全文获取类型
收费全文 | 598篇 |
免费 | 15篇 |
国内免费 | 6篇 |
专业分类
化学 | 401篇 |
晶体学 | 2篇 |
力学 | 27篇 |
数学 | 105篇 |
物理学 | 84篇 |
出版年
2018年 | 6篇 |
2017年 | 16篇 |
2016年 | 16篇 |
2015年 | 13篇 |
2014年 | 16篇 |
2013年 | 27篇 |
2012年 | 26篇 |
2011年 | 37篇 |
2010年 | 18篇 |
2009年 | 21篇 |
2008年 | 11篇 |
2007年 | 21篇 |
2006年 | 15篇 |
2005年 | 12篇 |
2004年 | 14篇 |
2003年 | 10篇 |
2002年 | 7篇 |
2001年 | 8篇 |
2000年 | 10篇 |
1999年 | 6篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 7篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1985年 | 8篇 |
1939年 | 6篇 |
1938年 | 10篇 |
1937年 | 10篇 |
1934年 | 10篇 |
1932年 | 9篇 |
1931年 | 7篇 |
1930年 | 6篇 |
1929年 | 9篇 |
1928年 | 7篇 |
1927年 | 9篇 |
1926年 | 11篇 |
1925年 | 5篇 |
1923年 | 7篇 |
1922年 | 6篇 |
1920年 | 5篇 |
1919年 | 6篇 |
1916年 | 8篇 |
1914年 | 5篇 |
1913年 | 7篇 |
1912年 | 7篇 |
1911年 | 6篇 |
1910年 | 5篇 |
1909年 | 8篇 |
1908年 | 9篇 |
排序方式: 共有619条查询结果,搜索用时 15 毫秒
121.
Jianghai Wo Anle Wang Jin Zhang Pengfei Du Yalan Wang Wenshan Cong Xin Xu Xiong Luo Lan Yu 《Optical and Quantum Electronics》2017,49(12):411
A wideband tunable optoelectronic oscillator (OEO) based on a dispersion compensated phase modulated microwave photonics filter (MPF) is proposed and experimentally demonstrated. The MPF, consisting of a tunable laser source (TLS), a phase modulator, a fiber Bragg grating Fabry–Perot filter and a photodetector, is used as the oscillating mode selection device. Dual-loop configuration is employed to make sure the OEO operates at a single oscillating mode. Theoretically analysis is carried out to demonstrate the tuning range limitation due to the phase-modulation to intensity-modulation conversion caused by long single mode fiber (SMF). To overcome this disadvantage and expand the signal tuning range, a dispersion compensation fiber is introduced followed by the SMF to compensate the fiber dispersion. By simply adjusting the wavelength of the TLS, an ultra-wideband tunable microwave signal with the frequency tuning range from 2 to 44 GHz is obtained. The phase noise is also investigated and measured to be less than ? 110 dBc/Hz at an offset of 10 kHz within the whole frequency range. 相似文献
122.
Wanda Ziemkowska Anna Kubiak Szymon Kucharski Robert Woźniak Romana Anulewicz-Ostrowska 《Polyhedron》2007
Reactions of Me5Al3[OC(C6H5)2C(C6H5)2O]2 (1) with alcohols ROH (R = Me, Et, tBu) in a 1:1 molar ratio afforded the compound Me2Al2[OC(C6H5)2C(C6H5)2O]2(C4H8O) (2) and a mixture of methylaluminum alkoxides. The alcohols acted as the factor formally eliminating a molecule of Me3Al (as a methylaluminum alkoxide) from compound 1. tBu3Al reacted with an equimolar amount of benzopinacol to form the monomeric complex tBuAl[OC(C6H5)2C(C6H5)2O](C4H8O) (3). Reactions of Me3Ga and Me3In with benzopinacol yielded trinuclear complexes Me5M3[OC(C6H5)2C(C6H5)2O]2 (4 (M = Ga), 5 (M = In)), isostructural to compound 1. In the presence of water and alcohols, compounds 4 and 5 underwent a decomposition reaction to benzopinacol and a mixture of metalloxanes and alkoxides. An unusual methylmethoxo indium benzopinacolate Me6In4[OC(C6H5)2C(C6H5)2O]2(OCH3)2 (6) was obtained in the reaction of benzopinacol with Me3In and Me2InOMe in a 1:1:1 molar ratio. Molecular structures of the compounds 3, 4 and 6 were determined by X-ray crystallography. 相似文献
123.
T. Czechowski M. Baranowski A. Wo?niak-Braszak K. Jurga J. Jurga P. K?dzia 《Applied magnetic resonance》2012,43(3):331-340
The design and construction of a high-performance, low-cost, and easy to assemble adiabatic extension set for homebuilt and commercial spectrometers is described. Described apparatus set was designed for the fast adiabatic passage generation and is based on direct digital synthesizer DDS. This solution gives generator high signal to noise ratio, phase stability even during frequency change which is only possible in expansive commercial high-end hardware. Critical synchronization and timing issues are considered and solutions are discussed. Different experimental conditions and techniques for the measurements are briefly discussed. The proposed system is very flexible and might be used for the measurement of low-frequency nuclear magnetic resonance. 相似文献
124.
Dr. Matthew D. Wodrich Miyeon Chang Simone Gallarati Dr. Łukasz Woźniak Prof. Dr. Nicolai Cramer Prof. Dr. Clemence Corminboeuf 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(41):e202200399
Group 9 metals, in particular RhIII complexes with cyclopentadienyl ligands, are competent C−H activation catalysts. Recently, a Cp*RhIII-catalyzed reaction of alkenes with N-enoxyphthalimides showed divergent outcome based on the solvent, with carboamination favored in methanol and cyclopropanation in 2,2,2-trifluoroethanol (TFE). Here, we create selectivity and activity maps capable of unravelling the catalyst-solvent interplay on the outcome of these competing reactions by analyzing 42 cyclopentadienyl metal catalysts, CpXMIII (M=Co, Rh, Ir). These maps not only can be used to rationalize previously reported experimental results, but also capably predict the behavior of untested catalyst/solvent combinations as well as aid in identifying experimental protocols that simultaneously optimize both catalytic activity and selectivity (solutions in the Pareto front). In this regard, we demonstrate how and why the experimentally employed Cp*RhIII catalyst represents an ideal choice to invoke a solvent-induced change in reactivity. Additionally, the maps reveal the degree to which even perceived minor changes in the solvent (e. g., replacing methanol with ethanol) influence the ratio of carboamination and cyclopropanation products. Overall, the selectivity and activity maps presented here provide a generalizable tool to create global pictures of anticipated reaction outcome that can be used to develop new experimental protocols spanning metal, ligand, and solvent space. 相似文献
125.
Zusammenfassung Das mittlere Dipolmoment und die dielektrische Orientierungspolarisation hochpolymerer Festkörper werden für den Fall berechnet, daß die Molekülketten durch Recken oder Stauchen eine Vorzugsorientierung besitzen und die Dipole nur in zur Molekülachse bevorzugten Ebenen beweglich sind.Es werden die Segmentdrehung und die Seitengruppendrehung unter Zugrundelegung derBoltzmann-Verteilung der Dipole in den möglichen Drehebenen unter dem Einfluß eines äußeren elektrischen Feldes durchgerechnet. Eine Behinderung durch Nachbarmoleküle ist nicht berücksichtigt.Die Rechnungen ergeben eine Anisotropie der Polarisation, die sich auch experimentell für die Segmentdrehung an PVC und für die Seitengruppendrehung am Beispiel des PMMA mit richtigem Vorzeichen in den dielektrischen Verlusten ausdrückt (Meßfrequenz 9,2 GHz; Temperaturbereich -196 ... + 22 °C).Kurzfassung des Vortrages anläßlich der Deutschen Rheologentagung in Bad Nauheim vom 6. bis 10. April 1965. 相似文献
126.
The aim of contribution is to formulate a certain extended version of the tolerance modelling technique for functionally graded composites. For the sake of simplicity, the considerations are restricted to the bidirectionally graded heat conductors. It is shown that the proposed approach enables to determine an entire class of mathematical models for which applications can be found in various specific problems. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
127.
Dr. Krzysztof Durka Paulina H. Marek-Urban Krzysztof Nowicki Jakub Drapała Dr. hab. Katarzyna N. Jarzembska Piotr Łaski Aleksandra Grzelak Dr. Marek Dąbrowski Prof. Krzysztof Woźniak Prof. Sergiusz Luliński 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(14):e202104492
The approach to a series of six- and seven-membered oxaboraheterocycles based on naphthalene or biphenyl backbones was developed. The key synthetic step involved Br/Li exchange in respective potassium (bromoaryl)trifluoroborates followed by quenching with selected electrophiles (CO2, DMF, Me2Si(H)Cl) and hydrolytic workup. Two ring-expanded benzoxaborole congeners were obtained by an additional reduction step with LiAlH4 or NaBH4. The obtained boracyclic compounds were characterized in detail by NMR spectroscopy and single-crystal X-ray diffraction. Specifically, biphenyl-based systems show dynamic behaviour interpreted in terms of inversion of non-planar seven-membered boraheterocycles. The acidity of the obtained compounds varies very strongly (pKa ranges from 3.1–9.6) depending on their structure. Due to the enhanced boron Lewis acidity, selected compounds were used as a basis for luminescent complexes with 8-hydroxyquinoline. A strong phase-dependent variation of emission-band maximum (480–527 nm) and photoluminescence quantum yield (10–95 %) was observed, which was rationalized in terms of specific aggregation effects. 相似文献
128.
129.
Iwona Włoch Urszula Bednarz Dorota Bród Andrzej Włoch Małgorzata Wołowiec-Musiał 《Discrete Applied Mathematics》2013,161(16-17):2695-2701
In this paper, we define a new kind of Fibonacci numbers generalized in the distance sense. This generalization is related to distance Fibonacci numbers and distance Lucas numbers, introduced quite recently. We also study distinct properties of these numbers for negative integers. Their representations and interpretations in graphs are also studied. 相似文献
130.