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   1393篇
  免费   75篇
  国内免费   5篇
化学   1208篇
晶体学   15篇
力学   17篇
数学   53篇
物理学   180篇
  2023年   16篇
  2022年   23篇
  2021年   29篇
  2020年   30篇
  2019年   26篇
  2018年   37篇
  2017年   20篇
  2016年   54篇
  2015年   45篇
  2014年   66篇
  2013年   77篇
  2012年   113篇
  2011年   162篇
  2010年   75篇
  2009年   67篇
  2008年   109篇
  2007年   75篇
  2006年   77篇
  2005年   64篇
  2004年   61篇
  2003年   47篇
  2002年   36篇
  2001年   19篇
  2000年   11篇
  1999年   7篇
  1998年   9篇
  1997年   7篇
  1996年   4篇
  1995年   10篇
  1994年   12篇
  1993年   5篇
  1992年   8篇
  1991年   6篇
  1990年   3篇
  1989年   6篇
  1988年   2篇
  1987年   7篇
  1986年   3篇
  1985年   5篇
  1984年   2篇
  1983年   4篇
  1981年   3篇
  1979年   4篇
  1977年   7篇
  1976年   3篇
  1975年   3篇
  1974年   2篇
  1973年   5篇
  1971年   2篇
  1967年   1篇
排序方式: 共有1473条查询结果,搜索用时 31 毫秒
21.
We study rule induction from two decision tables as a basis of rough set analysis of more than one decision tables. We regard the rule induction process as enumerating minimal conditions satisfied with positive examples but unsatisfied with negative examples and/or with negative decision rules. From this point of view, we show that seven kinds of rule induction are conceivable for a single decision table. We point out that the set of all decision rules from two decision tables can be split in two levels: a first level decision rule is positively supported by a decision table and does not have any conflict with the other decision table and a second level decision rule is positively supported by both decision tables. To each level, we propose rule induction methods based on decision matrices. Through the discussions, we demonstrate that many kinds of rule induction are conceivable.  相似文献   
22.
The thermal phase transition of RbMnFe(CN)6 has been observed by Mn and Fe 3p-1s X-ray emission spectroscopy (XES) and 1s X-ray absorption spectroscopy (XAS). The thermal variations of the spin states and the valences of Mn and Fe were determined to be Mn2+(S=5/2)-NC-Fe3+(S=1/2) for the high-temperature (HT) phase and Mn3+(S=2)-NC-Fe2+(S=0) for the low-temperature (LT) phase. These transitions are thus caused by charge transfer between Mn and Fe. The temperature dependences of Mn and Fe 3p-1s XES and 1s XAS were observed as the composition of the spectra of the HT and LT phases. The ratios of the HT component in each spectrum show good agreement with the thermal transition curves observed with magnetic susceptibility measurements.  相似文献   
23.
In the previous paper we studied the transport coefficients of quark–gluon plasma in finite temperature and finite density in vector and tensor modes. In this paper, we extend it to the scalar modes. We work out the decoupling problem and hydrodynamic analysis for the sound mode in charged AdS black hole and calculate the sound velocity, the charge susceptibility and the electrical conductivity. We find that Einstein relation among the conductivity, the diffusion constant and the susceptibility holds exactly.  相似文献   
24.
A semi-empirical calculation (PM3) was applied to elucidate the anodic oxidation mechanism of N,N-dimethylaniline (DMA) and the dimerization of a cation radical (A) derived from DMA was ruled out. The heat of reaction value of the dimerization of A was 42.43 kcal/mol. We propose the following. Cation radical A reacts with DMA to generate another cation radical (D). This reaction was exothermic and the heat of reaction value and the activation energy were -0.35 kcal/mol and 1.31 kcal/mol, respectively. Deprotonation of D by DMA gives neutral radical (E), which is oxidized to TMB by A. All these reactions were exothermic.  相似文献   
25.
The Mn (IV)(salen)(N 3) 2 complex ( 3) from Jacobsen's catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting Mn (III)(salen)(N 3)(CH 3OH) complex ( 2) are determined in order to investigate the conformation of the high-valent Mn (IV)(salen) molecule in comparison with that of Mn (III)(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N 3 ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH 3CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV-vis, electron paramagnetic resonance, (1)H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry.  相似文献   
26.
27.
The rotational spectrum of the NiS radical in the X(3)Sigma(-) state was observed by employing a source-modulation microwave spectrometer. The NiS radical was generated in a free space cell by a dc glow discharge in H(2)S diluted with Ar. The nickel atoms were supplied by the sputtering reaction from a nickel cathode. Rotational transitions with J = 11-10 to 25-24 were measured in the region between 135 and 314 GHz. Rotational, centrifugal distortion and several fine-structure constants were determined by a least-squares analysis. Other spectroscopic parameters such as dissociation energy, vibrational wavenumber and equilibrium bond length were also derived from the determined molecular constants. Excitation energies of the lowest (3)Pi and (1)Sigma(+) states were estimated from the fine-structure constants, lambda and gamma.  相似文献   
28.
The reaction of allyltitanocenes with five- to seven-membered cyclic enones proceeded with good to high diastereoselectivity depending on the ring size of enones. The stereochemistry of the major isomers produced by the reaction of cinnamyltitanocene was opposite to that of crotyltitanocene.  相似文献   
29.
Y Yan  L Zhang  T Ito  X Qu  Y Asakawa  T Awakawa  I Abe  W Liu 《Organic letters》2012,14(16):4142-4145
We herein report comparative analysis of two versions of the biosynthetic gene clusters of antimycins, a natural product family possessing up to 44 distinct entities. The biosynthetic pathway of antimycins is amenable to the high structural variation of the substrates, supported by successes in heterologous expression of the ant cluster and in fluorine incorporation. The latter facilitated the investigation of the structure-activity relationship into the usually invariable 3-formamidosalicylic acid moiety of the molecules.  相似文献   
30.
Large-scale, self-supporting ultrathin films composed of an elastomeric polyacrylate network interpenetrated by a silica (SiO2) network were synthesized and characterized. The organic network was first photopolymerized and the silica structure was subsequently developed in situ in the preformed organic gel. Composition and morphology of the hybrid interpenetrated network (IPN) nanofilms were characterized by infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy and compared with the case of zirconia (ZrO2) hybrid IPN reported earlier. Young modulus, ultimate tensile strength, and ultimate tensile elongation were determined for different organic/inorganic molar ratios and give some insights on how the composition of the nanofilms influence their robustness and self-supporting properties.  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