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排序方式: 共有226条查询结果,搜索用时 31 毫秒
21.
Delbaere S Micheau JC Frigoli M Vermeersch G 《The Journal of organic chemistry》2005,70(13):5302-5304
The kinetics of the thermal fading of four halogenated naphthopyrans (NP) have been analyzed using NMR spectroscopy in CD(3)CN. Two photomerocyanines (TT and TC) were detected after UV irradiation. The main relaxation process TC --> NP was coupled with TT/TC isomerization. The activation parameters of the various processes were rationalized by considering electronegativity and polarizability of the halogen substituents and their selective solvation by the electronegative nitrogen of acetonitrile. 相似文献
22.
The microwave surface resistance of a thin layer of copper in close contact with the surface of bulk lead is investigated. It is found that the basic ideas about proximity effects in superconductivity developed in recent years are capable of explaining the observed behaviour very satisfactorily. Our data give evidence of a complex excitation spectrum in the copper, with low energy excitations above a true gap and a group of high energy excitations which is usually observed in tunneling experiments. The response to a magnetic field shows that scattering at the free copper surface is diffuse. This leads at low fields, where a Meissner shielding current flows at the copper surface, to a decrease of the depairing energy by a large amount of first order in the vector potentialA instead of only a small second order amount. This also explains an initial decrease of the absorption when a weak magnetic field is applied. For fields over a large range below the critical field of lead superconductivity is effectively suppressed in copper. The data also give information on the increase of the penetration depth in lead because of the coating with copper and on the effective electron-electron interaction in copper: $$\lambda \to 2\lambda _L and (NV)_{Cu} \cong 0.05.$$ 相似文献
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24.
Résumé L'étude de la structure électronique d'une chaîne constituée par la répétition d'un motif polyatomique et l'interprétation des propriétés du polymère ainsi formé sont grandement facilités, s'il est possible de ramener les termes d'interaction entre unités voisines à un paramètre d'interaction unique. Ce problème a été traité en méthode des orbitales moléculaires par un procédé qui permet de tenir compte des modifications du monomère à l'intérieur du polymère et ne néglige que de faibles contributions.L'interaction entre deux monomèresi etj est représentée par la sous-matriceB
ij
de dimensionn ×n formée par les termes non-diagonaux correspondants de la matrice-énergie. Par des transformations unitaires appropriées, il est possible de mettre sous forme diagonale les sousmatricesB
ij
entre monomères voisins. En général, la diagonale de la matrice transformée contient un terme prépondérant qu'on peut considérer comme mesurant l'interaction (i, j). Une application numérique a été effectuée dans le cas d'un polymère fictif formé par des molécules H2 et dans le cas d'un polymère peptidique formé par des groupements HNCO en interaction par l'intermédiaire de liaisons hydrogène. La validité des approximations introduites selon l'importance de l'interaction peut être vérifiée sur l'exemple d'un dimère dont les deux unités sont placées à des distances variables. Dans le cas d'un polymère les transformations effectuées conduisent à remplacer les orbitales atomiques du monomère par des orbitales s'étendant sur deux unités voisines; en prenant comme motif structural de la chaîne le monomère double formé par deux unités successives, on peut représenter l'interaction entre deux motifs adjacents par un seul terme.La réduction de la matriceB
ij
à un seul terme fournit des bandes d'énergie dont la position est en très bon accord avec celles obtenues directement par les techniques classiques de calcul de l'état solide [7].
Gratias agimus Cl. Vir.Raphaeli Del Re, qui nos, quod ad usum linguae latinae attinet, consilio benigne adiuvit. 相似文献
The study of the electronic structure of a chain formed by repetition of polyatomic monomers and the interpretation of the resulting polymer are made much easier if the interaction terms relating to neighbouring units can be reduced to a single parameter. This problem has been treated in the MO scheme by a procedure which makes it possible to take into account the modifications of the monomer within the polymer and only neglects minor contributions.The interaction between two monomersi andj is represented by then ×n submatrixB ij formed by the corresponding non diagonal terms of the energy matrix. By appropriate unitary transformations it is possible to bring the submatricesB ij of neighbouring monomers to a diagonal form. In general, one of the diagonal elements thus obtained is much higher than the others, and can be taken as a measure of thei-j interaction. A numerical application has been made in the case of a fictitious polymer formed by H2 molecules, and in the case of a polypeptide formed by HNCO groups interacting through hydrogen bonds. The validity of the approximations introduced for different values of interaction can be tested on a dimer whose units are placed at different distances. In the case of a polymer the transformations mentioned above require that the atomic orbitals of a monomer be replaced by orbitals extending over two neighbouring units. Taking as a new unit the corresponding pair of monomers, the interaction between the units can be represented by a single term. The reduction of theB ij matrices to a single term each gives energy bands whose positions are in perfect agreement with those obtained directly by the classical techniques of solid-state physics [7].
Zusammenfassung Das Studium der elektronischen Struktur von linearen Polymeren vereinfacht sich sehr, wenn die Wechselwirkung zwischen benachbarten Monomeren durcheinen Parameter beschrieben werden kann. Entsprechend wird im MO-Schema ein Verfahren entwickelt, das auf der Veränderung der Monomeren im Polymeren fußt.Die Wechselwirkung zwischen zwei Monomereni, j des Polymer wird durch eine aus Nichtdiagonalelementen der Hamiltonmatrix bestehende UntermatrixB ij beschrieben. Für alle benachbarten Monomerenpaare lassen sich diese durch unitäre Transformationen diagonalisieren. I. a. ist eines der so erhaltenen Diagonalelemente vonB merklich größer und dient als Maß für die Wechselwirkung der beiden Monomeren. Angewendet wird die Methode auf ein fiktives Polymer aus H2-Molekülen und auf ein Polymer aus durch H-Brücken verbundenen Peptidgruppen. Der Einfluß der Näherungen wird zunächst an einem Dinieren für verschiedene Abstände getestet.Im Falle eines Polymeren sind als Monomere Einheiten von zwei benachbarten Molekülen mit entsprechend ausgedehnten Orbitalen zu betrachten. Die Rechnung führt hier auf Energiebänder, deren Lage voll mit der nach klassischen Methoden der Festkörperphysik [7] erhaltenen übereinstimmt.
Gratias agimus Cl. Vir.Raphaeli Del Re, qui nos, quod ad usum linguae latinae attinet, consilio benigne adiuvit. 相似文献
25.
Gaston Bonnecaze Albert Lichanot Simon Gromb 《Journal of Physics and Chemistry of Solids》1978,39(3):299-310
The study of electrogalvanic and electronic properties of the form β of silver sulfide has been performed on polycrystalline samples whose non stoichiometry is controled by coulometric titration with the cell Ag/RbAg4I5/Ag2+?S/Pt.The data obtained lead to the adoption of an electronic model according to which the Frenkel defects of the silver sublattice are completly ionized. The variation with temperature of the electronic properties near the stoichiometric composition between 23 and 176°C, the temperature of transition between the two forms β and α of Ag2S, enables the forbidden gap Ei = (1,35?1,5 × 10?3T) eV to be computed. The existence range has been drawn between 69°C and the α ? β transition temperature.The study of the ionic conductivity has made it possible to detect two ranges of preferential conduction and to determine the mechanism of migrations of the intersticial silver ions. 相似文献
26.
27.
[reaction: see text] The photochromic performance of a hybrid system connecting naphthopyran and dithienylethene was investigated, and the photochemistry of eight different isomers was explored by choosing an appropriate wavelength of light. 相似文献
28.
Qijun Meng Biaobiao Zhang Lizhou Fan Haidong Liu Mario Valvo Kristina Edstrm Maria Cuartero Roland de Marco Gaston A. Crespo Licheng Sun 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(52):19203-19209
Water‐splitting photoanodes based on semiconductor materials typically require a dopant in the structure and co‐catalysts on the surface to overcome the problems of charge recombination and high catalytic barrier. Unlike these conventional strategies, a simple treatment is reported that involves soaking a sample of pristine BiVO4 in a borate buffer solution. This modifies the catalytic local environment of BiVO4 by the introduction of a borate moiety at the molecular level. The self‐anchored borate plays the role of a passivator in reducing the surface charge recombination as well as that of a ligand in modifying the catalytic site to facilitate faster water oxidation. The modified BiVO4 photoanode, without typical doping or catalyst modification, achieved a photocurrent density of 3.5 mA cm?2 at 1.23 V and a cathodically shifted onset potential of 250 mV. This work provides an extremely simple method to improve the intrinsic photoelectrochemical performance of BiVO4 photoanodes. 相似文献
29.
30.
Cartigny B Azaroual N Imbenotte M Mathieu D Parmentier E Vermeersch G Lhermitte M 《Talanta》2008,74(4):1075-1078
The determination and quantification of glyphosate in serum using (1)H NMR spectroscopy is reported. This method permitted serum samples to be analysed without derivatization or any other sample pre-treatment, using 3-trimethylsilyl 2,2',3,3'-tetradeuteropropionic acid (TSP-d(4)) as a qualitative and quantitative standard. Characterization of the herbicide N-(phosphonomethyl)glycine was performed by analysing chemical shifts and coupling constant patterns. Quantification was performed by relative integration of CH(2)-P protons to the TSP-d(4) resonance peak. The method was tested for repeatability (n=5) and yielded coefficients of variation of 1% and 3%, respectively: detection and quantification limits were also determined and were 0.03 and 0.1mmol/L, respectively. The method was applied to the quantification of glyphosate in a case of acute poisoning. 相似文献