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   206715篇
  免费   17851篇
  国内免费   9352篇
电工技术   12679篇
技术理论   13篇
综合类   13376篇
化学工业   33948篇
金属工艺   11206篇
机械仪表   12961篇
建筑科学   16318篇
矿业工程   5875篇
能源动力   6181篇
轻工业   14038篇
水利工程   3704篇
石油天然气   12151篇
武器工业   1600篇
无线电   25006篇
一般工业技术   25085篇
冶金工业   10062篇
原子能技术   2297篇
自动化技术   27418篇
  2024年   730篇
  2023年   3582篇
  2022年   6234篇
  2021年   9103篇
  2020年   6898篇
  2019年   5546篇
  2018年   6267篇
  2017年   7020篇
  2016年   6304篇
  2015年   8451篇
  2014年   10601篇
  2013年   12867篇
  2012年   14000篇
  2011年   14781篇
  2010年   12597篇
  2009年   11997篇
  2008年   11518篇
  2007年   10708篇
  2006年   10929篇
  2005年   9439篇
  2004年   6323篇
  2003年   5391篇
  2002年   4915篇
  2001年   4380篇
  2000年   4384篇
  1999年   5044篇
  1998年   4341篇
  1997年   3662篇
  1996年   3453篇
  1995年   2805篇
  1994年   2239篇
  1993年   1709篇
  1992年   1327篇
  1991年   982篇
  1990年   765篇
  1989年   607篇
  1988年   483篇
  1987年   323篇
  1986年   265篇
  1985年   172篇
  1984年   152篇
  1983年   105篇
  1982年   110篇
  1981年   85篇
  1980年   78篇
  1979年   39篇
  1978年   32篇
  1977年   34篇
  1976年   38篇
  1975年   21篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
In this study, in vitro digestion and fermentation of Flammulina velutipes -derived polysaccharides (FVP) were investigated. It was found that FVP mainly consisted of 48.45% glucose, 15.40% mannose, 14.60% xylose, 11.80% fucose and 9.90% galactose. The -human saliva, simulated gastric and small intestinal juices conditions did not break down the FVP. Based on in vitro fermentation tests, FVP modulated the composition of gut microbiota by elevating the amounts of Bifidobacteriaceae and Bacteroidaceae and reducing the numbers of genera Lachnospiraceae and Enterococcaceae. Meanwhile, FVP affected the synthesis of short-chain fatty acids derived from gut microbiota.  相似文献   
102.
103.
104.
105.
106.
Due to the high health risks associated with indoor air pollutants and long-term exposure, indoor air quality has received increasing attention. In this study, we put emphasis on the molecular composition, source emissions, and chemical aging of air pollutants in a residence with designed activities mimicking ordinary Hong Kong homes. More than 150 air pollutants were detected at molecular level, 87 of which were quantified at a time resolution of not less than 1 hour. The indoor-to-outdoor ratios were higher than 1 for most of the primary air pollutants, due to emissions of indoor activities and indoor backgrounds (especially for aldehydes). In contrast, many secondary air pollutants exhibited higher concentrations in outdoor air. Painting ranked first in aldehyde emissions, which also caused great enhancement of aromatics. Incense burning had the highest emissions of particle-phase organics, with vanillic acid and syringic acid as markers. The other noteworthy fingerprints enabled by online measurements included linoleic acid, cholesterol, and oleic acid for cooking, 2,5-dimethylfuran, stigmasterol, iso-/anteiso-alkanes, and fructose isomers for smoking, C28-C34 even n-alkanes for candle burning, and monoterpenes for the use of air freshener, cleaning agents, and camphor oil. We showed clear evidence of chemical aging of cooking emissions, giving a hint of indoor heterogeneous chemistry. This study highlights the value of organic molecules measured at high time resolutions in enhancing our knowledge on indoor air quality.  相似文献   
107.
Li  Ruyi  Wang  Yang 《Catalysis Letters》2022,152(6):1742-1751
Catalysis Letters - Inspired by the discovery of the special structures of Ti-doped boron nitride fullerenes [(2019) Nat Commun 10: 4908], we herein present a computational investigation of...  相似文献   
108.
Flow field structure can largely determine the output performance of Polymer electrolyte membrane fuel cell. Excellent channel configuration accelerates electrochemical reactions in the catalytic layer, effectively avoiding flooding on the cathode side. In present study, a three-dimensional, multi-phase model of PEMFC with a 3D wave flow channel is established. CFD method is applied to optimize the geometry constructions of three-dimensional wave flow channels. The results reveal that 3D wave flow channel is overall better than straight channel in promoting reactant gases transport, removing liquid water accumulated in microporous layer and avoiding thermal stress concentration in the membrane. Moreover, results show the optimal flow channel minimum depth and wave length of the 3D wave flow channel are 0.45 mm and 2 mm, respectively. Due to the periodic geometric characteristics of the wave channel, the convective mass transfer is introduced, improving gas flow rate in through-plane direction. Furthermore, when the cell output voltage is 0.4 V, the current density in the novel channel is 23.8% higher than that of conventional channel.  相似文献   
109.
The degree of rate control (DRC) quantitatively identifies the kinetically relevant (sometimes known as rate-limiting) steps of a complex reaction network. This concept relies on derivatives which are commonly implemented numerically, for example, with finite differences (FDs). Numerical derivatives are tedious to implement, and can be problematic, and unstable or unreliable. In this study, we demonstrate the use of automatic differentiation (AD) in the evaluation of the DRC. AD libraries are increasingly available through modern machine learning frameworks. Compared with the FDs, AD provides solutions with higher accuracy with lower computational cost. We demonstrate applications in steady-state and transient kinetics. Furthermore, we illustrate a hybrid local-global sensitivity analysis method, the distributed evaluation of local sensitivity analysis, to assess the importance of kinetic parameters over an uncertain space. This method also benefits from AD to obtain high-quality results efficiently.  相似文献   
110.
The explosion venting duct can effectively reduce the hazard degree of a gas explosion and conduct the venting energy to the safe area. To investigate the flame quantitative propagation law of explosion venting with a duct, the effects of hydrogen fraction and explosion venting duct length on jet flame propagation characteristics of premixed H2-air mixtures were analyzed through experiment and simulation. The experiment results under initial conditions of room temperature and 1 atm show that when hydrogen fraction was high enough, part of the unburned hydrogen was mixed with air again to reach an ignitable concentration, resulting in the secondary combustion was easier produced and the duration of the secondary flame increased. With the increase of venting duct length, the flame front distance and propagation velocity increased. Meanwhile, the spatial distribution of pressure field and temperature field, and the propagation process and mechanism of the flame venting with a duct were analyzed using FLUENT software. The variation of the pressure wave and the pressure reflection oscillation law in the explosion venting duct was captured. Therefore, in the industrial explosion venting design with a duct, the hazard caused by the coupling of venting pressure and venting flame under different fractions should be considered comprehensively.  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