全文获取类型
收费全文 | 19237篇 |
免费 | 3744篇 |
国内免费 | 2625篇 |
专业分类
化学 | 9165篇 |
晶体学 | 262篇 |
力学 | 3370篇 |
综合类 | 162篇 |
数学 | 1625篇 |
物理学 | 6412篇 |
无线电 | 4610篇 |
出版年
2024年 | 54篇 |
2023年 | 310篇 |
2022年 | 501篇 |
2021年 | 585篇 |
2020年 | 757篇 |
2019年 | 651篇 |
2018年 | 634篇 |
2017年 | 780篇 |
2016年 | 940篇 |
2015年 | 885篇 |
2014年 | 1180篇 |
2013年 | 1721篇 |
2012年 | 1326篇 |
2011年 | 1310篇 |
2010年 | 1075篇 |
2009年 | 1146篇 |
2008年 | 1258篇 |
2007年 | 1265篇 |
2006年 | 1143篇 |
2005年 | 1018篇 |
2004年 | 926篇 |
2003年 | 854篇 |
2002年 | 654篇 |
2001年 | 650篇 |
2000年 | 592篇 |
1999年 | 490篇 |
1998年 | 445篇 |
1997年 | 375篇 |
1996年 | 349篇 |
1995年 | 293篇 |
1994年 | 287篇 |
1993年 | 230篇 |
1992年 | 191篇 |
1991年 | 146篇 |
1990年 | 101篇 |
1989年 | 98篇 |
1988年 | 82篇 |
1987年 | 61篇 |
1986年 | 53篇 |
1985年 | 34篇 |
1984年 | 36篇 |
1983年 | 14篇 |
1982年 | 29篇 |
1981年 | 11篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 10篇 |
1973年 | 5篇 |
1971年 | 6篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 359 毫秒
61.
The authors study the bifurcation problems of rough heteroclinic loop connecting three saddle points for the case β1 > 1, β2 > 1, β3 < 1 and β1β2β3 < 1. The existence, number, coexistence and incoexistence of 2-point-loop, 1-homoclinic orbit and 1-periodic orbit are studied. Meanwhile, the bifurcation surfaces and existence regions are given. 相似文献
62.
K. S. Birdi 《Colloid and polymer science》1997,275(6):561-566
The surface tension versus temperature data of homologous series of n-alkanes and n-alkenes is analyzed. Critical temperatures are shown to be predicted from these data, with a very high precision, after some
corrections are made. The corrections are shown to arise from the effect of the critical pressure on the extrapolated data
to surface tension approaching zero.
Received: 29 October 1996 Accepted: 13 December 1996 相似文献
63.
64.
A novel type of corner reflector for the reflection of surface plasmon polariton (SPP) is proposed. The reflector consists
of two layers of compensatory media whose permittivity and permeability take opposite signs to that of the corresponding layer
of the SP waveguide. By rigorously solving Maxwell's equations, the reflected SPP is proved to exist. The transverse wave
vector of the reflected SPP is always antiparallel to that of the incident SPP and no phase retardation is introduced. 相似文献
65.
有机发光显示(OLED)作为下一代显示器倍受关注。本文简要介绍OLED显示器的几项新技术和制作方法,以及其研究现状和今后的发展趋势。 相似文献
66.
67.
Many recent algorithmic approaches involve the construction of a differential equation model for computational purposes, typically
by introducing an artificial time variable. The actual computational model involves a discretization of the now time-dependent differential system, usually
employing forward Euler. The resulting dynamics of such an algorithm is then a discrete dynamics, and it is expected to be
“close enough” to the dynamics of the continuous system (which is typically easier to analyze) provided that small – hence
many – time steps, or iterations, are taken. Indeed, recent papers in inverse problems and image processing routinely report
results requiring thousands of iterations to converge. This makes one wonder if and how the computational modeling process
can be improved to better reflect the actual properties sought.
In this article we elaborate on several problem instances that illustrate the above observations. Algorithms may often lend
themselves to a dual interpretation, in terms of a simply discretized differential equation with artificial time and in terms
of a simple optimization algorithm; such a dual interpretation can be advantageous. We show how a broader computational modeling
approach may possibly lead to algorithms with improved efficiency.
AMS subject classification (2000) 65L05, 65M32, 65N21, 65N22, 65D18 相似文献
68.
An infinite row of periodically spaced, identical rigid circularcylinders is excited by an acoustic line source which is parallelto the generators of the cylinders. A method for calculatingthe scattered field accurately and efficiently is presented.When the cylinders are sufficiently close together, Rayleigh–Blochsurface waves that propagate energy to infinity along the arrayare excited. An expression is derived which enables the amplitudesof these surface waves to be computed without requiring thesolution to the full scattering problem. 相似文献
69.
The reaction between the Sn-Ag-Cu solders and Ni at 250°C for 10 min and 25 h was studied. Nine different Sn-Ag-Cu solders,
with the Ag concentration fixed at 3.9 wt.% and Cu concentrations varied between 0.0–3.0 wt.%, were used. When the reaction
time was 10 min, the reactions strongly depended on the Cu concentration. At low-Cu concentrations (≦0.2 wt.%), only a continuous
(Ni1−xCux)3Sn4 layer formed at the interface. When the Cu concentration increased to 0.4 wt.%, a continuous (Ni1−xCux)3Sn4 layer and a small amount of discontinuous (Cu1−yNiy)6Sn5 particles formed at the interface. When the Cu concentration increased to 0.5 wt.%, the amount of (Cu1−yNiy)6Sn5 increased and (Cu1−yNi6)6Sn5 became a continuous layer. Beneath this (Cu1−yNiy)6Sn5 layer was a very thin but continuous layer of (Ni1−xCux)3Sn4. At higher Cu concentrations (0.6–3.0 wt.%), (Ni1−xCux)3Sn4 disappeared, and only (Cu1−yNiy)6Sn5 was present. The reactions at 25 h also depended strongly on the Cu concentration, proving that the strong concentration
dependence was not a transient phenomenon limited to a short reaction time. The findings of this study were rationalized using
the Cu-Ni-Sn isotherm. This study shows that precise control over the Cu concentration in solders is needed to produce consistent
results. 相似文献
70.
为提高CCD摄像机的成像质量,同时使镜头结构紧凑、小型化,在大视场光学镜头的设计中,引入标准二次曲面和偶次非球面。根据初级像差理论,分析了非球面的位置、初始结构参数的求解规律。通过理论计算和ZEMAX光学设计软件的优化,给出工作波长为0.4~0.7μm、全视场角为80°,相对孔径为1∶1.5的镜头设计实例。该镜头由7块镜片组成,包括一个标准二次曲面和两个8次方非球面;在40lp/mm空间频率处的MTF值超过0.85,全视场畸变小于3%,像质优良 相似文献