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61.
Densification of boron carbide during sintering may be improved by a two-stage process, namely heating to 2000°C under vacuum and sintering at 2190°C under argon. This sintering regime allows achieving a relative density of the ceramic bodies fabricated from a fine powder higher than 95%. The nitrogen treatment of the boron carbide phase at 1900°C leads to the formation of the BN phase and precipitation of graphite. Vacuum treatment of these samples at 2000°C leads to decomposition of the boron nitride phase. The liberated free boron may again react with graphite to form in situ boron carbide particles. The experimental investigations of the sintering behavior of the boron carbide phase under various atmospheres supported the thermodynamic predictions regarding the phase transformation. No evidence, however, was found for enhanced sintering under a nitrogen atmosphere. 相似文献
62.
I. Levin T.G. AmosJ.C. Nino T.A. VanderahC.A. Randall M.T. Lanagan 《Journal of solid state chemistry》2002,168(1):69-75
Single-phase pyrochlore-type specimens of Bi1.5Zn0.92Nb1.5O6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements using neutron powder diffraction data confirmed an average pyrochlore structure A2B2O6O′ (Fd&3macr;m, a=10.5616(1) Å) with both Bi and Zn mixed on the A-sites. However, refinements revealed significant local deviations from the ideal pyrochlore arrangement which were caused by apparent displacive disorder on both the A and O′ sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by ∼0.39 Å from the ideal eight-fold coordinated positions. The displacements occur along the six 〈112〉 directions perpendicular to the O′-A-O′ links. In addition, the O′ ions were randomly displaced by ∼0.46 Å along all 12 〈110〉 directions. Crystal-chemical considerations suggest the existence of short-range correlations between the O′ displacements and both the occupancy of the A-sites (i.e., Bi or Zn) and the directions of the A-cation displacements. The combined A-cation and O′ displacements change the coordination sphere of the A-cations from 8 to (5+3); the resulting coordination environment of the A-cations bears similarities to that of the (5+1)coordinated Zn in zirconolite-like Bi2Zn2/3Nb4/3O7. The observed displacive disorder in the A2O′ network of the Bi1.5Zn0.92Nb1.5O6.92 structure involves atoms associated with the lowest-frequency vibrational bending mode, and is likely responsible for both the high dielectric constant and the dielectric relaxation reported for this compound. 相似文献
63.
The paper analyses the convergence of sequences of control polygons produced by a binary subdivision scheme of the form
相似文献
64.
65.
Dipole-moment derivatives, calculated by both the CNDO/2 method with different parameterizations and the INDO method, are compared to the experimental values determined from absolute infrared intensity measurements for the IR active modes of methane, ethane, ethylene and acetylene. A parameter refinement procedure is introduced in which the CNDO/2 molecular orbital parameters are adjusted through a damped least-squares treatment to give best agreement with the observed dipole-moment derivatives. It is found that the refinement does not substantially improve the agreement obtained with the original CNDO/2 parameterization. The INDO method gives somewhat poorer agreement than the CNDO/2 calculations. As an example of the applicability of the molecular orbital methods toward reproducing relative infrared intensities, the spectrum of cyclopropane in the gasphase is examined.
Zusammenfassung Die Ableitungen des Dipol-Moments, die nach der CNDO/2-Methode mit verschiedenen Parametrisierungen sowie der INDO-Methode berechnet wurden, werden mit den experimentellen Ergebnissen aus Messungen der absoluten Infrarot-Intensitäten für die IR-aktiven Schwingungen von Methan, Äthan, Äthylen und Azetylen verglichen. Die CNDO/2-Parameter werden mit einer Methode der kleinsten Quadrate den beobachteten Dipol-Moment-Ableitungen angepaßt. Die Ergebnisse sind jedoch nicht wesentlich von denen der ursprünglichen CNDO/2-Methode verschieden. Die INDO-Ergebnisse sind nicht so gut wie die CNDO/2-Ergebnisse. Als Beispiel der Anwendbarkeit der MO-Methoden zur Berechnung von relativen IR-Intensitäten wird das Spektrum des Cyclopropans in der Gasphase untersucht. Résumé Les dérivées du moment dipolaire, calculées par la méthode CNDO/2 avec différentes paramétrisations et par la méthode INDO, sont comparées aux valeurs expérimentales déterminées à partir de mesures d'intensité absolue pour les modes actifs dans l'infra-rouge dans le méthane, l'éthane, l'éthylène et l'acétylène. Les paramètres sont ajustés de manière à donner le meilleur accord avec les dérivées du moment dipolaire. Cet ajustement n'améliore pas sensiblement l'accord obtenu avec la paramétrisation CNDO/2 originale. La méthode INDO donne des résultats moins bons que les calculs CNDO/2. Le spectre du cyclopropane en phase gazeuse est étudié comme exemple de l'applicabilité de la méthode des orbitales moléculaires au calcul des intensités relatives infra-rouge.相似文献 66.
V. A. Mamedov N. A. Zhukova T. N. Beschastnova Ya. A. Levin A. T. Gubaidullin I. A. Litvinov 《Russian Chemical Bulletin》2007,56(11):2308-2314
The condensation of methyl phenylchloropyruvate with 1-phenyl-3-(2-pyridyl)thiourea and its 3-and 4-picolyl homologs affords
the corresponding 4-hydroxythiazolidines, which react with o-phenylenediamine to give one of two possible thiazolo[3,4-a]quinoxalines containing the pyridyl-or picolylimine substituents at position 1. 3a-Hydroxy-3-phenylimino-1-(2-pyridyl)thiazolo[3,4-a]quinoxalin-4-(3H,5H)-one, which is a covalent hydrate of the final product, was isolated as an intermediate in this reaction. 相似文献
67.
Mann DL Ware GM Bonnin E Eitenmiller RR Barna E Christiansen S De Borde JL DeVries J Gilliland P Hemmer J Kalman A Konings E Levin D Salvati L Woollard D 《Journal of AOAC International》2005,88(1):30-37
A liquid chromatographic (LC) method was validated for the determination of total vitamin B6 in infant formula. Total vitamin B6 was quantified by converting the phosphorylated and free vitamers into pyridoxine. Pyridoxine was determined by ion pair reversed-phase LC with fluorescence detection. The method was subjected to an AOAC collaborative study involving a factory-manufactured, milk- and soy-based infant formula. Each was spiked at 3 concentrations in the range of 0-1 microg/g and sent as blind duplicate to participant laboratories. Nine laboratories returned valid data which were statistically analyzed for outliers and precision parameters. The repeatability relative standard deviation (RSD(r)) ranges were 2.0-4.0 and 3.5-5.9% for fortified milk- and soy-based formulas, respectively. The reproducibility relative standard deviation (RSD(R)) ranges were 8.2-8.4 and 6.7-11.2% for fortified milk- and soy-based formulas, respectively. HORRAT values ranged from 0.42 to 0.53, indicating that the precision of the method is acceptable. The mean RSD(r):RSD(R) values were 0.60 and 0.55 for milk- and soy-based formulas, respectively. As expected, RSDs for the unfortified samples were higher, but their HORRAT values (0.81 and 2.06) helped define a realistic limit of quantitation as 0.05 microg/g. Recovery data were quantitative and varied between 81.4 and 98.0% (mean = 89.8%) for each of 6 spiked materials. 相似文献
68.
A. E. Kretov A. P. Momsenko A. S. Bespalyi Yu. A. Levin 《Chemistry of Heterocyclic Compounds》1973,9(5):592-594
1-Oxo-3-thioxo-5-hydroxy-2-acyl-2,3-dihydro-1H-1,3-thiazino[6,5-c]quinolines were obtained for the first time by condensation of 2-hydroxy-3-mercaptoquinoline-4-carboxylic acid with acyl isothiocyanates. Alkyl and acyl isothiocyanates react with 1-methyl-2-oxo-3-mercapto-1, 2-dihydroquinoline-4-carboxylic acid to give 1,5-dioxo-3-thioxo-2-alkyl (acyl)6-methyl-2, 3,5,6-tetrahydro-1H-1,3-thiazino [6,5-c] quinolines.See [1] for communication I.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 641–643, May, 1973. 相似文献
69.
Moses Salci Evgeny Yarevsky Sergey B. Levin Nils Elander 《International journal of quantum chemistry》2007,107(2):464-468
A three‐dimensional finite element method is applied to the ground states of the symmetric and asymmetric atomic helium trimers 4He3 and 4He2–3He. Three different He–He interaction potentials of hard‐core nature were studied. Two extrapolation procedures based on the convergence properties of the finite element method are investigated. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
70.
Tunnel junctions ofPb-I-Al andSn-I-Al were implanted at low temperatures with Mn ions in the ppm region. The tunnel characteristicdI/dV was measured on the pure sample and on the alloy with different concentrations. A bound state is found in both systems at an energy 0.7
0. Comparison with theory confirms, that the Kondo effect is not necessary for the existence of a bound state. 相似文献
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