首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1514655篇
  免费   26128篇
  国内免费   7127篇
电工技术   34588篇
综合类   6535篇
化学工业   272467篇
金属工艺   64943篇
机械仪表   43366篇
建筑科学   46907篇
矿业工程   11521篇
能源动力   50462篇
轻工业   115530篇
水利工程   15179篇
石油天然气   37789篇
武器工业   154篇
无线电   197256篇
一般工业技术   286358篇
冶金工业   159745篇
原子能技术   34111篇
自动化技术   170999篇
  2021年   15615篇
  2020年   11890篇
  2019年   14690篇
  2018年   15944篇
  2017年   15334篇
  2016年   21447篇
  2015年   17401篇
  2014年   28805篇
  2013年   88103篇
  2012年   35908篇
  2011年   48784篇
  2010年   43031篇
  2009年   51424篇
  2008年   45492篇
  2007年   42854篇
  2006年   45290篇
  2005年   40153篇
  2004年   42203篇
  2003年   42021篇
  2002年   41119篇
  2001年   37566篇
  2000年   36024篇
  1999年   34805篇
  1998年   42811篇
  1997年   37896篇
  1996年   34683篇
  1995年   30520篇
  1994年   28596篇
  1993年   28326篇
  1992年   26308篇
  1991年   23355篇
  1990年   23791篇
  1989年   22817篇
  1988年   21327篇
  1987年   19604篇
  1986年   18993篇
  1985年   22361篇
  1984年   22534篇
  1983年   20453篇
  1982年   19371篇
  1981年   19450篇
  1980年   18107篇
  1979年   18720篇
  1978年   17900篇
  1977年   17811篇
  1976年   19053篇
  1975年   16142篇
  1974年   15672篇
  1973年   15762篇
  1972年   13244篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
841.
842.
The concept and structure of the NANODEV simulation software are described. NANODEV deals with nanoelectronic devices that exploit single-electron tunneling, resonant tunneling, or quantum interference. It can use both simplified and sophisticated models and enables one to evaluate a wide variety of devices and configurations. The capabilities of NANODEV are illustrated by examples.  相似文献   
843.
A novel driver-assist stability system for all-wheel-drive electric vehicles is introduced. The system helps drivers maintain control in the event of a driving emergency, including heavy braking or obstacle avoidance. The system comprises a fuzzy logic system that independently controls wheel torque to prevent vehicle spin. Another fuzzy wheel slip controller is used to enhance vehicle stability and safety. A neural network is trained to generate the required reference for yaw rate. Vehicle true speed is estimated by a sensor data fusion method. The intrinsic robustness of fuzzy controllers allows the system to operate in different road conditions successfully. Moreover, the ease of implementing fuzzy controllers gives a potential for vehicle stability enhancement.  相似文献   
844.
A new dual-quantisation sigma-delta modulator is proposed, which introduces an additional feedback path in the input of the second integrator. In this way, unlike other dual-quantisation architectures, larger signal-to-noise ratios can be obtained by means of aggressive noise-shaping, like in a conventional multibit modulator. The proposed modulator is also shown to be more robust against non-idealities than other dual-quantisation architectures.  相似文献   
845.
Pozar  D.M. 《Electronics letters》2003,39(21):1490-1491
It is shown that the dominant factor controlling reflectarray bandwidth is the bandwidth of the radiating element, and that the effect of non-constant path delays over the surface of the flat reflector has little significance unless the aperture is electrically very large and the f/D ratio is small. An example of a polarisation-twist reflectarray using aperture coupled elements with time delay lines is used to demonstrate this conclusion.  相似文献   
846.
Optimal space-time constellations from groups   总被引:10,自引:0,他引:10  
We consider the design of space-time constellations based on group codes for fading channels with multiple transmit and receive antennas. These codes can be viewed as multiantenna extensions of phase-shift keying (PSK), in the sense that all codewords have equal energy, all are rotations of a fixed codeword, and there is a simple differential transmission rule that allows data to be sent without channel estimates at the transmitter or receiver. For coherent detection, we show that all optimal full-rank space-time group codes are unitary (each code matrix has equal-energy, orthogonal rows). This leads to a simpler code design criterion and suggests that unitary codes may play an important role in coherent as well as noncoherent communication. For any number of transmit antennas t, we then use the design criterion to characterize all full-rank unitary space-time group codes of minimum block length (also t) which have 2/sup p/ codewords. These results allow us to characterize all optimal 2/sup p/-ary unitary group codes with square code matrices. This restricted class of block codes matches the class proposed for differential modulation by Hughes (see IEEE Trans. Inform. Theory, vol.46, p.2567-78, Nov. 2000), and by Hochwald and Sweldens (see IEEE Trans. Commun., vol.48, p.2041-2052, Dec. 2000).  相似文献   
847.
848.
Reviews the book by Baruss (see record 2003-02669-000), which reviews the experimental and phenomenological research on alterations of consciousness, ranging from sleep and dreaming to mystical and near-death experience. The reviewer suggests there is a clear agenda announced by the book's subtitle, "An Empirical Analysis for Social Scientists." In the view of Baruss, all too often in states of consciousness research a preoccupation with theory has kept investigators from full engagement with the actual data. The book provides readable and at times appropriately controversial discussions of empirical literature on dreaming and lucid dreams, daydreaming and fantasy proneness, hypnosis, dissociative identity disorder, shamanism and possession states, psychedelic drug research, parapsychology, trance-chanelling and mediumship, the alien abduction syndrome, classical mystical experience, out-of-body and near-death experiences, and recent attempts by MacDonald and others to assess individual differences in spirituality through multifactor questionnaires. However, the reviewer believes that the breadth of coverage of both recent and past research is too often highly selective, and that the author's rejection of theory is both questionable in itself and more illusion than reality. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
849.
Can digital rights management be standardized?   总被引:1,自引:0,他引:1  
With increasing use of the Internet as a content-distribution platform, mere interoperability of the codec is no longer sufficient if content owners are not prepared to distribute their content online for fear of piracy. Therefore, one also needs interoperability in the area of managing and protecting related intellectual property rights. To provide interoperability of digital rights management (DRM) systems to protect the content rather than the distribution channel, MPEG has embarked on developing intellectual property management and protection (IPMP) specifications targeted at the content itself that are natively built into its standards. It is the hope that such standards will allow for interoperability of content commerce systems, giving consumers more choice, while providing rights holders with the tools to effectively protect their assets and to develop complex and attractive business offering to tempt consumers. While it remains uncertain whether standardization is in fact the right vehicle to provide the e-content market with an impetus to grow, it appears that MPEG is working on a set of specifications with relatively good chances of helping to achieve this goal.  相似文献   
850.
This paper presents a scheme for the efficient implementation of a low supply voltage continuous-time high-performance CMOS current mirror with low input and output voltage requirements. This circuit combines a shunt input feedback and a regulated cascode output stage to achieve low input resistance and very high output resistance. It can be used as a high-precision current mirror in analog and mixed signal circuits with a power supply close to a transistor's threshold voltage. The proposed current mirror has been simulated and a bandwidth of 40 MHz has been obtained. An experimental chip prototype has been sent for fabrication and has been experimentally verified, obtaining 0.15-V input-output voltage requirements, 100-/spl Omega/ input resistance, and more than 200-M/spl Omega/ (G/spl Omega/ ideally) output resistance with a 1.2-V supply in a standard CMOS technology.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号