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781.
准晶数学弹性力学和缺陷力学 总被引:4,自引:0,他引:4
对准晶数学弹性理论的基本概念和基本框架作了介绍,在此基础上分别针对目前已经发现的几类一维准晶、二维准晶和三维准晶讨论了其数学弹性的理论体系.为了求解准晶弹性的边值问题或初值一边值问题,还必须发展相应的方法论.物理工作者在研究准晶位错弹性问题中发展了Green函数方法.针对一维与二维准晶弹性中几类问题提出了分解与叠加程序,这一程序的使用,使极其复杂的准晶弹性问题得到简化,进而引进位移函数或应力函数,把数目。庞大的准晶弹性基本方程化成一个或少数几个高阶偏微分方程,进一步使求解步骤大为简化.对三维立方准晶弹性也采用了类似步骤使求解过程大为简化.在以上化简的基础上,发展了准晶弹性的边值问题或初值一边值问题的复交函数方法和 Fourier分析方法,求得了一系列准晶位错问题和裂纹问题的分析解(古典解).在研究准晶弹性的边值问题古典解的同时,也讨论了同这些边值问题相对应的变分问题和广义解(弱解)以及这种弱解的数值方法──有限元法.在物理学家工作基础上开展的这些工作可以看作对经典数学弹性理论和方法、经典Volterra位错理论、普通结构材料断裂力学和经典有限元的某些发展.此外,还把一维六方准晶弹性动力学的结果与统计物理的某些 相似文献
782.
C. Shekhar R. Giri R. S. Tiwari O. N. Srivastava 《Crystal Research and Technology》2004,39(8):718-725
We report the synthesis of La doped MgB2 superconductors with nominal compositions Mg1‐xLaxB2 (with x = 0.01, 0.03, 0.05, 0.07) by solid state reaction at ambient pressure. A special encapsulation technique has been used by us to prepare high quality superconducting MgB2 samples. The bulk polycrystalline samples possess superconducting transition temperature Tc(R=0) ranging between 36‐39 K. It has been found that transport critical current density Jc of the samples change significantly with the doping level of La. A high transport (Jc) value ∼1.9 x 103 A/cm2 at 15 K has been achieved for Mg0.97La0.03B2 sample. The XRD and TEM investigations indicate that the samples prepared by encapsulation method are devoid of MgO, which is generally found when synthesis of MgB2 is done through sintering of Mg and B powders. The detailed microstructural investigations of Mg0.97La0.03B2 specimens by transmission electron microscopy (TEM) reveal the presence of partial dislocation network, moiré fringes and superlattice structure in the as synthesized samples. The higher transport critical current density observed in Mg0.97La0.03B2 superconductor has been attributed to the partial dislocations which are capable of providing pinning centres. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Zeng Fanlin Sun Yi 《Acta Mechanica Solida Sinica》2006,19(4):283-288
A large-scale atom simulation of nanoindentation into a thin nickel film using thequasicontinuum method was performed. The initial stages of the plasticity deformation of nickelwere studied. Several useful results were obtained as follows: (1)The response of the load versusindentation depth—on the load versus indentation depth curve, besides the straight parts cor-responding to the elastic property of nickel, the sudden drop of the load occurred several times;(2) The phenomena of dislocation nucleation—the dislocation nucleation took place when theload descended, which makes it clear that dislocation nucleation causes the drop of the load;(3)The mechanism of the dislocation emission—the Peierls-Nabarro dislocation model and a pow-erful criterion were used to analyze the dislocation emission. And the computational value was ingood agreement with the predict value; (4) The density of geometrically necessary dislocations.A simple model was used to obtain the density of geometrically necessary dislocations beneaththe indenter. Furthermore, the influence of the boundary conditions on the simulation results wasdiscussed. 相似文献
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Liquid inclusions and various defects accordingly induced on a nonlinear optical material of CMTC crystal were investigated by atomic force microscopy. Liquid inclusions are chiefly caused by formation of macrosteps, which result from impurity‐induced inhibiting of step growth and meeting of step trains advancing along different directions. Liquid inclusions induce generation of dislocations and even cracks within the crystal by three‐dimensional nucleation growth. Liquid inclusions also provide screw dislocation growth sources, leading to formation of spiral hillock trains with ridged tails. Etching experiments reveal circular hollow cores, indicative of screw dislocation growth, and negative crystals resulting from further crystallization in the liquid inclusions. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献