全文获取类型
收费全文 | 215405篇 |
免费 | 26088篇 |
国内免费 | 18100篇 |
专业分类
化学 | 112161篇 |
晶体学 | 1683篇 |
力学 | 9835篇 |
综合类 | 886篇 |
数学 | 19853篇 |
物理学 | 56030篇 |
无线电 | 59145篇 |
出版年
2024年 | 402篇 |
2023年 | 4123篇 |
2022年 | 4677篇 |
2021年 | 6775篇 |
2020年 | 6882篇 |
2019年 | 6507篇 |
2018年 | 5709篇 |
2017年 | 5540篇 |
2016年 | 8092篇 |
2015年 | 8214篇 |
2014年 | 10149篇 |
2013年 | 13771篇 |
2012年 | 16734篇 |
2011年 | 17377篇 |
2010年 | 12327篇 |
2009年 | 12313篇 |
2008年 | 13592篇 |
2007年 | 12604篇 |
2006年 | 11711篇 |
2005年 | 10454篇 |
2004年 | 7893篇 |
2003年 | 6746篇 |
2002年 | 6311篇 |
2001年 | 5074篇 |
2000年 | 4818篇 |
1999年 | 4891篇 |
1998年 | 4184篇 |
1997年 | 3898篇 |
1996年 | 4046篇 |
1995年 | 3365篇 |
1994年 | 2989篇 |
1993年 | 2574篇 |
1992年 | 2376篇 |
1991年 | 1995篇 |
1990年 | 1651篇 |
1989年 | 1235篇 |
1988年 | 1050篇 |
1987年 | 852篇 |
1986年 | 766篇 |
1985年 | 807篇 |
1984年 | 592篇 |
1983年 | 435篇 |
1982年 | 403篇 |
1981年 | 310篇 |
1980年 | 275篇 |
1978年 | 227篇 |
1977年 | 218篇 |
1976年 | 229篇 |
1975年 | 231篇 |
1973年 | 216篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
991.
Shunkang Liu 《Journal of Infrared, Millimeter and Terahertz Waves》2004,25(3):471-480
Large signal operating equations in TWT have been solved by numerical solve method. The relations between interaction efficiency and design parameters in TWT are analyzed. A fast engineering design method of enhancing efficiency for folded waveguide TWT by velocity resynchronization is described. The computation is shown that the efficiency of Ka-band folded waveguide TWT with double taper can be enhanced over twice as original tube. This method of approach is very useful for tube designer. 相似文献
992.
Yikun Huang Nehorai A. Friedman G. 《Antennas and Propagation, IEEE Transactions on》2003,51(6):1307-1314
Coupling between two collocated orthogonal circular thin-wire loops is analyzed in this paper. Two coupled integral equations for the loop currents are derived and their solution in general form is found in terms of Fourier series. An analytical expression for currents induced through the mutual coupling is obtained for the case when all loop current harmonics higher than first can be ignored. It is found that strong coupling can exist for all loop current harmonics, except for the fundamental. It is also found that coupling for orthogonal collocated loop antennas depends on the relative locations of the loop terminals. 相似文献
993.
994.
Andre C.L. Carlin J.A. Boeckl J.J. Wilt D.M. Smith M.A. Pitera A.J. Lee M.L. Fitzgerald E.A. Ringel S.A. 《Electron Devices, IEEE Transactions on》2005,52(6):1055-1060
High-performance p/sup +//n GaAs solar cells were grown and processed on compositionally graded Ge-Si/sub 1-x/Ge/sub x/-Si (SiGe) substrates. Total area efficiencies of 18.1% under the AM1.5-G spectrum were measured for 0.0444 cm/sup 2/ solar cells. This high efficiency is attributed to the very high open-circuit voltages (980 mV (AM0) and 973 mV (AM1.5-G)) that were achieved by the reduction in threading dislocation density enabled by the SiGe buffers, and thus reduced carrier recombination losses. This is the highest independently confirmed efficiency and open-circuit voltage for a GaAs solar cell grown on a Si-based substrate to date. Larger area solar cells were also studied in order to examine the impact of device area on GaAs-on-SiGe solar cell performance; we found that an increase in device area from 0.36 to 4.0 cm/sup 2/ did not degrade the measured performance characteristics for cells processed on identical substrates. Moreover, the device performance uniformity for large area heteroepitaxial cells is consistent with that of homoepitaxial cells; thus, device growth and processing on SiGe substrates did not introduce added performance variations. These results demonstrate that using SiGe interlayers to produce "virtual" Ge substrates may provide a robust method for scaleable integration of high performance III-V photovoltaics devices with large area Si wafers. 相似文献
995.
Hawkins' isothermal model developed to study noise in bipolar junction transistors (BJTs) is modified to investigate bias-dependent noise in heterojunction bipolar transistors (HBTs) by incorporating thermal effects. It is shown that the inclusion of thermal effects into the high-frequency noise model of HBTs is necessary as the temperature of the device may become very different from the ambient temperature, especially at high bias current. Calculation of the noise figure by including the thermal effect shows that the isothermal calculation may underestimate the noise figure at high bias current. It is observed that noise at low bias is ideality factor n dependent whereas high bias noise is insensitive to the variation of n. Moreover, the common base current gain plays a major role in the calculation of the minimum noise figure. The excellent fit obtained between the theoretical calculation and the measured data are attributed to the inclusion of the bias-dependent junction heating as well as C/sub De/ and C/sub bc/ into the present calculation. 相似文献
996.
Chenn-Jung Huang Wei-Kuang Lai Sheng-Yu Hsiao Hao-Yu Liu Rui-Lin Luo 《International Journal of Wireless Information Networks》2004,11(3):131-146
In this paper, a routing protocol which utilizes the characteristics of Bluetooth technology is proposed for Bluetooth-based mobile ad hoc networks. The routing tables are maintained in the master devices and the routing zone radius for each table is adjusted dynamically by using evolving fuzzy neural networks. Observing there exists some useless routing packets which are helpless to build the routing path and increase the network loads in the existing ad hoc routing protocols, we selectively use multiple unicasts or one broadcast when the destination device is out of the routing zone radius coverage of the routing table. The simulation results show that the dynamic adjustment of the routing table size in each master device results in much less reply time of routing request, fewer request packets and useless packets compared with two representative protocols, Zone Routing Protocol and Dynamic Source Routing. 相似文献
997.
Chia-Ching Lin Gwo-Yun Lee Kin-Lu Wong 《Electronics letters》2003,39(18):1302-1304
A surface-mount dual-loop antenna suitable for dual-frequency WLAN operation is presented. For achieving dual-frequency operation with a compact size, the antenna comprises two loop strips of different sizes printed on a flexible printed circuit board, which is then bent and attached onto a foam base of compact size. The antenna shows an attractive feature of high antenna gain, about 4 and 5 dBi for frequencies across the 2.4 and 5 GHz WLAN bands, respectively. 相似文献
998.
999.
1000.
本文提出一种基于本地缓存的弹性分组环保护倒换机制(WrapProtectionBasedOnLocalBuffers,简称WP-LB)。其主要思想是当环路上任何位于源节点和故障区域之间的中间节点接收到含有故障信息的控制信令后,就不再向下游转发数据分组,而是在本地缓存这些数据。等到保护通路建立好之后,再把缓存的数据分组经过保护通路发送给目的节点。分析计算表明,与原有的保护倒换(WrapProtection,简称WP)机制相比,WP-LB机制不仅提高带宽利用率,减少平均时延,并且还能避免数据包的丢失和失序。 相似文献