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   260132篇
  免费   3192篇
  国内免费   823篇
电工技术   5130篇
综合类   330篇
化学工业   37248篇
金属工艺   10253篇
机械仪表   8487篇
建筑科学   6104篇
矿业工程   1076篇
能源动力   6389篇
轻工业   22851篇
水利工程   2562篇
石油天然气   3908篇
武器工业   68篇
无线电   34723篇
一般工业技术   50082篇
冶金工业   46543篇
原子能技术   5419篇
自动化技术   22974篇
  2021年   2040篇
  2019年   2032篇
  2018年   3468篇
  2017年   3534篇
  2016年   3728篇
  2015年   2320篇
  2014年   4049篇
  2013年   11286篇
  2012年   6564篇
  2011年   8877篇
  2010年   6983篇
  2009年   7849篇
  2008年   8680篇
  2007年   8617篇
  2006年   7804篇
  2005年   7142篇
  2004年   6886篇
  2003年   6735篇
  2002年   6372篇
  2001年   6470篇
  2000年   6170篇
  1999年   6256篇
  1998年   14244篇
  1997年   10270篇
  1996年   8115篇
  1995年   6412篇
  1994年   5711篇
  1993年   5583篇
  1992年   4553篇
  1991年   4214篇
  1990年   4048篇
  1989年   3781篇
  1988年   3625篇
  1987年   3171篇
  1986年   3065篇
  1985年   3611篇
  1984年   3383篇
  1983年   3032篇
  1982年   2850篇
  1981年   2952篇
  1980年   2762篇
  1979年   2648篇
  1978年   2499篇
  1977年   2945篇
  1976年   3554篇
  1975年   2316篇
  1974年   2312篇
  1973年   2318篇
  1972年   1849篇
  1971年   1745篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
We apply appropriately enhanced transition matrix and multicanonical methods to communication systems. Our procedure not only predicts time-independent quantities such as the bit-error-probability but can also be applied to dynamic effects such as the distribution of fading times.  相似文献   
132.
This paper presents the design and practical hardware implementation of optimal neurocontrollers that replace the conventional automatic voltage regulator (AVR) and the turbine governor of turbogenerators on multimachine power systems. The neurocontroller design uses a powerful technique of the adaptive critic design (ACD) family called dual heuristic programming (DHP). The DHP neurocontrollers' training and testing are implemented on the Innovative Integration M67 card consisting of the TMS320C6701 processor. The measured results show that the DHP neurocontrollers are robust and their performance does not degrade unlike the conventional controllers even when a power system stabilizer (PSS) is included, for changes in system operating conditions and configurations. This paper also shows that it is possible to design and implement optimal neurocontrollers for multiple turbogenerators in real time, without having to do continually online training of the neural networks, thus avoiding risks of instability.  相似文献   
133.
Motivated by the transmit antenna selection (TAS) concept, used in Multiple-Input-Multiple-Output systems, we argue for distributed transmit antenna selection (DTAS), which corresponds to a method of selecting a subset of available relays in cooperative diversity systems. Assuming amplify and forward relays, the proposed selection method represents a low-complexity tool for determining the optimum relaying set. Two optimization problems are studied: the error probability minimization subject to total energy consumption constraints, and the dual one, the total energy consumption minimization under error performance constraints. Numerical examples verify the advantage of the proposed method in adapting the number of relaying terminals to the desired performance-consumption tradeoff.  相似文献   
134.
Interval based MINLP superstructure synthesis of mass exchange networks   总被引:1,自引:1,他引:0  
A new simultaneous synthesis approach for mass exchange networks (MENs) is presented. The technique is adapted from the interval based mixed integer non-linear program (MINLP) superstructure (IBMS) synthesis of heat exchanger networks (HENs). The superstructure interval boundary compositions are defined by the supply and target compositions of either the rich or lean set of streams. Each rich and lean stream in the superstructure has the potential of exchanging mass with streams of opposite kind in each interval based on mass transfer feasibility. The model harnesses the strengths of the stagewise superstructure and the pinch technology methods for mass exchange network synthesis (MENS). The IBMS approach simultaneously trades-off the capital and operating costs for MENs. The superstructure composition interval defining approach introduced in this paper enforces the mixing of split streams at equal compositions, hence there is no need to include mixing equations in the model. Fixing the interval boundaries helps to eliminate the complexities involved in initialisations, thus the region of search for the optimum solution is reduced. The IBMS model is applied to MENS problems involving continuous contact and staged columns. It is also extended to problems involving multiple mass separating agents (MSAs) and regeneration. The results obtained compare well with those in the literature.  相似文献   
135.
Sibunit-supported Ru-catalysts promoted with cesium or rubidium compounds have been comparatively studied with XPS. The cesium promoter interacts both with support and with active component. The absence of the promoter–support interaction in the case of rubidium provides a stronger interaction between promoter and active component compared to the cesium-based catalysts. These differences in the promoter–support and promoter–metal interactions are exhibited when a sequence of ruthenium and alkali introduction are changed.  相似文献   
136.
Fe–Co bimetallic metals supported on CaCO3 support have been prepared using three different methods for the preparation of the catalysts; wet impregnation (IMP), deposition–precipitation (DP) and reverse micelle impregnation (RM). By careful choice and optimization of CVD reaction conditions and catalyst preparation procedures, we have tailored the nanofabrication of CNTs using 5 wt.% Fe–Co/CaCO3. High yields of ‘clean’ MWNTs with near 100% selectivity and excellent reproducibility were obtained (1215% and 835% yields for the IMP and DP methods respectively). The RM technique enabled more precise control over the size of the CNTs and a narrower diameter distribution (3–5 nm i.d. and 10–15 nm o.d.) was obtained.  相似文献   
137.
Hot filament and microwave plasma CVD micro- nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The sample grain size varies from 20 nm to 2 μm. The hydrogen concentration in samples is measured by SIMS and compared to the grain size, and to the ratio of sp2 carbon bonds determined by Raman spectroscopy from the 1332 cm 1 diamond peak and the sp2 1550 cm 1 G band. Hydrogen concentration appears to be proportional to the sp2 bonds ratio. The 3000 cm 1 CHx stretching mode band intensity observed on the Raman spectra is decreasing with the G band intensity. Thermal annealing modifies the sp2 phase structure and concentration, as hydrogen outdiffuses.  相似文献   
138.
Aiming at preparation of shape memory alloys (SMAs), we explored the SHS of Cu1 − x Zn1 − y Al1 − z alloys (0.29 < x < 0.30, 0.74 < y < 0.75, and 0.83 < z < 0.96). The most pronounced shape memory effect was exhibited by the alloys of the following compositions (wt %): (1) Cu(70.6)Zn(25.4)Al(4.0), (2) Cu(70.1)Zn(25.9)Al(4.0), and (3) Cu(69.9)Zn(26.1)Al(4.0). The effect of process parameters on the synthesis of CuZnAl alloys was studied by XRD, optical microscopy, and scanning electron microscopy (SEM). The grain size of CuZnAl was found to depend on the relative amount of the primary CuZn and AlZn phases. Changes in the transformation temperature and heat of transformation are discussed in terms of ignition intensity and compaction. Mechanism of the process depends on the level of the temperature attained relative to the melting point of components. At the melting point of AlZn, the process is controlled by the solid-state diffusion of AlZn into a product layer. The ignition temperature for this system depends on the temperature of the austenite-martensite transformation in CuZnAl alloys. The composition and structure of the products was found to markedly depend on process parameters. The SHS technique has been successfully used to prepare a variety of SMAs.   相似文献   
139.
The main rules and principles which govern the choice of new variants for chemical technology of combined processing of titanium-rare-metal raw materials are considered. The existing variants of the technology are characterized from the viewpoint of their technological, economical, and ecological efficiency and social importance for a given region. A realistic choice of a technology from a large number of available variants should take into account the entire combination of their advantages and disadvantages The implementation of a rational technological scheme will result in the creation of economically profitable and efficient domestic production satisfying the demands of Russian industry in rare-metal and titanium materials. Directions are noted in which it is expedient to continue studies on designing a rational technology of loparite concentrate or some other concentrate.  相似文献   
140.
A novel integrated vacuum field emission (VFE) differential amplifier (diff-amp) utilizing carbon nanotube (CNT) emitters has been developed. A dual-mask microfabrication process was employed to achieve a VFE diff-amp by integrating identical CNT VFE transistors with built-in split gates and integrated anodes. The identical pair of triode amplifiers was well-matched in their device characteristics. The measured ac small-signal characteristics of the diff-amp showed a common-mode-rejection ratio (CMRR) of ~ 320 (~ 50 dB). The proposed analytical model of the CMRR was verified to be in good agreement with the experimental data. The successful implementation of the CNT diff-amp demonstrates a new way to achieve temperature and radiation tolerant VFE integrated microelectronics.  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