全文获取类型
收费全文 | 763479篇 |
免费 | 10942篇 |
国内免费 | 2233篇 |
专业分类
电工技术 | 14533篇 |
综合类 | 992篇 |
化学工业 | 112272篇 |
金属工艺 | 32085篇 |
机械仪表 | 25200篇 |
建筑科学 | 18368篇 |
矿业工程 | 3760篇 |
能源动力 | 19575篇 |
轻工业 | 62702篇 |
水利工程 | 8017篇 |
石油天然气 | 13193篇 |
武器工业 | 111篇 |
无线电 | 88681篇 |
一般工业技术 | 149734篇 |
冶金工业 | 142292篇 |
原子能技术 | 14749篇 |
自动化技术 | 70390篇 |
出版年
2021年 | 6545篇 |
2020年 | 4825篇 |
2019年 | 6246篇 |
2018年 | 15573篇 |
2017年 | 15811篇 |
2016年 | 14048篇 |
2015年 | 7886篇 |
2014年 | 12557篇 |
2013年 | 33174篇 |
2012年 | 21135篇 |
2011年 | 31735篇 |
2010年 | 26094篇 |
2009年 | 27984篇 |
2008年 | 28575篇 |
2007年 | 29549篇 |
2006年 | 21199篇 |
2005年 | 21420篇 |
2004年 | 19648篇 |
2003年 | 19096篇 |
2002年 | 17604篇 |
2001年 | 17175篇 |
2000年 | 16424篇 |
1999年 | 17059篇 |
1998年 | 43009篇 |
1997年 | 30312篇 |
1996年 | 23246篇 |
1995年 | 17560篇 |
1994年 | 15381篇 |
1993年 | 15080篇 |
1992年 | 11153篇 |
1991年 | 10411篇 |
1990年 | 10147篇 |
1989年 | 9728篇 |
1988年 | 9127篇 |
1987年 | 8117篇 |
1986年 | 7990篇 |
1985年 | 8995篇 |
1984年 | 8446篇 |
1983年 | 7514篇 |
1982年 | 7103篇 |
1981年 | 7238篇 |
1980年 | 6837篇 |
1979年 | 6669篇 |
1978年 | 6340篇 |
1977年 | 7623篇 |
1976年 | 9787篇 |
1975年 | 5614篇 |
1974年 | 5354篇 |
1973年 | 5489篇 |
1972年 | 4433篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
T. Herrmannsdörfer J. Friebel F. Pobell J. Mrachkov M. Kirov E. Leyarovski 《Journal of Low Temperature Physics》1997,107(1-2):209-224
We have measured the 16 Hz susceptibility of the diluted Van Vleck paramagnets Pr1−x
Y
x
Ni5 withx=0.00, 0.05, 0.10. 0.20 at temperatures 1.5 K≤T≤50 K and magnetic fields up to 15T. Their Van Vleck type magnetic susceptibility χ(T, B, x) strongly depends on the field atB≥6 T. The temperatureT
max(B, x) of the maximum of χ(T, B, x) decreases at increasing the field from zero to 15 T by approximately one order of magnitude for all Pr1−x
Y
x
Ni5 compounds. Changing the dilutionx from zero to 0.20, the field whereT
max(B, x) strongly drops is increased from 9 T to 11 T. Our data agree qualitatively with the predictions of a point charge model
which considers the Zeeman term in addition to the electronic exchange and the dominating crystal field contributions. 相似文献
992.
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
993.
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.
Carin L. Felsen L.B. Kralj D.R. Oh H.S. Lee W.C. Unnikrishna Pillai S. 《Antennas and Propagation, IEEE Transactions on》1997,45(4):592-600
Phase-space data processing is receiving increased attention because or its potential for furnishing new discriminants relating to classification and identification of targets and other scattering environments. Primary emphasis has been on time-frequency processing because of its impact on transient, especially wideband, short-pulse excitations. Here, we investigate the windowed Fourier transform, the wavelet transform, and model based superresolution algorithms within the context of a fully quantified and calibrated test problem investigated by us previously: two-dimensional (2-D) short-pulse plane wave scattering by a finite periodic array of perfectly conducting coplanar flat strips. Because the forward problem has been fully calibrated and parametrized, some quantitative measures can be assigned with respect to the tradeoffs of these time-frequency algorithms, yielding tentative performance assessments of the tested processing algorithms 相似文献
998.
Hisamoto D. Umeda K. Nakamura Y. Kimura S. 《Electron Devices, IEEE Transactions on》1997,44(6):951-956
This paper describes the high performance of T-shaped-gate CMOS devices with effective channel lengths in the sub-0.1-μm region. These devices were fabricated by using selective W growth, which allows low-resistance gates smaller than 0.1 μm to be made without requiring fine lithography alignment. We used counter-doping to scale down the threshold voltage while still maintaining acceptable short-channel effects. This approach allowed us to make ring oscillators with a gate-delay time as short as 21 ps at 2 V with a gate length of 0.15 μm. Furthermore, we experimentally show that the high circuit speed of a sub-0.1-μm gate length CMOS device is mainly due to the PMOS device performance, especially in terms of its drivability 相似文献
999.
This paper presents a method for fast estimation of probabilities of rare events in stochastic networks, with a particular emphasis on coherent reliability systems. The method is based on the concepts of likelihood-ratios (LR), change of probability measure and the bottleneck-cut in the network. Both polynomial and exponential-time Monte Carlo estimators are defined, and conditions under which the time complexity of the proposed LR estimators is bounded by a polynomial are discussed. The accuracy of the method depends only on the size (cardinality) of the bottleneck-cut, not on the topology and actual size of the network. Supporting numerical results are presented, with the cardinality of the bottleneck-cut ⩽20 相似文献
1000.
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