全文获取类型
收费全文 | 3519篇 |
免费 | 121篇 |
国内免费 | 3篇 |
专业分类
化学 | 1902篇 |
晶体学 | 76篇 |
力学 | 112篇 |
数学 | 858篇 |
物理学 | 695篇 |
出版年
2023年 | 24篇 |
2022年 | 43篇 |
2021年 | 69篇 |
2020年 | 113篇 |
2019年 | 106篇 |
2018年 | 188篇 |
2017年 | 162篇 |
2016年 | 211篇 |
2015年 | 134篇 |
2014年 | 198篇 |
2013年 | 463篇 |
2012年 | 227篇 |
2011年 | 239篇 |
2010年 | 214篇 |
2009年 | 161篇 |
2008年 | 219篇 |
2007年 | 196篇 |
2006年 | 102篇 |
2005年 | 60篇 |
2004年 | 52篇 |
2003年 | 53篇 |
2002年 | 49篇 |
2001年 | 46篇 |
2000年 | 30篇 |
1999年 | 20篇 |
1998年 | 28篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1991年 | 6篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1986年 | 6篇 |
1985年 | 14篇 |
1984年 | 20篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1970年 | 4篇 |
1969年 | 5篇 |
1968年 | 4篇 |
1967年 | 5篇 |
排序方式: 共有3643条查询结果,搜索用时 187 毫秒
81.
82.
83.
A distance based measure of dependence is proposed for stable distributions that completely characterizes independence for a bivariate stable distribution. Properties of this measure are analyzed, and contrasted with the covariation and co-difference. A sample analog of the measure is defined and demonstrated on simulated and real data, including time series and distributions in the domain of attraction of a stable law. 相似文献
84.
Transfer hydrogenation of aryl ketones with homogeneous ruthenium catalysts containing diazafluorene ligands
下载免费PDF全文
![点击此处可从《应用有机金属化学》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Novel cationic ruthenium(II) complexes bearing a 4,5‐diazafluorene unit and p‐cymene as ligands have been synthesised. The complexes were characterised based on elemental analysis and Fourier transform infrared and nuclear magnetic resonance spectroscopies. The synthesised Ru(II) complexes were employed as pre‐catalysts for the transfer hydrogenation of aromatic ketones using 2‐propanol as both hydrogen source and solvent in the presence of NaOH. All complexes showed high catalytic activity as catalysts in the reduction of substituted acetophenones to corresponding secondary alcohols. The products of catalysis were obtained with conversion rates of between 80 and 99%. Among the seven new complexes investigated, the most efficient catalyst showed turnover frequencies in the range 255–291 h?1 corresponding to 85 to 97% conversion, respectively. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
85.
86.
87.
Hasan Yıldırım 《Physics letters. A》2019,383(12):1324-1329
InxGaN/ZnSnN2 quantum well structures are studied in terms of a binding energy of a donor atom. 1s and impurity states are considered. The Schrödinger's and Poisson's equations are solved self-consistently. A hydrogenic type wave function to represent each impurity state is assumed. The calculations include band-bending in the potential energy profile introduced by the built-in electric field existing along the structures. The binding energy and the energy of the transition between the impurity states are represented as a function of the quantum well width, the donor position, and the indium concentration. An external magnetic field up to 10 T is included into the calculations to compute the Zeeman splitting. The maximum value of the transition energy is around 30 meV (nearly 7.3 THz) which occurs in a 15-Å In0.3Ga0.7N/ZnSnN2 quantum well. Being strong, the built-in electric field makes the transition energy drop quickly with the decreasing well width. For the same reason, the energy curves are found to be highly asymmetric function of the donor position around the well center. Compared to the bulk value, the transition energy in the quantum well structures enhances nearly two-fold. 相似文献
88.
Firstly, poly[phenyl thiadiazole methacrylamide-co-divinylbenzene-co-2-acrylamido-2-methylpropane sulfonic acid] (PTMAAm-co-DVB-co-AMPS), a new polymer resin was synthesized. This polymer resin was characterized by elemental analyzer, X-ray diffractometer, scanning electron microscope (SEM) and IR spectrometer. The glass column packed with the synthesized polymer resin was used for solid phase extraction (SPE). At the same time, the analytes were separated and preconcentrated from various water, dried vegetables samples and standard reference material (CRM) with SPE and determined by flame atomic absorption spectrometer (FAAS). The experimental conditions of this method such as pH, flow rates of sample, flow rates of eluent, type / concentration / volume of eluent, sample volume and matrix ions were examined. The limits of detection (µg L?1) were calculated (3s) 0.9 for Mn(II), 1.4 for Cd(II), Co(II) and Zn(II), 1.5 for Cr(III), 2.2. for Cu(II), 1.9 for Pb(II),1.5 for Ni(II) and 1.9 for Fe(III) (n = 21). The low relative standard deviation, ≤ 2% (n = 11) and preconcentration factor as 75 for analytes were obtained. 相似文献
89.
Bünyamin Ak Duygu Elma Nermin Meriç Cezmi Kayan Uğur Işık Murat Aydemir Feyyaz Durap Akın Baysal 《Tetrahedron: Asymmetry》2013,24(20):1257-1264
A new and versatile class of unsymmetrical ferrocenyl-phosphinite ligands possessing a stereogenic center has been prepared from commercially available, inexpensive aminoacids such as, d-, l-phenylglycine and d-, l-phenylalanine, through a concise synthetic procedure. These ligands are not very sensitive to air and moisture, and display good enantioselectivities in the ruthenium-catalyzed asymmetric transfer hydrogenation of acetophenone derivatives, in which up to 91% ee was obtained. A comparison of the catalytic properties of amino alcohols and other analogues based on a ferrocenyl backbone is also discussed briefly. The structures of these ligands and their corresponding complexes have been elucidated by a combination of multinuclear NMR spectroscopy, IR spectroscopy, and elemental analysis. 相似文献
90.