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   554890篇
  免费   5362篇
  国内免费   1426篇
化学   290793篇
晶体学   8202篇
力学   26097篇
综合类   20篇
数学   66892篇
物理学   169674篇
  2021年   4871篇
  2020年   5314篇
  2019年   5936篇
  2018年   7952篇
  2017年   7960篇
  2016年   11404篇
  2015年   6700篇
  2014年   10824篇
  2013年   24788篇
  2012年   19192篇
  2011年   23408篇
  2010年   17083篇
  2009年   17085篇
  2008年   21728篇
  2007年   21566篇
  2006年   19892篇
  2005年   17754篇
  2004年   16573篇
  2003年   14699篇
  2002年   14707篇
  2001年   16624篇
  2000年   12518篇
  1999年   9812篇
  1998年   8295篇
  1997年   7945篇
  1996年   7613篇
  1995年   6892篇
  1994年   6802篇
  1993年   6632篇
  1992年   7199篇
  1991年   7539篇
  1990年   7194篇
  1989年   7077篇
  1988年   6974篇
  1987年   6863篇
  1986年   6520篇
  1985年   8410篇
  1984年   8498篇
  1983年   6865篇
  1982年   7022篇
  1981年   6912篇
  1980年   6394篇
  1979年   6994篇
  1978年   7226篇
  1977年   7219篇
  1976年   7232篇
  1975年   6664篇
  1974年   6631篇
  1973年   6758篇
  1972年   4941篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Amphiphilic tris(2,2′‐bipyridine)ruthenium‐cored star‐shaped polymers consisting of one polystyrene block and two poly(N‐isopropylacrylamide) blocks were prepared by the “arm‐first” method in which RAFT polymerization and nonconvalent ligand–metal complexation were employed. The prepared amphiphilic star‐shaped metallopolymers are able to form micelles in water. The size and distribution of the micelles were studied by dynamic light scattering and transmission electron microscopy techniques. Preliminary studies indicate that the polymer concentration and the hydrophilic poly(N‐isopropylacrylamide) block length can affect the morphologies of the formed metal‐interfaced core–shell micelles in water. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4204–4210, 2007  相似文献   
42.
The title compound, C12H21NO3, crystallizes in two polymorphic forms, viz. the tetragonal form described here and the monoclinic form described previously [Foces‐Foces, López‐Rodríguez, Pérez, Martín & Pérez‐Hernández (2007). Cryst. Growth Des. 7 , 905–911]. The differences in the conformations of the hydroxymethyl and methylaminocarbonyl substituents have important consequences in the hydrogen‐bond interaction motifs and, therefore, in the packing arrangements. These forms are concomitant polymorphs with melting points differing by 3 K.  相似文献   
43.
Glasses in SiO2–ZnO–BaO system with the different ZnO/BaO ratio were studied. In some cases, BaF2 was introduced to substitute for BaO on the equal base. Photoluminescent spectra showed that ZnO in glass matrices behaved somewhat differently from ZnO crystals. Especially, the introduction of fluorine ions led to dramatic shift of UV emission band of glasses closer to that of ZnO crystals. Raman spectral analysis provided consistent results. In particular, Raman bands in the high frequency region are sensitive to effects of different ZnO/BaO or BaF2/BaO ratio on structure of glasses.  相似文献   
44.
The growth of the isolated (100) face of a KDP crystal at exposure of the phase boundary to the initial ultrasound field and a standing acoustic wave has been investigated. A significant growth response of this face, exposed to sound normally along the acoustic axis in the near zone of a piston-like half-wave vibrator with f = 20 kHz and tangentially in the pulsed two-frequency (f = 600 and 900 kHz) standing-wave mode, has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are different. The results obtained show a fundamental possibility of controlling crystal growth and dissolution by varying the parameters of inhomogeneous acoustic field.  相似文献   
45.
The features of doping of KDP crystals with cerium ions and organocerium complexes with alizarin complexon and arsenazo III have been investigated. It is established that “direct” doping by introducing cerium salts into the initial solution cannot be implemented. The effect of organometallic complexes of cerium on the crystal growth has been studied. Organocerium complexes predominantly enter the prismatic or pyramidal growth sectors. It is shown that the complex arsenazo III + Ce blocks the growth of the prismatic sector. Cerium-doped KDP crystals exhibit a photoluminescence band peaking at the wavelength λmax= 350 nm.  相似文献   
46.
We propose a dislocation model of a nanotwin, which is used to calculate the stress fields in the nanotwin. We consider the role of these stresses in the process of interdislocation interaction and nanotwin growth.  相似文献   
47.
Fully developed laminar mixed convection of a nanofluid consists of water and Al2O3 in horizontal and inclined tubes has been studied numerically. Three-dimensional elliptic governing equations have been solved to investigate the flow behaviors over a wide range of the Grashof and Reynolds numbers. Comparisons with previously published experimental and numerical works on mixed convection in a horizontal and inclined tube are performed and good agreements between the results are observed. Effects of nanoparticles concentration and tube inclinations on the hydrodynamics and thermal parameters are presented and discussed. It is shown that the nanoparticles concentration does not have significant effects on the hydrodynamics parameters. Heat transfer coefficient increases by 15% at 4 Vol.% Al2O3. Skin friction coefficient continually increases with the tube inclination, but the heat transfer coefficient reaches a maximum at the inclination angle of 45°.  相似文献   
48.
The experimental and calculated values of the optical permittivity ε have been analyzed and generalized with allowance for the oxygen concentration and possible valences of Fe and O ions in YIG single crystals. It is shown that the deficit of oxygen ions in the samples is accompanied by a decrease in ε. A decrease in the gradient of distribution of Ba ions over the sample thickness decreases the difference between the calculated and experimental values. It is revealed that the ratio of the number of valence electrons to the number of core electrons, with regard to their polarizabilities, is proportional to the ratio of the incoherent and coherent X-ray scattering intensities, nk/k.  相似文献   
49.
This paper addresses the issue of energy propagation features in ribbed panels over a wide frequency range. First, a tool for estimating the wave propagation characteristics of one- and two-dimensional structures by k-space analysis is presented. This tool uses a concept of Inhomogeneous Wave Correlation with sparse measured or extracted data, and leads to the estimation of θ-dependent wavenumbers. Here, the method is employed with the sparse simulated and measured normal velocities of a set of ribbed panels and new insights into their k-space behavior are highlighted. Behavior is essentially characterized at low frequencies by structural orthotropy. Comparisons with homogenized data show very good agreement. At higher frequencies, a new behavior pattern is observed and explained both numerically and experimentally. This is a multi-modal waveguide type of propagation in a direction parallel to the ribs.  相似文献   
50.
The paper addresses the problem of calculation of the local stress field and effective elastic properties of a unidirectional fiber reinforced composite with anisotropic constituents. For this aim, the representative unit cell approach has been utilized. The micro geometry of the composite is modeled by a periodic structure with a unit cell containing multiple circular fibers. The number of fibers is sufficient to account for the micro structure statistics of composite. A new method based on the multipole expansion technique is developed to obtain the exact series solution for the micro stress field. The method combines the principle of superposition, technique of complex potentials and some new results in the theory of special functions. A proper choice of potentials and new results for their series expansions allow one to reduce the boundary-value problem for the multiple-connected domain to an ordinary, well-posed set of linear algebraic equations. This reduction provides high numerical efficiency of the developed method. Exact expressions for the components of the effective stiffness tensor have been obtained by analytical averaging of the strain and stress fields.  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