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Zusammenfassung Im System ZnO–Sb2O5 existieren zwei Spinellphasen (I) und (III) gleicher Zusammensetzung Zn7Sb2O12. Außerdem konnte noch eine weitere Modifikation (II) mit einer niedrigsymmetrischen Struktur aufgefunden werden.In (II) und (III) wurden Cu2+, Ni2+ und Co2+ als farbgebende Kationen eingebaut. Die spektralphotometrische Untersuchung ergab, daß Zn2+ in (II) sowohl oktaedrisch als auch tetraedrisch koordiniert ist. Im Spinell (III) wird Cu2+ sowohl in Tetraeder-und Oktaederlücken, Ni2+ nur in Oktaeder- und Co2+ vorwiegend in Oktaederlücken eingebaut.In the system ZnO–Sb2O5 exist two phases (I) and (III) with the spinel structure and the same composition Zn7Sb2O12. Besides these another modification (II) with a structure of lower symmetry could be found.The colouring cations Cu2+, Ni2+, Co2+ have been incorporated in (II) and (III). The spectrophotometrical investigation shows that Zn2+ occupies in (II) octahedral and tetrahedral sites. In the spinel (III) Cu2+ is incorporated tetrahedrally and octahedrally, Ni2+ only octahedrally and Co2+ predominantly octahedrally.Mit 4 Abbildungen1. Mitt.:H. Kasper, Z. anorg. allgem. Chem. (im Druck).  相似文献   
283.
A passive “badge-type” sampling device for the determination of gaseous ammonia was developed. The collection substrate is phosphoric acid. The sampler can be used for outdoor and indoor sampling of ammonia in the concentration range from 0.05 μg/m3 to 10 mg/m3. The performance was tested in the laboratory and in the field against an annualar denuder, a filter pack and an impinger technique. The intercalibration showed that the passive sampler compares very well with active samplers (r2=0.99; k=1.05). The average reproducibility of the sampler was 8%. Hence the badge sampler is well suited for the determination of ammonia in a wide range of concentrations and particularly for application under rural background conditions. The sampling rate of the device was calculated according to a simple multi-layer model.  相似文献   
284.
Single crystals of the so-called β-tetragonal (or tetragonal II or III) boron modification have been obtained from boron deposits prepared by hydrogen reduction of BBr3 on tantalum filaments at 1200°C. Chemical analysis of the samples shows that this phase can be regarded as a true modification of pure elemental boron in contrast to α-tetragonal phases which require small amounts of foreign atoms to stabilize their boron framework.The lattice parameters (a = 10.14(1)Å; c = 14.17(1)Å) were obtained and refined from single crystal data. The unit cell contains four chemical units, B21 · 2B12 · B2.5 resulting in dc = 2.34 g cm?3 (dm = 2.36(2) g cm?3). The systematic extinctions are compatible with space group P41 or P43.The structure was determined from 1009 independent reflexions using a model derived from the recently solved structure of α - AlB12 (a = 10.161Å; c = 14.283Å; space group P41212 or P43212). The final R value (unweighted data) is 9.6%.Basically, the structure of this tetragonal form of boron consists of the same three-dimensional boron skeleton, built upon simple and twinned icosahedra, as that of α-AlB12. However, the defective twinned icosahedral B19 units in α-AlB12 are now completed (B21 units) in the related tetragonal boron. A number of interstitial sites, located at positions different from those occupied by aluminum in α-AlB12, are totally or partially filled by boron atoms and very probably increase the stability of the boron framework.  相似文献   
285.
We demonstrate that commercially available unmodified carbocyanine dyes such as Cy5 (usually excited at 633 nm) can be used as efficient reversible single-molecule optical switch, whose fluorescent state after apparent photobleaching can be restored at room temperature upon irradiation at shorter wavelengths. Ensemble photobleaching and recovery experiments of Cy5 in aqueous solution irradiating first at 633 nm, then at 337, 488, or 532 nm, demonstrate that restoration of absorption and fluorescence strongly depends on efficient oxygen removal and the addition of the triplet quencher beta-mercaptoethylamine. Single-molecule fluorescence experiments show that individual immobilized Cy5 molecules can be switched optically in milliseconds by applying alternating excitation at 633 and 488 nm between a fluorescent and nonfluorescent state up to 100 times with a reliability of >90% at room temperature. Because of their intriguing performance, carbocyanine dyes volunteer as a simple alternative for ultrahigh-density optical data storage. Measurements on single donor/acceptor (tetramethylrhodamine/Cy5) labeled oligonucleotides point out that the described light-driven switching behavior imposes fundamental limitations on the use of carbocyanine dyes as energy transfer acceptors for the study of biological processes.  相似文献   
286.
Multidimensional cosmological models with a space-time consisting ofn (n 2) Einstein spaces are investigated for a special class of multicomponent perfect fluid as a matter source. The dynamical behaviour of the universe is described. In the case of static internal spaces the external space evolves like a Friedmann universe with changing effective equation of state. Some of the models considered are integrable and classical as well as quantum solutions are found. Some of them represent wormholes. Quantum wormholes have a discrete spectrum.  相似文献   
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This work describes the computation and accurate reproduction of subtle shifts in reduction potentials for two mutants of the iron-sulfur protein Pyrococcus furiosus ferredoxin. The computational models involved only first-sphere ligands and differed with respect to one ligand, either acetate (aspartate), thiolate (cysteine), or methoxide (serine). Standard procedures using vacuum optimization gave qualitatively wrong results and errors up to 0.07 V. Using electrostatically screened geometries and large basis sets for expanding the wave functions gave quantitatively correct results, with errors of only 0.03 V. Correspondingly, only this approach predicted a change in the coordination mode of aspartate (i.e., a carboxylate shift) accompanying the reduction of the wild-type cluster, confirming results from synthetic models and explaining why electrostatic screening is necessary. Hence, the carboxylate shift appears to occur in the proteins from which data were collected. The results represent the most accurate predictions of shifts in reduction potentials for modified proteins, the success in part being due to the similar nature of the three amino acid ligands involved. The predicted carboxylate shift is expected to tune aspartate's degree of electron donation to the cluster's two oxidation states, thus making the reversible redox reaction feasible.  相似文献   
290.
The naturally occurring heterocyclic amino acid ibotenic acid (Ibo) and the synthetic analogue thioibotenic acid (Thio-Ibo) possess interesting but dissimilar pharmacological activity at ionotropic and metabotropic glutamate receptors (iGluRs and mGluRs). Therefore, a series of Thio-Ibo analogues was synthesized. The synthesis included introduction of substituents by Suzuki and Grignard reactions on 4-halogenated 3-benzyloxyisothiazolols, reduction of the obtained alcohols, followed by introduction of the amino acid moiety by use of 2-(N-tert-butoxycarbonylimino)malonic acid diethyl ester. The obtained Thio-Ibo analogues (1, 2a-g) were characterized in functional assays on recombinant mGluRs and in receptor binding assays on native iGluRs. At mGluRs, the activity at Group II was retained for compounds with small substituents (2a-2d), whereas the Group I and Group III receptor activities for all new compounds were lost. Detection of NMDA receptor affinity prompted further characterization, and two-electrode voltage-clamp recordings at recombinant NMDA receptor subtypes NR1/NR2A-D expressed in Xenopus oocytes were carried out for compounds with small substituents (chloro, bromo, methyl or ethyl, compounds 2a-d). This series of Thio-Ibo analogues defines a structural threshold for NMDA receptor activation and reveals that the individual subtypes have different steric requirements for receptor activation. The compounds 2a and 2c are the first examples of agonists discriminating individual NMDA subtypes.  相似文献   
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