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排序方式: 共有167条查询结果,搜索用时 15 毫秒
1.
The contribution of configurational entropy to the effective hopping frequency of ionic transport in amorphous systems is discussed. The effective rate of ion hopping has been extracted from the onset frequency of the ac conductivity measured in ionically conducting silicate glasses. Both the onset frequency and the dc conductivity exhibit Arrhenius-type thermal activation with similar values for the activation energy, DeltaEa=0.65+/-0.3 eV. The prefactor of the onset frequency results in nu0'=(1.05+/-0.05)x10(11) Hz, which is much lower than characteristic vibrational frequencies (10(13) Hz). Following standard hopping percolation theory, the long-range motion is dominated by a fraction of high-energy barriers that connect clusters of faster sites. The multiplicity of equivalent sites for ion hop entails a retardation of the effective jumping time with respect to the elementary hop. This effect can be assimilated into a negative activation entropy term in the frequency prefactor of the ion hopping rate, which depends on the features of energy clustering and accounts for the wide dispersion of nu0' reported for many conducting glasses. The model implies an effective percolation length of Lc approximately 7 nm, in good agreement with previous works. 相似文献
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In a secret sharing scheme, some participants can lie about the value of their shares when reconstructing the secret in order to obtain some illicit benefit. We present in this paper two methods to modify any linear secret sharing scheme in order to obtain schemes that are unconditionally secure against that kind of attack. The schemes obtained by the first method are robust, that is, cheaters are detected with high probability even if they know the value of the secret. The second method provides secure schemes, in which cheaters that do not know the secret are detected with high probability. When applied to ideal linear secret sharing schemes, our methods provide robust and secure schemes whose relation between the probability of cheating and the information rate is almost optimal. Besides, those methods make it possible to construct robust and secure schemes for any access structure. 相似文献
5.
A single domain wall (DW) moves at linearly increasing velocity under an increasing homogeneous drive magnetic field. Present experiments show that the DW is braked and finally trapped at a given position when an additional antiparallel local magnetic field is applied. That position and its velocity are further controlled by suitable tuning of the local field. In turn, the parallel local field of small amplitude does not significantly affect the effective wall speed at long distance, although it generates tail-to-tail and head-to-head pairs of walls moving along opposite directions when that field is strong enough. 相似文献
6.
Iulian Preda Leonardo Soriano Daniel Díaz‐Fernández Guillermo Domínguez‐Cañizares Alejandro Gutiérrez Germán R. Castro Jesús Chaboy 《Journal of synchrotron radiation》2013,20(4):635-640
This work reports an X‐ray absorption near‐edge structure (XANES) spectroscopy study at the Ni K‐edge in the early stages of growth of NiO on non‐ordered SiO2, Al2O3 and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq‐ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq‐ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross‐linking bonds Ni—O—M (M = Si, Al, Mg) at the interface. 相似文献
7.
Humberto Bustos Rodriguez Dagoberto Oyola Lozano Yebrayl A. Rojas Martínez Germán A. Pérez Alcázar Stefan Flege Adam G. Balogh Louis J. Cabri Michael Tubrett 《Hyperfine Interactions》2007,175(1-3):195-206
X-ray diffraction (XRD), Mössbauer spectrometry (MS), secondary ions mass spectroscopy (SIMS) and laser-ablation microprobe–inductively coupled plasma–mass spectrometry (LAM–ICP–MS) were used to study mineral samples of Colombian auriferous ores collected from the “El Diamante” mine, located in the municipality of Guachavez-Nariño, in Colombia. The samples were prepared as polished thin sections and polished sections. From XRD data, quartz, sphalerite and pyrite were detected and their respective cell parameters were estimated. From MS analyses, pyrite, arsenopyrite and chalcopyrite were identified; their respective hyperfine parameters and respective texture were deduced. Multiple regions of approximately 200 × 200 μm in each sample were analyzed with SIMS; the occurrence of “invisible gold” associated mainly with pyrite and secondarily with arsenopyrite could thus be assigned. It was also found that pyrite is of the arsenious type. Spots from 30 to 40 μm in diameter were analyzed with LAM–ICP–MS for pyrite, arsenopyrite and sphalerite; Au is “homogeneously” distributed inside the structure of the arsenious pyrite and the arsenopyrite (not as inclusions); the chemical composition indicates similarities of this “invisible gold”, forming a solid solution with arsenious pyrite and arsenopyrite. One hundred nineteen and 62 ppm of ‘invisible gold’ was quantified in 21 spots analyzed on pyrite and in 14 spots on arsenopyrite, respectively. 相似文献
8.
Patricia W. Stege Germán A. Messina Guillermo Bianchi Roberto A. Olsina 《Journal of fluorescence》2010,20(2):517-523
Enzyme activities can provide indication for quantitative changes in soil organic matter (SOM). It is known that the activities
of most enzymes increase as native SOM content reflecting larger microbial communities and stabilization of enzymes on humic
materials. β-Gucosidase (β-Glu) activities have been frequently used as indicators of changes in quantity and quality of SOM.
In this study we propose a simple and very sensitive method, which has lower limit of detection compared with classic spectrophotometric
method with the aim of determinate the β-Glu activity in soil samples using Fluorescein mono-β-D-glucopyranoside (FMGlc) as
a substrate. The fluorescein released by the enzymatic reaction was quantified by capillary electrophoresis-laser induced
fluorescence (CE-LIF) method. The background electrolyte (BGE) consisted in 40 mM phosphate buffer, pH 6. The LOD and LOQ
for fluorescein were 1.3 10−7 mg mL−1 and 6.4 10−6 mg mL−1, respectively. This work deals with the minimization of the mixture for the enzymatic reaction and with the optimization
conditions of CE separation. To the best of our knowledge, this is the first time that an enzymatic activity was detected
in soil using CE-LIF system. 相似文献
9.
Patricia W. Stege Germán A. Messina Guillermo Bianchi Roberto A. Olsina Julio Raba 《Analytical and bioanalytical chemistry》2010,397(3):1347-1353
Soil microorganisms and enzymes are the primary mediators of soil biological processes, including organic matter degradation,
mineralization, and nutrient recycling. They play an important role in maintaining soil ecosystem quality and functional diversity.
Moreover, enzyme activities can provide an indication of quantitative changes in soil organic matter. β-Glucosidase (β-Glu)
activity has been found to be sensitive to soil management and has been proposed as a soil quality indicator because it provides
an early indication of changes in organic matter status and its turnover. The aims of the present study were to test and use
a simple and convenient procedure for the assay of β-Glu activity in agricultural soil. The method described here is based
on the enzymatic degradation of cellobiose by β-Glu present in the soil sample and the subsequent determination of glucose
produced by the enzymatic reaction using screen-printed carbon electrodes modified with multiwalled carbon nanotubes (SPCE-CNT)
equipped with coimmobilized glucose oxidase and horseradish peroxidase enzymes. The potential applied to the SPCE-CNT detection
was −0.15 V versus a Ag/AgCl pseudo-reference electrode. A linear calibration curve was obtained in the range 2.7–11.3 mM
with a correlation coefficient. In the present study, an easy and effective SPCE-CNT-modified electrode allowed an improved
amperometric response to be achieved and this is attributed to the increased surface area upon electrode modification. 相似文献
10.
Germán A. Enciso Morris W. Hirsch Hal L. Smith 《Journal of Dynamics and Differential Equations》2008,20(1):115-132
Classical results in the theory of monotone semiflows give sufficient conditions for the generic solution to converge toward
an equilibrium or toward the set of equilibria (quasiconvergence). In this paper, we provide new formulations of these results
in terms of the measure-theoretic notion of prevalence, developed in Christensen (Israel J. Math., 13, 255–260, 1972) and Hunt et al. (Bull. Am. Math. Soc., 27, 217–238, 1992). For monotone reaction–diffusion systems with Neumann boundary conditions on convex domains, we show the
prevalence of the set of continuous initial conditions corresponding to solutions that converge to a spatially homogeneous
equilibrium. We also extend a previous generic convergence result to allow its use on Sobolev spaces. Careful attention is
given to the measurability of the various sets involved. 相似文献