首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   398篇
  免费   21篇
  国内免费   1篇
化学   271篇
晶体学   1篇
力学   17篇
数学   22篇
物理学   59篇
无线电   50篇
  2023年   5篇
  2022年   9篇
  2021年   9篇
  2020年   8篇
  2019年   10篇
  2018年   19篇
  2017年   9篇
  2016年   16篇
  2015年   13篇
  2014年   14篇
  2013年   32篇
  2012年   44篇
  2011年   36篇
  2010年   26篇
  2009年   20篇
  2008年   18篇
  2007年   19篇
  2006年   19篇
  2005年   16篇
  2004年   19篇
  2003年   18篇
  2002年   13篇
  2001年   3篇
  2000年   2篇
  1999年   3篇
  1998年   1篇
  1997年   4篇
  1996年   5篇
  1994年   1篇
  1990年   1篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1976年   1篇
  1975年   1篇
  1971年   1篇
  1970年   2篇
排序方式: 共有420条查询结果,搜索用时 10 毫秒
381.
The UNIQUAC-NRF model for aqueous two-phase polymer–polymer systems is extended to correlate liquid–liquid equilibria of aqueous polymer–salt two-phase systems. The systems investigated are polyethylene glycol+ammonium sulfate+water and polyethylene glycol+potassium monohydrogen phosphate (K2HPO4)+water for five molecular weights of PEG. In this model, the nonrandom state is selected as a reference state. The model has six binary adjustable parameters that were determined by an optimization program. The Debye–Huckel equation based on Fowller–Guggenheim equation is used to calculate the long-range electrostatic interaction of the ions. The results obtained by this model are in good agreement with experimental data.  相似文献   
382.
The current study presents finite element simulations of shear localization along the interface between cohesionless granular soil and bounding structure under large shearing movement. Micro-polar (Cosserat) continuum approach is applied in the framework of elasto-plasticity in order to overcome the numerical problems of localization modeling seen in the conventional continuum mechanics. The effects of different micro-polar kinematic boundary conditions, along the interface, on the evolution and location of shear band are shown by the numerical results. Furthermore, shear band thickness is also investigated for its dependence on the initial void ratio, vertical pressure and mean grain size. Here, the distribution and evolution of static and kinematic quantities are the main focuses regarding infinite layer of micro-polar material during plane shearing, especially with advanced large movement of bounding structure. The influence of such movement has not been investigated yet in the literature. Based on the results obtained from this study, shear localization appears parallel to the direction of shearing. It occurs either in the middle of granular layer or near boundaries, regarding the assumed micro-polar kinematic boundary conditions at the bottom and top surfaces of granular soil layer. Narrower shear band is observed in lower rotation resistance of soil particles along the interface. It is emphasized that the displacement magnitude of bounding structure has significant effect on the distribution and evolution of state variables and polar quantities in the granular soil layer. However, continuous displacement has no meaningful effect on the thickness of shear band. Here, smooth distributions of void ratio and shear stress components are obtained within the shear band, what the other previous numerical investigations did not receive. Despite indirect linking of Lade’s model to the critical state soil mechanics, state variables tend towards asymptotical stationary condition in large shear deformation.  相似文献   
383.
We investigated the mechanical behavior of two-dimensional hierarchical honeycomb structures using analytical, numerical and experimental methods. Hierarchical honeycombs were constructed by replacing every three-edge vertex of a regular hexagonal lattice with a smaller hexagon. Repeating this process builds a fractal-appearing structure. The resulting isotropic in-plane elastic properties (effective elastic modulus and Poisson’s ratio) of this structure are controlled by the dimension ratios for different hierarchical orders. Hierarchical honeycombs of first and second order can be up to 2.0 and 3.5 times stiffer than regular honeycomb at the same mass (i.e., same overall average density). The Poisson’s ratio varies from nearly 1.0 (when planar ‘bulk’ modulus is considerably greater than Young’s modulus, so the structure acts ‘incompressible’ for most loadings) to 0.28, depending on the dimension ratios. The work provides insight into the role of structural organization and hierarchy in regulating the mechanical behavior of materials, and new opportunities for developing low-weight cellular structures with tailorable properties.  相似文献   
384.
It has been proposed that photosynthetic plankton can be used as a biological carbon pump to absorb and sequester carbon dioxide in the ocean. In this paper, plankton population dynamics are simulated in a single stratified water column to predict carbon dioxide sequestering due to surface iron fertilization in deep ocean. Using a predator-prey model and realistic parameter values, iron fertilization was found to only cause temporary blooms up to 5 months in duration, and relatively small increases in adsorption of atmospheric $\mathrm{CO_{2}}$.  相似文献   
385.
We develop a general theoretical framework for the recently proposed importance sampling method for enhancing the efficiency of rare-event simulations [W. Cai, M. H. Kalos, M. de Koning, and V. V. Bulatov, Phys. Rev. E 66, 046703 (2002)], and discuss practical aspects of its application. We define the success/fail ensemble of all possible successful and failed transition paths of any duration and demonstrate that in this formulation the rare-event problem can be interpreted as a "hit-or-miss" Monte Carlo quadrature calculation of a path integral. The fact that the integrand contributes significantly only for a very tiny fraction of all possible paths then naturally leads to a "standard" importance sampling approach to Monte Carlo (MC) quadrature and the existence of an optimal importance function. In addition to showing that the approach is general and expected to be applicable beyond the realm of Markovian path simulations, for which the method was originally proposed, the formulation reveals a conceptual analogy with the variational MC (VMC) method. The search for the optimal importance function in the former is analogous to finding the ground-state wave function in the latter. In two model problems we discuss practical aspects of finding a suitable approximation for the optimal importance function. For this purpose we follow the strategy that is typically adopted in VMC calculations: the selection of a trial functional form for the optimal importance function, followed by the optimization of its adjustable parameters. The latter is accomplished by means of an adaptive optimization procedure based on a combination of steepest-descent and genetic algorithms.  相似文献   
386.
A numerical study of the spectrum and eigenfunctions on a tubular arc   总被引:1,自引:0,他引:1  
The Hamiltonian for a particle constrained to move on the surface of a curved nanotube is derived using the methods of differential forms. A two-dimensional Gram–Schmidt orthonormalization procedure is employed to calculate basis functions for determining the eigenvalues and eigenstates of a tubular arc (a nanotube in the shape of a hyperbolic cosine) with several hundred scattering centers. The curvature of the tube is shown to induce bound states that are dependent on the curvature parameter and bend location of the tube.  相似文献   
387.
Karimi B  Ma'Mani L 《Organic letters》2004,6(26):4813-4815
[reaction: see text] A new silica-based scandium(III) interphase catalyst 2 efficiently catalyzes the cyanosilylation of a variety of aldehydes and ketones. The catalyst shows high thermal stability (up to 300 degrees C) and also is stable in both organic and polar solvents. It also could be recovered and reused for at least 10 reaction cycles without considerable loss of reactivity.  相似文献   
388.
This work describes a method for patterning a gold substrate with multiple, aligned self-assembled monolayers (SAMs) using light at different wavelengths. It describes the synthesis and characterization of an alkanethiolate SAM that is photosensitive to light at both 220 and 365 nm. A photomask acts as an area-selective filter for light at 220 and 365 nm, and a single set of exposures at these two wavelengths through one photomask, without steps of alignment between the exposures, can produce three aligned SAMs on one gold substrate. We demonstrate the versatility of this method of photopatterning by modifying individual aligned SAMs chemically to produce surfaces having different properties. We characterize the modified SAMs using immunolabeling, matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy, and surface plasmon resonance spectroscopy. We also describe the patterning of two aligned SAMs that resist the adsorption of proteins and a third region that does not resist the adsorption of proteins. The ability to produce multiple, aligned patterns of SAMs in a single step, without alignment of photomasks in separate steps, increases the versatility of SAMs for studying a range of physical phenomena.  相似文献   
389.
390.
The thermodynamic stabilities of various phases of the nitrides of the platinum-metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir, and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号