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   493188篇
  免费   5498篇
  国内免费   2027篇
电工技术   8897篇
综合类   691篇
化学工业   70848篇
金属工艺   18835篇
机械仪表   14574篇
建筑科学   11672篇
矿业工程   2293篇
能源动力   13122篇
轻工业   40415篇
水利工程   4868篇
石油天然气   8385篇
武器工业   56篇
无线电   62068篇
一般工业技术   98062篇
冶金工业   94341篇
原子能技术   11411篇
自动化技术   40175篇
  2021年   4009篇
  2020年   2993篇
  2019年   3730篇
  2018年   6311篇
  2017年   6400篇
  2016年   6737篇
  2015年   4358篇
  2014年   7637篇
  2013年   21626篇
  2012年   12185篇
  2011年   16878篇
  2010年   13266篇
  2009年   15289篇
  2008年   15987篇
  2007年   16002篇
  2006年   13958篇
  2005年   12721篇
  2004年   12492篇
  2003年   12405篇
  2002年   11824篇
  2001年   12309篇
  2000年   11402篇
  1999年   12196篇
  1998年   31064篇
  1997年   21664篇
  1996年   16588篇
  1995年   12552篇
  1994年   11111篇
  1993年   11154篇
  1992年   7908篇
  1991年   7583篇
  1990年   7356篇
  1989年   7168篇
  1988年   6785篇
  1987年   5947篇
  1986年   5859篇
  1985年   6458篇
  1984年   5951篇
  1983年   5608篇
  1982年   5275篇
  1981年   5173篇
  1980年   5109篇
  1979年   4774篇
  1978年   4651篇
  1977年   5398篇
  1976年   7289篇
  1975年   3923篇
  1974年   3846篇
  1973年   3818篇
  1972年   3287篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
Summary Starting with Z-2, 3-diamino-2-butenedinitrile as diamine component the synthesis of the low molecular N,N,O,O-chelate 3.Co and its ligand 3 is optimized. An easy synthesis of the chelate 8.Co covalently bound on macroreticular resins is presented. The chelates 8.Co are the most active heterogenous catalysts for the valence isomerization of quadricyclane to norbornadiene today.  相似文献   
72.
An all-optical multiplexing technique using wavelength division multiplexing (WDM)-time division multiplexing (TDM) conversion with an electroabsorption wavelength converter has been proposed and demonstrated. The effectiveness of this WDM-TDM conversion technique for various pulsewidth settings was experimentally investigated. The fluctuation of the signal performance, which was inevitably caused by the coherent crosstalk between adjacent pulses in the conventional optical time division multiplexing (OTDM) technique, were successfully suppressed, even in the case of wide pulse duration. High Q-factor performance has been maintained for a wide range of duty ration from 36% to 74%. By introducing this technique to the optical time division multiplexer, a highly stable and high-quality 40-Gb/s optical signal can be effectively produced without generating the short pulse or setting two tributaries at orthogonal polarization states, and without introducing high-speed electronics for signal multiplexing. The WDM-TDM conversion with an electroabsorption wavelength converter was extended to 60-Gb/s operation by using three 20-Gb/s tributaries. A clear eye opening was confirmed for a waveform after the WDM-TDM conversion of the 60-Gb/s signal  相似文献   
73.
The Six Sigma approach is one of the more recent initiatives adopted by organisations who seek to make a paradigm shift in performance improvement (attacking at least one of the cost, quality, delivery measurements for improved competitiveness). Is there anything new about Six Sigma, does it offer improvements that other approaches cannot, or is it just a clever delivery of repackaged goods? The author investigates the above questions related to the Six Sigma approach  相似文献   
74.
A simple apparatus for elongational test of molten polymers is presented. Its realiability is demonstrated by means of stress growth in constant stretching rate experiments and relaxation test on a low density polyethylene sample.  相似文献   
75.
A fast algorithm is proposed for estimating the auto- and cross-correlation functions of a large signal. The algorithm is based on the sectioning method by the fast Fourier transform. We determine the optimal length of the portion of data read from external memory into RAM which achieves Tmin—a minimum processing time. An estimate of Tmin is obtained.Translated from Kibernetika, No. 3, pp. 78–81, May–June, 1991.  相似文献   
76.
The maintenance of the MACRO (a Monopoles, Astrophysics and Cosmic Ray Observatory), a large-area detector that will be used to search for rare constituents or phenomena in cosmic radiation penetrating deep underground, is addressed. A real-time expert system for diagnosing detector and data acquisition system anomalies, which is based on the NEXPERT commercial tool, is described. It performs online diagnosis and, if an abnormal condition is identified, takes the appropriate action to reduce the unavailability of the apparatus. The data acquisition system is CAMAC-based, and the sensor modules, which gather the significant values for diagnosis, are implemented in the VME crate  相似文献   
77.
78.
Pre-metal-deposition reactive ion etching (RIE) was performed on an Al0.3Ga0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs  相似文献   
79.
80.
This paper describes a sequential tripping strategy used in a wide area back-up protection expert system (BPES) to combat situations in which protection relays have maloperated or information is missing. The BPES is an innovative back-up protection scheme designed to prevent the occurrence of widespread blackouts. The BPES evaluates the certainty that transmission lines are likely to be affected by the fault and uses a sequential tripping strategy to isolate the fault if a firm decision is not available due to maloperated relays and/or missing information. The mode of analysis and the sequential tripping strategy ensures that the BPES will clear a fault at minimum risk to the network. An example is included to demonstrate how the certainty factor based sequential tripping strategy is employed by the BPES to clear a fault which occurred on the South Western part of the UK National Grid System  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