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   3170篇
  免费   72篇
  国内免费   19篇
化学   2197篇
晶体学   35篇
力学   34篇
数学   123篇
物理学   872篇
  2022年   23篇
  2020年   29篇
  2019年   44篇
  2018年   21篇
  2017年   24篇
  2016年   50篇
  2015年   53篇
  2014年   62篇
  2013年   141篇
  2012年   144篇
  2011年   158篇
  2010年   94篇
  2009年   93篇
  2008年   165篇
  2007年   143篇
  2006年   169篇
  2005年   126篇
  2004年   147篇
  2003年   120篇
  2002年   108篇
  2001年   87篇
  2000年   67篇
  1999年   45篇
  1998年   30篇
  1997年   34篇
  1996年   49篇
  1995年   49篇
  1994年   34篇
  1993年   46篇
  1992年   55篇
  1991年   41篇
  1990年   32篇
  1989年   36篇
  1988年   42篇
  1987年   35篇
  1986年   24篇
  1985年   72篇
  1984年   61篇
  1983年   40篇
  1982年   48篇
  1981年   41篇
  1980年   36篇
  1979年   38篇
  1978年   39篇
  1977年   36篇
  1976年   34篇
  1975年   29篇
  1974年   17篇
  1973年   29篇
  1966年   17篇
排序方式: 共有3261条查询结果,搜索用时 15 毫秒
91.
The recent assignment of the vibrational spectrum of the quasilinear molecule HCNO revealed several near coincidences between vibrational energy levels involving the two bending modes, ν4 (skeletal bending mode) and ν5 (HCN-bending mode), and the lowest-lying stretching mode, ν3 (NO stretching mode). By considering the correlation between the energy levels of a linear and a bent molecular model of HCNO, it is seen that resonance interactions which would be of third or higher order in a linear molecule Hamiltonian would be of first or second order in the Hamiltonian of a bent molecule, and thus might be significant in the quasilinear molecule HCNO. In this way we were able to identify the type of observed interaction occurring between three pairs of nearly coincident levels, (00010, 00002), (00020, 00012), and (00100, 00004). Anomalous centrifugal distortion effects had been observed and reported earlier for the pure rotational transitions arising from molecules in the 00010, 00020, and 00002 levels. Rotational transitions arising from molecules in the 00004 and 00100 vibrational states of HCNO and the 00100 state of DCNO are reported here for the first time. For two pairs of levels, (00010, 00002) and (00100, 00004), we could determine the magnitude of the coefficients of the interaction matrix elements from an analysis of the centrifugal distortion effects.  相似文献   
92.
M Yamada  Y Murai 《显形杂志》2009,12(2):181-188
With the rapid advance in information technology, the applicability of computers has moved from the scientific field towards simulating human intelligence. We are already familiar with using computers to produce music and art and for language translation. A further use is in understanding traditional man-made products; best exemplified by literary works. In this study, we focus on enabling a computer to visualize the meaning of stories. Four world-famous plays by William Shakespeare have been chosen to demonstrate how the visualization scheme works in grasping the meaning of the stories. The scheme employs primitive keyword detection and ellipsoidal differential equations to create a visual imagery of the story. This methodology ensures uniqueness in the visualization of an individual work. In addition, color palettes obtained from pictures relevant to each story are used to enrich the consistency between the visual sense and the meaning of the story.  相似文献   
93.
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   
94.
A continuous investigation of secondary metabolites produced by the sponge-derived fungus, Gymnascella dankaliensis, has yielded a new polyketide tyrosine derivative, dankastatin C (3) and the known steroid, demethylincisterol A3 (4), which was originally found from a Homaxinella marine sponge. The stereostructure of the new compound has been determined based on the analysis of 1D and 2D NMR data. Dankastatin C (3) showed potent cell growth inhibitory activity against the murine P388 cell line.  相似文献   
95.
96.
The present paper focuses on (i) a new synthetic methodology to prepare vinyl ether-based synthetic glycoconjugates (glycopolymers) with well-controlled structure, and on (ii) the application of glycopolymers bearing modified disaccharide residue as thermotropic liquid crystalline (LC) materials. Two vinyl ethers (VEs) having pendant glucose residues with their hydroxyl functions protected by acetyl and isopropylidene groups, respectively, were found to undergo living cationic polymerization initiated by a HI/ZnI2-initiating system. Deprotection of the resultant monodisperse polymers led to water-soluble polymers bearing a pendant glucose residue. Sequential living block copolymerization of glucose-containing VE and alkyl VE, and subsequent deprotection afforded an amphiphilic block copolymer of well-controlled structure. Transmission electron microscopic observation of its cast thin film revealed microphase-separated surface morphologies that varied with varying segment composition ratio. A VE substituted with a heptadecanoated cellobiose pendant was independently prepared, and was cationically polymerized to give a thermotropic LC polymer. From X-ray analysis, the mesophase was assigned to a discotic columnar in type, in which each main chain, extended due to the steric repulsion between the neighboring bulky pendants, was surrounded by three discotic columns.  相似文献   
97.
98.
A novel chromatographic approach coupling anion-exchange (diethylaminoethylene) and hydrophilic-interaction (amide or zwitterionic type) columns was developed for the separating of 2-pyridylamino derivatives of N-glycans (PA-N-glycans). This is a kind of on-line, two-dimensional (2D) separation approach in hydrophilic-interaction chromatography (called the 2D-HILIC method), analogous to that of coupling cation- (or anion-, or mixed ion-) exchange and reversed-phase columns in hydrophobic interaction (reversed-phase) chromatography. The efficiency of the 2D-HILIC method was tested with biantennary neutral and sialylated PA-N-glycan standards by properly combining linear gradient elutions of water-acetonitrile and spiked-salt (ammonium acetate) elutions. The retention time RSDs of all the peaks in three sequential runs of a 100 min cycle are less than 0.52%, which indicates a reasonably good repeatability of the 2D-HILIC method. Then, the method was applied to a complex mixture of PA-N-glycans from human serum proteins. It was demonstrated that the neutral PA-N-glycans and mono-, di-, tri-, and tetrasialylated PA-N-glycans are able to be eluted in turn according to the number of sialic acids in an automated (programmed) single run.  相似文献   
99.
100.
Parametric x rays (PXR) produced by bombarding silicon and diamond crystals with electrons of 30 to 87 MeV were detected at 180 degrees relative to the direction of the electron beam. It was found that the dependence of the intensity on the orientation of the crystal agrees with the predictions of the kinematical theory of PXR. The absolute intensity is twice as large as predicted. These findings can be explained considering dynamical effects that govern the x-ray crystal interaction. Additionally, x rays caused by self-diffracted transition radiation have been observed.  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