全文获取类型
收费全文 | 602595篇 |
免费 | 5335篇 |
国内免费 | 1604篇 |
专业分类
化学 | 277892篇 |
晶体学 | 7712篇 |
力学 | 26386篇 |
综合类 | 17篇 |
数学 | 70433篇 |
物理学 | 165863篇 |
无线电 | 61231篇 |
出版年
2021年 | 5785篇 |
2020年 | 6423篇 |
2019年 | 7262篇 |
2018年 | 9361篇 |
2017年 | 9453篇 |
2016年 | 13063篇 |
2015年 | 6981篇 |
2014年 | 11814篇 |
2013年 | 25767篇 |
2012年 | 20313篇 |
2011年 | 24089篇 |
2010年 | 18253篇 |
2009年 | 18734篇 |
2008年 | 23827篇 |
2007年 | 24070篇 |
2006年 | 22094篇 |
2005年 | 19949篇 |
2004年 | 18642篇 |
2003年 | 16782篇 |
2002年 | 16654篇 |
2001年 | 17061篇 |
2000年 | 13725篇 |
1999年 | 10923篇 |
1998年 | 9699篇 |
1997年 | 9436篇 |
1996年 | 8695篇 |
1995年 | 7912篇 |
1994年 | 7996篇 |
1993年 | 7685篇 |
1992年 | 7907篇 |
1991年 | 8369篇 |
1990年 | 7981篇 |
1989年 | 7749篇 |
1988年 | 7464篇 |
1987年 | 6844篇 |
1986年 | 6529篇 |
1985年 | 8105篇 |
1984年 | 8400篇 |
1983年 | 7082篇 |
1982年 | 7302篇 |
1981年 | 6836篇 |
1980年 | 6477篇 |
1979年 | 6901篇 |
1978年 | 7170篇 |
1977年 | 7034篇 |
1976年 | 6924篇 |
1975年 | 6632篇 |
1974年 | 6458篇 |
1973年 | 6762篇 |
1972年 | 4919篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
41.
Antoniades N. Boskovic A. Tomkos I. Madamopoulos N. Lee M. Roudas I. Pastel D. Sharma M. Yadlowsky M.J. 《Selected Areas in Communications, IEEE Journal on》2002,20(1):149-165
This paper demonstrates the use of computer simulation for topological design and performance engineering of transparent wavelength-division multiplexing metropolitan-area networks. Engineering of these networks involves the study of various transport-layer impairments such as amplifier noise, component ripple, chirp/dispersion, optical crosstalk, waveform distortion due to filter concatenation, fiber nonlinearities, and polarization effects. A computer simulation methodology composed of three main simulation steps is derived and implemented. This methodology obtains performance estimations by applying efficient wavelength-domain simulations on the entire network topology, followed by time-/frequency-domain simulations on selected paths of the network and finally Q-budgeting on an identified worst case path. The above technique provides an efficient tool for topological design and network performance engineering. Accurate simulation models are presented for each of the performance impairments, and the computer simulation methodology is used for the design and engineering of a number of actual metro network architectures 相似文献
42.
Buttari D. Chini A. Meneghesso G. Zanoni E. Moran B. Heikman S. Zhang N.Q. Shen L. Coffie R. DenBaars S.P. Mishra U.K. 《Electron Device Letters, IEEE》2002,23(2):76-78
Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs 相似文献
43.
Nahler A. Irmer R. Fettweis G. 《Selected Areas in Communications, IEEE Journal on》2002,20(2):237-247
Since code division multiple access systems in multipath environments suffer from multiple access interference (MAI), multiuser detection schemes should be used in the receivers. Parallel interference cancellation (PIC) is a promising method to combat MAI due to its relatively low computational complexity and good performance. It is shown that the complexity of PIC is still high for realistic scenarios in terms of the symbol rate, the number of users, spreading gain, and multipath components. However, two novel methods are introduced to reduce significantly the complexity without sacrificing performance. The first approach, called reduced PIC, takes advantage of the composition of the interference to concentrate interference cancellation only on significant terms. The second approach, called differential PIC, exploits the multistage character of PIC to avoid unnecessary double calculations of certain terms in consecutive stages. It is shown that a combination of both approaches leads to a performance very close to the single-user bound whereas the complexity can be kept on the order of the conventional RAKE receiver 相似文献
44.
Losada M.A. Garces I. Mateo J. Salinas I. Lou J. Zubia J. 《Lightwave Technology, Journal of》2002,20(7):1160-1164
We have studied the optical power losses due to multiple curvatures in polymethylmetacrylate (PMMA) plastic optical fibers (POFs) of different numerical apertures (NAs) and attenuation. The fibers were tested for several configurations in order to assess the influence of different types of curved-to-straight fiber transitions in the amount of power radiation. We found that losses are below the standards for all tested fiber types, and thus, they are a suitable choice for local area network (LAN) applications. In addition, our results revealed the presence of modal interactions as confirmed using an experimental procedure to estimate the mode coupling strength for the same fibers. 相似文献
45.
A. Ivanov 《Journal of Electronic Testing》2002,18(4-5):361-362
46.
Integrated chaos generators 总被引:4,自引:0,他引:4
Delgado-Restituto M. Rodriguez-Vazquez A. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》2002,90(5):747-767
This paper surveys the different design issues, from mathematical model to silicon, involved in the design of analog CMOS integrated circuits for the generation of chaotic behavior 相似文献
47.
Ahn Jaeshin Stromsmoe Keith A. Lawson Ronald P. W. 《Industrial Electronics, IEEE Transactions on》1985,(4):405-409
A microprocessor-based system with 32 A/D, 24 D/A, and 16 ac load controllers, has been designed and built to monitor and control an ion beam thin-film deposition system. The A/D and D/A channels have electrical isolation of 7.5 kV between channels and between input and output. The microprocessor system keeps the ion beam deposition parameters stable for extended periods of operation and it is proposed as a means to greatly simplify switching from one deposition species to another to grow thin multilayer or alloy films. 相似文献
48.
A. A. Slavnov 《Theoretical and Mathematical Physics》1989,79(3):579-587
V. A. Steklov Mathematics Institute, USSR Academy of Sciences. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 79, No. 3, pp. 347–358, June, 1989. 相似文献
49.
The effect of secondary scattering that is produced by the induced current in an infinitely long, finitely conducting wire over a plane, finitely conducting earth from an incident electromagnetic pulse is investigated. This is accomplished by finding the field of a Hertzian dipole over the earth and identifying the part that is due to secondary scattering. Frequency-domain results for the overall induced current require that certain infinite integrals be numerically evaluated. Fourier inversion to the time domain in turn requires the numerical evaluation of another infinite integral. When the results are compared to those obtained when secondary scattering is ignored, it is found that secondary scattering slightly increases the current for the case of broadside incidence and reduces it for the case of grazing incidence by as much as two orders of magnitude. Peak currents are much smaller than previously thought. These general results hold for all of the earth parameters that were used in the study 相似文献
50.
Alsing P. Peterson P. Cardimona D. Gavrielides A. 《Quantum Electronics, IEEE Journal of》1987,23(5):557-568
We investigate the saturation effects of power broadening, Stark shifting, and population transfer on Stokes conversion in stimulated Raman scattering. We do not make the usual rotating wave approximation because the detuning from the next electronic state is assumed to be in the optical regime. Retaining the counter-rotating terms allows an exact determination of the pump and Stokes indexes of refraction. Steady-state solutions for the Stokes intensity and phase are obtained and the effects of making the rotating wave approximation (RWA) are discussed. Finally, we examine the behavior of these solutions for Stokes conversion in hydrogen gas when geometric propagation is appropriate. 相似文献