全文获取类型
收费全文 | 44725篇 |
免费 | 7238篇 |
国内免费 | 5455篇 |
专业分类
化学 | 23426篇 |
晶体学 | 565篇 |
力学 | 1677篇 |
综合类 | 342篇 |
数学 | 2963篇 |
物理学 | 11645篇 |
无线电 | 16800篇 |
出版年
2024年 | 192篇 |
2023年 | 1028篇 |
2022年 | 1423篇 |
2021年 | 1818篇 |
2020年 | 1778篇 |
2019年 | 1734篇 |
2018年 | 1481篇 |
2017年 | 1575篇 |
2016年 | 2017篇 |
2015年 | 2318篇 |
2014年 | 2684篇 |
2013年 | 3445篇 |
2012年 | 4015篇 |
2011年 | 4094篇 |
2010年 | 3104篇 |
2009年 | 2871篇 |
2008年 | 3198篇 |
2007年 | 2679篇 |
2006年 | 2470篇 |
2005年 | 2187篇 |
2004年 | 1660篇 |
2003年 | 1371篇 |
2002年 | 1437篇 |
2001年 | 1048篇 |
2000年 | 941篇 |
1999年 | 768篇 |
1998年 | 590篇 |
1997年 | 492篇 |
1996年 | 495篇 |
1995年 | 424篇 |
1994年 | 359篇 |
1993年 | 302篇 |
1992年 | 269篇 |
1991年 | 246篇 |
1990年 | 184篇 |
1989年 | 152篇 |
1988年 | 120篇 |
1987年 | 77篇 |
1986年 | 93篇 |
1985年 | 85篇 |
1984年 | 43篇 |
1983年 | 36篇 |
1982年 | 29篇 |
1981年 | 27篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1957年 | 10篇 |
1936年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
In this paper, the software cold-test simulation method to obtain RF dispersion, interaction impedance by MAFIA software is discussed considering effect of the conducting barrel in the millimeter-wave TWT. The dispersion simulation result is obtained more consistent with experiment data than the method that ignoring effect of the conducting barrel. By changing the structure parameter, high frequency characteristic will be affected. The change relationship of high frequency characteristic with structure parameter is obtained. The different calculation methods of the interaction impedance are discussed. These results are received and are consistent with experiment data. 相似文献
52.
53.
54.
The multihop optical network is the most appropriate solution to satisfy the increasing applications of Internet services. This paper extends the regular Kautz graph to one with multiple layers in order to produce more architectural variations. The connectivity between adjacent layers utilizes the systematic connection patterns of a regular Kautz graph. A routing algorithm based on its property is presented. Optical passive star (OPS) couplers are adopted to implement our new topologies. Three scheduling criteria that can solve the contention problem in the intermediate nodes are evaluated and compared in terms of their capability to improve the accessibility. 相似文献
55.
Jin Tae Kim Keun Byoung Yoon Choon-Gi Choi 《Photonics Technology Letters, IEEE》2004,16(7):1664-1666
A novel fabrication process using a hot embossing technique has been developed for micromechanical passive alignment of polymer planar lightwave circuit (PLC) devices. With only one step of embossing, single-mode waveguide straight channels and micropedestals for passive aligning are simultaneously defined on a polymer thin film with an accuracy of /spl plusmn/0.5 /spl mu/m. This process reduces the steps for fabricating alignment structures. A fabricated polymer PLC chip and fibers are combined on a v-grooved silicon optical bench (SiOB) in a flip-chip manner. The process provides a coupling loss as low as 0.67 dB per coupling face and a cost-effective packaging solution for various polymer PLC devices. 相似文献
56.
Shi-Jin Ding Hang Hu Lim H.F. Kim S.J. Yu X.F. Chunxiang Zhu Li M.F. Byung Jin Cho Chan D.S.H. Rustagi S.C. Yu M.B. Chin A. Dim-Lee Kwong 《Electron Device Letters, IEEE》2003,24(12):730-732
For the first time, we successfully fabricated and demonstrated high performance metal-insulator-metal (MIM) capacitors with HfO/sub 2/-Al/sub 2/O/sub 3/ laminate dielectric using atomic layer deposition (ALD) technique. Our data indicates that the laminate MIM capacitor can provide high capacitance density of 12.8 fF//spl mu/m/sup 2/ from 10 kHz up to 20 GHz, very low leakage current of 3.2 /spl times/ 10/sup -8/ A/cm/sup 2/ at 3.3 V, small linear voltage coefficient of capacitance of 240 ppm/V together with quadratic one of 1830 ppm/V/sup 2/, temperature coefficient of capacitance of 182 ppm//spl deg/C, and high breakdown field of /spl sim/6 MV/cm as well as promising reliability. As a result, the HfO/sub 2/-Al/sub 2/O/sub 3/ laminate is a very promising candidate for next generation MIM capacitor for radio frequency and mixed signal integrated circuit applications. 相似文献
57.
Combining ideal beamforming and Alamouti space-time block codes 总被引:3,自引:0,他引:3
The simplest Alamouti space-time block code is coupled with a larger number of transmit antennas via ideal beamforming to achieve higher diversity gain. It is shown that the combined system can remain both full diversity and full code rate without orthogonality loss. Simulation results show a significant performance gain over the conventional space-time block codes. 相似文献
58.
基于CATV的一站式电子政务系统的构建 总被引:2,自引:1,他引:1
电子政务是Internet网络上的综合应用,利用外交互式技术在CATV网络上构建一站式电子政务是充分利用现有的CATV网络资源的最好选择;在CATV网络上构建电子政务最关键的是要建立起安全的信息通道,安全的信息通道的建立必须借助于PKI技术. 相似文献
59.
In this letter, a new sharing mechanism, SRLG sharing, is proposed, which allows the links of the same shared risk link group (SRLG) in a primary light tree to share protections in WDM optical networks. In previous studies, how to share spare resources with SRLG constraints has not been studied in multicast optical networks. In this letter, considering SRLG sharing, we propose a novel algorithm –multicast with SRLG sharing (MSS)– to establish a protection light tree. Finally, the algorithm MSS and the algorithm multicast with no SRLG sharing (MNSS) are compared through a simulation to show that our new sharing scheme of SRLG sharing is more efficient than that of no SRLG sharing in terms of spare resource utilization and blocking probability. 相似文献
60.
This article describes the different methods to design regular low density parity-check (LDPC) codes with large girth. In graph terms, this corresponds to designing bipartite undirected regular graphs with large girth. Large girth speeds the convergence of iterative decoding and improves the performance at least in the high SNR range, by slowing down the onsetting of the error floor. We reviewed several existing constructions from exhaustive search to highly structured designs based on Euclidean and projective finite geometries and combinatorial designs. We describe GB and TS LDPC codes and compared the BER performance with large girth to the BER performance of random codes. These studies confirm that in the high SNR regime these codes with high girth exhibit better BER performance. The regularity of the codes provides additional advantages that we did not explore in this article like the simplicity of their hardware implementation and fast encoding. 相似文献