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   6022篇
  免费   453篇
  国内免费   266篇
电工技术   84篇
技术理论   1篇
综合类   393篇
化学工业   3336篇
金属工艺   86篇
机械仪表   150篇
建筑科学   523篇
矿业工程   58篇
能源动力   139篇
轻工业   890篇
水利工程   256篇
石油天然气   315篇
武器工业   18篇
无线电   52篇
一般工业技术   380篇
冶金工业   33篇
原子能技术   12篇
自动化技术   15篇
  2024年   25篇
  2023年   51篇
  2022年   108篇
  2021年   113篇
  2020年   145篇
  2019年   161篇
  2018年   141篇
  2017年   179篇
  2016年   203篇
  2015年   185篇
  2014年   260篇
  2013年   304篇
  2012年   448篇
  2011年   413篇
  2010年   313篇
  2009年   333篇
  2008年   305篇
  2007年   345篇
  2006年   414篇
  2005年   316篇
  2004年   319篇
  2003年   255篇
  2002年   219篇
  2001年   201篇
  2000年   188篇
  1999年   158篇
  1998年   129篇
  1997年   107篇
  1996年   87篇
  1995年   73篇
  1994年   56篇
  1993年   44篇
  1992年   35篇
  1991年   25篇
  1990年   16篇
  1989年   19篇
  1988年   8篇
  1987年   5篇
  1986年   3篇
  1985年   2篇
  1984年   4篇
  1982年   5篇
  1981年   4篇
  1980年   4篇
  1979年   7篇
  1978年   1篇
  1977年   3篇
  1976年   1篇
  1974年   1篇
排序方式: 共有6741条查询结果,搜索用时 0 毫秒
1.
Carboxyl‐terminated butadiene‐acrylonitrile rubber (CTBN) has often been used to improve the toughness of cyanate ester (CE) resin while sacrificing modulus and thermostability. In this paper, the addition of the appropriate amount of epoxy resin (EP) to the CE/CTBN system is shown to not only increase the modulus and thermostability of the blend, but also improve the toughness. The values of impact strength showed a maximum for the CE/CTBN/EP 100/5/5 blend. The temperature of 10 % weight loss (T10) improves from 376 °C for CE/CTBN 100/5 to 407 °C for the CE/CTBN/EP 100/5/2.5 blend. It is proposed that addition of the appropriate amount of EP can decrease the mobility and increase the stability of CTBN via the reaction between the terminal carboxyl group of CTBN and the hydroxyl group of EP. But a very high EP concentration will decrease the crosslinking density of CE, consequently reducing the mechanical properties and thermostability of the blends. Copyright © 2004 Society of Chemical Industry  相似文献   
2.
A series of new alternating aromatic poly(ester‐imide)s were prepared by the polycondensation of the preformed imide ring‐containing diacids, 2,2′‐bis(4‐trimellitimidophenoxy)biphenyl (2a) and 2,2′‐bis(4‐trimellitimidophenoxy)‐1,1′‐binaphthyl (2b) with various aromatic dihydroxy compounds in the presence of pyridine and lithium chloride. A model compound (3) was also prepared by the reaction of 2b with phenol, its synthesis permitting an optimization of polymerization conditions. Poly(ester‐imides) were fully characterized by FTIR, UV‐vis and NMR spectroscopy. Both biphenylene‐ and binaphthylene‐based poly(ester‐imide)s exhibited excellent solubility in common organic solvents such as tetrahydrofuran, m‐cresol, pyridine and dichloromethane. However, binaphthylene‐based poly(ester‐imide)s were more soluble than those of biphenylene‐based polymers in highly polar organic solvents, including N‐methyl‐2‐pyrrolidone, N,N‐dimethylacetamide, N,N‐dimethylformamide and dimethyl sulfoxide. From differential scanning calorimetry thermograms, the polymers showed glass‐transition temperatures between 261 and 315 °C. Thermal behaviour of the polymers obtained was characterized by thermogravimetric analysis, and the 10 % weight loss temperatures of the poly(ester‐imide)s was in the range 449–491 °C in nitrogen. Furthermore, crystallinity of the polymers was estimated by means of wide‐angle X‐ray diffraction. The resultant poly(ester‐imide)s exhibited nearly an amorphous nature, except poly(ester‐imide)s derived from hydroquinone and 4,4′‐dihydroxybiphenyl. In general, polymers containing binaphthyl units showed higher thermal stability but lower crystallinity than polymers containing biphenyl units. Copyright © 2005 Society of Chemical Industry  相似文献   
3.
α,β‐Unsaturated nitriles have been synthesized by Knoevenagel condensation of a carbonylic compound with malononitrile, assisted by sonochemical irradiation. Two alkaline‐promoted clays (Li+‐ and Cs+‐exchanged saponites) have been employed as catalysts. The influence of the carbonylic compound (benzaldehyde or cyclohexanone) and the use of a solvent on the catalytic activity have been studied. Remarkable increase in the conversion values has been found when the reaction is activated by ultrasound, as compared with the thermal activation. In this green, solvent‐free procedure, α,β‐unsaturated nitriles have been produced in very high yields (97%) when the Cs+‐saponite is used as catalyst. Copyright © 2004 Society of Chemical Industry  相似文献   
4.
尿素醇解合成碳酸酯类化合物技术进展   总被引:12,自引:1,他引:11  
综述了用尿素醇解合成碳酸酯类化合物的技术进展 ,重点介绍尿素醇解合成碳酸二甲酯研究现状以及与其它工艺路线的比较 ,另外也介绍了尿素醇解合成长碳链脂肪族和芳香族类碳酸酯的研究进展。  相似文献   
5.
Synthesis of oleyl oleate as a jojoba oil analog   总被引:2,自引:0,他引:2  
Synthesis of a wax ester analog of jojoba oil was accomplished from oleic acid and oleyl alcohol with a zeolite as catalyst. A full 23 factorial design at two levels has been used in the synthesis. The variables selected were temperature, reduced pressure and initial catalyst concentration. The most important variable within the range studied was temperature. Reduced pressure had a negative influence, and initial catalyst concentration showed a positive influence on the process. A response equation has been determined for the yield of ester. The properties of the synthesized product are similar to those of natural jojoba oil.  相似文献   
6.
A dispersed flow model previously developed to study substrate utilization in unsaturated media was experimentally verified and its practical application was considered. For this purpose, measurements were made using tap water and a synthetic feed solution. The importance of the change in fluid regime as regards to the simultaneous transport and reaction within biological filters were demonstrated. The effect of drop formation and the breakage of liquid jets inside the filters on substrate utilization was also shown.  相似文献   
7.
以椰油酸甲酯、单乙醇胺,P2O5等为原料制备一种新型阴离子表面活性剂-烷醇酰胺磷酸酯盐。初步探讨了工艺条件,测定了其物化性能。  相似文献   
8.
Statistical studies of branched ester latex and paint properties   总被引:3,自引:0,他引:3  
Vinyl versatate, butyl acrylate and 2-ethylhexyl acrylate were compared as comonomer, for colloid-stabilized vinyl acetate latex polymers to assess their impact on latex coating properties. Vinyl versatate afforded advantages in scrub resistance, gloss, hydrophobicity and higher glass transition temperatures. Butyl acrylate developed better wet adhesion efficiency, gel content and hiding efficiency. 2-Ethylhexyl acrylate offered performance comparable to butyl acrylate in hiding efficiency, lowering copolymer glass transition temperatures, and gloss development. Combinations of these monomers, in many instances, afforded a better balance of performance properties than individual monomers alone. In the subject study, a simplex-centroid design was utilized to statistically map polymer compositions for determining the effect of comonomer composition on latex and paint film properties. In addition to developing contour maps of polymer and paint properties as a function of composition, a technique for studying structure-property relationships is presented.  相似文献   
9.
Racemic amino acid esters were optically resolved via hydrolysis in organic solvents by the catalysis of an industrial alkaline protease, “Alcalase”. The products which were composed mainly of L-amino acids were insoluble and easily separated by filtration. The activity of the enzyme and enantiomeric excess of the products were significantly dependent on the nature of solvent and the water content in the reaction media. Generally, high values of enantiomeric excess were obtained at low water contents. Many natural and unnatural amino acids were resolved by this method.  相似文献   
10.
Lutein and lutein esters in marigold flowers was quantitatively determined by high performance chromatography (HPLC) with ODS-C18 column. A mixture of CH3CN-CH3OH-CH3COOCH2CH2 with volume ratio of 55:1:44 was used as mobile phase at a flow rate of 1.0mL/min and detection was carried out at 480 nm. The column temperature was about 20℃. The contents of lutein and lutein esters were determined by analytical curve of lutein since lutein and lutein esters have the same spectral characteristics. Determination results of hexane extracts and saponified samples of lutein show that the saponification transforms the esters into free lutein. The increase of the content of dipalmitate and palmitate stearate reveals that the reaction includes transesterifications.  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