全文获取类型
收费全文 | 2405篇 |
免费 | 268篇 |
国内免费 | 141篇 |
专业分类
化学 | 64篇 |
晶体学 | 2篇 |
力学 | 979篇 |
综合类 | 38篇 |
数学 | 188篇 |
物理学 | 306篇 |
无线电 | 1237篇 |
出版年
2024年 | 10篇 |
2023年 | 43篇 |
2022年 | 71篇 |
2021年 | 62篇 |
2020年 | 89篇 |
2019年 | 92篇 |
2018年 | 59篇 |
2017年 | 84篇 |
2016年 | 130篇 |
2015年 | 127篇 |
2014年 | 185篇 |
2013年 | 168篇 |
2012年 | 155篇 |
2011年 | 188篇 |
2010年 | 129篇 |
2009年 | 131篇 |
2008年 | 133篇 |
2007年 | 138篇 |
2006年 | 121篇 |
2005年 | 111篇 |
2004年 | 90篇 |
2003年 | 89篇 |
2002年 | 56篇 |
2001年 | 59篇 |
2000年 | 46篇 |
1999年 | 39篇 |
1998年 | 23篇 |
1997年 | 31篇 |
1996年 | 23篇 |
1995年 | 28篇 |
1994年 | 14篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1971年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有2814条查询结果,搜索用时 0 毫秒
41.
42.
AbstractThe present work is concerned with the 2D deformation in a nonhomogeneous, isotropic, micropolar, magneto-thermoelastic medium in the context of Lord-Shulman theory as a result of an inclined load. The inclined load is supposed to be a linear combination of normal load and tangential load. Material properties are assumed to be graded in x-direction. Normal mode technique is proposed to obtain the analytical expressions for the temperature field, displacement components, and stresses. These are also calculated numerically and depicted graphically to observe the variations of the considered physical variables.Communicated by Seonho Cho. 相似文献
43.
对于一般任意支撑的连续梁结构动力稳定性问题,已有的计算方法求解过程都很复杂,给工程设计带来极大的不便.本文提出了一个简化的分析方法,利用现有的商业软件,只需求得连续梁的自然频率及静力屈曲(失稳)荷载,就可容易得到结构的动力失稳区域,当考虑结构阻尼对不稳定区域的影响时,可将阻尼矩阵表达为Rayleigh阻尼的形式.研究结果表明:采用本文计算方法与已有的理论计算方法得到的连续梁主参数共振的不稳定边界非常吻合,而本文计算方法更为简单,计算结果可靠,计算精度高,可满足工程设计的需要. 相似文献
44.
IPTV是下一代网络中最重要的业务之一,IPTV承载网的技术要求对国内运营商的宽带网络提出了新的挑战,通过以GPON作为IPTV等新业务的承载层网络,对承载方案进行了设计,随着IPTV用户的不断增加,必将成为xDSL技术向GPON网络演进的关键驱动因素. 相似文献
45.
46.
Functional load (FL) quantifies the contributions by phonological contrasts to distinctions made across the lexicon. Previous research has linked particularly low values of FL to sound change. Here, we broaden the scope of enquiry into FL to its evolution at higher values also. We apply phylogenetic methods to examine the diachronic evolution of FL across 90 languages of the Pama–Nyungan (PN) family of Australia. We find a high degree of phylogenetic signal in FL, indicating that FL values covary closely with genealogical structure across the family. Though phylogenetic signals have been reported for phonological structures, such as phonotactics, their detection in measures of phonological function is novel. We also find a significant, negative correlation between the FL of vowel length and of the following consonant—that is, a time-depth historical trade-off dynamic, which we relate to known allophony in modern PN languages and compensatory sound changes in their past. The findings reveal a historical dynamic, similar to transphonologization, which we characterize as a flow of contrastiveness between subsystems of the phonology. Recurring across a language family that spans a whole continent and many millennia of time depth, our findings provide one of the most compelling examples yet of Sapir’s ‘drift’ hypothesis of non-accidental parallel development in historically related languages. 相似文献
47.
Scenario Reduction Algorithms in Stochastic Programming 总被引:4,自引:0,他引:4
We consider convex stochastic programs with an (approximate) initial probability distribution P having finite support supp P, i.e., finitely many scenarios. The behaviour of such stochastic programs is stable with respect to perturbations of P measured in terms of a Fortet-Mourier probability metric. The problem of optimal scenario reduction consists in determining a probability measure that is supported by a subset of supp P of prescribed cardinality and is closest to P in terms of such a probability metric. Two new versions of forward and backward type algorithms are presented for computing such optimally reduced probability measures approximately. Compared to earlier versions, the computational performance (accuracy, running time) of the new algorithms has been improved considerably. Numerical experience is reported for different instances of scenario trees with computable optimal lower bounds. The test examples also include a ternary scenario tree representing the weekly electrical load process in a power management model. 相似文献
48.
于宝中 《电信工程技术与标准化》2007,20(6):36-39
如何建设高品质通信电源系统,对于保证网络运行起到非常关键的作用。本文通过分析通信系统用电负载类别、负载对电源系统的要求及配制方式进行探讨,提出解决方案。 相似文献
49.
本文针对8阵元智能天线的特点,讨论了现网条件下智能天线应用与TD-SCDMA系统的各种安装方式,并从建筑力学的角度分析了智能天线在各种安装条件下的风载指标,并根据这些指标分析了各种安装方式的可行性. 相似文献
50.
R. Nieva G. S. Christensen M. E. El-Hawary 《Journal of Optimization Theory and Applications》1981,35(2):261-275
This paper considers the problem of short-term optimal operation of nuclear-hydro-thermal electric power systems. The solution is obtained by use of a functional analytic optimization technique that employs the minimum norm formulation.A power system with an arbitrary number of generating stations is considered. The limited flexibility exhibited by the thermal nuclear reactors, when operating in a load-following mode, is accounted for by means of a model of the xenon concentration in their cores. The nonlinear effects induced by trapezoidal water reservoirs and the time delay of the water flow between upstream and downstream hydroplants is taken into consideration as well.A two-level iterative scheme of the feasible type is proposed for implementing the optimal solution.This work was supported in part by the National Research Council of Canada, Grant No. A-4146. 相似文献