全文获取类型
收费全文 | 135854篇 |
免费 | 20590篇 |
国内免费 | 13705篇 |
专业分类
化学 | 71225篇 |
晶体学 | 1218篇 |
力学 | 6920篇 |
综合类 | 533篇 |
数学 | 11909篇 |
物理学 | 41714篇 |
无线电 | 36630篇 |
出版年
2024年 | 549篇 |
2023年 | 3277篇 |
2022年 | 4046篇 |
2021年 | 5241篇 |
2020年 | 5210篇 |
2019年 | 4885篇 |
2018年 | 4128篇 |
2017年 | 4071篇 |
2016年 | 5983篇 |
2015年 | 6140篇 |
2014年 | 7494篇 |
2013年 | 9606篇 |
2012年 | 11418篇 |
2011年 | 11644篇 |
2010年 | 8163篇 |
2009年 | 8106篇 |
2008年 | 8566篇 |
2007年 | 7762篇 |
2006年 | 7267篇 |
2005年 | 6323篇 |
2004年 | 4375篇 |
2003年 | 3670篇 |
2002年 | 3290篇 |
2001年 | 2744篇 |
2000年 | 2593篇 |
1999年 | 3017篇 |
1998年 | 2658篇 |
1997年 | 2317篇 |
1996年 | 2565篇 |
1995年 | 2032篇 |
1994年 | 1930篇 |
1993年 | 1551篇 |
1992年 | 1448篇 |
1991年 | 1276篇 |
1990年 | 988篇 |
1989年 | 692篇 |
1988年 | 559篇 |
1987年 | 442篇 |
1986年 | 428篇 |
1985年 | 371篇 |
1984年 | 276篇 |
1983年 | 197篇 |
1982年 | 165篇 |
1981年 | 114篇 |
1980年 | 88篇 |
1979年 | 53篇 |
1978年 | 40篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 50篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
932.
933.
文章讨论的基于“替换法”的汉字信息加、解密实现技术,改变了传统的汉字加密结果,使加密后的密文仍是可以识别和再利用的汉字,发送者可以在任何可编辑汉字的通讯设备上发送和接收密文,确保中文信息传输过程中的安全性。加、解密算法实现的代码语言为VFP6.0。 相似文献
934.
静电驱动悬臂梁数模转换器设计、分析与模拟 总被引:1,自引:0,他引:1
提出了一种新型的微电子机械数模转换器 (MEMDAC)———静电驱动悬臂梁数模转换器 .与传统的静电驱动悬臂梁执行器相比 ,新的结构能更精确地控制梁的挠度 .文中给出了此MEMDAC的设计、分析和模拟过程 .有限元(FEA)分析表明此MEMDAC的最大输出位移为 1 5 1μm ,分辨率为 0 1μm ,其最大非线性误差为 0 0 3μm . 相似文献
935.
SeongKu Kim Geary K. Fetterman H.R. Zhang C. Wang C. Steier W.H. 《Electronics letters》2003,39(18):1321-1322
Electro-optic (EO) polymeric Mach-Zehnder (MZ) modulators with photo-bleaching (PB) induced waveguides and dual driving electrodes operating at 1.55 /spl mu/m wavelength have been demonstrated. The half-wave voltage of the integrated polymeric modulator was 4.5 V in a push-pull configuration with a 1.5 cm interaction length. The extinction ratio was greater than 20 dB, and the fibre-to-fibre insertion loss was 8 dB for the TM polarisation. The achieved fibre-to-fibre insertion loss and driving voltage are the best, to the authors knowledge, in the reported PB induced MZ EO polymeric modulators. 相似文献
936.
937.
938.
939.
In upstream transmission of the DOCSIS (data over cable service interface specification) 1.0/1.1 and the TDMA mode of DOCSIS 2.0 standard, a packet begins with a preamble, the length and value of which are programmable by the headend. The acquisition process includes packet detection, symbol timing estimation, carrier frequency offset estimation and carrier phase estimation by means of the preamble so that the following information packet can be correctly demodulated. Methods to solve these tasks are not subject to standardization and are left to the system designer. However, they have a great impact on the system performance and thus are key elements for implementation. We present dedicated algorithms. A specific preamble is created and the corresponding scheme for packet acquisition is described in some detail. The packet detecting algorithm is independent of the carrier and symbol timing. The symbol timing estimate is obtained by using the minimum mean square algorithm regardless of the carrier frequency deviation and carrier phase. Algorithms for estimating the carrier frequency offset and carrier phase are proposed. Computer simulation results are given for all algorithms presented. 相似文献
940.
Hayat M.M. Sakoglu U. Oh-Hyun Kwon Shuling Wang Campbell J.C. Saleh B.E.A. Teich M.C. 《Quantum Electronics, IEEE Journal of》2003,39(1):179-185
The recurrence theory for the breakdown probability in avalanche photodiodes (APDs) is generalized to heterostructure APDs that may have multiple multiplication layers. The generalization addresses layer-boundary effects such as the initial energy of injected carriers as well as the layer-dependent profile of the dead space in the multiplication region. Reducing the width of the multiplication layer serves to both downshift and sharpen the breakdown probability curve as a function of the applied reverse-bias voltage. In structures where the injected carriers have an initial energy that is comparable to the ionization threshold energy, the transition from linear mode to Geiger-mode is more abrupt than in structures in which such initial energy is negligible. The theory is applied to two recently fabricated Al/sub 0.6/Ga/sub 0.4/As-GaAs heterostructure APDs and to other homostructure thin GaAs APDs and the predictions of the breakdown-voltage thresholds are verified. 相似文献