全文获取类型
收费全文 | 851713篇 |
免费 | 9730篇 |
国内免费 | 2767篇 |
专业分类
化学 | 401902篇 |
晶体学 | 10665篇 |
力学 | 32290篇 |
综合类 | 23篇 |
数学 | 91509篇 |
物理学 | 216797篇 |
无线电 | 111024篇 |
出版年
2021年 | 6314篇 |
2020年 | 7084篇 |
2019年 | 7483篇 |
2018年 | 9096篇 |
2017年 | 8806篇 |
2016年 | 14303篇 |
2015年 | 9472篇 |
2014年 | 14276篇 |
2013年 | 36216篇 |
2012年 | 27309篇 |
2011年 | 33473篇 |
2010年 | 23592篇 |
2009年 | 24615篇 |
2008年 | 33066篇 |
2007年 | 33974篇 |
2006年 | 32236篇 |
2005年 | 29717篇 |
2004年 | 27379篇 |
2003年 | 24692篇 |
2002年 | 24172篇 |
2001年 | 26275篇 |
2000年 | 20971篇 |
1999年 | 17142篇 |
1998年 | 14633篇 |
1997年 | 14229篇 |
1996年 | 13908篇 |
1995年 | 12784篇 |
1994年 | 12478篇 |
1993年 | 12000篇 |
1992年 | 13168篇 |
1991年 | 13154篇 |
1990年 | 12397篇 |
1989年 | 11912篇 |
1988年 | 11774篇 |
1987年 | 10455篇 |
1986年 | 9886篇 |
1985年 | 12943篇 |
1984年 | 13115篇 |
1983年 | 10726篇 |
1982年 | 11003篇 |
1981年 | 10809篇 |
1980年 | 10266篇 |
1979年 | 10776篇 |
1978年 | 10957篇 |
1977年 | 10869篇 |
1976年 | 10631篇 |
1975年 | 10019篇 |
1974年 | 9921篇 |
1973年 | 10102篇 |
1972年 | 6798篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
A theoretical expression is derived for the far fields of semiconductor lasers with coated facets. It is shown that the far fields are different for uncoated, antireflection-coated and high-reflection-coated lasers. The correction factor for the far fields is shown to depend on the transmission coefficient of the facets. 相似文献
282.
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
283.
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
284.
Boutros J. Caire G. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(7):1772-1793
We present a framework for iterative multiuser joint decoding of code-division multiple-access (CDMA) signals, based on the factor-graph representation and on the sum-product algorithm. In this framework, known parallel and serial, hard and soft interference cancellation algorithms are derived in a unified way. The asymptotic performance of these algorithms in the limit of large code block length can be rigorously analyzed by using density evolution. We show that, for random spreading in the large-system limit, density evolution is considerably simplified. Moreover, by making a Gaussian approximation of the decoder soft output, we show that the behavior of iterative multiuser joint decoding is approximately characterized by the stable fixed points of a simple one-dimensional nonlinear dynamical system 相似文献
285.
286.
It is essential in the simulation of power electronics applications to model magnetic components accurately. In addition to modeling the nonlinear hysteresis behavior, eddy currents and winding losses must be included to provide a realistic model. In practice the losses in magnetic components give rise to significant temperature increases which can lead to major changes in the component behavior. In this paper a model of magnetic components is presented which integrates a nonlinear model of hysteresis, electro-magnetic windings and thermal behavior in a single model for use in circuit simulation of power electronics systems. Measurements and simulations are presented which demonstrate the accuracy of the approach for the electrical, magnetic and thermal domains across a variety of operating conditions, including static thermal conditions and dynamic self heating 相似文献
287.
Medard M. Lumetta S. Liuyang Li 《Selected Areas in Communications, IEEE Journal on》2002,20(4):822-833
We describe an architecture for an optical local area network (LAN) or metropolitan area network (MAN) access. The architecture allows for bandwidth sharing within a wavelength and is robust to both link and node failures. The architecture can be utilized with an arbitrary, link-redundant mesh network (node-redundancy is necessary only to handle all node failures), and assumes neither the use of a star topology nor the ability to embed such a topology within the physical mesh. Reservation of, bandwidth is performed in a centralized fashion at a (replicated) head end node, simplifying the implementation of complex sharing policies relative to implementation on a distributed set of routers. Unlike a router, however, the head end does not take any action on individual packets and, in particular, does not buffer packets. The architecture thus avoids the difficulties of processing packets in the optical domain while allowing for packetized shared access of wavelengths. We describe the route construction scheme and prove its ability to recover from single link and single node failures, outline a flexible medium access protocol and discuss the implications for implementing specific policies, and propose a simple implementation of the recovery protocol in terms of state machines for per-link devices 相似文献
288.
We study the induced measure obtained from a 1-step Markov measure, supported by a topological Markov chain, after the mapping of the original alphabet onto another one. We give sufficient conditions for the induced measure to be a Gibbs measure (in the sense of Bowen) when the factor system is again a topological Markov chain. This amounts to constructing, when it does exist, the induced potential and proving its Hölder continuity. This is achieved through a matrix method. We provide examples and counterexamples to illustrate our results. 相似文献
289.
M. A. Nikitina 《Russian Physics Journal》2003,46(3):296-304
The h
2-persistent multiple-access communication networks with discrete and continuous strategy of control of conflict messages are investigated. 相似文献
290.
X-ray diffraction patterns of nanocrystalline Fe-Cu-Nb-Si-B (FINEMET) alloys reveal that bcc α-Fe/α-FeSi crystallites with
the average grain size of 20(5) nm are dispersed in amorphous matrix. Enhanced electron—electron interaction (EEI) and quantum
interference (QI) effects as well as electron-magnon (and/or electron-spin fluctuation) scattering turn out to be the main
mechanisms that govern the temperature dependence of resistivity. Of all the inelastic scattering processes, inelastic electron-phonon
scattering is the most effective mechanism to destroy phase coherence of electron wave functions. The diffusion constant,
density of states at the Fermi level and the inelastic scattering time have been estimated, for the first time, for the alloys
in question
Article presented at the International Symposium on Advances in Superconductivity and Magnetism: Materials, Mechanisms and
Devices, ASMM2D-2001, 25–28 September 2001, Mangalore, India. 相似文献