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   19370篇
  免费   3336篇
  国内免费   2322篇
化学   13670篇
晶体学   181篇
力学   1128篇
综合类   121篇
数学   2731篇
物理学   7197篇
  2024年   59篇
  2023年   418篇
  2022年   548篇
  2021年   655篇
  2020年   849篇
  2019年   769篇
  2018年   674篇
  2017年   631篇
  2016年   966篇
  2015年   907篇
  2014年   1059篇
  2013年   1440篇
  2012年   1681篇
  2011年   1764篇
  2010年   1240篇
  2009年   1106篇
  2008年   1258篇
  2007年   1111篇
  2006年   985篇
  2005年   890篇
  2004年   708篇
  2003年   622篇
  2002年   688篇
  2001年   544篇
  2000年   447篇
  1999年   436篇
  1998年   343篇
  1997年   314篇
  1996年   336篇
  1995年   295篇
  1994年   236篇
  1993年   169篇
  1992年   166篇
  1991年   158篇
  1990年   125篇
  1989年   100篇
  1988年   57篇
  1987年   56篇
  1986年   71篇
  1985年   48篇
  1984年   32篇
  1983年   27篇
  1982年   15篇
  1981年   11篇
  1980年   4篇
  1976年   1篇
  1975年   2篇
  1974年   1篇
  1969年   1篇
  1957年   4篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
861.
Two polymorphic structures have been well determined in a valence tautomeric (VT) dinuclear cobalt complex. These polymorphs showed distinct thermal‐ and photomagnetic behavior, and are thus ideal for studying the “pure” intermolecular factors to VT transitions. In polymorph 1A , the VT cations are arranged head‐to‐waist with their neighbors and exhibit weak π???π interactions, resulting in a gradual and incomplete thermal VT transition. In contrast, the cations in polymorph 1B are arranged head‐to‐tail and exhibit relatively strong π???π interactions, leading to an abrupt and complete thermal VT transition with adjustable hysteresis loop at around room temperature. The VT process for both polymorphs can be induced by light, but the light‐excited state of 1B? 2H2O has a higher thermal relaxation temperature than that of 1A? 3H2O.  相似文献   
862.
863.
864.
To simplify the assessment method of soy protein isolate(SPI) functionalities,the viscosity and functionalities of commercial SPI products were studied.Viscosity value(y) increases with increasing concentration(x) and exhibits a highly significant correlation with the exponential equation y = a·ebx.The b values of products are gradually enhanced from dispersion,emulsion and injected to gel type.Products with low b values(<0.2),and high dispersivity were dispersion-type.Products having high b values (>0.4) and gel springiness were gel-type.The other products with centered b value(0.2-0.4),high solubility and emulsifying capacity were emulsion-type.  相似文献   
865.
A novel NH4OAc/CuBr(PPh3)3-induced intermolecular Pummerer-type reaction between free (NH)-indoles and dimethylsulfoxide is first reported, which offers the 3-methylthiomethyl substituted indoles conveniently and efficiently.  相似文献   
866.
The acyclic alkyl aromatic ketones, 2-isobutyl-4-methyl-1-arylpentan-1-one derivatives were designed and synthesized for photochemical investigations. Irradiation of such compounds in acetonitrile solution led to the Yang cyclization and Norrish type II cleavage photoproducts with a ratio of 1:1, whereas the reaction conducted in the solid state using the ionic chiral auxiliary method led to only the Yang cyclization product with as high as 99% ee. Furthermore, the conformational transitions were first observed in the reaction.  相似文献   
867.
A hexagonal porphyrin‐based porous organic polymer, namely, CPF‐1, was constructed by 3+2 ketoenamine condensation of the C2‐symmetric porphyrin diamine 5,15‐bis(4‐aminophenyl)‐10,20‐diphenylporphyrin and 1,3,5‐triformylphloroglucinol. This material exhibits permanent porosity and excellent thermal and chemical stability. CPF‐1 can be employed as a superior supporting substrate to immobilize Au nanoparticles (NPs) as a result of the strong interactions between Au NPs and the CPF support. An Au@CPF‐1 hybrid was synthesized by an interfacial solution infiltration method with NaBH4 as reducing agent. Au NPs (5 nm) grew on CPF‐1 and were distributed without aggregation. Moreover, Au@CPF‐1 exhibits superior catalytic activity compared to many other reported Au‐based catalysts for the reduction of 4‐nitrophenol in the presence of NaBH4. In addition, Au@CPF‐1 has excellent stability and recyclability, and it can be reused for three successive reaction cycles without loss of activity. The dense distribution of phenyl rings on the channel walls of the CPF support can reasonably be regarded as the active sites that adsorb the 4‐nitrophenol molecule through hydrogen‐bonding and C?H ??? π interactions, as was confirmed by the X‐ray structure of model compound DAPP‐Benz.  相似文献   
868.
Li  Chuanjiang  Chen  Liangming  Guo  Yanning  Ma  Guangfu 《Nonlinear dynamics》2018,91(2):1307-1320
Nonlinear Dynamics - This paper studies the formation–containment control problem for networked nonlinear Euler–Lagrange systems with input saturation. To realize the concurrency of...  相似文献   
869.
870.
Guo Bin  Qiu Xiao-Ming 《Optik》2012,123(15):1390-1392
An approach of analytical calculation for photonic band gap structure (PBGS) of one-dimensional periodic media, such as non-uniform distribution plasma photonic crystal (PPC), is given on the basis of the recently reported differential transfer matrix method. The dispersion equation of electromagnetic wave propagating in one dimensional non-uniform distribution PPC is derived and the PBGSs for several numerical cases, such as linear distribution, exponential distribution and Epstein distribution, are calculated and discussed.  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