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921.
We present exact diagonalization results on a modified Peierls-Hubbard model for the neutral-ionic phase transition. The ground state potential energy surface and the infrared intensity of the Peierls mode point to a strong, nonlinear electron-phonon coupling, with effects that are dominated by the proximity to the electronic instability rather than by electronic correlations. The huge infrared intensity of the Peierls mode at the ferroelectric transition is related to the temperature dependence of the dielectric constant of mixed-stack organic crystals. 相似文献
922.
We describe a very rapid high-performance capillary electrophoresis method for the separation and quantification of reduced (GSH) and oxidized (GSSG) glutathione in red blood cells. Two procedures for sample preparation have been compared, Microcon-10 membrane filtration and acid precipitation. The separation is obtained in an uncoated capillary using a high ionic strength borate buffer at pH 7.8. The intra-assay coefficients of variation (CVs%) are 1.53 and 1.66 for GSH and GSSG, respectively. The run is shorter than 90 s and the migration time is highly reproducible both for GSH (CV% 0.22) and GSSG (CV% 0.17). When the filtration step is used only GSH is found, whereas both GSH and GSSG are detectable after acid precipitation, suggesting that GSSG revealed after acid treatment may be an artefact due to GSH oxidation. Because of its good analytical performance this method could be used for routine red blood cell glutathione measurement in healthy or pathological conditions. 相似文献
923.
Let M be a manifold of X = C
n
, A a small analytic disc attached to M, z
o
a point of A where A is tangent to M, z
1 another point of A where M extends to a germ of manifold M
1 with boundary M. We prove that CR functions on M which extend to M
1 at z
1 also extend at z
o
to a new manifold M
2. The directions M
1 and M
2 point to, are related by a sort of connection associated to A which is dual to the connection obtained by attaching 'partial analytic lifts' of A to the co-normal bundle to M in X. 相似文献
924.
925.
We propose a new measure of conditioning for the exponential of a block triangular matrix. We also show that different condition numbers must be used to assess the accuracy of different algorithms which implement diagonal Padé with scaling and squaring. 相似文献
926.
The Chaboche nonlinear kinematic hardening model: calibration methodology and validation 总被引:2,自引:0,他引:2
This work studies how a nonlinear kinematic model aimed for cyclic plasticity could be put into effect and used within a FEM code. A correct modeling of cyclic elasto-plastic behavior can be exploited in low-cycle fatigue life investigation as well as in manufacturing problems related to springback prediction. The chosen formulation has been proposed by Chaboche, and it is implemented in most of the commercial codes used for nonlinear FEM simulations. At first, a procedure for the proper identification of unknown material model parameters has been put forward. This calibration, based on the data collected from experimental low-cycle fatigue tests, has been performed by means of an inverse method. Laboratory tests differ according to the type of material under investigation. A classification can be operated distinguishing between specimens obtained from bulk materials or from sheet metals. For the former, standard tension-compression tests have been performed, while for the latter, a dedicated testing equipment for three-point bend cyclic tests has been devised. Then, further experimental tests have been run to check model transferability: different strain per cycle amplitudes, asymmetric strain cycling and different stress triaxiality levels have been investigated. For each of these tests, experimental vs. FEM results have been analyzed to show the level of agreement that has been reached. 相似文献
927.
Summary. By providing a matrix version of Koenig's theorem we reduce the problem of evaluating the coefficients of a monic factor
r(z) of degree h of a power series f(z) to that of approximating the first h entries in the first column of the inverse of an Toeplitz matrix in block Hessenberg form for sufficiently large values of n. This matrix is reduced to a band matrix if f(z) is a polynomial. We prove that the factorization problem can be also reduced to solving a matrix equation for an matrix X, where is a matrix power series whose coefficients are Toeplitz matrices. The function is reduced to a matrix polynomial of degree 2 if f(z) is a polynomial of degreeN and . These reductions allow us to devise a suitable algorithm, based on cyclic reduction and on the concept of displacement rank,
for generating a sequence of vectors that quadratically converges to the vector having as components the coefficients of the factor r(z). In the case of a polynomial f(z) of degree N, the cost of computing the entries of given is arithmetic operations, where is the cost of solving an Toeplitz-like system. In the case of analytic functions the cost depends on the numerical degree of the power series involved
in the computation. From the numerical experiments performed with several test polynomials and power series, the algorithm
has shown good numerical properties and promises to be a good candidate for implementing polynomial root-finders based on
recursive splitting strategies. Applications to solving spectral factorization problems and Markov chains are also shown.
Received September 9, 1998 / Revised version received November 14, 1999 / Published online February 5, 2001 相似文献
928.
Summary: Three pyridine strong base anion exchangers as beads were obtained by quaternization reactions of a 4-vinylpyridine : 8% divinylbenzene copolymer of gel type. These resins possess methyl / ethyl / butyl radicals as substituents on N+ atoms and have exchange capacities of 4.80 mEq/g and 2.10 mEq/mL. For pyridine strong base anion exchangers, the behaviours in the retention processes of Cr(VI) as oxyanions and Ga(III) as [GaCl4]− complex anion were evaluated with the bath method. All the resins exhibited retention properties, but the retained amounts of the metal cations are different as a function of the alkyl length as substituent on N+ atoms and the complex anion nature. Thus, Cr(VI) oxyanions are best retained by the resin with CH3 as substituent on N+ atoms while [GaCl4]− complex anion by the resin with C4H9 as substituent on N+ atoms. By aminolysis reaction of an ethylacrylate : acrylonitrile : divinylbenzene copolymer as beads of macroporous type with NH2OH · HCl in the presence of C2H5OH a new chelating ion exchanger was performed which contains both amidoxime and hydroxamic acid functional groups. This ion exchanger has the retention property for different metal cations but its retention capacities values are strongly dependent of the nature of metal cation and the counterion as well as pH of the solution. Thus, in the static conditions Zn(II) cation with NOequation/tex2gif-stack-1.gif anion as counterion is retained with the best result at pH = 5. As an example, for the aqueous metal cation solution of 10−2 M concentration for Zn(NO3)2 the resin possess at equilibrium a retention capacity of 6.70 mmol Zn/g dry resin and for Cu(II) from Cu(NO3)2 solution of same concentration, the retention capacity is 0.22 mmol Cu/g dry resin and Fe(III) from Fe(NO3)3 solution is not retained. 相似文献
929.
In recent years several proposals for the step-size selection have largely improved the gradient methods, in the case of both constrained and unconstrained nonlinear optimization. We introduce a new step-size rule with some crucial properties. We design step-size selection strategies where the new rule and a standard Barzilai-Borwein (BB) rule can be adaptively alternated to get meaningful convergence rate improvements in comparison with other BB-like gradient schemes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
930.
Dietmar Rakowitz Gottfried Heinisch Peter Lukavsky Sigrid Kiendler Cornelia Trenkwalder Daniela Barlocco Giulio Rastelli Luca Costantino 《Journal of heterocyclic chemistry》2000,37(5):1089-1096
In the course of our research aimed at the discovery of new compounds acting as aldose reductase inhibitors, we tested a series of some (E)‐ and (Z)‐ω‐[[(aryldiazinylmethylene)amino]oxy]alkanoic acids, which were found to have moderate in vitro inhibitory activity. On this basis we have now prepared several new derivatives modified both at the length of the chain and at its terminal carboxylic group, together with compounds carrying various substituents at the phenyl ring. This paper describes their synthesis and biological properties. 相似文献