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   1062篇
  免费   26篇
  国内免费   4篇
化学   580篇
晶体学   3篇
力学   40篇
数学   310篇
物理学   159篇
  2023年   9篇
  2022年   5篇
  2021年   33篇
  2020年   40篇
  2019年   23篇
  2018年   26篇
  2017年   19篇
  2016年   40篇
  2015年   28篇
  2014年   36篇
  2013年   60篇
  2012年   78篇
  2011年   94篇
  2010年   69篇
  2009年   51篇
  2008年   54篇
  2007年   61篇
  2006年   56篇
  2005年   55篇
  2004年   50篇
  2003年   24篇
  2002年   26篇
  2001年   21篇
  2000年   12篇
  1999年   16篇
  1998年   10篇
  1997年   5篇
  1996年   13篇
  1995年   10篇
  1994年   4篇
  1993年   6篇
  1992年   7篇
  1991年   4篇
  1990年   9篇
  1989年   2篇
  1988年   4篇
  1987年   2篇
  1986年   2篇
  1985年   8篇
  1984年   1篇
  1983年   3篇
  1982年   5篇
  1981年   4篇
  1980年   1篇
  1979年   3篇
  1978年   1篇
  1968年   1篇
  1932年   1篇
排序方式: 共有1092条查询结果,搜索用时 15 毫秒
61.
The separation of cis and trans isomers is relevant for biological and nutritional applications, silver-exchanged zeolite Y was prepared and applied for the treatment of mixtures of cis and trans geometrical isomers of mono- and polyunsaturated fatty acid methyl esters (FAMEs). cis FAMEs were adsorbed into zeolite with a high degree of selectivity (cis/trans ratio in the range of 1.9-3.2). The effectiveness was due to the synergism of the π-complexation between the silver ion and the double bonds and the different FAME structures trapped into the zeolite cages. Some indication of the complex stabilities came from theoretical studies using unsaturated lipids. A prototype cartridge was also designed for application in the fractionation of cis/trans FAME mixtures.  相似文献   
62.
We present a semigrand ensemble Monte Carlo and Brownian dynamics simulation study of structural and dynamical properties of polydisperse soft spheres interacting via purely repulsive power-law potentials with a varying degree of "softness." Comparisons focus on crystal and amorphous phases at their coexistence points. It is shown through detailed structural analysis that as potential interactions soften, the "quality of crystallinity" of both monodisperse and polydisperse systems deteriorates. In general, polydisperse crystalline phases are characterized by a more ordered structure than the corresponding monodisperse ones (i.e., for the same potential softness). This counter-intuitive feature originates partly from the fact that particles of different sizes may be accommodated more flexibly in a crystal structure and from the reality that coexistence (osmotic) pressure is substantially higher for polydisperse systems. These trends diminish for softer potentials. Potential softness eventually produces substitutionally disordered crystals. However, substitutional order is apparent for the hard-spherelike interactions. Diffusionwise, crystals appear quite robust with a slight difference in the vibrational amplitudes of small and large particles. This difference, again, diminishes with potential softness. Overcrowding in amorphous polydisperse suspensions causes "delayed" diffusion at intermediate times.  相似文献   
63.
A maximum likelihood method for estimating remote surface orientation from multi-static acoustic, optical, radar, or laser images is presented. It is assumed that the images are corrupted by signal-dependent noise, known as speckle, arising from complex Gaussian field fluctuations, and that the surface properties are effectively Lambertian. Surface orientation estimates for a single sample are shown to have biases and errors that vary dramatically depending on illumination direction. This is due to the signal-dependent nature of speckle noise and the nonlinear relationship between surface orientation, illumination direction, and fluctuating radiance. The minimum number of independent samples necessary for maximum likelihood estimates to become asymptotically unbiased and to attain the lower bound on resolution of classical estimation theory are derived, as are practical design thresholds.  相似文献   
64.
We present artificial boundary conditions for the numerical simulation of compressible flows using high-order accurate discretizations with the discontinuous Galerkin (DG) finite element method. The construction of the proposed boundary conditions is based on characteristic analysis and applied for boundaries with arbitrary shape and orientation. Numerical experiments demonstrate that the proposed boundary treatment enables to convect out of the computational domain complex flow features with little distortion. In addition, it is shown that small-amplitude acoustic disturbances could be convected out of the computational domain, with no significant deterioration of the overall accuracy of the method. Furthermore, it was found that application of the proposed boundary treatment for viscous flow over a cylinder yields superior performance compared to simple extrapolation methods.  相似文献   
65.
Massive photoinduced short- and medium-range structural changes (photopolymerization) in As-S glasses are induced by near-bandgap light and studied by Raman scattering. Structural changes involve bond restructuring in sulfur-rich nanodomains of these nanoscale-phase-separated glasses. The spectral dependence of the photopolymerization effect demonstrates that various wavelengths can be used to optically change the structure of As-S glasses. The immense structural changes are relevant to recent findings about the role of bandgap light illumination for fabricating channel waveguides in noncrystalline arsenic sulfides.  相似文献   
66.
The analysis of information derived from magnetic resonance imaging (MRI) and spectroscopy (MRS) has been identified as an important indicator for discriminating among different brain pathologies. The purpose of this study was to investigate the efficiency of the combination of textural MRI features and MRS metabolite ratios by means of a pattern recognition system in the task of discriminating between meningiomas and metastatic brain tumors. The data set consisted of 40 brain MR image series and their corresponding spectral data obtained from patients with verified tumors. The pattern recognition system was designed employing the support vector machines classifier with radial basis function kernel; the system was evaluated using an external cross validation process to render results indicative of the generalization performance to “unknown” cases. The combination of MR textural and spectroscopic features resulted in 92.15% overall accuracy in discriminating meningiomas from metastatic brain tumors. The fusion of the information derived from MRI and MRS data might be helpful in providing clinicians a useful second opinion tool for accurate characterization of brain tumors.  相似文献   
67.
We study groups whose cohomology functors commute with filtered colimits in high dimensions. We relate this condition to the existence of projective resolutions which exhibit some finiteness properties in high dimensions, and to the existence of Eilenberg–Mac Lane spaces with finitely many n-cells for all sufficiently large n. To that end, we determine the structure of completely finitary Gorenstein projective modules over group rings. The methods are inspired by representation theory and make use of the stable module category, in which morphisms are defined through complete cohomology. In order to carry out these methods, we need to restrict ourselves to certain classes of hierarchically decomposable groups.  相似文献   
68.
69.
We use nondiscrete mathematical induction to extend the applicability of the Secant methods for solving equations in a Banach setting. Our approach has the following advantages over earlier works under the same information: weaker sufficient convergence conditions; tighter error bounds on the distances involved and more information on the location of the solution. Numerical examples where our results apply but earlier ones fail to solve nonlinear equation as well as tighter error bounds are also provided in this study.  相似文献   
70.
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