全文获取类型
收费全文 | 5083篇 |
免费 | 520篇 |
国内免费 | 314篇 |
专业分类
化学 | 336篇 |
晶体学 | 13篇 |
力学 | 1874篇 |
综合类 | 111篇 |
数学 | 2312篇 |
物理学 | 1271篇 |
出版年
2024年 | 12篇 |
2023年 | 38篇 |
2022年 | 83篇 |
2021年 | 74篇 |
2020年 | 120篇 |
2019年 | 109篇 |
2018年 | 126篇 |
2017年 | 117篇 |
2016年 | 165篇 |
2015年 | 125篇 |
2014年 | 219篇 |
2013年 | 285篇 |
2012年 | 175篇 |
2011年 | 266篇 |
2010年 | 198篇 |
2009年 | 271篇 |
2008年 | 304篇 |
2007年 | 337篇 |
2006年 | 317篇 |
2005年 | 302篇 |
2004年 | 251篇 |
2003年 | 251篇 |
2002年 | 214篇 |
2001年 | 165篇 |
2000年 | 169篇 |
1999年 | 174篇 |
1998年 | 180篇 |
1997年 | 154篇 |
1996年 | 101篇 |
1995年 | 94篇 |
1994年 | 81篇 |
1993年 | 70篇 |
1992年 | 72篇 |
1991年 | 59篇 |
1990年 | 65篇 |
1989年 | 28篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 20篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有5917条查询结果,搜索用时 406 毫秒
1.
Recyclable and reusable Pd(OAc)2/PPh3/PEG‐2000 system for homocoupling reaction of arylboronic acids under air without base 下载免费PDF全文
A stable and efficient Pd(OAc)2/PPh3/PEG‐2000 catalytic system for homocoupling of arylboronic acids has been developed. In the presence of Pd(OAc)2 and PPh3, the homocoupling reaction of arylboronic acids was carried out smoothly in PEG‐2000 at 70 °C under air without base to afford a variety of symmetric biaryls in good to excellent yields. The isolation of the products was readily performed by extraction with diethyl ether, and the Pd(OAc)2/PPh3/PEG‐2000 system could be easily recycled and reused six times without significant loss of catalytic activity. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
2.
G. Montero L. Gonzlez E. Flrez M. D. García A. Surez 《Numerical Linear Algebra with Applications》2002,9(3):239-247
Parallel preconditioners are presented for the solution of general linear systems of equations. The computation of these preconditioners is achieved by orthogonal projections related to the Frobenius inner product. So, minM∈??∥AM?I∥ F and matrix M0∈?? corresponding to this minimum (?? being any vectorial subspace of ??n(?)) are explicitly computed using accumulative formulae in order to reduce computational cost when subspace ?? is extended to another one containing it. Every step, the computation is carried out taking advantage of the previous one, what considerably reduces the amount of work. These general results are illustrated with the subspace of matrices M such that AM is symmetric. The main application is developed for the subspace of matrices with a given sparsity pattern which may be constructed iteratively by augmenting the set of non‐zero entries in each column. Finally, the effectiveness of the sparse preconditioners is illustrated with some numerical experiments. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
3.
Zhong Yin Xiao 《International Journal of Infrared and Millimeter Waves》2006,27(6):847-855
A new compact bandpass filter composed of two coupled linear tapered line resonators (LTLRs) is designed. The two resonators
are arranged in different layers. The filter has two transmission zeros on both sides of the passband. It has small size and
steep response below its passband. It is also shown that this filter has 0.6 dB low insertion loss at the centre frequency
and 10.5% 3dB bandwidth.
This work was supported by the Development Foundation of Shanghai Educational Committee (217608) and Shanghai Leading Academic
Discipline Project (T0102). 相似文献
4.
Huafei Sun 《Geometriae Dedicata》2002,90(1):63-75
We study minimal submanifolds in the locally symmetric and conformally flat Riemannian manifold and generalize Yau's result obtained in J. Amer. Math. 97 (1975), 76–100. 相似文献
5.
S. K. Godunov V. T. Zhukov O. B. Feodoritova 《Computational Mathematics and Mathematical Physics》2006,46(6):971-982
An algorithm is constructed for calculating invariant subspaces of symmetric hyperbolic systems arising in electromagnetic, acoustic, and elasticity problems. Discrete approximations are calculated for subspaces that correspond to minimal eigenvalues and smooth eigenfunctions. Difficulties related to the presence of an infinite-dimensional kernel in the differential operator are successfully handled. The efficiency of the algorithm is demonstrated using acoustics equations. 相似文献
6.
Zhao Hanzhong 《Acta Mechanica Solida Sinica》2002,15(2):156-162
A finite difference/boundary integral procedure to determine the acoustic reflected pressure from a fluid-loaded bi-laminated
plate is described. The bi-laminate is composed of a piezoelectric layer and an elastic layer in contact with the fluid, and
is held by an acoustically hard baffle. In the numerical model, the fluid pressure at fluid/solid interface is replaced by
a continuum of point sources weighted by the normal acceleration of the elastic plate, and the governing equation system is
solved in the solid domain. With the normal acceleration found, the reflected pressure in the fluid is determined by an integral
expression involving the Green's function. It is demonstrated that an appropriate applied voltage potential across the piezoelectric
layer has the effect of cancelling either the reflected or scattered pressure of the plate at any chosen field points in the
fluid.
Project supported by the National Natural Science Foundation of China (No. 10172039). 相似文献
7.
A new method is presented for the determination of Ricci Collineations (RC) and Matter Collineations (MC) of a given spacetime, in the cases where the Ricci tensor and the energy momentum tensor are non-degenerate and have a similar form with the metric. This method reduces the problem of finding the RCs and the MCs to that of determining the KVs whereas at the same time uses already known results on the motions of the metric. We employ this method to determine all hypersurface homogeneous locally rotationally symmetric spacetimes, which admit proper RCs and MCs. We also give the corresponding collineation vectors. These results conclude a long due open problem, which has been considered many times partially in the literature. 相似文献
8.
9.
乐毅 《宁波大学学报(理工版)》2004,17(4):437-440
从正交各向异性三维弹性动力学的控制方程出发,在求得四边简支矩形板自由振动频率和位移振型的基础上,构造了受迫振动的位移函数;利用自由振动位移振型的正交性,将控制方程的空间变量和时间变量分离,得到了广义质量、广义力和频率表示的关于时间的2阶常微分方程,从而得到了正交各向异性弹性矩形板在受迫振动下的位移场和应力场.给出了本文方法与薄板理论的算例. 相似文献
10.
Tiegang Fang 《International Journal of Non》2008,43(9):1007-1011
The unsteady boundary layer over a semi-infinite flat plate was investigated in this paper. The flow involves the unsteady flow over a flat plate with leading edge accretion or ablation. The momentum boundary layer was further analyzed and it was shown that the leading edge ablation had a similar effect to the wall mass injection or upstream wall movement making the fluid blown away from the wall. The thermal boundary layer of the same flow was also studied. Results show that the leading edge accretion or ablation can greatly change the fluid motion and the heat transfer characteristics. 相似文献