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   1364篇
  免费   226篇
  国内免费   290篇
化学   1257篇
晶体学   10篇
力学   48篇
综合类   18篇
数学   169篇
物理学   378篇
  2023年   16篇
  2022年   25篇
  2021年   34篇
  2020年   52篇
  2019年   46篇
  2018年   43篇
  2017年   41篇
  2016年   58篇
  2015年   69篇
  2014年   81篇
  2013年   88篇
  2012年   87篇
  2011年   98篇
  2010年   88篇
  2009年   85篇
  2008年   88篇
  2007年   67篇
  2006年   89篇
  2005年   73篇
  2004年   67篇
  2003年   68篇
  2002年   97篇
  2001年   102篇
  2000年   67篇
  1999年   51篇
  1998年   38篇
  1997年   16篇
  1996年   9篇
  1995年   15篇
  1994年   13篇
  1993年   14篇
  1992年   9篇
  1991年   17篇
  1990年   6篇
  1989年   2篇
  1988年   3篇
  1987年   2篇
  1986年   3篇
  1985年   2篇
  1984年   4篇
  1982年   2篇
  1974年   2篇
  1973年   4篇
  1969年   4篇
  1968年   7篇
  1967年   3篇
  1966年   5篇
  1962年   2篇
  1936年   1篇
  1903年   2篇
排序方式: 共有1880条查询结果,搜索用时 15 毫秒
81.
82.
83.
This paper summarizes the main achievements about the structure–activity relationships of anthocyanidin glycosylation. Anthocyanidin glycosylation is the essential step of anthocyanin biosynthesis and also the prerequisite of the further modifications of anthocyanins, which is jointly characterized by the glycosylation site, the type and number of the glycosyl as well as the glycosidic bond type. It generally enhances the stability, results in the hypsochromic effect and blueing, decreases the bioavailability and anticancer activity, and decreases, increases, or does not change the antioxidant activity of the anthocyanidins or anthocyanins, which is synergetically determined by the glycosylation site and the type and number of the glycosyl. Thereinto, in nature, the blue hues caused by the glycosylation may also be reinforced by the formation of the anthocyanic vacuolar inclusions. This review could provide a reference for the research of the structure-optimizing and function-exploiting of anthocyanins.  相似文献   
84.
Sampling is the most important procedure in arriving at the value of an electronic scrap. Inappropriate sampling makes other procedures, including assaying, valueless, as the sample must really represent the electronic scrap from which it was taken. The aim of this study was to develop the procedure to investigate the composition of heterogeneous printed circuit boards (PCBs) material and to show its applicability as well as its related limitations. The contents of Cu, Pb, Ni, and Zn in PCBs from wasted mobile phones were determined. Mixtures of aqua regia, hydrogen peroxide and hydrofluoric acid were applied to sample digestion. Open and closed vessel wet digestion with or without heating and microwave-assisted one was realized. For these purposes different sample amounts (0.2, 0.4, and 0.5 g) were applied and on the basis of calculated relative standard deviation values (RSD), the precision of repeated the most effective digestion was evaluated. The microwave-assisted wet digestion and mixture of aqua regia and hydrogen peroxide was found as the most effective one of the compared mixtures and applied conditions. A sample amount of 0.5 g was found as the most suitable. RSD values confirm the importance of electronic waste material sample preparation.  相似文献   
85.
采用聚丙烯酰胺(PAM)为软模板,在CuSO4、Na2SO3体系中调控合成了片状、海胆状、一维线、针状聚集体氧化亚铜晶体。研究了不同浓度Cu2+、不同PAM含量及不同温度对晶体生长的影响。结果表明,PAM促使氧化亚铜异相成核,氧化亚铜形貌随PAM质量分数的变化而变化:当c(Cu2+)=0.5mol.L-1时,PAM质量分数为3.0%(常温引入体系),产物主要为片状晶体;PAM质量分数为1.0%、3.0%时(高温引入体系),分别获得一维线状、海胆状晶体;当Cu2+浓度为0.1 mol.L-1时,体系中高温引入质量分数3.0%的PAM,得到片层状晶体;当Cu2+浓度为0.3、0.4 mol.L-1时,得到针状聚集体氧化亚铜晶体。分别用X射线粉末衍射、冷场发射扫描电子显微镜和激光粒度仪对所制备的产品进行了表征。  相似文献   
86.
The chiral separations of four pharmaceutical racemates which contain N-alkyl groups were satisfactorily resolved using SBE-β-CD as a chiral mobile phase additive(CMPA)in a RP-HPLC system(the resolution is 2.701 for ondansetron hydrochloride,1.996 for sulpiride,1.293 for clenbuterol hydrochloride and 0.816 for omeprazole).In addition,the effects of different parameters such as CD type and CD concentration were investigated.The separation mechanism arises through the combination of several potential interactions,including electrostatic interactions as well as hydrogen bonding interactions and hydrophobic inclusion interactions,which allow for the SBE-β-CD–drug complexation with strong stereoselectivity and stability.The resolution also relates to the number and location of N atoms in the enantiomers.This method will be applicable to the isolation of various types of biologically important enantiomers containing N-alkyl groups.  相似文献   
87.
以N,N-二甲基-1,3-丙二胺、尿素及1,4-二氯丁烷为原料,通过共缩聚反应两步法制备了一种新的主链型的水溶性聚季铵盐.采用正交试验获得了制备聚季铵盐的最佳条件:N,N-二甲基-1,3-丙二胺与尿素的物质的量比为2∶1,反应温度为135℃,反应时间为11h,得到中间体双[3-(N,N-二甲基丙胺基)]丙脲,产率高达99%;中间体与1,4-二氯丁烷的物质的量比为1∶1,于90℃下反应6h.采用红外光谱和核磁氢谱对产物结构进行表征,测定了其热稳定性和特性粘数.  相似文献   
88.
The self‐assembly of 4 ‐ MTPP [ 4 ‐ MTPP = 2‐(methylthio)‐4‐(pyridin‐4‐yl)pyrimidine] with Cu(NO3)2 and AgNO3 was structurally investigated. For Cu(NO3)2, a discrete mononuclear CuII coordination compound, [Cu( 4 ‐ MTPP )2(NO3)2] ( 1 ), resulted that is exclusively based on Cu–N coordination. For AgNO3, a unique one‐dimensional double‐chain structure ( 2 ) was obtained with the Ag–N distances varying from 2.181(9) to 2.223(9) Å, and the average Ag–S distance being 2.98 Å. Compared to zero‐dimensional 1 , the extension to one‐dimensional 2 is considered to result from the specific affinity between Ag+ and the ligand 4 ‐ MTPP that is attributed to the strong coordinating tendency of silver for aromatic nitrogen and thioether sulfur atoms.  相似文献   
89.
Nanometer‐sized flakes of MnV2O6 were synthesized by a hydrothermal method. No surfactant, expensive metal salt, or alkali reagent was used. These MnV2O6 nanoflakes present a high discharge capacity of 768 mA h g?1 at 200 mA g?1, good rate capacity, and excellent cycling stability. Further investigation demonstrates that the nanoflake structure and the specific crystal structure make the prepared MnV2O6 a suitable material for lithium‐ion batteries.  相似文献   
90.
The title CdII coordination framework, [Cd(C15H8O5)(H2O)]n or [Cd(bpdc)(H2O)]n [H2bpdc is 2‐(4‐carboxybenzoyl)benzoic acid], has been prepared and characterized using IR spectroscopy, elemental analysis, thermal analysis and single‐crystal X‐ray diffraction. Each CdII centre is six‐coordinated by two O atoms from one 2‐(4‐carboxylatobenzoyl)benzoate (bpdc2−) ligand in chelating mode, three O‐donor atoms from three other bpdc2− anions and one O atom from a coordinated water molecule in an octahedral coordination environment. Two crystallographically equivalent CdII cations are bridged by one O atom of the 2‐carboxylate group of one bpdc2− ligand and by both O atoms of the 4‐carboxylate group of a second bpdc2− ligand to form a binuclear [(Cd)2(O)(OCO)] secondary building unit. Adjacent secondary building units are interlinked to form a one‐dimensional [Cd(OCO)2]n chain. The bpdc2− ligands link these rod‐shaped chains to give rise to a complex two‐dimensional [Cd(bpdc)]n framework with a 4,4‐connected binodal net topology of point symbol {43.62.8}. The compound exhibits a strong fluorescence emission and typical ferroelectric behaviour in the solid state at room temperature.  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