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991.
Abbas‐Ali Shaik Martin Richter Hans R. Kricheldorf Ralph‐P. Krüger 《Journal of polymer science. Part A, Polymer chemistry》2001,39(19):3371-3382
Copolyesters with an alternating sequence of terephthalic acid and aliphatic dicarboxylic acids were prepared with three different methods. First, dicarboxylic acid dichlorides were reacted with bis(2‐hydroxyethyl)terephthalate (BHET) in refluxing 1,2‐dichlorobenzene. Second, the same monomers were polycondensed at 0–20 °C in the presence of pyridine. Third, dicarboxylic acid dichlorides and silylated BHET were polycondensed in bulk. Only this third method gave satisfactory molecular weights. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry revealed that the copolyesters prepared by the pyridine and silyl methods might have contained considerable fractions of cyclic oligoesters and polyesters despite the absence of transesterification and backbiting processes. The alternating sequences and thermal properties were characterized with 1H NMR spectroscopy and differential scanning calorimetry measurements, respectively. In agreement with the alternating sequence, all copolyesters proved to be crystalline, but the crystallization was extremely slow [slower than that of poly(ethylene terephthalate)]. A second series of alternating copolyesters was prepared by the polycondensation of silylated bis(4‐hydroxybut‐ yl)terephthalate with various aliphatic dicarboxylic acid dichlorides. The resulting copolyesters showed significantly higher rates of crystallization, and the melting temperatures were higher than those of the BHET‐based copolyesters. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3371–3382, 2001 相似文献
992.
W. Ihra M. Leadbeater J.L. Vega K. Richter 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(3):425-435
We study the effect on the density of states in mesoscopic ballistic billiards to which a superconducting lead is attached.
The expression for the density of states is derived in the semiclassical S-matrix formalism shedding light onto the origin of the differences between the semiclassical theory and the corresponding
result derived from random matrix models. Applications to a square billiard geometry and billiards with boundary roughness
are discussed. The saturation of the quasiparticle excitation spectrum is related to the classical dynamics of the billiard.
The influence of weak magnetic fields on the proximity effect in rough Andreev billiards is discussed and an analytical formula
is derived. The semiclassical theory provides an interpretation for the suppression of the proximity effect in the presence
of magnetic fields as a coherence effect of time reversed trajectories. It is shown to be in good agreement with quantum mechanical
calculations.
Received 21 August 1999 and Received in final form 21 March 2001 相似文献
993.
The structure of the active site of human glyoxalase I and the reaction mechanism of the enzyme-catalyzed conversion of the thiohemiacetal, formed from methylglyoxal and glutathione, to S-D-lactoylglutathione has been investigated by ab initio quantum chemical calculations. To realistically represent the environment of the reaction center, the effective fragment potential methodology has been employed, which allows systems of several hundred atoms to be described quantum mechanically. The methodology and the active site model have been validated by optimizing the structure of a known enzyme-inhibitor complex, which yielded structures in good agreement with the experiment. The same crystal structure has been used to obtain the quantum motif for the investigation of the glyoxalase I reaction. The results of our study confirm that the metal center of the active site zinc complex plays a direct catalytic role by binding the substrate and stabilizing the proposed enediolate reaction intermediate. In addition, our calculations yielded detailed information about the interactions of the substrate, the reaction intermediates, and the product with the active site of the enzyme and about the mechanism of the glyoxalase I reaction. The proton transfers of the reaction proceed via the two highly flexible residues Glu172 and Glu99. Information about the structural and energetic effect of the protein on the first-shell complex has been attained by comparison of the structures optimized in the local protein environment and in a vacuum. The environment of the zinc complex disturbs the Cs symmetry found for the complex in a vacuum, which suggests an explanation for the stereochemical behavior of glyoxalase I. 相似文献
994.
J. C. Hostetter H. S. Roberts R. B. Sosman A. Stähler E. Tiede F. Richter R. Peters H. M. Goodwin L. L. de Koninck J. M. Kolthoff W. D. Treadwell L. Weiss Schindler H. Willard Florence Fenwick J. F. King R. N. Washburne R. G. Gustavson C. W. Knudson Th. F. Buehrer O. E. Schupp B. S. Evans E. Zintl und P. Drossbach 《Fresenius' Journal of Analytical Chemistry》1928,75(1-2):47-63
Ohne Zusammenfassung 相似文献
995.
996.
M. Richter 《Isotopes in environmental and health studies》2013,49(9):298-301
A review about the properties of several types of tracerinput signals is given and in particular the stochastic signals are discussed. An estimation of the necessary measuring time shows, that true stochastic signals are unimportant for tracer experiments. The use of periodic pseudo-random binary signals (PRBS) permits a strong reduction of the measuring time. Therefore their practical application for tracer experiments is possible. The PRBS-method has special advantages in comparison with impulse methods, if fluctuations and disturbances exist within the investigated systems. Correlations for the estimation of parameters, needed for the realization of PRBS-measurements, are given and several measuring arrangements are discussed. The application of the PRBS-method for measurements of residence time distributions (RDC) of a mixer-settler extractor is described. 相似文献
997.
Certification aspects of the fast gradient method for solving the dual of parametric convex programs
Stefan Richter Colin Neil Jones Manfred Morari 《Mathematical Methods of Operations Research》2013,77(3):305-321
This paper examines the computational complexity certification of the fast gradient method for the solution of the dual of a parametric convex program. To this end, a lower iteration bound is derived such that for all parameters from a compact set a solution with a specified level of suboptimality will be obtained. For its practical importance, the derivation of the smallest lower iteration bound is considered. In order to determine it, we investigate both the computation of the worst case minimal Euclidean distance between an initial iterate and a Lagrange multiplier and the issue of finding the largest step size for the fast gradient method. In addition, we argue that optimal preconditioning of the dual problem cannot be proven to decrease the smallest lower iteration bound. The findings of this paper are of importance in embedded optimization, for instance, in model predictive control. 相似文献
998.
Sven C. Richter Prof. Dr. Martin Oestreich 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(36):8508-8512
A sequence of a Baeyer–Villiger oxidation and a Lewis acid-promoted reduction of the resulting formate with Et3SiH enabled the metal-free formal decarbonylation of tertiary and secondary aliphatic aldehydes. The new methodology mimics the biosynthetic decarbonylation pathway through oxidative C−C bond cleavage rather than the C(O)−H bond activation known from conventional Tsuji–Wilkinson-type reactions. The substrate scope is complementary to existing transition-metal-catalyzed protocols. 相似文献
999.
A. Gautier F. G. Hawley Erich Richter 《Analytical and bioanalytical chemistry》1918,57(11-12):546-548
1000.