全文获取类型
收费全文 | 351004篇 |
免费 | 6818篇 |
国内免费 | 3168篇 |
专业分类
化学 | 196638篇 |
晶体学 | 5463篇 |
力学 | 15392篇 |
综合类 | 145篇 |
数学 | 36508篇 |
物理学 | 106844篇 |
出版年
2020年 | 2631篇 |
2019年 | 2754篇 |
2018年 | 2748篇 |
2017年 | 2711篇 |
2016年 | 4991篇 |
2015年 | 4052篇 |
2014年 | 5415篇 |
2013年 | 15533篇 |
2012年 | 11944篇 |
2011年 | 14669篇 |
2010年 | 9431篇 |
2009年 | 9472篇 |
2008年 | 13107篇 |
2007年 | 13183篇 |
2006年 | 12571篇 |
2005年 | 11511篇 |
2004年 | 10353篇 |
2003年 | 9128篇 |
2002年 | 8924篇 |
2001年 | 10874篇 |
2000年 | 8357篇 |
1999年 | 6643篇 |
1998年 | 5144篇 |
1997年 | 5158篇 |
1996年 | 5108篇 |
1995年 | 4598篇 |
1994年 | 4383篇 |
1993年 | 4108篇 |
1992年 | 4850篇 |
1991年 | 4708篇 |
1990年 | 4357篇 |
1989年 | 4402篇 |
1988年 | 4309篇 |
1987年 | 4249篇 |
1986年 | 3991篇 |
1985年 | 5614篇 |
1984年 | 5608篇 |
1983年 | 4604篇 |
1982年 | 4926篇 |
1981年 | 4762篇 |
1980年 | 4627篇 |
1979年 | 4725篇 |
1978年 | 4992篇 |
1977年 | 4813篇 |
1976年 | 4695篇 |
1975年 | 4547篇 |
1974年 | 4372篇 |
1973年 | 4514篇 |
1972年 | 2799篇 |
1967年 | 2735篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
A simple and accurate algorithm (phase scanning method) is proposed for 3D surface contouring and dynamic response determination of a vibrating object. A sinusoidal fringe pattern is projected onto a low-frequency vibrating object by a programmable liquid crystal display projector. The fringe patterns are captured by a high-speed CCD camera with a telecentric gauging lens. Phase values are evaluated point by point using phase scanning method. From the phase values of each point on the object, the contour of the specimen at different instants of vibration can be retrieved. In this paper, a small vibrating coin is used to demonstrate the validity of the method and the experimental results are compared with test results on a stationary coin using four-step phase shifting and fast Fourier transform methods. The technique is especially useful in applications where the vibrating object has a complicated shape. 相似文献
992.
F. R. Ling B. Wang T. Geng S. Q. Fang W. Yuan D. D. Teng C. X. Guan 《Optics & Laser Technology》2004,36(7):541-544
In this paper experimental studies of nonvolatile photorefractive holographic recording in Ce:Cu:LiNbO3 crystals doped with Sc(0,1,2,3 mol%) were carried out. The Sc:Ce:Cu:LiNbO3 crystals were grown by the Czochralski method and oxidized in Nb2O5 powders. The nonvolatile holographic recording in Sc:Ce:Cu:LiNbO3 crystals was realized by the two-photon fixed method. We found that the recording time of Sc:Ce:Cu:LiNbO3 crystal became shorter with the increase of Sc doping concentration, especially doping with Sc(3 mol%), which exceeds the so-called threshold, and there was little loss of nonvolatile diffraction efficiencies between Sc(3 mol%):Ce:Cu:LiNbO3 and Ce:Cu:LiNbO3 crystals. 相似文献
993.
M. Ismail A. Farraga H. A. Abdusalam Th. El-Sherbini 《International Journal of Theoretical Physics》2004,43(1):111-125
The scattering of electrons by atomic copper has been studied using Born approximation and the concept of the generalized oscillator strength (GOS). Differential and total cross-sections for the excitation of the 3d10 4p2 P state are calculated at incident energies of 100 eV and are compared with other available experimental and theoretical data. The agreement between our calculation for the differential cross-section and the available experimental results is fairly good at the forward angles, while the agreement at large angles is poor. The calculated total cross-sections are compared with the experimental data and those predicted by several theories. It is found that our calculation for the total cross-sections are in a good agreement with the close coupling calculation of Msezane and Henry (1986a, Physical Review A
33, 1631) for incident energies greater than 20 eV. The integrated cross-section measurements of Ismail and Teubner (1995, Journal of Physics B: Atomic, Molecular and Optical Physics
28, 4164) are in good agreement with the present calculation. 相似文献
994.
A method for the construction of the second constant of motion in fourth order is carried out. Correspondingly the fourth order potential equation is obtained whose solutions directly provide the classical integrable systems. Second constant of motion is obtained for a large class of potentials. Quantum invariants are also obtained with second order quantum corrections of the order O(?2) to the corresponding classical invariants. The phase space diagrams for these cases are drawn using a mathematical computer software package in two dimensions. 相似文献
995.
Gábor Házi 《Central European Journal of Physics》2003,1(3):453-462
A general class of analytical solutions of the lattice Boltzmann equation is derived for two-dimensional, steady-state unidirectional
flows. A subset of the solutions that verifies the corresponding Navier-Stokes equations is given. It is pointed out that
this class includes, e.g., the Couette and the Poiseuille flow but not, e.g., the basic Kolmogorov flow. For steady-state
non-unidirectional flows, first and second order solutions of the lattice Boltzmann equation are derived. Practical consequences of the analysis
are mentioned. Differences between the technique applied here and those used in some earlier works are emphasized. 相似文献
996.
1D-nanostructural zinc oxide (ZnO) with different shapes have been synthesized on p-type Si(1 0 0) and glass substrates via vapor phase growth by heating pure zinc powder at temperatures between 480 and 570 °C. The different ZnO nanostructures depend on the substrates and the growth temperatures. Scanning electron microscopy and X-ray diffraction revealed that a well-aligned nanowires array, which are vertical to the substrate of Si(1 0 0) with 18 sides on their heads, but six sides on their stems, has been formed at 480 °C. Raman study on the ZnO nanostructures shows that the coupling strength between electron and phonon determined by the ratio of the second- to the first-order Raman scattering cross-sections declines with decreasing diameter of the nanowires. However, a little changes of the coupling strength in terms of the width of the nanobelts have been observed. 相似文献
997.
998.
M. De Falco F. De Giovanni C. Musella R. Schmidt 《Rendiconti del Circolo Matematico di Palermo》2003,52(1):70-76
A subgroupH of a groupG is said to bepermutable ifHX=XH for every subgroupX ofG. In this paper the structure of groups in which every subgroup either is abelian or permutable is investigated.
This work was done while the last author was visiting the University of Napoli Federico II. He thanks the “Dipartimento di
Matematica e Applicazioni” for its financial support. 相似文献
999.
C. M. Zhang 《General Relativity and Gravitation》2003,35(8):1467-1472
In the framework of spacetime with torsion and without curvature, the Dirac particle spin precession in the rotational system is studied. We write out the equivalent tetrad of the rotating frame, in the polar coordinate system, through considering the relativistic factor, and the resultant equivalent metric is a flat Minkowski one. The obtained rotation-spin coupling formula can be applied to the high speed rotating case, which is consistent with the expectation. 相似文献
1000.
C. Soize 《Journal of sound and vibration》2003,263(4):893-916
Recently, a new approach, called a non-parametric model of random uncertainties, has been introduced for modelling random uncertainties in linear and non-linear elastodynamics in the low-frequency range. This non-parametric approach differs from the parametric methods for random uncertainties modelling and has been developed in introducing a new ensemble of random matrices constituted of symmetric positive-definite real random matrices. This ensemble differs from the Gaussian orthogonal ensemble (GOE) and from the other known ensembles of the random matrix theory. The present paper has three main objectives. The first one is to study the statistics of the random eigenvalues of random matrices belonging to this new ensemble and to compare with the GOE. The second one is to compare this new ensemble of random matrices with the GOE in the context of the non-parametric approach of random uncertainties in structural dynamics for the low-frequency range. The last objective is to give a new validation for the non-parametric model of random uncertainties in structural dynamics in comparing, in the low-frequency range, the dynamical response of a simple system having random uncertainties modelled by the parametric and the non-parametric methods. These three objectives will allow us to conclude about the validity of the different theories. 相似文献