全文获取类型
收费全文 | 238489篇 |
免费 | 1958篇 |
国内免费 | 680篇 |
专业分类
化学 | 102305篇 |
晶体学 | 3340篇 |
力学 | 9287篇 |
综合类 | 5篇 |
数学 | 22470篇 |
物理学 | 69000篇 |
无线电 | 34720篇 |
出版年
2021年 | 1847篇 |
2020年 | 2014篇 |
2019年 | 2270篇 |
2018年 | 2998篇 |
2017年 | 2869篇 |
2016年 | 4163篇 |
2015年 | 2427篇 |
2014年 | 3986篇 |
2013年 | 9718篇 |
2012年 | 7671篇 |
2011年 | 9193篇 |
2010年 | 6613篇 |
2009年 | 6837篇 |
2008年 | 9132篇 |
2007年 | 9597篇 |
2006年 | 9285篇 |
2005年 | 8425篇 |
2004年 | 7830篇 |
2003年 | 7044篇 |
2002年 | 6724篇 |
2001年 | 7705篇 |
2000年 | 6209篇 |
1999年 | 4985篇 |
1998年 | 4168篇 |
1997年 | 4169篇 |
1996年 | 3839篇 |
1995年 | 3646篇 |
1994年 | 3492篇 |
1993年 | 3326篇 |
1992年 | 3687篇 |
1991年 | 3598篇 |
1990年 | 3484篇 |
1989年 | 3209篇 |
1988年 | 3127篇 |
1987年 | 2851篇 |
1986年 | 2636篇 |
1985年 | 3337篇 |
1984年 | 3563篇 |
1983年 | 2938篇 |
1982年 | 3091篇 |
1981年 | 3052篇 |
1980年 | 2909篇 |
1979年 | 2964篇 |
1978年 | 3048篇 |
1977年 | 3051篇 |
1976年 | 3073篇 |
1975年 | 2879篇 |
1974年 | 2854篇 |
1973年 | 3010篇 |
1972年 | 1937篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Lourtioz J.-M. Chusseau L. Brun E. Hamaide J.-P. Lesterlin D. Leblond F. 《Electronics letters》1992,28(16):1499-1500
Fourier-transform-limited pulses ( Delta tau . Delta nu approximately=0.35) have been obtained from a microwave-modulated DBR laser at 1.56 mu m. The chirp of the gain-switched pulse is compensated for by applying a fraction of the microwave signal to the phase section, thereby creating an instantaneous frequency shift of opposite sign. The generation of short coherent pulses from such a monolithic source can be of great interest for long haul soliton transmission.<> 相似文献
152.
A new 3-D graded TLM model for thin radiating wires is presented. The model differs from previous TLM models because the wire is modelled at the interfaces between cells, rather than through the centre of the node. The connection between the TLM symmetrical condensed-node link lines and the line forms a 'wire interface'. The scattering algorithm for the interface and simulation results are given.<> 相似文献
153.
D. J. Hartfiel 《Linear and Multilinear Algebra》1994,36(4):293-304
In this paper we consider the nonnegative matrix system ciA+uiB=ci+1 where the nonnegative matrix A is allowed to vary, within bounds. The cone control problem is to find a nonnegative matrix B such that if Ci is a nonnegative vector in a specified cone, then there is a nonnegative vector ui such that ci+1 is in that cone. We extend this problem to input control by finding a B such that the cone, generated by the rows of B, is as small as possible. Thus, the percent distribution of ∣uiB∣ through the states of the sustem by uiB is either constant or varies little. 相似文献
154.
155.
It is shown that the unique coherence properties of mode-locked lasers can be exploited to provide new opportunities for high-speed data transmission. Using passive Mach-Zehnder optical delay stages, very high bit rate optical data streams constructed from asynchronously combined transmitters can be demultiplexed directly in the optical domain, without imposing restrictions on the source wavelength. Potential implementation and performance of the technique are discussed.<> 相似文献
156.
V. D. Levchenko A. I. Morozov A. S. Sigov 《Journal of Experimental and Theoretical Physics》2002,94(5):985-991
Multilayer ferromagnet-layered antiferromagnet (Fe/Cr) structures frustrated because of roughness of interlayer boundaries were studied by mathematical modeling methods. The phase diagram of a three-layer system (plotted as film thickness versus the degree of roughness of the interfaces) was obtained, and the order parameter distributions in each phase were determined. The character of phase transitions in this system was studied. The applicability range of the Slonczewski magnetic proximity model was determined. 相似文献
157.
158.
The feasibility of using a thick ground plane with an aperture coupled microstrip patch antenna while maintaining reasonable antenna performance is demonstrated using moment method and reciprocity analyses in the spectral domain. The thick ground plane, which may serve as a heatsink for active MMIC circuitry, or as a mechanical support for thin substrates, is particularly advantageous in millimetre wave phased array applications. It is found that the effect of ground plane thickness is to reduce the level of coupling from the feed line to the patch 相似文献
159.
Results from a unique analytical and experimental effort to assess low-altitude, short-range, radar detection capabilities in an evaporation ducting environment are presented. Within the horizon, the duct may shift the location of the last interference null several kilometers in range, which may cause nondetection at ranges where detection is expected and detection at ranges where detection is not expected. In addition, the evaporation duct may reduce the signal strength at ranges near the last interference peak so that detection of low-altitude, small-size, targets may not be possible until the target is much closer. At ranges near and beyond the horizon, radar signal strength depends both on the surface layer and on the mixed layer. To accurately model propagation in this region, knowledge of both surface layer and upper air meteorology Is required. An empirical model to merge the surface layer with the mixed layer has been developed. Comparisons of measured to calculated detection range indicate that the assumption of a standard atmosphere minimizes the variance of the difference between the detection ranges whereas ranges calculated using the measured surface meteorological conditions and a modified surface layer model minimizes the median of the difference 相似文献
160.
Richardson R.D. McNerney G.M. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1993,81(3):378-389
Wind energy has matured to a level of development where it is ready to become a generally accepted utility generation technology. A brief discussion of this development is presented, and the operating and design principles are discussed. Alternative designs for wind turbines and the tradeoffs that must be considered are briefly compared. Development of a wind energy system and the impacts on the utility network including frequency stability, voltage stability, and power quality are discussed. The assessment of wind power station economics and the key economic factors that determine the economic viability of a wind power plant are presented 相似文献