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   1545篇
  免费   50篇
  国内免费   2篇
化学   1056篇
晶体学   20篇
力学   64篇
数学   137篇
物理学   320篇
  2024年   11篇
  2023年   22篇
  2022年   52篇
  2021年   33篇
  2020年   52篇
  2019年   54篇
  2018年   53篇
  2017年   43篇
  2016年   76篇
  2015年   39篇
  2014年   65篇
  2013年   131篇
  2012年   141篇
  2011年   163篇
  2010年   89篇
  2009年   63篇
  2008年   89篇
  2007年   68篇
  2006年   62篇
  2005年   56篇
  2004年   36篇
  2003年   24篇
  2002年   32篇
  2001年   9篇
  2000年   10篇
  1999年   5篇
  1997年   9篇
  1996年   7篇
  1995年   4篇
  1994年   6篇
  1993年   3篇
  1992年   2篇
  1991年   4篇
  1990年   3篇
  1988年   2篇
  1986年   7篇
  1985年   6篇
  1984年   5篇
  1983年   4篇
  1982年   5篇
  1981年   10篇
  1980年   2篇
  1979年   3篇
  1978年   4篇
  1977年   3篇
  1976年   7篇
  1975年   4篇
  1974年   3篇
  1973年   6篇
  1971年   2篇
排序方式: 共有1597条查询结果,搜索用时 0 毫秒
1.
2.
We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles (θH2O≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH3+, SiOH+ and COOH. We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates.  相似文献   
3.
The Schiff base [1,2-bis(salicylidene amino)phenylene]cobalt(II) complex, chemically bonded to a carbamate-modified silica gel catalyst, has been prepared by a four step procedure. The oxidation of cyclohexane was studied in the presence of this catalyst under relatively mild conditions (150–200 °C, 15–20 atm) using molecular oxygen. The catalyst was found to be very selective for the production of cyclohexanol, with cyclohexanone formed in only a small amount (45:1). This is in contrast to the commercially available processes in which cyclohexanol and cyclohexanone are both formed in appreciable amounts. The t.g.a. analysis shows the catalyst to be stable up to 211 °C and atomic absorption spectroscopy indicated negligible metal loss during 50 h use of the catalyst up to 180 °C.  相似文献   
4.
1R,2S,5R,8R; 1R,2S,5R,8S; 1S,2S,5R,8R; and 1S,2S,5R,8S-Iridodials have been prepared in five steps from 4aS,7S,7aR and 4aS,7S,7aS-nepetalactones, major components of catnip oil. 1R,2S,5R,8R-Iridodial has been identified as a male-produced male-aggregation pheromone for Chrysopa oculata, the first pheromone of any kind identified for lacewings.  相似文献   
5.
New mononuclear and dinuclear complexes [3-hydroxyethyl-1,3,5,8,11pentaazacyclotridecane]copper(II) (1)/nickel(II) (2) perchlorate and O,O ethane bridged bis-copper(II) (3)/nickel(II) (4) macrocycles have been synthesized and characterized by various spectroscopic techniques, viz. i.r., n.m.r., e.p.r., u.v.–vis. and conductance measurements. Spectral data and conductance measurements reveal that all the complexes are consistent with square-planar geometry and are ionic in nature. The catalytic activity of the dinuclear Cu(II) complex (3) in the presence of pyrocatechol was determined spectrometrically by monitoring the increase of the o-benzoquinone characteristic absorption band at 25,000 cm−1 with respect to time in DMF saturated with molecular oxygen. The kinetic parameters Vmax (2.8×10−3 M s−1) and KM (1.4×10−3 mm) have been determined by Michaelis–Menten method. Electrochemistry of the dinuclear Cu(II) complex has been studied in the presence of molecular oxygen with pyrocatechol and without pyrocatechol at a scan rate of 0.1 V s−1 by cyclic voltammetry. On addition of pyrocatechol, complex shows a shift in Epc, Epa and E1/2 values indicating the oxidation of substrate (pyrocatechol).  相似文献   
6.
A novel method for effecting the aza-Michael reactions of N-alkyl- and N-arylpiperazines with acrylonitrile using Cu-nanoparticles is described. The method features the use of 10 mol % Cu (14-17 nm) nanoparticles under mild reaction conditions to afford the addition products in good to excellent yields. The Cu-nanoparticles selectively catalysed the aza-Michael reaction of N-alkyl- and N-arylpiperazines in the presence of aromatic amino or aliphatic hydroxy groups.  相似文献   
7.
The X-ray structure analysis of bis(8-dimethylamino-l-naphthyl)phenylphosphane (3) and of the corresponding sulphide 4 has revealed hexacoordination at phosphorus in both cases, the N … P separations being less than the sum of the van der Waal radii. Furthermore, in both cases the overall geometry corresponds to a distorted bicapped tetrahedron. The optimum geometry calculated for 4 via the program developed by Autodesk (MM + method) suggests that the structure of the molecule is a function not only of steric requirements but also of electronic effects.  相似文献   
8.
Algebras and Representation Theory - We introduce the concept of a prime band in a string algebra Λ and use it to associate to Λ its finite bridge quiver. Then we introduce a new...  相似文献   
9.
10.
X-band electron spin resonance (ESR) and UV-vis spectra of a homobinuclear [(Bipy)2Cu-E-Im-Cu(Bipy)2](BF4)3 and a heterobinuclear [(Bipy)2Cu-E-Im-Zn(Bipy)2](BF4)3 complexes, E-Im=2-ethylimidazolate ion have been described as possible models for superoxide dismutase (SOD). Magnetic moment and ESR spectral measurements of the homobinuclear complex have shown an antiferromagnetic exchange interaction. From pH-dependent ESR and UV-vis spectral measurements studies, these complexes have been found to be stable over 8.5-10.5 pH range. These complexes catalyze the dismutation of superoxide (O2-) at biological pH. All the observations indicate that these complexes act as good possible models for superoxide dismutase.  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