全文获取类型
收费全文 | 731256篇 |
免费 | 9505篇 |
国内免费 | 1676篇 |
专业分类
电工技术 | 12969篇 |
综合类 | 668篇 |
化学工业 | 109925篇 |
金属工艺 | 34249篇 |
机械仪表 | 24255篇 |
建筑科学 | 16494篇 |
矿业工程 | 5335篇 |
能源动力 | 17560篇 |
轻工业 | 53948篇 |
水利工程 | 8800篇 |
石油天然气 | 18386篇 |
武器工业 | 50篇 |
无线电 | 76377篇 |
一般工业技术 | 154169篇 |
冶金工业 | 128144篇 |
原子能技术 | 18045篇 |
自动化技术 | 63063篇 |
出版年
2021年 | 6675篇 |
2020年 | 4902篇 |
2019年 | 6370篇 |
2018年 | 15209篇 |
2017年 | 15376篇 |
2016年 | 14367篇 |
2015年 | 7847篇 |
2014年 | 12812篇 |
2013年 | 31111篇 |
2012年 | 20780篇 |
2011年 | 30056篇 |
2010年 | 24610篇 |
2009年 | 26246篇 |
2008年 | 26904篇 |
2007年 | 27461篇 |
2006年 | 19743篇 |
2005年 | 19450篇 |
2004年 | 18381篇 |
2003年 | 18085篇 |
2002年 | 16612篇 |
2001年 | 16272篇 |
2000年 | 15348篇 |
1999年 | 15491篇 |
1998年 | 37710篇 |
1997年 | 26934篇 |
1996年 | 20547篇 |
1995年 | 15651篇 |
1994年 | 13766篇 |
1993年 | 13632篇 |
1992年 | 10174篇 |
1991年 | 9785篇 |
1990年 | 9741篇 |
1989年 | 9389篇 |
1988年 | 8765篇 |
1987年 | 8049篇 |
1986年 | 7854篇 |
1985年 | 8621篇 |
1984年 | 8158篇 |
1983年 | 7416篇 |
1982年 | 6947篇 |
1981年 | 7118篇 |
1980年 | 6863篇 |
1979年 | 6902篇 |
1978年 | 6878篇 |
1977年 | 7781篇 |
1976年 | 9760篇 |
1975年 | 6064篇 |
1974年 | 5908篇 |
1973年 | 5984篇 |
1972年 | 5116篇 |
排序方式: 共有10000条查询结果,搜索用时 990 毫秒
991.
Douay M. Xie W.X. Taunay T. Bernage P. Niay P. Cordier P. Poumellec B. Dong L. Bayon J.F. Poignant H. Delevaque E. 《Lightwave Technology, Journal of》1997,15(8):1329-1342
A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models 相似文献
992.
993.
Single-phase voltage source power converters (VSCs) under consideration are AC-DC current-controlled boost-type power converters with bidirectional power-handling capability. Equivalence between two series-connected two-level power converters and a single three-level power converter is considered here. Further considered is the series operation of three-level power converters. Simulation results and experimental verification for both are provided. Economical configurations of three-level power converters leading to multilevel waveforms are presented thereafter 相似文献
994.
Watanabe H. Komori J. Higashitani K. Sekine M. Koyama H. 《Semiconductor Manufacturing, IEEE Transactions on》1997,10(2):228-232
A novel monitoring method for plasma-charging damage is proposed. This method performs a quick and accurate evaluation using antenna PMOSFET. It was found that not only hot-carrier (HC) lifetime but transistor parameters such as initial gate current and substrate current were changed according to the degree of plasma-charging damage. However, the present work suggests that monitoring the shift of drain current after a few seconds of HC stress is a more accurate method to indicate plasma-charging damage. The monitoring method using the present test structure is demonstrated to be useful for realizing highly reliable devices 相似文献
995.
The European Telecommunications Standards Institute (ETSI) has recently defined a European standard for a high performance radio LAN (known as HIPERLAN). To operate as wired LAN replacements, these systems will operate at 5.2 GHz and support instantaneous bit rates of just under 24 Mb/s. To counteract the time dispersive nature of the indoor radio channel, the use of adaptive equalisation is suggested. In this paper a number of possible modulation and equalisation techniques are presented and, in particular, the Bit Error Rate (BER) performance of quasi-coherent GMSK combined with Decision Feedback Equalisation is explored through computer simulation. The trade off between symbol spaced and fractionally spaced equalisation is considered together with the importance of feedfoward and feedback synchronisation to the channel's power delay profile. The paper also includes a comparison of the RLS and LMS based training algorithms and compares the modem developed under the ESPRIT III LAURA project with that specified in HIPERLAN.The application of dual antenna diversity is investigated and its impact on the number of received error free data packets obtained as a function of signal leval and rms delay spread. The use of such diversity is shown to greatly improve the BER performance of a HIPERLAN modem. The problem of frequency offset is considered and modifications are proposed to the HIPERLAN frame structure to improve the receiver's tolerance to such errors. Important practical issues such as frame and symbol synchronisation, frequency offset correction and hardware implementation are discussed from both the LAURA and HIPERLAN viewpoint. 相似文献
996.
A simple phenomenological model for giant magnetoresistance (GMR) is employed for current and field parallel (CIP) to the magnetic multilayer planes in ordinary and discontinuous multilayer films. To our knowledge, it is the first model to include hysteresis in the field (H) dependence of the GMR. The computed GMR versus H curves qualitatively reproduce the GMR hysteresis seen experimentally. In particular, two GMR peaks are found to be symmetrically placed about H=0, and the GMR hysteresis curve itself is found to have an inverted butterfly shape. Also seen in the computed results is the general increase in GMR magnitude found for annealed discontinuous multilayer films. Various parameter variations are examined in the computed results. While the model reproduces GMR hysteresis quite well and the general increase in GMR for discontinuous multilayer films, it does not, in its present form, account for the oscillations seen in the GMR when the nonmagnetic layer thicknesses are varied, which is expected as a strictly quantum mechanical result 相似文献
997.
C.H. Chu C.I. Hung Y.H. Wang M.P. Houng 《Photonics Technology Letters, IEEE》1997,9(9):1262-1264
We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques. 相似文献
998.
Yu. B. Vasil’ev S. D. Suchalkin S. V. Ivanov B. Ya. Mel’tser A. F. Tsatsul’nikov P. V. Neklyudov P. S. Kop’ev 《Semiconductors》1997,31(10):1071-1073
Cyclotron resonance is measured in solitary type-II InAs-AlGaSb quantum wells grown by molecular-beam epitaxy under various
growth conditions. Quantum oscillations observed in the cyclotron resonance spectra in InAs-GaSb samples are attributed to
scattering by a short-range potential due to roughness of the heterointerface. A new method based on measurement of the cyclotron
resonance spectra is proposed for assessing the quality of the heterointerface.
Fiz. Tekh. Poluprovodn. 31, 1246–1248 (October 1997) 相似文献
999.
Sadka A.H. Eryurthlu F. Kondoz A.M. 《Vision, Image and Signal Processing, IEE Proceedings -》1997,144(6):369-376
An algorithm is presented to enhance the resilience of block-based video-coding algorithms against channel errors. The error-resilience algorithm described suggests the use of fixed length coding to alleviate the effect of Huffman coding on the synchronisation of the decode under erroneous conditions. The proposed mechanism modifies the order of transmission of the fixed-length video parameters to increase the chance of their arrival. Synchronisation words are sent at fixed-length intervals within the bit stream to reduce the possibility of the decoder falling on similar bit patterns within the bit stream. To limit the effect of error accumulation, the motion prediction process is halted and the differential coding of motion vectors is not applied. FEC techniques are applied on some error-sensitive segments of the reordered video bitstream. The effectiveness of the proposed error-resilience algorithm is evidenced by both subjective and objective results 相似文献
1000.
J.P.R. Lacey S.J. Madden M.A. Summerfield 《Photonics Technology Letters, IEEE》1997,9(10):1355-1357
Multichannel wavelength converters may be important components in the cross-connects in future wavelength-division multiplexed (WDM) transport networks. We demonstrate a multichannel, polarization-insensitive, optically transparent wavelength converter, based on four-wave mixing in two semiconductor optical amplifiers in a polarization-diversity arrangement. Bit-error-rate (BER) measurements with four input 2.5-Gb/s WDM channels, spaced by 2 nm, show penalties for wavelength conversion less than 2.6 dB at 10/sup -9/ BER. Changes in the state of polarization of the input signals cause the output power to change by less than 1.2 dB, and the corresponding power penalties change by less than 0.9 dB. 相似文献