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21.
We present an extension of the density-functional theory (DFT) formalism for lattice gases to systems with internal degrees of freedom. In order to test approximations commonly used in DFT approaches, we investigate the statics and dynamics of occupation (density) profiles in the one-dimensional Potts model. In particular, by taking the exact functional for this model we can directly evaluate the quality of the local equilibrium approximation used in time-dependent density-functional theory (TDFT). Excellent agreement is found in comparison with Monte Carlo simulations. Finally, principal limitations of TDFT are demonstrated.  相似文献   
22.
In this work, the redispersion of three nanocrystalline TiO2 colloids is studied: one pure and two Fe-doped titania. These three colloids are produced by an easy aqueous sol–gel synthesis using precipitation-acidic peptization of Ti precursor. For the two Fe-doped TiO2, one is doped during synthesis (primary doping) and the other is doped after the synthesis (secondary doping). The initial colloids are composed of crystalline TiO2 particles around 7?nm with good photocatalytic properties, tested on PNP degradation under visible light (wavelength >390?nm). The powders obtained by air drying of these three colloids are redispersed in water to produce colloids, which are compared to the initial colloid produced. For each colloid, five cycles of drying redispersion are achieved. The colloids are characterized by dynamic light scattering, zeta potential measurements, inductively coupled plasma–atomic emission spectroscopy, X-ray diffraction, nitrogen adsorption–desorption measurements, Mössbauer spectroscopy, diffuse reflectance spectroscopy, and photocatalytic tests. The results show that similar products are obtained between the cycles, maintaining homologous properties of colloids. This property of redispersion is mainly due to the acid (HNO3, HCl, or H2SO4) which protonates the surface of the TiO2 nanoparticle leading to high-surface charges and electrostatic repulsions between aggregates. This property can be very useful for industrial applications of this synthesis, especially as it allows the volume and weight to be reduced for transportation and storage. Moreover, results show that the pure TiO2 powder can be doped during its redispersion step. The redispersion of the TiO2 developed here is possible without surface functionalization or multiple step processes, contrary to commercial Degussa P25. A 2-year stability study of all the produced colloids has been performed by following the evolution of the macroscopic aspect and the physicochemical properties of these sols. This study showed high stability of the produced colloids.  相似文献   
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Continuum limits of various tight binding linear chain lattices used in the author's recent study of quantum transport in the presence of dynamical disorder, are analyzed from the point of view of their energy level spectra when disorder is absent. These spectra show a linear dispersion similar to that in the Luttinger model, and describe the energy levels of the corresponding discrete systems in the range of midband wavevectors. Next, the more conventional longwavelength continuum limit, which describes the energy levels of the actual discrete systems near the bottom of the bands is discussed. On the basis of these properties it is argued that the applicability of the continuum models to the study of dynamical properties is restricted to low frequencies in the range of low lying excitations, near the midband Fermi level in half-filled band situations in the case of the midband models, and near the bottom of a nearly empty band for the longwavelength models. Finally, it is shown that in the presence of dynamic disorder the longwavelength continuum limit of a single-band tight-binding model leads to nondiffusive motion, with a mean squared displacement <x 2(t)>t 3, fort.  相似文献   
25.
Samples of low density xerogels were submitted to mercury porosimetry at pressures up to 200 MPa. These samples show an unusual behavior: they are first crushed by the isostatic mercury pressure without mercury intrusion and are then intruded by the mercury above a certain pressure. This transition allows the easy determination of the one constant found in the buckling model that is used to interpret the crushing part of the mercury porosimetry experiment. The relation between this constant and the structure of the xerogels determined by TEM and nitrogen adsorption is discussed.  相似文献   
26.
From the literature it is known that spectral least-squares schemes perform poorly with respect to mass conservation and compensate this lack by a superior conservation of momentum. This should be revised, since the here presented new least-squares spectral collocation scheme leads to an outstanding performance with respect to conservation of momentum and mass. The reasons can be found in using only a few elements, each with high polynomial degree, avoiding normal equations for solving the overdetermined linear systems of equations and by introducing the Clenshaw–Curtis quadrature rule for imposing the average pressure to be zero. Furthermore, we combined the transformation of Gordon and Hall (transfinite mapping) with our least-squares spectral collocation scheme to discretize the internal flow problems.  相似文献   
27.
An environmentally-friendly aqueous sol–gel process for producing undoped and Cu2+, Ni2+, Zn2+ or Pb2+-doped TiO2 photocatalysts exhibiting a remarkably high photocatalytic activity without requiring any calcination step has been developed. The physicochemical properties of the catalysts were characterized by ICP-AES, XRD, UV–Vis spectroscopy and nitrogen adsorption–desorption. It has been found that the catalysts are composed of nanocrystallites of anatase with a size of 6–7 nm and a specific surface area varying from 184 to 275 m2 g?1. A screening of the photocatalytic activity of the undoped and doped photocatalysts has been performed by evaluating the degradation of 4-nitrophenol under artificial light (330 nm < λ < 800 nm) after 7 h of illumination using a custom-designed multisample photoreactor. The activity measured for the TiO2-Undoped catalyst was found to be five times higher than the activity measured for uncalcined TiO2 catalysts produced by other sol–gel methods. We propose that this interesting result is due to the particular morphology of the xerogels obtained. It has also been demonstrated that the presence of the dopant leads to an enhancement of the photocatalytic activity in all cases. The role of particular dopants in modulating the photocatalytic activity will be discussed. Finally, the possibility of producing undoped and Zn2+-doped films presenting a higher activity than the commercial photocatalytic coating (Saint Gobain Glass Bioclean®) without requiring any calcination step has been demonstrated. These preliminary results constitute an important step forward in the development of photocatalytic films using a sol–gel process compatible with the constraints associated with large-scale industrial processing.  相似文献   
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The role of deviations from perfect screening in the Van der Waals interaction between two metal half-spaces a distance d apart is analyzed for a simple model. The interaction energy which is finite for d → 0 includes asymptotic corrections to the well-known d?2 law. These corrections lead to a reduction of the order of 30 to 40% of the interaction in the vicinity of equilibrium distances for particle adhesion.  相似文献   
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