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181.
182.
S. Carturan A. Quaranta M. Bonafini A. Vomiero G. Maggioni G. Mattei C. de Julián Fernández M. Bersani P. Mazzoldi G. Della Mea 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,42(2):243-251
Silver nanoclusters embedded in two transparent
fluorinated polyimides, 4,4'-hexafluoroisopropylidene diphthalic anhydride –
2,3,5,6-tetramethyl paraphenylene diamine (6FDA-DAD) and 3,3',4,4' –
biphenyltetracarboxylic acid dianhydride – 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (BPDA-3F), have been
produced by surface modification with KOH aqueous solution followed by
K-assisted Ag doping and thermal reduction in hydrogen atmosphere. The
reaction rate of the nucleophilic hydrolysis in KOH, studied by Fourier
transform infrared spectroscopy (FT-IR) and Rutherford backscattering
spectrometry (RBS), depends on the polyimide chemical structure. After
ion-exchange in AgNO3 solution and subsequent annealing, the polyimide
structure recovery was monitored by FT-IR whereas the characteristic surface
plasmon absorption band of silver nanoparticles was evidenced by optical
absorption measurements. The structure of silver nanoclusters as related to
size and size distribution in the different polyimide matrices was
thoroughly investigated by Transmission electron microscopy (TEM) and X-ray
diffraction (XRD). The collected data evidenced a uniform distribution of Ag
clusters of nanometric size after thermal treatment at 300 ○C in both
polyimides. For the same ion-exchange treatment parameters and annealing
temperature, XRD analyses evidenced the presence of crystallites with
similar sizes. 相似文献
183.
184.
185.
In this paper we study the spectral counting function for the weightedp-laplacian in one dimension. First, we prove that all the eigenvalues can be obtained by a minimax characterization and then
we show the existence of a Weyl-type leading term. Finally we find estimates for the remainder term.
The first author is supported by Universidad de Buenos Aires grant TX48, by ANPCyT PICT No. 03-05009. The second author is
supported by CONICET and Universidad de San Andres. 相似文献
186.
187.
Alvarez-Puebla RA Valenzuela-Calahorro C Garrido JJ 《Langmuir : the ACS journal of surfaces and colloids》2004,20(9):3657-3664
Brown humic acids (BHAs) constitute the most polar and soluble fraction of humic acids. Their colloidal character and their high number of functional surface groups justify their higher reactivity as against metallic cations with respect to other humic fractions (i.e., gray humic acids and humins). The aim of this work is to study the retention mechanisms of Cu(II), Ni(II), and Co(II) on a BHA by means of a proper combination of physical and chemical techniques: sorption isotherms, mathematical modeling of these isotherms, molecular modeling, FTIR, and N2 (77 K) and CO2 (273 K) adsorption. Electrostatic retention for the three cations is an important mechanism at very low concentrations. Its magnitude is higher than that of the specific retention in the initial stages of the retention but it decreases progressively with respect to the former as the initial metal concentration increases. The BHA surface area varies with the amount of retained metal. When the initial amount of added metal is low (n0 < 80 mmol kg(-1)), the cations form 2:1 complexes, which are energetically favored due to the chelate effect. To obtain this coordination, the BHA slightly modifies its conformation by decreasing its area. When the initial amount of added metal is sufficiently high to occupy most of the surface functional groups (n0 > 1280 mmol kg(-1)), the cations are heterogeneously retained over the whole surface, thus preventing the available groups at low n0 from giving place to the 2:1 complexes due to the fact that they are already occupied. 相似文献
188.
Carriedo GA García Alonso FJ Gómez Elipe P Brillas E Labarta A Juliá L 《The Journal of organic chemistry》2004,69(1):99-104
Stable cyclotriphosphazenes 5 and 6, with three and four carbon radical centers, have been prepared by condensation of (4-hydroxy-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl radical (4) with tetrachloro-2,2'-dioxybiphenylcyclotriphosphazene (7). EPR studies of both polyradicals in fluid solution suggest an electronic communication through the PN multiple bonds of the cycle. EPR spectral results in frozen solutions and magnetic susceptibility measurements in the solid are consistent with very weak electron-electron dipolar interactions. Reductive cyclic voltammetry shows a single three-electron redox couple for triradical 5 and a single four-electron redox couple for tetraradical 6. Both polyradicals 5 and 6 have been chemically oxidized to a stable trication 5(3+) and a tetracation 6(4+), respectively, by electron-transfer reactions. 相似文献
189.
Castro M Cruz J López-Sandoval H Barba-Behrens N 《Chemical communications (Cambridge, England)》2005,(30):3779-3781
The ephedrine derivative, (H2ceph), yields [Cu(Hceph)2], showing a CH...Cu(II) agostic interaction; while in the analogous compound [Cu(Hcpse)2], with pseudoephedrine (H2cpse), that interaction is absent, despite the fact that these two diasteromers differ only in the orientation of the methyl and phenyl groups: erythro in H2ceph and threo in H2cpse. The X-ray crystal structure of [Cu(Hceph)2], indicates a Cu...HC length of 2.454 A and the theoretical study reveals the formation of a Cu...HC bond since the associated electronic density shows both a bond critical point and a bond ring critical point. 相似文献
190.
A new method to test the antioxidant activity of polyphenols by electron transfer reactions to a stable organic free radical, tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl radical (HNTTM), is reported. Therefore, the activity of the natural flavanols, (-)-epicatechin, and two synthetic derivatives, 4beta-(S-cysteinyl)epicatechin and 4beta-(2-aminoethylthio)epicatechin, can be differentiated by their capacity to transfer hydrogen atoms to 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) and to transfer electrons to HNTTM. [structure: see text] 相似文献