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81.
82.
Ion-exchange resins have been often used as catalysts especially those based on styrene-divinylbenzene copolymers with sulfonic acid groups in the aromatic rings of polymer chains. That is due to the advantages of heterogenous catalysis over the homogeneous acid catalysis. Moreover, resin catalysts can often lead to high selectivity in organic reactions due to the matrix effects. Therefore, the study of copolymers synthesis conditions to determine the type of polymer structure produced as well as the characterization of sulfonic resins obtained thereof are of great interest. The current paper describes the synthesis, characterization and evaluation as catalysts of sulfonic resins derived from polymer supports synthesized by aqueous suspension polymerization of styrene and divinylbenzene. The reaction conditions were varied and polymer supports with different physical properties and morphological characteristics were obtained. The polymer supports were chemically modified by sulfonation. The resultant sulfonic resins had their catalyst activity evaluated in the esterification of acetic acid with n-butanol. 相似文献
83.
Neusa M.T. Pires Fernanda M.B. Coutinho Marcos A.S. Costa 《European Polymer Journal》2004,40(11):2599-2603
This paper reports the study of the dependence of reaction conversion, catalyst activity, polymer microstructure, molecular weight, molecular weight distribution curves and Mooney viscosity on reaction temperature and monomer concentration in the reaction medium used in the synthesis of high cis-polybutadiene. A ternary catalyst system composed by neodymium versatate, trans-butyl chloride and diisobutylaluminum hydride was used in its synthesis. The highest molecular weights were obtained at polymerization temperatures in the range from 70 to 80 °C. The highest content of cis-1,4 repeating units (about 99%) was observed when the polymerization was carried out at the lowest initial monomer concentration (0.56 mol/l). 相似文献
84.
Fernanda M. B. Coutinho Luiz Cludio Rezende Juan Raul Quijada 《Journal of polymer science. Part A, Polymer chemistry》1986,24(11):3021-3032
A comparative kinetic study of the reaction of three different hydroxylated liquid poly-butadienes (M?n ? 3000)–R-45M, R-45HT and H-034–with 3-isocyanatomethyl–3,5,5-trimethylclohexylisocyanate (IPDI) and dimer diacid diisocyanate (DDI) was carried out in toluene solution. An analytical method was used to follow the kinetics of the reactions, at four different temperatures. In the second-order plots, a discontinuity was observed in the reaction with IPDI, in contrast to the reactions with DDI which showed straight-line plots. In all studied reactions, the R-45M polybutadiene was about twice as reactive as R-45HT and H-034. The latter hydroxylated polybutadienes (R-45HT and H-034) showed similar reactivities. 相似文献
85.
Marta C. D.?Silva J. R.?Botelho Marta M.?Conceio B. F.?Lira Monyque A.?Coutinho A. F.?Dias A. G.?Souza P. F. A.?Filho 《Journal of Thermal Analysis and Calorimetry》2005,79(2):277-281
Conventional methods have been proposed to determine thermal properties of edible vegetable oils. The evaluation of the applicability of DSC and microwave oven (MO) methods to determine the specific heat capacities of the edible vegetable oils was performed. It was observed that the specific heat capacities of each edible oil increased as a function of the saturation of the fatty acids. 相似文献
86.
Sequential Monte Carlo/quantum mechanical calculations are performed to study the solvent effects on the electronic absorption
spectrum of formamide (FMA) in aqueous solution, varying from hydrogen bonds to the outer solvation shells. Full quantum-mechanical
intermediate neglect of differential overlap/singly excited configuration interaction calculations are performed in the supermolecular
structures generated by the Monte Carlo simulation. The largest calculation involves the ensemble average of 75 statistically
uncorrelated quantum mechanical results obtained with the FMA solute surrounded by 150 water solvent molecules. We find that
the n → π* transition suffers a blueshift of 1,600 cm−1 upon solvation and the π → π* transition undergoes a redshift of 800 cm−1. On average, 1.5 hydrogen bonds are formed between FMA and water and these contribute with about 20% and about 30% of the
total solvation shifts of the n → π* and π → π* transitions, respectively. The autocorrelation function of the energy is used
to sample configurations from the Monte Carlo simulation, and the solvation shifts are shown to be converged values.
Received: 14 March 2002 / Accepted: 3 April 2002 / Published online: 24 June 2002 相似文献
87.
Microalgae have an outstanding capacity to efficiently produce value-added compounds. They have been inspiring researchers worldwide to develop a blue biorefinery, supporting the development of the bioeconomy, tackling the environmental crisis, and mitigating the depletion of natural resources. In this review, the characteristics of the carotenoids produced by microalgae are presented and the downstream processes developed to recover and purify them are analyzed, considering their main applications. The ongoing activities and initiatives taking place in Portugal regarding not only research, but also industrialization under the blue biorefinery concept are also discussed. The situation reported here shows that new techniques must be developed to make microalgae production more competitive. Downstream pigment purification technologies must be developed as they may have a considerable impact on the economic viability of the process. Government incentives are needed to encourage a constructive interaction between academics and businesses in order to develop a biorefinery that focuses on high-grade chemicals. 相似文献
88.
89.
F.?M.?L.?AlmeidaJr. Y.?A.?Coutinho J.?A.?Martins Sim?esEmail author J.?Ponciano A.?J.?Ramalho S.?Wulck M.?A.?B.?Vale 《The European Physical Journal C - Particles and Fields》2004,38(1):115-122
It was recently shown that left-right symmetric models for elementary particles can be built with only two Higgs doublets. The general consequence of these models is that the left and right fermionic sectors can be connected by a new neutral gauge boson Z having its mass as the only additional new parameter. In this paper we study the influence of the fundamental fermionic representation for this new neutral gauge boson. Signals of possible new heavy neutral gauge bosons are investigated for the future electron-positron colliders at
GeV, 1 TeV and 3 TeV. The total cross sections, forward-backward and left-right asymmetries and model differences are calculated for the process
. Bounds on Z masses are estimated.Received: 4 May 2004, Revised: 22 September 2004, Published online: 9 November 2004 相似文献
90.
The effect of the electron-magnon interaction on the Raman scattering by magnons is discussed. It is shown that in addition to Loudon's spin-orbit coupling mechanism there is another, carrier-mediated, contribution to the Raman line. The frequency dependence of the resonant scattering is discussed and application to CdCr2Se4 is made. 相似文献