全文获取类型
收费全文 | 2883篇 |
免费 | 261篇 |
国内免费 | 16篇 |
专业分类
化学 | 2146篇 |
晶体学 | 11篇 |
力学 | 61篇 |
数学 | 422篇 |
物理学 | 520篇 |
出版年
2023年 | 52篇 |
2022年 | 65篇 |
2021年 | 118篇 |
2020年 | 166篇 |
2019年 | 186篇 |
2018年 | 83篇 |
2017年 | 69篇 |
2016年 | 198篇 |
2015年 | 149篇 |
2014年 | 160篇 |
2013年 | 193篇 |
2012年 | 275篇 |
2011年 | 267篇 |
2010年 | 155篇 |
2009年 | 127篇 |
2008年 | 193篇 |
2007年 | 155篇 |
2006年 | 149篇 |
2005年 | 97篇 |
2004年 | 59篇 |
2003年 | 33篇 |
2002年 | 29篇 |
2001年 | 27篇 |
2000年 | 19篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 5篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1971年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
1932年 | 2篇 |
1928年 | 1篇 |
排序方式: 共有3160条查询结果,搜索用时 687 毫秒
991.
992.
Sebastian Reich 《BIT Numerical Mathematics》1996,36(1):122-134
Recent observations [5] indicate that energy-momentum methods might be better suited for the numerical integration of highly oscillatory Hamiltonian systems than implicit symplectic methods. However, the popular energy-momentum method, suggested in [3], achieves conservation of energy by a global scaling of the force field. This leads to an undesirable coupling of all degrees of freedom that is not present in the original problem formulation. We suggest enhancing this energy-momentum method by splitting the force field and using separate adjustment factors for each force. In case that the potential energy function can be split into a strong and a weak part, we also show how to combine an energy conserving discretization of the strong forces with a symplectic discretization of the weak contributions. We demonstrate the numerical properties of our method by simulating particles that interact through Lennard-Jones potentials and by integrating the Sine-Gordon equation.This work was partly supported by NIH Grant P41RR05969, DOE/NSF Grant DE-FG02-91-ER25099/DMS-9304268, and NSF GCAG/HPCC ASC-9318159. 相似文献
993.
Dirichlet integrals and the associated Dirichlet statistical densities are widely used in various areas. Generalizations of
Dirichlet integrals and Dirichlet models to matrix-variate cases, when the matrices are real symmetric positive definite or
hermitian positive definite, are available [4]. Real scalar variables case of the Dirichlet models are generalized in various
directions. One such generalization of the type-2 or inverted Dirichlet is looked into in this article. Matrix-variate analogue,
when the matrices are hermitian positive definite, are worked out along with some properties which are mathematically and
statistically interesting. 相似文献
994.
Wavelength stabilized 980 nm uncooled pump laser modules for erbium-doped fiber amplifiers 总被引:2,自引:0,他引:2
Tomas Pliska Sebastian Arlt Rainer Bttig Tim Kellner Isabella Jung Nicolai Matuschek Pascal Mauron Bernd Mayer Stefan Mohrdiek Jürgen Müller Susanne Pawlik Hans-Ulrich Pfeiffer Berthold Schmidt Boris Sverdlov Stefan Teodoropol Jrg Troger Bernd Valk Christoph Harder 《Optics and Lasers in Engineering》2005,43(3-5):271
We review the development of wavelength stabilized 980 nm pump laser modules without active temperature stabilization for applications in erbium-doped fiber amplifiers. Operation over a wide temperature range with an output power exceeding 400 mW at an ambient temperature of 70°C is demonstrated. The overall reliability of uncooled modules is estimated to be well below 500 FIT at all operating conditions. Such devices are made possible by continuous development and steady improvement of the pump laser chip, the optimization of the fiber Bragg grating stabilization scheme, careful design of the module package, and extended reliability analysis on the basis of stress tests as well as field data. 相似文献
995.
Sebastian Zeck 《Fluid Phase Equilibria》1991,70(2-3):125-140
Phase equilibrium thermodynamics is one of the most important fundamental sciences for process development in the chemical industry. The outstanding results achieved by thermodynamics in the last decades have let to a profound change in working methods for the development of thermal separation processes. However, there remains a large number of unsolved problems in the field of phase equilibrium thermodynamics which often represent a significant restriction to process development for the chemical engineer. Current deficiencies are illustrated with some significant examples and desired developments for the future are suggested. 相似文献
996.
Maria Martinez-Lara Jose A. Barea-Aranda Laureano-Moreno Real Sebastian Bruque 《Journal of inclusion phenomena and macrocyclic chemistry》1990,9(4):287-299
The layered acid solids of formula H3OUO2XO4· 3 H2O (X = As, P) intercalate aniline and benzidine arylamines, by protonation of the guest molecules. The intercalates maintain the original laminar structure.The insertion of aniline and benzidinein the metal derivativesM(UO2XO4)2·n H2O (M = Cu, VO, Fe) requires rather drastic conditions. Near and medium infrared spectra of intercalates in which M = VO2+ and Cu2+, indicate that the polymerization and/or oxidation of sorbed amines occurs; however, the guest-host reactions for Fe2+-derivatives are of the acid-base type. 相似文献
997.
Compositions of saturated vapor and liquid phases at equilibrium were measured for hydrogen + carbon dioxide + 1-methylnaphthalene mixtures at 543 and 704 K over a pressure range of 50–250 atm. Measurements were made at three relative concentrations of hydrogen to carbon dioxide at each condition of temperature and pressure. A significant variation of the K-value of 1-methylnaphthalene with gas composition was observed in the high pressure region. 相似文献
998.
Formation Mechanism of the First Carbon–Carbon Bond and the First Olefin in the Methanol Conversion into Hydrocarbons 下载免费PDF全文
Dr. Yue Liu Dr. Sebastian Müller Dr. Daniel Berger Dr. Jelena Jelic Prof. Dr. Karsten Reuter Dr. Markus Tonigold Dr. Maricruz Sanchez‐Sanchez Prof. Dr. Johannes A. Lercher 《Angewandte Chemie (International ed. in English)》2016,55(19):5723-5726
The elementary reactions leading to the formation of the first carbon–carbon bond during early stages of the zeolite‐catalyzed methanol conversion into hydrocarbons were identified by combining kinetics, spectroscopy, and DFT calculations. The first intermediates containing a C?C bond are acetic acid and methyl acetate, which are formed through carbonylation of methanol or dimethyl ether even in presence of water. A series of acid‐catalyzed reactions including acetylation, decarboxylation, aldol condensation, and cracking convert those intermediates into a mixture of surface bounded hydrocarbons, the hydrocarbon pool, as well as into the first olefin leaving the catalyst. This carbonylation based mechanism has an energy barrier of 80 kJ mol?1 for the formation of the first C?C bond, in line with a broad range of experiments, and significantly lower than the barriers associated with earlier proposed mechanisms. 相似文献
999.
Formamides as Lewis Base Catalysts in SN Reactions—Efficient Transformation of Alcohols into Chlorides,Amines, and Ethers 下载免费PDF全文
Dr. Peter H. Huy Sebastian Motsch Sarah M. Kappler 《Angewandte Chemie (International ed. in English)》2016,55(34):10145-10149
A simple formamide catalyst facilitates the efficient transformation of alcohols into alkyl chlorides with benzoyl chloride as the sole reagent. These nucleophilic substitutions proceed through iminium‐activated alcohols as intermediates. The novel method, which can be even performed under solvent‐free conditions, is distinguished by an excellent functional group tolerance, scalability (>100 g) and waste‐balance (E‐factor down to 2). Chiral substrates are converted with excellent levels of stereochemical inversion (99 %→≥95 % ee). In a practical one‐pot procedure, the primary formed chlorides can be further transformed into amines, azides, ethers, sulfides, and nitriles. The value of the method was demonstrated in straightforward syntheses of the drugs rac‐Clopidogrel and S‐Fendiline. 相似文献
1000.
Wave‐function frozen‐density embedding: Approximate analytical nuclear ground‐state gradients 下载免费PDF全文
We report the derivation of approximate analytical nuclear ground‐state uncoupled frozen density embedding (FDEu) gradients for the resolution of identity (RI) variant of the second‐order approximate coupled cluster singles and doubles (RICC2) as well as density functional theory (DFT), and an efficient implementation thereof in the KOALA program. In order to guarantee a computationally efficient treatment, those gradient terms are neglected which would require the exchange of orbital information. This approach allows for geometry optimizations of single molecules surrounded by numerous molecules with fixed nuclei at RICC2‐in‐RICC2, RICC2‐in‐DFT, and DFT‐in‐DFT FDE level of theory using a dispersion correction, required due to the DFT‐based treatment of the interaction in FDE theory. Accuracy and applicability are assessed by the example of two case studies: (a) the Watson‐Crick pair adenine‐thymine, for which the optimized structures exhibit a maximum error of about 0.08 Å for our best scheme compared to supermolecular reference calculations, (b) carbon monoxide on a magnesium oxide surface model, for which the error amount up to 0.1 Å for our best scheme. Efficiency is demonstrated by successively including environment molecules and comparing to an optimized conventional supermolecular implementation, showing that the method is able to outperform conventional RICC2 schemes already with a rather small number of environment molecules, gaining significant speed up in computation time. © 2016 Wiley Periodicals, Inc. 相似文献