全文获取类型
收费全文 | 3932篇 |
免费 | 945篇 |
国内免费 | 469篇 |
专业分类
化学 | 1292篇 |
晶体学 | 123篇 |
力学 | 415篇 |
综合类 | 51篇 |
数学 | 902篇 |
物理学 | 2563篇 |
出版年
2024年 | 20篇 |
2023年 | 65篇 |
2022年 | 117篇 |
2021年 | 130篇 |
2020年 | 144篇 |
2019年 | 137篇 |
2018年 | 152篇 |
2017年 | 187篇 |
2016年 | 203篇 |
2015年 | 167篇 |
2014年 | 260篇 |
2013年 | 369篇 |
2012年 | 233篇 |
2011年 | 310篇 |
2010年 | 232篇 |
2009年 | 279篇 |
2008年 | 274篇 |
2007年 | 263篇 |
2006年 | 210篇 |
2005年 | 188篇 |
2004年 | 193篇 |
2003年 | 165篇 |
2002年 | 162篇 |
2001年 | 112篇 |
2000年 | 131篇 |
1999年 | 109篇 |
1998年 | 103篇 |
1997年 | 80篇 |
1996年 | 68篇 |
1995年 | 39篇 |
1994年 | 39篇 |
1993年 | 22篇 |
1992年 | 26篇 |
1991年 | 21篇 |
1990年 | 18篇 |
1989年 | 18篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有5346条查询结果,搜索用时 23 毫秒
1.
2.
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
3.
固体力学有限元体系的结构拓扑变化理论 总被引:2,自引:1,他引:1
本文是文[1]的继续.文[1]提出了杆件系统的结构拓扑变化理论和拓扑变化法本文将这一理论和方法推进到连续体有限元体系;且在此基础上揭示出有限元体系的一个新性质,称为基本位移之梯度的正交性定理,从而给出一套设计敏度的显式表达式,可直接用于计算. 相似文献
4.
5.
采用交流法测量大功率商用钛酸钡(BT)陶瓷加热器的电阻和加热功率随温度的变化关系.结果显示,BT陶瓷的电导特性在80℃附近出现了明显的转变,从低温时的极化子跳跃导电转变为高温时的能带导电,此时,电阻出现极小值,而加热功率出现极大值. 相似文献
6.
Shougo Higashi 《Surface science》2006,600(3):591-597
We have studied the individual adsorption of Mn and Bi, and their coadsorption on Cu(0 0 1) by low-energy electron diffraction (LEED). For Mn, we have determined the c(2 × 2) structure formed at 300 K, whose structure had been determined by several methods. We reconfirmed by a tensor LEED analysis that it is a substitutional structure and that a previously reported large corrugation (0.30 Å) between substitutional Mn and remaining surface Cu atoms coincides perfectly with the present value. In the individual adsorption of Bi, we have found a c(4 × 2) structure, which is formed by cooling below ∼250 K a surface prepared by Bi deposition of ∼0.25 ML coverage at 300 K where streaky half-order LEED spots appear. The c(4 × 2) structure has been determined by the tensor LEED analysis at 130 K and it is a substitutional structure. In the coadsorption, we found a c(6 × 4) structure, which has been determined by the tensor LEED analysis. It is very similar to the previously determined structure of the c(6 × 4) formed by coadsorption of Mg and Bi, and embedded MnBi4 clusters are arranged in the top Cu layer instead of MgBi4. Large lateral displacements of Bi atoms in the c(6 × 4)-(Mn + Bi) suggest that the Mn atoms undergo the size-enhancement caused by their large magnetic moment. 相似文献
7.
We give necessary and sufficient conditions for an operator on a separable Hilbert space to satisfy the hypercyclicity criterion.
This paper is a part of the second author’s Doctoral thesis, written at Shiraz University under the direction of the first
author. 相似文献
8.
弱拓扑和强可容许拓扑之间的一个关系 总被引:2,自引:0,他引:2
本文对局部凸空间X及其对偶X′,建立了弱拓扑σ(X,X′)和强拓扑β(X,X′)之间的一个新关系,改进了有关(X,X′)--可容许拓扑全体上的不变性结果。 相似文献
9.
The aim of this paper is to study I-fuzzy topologies (fuzzy topologies in
ostak's sense) derived from decreasing families of I-topologies (fuzzy topologies in Chang's sense), defined on a set X. 相似文献
10.
For a topological space X we denote by CL(X) the collection of all nonempty closed subsets of X. Suppose we have a map T which assigns in some coherent way to every topological space X some topology T(X) on CL(X). In this paper we study continuity and inverse continuity of the map iA,X :(CL(A),T{A)) → (CL(X),T(X)) defined by iA,x(F) = F whenever F ∈CL(A), for various assignment T; in particular, for locally finite topology, upper Kuratowski topology, and Attouch-Wets topology, etc. 相似文献