全文获取类型
收费全文 | 227篇 |
免费 | 39篇 |
国内免费 | 33篇 |
专业分类
化学 | 86篇 |
晶体学 | 1篇 |
力学 | 89篇 |
综合类 | 3篇 |
数学 | 37篇 |
物理学 | 83篇 |
出版年
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 5篇 |
2020年 | 13篇 |
2019年 | 10篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 11篇 |
2015年 | 9篇 |
2014年 | 13篇 |
2013年 | 15篇 |
2012年 | 16篇 |
2011年 | 12篇 |
2010年 | 10篇 |
2009年 | 21篇 |
2008年 | 13篇 |
2007年 | 11篇 |
2006年 | 15篇 |
2005年 | 12篇 |
2004年 | 13篇 |
2003年 | 13篇 |
2002年 | 3篇 |
2001年 | 5篇 |
2000年 | 2篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 3篇 |
1996年 | 9篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 8篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1988年 | 2篇 |
1985年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有299条查询结果,搜索用时 0 毫秒
231.
232.
研究了一种X波段高功率宽带回旋行波放大器,它由改变了局部周期的矩形盘荷波导组成,并采用锥化引导磁场和盘的高度来提高增益和带宽。分析了器件的线性特性。利用全电磁(PIC)三维模拟程序EMC进行了数值模拟,结果表明:可以得到功率介于88~103kW,相对带宽介于16.63%~24.11%的不同微波输出。 相似文献
233.
粘弹性轴向运动梁的非线性动力学行为 总被引:3,自引:0,他引:3
本文研究了带有小脉动的轴向运动粘弹性梁的分岔及混沌现象。建立了系统的动力学模型。通过二阶Galerkin截断,把描述系统运动的偏微分方程离散化。利用数值方法分别分析了几种运动脉动频率时,梁随轴向运动脉动幅值,平均速度及粘弹性系数等几个参数变化时的运动分岔行为。利用Lyapunov指数识别系统的动力学行为,区分准周期振动和混沌运动。 相似文献
234.
235.
针对薄壁圆筒壳结构轴压屈曲载荷的缺陷敏感性以及真实几何缺陷的不确定性,提出一种基于实测缺陷数据和极大熵原理的初始缺陷建模与屈曲载荷预测方法。首先,将初始几何缺陷视为二维随机场,并利用实测缺陷数据和Karhunen-Loève展开法将初始缺陷的随机场建模转化为随机向量的建模;其次,利用极大熵方法确定随机向量的概率分布;最后,基于所构建的初始缺陷随机模型,利用MCMC抽样方法和确定性屈曲分析方法,进行随机屈曲分析并给出基于可靠度的屈曲载荷折减因子。数值算例表明,与直接假设随机场相关结构的方法相比,本文方法的结果是对薄壁圆筒壳屈曲载荷的一个更无偏估计。 相似文献
236.
In this paper,the problem of axially symmetric deformation is examined for a composite cylindrical tube under equal axial loads acting on its two ends,where the tube is composed of two different incompressible neo-Hookean materials.Significantly,the implicit analytical solutions describing the deformation of the tube are proposed.Numerical simulations are given to further illustrate the qualitative properties of the solutions and some meaningful conclusions are obtained.In the tension case,with the increasing axial loads or with the decreasing ratio of shear moduli of the outer and the inner materials,it is proved that the tube will shrink more along the radial direction and will extend more along the axial direction.Under either tension or compression,the deformation along the axial direction is obvious near the two ends of the tube,while in the rest,the change is relatively small.Similarly,for a large domain of the middle part,the axial elongation is almost constant;however,the variation is very fast near the two ends.In addition,the absolute value of the axial displacement increases gradually from the central cross-section of the tube and achieves the maximum at the two endpoints. 相似文献
237.
Hong Song Ya Li Qi‐Jun Yao Liang Jin Lei Liu Yan‐Hua Liu Bing‐Feng Shi 《Angewandte Chemie (International ed. in English)》2020,59(16):6576-6580
The atroposelective synthesis of axially chiral styrenes remains a formidable challenge due to their relatively lower rotational barriers compared to the biaryl atropoisomers. Herein, we describe the construction of axially chiral styrenes through PdII‐catalyzed atroposelective C?H olefination, using a bulky amino amide as a transient chiral auxiliary. Various axially chiral styrenes were produced with good yields and high enantioselectivity (up to 95 % yield and 99 % ee). Carboxylic acid derivatives of the resulting axially chiral styrenes showed superior enantiocontrol over the biaryl counterparts in CoIII‐catalyzed enantioselective C(sp3)?H amidation of thioamide. Mechanistic studies suggest that C?H cleavage is the enantioselectivity‐determining step. 相似文献
238.
239.
M. Ghaedi S. Haghdoust S. Nasiri Kokhdan A. Mihandoost R. Sahraie A. Daneshfar 《光谱学快报》2013,46(7):500-510
Activated carbon (AC), multiwalled carbon nanotube (MWCNT), and cadmium hydroxide nanowire loaded on activated carbon (Cd(OH)2-NW-AC) have been used for the removal of safranine O (SO) from wastewater. The effects of various parameters including pH, temperature, concentration of the dye, amount of adsorbents, and contact time on the SO adsorption efficiency for all adsorbents has been investigated. Graphical correlation of fitting experimental data to various adsorption isotherm models like those of Langmuir, Freundlich, Tempkin, and Dubinin–Radushkevich for all adsorbents have been calculated. It was found that safranine O adsorption on all adsorbents was endothermic and feasible in nature. Fitting the experimental data to different kinetic models suggests that the adsorption process follows pseudo-second-order kinetics with involvement of the particle diffusion mechanism. 相似文献
240.
Masayuki Shirai Kazuo Torii Masahiko Arai 《Molecular Crystals and Liquid Crystals》2013,570(2):321-326
Abstract Mesoporous smectite-type materials (SM) were synthesized with a hydrothermal method from water glass, magnesium chloride and alkyl ammonium salt. The mesopore diameter of SM was controlled from 30 to 130 Å by changing the reaction temperature, pH and the template materials during hydrothermal treatment. Palladium metal particles were intercalated in the mesopores of SM with an ion-exchanging of [Pd(NH3)4Cl2] on them and hydrogen reduction treatment. The size-selective hydrogenation of butadiene-acrylonitrile rubbers (NBR) in carbon tetrachloride was studied using palladium loaded smectite catalysts (Pd-SM). The reaction was controlled by the sizes of mesopore of Pd-SM, which determine the diffusion of polymer molecules onto the dispersed palladium particles within the pores. 相似文献