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891.
Summary Thermal density fluctuations in amorphous polymethyl methacrylate, polycarbonate and polyethylene terephthalate were studied by means of small angle X-ray scattering. The measurements were performed in a temperature range from 20 °C up to about 50 °C above the glass transition temperatureT
g
of the individual sample.AboveT
g
the experimental values of the fluctuations are proportional to the isothermal compressibility and the temperature of the sample as predicted by the fluctuation theory for a system in thermodynamic equilibrium.At temperatures belowT
g
this relation is no longer correct. The experimentally determined fluctuations are now proportional to the compressibility of the sample in the equilibrium state atT
g
and to the actual temperature of the glassy sample. By considering a statistical ensemble with exchange of energy, particles and order between the systems of the ensemble an equation can be derived for the fluctuations of the number of particles per given volume which predicts the observed behaviour. The order parameter takes into account the fact that the properties of the glassy state depend on the way by which the state was produced.
Zusammenfassung Es wurden thermische Dichtefluktuationen in amorphem Polymethylmethacrylat, Polycarbonat und Polyäthylenterephthalat mit Hilfe der Röntgenkleinwinkelstreuung untersucht. Die Messungen erstrecken sich über einen Temperaturbereich von 20 °C bis zu etwa 50 °C oberhalb der GlastemperaturT g der jeweiligen Probe.In Übereinstimmung mit den Ergebnissen der Fluktuationstheorie sind die Fluktuationen oberhalb vonT g proportional zur isothermen Kompressibilität und zur Temperatur der Probe.Unterhalb der Glastemperatur ist diese Beziehung nicht mehr erfüllt. Hier ist die Fluktuation proportional zur Kompressibilität der Probe im Gleichgewichtszustand beiT g und zur Temperatur. Auf der Basis einer statistischen Gesamtheit mit der Austauschmöglichkeit von Energie, Partikel und Ordnung zwischen den Systemen der Gesamtheit kann eine Beziehung für die Dichtefluktuation abgeleitet werden, die mit den experimentellen Ergebnissen übereinstimmt. Der zusätzliche Ordnungsparameter berücksichtigt die Tatsache, daß die Eigenschaften im Glaszustand wegabhängig sind.相似文献
892.
The valence ionization potentials (IP's) of cyclopentadiene and 1-sila-cyclopentadiene-(2,4) are studied by an ab initio many-body approach which includes the effect of electron correlation and reorganization beyond the Hartree-Fock approximation. The Hartree-Fock approximation gives the correct ordering of the IP's for cyclopentadiene but this ordering does not agree with the results of the previous experimental and theoretical studies. The ordering is 1a2(π), 2b1(π), 4b2, 6a1, 5a1, 3b2, 1b1 (π), 4a1, 2b2, 3a1. For sila-cyclopentadiene the ordering of the IP's is: 1a2(π), 4b2, 2b1(π), 6a1, 1b1(π), 5a1, 3b2, 4a1, 3a1, 2b2. The Hartree-Fock approximation is found to be incorrect with respect to the ordering of the 4b2 and 2b1(π) IP's. A number of one-electron properties are calculated in the one-particle approximation and compared with the available experimental data. 相似文献
893.
Professor J. R. Sabin R. E. Harris T. W. Archibald P. A. Kollman L. C. Allen 《Theoretical chemistry accounts》1970,18(3):235-238
Ab initio calculations are carried out on the planar hexamer model of anomalous water using a variety of basis sets. The results are compared to results of similar calculations on free water and water in the ice I-like puckered ring configurations.
Zusammenfassung Es werden ab initio-Rechnungen für das planare Hexamer-Modell von anomalem Wasser unter Verwendung mehrerer BasissÄtze durchgeführt. Die Ergebnisse werden mit Resultaten Ähnlicher Rechnungen über freies Wasser und Wasser in der I-Eis Ähnlichen Konfiguration verzerrter Ringe verglichen.
Résumé Calculs ab initio sur le modèle hexamérique plan de l'eau anormale en utilisant différentes bases. Les résultats sont comparés à ceux de calculs similaires sur l'eau libre et sur l'eau dans une configuration cyclique compacte analogue à celle de la glace I.相似文献
894.
The major product from 2,3-naphthoquinodimethane formed by cyclisation of -dipropadienylbenzene was found to be the dimer 5 containing an eight-membered ring, for which the inversion barrier was determined by dynamic 1H NMR spectrometry, ΔG3 = 18 kcal/mole. 相似文献
895.
896.
Platinum Iodides PtI2 and Pt3I8 Powder samples of tetragonal Pt3I8 (a = 1166.4(1); c = 1068.2(2) pm) and the cubic α-form of PtI2 (a = 1109 pm) were obtained by thermal decomposition of PtI4 in closed quarz ampoules at a I2 pressure of 2–3 bar and 8 bar, respectively, and at 320°C and 430°C and 430°C, respectively. Single crystals of the monoclinic α-Form of PtI2 (a = 658.77(6); b = 871.50(34), c = 688.94(11) pm; β = 102.76°; Z = 4) were formed by hydrothermal synthesis from PtI4, KI, and I2 at 420°C. In the crystal structure of β-PtI2 two square planar PtI4-units are connected by a common edge to planar Pt2I6 groups which are linked by common corners to puckered layers. Crystals of Pt3I8 synthesized by hydrothermal synthesis from PtI4, KI, and I2 at 350°C were twinned and showed defects. As the result of X-ray studies the compound can be formulated as a mixed-valence platinum(II,IV) iodide PtI4 · 2 PtI2. Octahedral PtI6 and square planar PtI4 units are linked together to a three dimensional skeleton. 相似文献
897.
A stable nitronic acid, 2-aci-nitro-1,3-propanediol, may be prepared from nitromethane and paraformaldehyde under fluoride ion catalysis. 相似文献
898.
J. Zhuang W. Kunnmann L.M. Corliss J.M. Hastings R.O. Moyer 《Journal of solid state chemistry》1983,48(1):117-120
A powder neutron diffraction study of Eu1532IrD5 has shown it to be isostructural with Sr2IrD5 at room temperature. Unlike the latter compound, it does not undergo a deuterium-ordering transition at low temperatures. Magnetic scattering at helium temperatures confirms the suggestion that it becomes ferromagnetic below 20 K with a moment corresponding to that of divalent europium. 相似文献
899.
W. Kunz J. Barthel L. Klein T. Cartailler P. Turq B. Reindl 《Journal of solution chemistry》1991,20(9):875-891
A variety of methods has been used for the study of lithium bromide solutions in acetonitrile yielding by their combination reliable information on different levels of approximation. Osmotic coefficients based on precise vapor pressure measurements are reproduced by CM (chemical model) and HNC (hypernetted chain) calculations and by BD (brownian dynamics) simulations. The results of neutron scattering experiments are treated with the help of HNC and BD methods. Hartree-Fock calculations on isolated LiBr pairs and solvated lithium ions yield reliable particle distances and reveal the geometry of the lithium solvation sphere. 相似文献
900.
This review addresses the special problems associated with the micro-structural characterization of thin and ultrathin organic films, primarily by optical spectroscopies. Films which are deposited by Langmuir-Blodgett techniques, self-assembly chemistry, and bulk film deposition techniques are considered. The use of enhanced optical excitation using surface phasma resonances and integrated optical structures is discussed extensively, as is the use of ellipsometry. Discussion of the spectroscopies used is broken into a section on electronic spectroscopies and an extensive discussion of vibrational spectroscopies. Vibrational information may be obtained with photons (absorption or scattering) or electrons (loss spectra), and the types of experimental systems amenable to each, along with the advantages and disadvantages of each are explored. 相似文献