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   2394篇
  免费   260篇
  国内免费   252篇
化学   2185篇
晶体学   40篇
力学   18篇
综合类   13篇
数学   4篇
物理学   646篇
  2024年   3篇
  2023年   24篇
  2022年   39篇
  2021年   59篇
  2020年   100篇
  2019年   72篇
  2018年   64篇
  2017年   64篇
  2016年   104篇
  2015年   97篇
  2014年   116篇
  2013年   221篇
  2012年   131篇
  2011年   137篇
  2010年   102篇
  2009年   150篇
  2008年   145篇
  2007年   149篇
  2006年   181篇
  2005年   128篇
  2004年   124篇
  2003年   125篇
  2002年   89篇
  2001年   58篇
  2000年   72篇
  1999年   63篇
  1998年   52篇
  1997年   43篇
  1996年   39篇
  1995年   40篇
  1994年   19篇
  1993年   19篇
  1992年   23篇
  1991年   7篇
  1990年   7篇
  1989年   5篇
  1988年   8篇
  1987年   9篇
  1986年   3篇
  1985年   4篇
  1984年   1篇
  1983年   1篇
  1982年   3篇
  1980年   2篇
  1979年   2篇
  1977年   1篇
  1975年   1篇
排序方式: 共有2906条查询结果,搜索用时 15 毫秒
21.
22.
电化学超级电容器电极材料的研究进展   总被引:9,自引:0,他引:9  
苗小丽  邓正华 《合成化学》2002,10(2):106-109,119
回顾了电化学超级电容器电极材料的研究进展,并对不同电极材料的储能原理和性能特点进行了简要的阐述。参考文献29篇。  相似文献   
23.
由微乳凝胶法制备了一系列不同MoO3含量的MoO3/SiO2纳米复合氧化物,运用DSC-TG,TEM,XRD和FT-IR等技术对其进行了相应表征.结果表明:所制备复合氧化物适宜的焙烧温度为300℃;样品粒径分布在20~24 nm;样品中MoO3在SiO2表面分散良好,其单层分散阈值在12~12.5%之间;表面Mo与Si原子以Mo—O—Si键联接.  相似文献   
24.
It has been recently proved that RuO2 can act as an effective surface activator of aluminum alloy sacrificial anodes. TiO2 has the property of stabilizing RuO2 coating and resisting biofouling on metal surfaces. Hence, a mixed oxide catalytic coating of TiO2 and RuO2 can enhance the galvanic performance of aluminum alloy sacrificial anodes and resists biofouling on the anode surface. In the present work RuO2–TiO2 mixed oxide was coated on aluminum alloy sacrificial anodes. The large and uniform porous nature of the coating was found to facilitate efficient ion diffusion. The coating was found to persist on the anode even after 3 months of galvanic exposure. The anode having an optimum combination of the mixed oxide had 70% TiO2 as the major component in the coating. The catalytic coating significantly improved the performance of the anodes to a large extent.  相似文献   
25.
Two mixed oxide systems La2-xSrxCuO λ (0.0⩽x⩽1. 0) and La2-xThxCuO λ (O. O⩽x⩽ 0.4) with K2NiF4 structure were prepared by varyingx values. Their crystal structures were studied by means of XRD and IR spectra. The average valence of Cu ion at B site, nonstoichiometric oxygen (λ) and the chemical composition in the bulk and on the surface of the catalysts were measured by means of chemical analysis and XPS. The catalytic behavior in reaction CO+NO was investigated under the regular change of average valence of Cu ion at B site and nonstoichiometric oxygen (λ). Meanwhile, the adsorption and activation of the small molecules NO and the mixture of NO+CO over the mixed oxide catalysts were studied by means of MS-TPD. The catalytic mechanism of reaction NO+CO over these oxide catalysts were proposed; and it has been found that, at lower temperatures the activation of NO is the rate determining step and the catalytic activity is related to the lower valent metallic ion and its concentration, while at higher temperatures the adsorption of NO is the rate determining step and the catalytic activity is related to the oxygen vacancy and its concentration. Project supported by the National Natural Science Foundation of China.  相似文献   
26.
采用共沉淀和水热法合成了不同阴离子粘土前驱型复合氧化物催化材料,样品经XRD,FTIR,TPR,TG-DSC,SEM进行表征.同时考察了铜含量在掺杂和不掺杂稀土时对甲烷催化燃烧的影响,结果发现随含铜量增加催化活性增加;未掺杂La的催化剂在高温下出现失活现象,而掺杂型催化剂稳定性良好.同时,制备方法对催化剂的性能有显著影响,在不同制备前驱物pH的条件下,使用共沉淀和水热法合成的阴离子粘土材料,合成时的pH均对催化剂的活性影响较大;同时焙烧温度对催化活性有不同程度的影响.  相似文献   
27.
研究了钙钛矿型LaSrCoCu_xO_3催化剂对CO氧化反应的催化活性及其表面氧的催化作用.结果表明,x=0.4的催化剂对CO氧化具有最高催化活性,常压及本实验条件下CO完全氧化的最低温度为168℃;催化剂均为氧缺陷化合物,吸附于表面晶格氧缺陷上的吸附氧是CO氧化反应的活性氧物种;并发现催化剂中存在有非常价态的C04 ,认为CO氧化反应是通过吸附氧调变Co3 和Co4 价态而进行.  相似文献   
28.
Fumed silica, silica gel, silica-alumina and cross-linked (5.5%) polystyrene have been functionalized with quaternary ammonium groups and the Chini cluster [Pt12(CO)24]2− has been anchored onto these functionalized materials by ion pairing. A catalyst has also been prepared by the adsorption of Na2[Pt12(CO)24] on unfunctionalized fumed silica. The catalytic activities of the resultant materials, and that of commercially purchased 5% platinum on alumina have been studied for the hydrogenation of a variety of unsaturated compounds. The substrates studied are: α-acetamidocinnamic acid, cyclohexanone, acetophenone, methyl pyruvate, ethyl acetoacetate, nitrobenzene and benzonitrile. Compared to the polystyrene supported catalyst, the inorganic oxide supported catalysts have higher surface areas and for most of the substrates have notably higher activities. The functionalized fumed silica-based catalyst gives higher conversions than functionalized silica gel and silica-alumina-based catalysts. In the hydrogenation of acetophenone and ethyl acetoacetate, the functionalized fumed silica-based catalyst show superior activity compared to the commercial platinum catalyst, and the catalyst made by conventional adsorption method. In benzonitrile hydrogenation with all the cluster-derived catalysts a hydrazine derivative is selectively formed, but when the commercial platinum catalyst is used benzyl amine is the main product.  相似文献   
29.
A rapid access to carbocyclic nucleosides containing a fused isoxazoline ring is proposed starting from cyclopentadiene. The route involves an hetero Diels-Alder cycloaddition reaction of nitrosocarbonylbenzene followed by a 1,3-dipolar cycloaddition of nitrile oxides, cleavage of the N-O tether and elaboration of the heterocyclic aminols into nucleosides via linear construction of purine and pyrimidine heterocycles.  相似文献   
30.
Magnetic phase transitions of the first and second order were revealed by Mössbauer spectroscopy in nanosystems of - and -ferric oxides and metallic europium subjected to shear stress (240°) under high pressure (20 kbar). For - and -ferric oxide nanoclusters, the Curie (Neel) points decreased to 300 K, whereas for nanostructured europium the Neel point increased from 90 to 100 K. The thermodynamic model of magnetic phase transitions predicting a change in the character of magnetic phase transitions and a decrease (increase) in the critical Neel (Curie) points in nanoclusters was developed. The type of magnetic phase transitions and the change in the critical points were caused by defects in nanoclusters, whose maximum concentration was observed for the clusters with the 20—50 nm size range.  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