全文获取类型
收费全文 | 17390篇 |
免费 | 1952篇 |
国内免费 | 2301篇 |
专业分类
化学 | 8042篇 |
晶体学 | 175篇 |
力学 | 2175篇 |
综合类 | 267篇 |
数学 | 6869篇 |
物理学 | 4115篇 |
出版年
2024年 | 56篇 |
2023年 | 198篇 |
2022年 | 393篇 |
2021年 | 457篇 |
2020年 | 577篇 |
2019年 | 533篇 |
2018年 | 520篇 |
2017年 | 621篇 |
2016年 | 728篇 |
2015年 | 492篇 |
2014年 | 774篇 |
2013年 | 1470篇 |
2012年 | 980篇 |
2011年 | 979篇 |
2010年 | 836篇 |
2009年 | 1061篇 |
2008年 | 1108篇 |
2007年 | 1177篇 |
2006年 | 962篇 |
2005年 | 866篇 |
2004年 | 790篇 |
2003年 | 712篇 |
2002年 | 652篇 |
2001年 | 572篇 |
2000年 | 530篇 |
1999年 | 522篇 |
1998年 | 423篇 |
1997年 | 369篇 |
1996年 | 324篇 |
1995年 | 312篇 |
1994年 | 258篇 |
1993年 | 201篇 |
1992年 | 180篇 |
1991年 | 175篇 |
1990年 | 123篇 |
1989年 | 107篇 |
1988年 | 84篇 |
1987年 | 75篇 |
1986年 | 68篇 |
1985年 | 64篇 |
1984年 | 57篇 |
1983年 | 33篇 |
1982年 | 42篇 |
1981年 | 41篇 |
1980年 | 32篇 |
1979年 | 32篇 |
1978年 | 15篇 |
1977年 | 21篇 |
1976年 | 16篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
In view of immense importance of silylenes and the fact that their properties undergo significant changes on substitution with halogens, here, we have used B3LYP/6-311++G** level of theory to access the effects of 1–4 halogens (X = F, Cl, Br, and I) on four unprecedented sets of cyclopentasilylene-2,4-dienes; with the following formulas: SiC4H3X ( 1 X ), SiC4H2X2 ( 2 X ), SiC4HX3 ( 3 X ), and SiC4X4 ( 4 X ). In going down from F to I, the singlet (s)-triplet (t) energy gap (ΔEs-t, a possible indication of stability), and band gap (ΔEH-L) decrease while nucleophilicity (N), chemical potential (μ), and proton affinity (PA) increase. The overall order of N, μ, and PA for each X is 2 X > 1 X > 3 X > 4 X . Precedence of 2 X over 1 X is attributed to the symmetric cross conjugation in the former. The highest and lowest N are shown by 2 I and 4 F . The trend of divalent angle () for each X is 4 X > 1 X > 3 X > 2 X . The results show that in going from electron withdrawing groups (EWGs) to electron donating groups (EDGs), the ΔEs-t and ΔEH-L decrease while N, μ, and PA increase. Also, rather high N of our scrutinized silylenes may suggest new promising ligands in organometallic chemistry. 相似文献
2.
Rafael López 《Annals of Global Analysis and Geometry》1997,15(3):201-210
In this paper we study constant mean curvature compact surfaces with two Jordan curves in parallel planes as boundary and we investigate the point at which the surface inherits the symmetries of its boundary. 相似文献
3.
Daniel L. Feldheim Del R. Lawson Charles R. Martin 《Journal of Polymer Science.Polymer Physics》1993,31(8):953-957
We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc. 相似文献
4.
Mark Webber 《Mathematical Methods in the Applied Sciences》2006,29(7):819-838
We investigate the influence of slip boundary conditions on the onset of Bénard convection in an infinite fluid layer. It is shown that the critical Rayleigh number is a decreasing function of the slip length, and therefore boundary slip is seen to have a destabilizing effect. Chebyshev‐tau and compound matrix formulations for solving the eigenvalue problem are presented. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
5.
Michael R. Caputo 《Natural Resource Modeling》1989,3(2):241-259
This paper analyzes the extent to which standard dynamic renewable resource models possess refutable implications. Both the steady state comparative static and local comparative dynamic properties of the standard model are studied. A unified framework is developed which enables one to analyze the qualitative properties of any standard renewable resource model. This is achieved by explicitly linking the local stability, steady state comparative static, and local comparative dynamic properties of the model. 相似文献
6.
Kasper K. Berthelsen Jesper Møller 《Bulletin of the Brazilian Mathematical Society》2002,33(3):351-367
This primer provides a self-contained exposition of the case where spatial birth-and-death processes are used for perfect
simulation of locally stable point processes. Particularly, a simple dominating coupling from the past (CFTP) algorithm and
the CFTP algorithms introduced in [13], [14], and [5] are studied. Some empirical results for the algorithms are discussed.
Received: 30 June 2002 相似文献
7.
A. A. Abramov S. V. Kurochkin 《Computational Mathematics and Mathematical Physics》2006,46(6):949-954
A new method for calculating the radial spheroidal functions of the first kind is proposed for the arguments that are greater than unity in modulus. A well-known representation of these functions is refined and used for this purpose. The constructs and the software implementation proposed in the paper provide an efficient tool for the calculation of the functions with a desired accuracy in a wide range of parameters. 相似文献
8.
本文对具有有限时滞的泛函微分方程建立了关于依照两种测度的实际稳定性的Razumikhin型判定定理,其中未采用通常的辅助函数,且可运用多个含有状态变量x的部分变元的Lyapunov函数,得出部分变远实际稳定性的判定定理,从而改进了已有的结果。 相似文献
9.
V. B. Bekezhanova 《Fluid Dynamics》2006,41(3):343-350
The problem of the stability of steady convective viscous incompressible fluid flow in a vertical layer with boundaries at different temperatures is considered in the presence of transverse blowing through the layer. The complete spectral problem is solved for a silicon melt. The neutral curve is constructed and the critical Grashof number is found. The numerical calculations show that the presence of transverse blowing significantly affects the flow stability. As compared with the Oberbeck-Boussinesq model, in the microconvection model the instability develops at lower wavenumbers. 相似文献
10.
N. A. Lockerbie 《General Relativity and Gravitation》2004,36(3):593-600
By using a novel free-flying, rotating, optical bench aboard the International Space Station (ISS) as the basis for a measurement of differential acceleration between two gravitating bodies, in principle the Newtonian inverse-square law and the constant of gravitation, G, can be determined at the parts in 106 level. 相似文献