全文获取类型
收费全文 | 243791篇 |
免费 | 2957篇 |
国内免费 | 1243篇 |
专业分类
电工技术 | 4845篇 |
综合类 | 264篇 |
化学工业 | 35482篇 |
金属工艺 | 10866篇 |
机械仪表 | 7073篇 |
建筑科学 | 6247篇 |
矿业工程 | 481篇 |
能源动力 | 6302篇 |
轻工业 | 24159篇 |
水利工程 | 1838篇 |
石油天然气 | 1335篇 |
武器工业 | 3篇 |
无线电 | 31078篇 |
一般工业技术 | 44929篇 |
冶金工业 | 50159篇 |
原子能技术 | 3863篇 |
自动化技术 | 19067篇 |
出版年
2021年 | 1576篇 |
2019年 | 1433篇 |
2018年 | 2338篇 |
2017年 | 2228篇 |
2016年 | 2449篇 |
2015年 | 1911篇 |
2014年 | 3166篇 |
2013年 | 10820篇 |
2012年 | 5457篇 |
2011年 | 7794篇 |
2010年 | 6293篇 |
2009年 | 6970篇 |
2008年 | 7561篇 |
2007年 | 7668篇 |
2006年 | 7034篇 |
2005年 | 6524篇 |
2004年 | 6250篇 |
2003年 | 6273篇 |
2002年 | 6247篇 |
2001年 | 6376篇 |
2000年 | 5808篇 |
1999年 | 6421篇 |
1998年 | 16572篇 |
1997年 | 11605篇 |
1996年 | 8957篇 |
1995年 | 6698篇 |
1994年 | 5913篇 |
1993年 | 5738篇 |
1992年 | 4057篇 |
1991年 | 3998篇 |
1990年 | 3583篇 |
1989年 | 3506篇 |
1988年 | 3443篇 |
1987年 | 2915篇 |
1986年 | 2846篇 |
1985年 | 3414篇 |
1984年 | 3056篇 |
1983年 | 2746篇 |
1982年 | 2549篇 |
1981年 | 2571篇 |
1980年 | 2433篇 |
1979年 | 2275篇 |
1978年 | 2238篇 |
1977年 | 2812篇 |
1976年 | 4173篇 |
1975年 | 1899篇 |
1974年 | 1797篇 |
1973年 | 1783篇 |
1972年 | 1460篇 |
1971年 | 1317篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
H Neudeck M Joncic C Schuster S Bisson R Hildebrandt T Oney B Stiemer H Hopp R Graf 《Canadian Metallurgical Quarterly》1997,37(6):449-458
Nitric oxide (NO) acts as a modulator of neuronal transmission in mature neuronal systems, including the retina. Recently, NO has also been suggested to have a trophic function during development. We examined immunocytochemically the distribution of NO-producing cells in developing and transplanted rabbit retinas. An antibody detecting the neuronal isoform of its biosynthetic enzyme, nitric oxide synthase (NOS), was used on normal developing retinas [starting at embryonic day (E) 15] and on rabbit retinal transplants after various survival times (1-139 days after surgery). Weakly stained cell bodies were first observed in the proximal margin of the neuroblastic layer at E 29. Stained processes projecting towards a developing inner plexiform layer were also visible at this time point. Immunoreactive cells were located at later stages in the innermost part of the inner nuclear layer and in the ganglion cell layer, and are likely to correspond mainly to amacrine cells. NOS-labelled cells were also found in retinal transplants. The first NOS-labelled cells appeared, as in normal developing retinas, in ages corresponding to E 29 and were still detected in transplants corresponding to postnatal day 123. NOS-labelled cells were seen in areas between rosettes, where amacrine cells are located. NOS-labelled processes were at times seen to project for long distances, forming very distinct plexuses. NOS-containing amacrine cells thus appear both in the transplants and in developing retinas in the embryonic stages, long before synaptic function involving these cells can be expected, suggesting a role for NO not only in neuromodulation but also in retinal development. 相似文献
72.
Large-scale simulation studies in image pattern recognition 总被引:1,自引:0,他引:1
Tin Kam Ho Baird H.S. 《IEEE transactions on pattern analysis and machine intelligence》1997,19(10):1067-1079
Many obstacles to progress in image pattern recognition result from the fact that per-class distributions are often too irregular to be well-approximated by simple analytical functions. Simulation studies offer one way to circumvent these obstacles. We present three closely related studies of machine-printed character recognition that rely on synthetic data generated pseudo-randomly in accordance with an explicit stochastic model of document image degradations. The unusually large scale of experiments - involving several million samples that makes this methodology possible have allowed us to compute sharp estimates of the intrinsic difficulty (Bayes risk) of concrete image recognition problems, as well as the asymptotic accuracy and domain of competency of classifiers 相似文献
73.
We predict that, for wavelength division multiplexing optical-network applications, an asymmetrically dilated configuration of a 2×2 cross-connect is significantly better in terms of overall crosstalk when the levels of the bar-port crosstalk and the cross-port crosstalk are significantly different from each other, as is the case with optical-frequency filters which utilize grating-assisted coupling. As a verification, we present a simulation study with 2×2 polarization-diversified acousto-optic tunable filters. We present a recursive method to extend the principle of asymmetric dilation to larger-size cross-connect switches, and make a recommendation for an asymmetrically dilated 4×4 cross-connect configuration 相似文献
74.
The application of a general self-calibration procedure, the so-called TNA (through, network, attenuator) method, to a dual six-port network analyzer is presented for the first time. This innovation results from strictly separating the establishment of the complex measurement ability and the determination of the analyzers system error parameters. As will be seen, the well-known TRL (through, reflect, line) method is a special case of the general TNA technique 相似文献
75.
Fahmy H.I. Develekos G. Douligeris C. 《Selected Areas in Communications, IEEE Journal on》1997,15(2):226-237
Communication network design is becoming increasingly complex, involving making networks more usable, affordable, and reliable. To help with this, we have proposed an expert network designer (END) for configuring, modeling, simulating, and evaluating large structured computer networks, employing artificial intelligence, knowledge representation, and network simulation tools. We present a neural network/knowledge acquisition machine-learning approach to improve the END's efficiency in solving the network design problem and to extend its scope to acquire new networking technologies, learn new network design techniques, and update the specifications of existing technologies 相似文献
76.
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
77.
Two novel configurations for digitally tunable optical filters based on arrayed-waveguide grating (AWG) multiplexers are described in detail with emphasis on the connection of the AWG multiplexer and optical switches. Performance comparisons show that conventional configurations are disadvantaged by the switch size required and loss imbalance among the optical frequency-division-multiplexed (FDM) channels; the proposed configurations require only O(√(N)) switch elements to select one of N FDM channels, and the loss imbalance is lower by up to 75% in decibel 相似文献
78.
Sugawara F. Aoki K. Yamaguchi H. Sasaki K. Sasaki T. Fujisaki H. 《Electron Device Letters, IEEE》1997,18(10):483-485
A new lateral MOS-gated thyristor, called the Base-Current-Controlled Thyristor, is described. This device is designed so that most holes at the on-stage reach the P base through the floating P+ region adjacent to the P base and the on-state MOSFET. At the turn-off stage, the interruption of the hole current to the P base due to switching off the above MOSFET occurs simultaneously with the conventional turn-off operation. The concept of this device is verified experimentally by using the fabricated lateral device with the external MOSFET. This device exhibits a better trade-off relation between the on-state voltage and the turn-off time compared uith the conventional MOS-gated thyristor 相似文献
79.
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
80.
Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0×1015cm-2 at 72 keV (1 at% peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3×1015cm-2 at 87 keV (1 at% peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen
at 1100 and 1300°C for 2–6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation
led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when
aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can
be attributed to their different roles in modifying the structure and properties of the oxide formed.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献