全文获取类型
收费全文 | 287223篇 |
免费 | 3528篇 |
国内免费 | 814篇 |
专业分类
电工技术 | 5033篇 |
综合类 | 162篇 |
化学工业 | 45420篇 |
金属工艺 | 11895篇 |
机械仪表 | 8605篇 |
建筑科学 | 6849篇 |
矿业工程 | 1834篇 |
能源动力 | 6737篇 |
轻工业 | 25907篇 |
水利工程 | 3190篇 |
石油天然气 | 7086篇 |
武器工业 | 16篇 |
无线电 | 31464篇 |
一般工业技术 | 56943篇 |
冶金工业 | 51378篇 |
原子能技术 | 7534篇 |
自动化技术 | 21512篇 |
出版年
2021年 | 2102篇 |
2018年 | 3676篇 |
2017年 | 3622篇 |
2016年 | 3908篇 |
2015年 | 2468篇 |
2014年 | 4237篇 |
2013年 | 12022篇 |
2012年 | 6833篇 |
2011年 | 9221篇 |
2010年 | 7567篇 |
2009年 | 8649篇 |
2008年 | 8936篇 |
2007年 | 8815篇 |
2006年 | 7830篇 |
2005年 | 7312篇 |
2004年 | 6803篇 |
2003年 | 6558篇 |
2002年 | 6628篇 |
2001年 | 6499篇 |
2000年 | 6176篇 |
1999年 | 6230篇 |
1998年 | 14756篇 |
1997年 | 11031篇 |
1996年 | 8569篇 |
1995年 | 6523篇 |
1994年 | 5915篇 |
1993年 | 5787篇 |
1992年 | 4480篇 |
1991年 | 4443篇 |
1990年 | 4287篇 |
1989年 | 4302篇 |
1988年 | 4255篇 |
1987年 | 3598篇 |
1986年 | 3595篇 |
1985年 | 4161篇 |
1984年 | 3965篇 |
1983年 | 3621篇 |
1982年 | 3422篇 |
1981年 | 3558篇 |
1980年 | 3422篇 |
1979年 | 3365篇 |
1978年 | 3445篇 |
1977年 | 3923篇 |
1976年 | 5038篇 |
1975年 | 3158篇 |
1974年 | 3008篇 |
1973年 | 3024篇 |
1972年 | 2651篇 |
1971年 | 2473篇 |
1970年 | 2104篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
291.
Exact pairwise error probability of space-time codes 总被引:9,自引:0,他引:9
Taricco G. Biglieri E. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2002,48(2):510-513
We describe a simple technique for the numerical calculation, within any desired degree of accuracy, of the pairwise error probability (PEP) of space-time codes over fading channels. This method applies also to the calculation of E[Q(√ξ)] for any nonnegative random variable ξ whose moment-generating function Φξ(s)=E[exp(-sξ)] is known. Its application to the multiple antenna independent Rayleigh-fading channel and to the Rayleigh block fading channel is discussed, and illustrated by two simple examples 相似文献
292.
K. P. Lee S. J. Pearton M. E. Overberg C. R. Abernathy R. G. Wilson S. N. G. Chu N. Theodoropolou A. F. Hebard J. M. Zavada 《Journal of Electronic Materials》2002,31(5):411-415
In p-GaN implanted with Mn (3×1016 cm−2 at 250 keV), the material after annealing shows ferromagnetic properties below 250 K. Cross-sectional transmission electron
microscopy (TEM) revealed the presence of platelet structures with hexagonal symmetry. These regions are most likely GaxMn1−xN, which produce the ferromagnetic contribution to the magnetization. In p-GaN implanted with Fe, the material after annealing
showed ferromagnetic properties at temperatures that were dependent on the Fe dose, but were below 200 K in all cases. In
these samples, TEM and diffraction analysis did not reveal any secondary phase formation. The results for the Fe implantation
are similar to those reported for Fe doping during epitaxial growth of GaN. 相似文献
293.
294.
295.
A method is given for calculating the monitoring reliability parameters (MRP) for an engineering system with allowance for the drift in the measurement error of the means of measurement (MM). The approach is based on introducing a biased MM error distribution into the traditional formulas for the conditional probabilities of spurious and unobserved failure. The bias at the center of the distribution is determined by the systematic error at the given instant. An example is given to illustrate the performance in MRP calculation.Translated from Izmeritel'naya Tekhnika, No. 1, pp. 12–13, January, 1994. 相似文献
296.
An adaptive antenna array is incorporated into a decorrelatingmulti-user detector to effectively increase the DS-CDMA system capacity.Capacity improvement is due to beamforming gain, spatial diversity gain(assuming large angle spread), and the decorrelating effect. Thereceiver has been analyzed for the cases of sufficiently andinsufficiently spaced receiving antennas. The receiver consists of afront-end beamformer for every user in the cell and has knowledge of allusers' signature sequences. The beamformer estimates the desired userchannel vector, enhancing its signal and reducing the co-channelinterference from other directions. The multi-user detection, exploitingknowledge of other users, rejects those interferers whose arrival anglesare close to that of the desired user. The average uncoded Bit ErrorRate (BER) as a function of the number of in-beam active users, theaverage antenna Signal-to-Noise Ratio (SNR), and the number of receivingantennas is examined in both single-path and multi-path Rayleigh fadingchannels. Analysis shows an increase in system capacity proportional tothe number of receiving antennas. 相似文献
297.
298.
Montanari D. Van Houdt J. Groeseneken G. Maes H.E. 《Solid-State Circuits, IEEE Journal of》1998,33(7):1090-1095
This paper presents a high-speed, small-area circuit specifically designed to identify the levels in the read out operation of a flash multilevel memory. The circuit is based on the analog computation of the Euclidean distance between the current read out from a memory cell and the reference currents that represent the different logic levels. An experimental version of the circuit has been integrated in a standard double-metal 0.7-μm CMOS process with a die area of only 140×100 μm2. Operating under a 5-V power supply, this circuit identifies the read-out current of a memory cell, and associates it with the appropriate logic level in 9 ns 相似文献
299.
A modification to the `shape-invariant' sinusoidal speech model is proposed, whereby the phases of the component sinewaves used for the excitation are made to add coherently at each glottal closure. Applied to pitch and time-scale modification, higher quality synthetic speech is produced when large changes are required 相似文献
300.
The authors report the design of a new current-mode A/D converter, based on a modified successive-approximations model, in 1.2 μm CMOS technology. The proposed circuit is characterised by good accuracy and fast dynamic performance, low power consumption and small occupation area. SPICE simulations allow the design approach to be validated and the electrical performance of the ADC to be predicted 相似文献