全文获取类型
收费全文 | 739899篇 |
免费 | 11818篇 |
国内免费 | 6438篇 |
专业分类
化学 | 339278篇 |
晶体学 | 9597篇 |
力学 | 33737篇 |
综合类 | 282篇 |
数学 | 86151篇 |
物理学 | 203693篇 |
无线电 | 85417篇 |
出版年
2021年 | 7916篇 |
2020年 | 8447篇 |
2019年 | 9427篇 |
2018年 | 11816篇 |
2017年 | 11898篇 |
2016年 | 16315篇 |
2015年 | 9612篇 |
2014年 | 15661篇 |
2013年 | 32493篇 |
2012年 | 26095篇 |
2011年 | 30454篇 |
2010年 | 23345篇 |
2009年 | 24346篇 |
2008年 | 29970篇 |
2007年 | 29814篇 |
2006年 | 27317篇 |
2005年 | 24560篇 |
2004年 | 22714篇 |
2003年 | 20371篇 |
2002年 | 20114篇 |
2001年 | 20983篇 |
2000年 | 16887篇 |
1999年 | 13568篇 |
1998年 | 11870篇 |
1997年 | 11633篇 |
1996年 | 10801篇 |
1995年 | 9791篇 |
1994年 | 9807篇 |
1993年 | 9431篇 |
1992年 | 9851篇 |
1991年 | 10152篇 |
1990年 | 9759篇 |
1989年 | 9453篇 |
1988年 | 9095篇 |
1987年 | 8266篇 |
1986年 | 7916篇 |
1985年 | 9974篇 |
1984年 | 10267篇 |
1983年 | 8645篇 |
1982年 | 8704篇 |
1981年 | 8263篇 |
1980年 | 7823篇 |
1979年 | 8372篇 |
1978年 | 8593篇 |
1977年 | 8532篇 |
1976年 | 8479篇 |
1975年 | 8090篇 |
1974年 | 7894篇 |
1973年 | 8282篇 |
1972年 | 6062篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Multiuser detection for DS-CDMA UWB in the home environment 总被引:20,自引:0,他引:20
We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that the adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate the adaptive MMSE is able to reject a narrowband IEEE 802.11a OFDM interferer, even for signal-to-interference ratio as severe as -30 dB. We show the adaptive MMSE exhibits only a 6 dB penalty relative to the single user case for the heavy multi-access interference (number of asynchronous users equal to spreading code length). The practical RAKE receivers were incapable of effectively rejecting either the strong narrowband interference or the heavily loaded wideband interference. Even more moderate levels of interference caused significant degradation in the performance of the practical RAKE receivers. 相似文献
132.
Krahenbuhl R. Howerton M.M. Dubinger J. Greenblatt A.S. 《Lightwave Technology, Journal of》2002,20(1):92-99
High-performance Y-branch digital optical switches realized in Ti:LiNbO3 are presented. Their switching response functions have been optimized in terms of switch voltage and crosstalk ratio. The optimization is based on analyzing different types of waveguide shaping and switching arrangements using coupled mode theory and computer simulations. Excellent switching characteristics are achieved with devices exploiting a specially shaped waveguide branch in a dilated switch arrangement. Demonstrated performances include switching voltage as low as 9 V with crosstalk suppression better than 45 dB and fiber-to-fiber losses as low as 4 dB. Polarization independence with crosstalk suppression better than 40 dB over a 1520- to 1570-nm wavelength range is achieved for any applied switch voltage greater than 18 V. These optimized digital optical switches have further demonstrated the capability to reshape electrical input signals at switching rates of several hundred megahertz 相似文献
133.
Beyond stabilizer codes II: Clifford codes 总被引:1,自引:0,他引:1
Klappenecker A. Rotteler M. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(8):2396-2399
For pt. I see ibid., vol.48, no.8, p.2392-95 (2002). Knill (1996) introduced a generalization of stabilizer codes, called Clifford codes. It remained unclear whether or not Clifford codes can be superior to stabilizer codes. We show that Clifford codes are stabilizer codes provided that the abstract error group has an Abelian index group. In particular, if the errors are modeled by tensor products of Pauli matrices, then the associated Clifford codes are necessarily stabilizer codes. 相似文献
134.
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
135.
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
136.
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
137.
Jeroen J. L. M. Cornelissen W. Sander Graswinckel Alan E. Rowan Nico A. J. M. Sommerdijk Roeland J. M. Nolte 《Journal of polymer science. Part A, Polymer chemistry》2003,41(11):1725-1736
The conformational properties of polymers derived from isocyanodipeptides have been investigated with a combination of model calculations, X‐ray diffraction, and circular dichroism spectroscopy. Depending on the configuration of the side chains, defined arrays of hydrogen bonds along the polymeric backbone are formed. This leads to a well‐defined conformation as, for example, expressed in the formation of lyotropic liquid‐crystalline phases and increased helical stability. Upon the disruption of the hydrogen bonds by a strong acid, a less well‐defined macromolecular conformation is observed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1725–1736, 2003 相似文献
138.
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
139.
140.
Widely tunable bottom-emitting vertical-cavity SOAs 总被引:1,自引:0,他引:1
Cole G.D. Bjorlin E.S. Wang C.S. MacDonald N.C. Bowers J.E. 《Photonics Technology Letters, IEEE》2005,17(12):2526-2528
We present bottom-emitting tunable vertical-cavity semiconductor optical amplifiers (VCSOAs) with an effective wavelength tuning range of >20 nm. These devices utilize a high reflectivity micromechanically tunable Bragg mirror as the back reflector. Compared with our first generation tunable VCSOAs, the bottom-emitting devices exhibit a two-fold increase in the effective tuning range as well as a five-fold reduction in the required tuning voltage. 相似文献