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   3031篇
  免费   83篇
  国内免费   10篇
化学   2240篇
晶体学   33篇
力学   34篇
数学   159篇
物理学   658篇
  2023年   12篇
  2021年   17篇
  2020年   38篇
  2019年   51篇
  2018年   40篇
  2017年   20篇
  2016年   59篇
  2015年   48篇
  2014年   71篇
  2013年   155篇
  2012年   150篇
  2011年   176篇
  2010年   106篇
  2009年   93篇
  2008年   157篇
  2007年   154篇
  2006年   147篇
  2005年   148篇
  2004年   124篇
  2003年   136篇
  2002年   101篇
  2001年   70篇
  2000年   70篇
  1999年   38篇
  1998年   23篇
  1997年   30篇
  1996年   29篇
  1995年   24篇
  1994年   32篇
  1993年   40篇
  1992年   37篇
  1991年   49篇
  1990年   46篇
  1989年   32篇
  1988年   43篇
  1987年   44篇
  1986年   36篇
  1985年   63篇
  1984年   43篇
  1983年   21篇
  1982年   34篇
  1981年   35篇
  1980年   24篇
  1979年   36篇
  1978年   33篇
  1977年   42篇
  1976年   31篇
  1975年   23篇
  1974年   18篇
  1973年   22篇
排序方式: 共有3124条查询结果,搜索用时 15 毫秒
71.
本文提供了一种测定火炸药冲击感度,并根据测定值判定其冲击爆炸危险性的新方法测定水下爆炸气泡能法。通过对30几种炸药的试用,判定的结果与文献报道的用其他方法判定的结果相当好的一致。而且本法的优点还在于同时可得到试样的动、静态威力,操作简便、安全、噪音小。此外,利用本试验法还探讨了炸药形态、惰性介质(水)对冲击感度的影响。这些对于处理炸药时应如何采取安全对策具有指导意义。  相似文献   
72.
Optical Review - To evaluate multi-spectral images of the absorption and scattering properties in the cerebral cortex of rat brain, we investigated spectral reflectance images estimated by the...  相似文献   
73.
Unsaturated nitriles have been synthesized from trisubstituted olefins via ene-type chlorination and regioselective substitution with cyanide ion followed by double bond migration.  相似文献   
74.
Abstract

The epoxidation of allylic alcohols with H2O2 catalyzed by a new Mo-species, [PMo12O40]3?[C5H5N+(CH2)15CH3]3 (MPCP), prepared from 12-molybdatophosphoric acid (H3PMo12O40) and cetylpyridinium chloride [C5H5N+(CH2)15CH3 · Cl?], has recently rep0rted.) The high regioselectivity for the epoxidation by this MPCP-H2O2 system was comparable to that of Sharpless method using t-BuOOH as oxidant.  相似文献   
75.
Optically active aromatic ether ketone macrocycles containing 2,2′-dimethoxy-1,1′-binaphthyl-6,6′-diyl moieties were synthesized through electrophilic aromatic substitution reaction and stepwise nucleophilic aromatic substitution reactions. Structures of the resulting macrocycles were confirmed by NMR and matrix assisted laser desorption/ionization time of flight (MALDI-TOF) mass measurements.  相似文献   
76.
Molydbenum and tungsten-catalyzed oxidations of alcohols with dilute hydrogen peroxide under phase-transfer conditions have been focused in recent years 1–3. In a previous paper, we have reported that tricetylpyridinium-12-molybdophosphate(CMP), [π - C5H5N+(CH2 15CH3]3 (PMO12O40)3-, prepared from 12-molybdophosphoric acid (MPA) and cetylpyridinium chloride (CPC), catalyzed the oxidation of secondary alcohols to carbonyl compounds with tert-butylhydroperoxide(t-BuOOH)4. When hydrogen peroxide in place of t-BuOOH was employed as the oxidant for the above oxidation, however, alcohols were oxidized with difficulty to give carbonyl compounds in poor yields. In continuation of this study, we wish to report here the selective oxidation of secondary hydroxy group of alcohols and diols and diols with H2O2 under the influence to tricetylpyridinium-12 tungstophosphate (CWP), [π-C5H5N+ (CH2) 15 CH3]3(PW12O40)3-, derived from 12-tungstophosporic acid (WPA) and CPC.  相似文献   
77.
Head-to-head-type styrene and substituted styrene dimers bearing two fluoroalkyl end-groups have been efficiently synthesized by a simple reaction of perfluoroalkyl iodide with styrene under radical conditions as a mixture of meso and racemic forms. The meso form obtained from the mixture by recrystallization gave a crystal suitable for X-ray diffraction study and the crystal structure was found to be based on π-stacking of benzene rings and aggregation of fluoroalkyl chains. Dynamic light scattering measurements showed that meso-styrene dimers bearing two fluoroalkyl end-groups can form the nanometer size-controlled self-assemblies through the intermolecular π-stacking of benzene rings and aggregation of end-capped fluoroalkyl groups in methanol.
Figure
Self-assembled meso-perfluorohexylated styrene dimer [C6F13–CH2CHPh–CHPh–CH2–C6F13] based on π-stacking of benzene rings and aggregation of fluoroalkyl chains: Fluorous domains are constructed by self-assembly of fluoroalkyl chains.  相似文献   
78.
Fluoro(silyl)acetylenes, which were prepared by reaction of 1,1-difluoroethylene with silyl chlorides, reacted with triethylamine to give dark-brown colored polyfluoro(silyl)acetylene powders in excellent to moderate isolated yields. In contrast, the corresponding nonfluorinated acetylene was unable to react with triethylamine at all to afford poly(silyl)acetylene under similar conditions. Polyfluoro(silyl)acetylenes thus obtained were nanometer size-controlled cubic fine particles with a good dispersibility and stability in a variety of solvents. These polyfluoro(silyl)acetylene nanoparticles exhibited clear absorption and emission spectra related to the conjugated units in polymer main chain. Furthermore, these polyfluoro(silyl)acetylene nanoparticles were applied to the surface modification of poly(methyl methacrylate) [PMMA] film to exhibit a higher oleophobicity imparted by fluorine on their surface, compared to that on the reverse side.
Figure
New polyfluoro(silyl)acetylenes were prepared by reaction of the creesponding fluoro(silyl)acetylenes with triethylamine in excellent to moderate isolated yields. In contrast, the corresponding nonfluorinated silylacetylene was unable to give poly(silyl)acetylene under similar conditions. These polyfluoro(silyl)acetylenes thus obtained can form the nanometer size-controlled cubic fine particles (within 100 nm) in a variety of solvents.  相似文献   
79.
Abstract

The deformation electron-density distributions of 6a-thia-1,3,4,6-tetraazapentalene derivatives (I, II) and bis(phenylthio)dibenzothiophene (III) were investigated by the X-ray diffraction method. For I and II, a structure around the hypervalent S-N bonds is a trigonal bipyramid with equatorial sp2 hybrid of S-C bond and lone-pair electrons and apical polarized S-N bonds. In 111 lone-pair electron densities are observed perpendicular to the thiophene ring and the C-S-C sulfide planes.  相似文献   
80.
In this study, we combined a column‐switching system with a fluorous scavenging derivatization method to develop a fully automated reagent peak‐free LC fluorescence detection protocol for the analysis of highly polar carboxylic acids. In this method, highly polar carboxylic acids were derivatized with fluorescent 1‐pyrenemethylamine in the presence of 1‐ethyl‐3‐(3‐dimethylaminopropyl)carbodiimide and 1‐hydroxy‐1H‐benzotriazole. Residual excess of the unreacted reagent was tagged with 2‐(perfluorooctyl)ethyl isocyanate and then removed selectively using a fluorous column‐switching system placed in front of an analytical reversed‐phase column. The signal of the fluorous‐tagged unreacted reagent was completely absent in the resulting chromatograms; therefore, it did not interfere with the quantification of each acid especially those eluted before 20 min. The detection limits (S/N = 3) for the examined acids were in the range from 4.0 to 22 fmol per injection. We have applied this method to comparative analysis of highly polar carboxylic acids in urine samples obtained from diabetes mellitus type‐II model mice and their control.  相似文献   
[首页] « 上一页 [3] [4] [5] [6] [7] 8 [9] [10] [11] [12] [13] 下一页 » 末  页»
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