全文获取类型
收费全文 | 318137篇 |
免费 | 1460篇 |
国内免费 | 356篇 |
专业分类
化学 | 136859篇 |
晶体学 | 4651篇 |
力学 | 16868篇 |
综合类 | 3篇 |
数学 | 42001篇 |
物理学 | 99378篇 |
无线电 | 20193篇 |
出版年
2021年 | 3377篇 |
2020年 | 3802篇 |
2019年 | 4667篇 |
2018年 | 6742篇 |
2017年 | 6926篇 |
2016年 | 8548篇 |
2015年 | 3663篇 |
2014年 | 7370篇 |
2013年 | 12804篇 |
2012年 | 10619篇 |
2011年 | 12431篇 |
2010年 | 10598篇 |
2009年 | 10974篇 |
2008年 | 12245篇 |
2007年 | 12216篇 |
2006年 | 10878篇 |
2005年 | 9417篇 |
2004年 | 9128篇 |
2003年 | 8563篇 |
2002年 | 8761篇 |
2001年 | 8348篇 |
2000年 | 6764篇 |
1999年 | 5121篇 |
1998年 | 4869篇 |
1997年 | 4708篇 |
1996年 | 4084篇 |
1995年 | 3636篇 |
1994年 | 3702篇 |
1993年 | 3680篇 |
1992年 | 3630篇 |
1991年 | 4023篇 |
1990年 | 4083篇 |
1989年 | 4075篇 |
1988年 | 3772篇 |
1987年 | 3798篇 |
1986年 | 3476篇 |
1985年 | 3997篇 |
1984年 | 4224篇 |
1983年 | 3576篇 |
1982年 | 3758篇 |
1981年 | 3444篇 |
1980年 | 3209篇 |
1979年 | 3672篇 |
1978年 | 3751篇 |
1977年 | 3904篇 |
1976年 | 3973篇 |
1975年 | 3716篇 |
1974年 | 3565篇 |
1973年 | 3727篇 |
1972年 | 3206篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
V. YILDIRIM 《Journal of sound and vibration》2002,252(3):479-491
Numerical and analytical studies are performed for the free vibration analysis of non-cylindrical (conical, barrel and hyperboloidal types) helical springs. The stiffness matrix method is used in the numerical analysis. A total of 12 degrees of freedom (six displacements and six rotations) is described for an element. The exact element stiffness matrix and the exact concentrated element inertia matrix are used in the formulation. The rotary inertia, the shear and extensional deformation effects are considered in the analysis. Comparison of the numerical results with the reported results obtained numerically and experimentally gives satisfactory values. After verification of the numerical frequencies, the non-dimensional fundamental frequencies of fixed-fixed non-cylindrical helical springs with circular section are expressed in a simple formula with a maximum absolute relative error of 5% using those numerical values for the constant helix pitch angles (5°, 10°, and 15°). These expressions restricted to the fundamental frequencies are also verified with ANSYS results. 相似文献
42.
The possible application of adaptive antennas in microcellular operational environments is studied in this paper with the help of a site-specific ray-tracing propagation model. First are examined the statistics of propagation parameters that play an important role for the performance of adaptive antennas. Particular emphasis is put on the spatial characteristics of the radio channel under line-of-sight (LOS) and non-LOS conditions. For this reason, area statistics are produced for a typical small-cell environment for parameters such as the number of multipath clusters, number of rays, angular size, and excess delay of each impulse response, as well as the number of rays, angular size and power of the multipath clusters. Results show that contrary to the common belief for such environments, the signal is not "almost" uniformly distributed in the spatial domain, instead is contained in a few narrow clusters. Then the effect of the microcellular environment on the uplink performance of some well-known techniques like conventional beamforming, maximum entropy, switched beams, maximum ratio and optimum combining, and beam-space optimum combining is examined . The results show that for typical time-division multiple-access (TDMA) scenarios with one and two users, the beam space optimum combining (strongest eigenvalue version) and the switched beams methods, offer the best performance 相似文献
43.
V.R. Velasco 《Microelectronics Journal》2002,33(4):361-364
We study the electronic states of different GaAs-AlAs Fibonacci quasi-perodic heterostructures grown along the (001) direction. We employ an empirical tight-binding Hamiltonian including spin-orbit coupling together with the surface Green function matching method. We present results for the eighth Fibonacci generation formed from different building blocks. We compare these results with those of the constituent quantum wells and with those of heterostructures containing the same number of GaAs and AlAs slabs after periodic repetition of the building blocks. No Fibonacci spectrum is found in the energy regions near the conduction and valence band edges of GaAs. A selective localization of the local density of states in the GaAs layers is found for many electronic states. 相似文献
44.
Low-loss (<6 dB), flat-top, 16-wavelength, 100-GHz channel spacing DWDM (de)multiplexers are reported. The bulk-optics architecture used allows excellent thermal stability for operating temperature [-10, 60°C] 相似文献
45.
Tonchev V.D. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(4):975-977
It is proved that a class of q-ary (2n,n) formally self-dual codes obtained from symmetric matrices over GF (q), contains codes that meet the Varshamov-Gilbert bound. The codes are self-dual with respect to the symplectic inner product and yield quantum codes encoding one state with n q-ary qubits and having minimum distance proportional to n 相似文献
46.
Self-similarity in diffraction by a self-similar fractal screen 总被引:1,自引:0,他引:1
Lakhtakia A. Holter N. Varadan V. 《Antennas and Propagation, IEEE Transactions on》1987,35(2):236-239
Using the Sierpinski gasket as an example, it has been shown here that the paraxial Fraunhoffer-zone diffracted field of a self-similar fractal screen also exhibits self-similarity. This also establishes that fractal structures can be used with great profit in problems involving array syntheses. Recently, it has been shown that the Sierpinski gasket is itself a member of a much wider class of gaskets, and the potential for the use of fractal structures in electromagnetic (EM) problems may be vast indeed. 相似文献
47.
The purpose of this paper is to investigate problems of the Navier-Stokes approximation to kinetic equations in terms of the
so-called Chapman-Enskog projection. One considers properties of the Chapman-Enskog projection for the Cauchy problem for
moment approximations of the kinetic equation and primarily the Chapman-Enskog projection for the Boltzmann-Peierls kinetic
equation. The existence of the Chapman-Enskog projection for the Cauchy problem is proved for the phase space of conservative
variables (phenomena of nonlinear diffusion) and for the phase space of physical variables (the second sound projection).
__________
Translated from Trudy Seminara imeni I. G. Petrovskogo, No. 25, pp. 184–225, 2005. 相似文献
48.
I. J. Lozada J. C. Osorio D. V. Griffiths M. Cerrolaza 《Numerical Methods for Partial Differential Equations》2006,22(2):296-316
The semi‐analytical integration of an 8‐node plane strain finite element stiffness matrix is presented in this work. The element is assumed to be super‐parametric, having straight sides. Before carrying out the integration, the integral expressions are classified into several groups, thus avoiding duplication of calculations. Symbolic manipulation and integration is used to obtain the basic formulae to evaluate the stiffness matrix. Then, the resulting expressions are postprocessed, optimized, and simplified in order to reduce the computation time. Maple symbolic‐manipulation software was used to generate the closed expressions and to develop the corresponding Fortran code. Comparisons between semi‐analytical integration and numerical integration were made. It was demonstrated that semi‐analytical integration required less CPU time than conventional numerical integration (using Gaussian‐Legendre quadrature) to obtain the stiffness matrix. © 2005 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2006 相似文献
49.
Nonlinear Theory of Spectra of Thermally Stimulated Currents in Complex Crystals with Hydrogen Bonds
Tonkonogov M. P. Ismailov Zh. T. Timokhin V. M. Fazylov K. K. Kalitka V. A. Baimukhanov Z. K. 《Russian Physics Journal》2002,45(10):1008-1018
The nonlinear theory of thermally stimulated depolarization currents is developed. The theory explains the processes of hetero- and homocharge relaxation in complex crystals with hydrogen bonds and allows the relaxation oscillator parameters to be calculated using the quadratic approximation for the external electric field. 相似文献
50.
The results of experimental investigations on thermal nonequilibrium ionization in CO2: N2: He mixtures are presented. Measurements of electron density, ne, in vibrationally excited nitrogen were made in a supersonic flow with different CO2 contents as well as in a CO2: N2: He = 1 : 5 : 4 mixture laser gas. The mixtures were heated in a shock tube and expanded through a supersonic nozzle. Furthermore, supersonic mixing of N2 and CO2 + He was used in some experiments. The measured values of ne in the plenum chamber and in the supersonic nozzle are reported, and the processes responsible for nonequilibrium ionization in a laser-active medium are discussed. 相似文献