全文获取类型
收费全文 | 146495篇 |
免费 | 2494篇 |
国内免费 | 1434篇 |
专业分类
电工技术 | 2482篇 |
综合类 | 833篇 |
化学工业 | 15315篇 |
金属工艺 | 6101篇 |
机械仪表 | 4132篇 |
建筑科学 | 2987篇 |
矿业工程 | 588篇 |
能源动力 | 3507篇 |
轻工业 | 8341篇 |
水利工程 | 1303篇 |
石油天然气 | 746篇 |
武器工业 | 21篇 |
无线电 | 17852篇 |
一般工业技术 | 25336篇 |
冶金工业 | 46182篇 |
原子能技术 | 1580篇 |
自动化技术 | 13117篇 |
出版年
2023年 | 417篇 |
2022年 | 547篇 |
2021年 | 824篇 |
2020年 | 608篇 |
2019年 | 797篇 |
2018年 | 1663篇 |
2017年 | 1824篇 |
2016年 | 2292篇 |
2015年 | 1732篇 |
2014年 | 1975篇 |
2013年 | 5255篇 |
2012年 | 4757篇 |
2011年 | 6290篇 |
2010年 | 3824篇 |
2009年 | 4563篇 |
2008年 | 4575篇 |
2007年 | 4775篇 |
2006年 | 4319篇 |
2005年 | 6661篇 |
2004年 | 5767篇 |
2003年 | 5137篇 |
2002年 | 3648篇 |
2001年 | 3814篇 |
2000年 | 3044篇 |
1999年 | 3715篇 |
1998年 | 15427篇 |
1997年 | 9863篇 |
1996年 | 7209篇 |
1995年 | 4587篇 |
1994年 | 3843篇 |
1993年 | 3799篇 |
1992年 | 1875篇 |
1991年 | 1898篇 |
1990年 | 1814篇 |
1989年 | 1593篇 |
1988年 | 1458篇 |
1987年 | 1076篇 |
1986年 | 1084篇 |
1985年 | 1091篇 |
1984年 | 875篇 |
1983年 | 783篇 |
1982年 | 827篇 |
1981年 | 854篇 |
1980年 | 757篇 |
1979年 | 536篇 |
1978年 | 513篇 |
1977年 | 1141篇 |
1976年 | 2274篇 |
1975年 | 396篇 |
1974年 | 317篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
191.
A beam propagation method (BPM) based on the finite element method (FEM) is described for longitudinally varying three-dimensional (3-D) optical waveguides. In order to avoid nonphysical reflections from the computational window edges, the transparent boundary condition is introduced. The present algorithm using the Pade approximation is, to our knowledge, the first wide-angle finite element beam propagation method for 3-D waveguide structures. To show the validity and usefulness of this approach, numerical results are shown for Gaussian-beam excitation of a straight rib waveguide and guided-mode propagation in a Y-branching rib waveguide 相似文献
192.
This paper presents the design criteria, procedure, and implementation of a soft-switched power-factor-correction (PFC) circuit based on the extended-period quasi-resonant (EPQR) principles. All power electronic devices including switches and diodes in the circuit are fully soft switched. The design method is demonstrated in a prototype circuit. The operating principles are confirmed with computer simulation and experimental results. A comparison of the EP-QR operation and zero-voltage-transition (ZVT) pulse-width modulation (PWM) method 相似文献
193.
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
194.
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
195.
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
196.
Z. Yu Y. F. Hsia X. Z. You H. Spiering P. Gutlich 《Journal of Materials Science》1997,32(24):6579-6581
A light-induced excited spin state trapping (LIESST) experiment for a thermal gradual spin crossover complex, Fetris (2-pyridylmethyl)
amine(NCS)2 or Fe(tpa) (NCS)2, was attempted for the first time. The high spin (HS) state after light inducement stayed metastable
over a period of days without relaxation at 10 K. Intersystem relaxation from a high to a low spin (LS) complex occurred at
50 K after bleaching at 10 K. Investigation of the Mossbauer spectra of the LIESST and relaxation experiment indicated that
the Debye–Waller factor was a correlation parameter of the HS fraction and that the co-operative effect played a role in the
relaxation process for such a solid compound.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献
197.
Yamada T. Kawakami Y. Nakano T. Mutoh N. Orihara K. Teranishi N. 《Electron Devices, IEEE Transactions on》1997,44(10):1580-1587
This study reports an optimum design for a two-phase charge-coupled device (CCD) and limitations on its driving voltage reduction. The two-phase CCD to be used as a horizontal-CCD (H-CCD) in a CCD image sensor requires low-voltage and high-speed operation. Reducing the driving voltage, however, may induce potential pockets in the channel under the inter-electrode gaps which results in a fatal decrease in charge-transfer efficiency. In this case it is necessary to optimize the CCD design to be free of pocket generation. For this requirement, we conducted two-dimensional (2-D) device simulations for the two-phase CCD, whose potential barriers are formed by boron ion-implantation. Our simulations indicated that the edge position of the potential barrier region and the dose of boron-ion implantation would be important parameters for controlling the size of potential pockets. At an optimum edge position and a boron dose, the minimum driving voltage appears to be reducible to 1.1 V. Characteristics of potential pockets and methods of their suppression are also discussed 相似文献
198.
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
199.
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
200.
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