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61.
Leandro E. Roth Enrique M. Valls Marcelo A. Villar 《Journal of polymer science. Part A, Polymer chemistry》2003,41(8):1099-1106
Model silicone networks obtained by curing linear poly(dimethylsiloxane) (PDMS) chains with end‐vinyl groups, (B2), with a polyfunctional silane‐terminated crosslinker of functionality f, (Af), through a hydrosilylation reaction have been widely used. In these networks, the principal characteristics of their ultimate molecular structure are strongly affected by the final extent of reaction reached during the crosslinking reaction. This work analyzes the effect of the initial concentration of the reactive end groups on the maximum attainable extent of reaction under normal bulk crosslinking conditions. This was accomplished by examining the reaction between linear B2 PDMS chains with difunctional and trifunctional silanes. The experimental results were fitted by an exponential equation to have an empirical equation able to predict the maximum extent of reaction to be obtained as a function of the initial concentration of reactive groups. Molecular parameters relevant to this study, such as the degree of polymerization, the weight‐average molecular weight for the A2 + B2 system, or the weight fraction of solubles for the A3 + B2 system, were calculated with a mean field theory (recursive approach). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1099–1106, 2003 相似文献
62.
Craig GA Costa JS Roubeau O Teat SJ Aromí G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(11):3120-3127
A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H4L, has led to an FeII spin‐crossover (SCO) complex, [Fe(H4L)2][ClO4]2 ? H2O ? 2 (CH3)2CO ( 1 ), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T0.5 of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high‐spin state may also be thermally trapped, exhibiting a very large TTIESST (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO4? and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe? N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order–disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising‐like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions. 相似文献
63.
In this note, we give a positive answer to a question of Dimca and Greuel about the quotient between the Milnor and Tjurina numbers of an isolated plane curve singularity in the cases of one Puiseux pair and semi-quasi-homogeneous singularities. 相似文献
64.
Diego Cortes José Luis Rios Angel Villar Serafin Valverde 《Tetrahedron letters》1984,25(30):3199-3202
The structures of Cherimolin and Dihydrocherimolin, two new antimicrobial γ-lactone-bis-tetrahydrofuranes from Mill., Annonaceae, have been elucidated by spectroscopic analysis. 相似文献
65.
Aromí G Bouwman E Burzurí E Carbonera C Krzystek J Luis F Schlegel C van Slageren J Tanase S Teat SJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(35):11158-11166
A highly asymmetric Ni(II) cluster [Ni(4)(OH)(OMe)(3)(Hphpz)(4)(MeOH)(3)](MeOH) (1) (H(2)phpz=3-methyl-5-(2-hydroxyphenyl)pyrazole) has been prepared and its structure determined by means of single-crystal X-ray diffraction by using synchrotron radiation. Variable-temperature bulk-magnetization measurements show that the complex exhibits intramolecular-ferromagnetic interactions leading to a spin ground state S=4 with close-lying excited states. Magnetization and high-frequency EPR measurements suggest the presence of sizable Ising-type magnetic anisotropy, with zero-field splitting parameters D=-0.263 cm(-1) and E=0.04 cm(-1) for the spin ground state, and an isotropic g value of 2.25. The presence of both axial and transverse anisotropy was confirmed through low-temperature specific heat determinations down to 300 mK, but no slow relaxation of the magnetization was observed by AC measurements down to 1.8 K. Interestingly, AC susceptibility measurements down to temperatures as low as 23 mK showed no indication of slow relaxation of the magnetization in 1. Thus, despite the presence of an anisotropy barrier (U approximately 4.21 cm(-1) for the purely axial limit), the magnetization relaxation remains extremely fast down to the lowest temperatures. The estimated quantum tunneling rate, Gamma>0.667 MHz, makes this complex a prime candidate for observation of coherent tunneling of the magnetization. 相似文献
66.
A. O. Gattone D. Abriola F. T. Avignone R. L. Brodzinski J. I. Collar R. J. Creswick D. E. Di Gregorio H. A. Farach C. K. Gurard F. Hasenbalg H. Huck H. S. Miley A. Morales J. Morales S. Nussinov A. Ortiz de Solrzano J. H. Reeves J. A. Villar K. Zioutas The SOLAX Collaboration 《Nuclear Physics B - Proceedings Supplements》1999,70(1-3)
A new technique has been used to search for solar axions using a single crystal germanium detector. It exploits the coherent conversion of axions into photons when their angle of incidence satisfies a Bragg condition with a crystalline plane. The analysis of approximately 1.94 kg.yr of data from the 1 kg DEMOS detector in Sierra Grande, Argentina, yields a new laboratory bound on axion-photon coupling of gαγγ < 2.7 × 10−9GeV−1, independent of axion mass up to 1 keV. 相似文献
67.
D. A. Vega M. A. Villar J. L. Alessandrini E. M. Valls 《Journal of Polymer Science.Polymer Physics》1999,37(11):1121-1130
A model to describe the dynamics of networks with linear pendant chains has been formulated based on the properties of ensembles of micronetworks, using the Rouse model. This development indicates that the terminal relaxation time of pendant chains with relatively large molecular weight scales with the square of the molecular weight of those chains. On the other hand, when the molecular weight of pendant and elastically active chains are comparable, a nearly exponential growth of the terminal relaxation time with the molecular weight is predicted. The main predictions of the model are compared with experimental results of model poly(dimethyl siloxane) (PDMS) networks, with controlled amounts of linear pendant chains of known molecular weight. The terminal relaxation time of these networks was estimated from the values of the loss modulus G″(ω) measured experimentally. An exponential dependence on the molecular weight of pendant chains was derived for the terminal relaxation time. This behavior is in good agreement with the predictions of our model for micronetworks, provided that the friction coefficient scales linearly with the number of entanglements. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1121–1130, 1999 相似文献
68.
This paper contains three parts where each part triggered and motivated the subsequent one. In the first part (Proper Secrets) we study the Shamir’s “k-out-of-n” threshold secret sharing scheme. In that scheme, the dealer generates a random polynomial of degree k−1 whose free coefficient is the secret and the private shares are point values of that polynomial. We show that the secret
may, equivalently, be chosen as any other point value of the polynomial (including the point at infinity), but, on the other
hand, setting the secret to be any other linear combination of the polynomial coefficients may result in an imperfect scheme.
In the second part ((t, k)-bases) we define, for every pair of integers t and k such that 1 ≤ t ≤ k−1, the concepts of (t, k)-spanning sets, (t, k)-independent sets and (t, k)-bases as generalizations of the usual concepts of spanning sets, independent sets and bases in a finite-dimensional vector
space. We study the relations between those notions and derive upper and lower bounds for the size of such sets. In the third
part (Linear Codes) we show the relations between those notions and linear codes. Our main notion of a (t, k)-base bridges between two well-known structures: (1, k)-bases are just projective geometries, while (k−1, k)-bases correspond to maximal MDS-codes. We show how the properties of (t, k)-independence and (t, k)-spanning relate to the notions of minimum distance and covering radius of linear codes and how our results regarding the
size of such sets relate to known bounds in coding theory. We conclude by comparing between the notions that we introduce
here and some well known objects from projective geometry.
相似文献
69.
Velázquez E Rivas R del Villar M Valverde A Peix A Mateos PF Velázquez E Martínez-Molina E 《Electrophoresis》2006,27(9):1732-1738
Low-molecular-weight (LMW) RNA profiles, which include ribosomal and transfer RNA molecules with similar small sizes, are molecular signatures of microorganisms with a great potential in microbial identification. The greatest resolution of these profiles was achieved by staircase electrophoresis in sequencing gels. Nevertheless, this technique is difficult to use because it takes 7 h, the gels have large sizes and it is necessary to heat the system and to recycle the buffer to maintain the denaturing conditions and avoid smile effects. Most available sequencing slabs have no internal temperature control or homogenizing devices, which by contrast are present in some newly designed non-sequencing slabs. Nevertheless, these slabs present two important problems for separating LMW RNA molecules, the size of gels is only 20 cm (instead of 40 cm) and the maximum voltage that can be reached is only 840 V (instead 2400 V). Staircase electrophoresis follows a model in which the external polarization is incrementally modified with a constant time step value. In the present work, we experimentally confirmed that by reducing the time step and increasing the total number of steps a suitable resolution is achieved. Under these conditions, despite the smaller size of the gels and the lower values of the electric field, the intensity reaches higher values than in sequencing gels and the LMW RNA profiles are correctly separated in 5 h. The resolution of these profiles obtained in non-sequencing gels is similar to that obtained in sequencing ones facilitating the analysis of large populations of microorganisms in any laboratory. 相似文献
70.
Companyó X Valero G Pineda O Calvet T Font-Bardía M Moyano A Rios R 《Organic & biomolecular chemistry》2012,10(2):431-439
An enantioselective α-oxyamination of unprotected 3-substituted oxindoles with nitrosobenzene catalyzed by tertiary amine-thiourea bifunctional organocatalysts has been developed and affords the corresponding 3-amino-2-oxindole derivatives in good yields and with moderate to excellent enantioselectivities (up to > 99.9?:?0.1 er when the product is isolated by direct filtration from the reaction mixture). The absolute configuration of the major enantiomers of the products has been established both by chemical correlation and by comparison between the theoretically calculated and the experimental ECD. 相似文献