全文获取类型
收费全文 | 3863篇 |
免费 | 83篇 |
国内免费 | 22篇 |
专业分类
化学 | 2640篇 |
晶体学 | 31篇 |
力学 | 75篇 |
数学 | 593篇 |
物理学 | 629篇 |
出版年
2022年 | 19篇 |
2021年 | 28篇 |
2020年 | 47篇 |
2019年 | 54篇 |
2018年 | 41篇 |
2017年 | 31篇 |
2016年 | 69篇 |
2015年 | 63篇 |
2014年 | 102篇 |
2013年 | 197篇 |
2012年 | 162篇 |
2011年 | 190篇 |
2010年 | 130篇 |
2009年 | 122篇 |
2008年 | 177篇 |
2007年 | 162篇 |
2006年 | 159篇 |
2005年 | 152篇 |
2004年 | 127篇 |
2003年 | 118篇 |
2002年 | 119篇 |
2001年 | 68篇 |
2000年 | 80篇 |
1999年 | 50篇 |
1998年 | 64篇 |
1997年 | 53篇 |
1996年 | 69篇 |
1995年 | 65篇 |
1994年 | 54篇 |
1993年 | 58篇 |
1992年 | 55篇 |
1991年 | 69篇 |
1990年 | 47篇 |
1989年 | 54篇 |
1988年 | 63篇 |
1987年 | 48篇 |
1986年 | 61篇 |
1985年 | 56篇 |
1984年 | 65篇 |
1983年 | 43篇 |
1982年 | 35篇 |
1981年 | 51篇 |
1980年 | 56篇 |
1979年 | 43篇 |
1978年 | 39篇 |
1977年 | 37篇 |
1976年 | 32篇 |
1975年 | 27篇 |
1974年 | 24篇 |
1973年 | 23篇 |
排序方式: 共有3968条查询结果,搜索用时 11 毫秒
161.
Joachim Escher 《Applicable analysis》2013,92(7):1474-1494
We consider an incompressible ferrofluid in a vertical Hele–Shaw cell and develop a proper analytic framework for the free interface and the velocity potential of the fluid in a periodic geometry. The flow is assumed to obey a non-Newtonian Darcy law. The forces influencing the fluid are gravity, surface tension and the response to a magnetic field induced by a current. In addition, the flow is stabilized at the lower boundary component by an external source b. We prove a well-posedness result for the flow near flat solutions. Moreover, we find conditions on the parameters and on the slope of b for the exponential stability and instability of flat interfaces. Furthermore, we identify values for the current's intensity ι where critical bifurcation of nontrivial finger-shaped solutions from the branch of trivial (flat) solutions takes place. 相似文献
162.
Carsten Collon Stephan Eckhardt Joachim Rudolph 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2013,19(5):455-469
The modelling of active magnetic bearings based on a network approach is considered. Unlike in the standard modelling approach, where a linearization of the current-force relation for the centred shaft position is used, network models permit to include the position dependence of the bearing force in the force model. This becomes necessary when model based controllers are used to stabilize a magnetically supported shaft in tracking applications. The approach is based on the well known application of network models to magnetic circuits. Further simplifying assumptions are discussed which allow one to obtain a network with a limited number of lumped parameters describing the magnetic behaviour of a magnetic bearing. The modelling of a combined radial and axial bearing serves as an example for the application of the proposed approach. Furthermore, the fitting of the network based model to measured characteristic force curves is discussed. In this context, a method for including saturation effects in the model is sketched. 相似文献
163.
Marcin Rojkiewicz Piotr Ku Maria Ksiek Joachim Kusz 《Acta Crystallographica. Section C, Structural Chemistry》2022,78(1):56-62
Cathinones belong to a group of compounds of great interest in the new psychoactive substances (NPS) market. Constant changes to the chemical structure made by the producers of these compounds require a quick reaction from analytical laboratories in ascertaining their characteristics. In this article, three cathinone derivatives were characterized by X-ray crystallography. The investigated compounds were confirmed as: 1-[1-(4-methylphenyl)-1-oxohexan-2-yl]pyrrolidin-1-ium chloride ( 1 , C17H26NO+·Cl?, the hydrochloride of 4-MPHP), 1-(4-methyl-1-oxo-1-phenylpentan-2-yl)pyrrolidin-1-ium chloride ( 2 ; C16H24NO+·Cl?, the hydrochloride of α-PiHP) and methyl[1-(4-methylphenyl)-1-oxopentan-2-yl]azanium chloride ( 3 ; C13H20NO+·Cl?, the hydrochloride of 4-MPD). All the salts crystallize in a monoclinic space group: 1 and 2 in P21/c, and 3 in P21/n. To the best of our knowledge, this study provides the first detailed and comprehensive crystallographic data on salts 1 – 3 . 相似文献
164.
Reza Kordnezhadian Bing-Yu Li Armir Zogu Dr. Joachim Demaerel Prof. Dr. Wim M. De Borggraeve Dr. Ermal Ismalaj 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(60):e202201491
Pentafluorosulfanyl (SF5)-containing compounds and corresponding analogs are a highly valuable class of fluorine-containing building blocks owing to their unique properties. The reason for that is the set of peculiar and tremendously beneficial characteristics they can impart on molecules once introduced onto them. Despite this, their application in distinct scientific fields remains modest, given the extremely harsh reaction conditions needed to access such compounds. The recent synthetic approaches via S−F, and C−SF5 bond formation as well as the use of SF5-containing building blocks embody a “stairway-to-heaven” loophole in the synthesis of otherwise-inaccessible chemical scaffolds only a few years ago. Herein, we report and evaluate the properties of the SF5 group and analogs, by summarizing synthetic methodologies available to access them as well as following applications in material science and medicinal chemistry since 2015. 相似文献
165.
Single‐Layered Ultrasmall Nanoplates of MoS2 Embedded in Carbon Nanofibers with Excellent Electrochemical Performance for Lithium and Sodium Storage 下载免费PDF全文
Dr. Changbao Zhu Dr. Xiaoke Mu Prof. Peter A. van Aken Prof. Yan Yu Prof. Joachim Maier 《Angewandte Chemie (International ed. in English)》2014,53(8):2152-2156
The preparation and electrochemical storage behavior of MoS2 nanodots—more precisely single‐layered ultrasmall nanoplates—embedded in carbon nanowires has been studied. The preparation is achieved by an electrospinning process that can be easily scaled up. The rate performance and cycling stability of both lithium and sodium storage were found to be outstanding. The storage behavior is, moreover, highly exciting from a fundamental point of view, as the differences between the usual storage modes—insertion, conversion, interfacial storage—are beneficially blurred. The restriction to ultrasmall reaction domains allows for an almost diffusion‐less and nucleation‐free “conversion”, thereby resulting in a high capacity and a remarkable cycling performance. 相似文献
166.
M.Sc. Johannes E. M. N. Klein Dr. Burkhard Miehlich Dr. Michael S. Holzwarth Prof. Dr. Matthias Bauer Dipl.‐Chem. Magdalena Milek Dr. Marat M. Khusniyarov Dr. Gerald Knizia Prof. Dr. Hans‐Joachim Werner Prof. Dr. Bernd Plietker 《Angewandte Chemie (International ed. in English)》2014,53(7):1790-1794
During the past 10 years iron‐catalyzed reactions have become established in the field of organic synthesis. For example, the complex anion [Fe(CO)3(NO)]?, which was originally described by Hogsed and Hieber, shows catalytic activity in various organic reactions. This anion is commonly regarded as being isoelectronic with [Fe(CO)4]2?, which, however, shows poor catalytic activity. The spectroscopic and quantum chemical investigations presented herein reveal that the complex ferrate [Fe(CO)3(NO)]? cannot be regarded as a Fe?II species, but rather is predominantly a Fe0 species, in which the metal is covalently bonded to NO? by two π‐bonds. A metal–N σ‐bond is not observed. 相似文献
167.
Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations 下载免费PDF全文
Dr. Joachim Reimer Prof. Dr. Frédéric Vogel Prof. Dr. Matthew Steele‐MacInnis 《Chemphyschem》2016,17(10):1446-1453
Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular‐scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na?SO4 exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K?SO4 show significantly less clustering under equivalent conditions. In mixed systems (NaxK2?xSO4), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure‐breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K+ and SO42?, compared with the analogous Na+?SO42? interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na?K?SO4. Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium–sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting‐in in ternary hydrothermal solutions of Na?K?SO4 reflects the opposing, but complementary, natures of Na?SO4 versus K?SO4 interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system. 相似文献
168.
Regina Kalhne Mohamed Amin Joachim Sobottka Robert Sauerbrey 《Analytica chimica acta》2000,420(2):205-216
Fen soils from two sites of the Rhin-Havel-Luch, a peatland in the north-east of Germany, have been investigated. The samples have been collected in two horizons, representing different degrees of degradation and mineralisation of peat. Gravimetric measurements, energy dispersive X-ray fluorescence (EDXRF), elemental analysis, and 1H low resolution nuclear magnetic resonance (LR-NMR) of the fen soil samples have been performed. By multivariate analysis of all the experimental data, especially by the principal component analysis (PCA) and by the cluster analysis, respectively, it was possible to classify the fen soils, to identify their characteristic properties, to detect temporal and local variations, and to prove representative field sampling. Furthermore, the correlation between variables of the applied analytical methods could be interpreted in context to the composition of fen soils and mutual influences of their properties. 相似文献
169.
Dzubiella J 《Journal of the American Chemical Society》2008,130(42):14000-14007
The structure of a single alanine-based Ace-AEAAAKEAAAKA-Nme peptide in explicit aqueous electrolyte solutions (NaCl, KCl, NaI, and KF) at large salt concentrations (3-4 M) is investigated using approximately 1 mus molecular dynamics (MD) computer simulations. The peptide displays 71% alpha-helical structure without salt and destabilizes with the addition of NaCl in agreement with experiments of a somewhat longer version. It is mainly stabilized by direct and indirect (" i + 4")EK salt bridges between the Lys and Glu side chains and a concomitant backbone shielding mechanism. NaI is found to be a stronger denaturant than NaCl, while the potassium salts hardly show influence. Investigation of the molecular structures reveals that consistent with recent experiments Na (+) has a much stronger affinity to side chain carboxylates and backbone carbonyls than K (+), thereby weakening salt bridges and secondary structure hydrogen bonds. At the same time, the large I (-) has a considerable affinity to the nonpolar alanine in line with recent observations of a large propensity of I (-) to adsorb to simple hydrophobes, and thereby "assists" Na (+) in its destabilizing action. In the denatured states of the peptide, novel long-lived (10-20 ns) "loop" configurations are observed in which single Na (+) ions and water molecules are hydrogen-bonded to multiple backbone carbonyls. In an attempt to analyze the denaturation behavior within the preferential interaction formalism, we find indeed that for the strongest denaturant, NaI, the protein is least hydrated. Additionally, a possible indication for protein denaturation might be a preferential solvation of the peptide backbone by the destabilizing cosolute (sodium). The mechanisms found in this work may be of general importance to understand salt effects on protein secondary structure stability. 相似文献
170.
Homogeneous hydrogenation of barbituric acid derivatives with parahydrogen yields a substantial increase of the (1)H NMR signals of the reaction products. These physiologically relevant compounds were hydrogenated at both ambient and elevated temperatures and pressures using a standard cationic rhodium catalyst. The resulting nonthermal nuclear spin polarization (hyperpolarization) is limited by the spin-lattice relaxation time T(1) of the corresponding nuclei in the products, being shorter than the time constant of the hydrogenation. The signal-to-noise ratio of the NMR spectra could be further increased upon signal averaging the antiphase PHIP signals of 25 successive scans following 30 degrees pulse experiments and a delay of 10 s. 相似文献