首页 | 官方网站   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   2176篇
  免费   108篇
  国内免费   7篇
数理化   2291篇
  2024年   4篇
  2023年   35篇
  2022年   74篇
  2021年   84篇
  2020年   93篇
  2019年   67篇
  2018年   35篇
  2017年   42篇
  2016年   84篇
  2015年   92篇
  2014年   86篇
  2013年   123篇
  2012年   161篇
  2011年   199篇
  2010年   111篇
  2009年   90篇
  2008年   141篇
  2007年   118篇
  2006年   105篇
  2005年   86篇
  2004年   80篇
  2003年   56篇
  2002年   41篇
  2001年   18篇
  2000年   11篇
  1999年   12篇
  1998年   10篇
  1997年   7篇
  1996年   15篇
  1995年   6篇
  1994年   11篇
  1993年   14篇
  1992年   7篇
  1991年   10篇
  1990年   7篇
  1988年   10篇
  1987年   5篇
  1986年   4篇
  1985年   14篇
  1984年   11篇
  1982年   18篇
  1981年   8篇
  1980年   7篇
  1979年   11篇
  1978年   7篇
  1977年   6篇
  1976年   14篇
  1975年   5篇
  1974年   5篇
  1973年   9篇
排序方式: 共有2291条查询结果,搜索用时 10 毫秒
1.
A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards.  相似文献   
2.
Franz  H.  Asthalter  T.  Dommach  M.  Ehnes  A.  Messel  K.  Sergueev  I. 《Hyperfine Interactions》2002,141(1-4):131-134
The present status of the new nuclear resonance beamline PETRA 1 at HASYLAB, DESY, Hamburg is described. Besides an overview of the experimental setup some examples of recent experiments are given. Those cover the main applications, i.e., inelastic scattering from iron alloys and quasielastic scattering from glass-forming liquids. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
3.
The theory for the calculation of the frequency-dependent hyperpolarizabilities β(?2ω; 0, ω), β(?ω; 0, ω), and β(0; ω, ?ω) is discussed. New relations between these tensors are derived for those wave functions that obey the time-dependent Hellmann–Feynman theorem (e.g., the self-consistent field [SCF] or the exact wave function). Using second-order Møller–Plesset perturbation theory (MP2), expressions are obtained for the hyperpolarizabilities in terms of derivatives of appropriately defined linear polarizability tensors with respect to a static electric field. Results are presented for ammonia and formaldehyde for the optical Kerr effect and for secondharmonic generation. These results indicate that it is desirable to determine the frequency-dependent contribution to the hyperopolarizability at the MP2 rather than the SCF level of theory, in cases where the static hyperpolarizability has a large contribution from electron correlation and/or where the frequency-dependent contribution may be more significant, such as for secondharmonic generation.  相似文献   
4.
Light‐emitting diodes based on organic materials [organic light‐emitting diodes (OLEDs)] have attracted much interest over the past decade. Several different attempts have been made to realize multicolor OLEDs. This article describes a new approach based on energy transfer in a donor/acceptor system. A copolymer containing both donor and acceptor compounds as comonomer units is prepared. The polymer consists of a derivative of a luminescent dye [4‐dicyanmethylene‐2‐methyl‐6‐4H‐pyran (DCM); acceptor compound], which is copolymerized with fluorene (donor compound) to combine the properties of an electroactive polymer with a highly luminescent dye. Photochemical processing is achieved by UV irradiation of this copolymer in the presence of gaseous trialkylsilanes. This reagent selectively saturates the C?C bonds in the DCM comonomer units while leaving the fluorene units essentially unaffected. As a result of the photochemical process, the red electroluminescence of the acceptor compound vanishes, and the blue‐green electroluminescence from the polyfluorene units is recovered. Compared with previous approaches based on polymer blends, this copolymer approach avoids problems associated with phase‐separation phenomena in the active layer of OLEDs. © 2006Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4317–4327, 2006  相似文献   
5.
6.
7.
Mononuclear (Me3TACN)MnX3 compounds, where X is Cl, Br, or N3, and Me3TACN is 1,4,7-N,N′,N″-trimethyl-1,4,7-triazacyclononane, have been tested for catalyzing both sulfide oxygenation and styrene epoxidation by tert-butyl hydroperoxide (TBHP) and display turnover frequencies (TOF) up to 200 h−1 at room temperature. Sulfoxides or sulfones may be produced selectively by varying reaction conditions. Product distribution from the oxygenation reactions of ethyl phenyl sulfide, 2-chloroethyl phenyl sulfide, and styrene is consistent with a mechanism involving an early single-electron transfer (SET) step.  相似文献   
8.
During the condensation of 4-methyl-4-phenyl-3-thiosemicarbazide (3) with 2,3-butanedione an unexpected by-product, 8a-methyl-6,8a-dihydro-8-methyl-2-[N-methyl-N-phenylamino]-5H-1,3,4-thiadiazolo[3,2-d][1,2,4]triazine-5-thione, (5) was obtained in addition to the desired bis(thiosemicarbazone) (4). An X-ray crystal structure was obtained and is presented together with a proposed mechanism for its formation. A brief investigation of the coordination chemistry of bicycle (5) was carried out. The formation of cyclic compounds during the synthesis of analogues with extended backbones is also described.  相似文献   
9.
The photolysis of virgin PVC powder suspended in water, methanol, n-hexane, aqueous NH4OH (30 wt-%), and 0.1 wt-% iodine in methanol and also as dry powder was studied. The mechanism of photolysis of PVC powder has been investigated by using ESR spectroscopy, conductivity titration, gel-permeation chromatography (GPC), and absorption spectroscopy. Photolysis of PVC has been found to occur by a free-radical mechanism. ESR spectroscopy permits a partial identification of several different types of free radicals in PVC such as alkyl, polyenyl, and peroxy radicals. An interpretation is proposed of the mechanism of formation of conjugated polyene structures, and also a new explanation of the crosslinking mechanism, in which transfer of unpaired electrons to double bonds occurs, is suggested. It has also been found that conjugated double bonds can photosensitize free-radical formation as a result of increased ultraviolet absorption due to polyene structures.  相似文献   
10.
The new thiooligomers of hydroxyphenylene and hydroxymethoxyphenylene sulphides have been used as photo-stabilizers for cis-1,4-polybutadiene. The photo-stabilizing properties of these thiooligomers were compared with commercial sulphur-containing compounds such as bis(4-hydroxyphenylene)sulphide, 2,2′-thiobis(4-methyl-6-tert.butylphenol) (Irganox 1081), bis((beta-(3,5-di-tert,butyl-4-hydroxybenzyl)-methylcarboxyethyl))sulphide (Irganox 1035) and dilaurylthiodipropionate (Irganox PS800). The photo-stabilizing mechanisms of these compounds have been discussed in detail.  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-23

京公网安备 11010802026262号