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   282250篇
  免费   3550篇
  国内免费   807篇
电工技术   4963篇
综合类   161篇
化学工业   43007篇
金属工艺   11804篇
机械仪表   8500篇
建筑科学   6410篇
矿业工程   1833篇
能源动力   6683篇
轻工业   25226篇
水利工程   3168篇
石油天然气   6970篇
武器工业   16篇
无线电   31283篇
一般工业技术   56357篇
冶金工业   51357篇
原子能技术   7532篇
自动化技术   21337篇
  2021年   1988篇
  2018年   3676篇
  2017年   3620篇
  2016年   3905篇
  2015年   2466篇
  2014年   4230篇
  2013年   12019篇
  2012年   6825篇
  2011年   9214篇
  2010年   7565篇
  2009年   8642篇
  2008年   8930篇
  2007年   8813篇
  2006年   7826篇
  2005年   7311篇
  2004年   6792篇
  2003年   6484篇
  2002年   6294篇
  2001年   6100篇
  2000年   5703篇
  1999年   5846篇
  1998年   14432篇
  1997年   10723篇
  1996年   8234篇
  1995年   6448篇
  1994年   5840篇
  1993年   5708篇
  1992年   4401篇
  1991年   4391篇
  1990年   4223篇
  1989年   4213篇
  1988年   4173篇
  1987年   3530篇
  1986年   3550篇
  1985年   4088篇
  1984年   3900篇
  1983年   3554篇
  1982年   3366篇
  1981年   3495篇
  1980年   3374篇
  1979年   3308篇
  1978年   3406篇
  1977年   3867篇
  1976年   4989篇
  1975年   3114篇
  1974年   2968篇
  1973年   2988篇
  1972年   2621篇
  1971年   2432篇
  1970年   2053篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Automation and Remote Control - In this survey, we discuss various approaches to control theory that have arisen in the recent decades and reflect the desire to reach a trade-off between the $$LQG...  相似文献   
82.
The lithium-doped phases Gd1.7Li0.3Zr2O6.7 and Gd2Zr1.7Li0.3O6.55 with a pyrochlore structure were prepared by the modified Pechini method using citric acid and glycerol. Monitoring of the lithium content by using a nuclear microanalysis showed that a significant loss of lithium occurred after heat treatment above 1200 °C. Dense ceramics with a stoichiometric lithium content can be prepared by a low temperature microwave sintering (1100 °C). The introduction of lithium in the Gd-sublattice was accompanied by a decrease in the unit cell parameter (a = 10.5208 (1) Å vs 10.5346 (2) Å for Gd2Zr2O7) and during doping at the Zr-sites with lithium, the cell parameter increased (10.5720 (1) Å). The doping in both cases led to an increase in the free cell volume. The impedance spectroscopy results showed that the bulk conductivity can be enhanced by the Li+-doping at the Gd3+-site by almost an order of magnitude. The sample Gd2Zr1.7Li0.3O6.55 had a conductivity lower than that of Gd1.7Li0.3Zr2O6.7 due to the possible trapping of oxygen vacancies by a high-charged acceptor defect LiZr???. The conductivity?pO2 measurements showed that the Li-containing phase was a pure oxide-ion conductor at T < 800 °C.  相似文献   
83.
Nano-sized light rare-earth (La, Pr, Nd, and Sm) doped Ba0.90Ca0.10Ti0.90Zr0.10O3 ceramics were synthesized to enhance the energy storage performance. The Rietveld study of bare and doped samples has shown tetragonal crystal symmetry and a single-phase perovskite structure. The rare-earth addition in Ba0.90Ca0.10Ti0.90Zr0.10O3 has resulted in a remarkable change in the microstructure of the doped samples. The addition of Nd in Ba0.90Ca0.10Ti0.90Zr0.10O3 lattice has resulted in optimum grain size and density among the five compositions. As a result of improvement in morphological characteristics in the Nd-doped sample, the dielectric, ferroelectric and piezoelectric characteristics were significantly enhanced. The Nd-doped sample has shown a relaxor behavior with a maximum dielectric constant of 10788 combined with a high saturation polarization of 41.88 μC/cm2. Further, the material has shown optimum electromechanical behavior with excellent aging characteristics. The obtained properties of Nd-doped Ba0.90Ca0.10Ti0.90Zr0.10O3 sample justifies its potential application in multi-layer energy storage capacitors.  相似文献   
84.
Journal of Applied Electrochemistry - In this study, within the defined orthogonal array of Taguchi design, the hydrothermal process parameters have been optimized for fabricating the smallest...  相似文献   
85.
Journal of Porous Materials - The present study reports a systematic analysis of morphology and hydrogen sorption capacity of mesoporous organic-inorganic silica prepared by varying the silica...  相似文献   
86.
The valuable aroma compound piperonal with its vanilla-like olfactory properties is of high interest for the fragrance and flavor industry. A lipoxygenase (LOXPsa1) of the basidiomycete Pleurotus sapidus was identified to convert piperine, the abundant pungent principle of black pepper (Piper nigrum), to piperonal and a second volatile product, 3,4-methylenedioxycinnamaldehyde, with a vanilla-like odor through an alkene cleavage. The reaction principle was co-oxidation, as proven by its dependence on the presence of linoleic or α-linolenic acid, common substrates of lipoxygenases. Optimization of the reaction conditions (substrate concentrations, reaction temperature and time) led to a 24-fold and 15-fold increase of the piperonal and 3,4-methylenedioxycinnamaldehyde concentration using the recombinant enzyme. Monokaryotic strains showed different concentrations of and ratios between the two reaction products.  相似文献   
87.
Agelaia-MPI and protonectin are antimicrobial peptides isolated from the wasp Parachartergus fraternus that show antimicrobial and neuroactive activities. Previously, two analogues of these peptides, neuroVAL and protonectin-F, were designed to reduce nonspecific toxicity and improve potency. Here, the three-dimensional structures of neuroVAL, protonectin and protonectin-F were determined by using circular dichroism and NMR spectroscopy. Antibacterial, antifungal, cytotoxic and hemolytic activities were tested for the parent peptides and analogues. All peptides showed moderate antimicrobial activity against Gram-positive bacteria, with agelaia-MPI being the most active. Protonectin and protonectin-F were found to be toxic to cancerous and noncancerous cell lines. Internalization experiments revealed that these peptides accumulate inside both cell types. By contrast, neuroVAL was nontoxic to all tested cells and was able to enter cells without accumulating. In summary, neuroVAL has potential as a nontoxic cell-penetrating peptide, while protonectin-F needs further modification to realize its potential as an antitumor peptide.  相似文献   
88.
A proper detection and classification of defects in steel sheets in real time have become a requirement for manufacturing these products, largely used in many industrial sectors. However, computers used in the production line of small to medium size companies, in general, lack performance to attend real-time inspection with high processing demands. In this paper, a smart deep convolutional neural network for using in real-time surface inspection of steel rolling sheets is proposed. The architecture is based on the state-of-the-art SqueezeNet approach, which was originally developed for usage with autonomous vehicles. The main features of the proposed model are: small size and low computational burden. The model is 10 to 20 times smaller when compared to other networks designed for the same task, and more than 700 times smaller than general networks. Also, the number of floating-point operations for a prediction is about 50 times lower than the ones used for similar tasks. Despite its small size, the proposed model achieved near-perfect accuracy on a public dataset of 1800 images of six types of steel rolling defects.  相似文献   
89.
Tian  J. Y.  Xu  G.  Hu  H. J.  Zhou  M. X. 《Strength of Materials》2019,51(3):439-449
Strength of Materials - A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation,...  相似文献   
90.
Russian Microelectronics - The recently developed nanomaterials and their production technologies as intellectual property objects (IPOs) are considered. The role of the informational-analytical...  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