The avoidance of such weight functions, which may exhibit singular behaviour, guarantees satisfaction of the boundary conditions. The MOS is applied to data sets recently obtained from the direct simulation of turbulence in a channel in which viscoelasticity is imparted to the fluid using a Giesekus model. The analysis reveals a steep drop in the dimensionality of the turbulence as viscoelasticity is increased. This is consistent with the results that have been obtained with other viscoelastic models, thus revealing an essential generic feature of polymer‐induced drag reduced turbulent flows. Published in 2006 by John Wiley & Sons, Ltd. 相似文献
62.
Mass distributions of fragments in the low-energy fission of nuclei from 187Ir to 213At have been analysed. This analysis has shown that shell effects in symmetric-mode fragment mass yields from the fission of pre-actinide nuclei could be described if one assumes the existence of two strongly deformed neutron shells in the arising fragments with neutron numbers N1 ≈ 52 and N2 ≈ 68. A new method has been proposed for quantitatively describing the mass distributions of the symmetric fission mode for pre-actinides with A ≈ 180–220. 相似文献
63.
Flowing and static gas-phase samples of HNO3 in O2 and N2 were analyzed by long-path ultraviolet/visible (UV/VIS) spectroscopy to reveal the presence of both NO2 and NO3, the concentrations of which were calculated using differential absorption cross sections. NO2 is produced predominantly by the heterogeneous decomposition of HNO3, whereas NO3 is generated in the gas phase by the thermal decomposition of N2O5, a product of the self-disproportionation of liquid HNO3. © 1993 John Wiley & Sons, Inc. 相似文献
64.
65.
O. A. Dubovskii 《Physics of the Solid State》1998,40(11):1937-1941
Solutions of the dispersion equations for polariton states in finite one-dimensional crystals of arbitrary length are obtained.
The appearance and evolution of the radiative and nonradiative polariton branches are traced as the length varies from two
monomers to limiting values, above which the spectrum no longer undergoes significant changes. The dependences of the frequencies
and radiative widths on the polariton wave vector are found for various orientations of the dipole moment of the quantum transition.
The evolution of superradiance as the length of the crystal increases is traced. Some previously unknown significant features
of the polariton spectrum are noted particularly the damping of the branch traditionally termed nonradiative as a consequence
of emission from the end faces.
Fiz. Tverd. Tela (St. Petersburg) 40, 2136–2140 (November 1998) 相似文献
66.
67.
68.
69.
The statistical characteristics of conditional time before ruin of an insurance company, including the distribution function of conditional time and its moments are examined. 相似文献
70.
The long-range sound propagation from a deep ocean to a receiving system located on the shelf is modeled. The waveguide model is constructed on the basis of the data of an acoustic-oceanographic experiment carried out in the northwestern Pacific. The sensitivity and the frequency dependence of the difference in the sound field levels at the crossing of the frontal zone on the geoacoustic characteristics of the bottom of the shelf and the continental slope are investigated. It is shown that the level difference decreases by 8.2 dB as the velocity of longitudinal waves increases by 100 m/s in the range within 1490–1820 m/s. 相似文献