全文获取类型
收费全文 | 135506篇 |
免费 | 20293篇 |
国内免费 | 15350篇 |
专业分类
化学 | 72476篇 |
晶体学 | 1136篇 |
力学 | 5878篇 |
综合类 | 667篇 |
数学 | 12640篇 |
物理学 | 39566篇 |
无线电 | 38786篇 |
出版年
2024年 | 555篇 |
2023年 | 3157篇 |
2022年 | 4194篇 |
2021年 | 5153篇 |
2020年 | 5199篇 |
2019年 | 4702篇 |
2018年 | 4433篇 |
2017年 | 4156篇 |
2016年 | 5884篇 |
2015年 | 6245篇 |
2014年 | 7659篇 |
2013年 | 9747篇 |
2012年 | 11407篇 |
2011年 | 11600篇 |
2010年 | 8590篇 |
2009年 | 8516篇 |
2008年 | 8772篇 |
2007年 | 7948篇 |
2006年 | 7542篇 |
2005年 | 6622篇 |
2004年 | 4824篇 |
2003年 | 4029篇 |
2002年 | 3628篇 |
2001年 | 3102篇 |
2000年 | 2811篇 |
1999年 | 2933篇 |
1998年 | 2503篇 |
1997年 | 2169篇 |
1996年 | 2143篇 |
1995年 | 1901篇 |
1994年 | 1617篇 |
1993年 | 1339篇 |
1992年 | 1175篇 |
1991年 | 996篇 |
1990年 | 827篇 |
1989年 | 593篇 |
1988年 | 463篇 |
1987年 | 379篇 |
1986年 | 383篇 |
1985年 | 296篇 |
1984年 | 222篇 |
1983年 | 191篇 |
1982年 | 141篇 |
1981年 | 84篇 |
1980年 | 66篇 |
1979年 | 27篇 |
1978年 | 28篇 |
1976年 | 24篇 |
1975年 | 23篇 |
1957年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 24 毫秒
981.
IP电话的实现及其关键技术 总被引:1,自引:0,他引:1
IP电话在推出后短短十年的时间以其低价、高效、灵活的特点迅速发展起来。本文介绍了IP电话的基本结构,比较了IP电话与普通电话的优缺点,讨论了IP电话的相关协议及关键技术,在这些基础上进一步指出IP电话的发展趋势,相信IP将成为未来信息通信的主导技术。 相似文献
982.
基于主元分析的多变量统计过程的故障辨识技术 总被引:2,自引:0,他引:2
杨莉 《太赫兹科学与电子信息学报》2004,2(4):256-259
为了更好的进行故障检测与诊断,介绍了主元分析理论,给出了基于主元分析的过程故障辨识机理及策略。仿真实例表明,利用此方法建立的故障诊断模型,能够在不依赖过程机理的前提下高效抽取原始数据空间的主要变化信息,对过程的非正常变化做出反应,同时还能较正确地找出发生故障的原因以及相应环节。 相似文献
983.
Packaging represents a significant and expensive obstacle in commercializing microsystem technology (MST) devices such as microelectromechanical systems (MEMS), microopticalelectromechanical systems (MOEMS), microsensors, microactuators, and other micromachined devices. This paper describes a novel wafer-level protection method for MSTs which facilitate improved manufacturing throughput and automation in package assembly, wafer-level testing of devices, and enhanced device performance. The method involves the use of a wafer-sized microcap array. This array consists of an assortment of small caps molded onto a material with adjustable shapes and sizes to serve as protective structures against the hostile environments associated with packaging. It may also include modifications which enhance its adhesion to the MST wafer or increase the MST device function. Depending on the application, the micromolded cap can be designed and modified to facilitate additional functions, such as optical, electrical, mechanical, and chemical functions, which are not easily achieved in the device by traditional means. The fabrication and materials selection of the microcap device is discussed in this paper. The results of wafer-level microcap packaging demonstrations are also presented. 相似文献
984.
In this paper we propose a new protocol called time-space label switching protocol (TSL-SP) in optical burst switching (OBS), and define the terms time-space label (TSL) and time-space routing (TSR). An important concept of response time is introduced in the time label mechanism. The TSL-SP is a new technology that can quickly and efficiently forward data with a label on the optical networks. A two-dimension label switched path (TD-LSP) can be set up, that is maintained and deleted by the TSL-SP. For clearly illuminating the operation principles of the TSL-SP, we propose a new approach of orthogonal time-space coordinates in which the vertical coordinate is the space label and the horizontal coordinate is the time label. The proposed TD-LSP can dramatically reduce the routing failure probability and greatly improve the link network efficiency compared with other signaling protocols. Moreover, we define the time-space label control plane that can achieve the higher efficiency. When the TSL-SP is applied to networks, switching performance can be improved by two orders compared to the switching performance with the conventional OBS signaling protocols. The fundamental goal of TSL-SP is to band the signaling and routing functions together closely. Also, the TSL-SP can reduce the complexity of the network, support automatic service offering, and provide traffic engineering. 相似文献
985.
986.
H. Nagata Yagang Li K.R. Voisine W.R. Bosenberg 《Photonics Technology Letters, IEEE》2004,16(11):2457-2459
Reliability for nonhermetic bias-free LiNbO/sub 3/ optical modulators is estimated from aging data in both dry and damp heat conditions. The two dominant failure modes for these devices are: 1) device performance degradation due to temperature-activated drift of the annealed proton-exchange waveguide; and 2) optical insertion loss increase due to humidity-induced deterioration of glued fiber joints. A total failure rate of 30 failures in time (FITs) is predicted for 20 years operation at 45/spl deg/C and moderate humidity conditions (40% RH). This estimate based on laboratory-test data is consistent with a field failure rate <5 FITs observed in fielded devices. 相似文献
987.
988.
989.
MuShanxiang JiXiaoli LiMing LiXingguo 《电子科学学刊(英文版)》2003,20(6):407-413
Linear Modulated Frequency (LMF) Costas Stepped Frequency Radar (SFR) signal is introduced. Its ambiguity function is derived and analyzed in detail and its feasibility is validated in theory. The scheme of the proposed signal processing is also presented. The results of theoretic analysis and simulation show that, by using the proposed signal and increasing the bandwidth of the total stepped frequency, the ambiguity sidelobe is well suppressed and the range-velocity coupling in the stepped frequency radar is also greatly weakened. 相似文献
990.