全文获取类型
收费全文 | 151868篇 |
免费 | 1888篇 |
国内免费 | 656篇 |
专业分类
电工技术 | 3115篇 |
综合类 | 187篇 |
化学工业 | 24074篇 |
金属工艺 | 5781篇 |
机械仪表 | 4943篇 |
建筑科学 | 4418篇 |
矿业工程 | 377篇 |
能源动力 | 3928篇 |
轻工业 | 17308篇 |
水利工程 | 1140篇 |
石油天然气 | 618篇 |
武器工业 | 5篇 |
无线电 | 20493篇 |
一般工业技术 | 28846篇 |
冶金工业 | 23755篇 |
原子能技术 | 2341篇 |
自动化技术 | 13083篇 |
出版年
2019年 | 834篇 |
2018年 | 1081篇 |
2017年 | 1131篇 |
2016年 | 1268篇 |
2015年 | 1063篇 |
2014年 | 1796篇 |
2013年 | 6576篇 |
2012年 | 3193篇 |
2011年 | 4609篇 |
2010年 | 3595篇 |
2009年 | 4151篇 |
2008年 | 4647篇 |
2007年 | 4913篇 |
2006年 | 4350篇 |
2005年 | 4108篇 |
2004年 | 4002篇 |
2003年 | 3899篇 |
2002年 | 3928篇 |
2001年 | 3980篇 |
2000年 | 3741篇 |
1999年 | 3691篇 |
1998年 | 6633篇 |
1997年 | 5227篇 |
1996年 | 4460篇 |
1995年 | 3709篇 |
1994年 | 3358篇 |
1993年 | 3184篇 |
1992年 | 2783篇 |
1991年 | 2690篇 |
1990年 | 2628篇 |
1989年 | 2614篇 |
1988年 | 2459篇 |
1987年 | 2166篇 |
1986年 | 2115篇 |
1985年 | 2556篇 |
1984年 | 2316篇 |
1983年 | 2195篇 |
1982年 | 2068篇 |
1981年 | 1991篇 |
1980年 | 1860篇 |
1979年 | 1874篇 |
1978年 | 1770篇 |
1977年 | 2085篇 |
1976年 | 2562篇 |
1975年 | 1583篇 |
1974年 | 1430篇 |
1973年 | 1453篇 |
1972年 | 1196篇 |
1971年 | 1115篇 |
1970年 | 948篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The recent growth in the use of digital radio is reviewed. The technology used to implement low-power digital radio in the local exchange loop plant is discussed. The integration of digital radio subscriber loops withnetwork intelligence is explored. The relationship between low-power digital radio loops and broadband fiber loops is briefly examined, and standards and frequency allocation activities are summarized 相似文献
992.
Design aspects of MOS-controlled thyristor elements: technology,simulation, and experimental results
Bauer F. Halder E. Hofmann K. Haddon H. Roggwiller P. Stockmeier T. Burgler J. Fichtner W. Muller S. Westermann M. Moret J.-M. Vuilleumier R. 《Electron Devices, IEEE Transactions on》1991,38(7):1605-1611
2.5-kV thyristor devices have been fabricated with integrated MOS controlled n+-emitter shorts and a bipolar turn-on gate using a p-channel DMOS technology. Square-cell geometries with pitch variations ranging from 15 to 30 μm were implemented in one- and two-dimensional arrays with up to 20000 units. The impact of the cell pitch on the turn-off performance and the on-state voltage was studied for arrays with constant cathode area as well as for single-cell structures. By realizing MOS components with submicrometer channel lengths, scaled single cells are shown to turn off with current densities of several kiloamperes per square centimeter at a gate bias of 5 V. In the case of multi-cell ensembles, turn-off performance is limited due to inhomogeneous current distribution. Critical process parameters as well as the device behavior were optimized through multidimensional numerical simulation 相似文献
993.
Concerns about toxicity of cholesterol oxidation products (COPS) prompted this study. Two restaurants were selected which use animal-vegetable (A/V) shortening for deep-frying. The survey of COPS for 30 days indicated values for total COPS in French fried potatoes were 20 ± 9 ppm to 24 ± 6 ppm. 5α-Cholestane-3β,5,6β-triol (triol) was identified in French fried potatoes from one restaurant. The mean for triol was 9 ± 3 ppm. Triol, 7α-hydroxycholesterol, 7β-hydroxy-cholesterol and 7-ketocholesterol were confirmed by co-chromatography and mass spectrometry. Triol is one of the most potent of angiotoxic COPS. This and other studies suggested French fried potatoes and other deep fried foods cooked in A/V shortening are a major source of COPS. 相似文献
994.
P. J. M. Sonnemans P. M. W. Krvers A. C. Brombacher P. C. van Beek J. E. A. Reinders 《Quality and Reliability Engineering International》2003,19(3):183-196
Often companies in the (petro‐) chemical industry claim that all possible countermeasures against potential accidents have been taken and therefore accidents are unforeseeable. In this paper we question this statement by analysing the pre‐warning signals (precursors) preceding a number of industrial accidents. 17 accidents that occurred in the (petro‐) chemical industry have been investigated by exploring FACTS, an accident database containing information about industrial accidents worldwide. This paper will demonstrate that the existence of precursor information could have been used to foresee and even prevent these accidents if a proper control action had been initiated. The accidents are analysed further, according to a control model, which was adapted from that of C. Argyris. It demonstrates the ineffectiveness of several elements of business process control loops and that the so‐called ‘double‐loop learning’ cycle is more important than the ‘single‐loop learning’ cycle if one considers safety improvement. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
995.
H.W. Choi C.W. Jeon M.D. Dawson P.R. Edwards R.W. Martin 《Photonics Technology Letters, IEEE》2003,15(4):510-512
High-performance, two-dimensional arrays of parallel-addressed InGaN blue micro-light-emitting diodes (LEDs) with individual element diameters of 8, 12, and 20 /spl mu/m, respectively, and overall dimensions 490 /spl times/490 /spl mu/m, have been fabricated. In order to overcome the difficulty of interconnecting multiple device elements with sufficient step-height coverage for contact metallization, a novel scheme involving the etching of sloped-sidewalls has been developed. The devices have current-voltage (I-V) characteristics approaching those of broad-area reference LEDs fabricated from the same wafer, and give comparable (3-mW) light output in the forward direction to the reference LEDs, despite much lower active area. The external efficiencies of the micro-LED arrays improve as the dimensions of the individual elements are scaled down. This is attributed to scattering at the etched sidewalls of in-plane propagating photons into the forward direction. 相似文献
996.
997.
Service Load Effective Compression Flange Width in Fiber Reinforced Polymer Deck Systems Acting Compositely with Steel Stringers 总被引:1,自引:0,他引:1
This paper reports on the field study of a steel stringer-fiber reinforced polymer (FRP) deck composite bridge in Pennsylvania. The objective of the study is to assess the effective compression flange width in the FRP deck and floor systems when they act compositely with underlying steel girders at service conditions. The research results reported herein support the notion of employing a design approach, for both interior and exterior girders of a composite floor system, that is philosophically consistent with current practice related to steel girders acting compositely with concrete decking. It appears from the results presented herein that FRP decks and floors acting compositely with underlying steel girders exhibit an effective width that is close to the actual girder spacing for interior beams, and approximately one-half this value for exterior beams. 相似文献
998.
A near-optimal multiuser detector for DS-CDMA systems using semidefinite programming relaxation 总被引:1,自引:0,他引:1
A multiuser detector for direct-sequence code-division multiple-access systems based on semidefinite programming (SDP) is proposed. It is shown that maximum likelihood (ML) detection can be carried out by "relaxing" the associated integer programming problem to a dual SDP problem, which leads to a detector of polynomial complexity. Computer simulations that demonstrate that the proposed detector offers near-optimal performance with considerably reduced computational complexity compared with that of existing primal-SDP-relaxation based detectors are presented. 相似文献
999.
Specification of uncertainty has historically been done by a variety of methods with differences in results, especially for instruments and standards that achieve small uncertainty values. A relatively simple but practical approach is used in newly published standard IEC 62008 1st Edition, "Performance characteristics and calibration methods for digital data acquisition systems and relevant software." The standard currently covers only relevant manufacturer's specifications for direct current (dc) data acquisition (DAQ) devices. These are plug-in cards without the associated computer for readout and control or power supply. Rather than complete instruments, the cards require an associated computer for readout and control. The history of uncertainty is reviewed; some popular current techniques for specifying uncertainty are compared. The components of uncertainty chosen for DAQ plug-in cards are reviewed, and their effects are discussed. 相似文献
1000.
Leppelmeier G.W. Aulamo O. Hassinen S. Malkki A. Riihisaari T. Tajakka R. Tamminen J. Tanskanen A. 《Geoscience and Remote Sensing, IEEE Transactions on》2006,44(5):1283-1287
The Ozone Monitoring Instrument (OMI) operates onboard the National Aeronautics and Space Administration's Earth Observing System Aura satellite, which was launched in July 2004. Like its sister spacecraft Terra and Aqua, Aura's capabilities include direct broadcast (DB), i.e., the ability to broadcast data at the same time as they are being measured and stored in the spacecraft's memory for later transmission to Earth. The Finnish Meteorological Institute's Satellite Data Centre at Sodankyla/spl uml/ in Finnish Lapland is exploiting this capability to receive OMI data while Aura is in sight of the receiver, which enables nearly immediate production of OMI data products for a region that includes a large part of Europe, stretching from the North Pole to the Italian Alps. The current OMI Very Fast Delivery (VFD) products include maps of surface UV-B, ozone columns, and cloud coverage. 相似文献