全文获取类型
收费全文 | 6853篇 |
免费 | 847篇 |
国内免费 | 214篇 |
专业分类
化学 | 1055篇 |
晶体学 | 31篇 |
力学 | 124篇 |
综合类 | 104篇 |
数学 | 553篇 |
物理学 | 1528篇 |
无线电 | 4519篇 |
出版年
2024年 | 15篇 |
2023年 | 90篇 |
2022年 | 278篇 |
2021年 | 273篇 |
2020年 | 227篇 |
2019年 | 160篇 |
2018年 | 145篇 |
2017年 | 264篇 |
2016年 | 285篇 |
2015年 | 278篇 |
2014年 | 503篇 |
2013年 | 423篇 |
2012年 | 510篇 |
2011年 | 516篇 |
2010年 | 306篇 |
2009年 | 389篇 |
2008年 | 403篇 |
2007年 | 415篇 |
2006年 | 386篇 |
2005年 | 378篇 |
2004年 | 293篇 |
2003年 | 272篇 |
2002年 | 207篇 |
2001年 | 178篇 |
2000年 | 113篇 |
1999年 | 116篇 |
1998年 | 73篇 |
1997年 | 83篇 |
1996年 | 90篇 |
1995年 | 56篇 |
1994年 | 43篇 |
1993年 | 35篇 |
1992年 | 25篇 |
1991年 | 15篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1987年 | 17篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有7914条查询结果,搜索用时 15 毫秒
991.
粗糙波面中心点亮度比像质评价准则 总被引:1,自引:1,他引:0
建立了兼含粗糙与波面像差影响的中心点亮度比公式及其像质评价准则,根据粗糙波面中心点亮度比的大小,把像质标准划分为“极佳”“良好”和“不理想”等三个等级,粗糙度公差范围为λ/20~λ/80。 相似文献
992.
Summary A study on solar microclimates in eight medium and large European cities, supported by EEC from 1986 to 1988, could allow
to mark the main resemblances and anomalies in the solar-radiation features in different geographical and climatic regions
in Northern, Central and Southern Europe and also to achieve reliable estimates of available energy in thermal-conversion
processes of solar radiation. This paper presents the results of an analysis of solar-radiation observations taken during
the EEC study and discusses the main solar climatic peculiarities of the urban area of Turin (one of the eight cities involved
in this project), comparing them to other European regions (south and north of Turin). 相似文献
993.
激光精细加工及其质量检测的最新进展 总被引:2,自引:0,他引:2
本文综述了目前激光精细加工的状况及其质量检测的最新发展,指出带有高精度质量检测的激光精细加工系统在工业应用中的重要性。 相似文献
995.
Veselin Rakocevic 《International Journal of Communication Systems》2004,17(7):723-734
This paper provides a parallel review of two important issues for the next‐generation multimedia networking. Firstly, the emerging multimedia applications require a fresh approach to congestion control in the Internet. Currently, congestion control is performed by TCP; it is optimised for data traffic flows, which are inherently elastic. Audio and video traffic do not find the sudden rate fluctuations imposed by the TCP multiplicative‐decrease control algorithm optimal. The second important issue is the mobility support for multimedia applications. Wireless networks are characterized by a substantial packet loss due to the imperfection of the radio medium. This increased packet loss disturbs the foundation of TCP's loss‐based congestion control. This paper contributes to the ongoing discussion about the Internet congestion control by providing a parallel analysis of these two issues. The paper describes the main challenges, design guidelines, and existing proposals for the Internet congestion control, optimised for the multimedia traffic in the wireless network environment. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
996.
Proportional delay differentiation provision by bandwidth adaptation of class‐based queue scheduling
Angelos Michalas Malamati Louta Paraskevi Fafali George Karetsos Vassilios Loumos 《International Journal of Communication Systems》2004,17(7):743-761
Supporting quality of service (QoS) over the Internet is a very important issue and many mechanisms have already been devised or are under way towards achieving this goal. One of the most important approaches is the so‐called Differentiated Services (DiffServ) architecture, which provides a scalable mechanism for QoS support in a TCP/IP network. The main concept underlying DiffServ is the aggregation of traffic flows at an ingress (or egress) point of a network and the marking of the IP packets of each traffic flow according to several classification criteria. Diffserv is classified under two taxonomies: the absolute and the relative. In absolute DiffServ architecture, an admission control scheme is utilized to provide QoS as absolute bounds of specific QoS parameters. The relative DiffServ model offers also QoS guarantees per class but in reference to the guarantees given to the other classes defined. In this paper, relative proportional delay differentiation is achieved based on class‐based queueing (CBQ) scheduler. Specifically, the service rates allocated to the classes of a CBQ scheduler are frequently adjusted in order to obtain relative delay spacing among them. The model presented can also be exploited in order to meet absolute delay constraints in conjunction with relative delay differentiation provision. Simulation experiments verify that our model can attain relative as well as absolute delay differentiation provided that the preconditions posed are satisfied. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
997.
研究一种语音伪装新方法,将保密语音替换成另外一种语音以便于在公网上安全传输。保密语音经2.4kb/低速率压缩编码后,依据设计的客观音质评估模型,将参数隐蔽地嵌入普通语音;检测时利用逆过程恢复保密语音。实验表明算法的信噪比较传统算法提高约3.5dB,并在受到压缩、滤波等攻击时具有较高的鲁棒性。 相似文献
998.
999.
一种基于资源动态分配的光突发装配新技术 总被引:4,自引:4,他引:0
在粗颗粒分类基础上提出了一种新的光突发装配技术。它包括两个方面:映射规则,主要解决某一优先级光突发可以装配的类别分组,提出了S映射、Q映射和TQ映射3种规则,其差别主要在于映射是否具有自适应性;动态门限装配能力分配方案,即ATQ-DT,主要解决如何动态、合理地分配光突发装配能力。TQ映射和ATQ-DT可随服务质量(QoS)要求和负载情况的变化而自动调整映射关系和装配门限。经仿真表明:它们的组合,即“TQ映射 ATQ-DT”在保证各分组类别的分组损失率(PLP)上效果最为明显。 相似文献
1000.
In terms of the so-called irradiance moments of a light field, the beam quality change, ΔQ, of radially and azimuthally polarized beams caused by propagation through a quartic phase plate (as occurs, for example, in strongly pumped laser rods used in high-power solid-state lasers) is studied. Analytical expressions for ΔQ are given, and a comparison between the scalar and vectorial regimes is also shown. The results are applied to several cases of interest. 相似文献