全文获取类型
收费全文 | 372篇 |
免费 | 10篇 |
国内免费 | 3篇 |
专业分类
化学 | 283篇 |
晶体学 | 1篇 |
力学 | 9篇 |
数学 | 35篇 |
物理学 | 57篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 18篇 |
2020年 | 8篇 |
2019年 | 6篇 |
2018年 | 12篇 |
2017年 | 7篇 |
2016年 | 7篇 |
2015年 | 18篇 |
2014年 | 11篇 |
2013年 | 25篇 |
2012年 | 22篇 |
2011年 | 33篇 |
2010年 | 16篇 |
2009年 | 24篇 |
2008年 | 30篇 |
2007年 | 27篇 |
2006年 | 27篇 |
2005年 | 21篇 |
2004年 | 15篇 |
2003年 | 12篇 |
2002年 | 6篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 1篇 |
1997年 | 6篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1991年 | 2篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1971年 | 2篇 |
1969年 | 1篇 |
排序方式: 共有385条查询结果,搜索用时 781 毫秒
31.
Michael Lukaschewitsch Peter Maass Michael Pidcock Cristiana Sebu 《Mathematical Methods in the Applied Sciences》2009,32(2):206-222
The forward problem of electrical impedance tomography on unbounded domains can be studied by introducing appropriate function spaces for this setting. In this paper we derive the point‐wise asymptotic behaviour of weak solutions to this problem in the three‐dimensional case. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
32.
33.
This is a review of the derivation of the Landauer conductance using the Keldysh non-equilibrium Green's function (NEGF) formalism and the equations-of-motion (EOM) method. We consider the elastic quantum electronic transport through a multi-lead device and treat the conductor in the mean-field approximation. This is suitable for open quantum dots as well as for several molecular systems where charging effects are negligible. The focus of the presentation is to unveil the technical issues involved in the formalism. We show how the Landauer conductance emerges as a linear term in the current-voltage I-V characteristics and indicate how to go beyond this regime. We address the connection of the NEGF approach to recent developments in molecular transport and discuss the problems that arise when one tries to include interaction effects beyond the mean field. 相似文献
34.
Pinheiro AV Parola AJ Baptista PV Lima JC 《The journal of physical chemistry. A》2010,114(49):12795-12803
There are numerous reports of coumarin ester derivatives, in particular phosphate esters, as photocleavable cages in biological systems. Despite the comprehensive analysis of the photocleavage mechanism, studies of 4-methylcoumarin caged phosphates and/or nucleotides were always performed at constant pH. In this work, we present the study of the pH effect on the photochemistry of (7-diethylaminocoumarin-4-yl)methyl phosphate (DEACM-P). Fluorescence and photocleavage quantum yields, as well as the fluorescence decay times were measured as a function of the pH. It was found that the pH produces significant changes in the overall photochemical quantum yield of DEACM-P, and the observed changes are complementary to those obtained from the fluorescence quantum yield. Deprotonation of DEACM-HPO(4)(-) to yield DEACM-PO(4)(2-), produces a decrease in the photochemical quantum yield (from 0.0045 to 0.0003) and an increase in the fluorescence quantum yield (from 0.072 to 0.092). Moreover, from the analysis of the decay times, we have also found that hydroxyl ion is not only relevant, but it is mechanistically involved in the photoreaction of DEACM-HPO(4)(-). 相似文献
35.
Andressa H. de Morais Batista Francisco F. de Sousa Sara B. Honorato Alejandro P. Ayala Josue M. Filho Francisco W. de Sousa Antonio N. Pinheiro J.C.S. de Araujo Ronaldo F. Nascimento Antoninho Valentini Alcineia C. Oliveira 《Journal of molecular catalysis. A, Chemical》2010,315(1):86-98
Isomorphously substituted (MeDM) and impregnated metal-containing MCM-41 (MeOx/IM) catalysts, in which Me = Co, Cu, Cr, Fe or Ni, have been prepared. Structural and textural characterizations of the catalysts were performed by means of X-ray diffraction (XRD), chemical analysis, Raman spectroscopy, electron paramagnetic resonance (EPR), N2 adsorption isotherms and temperature programmed reduction (TPR). Cu2+, Co2+, and Cr4+/Cr3+ species were found over the catalysts as cations incorporated in the MCM-41 structure (MeDM) or highly dispersed oxides on the surface (MeOx/IM). The MeDM catalysts exhibited a good performance in the dehydrogenation of ethylbenzene with CO2. However, MeOx/IM catalysts had a low performance in styrene production (activity less than 15 × 10?3 mmol h?1 and selectivity for styrene less than 80%) due to the high reducibility of the metals species. However, Ni2+ or Fe3+ coordinated with the MCM-41 framework, as well as NiOx and Fe2O3 extra-framework species, is continuously oxidized by the CO2 to maintain the active sites for dehydrogenating ethylbenzene. Deactivation studies on the FeDM sample showed that Fe3+ species produced active sp2 carbon compounds, which are removed by CO2; the referred sample is catalytically selective for styrene and stable over 24 h of reaction. In contrast, highly active Ni2+ and Ni0 species produced a large amount of polyaromatic carbonaceous deposits from styrene, as identified by TPO, TG and Raman spectroscopy. An acid–base mechanism is proposed to operate to adsorb ethylbenzene and abstract the β-hydrogen. CO2 plays a role in furnishing the lattice oxygen to maintain the Fe3+ active sites in the dehydrogenation of ethylbenzene to form styrene. 相似文献
36.
Francesco Napoli Dr. Mario Chiesa Dr. Elio Giamello Prof. Gloria Preda Dr. Cristiana Di Valentin Dr. Gianfranco Pacchioni Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(23):6776-6785
The formation of O2? radical anions by contact of O2 molecules with a Na pre‐covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O2. The typical EPR signal of isolated Na atoms disappears when the reaction with O2 takes place and new paramagnetic species are observed, which are attributed to different surface‐stabilised O2? radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O2?Na+ species to be determined, demonstrating the direct coordination of the O2? adsorbate to surface Na+ cations. DFT calculations enable the structural details of the formed species to be determined. Matrix‐isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface‐stabilised Na+? O2? complexes. 相似文献
37.
A combined experimental and first principles study of Cu defects in bulk ZnO is presented. Cu particles are epitaxially deposited on the polar O-ZnO(0001) surface at room temperature. Upon heating, a broadening of the quasielastic peak in high resolution electron energy loss spectra is observed, corresponding to an electronic doping effect of Cu atoms in bulk ZnO with an ionization energy of 88 meV. Cu impurities in ZnO, although commonly acting as acceptors, are presently observed to induce shallow donor states. We assign these to interstitial Cu species on the basis of a hybrid density functional study. 相似文献
38.
39.
Alcântara Antônio F. C. Dos Santos Hélio F. De Almeida Wagner B. Vaz Maria G. F. Pinheiro Luíza M. M. Stumpf Humberto O. 《Structural chemistry》1999,10(5):367-374
2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide radical 2 and the 2-[2-(N-alkyl)-pyrid-inium] derivatives (3a and 3b) were synthesized and characterized by elemental analysis, melting points, and spectroscopy. The density functional theory (DFT) was used in order to obtain the structures and electronic properties of the new nitronyl nitroxide radicals. Modified reaction procedures of the intermediates are described with better yield and purity of the final products. The magnetic properties of the compounds 2, 3a, and 3b are predicted for the first time using a simple model of charge transference, in the framework of the molecular orbital calculations. The obtained results show that the substituent at the central carbon atom of the imidazoline is important to determine the spin distribution and consequently the nature of the magnetic interaction. 相似文献
40.
Helio S. Migon Mariane Branco Alves André F. B. Menezes Eduardo G. Pinheiro 《商业与工业应用随机模型》2023,39(3):471-493
We briefly review the main developments of Bayesian dynamic models. The emphasis is on marketing applications. Typical examples in this area are discussed. The concepts of monitoring and intervention are carefully explained with illustrative examples and open source computational routines. We avoid algebraic developments and instead use graphical examples to illustrate theoretical aspects. Two real-world problems using Bayesian dynamic models are discussed. Finally, we describe recent developments and alternative proposals to formally address the dependence when dealing with the modeling of multiple time series. 相似文献