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   87443篇
  免费   6615篇
  国内免费   2601篇
电工技术   2003篇
综合类   5664篇
化学工业   35136篇
金属工艺   2623篇
机械仪表   1930篇
建筑科学   4402篇
矿业工程   2818篇
能源动力   2031篇
轻工业   20749篇
水利工程   1022篇
石油天然气   4759篇
武器工业   253篇
无线电   1751篇
一般工业技术   4655篇
冶金工业   3431篇
原子能技术   681篇
自动化技术   2751篇
  2024年   383篇
  2023年   1133篇
  2022年   2226篇
  2021年   2702篇
  2020年   2505篇
  2019年   2444篇
  2018年   2164篇
  2017年   2686篇
  2016年   3010篇
  2015年   3028篇
  2014年   4533篇
  2013年   4936篇
  2012年   5882篇
  2011年   6239篇
  2010年   4607篇
  2009年   4910篇
  2008年   4300篇
  2007年   5490篇
  2006年   5050篇
  2005年   4421篇
  2004年   3727篇
  2003年   3265篇
  2002年   2857篇
  2001年   2474篇
  2000年   2106篇
  1999年   1843篇
  1998年   1415篇
  1997年   1147篇
  1996年   989篇
  1995年   832篇
  1994年   705篇
  1993年   555篇
  1992年   427篇
  1991年   342篇
  1990年   260篇
  1989年   183篇
  1988年   131篇
  1987年   153篇
  1986年   86篇
  1985年   104篇
  1984年   112篇
  1983年   79篇
  1982年   49篇
  1981年   27篇
  1980年   42篇
  1979年   19篇
  1978年   11篇
  1977年   11篇
  1974年   12篇
  1951年   15篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
对称配筋小偏心受压构件的截面设计   总被引:4,自引:1,他引:3  
对小偏心受压构件在对称配筋情况下的截面配筋计算,提出用最佳一次逼近多项式简化基本公式,直接按二次方程求解ξ的近似方法,具有精度高,相对误差小,计算方便等优点,可供工程设计人员参考。  相似文献   
12.
LaNiO3 was synthesized by sol-gel method in which lanthanum nitrate and nickel nitrate were used as start materials and citric acid was used as complex for gel formation.The precursor was dried and subsequently heated at elevated temperature to form the desired product.XRD analysis shows that pure LaNiO3 was synthesized.Electrical conductivity and electrochemical performance of the material were tested.The electrical conductivity decreases from 34.5that there are current peaks in the curve, which is the evidence of the electrochemical activity of LaNiO3.  相似文献   
13.
氯化铁催化合成富马酸二甲酯   总被引:9,自引:0,他引:9  
俞善信  雷焕文 《化学试剂》1993,15(6):374-376
氯化铁可代替硫酸催化酯化反应,探讨了氯化铁催化合成富马酸二甲酯的条件,操作简便,反应温和,无腐蚀,减少了污染与产品纯化。  相似文献   
14.
The hydrolysis of isocyanic acid in the gaseous phase has been investigated at temperatures between 553 and 613 K by mass spectrometry and evaluated to obtain the corresponding kinetic data. The reaction order and reaction constant have been determined. Finally, the influence of water on the catalysed formation of melamine from isocyanic acid under the operating conditions employed has been investigated in order to determine whether there is a need to try the process gas.  相似文献   
15.
In order to produce highly concentrated bioethanol by pervaporation using an ethanol‐permselective silicalite membrane, techniques to suppress adsorption of succinic acid, which is a chief by‐product of ethanol fermentation and causes the deterioration in pervaporation performance, onto the silicalite crystals was investigated. The amount adsorbed increased as the pH of the aqueous succinic acid solution decreased. The pervaporation performance also decreased with decreasing pH when the ternary mixtures of ethanol/water/succinic acid were separated. Using silicalite membranes individually coated with two types of silicone rubber, pervaporation performance was significantly improved in the pH range of 5 to 7, when compared with that of non‐coated silicalite membranes in ternary mixtures of ethanol/water/succinic acid. Moreover, when using a silicalite membrane double‐coated with the two types of silicone rubber, pervaporation performance was stabilized at lower pH values. In the separation of bioethanol by pervaporation using the double‐coated silicalite membrane, removal of accumulated substances having an ultraviolet absorption maximum at approximately 260 nm from the fermentation broth proved to be vital for efficient pervaporation. Copyright © 2005 Society of Chemical Industry  相似文献   
16.
An optimal control problem with nonsmooth performance criterion described by a system of ordinary differential equations is considered. Necessary first-order optimality conditions are obtained.  相似文献   
17.
A centrifugal partition chromatograph (CPC) was used as a liquid-liquid catalytic reactor for the isomerisation of hexen-3-ol into ethylpropylketone with a water soluble rhodium catalyst. Global mass transfer coefficients were measured and shown to depend on both the nature of the solute and the flow rate. Liquid-liquid partition isotherms were also determined with the CPC using elution chromatography. Finally, a reactor model was derived to account for the experimental results obtained both under stationary and transient (pulse) conditions. A parameter sensitivity evaluation is also presented.  相似文献   
18.
Tomato seed oil was extracted with supercritical carbon dioxide in a semibatch-flow extractor within the temperature range 313–343 K and the pressure range 10.8–24.5MPa. The extraction rates increased with pressure, but decreased with temperature increase because of the variation in solvent density and resultant differences in oil solubility. The fatty acid composition of the extracted oil was similar to that of soybean oil.  相似文献   
19.
以水杨酸和对硝基苯酚为原料,在三氯氧磷存在下进行酯化,然后催化加氢还原硝基,最后用乙酐进行二酰化制得扑炎痛,三步收率分别为80.8%、90.4%和88.5%。  相似文献   
20.
Analysis of characteristic odors from human male axillae   总被引:7,自引:0,他引:7  
A number of studies concerning the analysis of axillary odors have assumed that the characteristic odor produced in the axillae is due to volatile steroids and isovaleric acid. Organoleptic evaluation of Chromatographic eluants from axillary extracts was employed to isolate the region in the chromatogram where the characteristic odor eluted. The odor of the dissolved eluant was eliminated when it was treated with base, suggesting that acids make up the characteristic axillary odor. Subsequent extraction of the pH-adjusted axillary extract in conjunction with organoleptic evaluation of the Chromatographic eluant, preparative gas chromatography, and analysis by GC-MS as well as GC-FTIR showed the presence of a number of C6 to C11 straight-chain, branched, and unsaturated acids as important contributors to the axillary odor. The major odor component is (E)-3-methyl-2-hexenoic acid. Three homologous series of minor components are also important odor contributors; these consist of the terminally unsaturated acids, the 2-methyl-C6 to -C10 acids and the 4-ethyl-C5 to -C11 acids. These types of acids have not been reported previously as components of the human axillary secretions and have not been proposed previously as part of the principal odor components in this area.  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