全文获取类型
收费全文 | 20396篇 |
免费 | 3419篇 |
国内免费 | 1719篇 |
专业分类
化学 | 3869篇 |
晶体学 | 236篇 |
力学 | 3170篇 |
综合类 | 163篇 |
数学 | 1979篇 |
物理学 | 8436篇 |
无线电 | 7681篇 |
出版年
2024年 | 49篇 |
2023年 | 177篇 |
2022年 | 457篇 |
2021年 | 545篇 |
2020年 | 545篇 |
2019年 | 469篇 |
2018年 | 461篇 |
2017年 | 656篇 |
2016年 | 848篇 |
2015年 | 905篇 |
2014年 | 1358篇 |
2013年 | 1777篇 |
2012年 | 1582篇 |
2011年 | 1583篇 |
2010年 | 1271篇 |
2009年 | 1284篇 |
2008年 | 1432篇 |
2007年 | 1492篇 |
2006年 | 1289篇 |
2005年 | 1119篇 |
2004年 | 948篇 |
2003年 | 847篇 |
2002年 | 700篇 |
2001年 | 642篇 |
2000年 | 526篇 |
1999年 | 417篇 |
1998年 | 442篇 |
1997年 | 276篇 |
1996年 | 234篇 |
1995年 | 195篇 |
1994年 | 184篇 |
1993年 | 152篇 |
1992年 | 104篇 |
1991年 | 97篇 |
1990年 | 84篇 |
1989年 | 77篇 |
1988年 | 61篇 |
1987年 | 41篇 |
1986年 | 57篇 |
1985年 | 46篇 |
1984年 | 29篇 |
1983年 | 14篇 |
1982年 | 20篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1976年 | 5篇 |
1972年 | 3篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
111.
提出了一种新的器件结构——非对称Halo L DD低功耗器件,该器件可以很好地抑制短沟效应,尤其可以很好地改善DIBL效应、热载流子效应以及降低功耗等,是低功耗高集成度电路的优选结构之一.分析了非对称HaloL DD器件的主要特性,并将其与常规结构、非对称L DD结构、非对称Halo结构的器件进行了比较并进行了参数优化分析. 相似文献
112.
通信技术的发展,通信网络结构日益复杂,一个网络中会同时存在PDH、SDH、ATM、DWDM、无线业务和IP等多种技术。使用传统网管系统已无法胜任,只有应用针对多厂商、多技术网络的集中网管系统才能满足管理要求。现提出了一种新型的针对多厂商、多技术网络的集中网管软件的设计模型及实现方法,并详细讨论了各模块的数据结构、实现功能,以及用Java实现该软件的具体方法。特别根据网管任务的实际需要,提出了故障屏蔽的概念及实现方法。经仿真试验,证明该网管软件已达到设计要求。 相似文献
113.
Claudio Sacchi Gianluca Gera Carlo S. Regazzoni 《International Journal of Satellite Communications and Networking》2004,22(2):193-215
This paper aims at focusing on the aspects concerning the physical layer design for an innovative satellite communication experiment. Such an experiment, denoted by the acronym DAVID–DCE (Data and Video Interactive Distribution—Data Collection Experiment) is based on the exploitation of the W‐band (75–110 GHz) for high‐bit‐rate satellite transmission. The potential advantages of using of the W‐band are mainly related to the great bandwidth availability, and to the absence of interference. Moreover, an expected result of the experiment is a substantive improvement in the communication system's performances in the presence of meteorological phenomena (e.g. rain) as compared with the more conventional Ka‐band satellite transmission. On the other hand, problems to be faced concern the non‐ideal behaviours of hardware devices employed for high‐frequency digital transmission. In particular, carrier recovery and timing recovery are the most crucial signal‐processing tasks to be carefully considered in the design of the physical level of the system, because they considerably suffer from hardware impairments. The purpose of this work is to illustrate the proposed solutions in terms of the most critical modulation, demodulation and synchronization design issues, together with the effects of non‐ideal behaviours of hardware components on BER performances. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
114.
115.
E. de A. Gonçalves V. M. de Oliveira A. Rosas P. R.A. Campos 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(1):127-132
Adaptation of populations takes place with the occurrence and subsequent fixation of mutations that confer some selective
advantage
to the individuals which acquire it. For this reason, the study of the process of fixation of advantageous mutations has a
long history
in the population genetics literature. Particularly, the previous investigations aimed to find out the main evolutionary forces
affecting the strength of natural selection in the populations.
In the current work, we investigate the dynamics of fixation of beneficial mutations in a subdivided population. The subpopulations
(demes)
can exchange migrants
among their neighbors, in a migration network which is assumed to have either a random graph or a scale-free topology. We
have observed that the migration rate
drastically affects the dynamics of mutation fixation, despite of the fact that the probability of fixation is invariant on
the
migration rate, accordingly to
Maruyama's conjecture. In addition, we have noticed a topological dependence of the adaptive evolution of the population when
clonal interference
becomes effective. 相似文献
116.
A general framework is developed to treat inverse problems with parameters that are random fields. It involves a sampling
method that exploits the sensitivity derivatives of the control variable with respect to the random parameters. As the sensitivity
derivatives are computed only at the mean values of the relevant parameters, the related extra cost of the present method
is a fraction of the total cost of the Monte Carlo method. The effectiveness of the method is demonstrated on an example problem
governed by the Burgers equation with random viscosity. It is specifically shown that this method is two orders of magnitude
more efficient compared to the conventional Monte Carlo method. In other words, for a given number of samples, the present
method yields two orders of magnitude higher accuracy than its conventional counterpart. 相似文献
117.
118.
本文介绍了可在微机上实现大规模电信网践模拟的实用化软件SIMUNET,及基所采用的无时钟事件追踪法。该软件可用规划中的电信网络和实际网路的模拟及性能评价,也可用于不同流量控制算法的评价。文中给出一个评价实例。 相似文献
119.
自由电子激光振荡器的二维数值模拟 总被引:2,自引:2,他引:0
在柱二维坐标系中,用数值计算方法求解了摇摆器内电子模拟方程组、光场方程组和无源区(不含摇摆器区)经非自适应方法变换后的无源旁轴波动方程并设计了相应的程序R_2D。用该程序计算的数值结果表明,在有源条件下二维数值模拟结果与高斯光束近似下的数值模拟结果符合较好;取完全相同的计算条件R-2D程序与洛斯阿拉莫斯实验室(LANL)的FELEX程序的计算结果符合较好;初步认为LANL实验数据的物理图象合理,数值结果基本可信;最后给出北京自由电子激光器(BFEL)的一些物理数据。 相似文献
120.
J. Schulte 《Journal of statistical physics》1993,70(5-6):1343-1347
The thermodynamic stability of4He4–13 at 3.2 K is investigated with the classical Monte Carlo method, with the semiclassical path-integral Monte Carlo (PIMC) method, and with the semiclassical all-order many-body method. In the all-order many-body simulation the dipole-dipole approximation including short-range correction is used. The resulting stability plots are discussed and related to recent TOF experiments by Stephens and King. It is found that with classical Monte Carlo of course the characteristics of the measured mass spectrum cannot be resolved. With PIMC, switching on more and more quantum mechanics. by raising the number of virtual time steps results in more structure in the stability plot, but this did not lead to sufficient agreement with the TOF experiment. Only the all-order many-body method resolved the characteristic structures of the measured mass spectrum, including magic numbers. The result shows the influence of quantum statistics and quantum mechanics on the stability of small neutral helium clusters. 相似文献