全文获取类型
收费全文 | 37869篇 |
免费 | 5933篇 |
国内免费 | 4484篇 |
专业分类
化学 | 20319篇 |
晶体学 | 391篇 |
力学 | 1608篇 |
综合类 | 258篇 |
数学 | 3217篇 |
物理学 | 10253篇 |
无线电 | 12240篇 |
出版年
2024年 | 183篇 |
2023年 | 843篇 |
2022年 | 1212篇 |
2021年 | 1399篇 |
2020年 | 1367篇 |
2019年 | 1386篇 |
2018年 | 1171篇 |
2017年 | 1210篇 |
2016年 | 1528篇 |
2015年 | 1781篇 |
2014年 | 2171篇 |
2013年 | 2789篇 |
2012年 | 3194篇 |
2011年 | 3421篇 |
2010年 | 2469篇 |
2009年 | 2567篇 |
2008年 | 2730篇 |
2007年 | 2522篇 |
2006年 | 2179篇 |
2005年 | 1794篇 |
2004年 | 1369篇 |
2003年 | 1101篇 |
2002年 | 1108篇 |
2001年 | 944篇 |
2000年 | 840篇 |
1999年 | 719篇 |
1998年 | 579篇 |
1997年 | 512篇 |
1996年 | 538篇 |
1995年 | 443篇 |
1994年 | 390篇 |
1993年 | 314篇 |
1992年 | 282篇 |
1991年 | 239篇 |
1990年 | 202篇 |
1989年 | 159篇 |
1988年 | 102篇 |
1987年 | 93篇 |
1986年 | 89篇 |
1985年 | 73篇 |
1984年 | 55篇 |
1983年 | 47篇 |
1982年 | 27篇 |
1981年 | 24篇 |
1980年 | 13篇 |
1979年 | 18篇 |
1978年 | 8篇 |
1976年 | 10篇 |
1975年 | 8篇 |
1974年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A continuous-wave hybrid AlGaInAs-silicon evanescent laser 总被引:1,自引:0,他引:1
A.W. Fang Hyundai Park R. Jones O. Cohen M.J. Paniccia J.E. Bowers 《Photonics Technology Letters, IEEE》2006,18(10):1143-1145
We report a novel laser architecture, the hybrid silicon evanescent laser (SEL), that utilizes offset AlGaInAs quantum wells (QWs) bonded to a silicon waveguide. The silicon waveguide is fabricated on a silicon-on-insulator wafer using a complimentary metal-oxide-semiconductor-compatible process, and is subsequently bonded with the AlGaInAs QW structure using low temperature O/sub 2/ plasma-assisted wafer bonding. The optical mode in the SEL is predominantly confined in the passive silicon waveguide and evanescently couples into the III-V active region providing optical gain. The SEL lases continuous wave (CW) at 1568 nm with a threshold of 23 mW. The maximum temperature for CW operation is 60/spl deg/C. The maximum single-sided fiber-coupled CW output power at room temperature is 4.5 mW. 相似文献
992.
Virtual backbone routing (VBR) is a scalable hybrid routing framework for ad hoc networks, which combines local proactive and global reactive routing components over a variable-sized zone hierarchy. The zone hierarchy is maintained through a novel distributed virtual backbone maintenance scheme, termed the distributed database coverage heuristic (DDCH), also presented in this paper. Borrowing from the design philosophy of the zone routing protocol, VBR limits the proactive link information exchange to the local routing zones only. Furthermore, the reactive component of VBR restricts the route queries to within the virtual backbone only, thus improving the overall routing efficiency. Our numerical results suggest that the cost of the hybrid VBR scheme can be a small fraction of that of either one of the purely proactive or purely reactive protocols, with or without route caching. Since the data routes do not necessarily pass through the virtual backbone nodes, traffic congestion is considerably reduced. Yet, the average length of the VBR routes tends to be close to optimal. Compared with the traditional one-hop hierarchical protocols, our results indicate that, for a network of moderate to large size, VBR with an optimal zone radius larger than one can significantly reduce the routing traffic. Furthermore, we demonstrate VBR's improved scalability through analysis and simulations. 相似文献
993.
A novel frequency offset estimation method for OFDM systems with large estimation range 总被引:4,自引:0,他引:4
Chunlin Yan Shaoqian Li Youxi Tang Xiao Luo Jiayi Fang 《Broadcasting, IEEE Transactions on》2006,52(1):58-61
In this paper, we proposed a novel frequency synchronization method which has larger estimation range than conventional method proposed by Tufvesson. A two steps frequency offset estimation method is performed in the new method. In the first step the received signal is correlated with local training sequence to eliminate the influence of training sequence which makes the estimation range of frequency offset is independent of period of the repeated data in the training sequence. The received signal is correlated with its delay in the second step to obtain the fine estimation of frequency offset. Precise estimation can be achieved in the proposed method than Tufvesson's method. The period of the repeated data in the training sequence determines the estimation range of the frequency offset in the conventional method, while in the new method they are irrelevant. Tens of subcarriers spacing can be estimated by the new method. The new method can be used in other cases as long as a known sequence is transmitted, which will introduce valuable flexibility in training sequence design. The validity of the algorithm is verified in AWGN channel and multipath fading channel. 相似文献
994.
详细介绍了基于4OQAM技术搭建的MDG无线移动数字图像传输系统,分析了其性能指标,提出了无线移动数字图像传输系统的系统解决方案,为广电系统突发新闻、文体活动等电视现场直播开辟了新的传输方式. 相似文献
995.
通过理论与实例介绍如何针对移动预付费增值业务多样化、复杂化以及业务不断变更的需求。快速、灵活实现各种新增值业务的实时扣费,为广大移动预付费用户提供灵活费率资费的解决方案。 相似文献
996.
997.
介绍了多载波通信中一种设计高效宽带锐截止FIR(有限冲击响应)滤波器的新方法———频障法。通过选择特定的原型滤波器来降低实现宽带锐截止FIR滤波器的复杂度。首先设计一个合适的宽带低通原型FIR滤波器Ha(z),然后得到其互补滤波器Hc(z)。再对它们进行L倍内插,得到两个互补的多带滤波器Ha(zL)、Hc(zL),然后用合适的频障滤波器HMa(z)和HM c(z)分别去消除Ha(zL)、Hc(zL)中不需要的频带,相应的输出加在一起就是所要设计的滤波器H(z)。详细研究了该方法的实现结构和设计方法,并通过设计实例证明了实现同样的性能时频障法比其他方法能节省很大的运算量,因而具有较高的应用价值。 相似文献
998.
介绍了用热壁反应炉在50mm SiC半绝缘衬底上制备的SiC MESFET外延材料.其沟道层厚度约为0.35μm,掺杂浓度约为1.7×1017cm-3.沟道和衬底之间的缓冲层为非有意掺杂的弱n型.欧姆接触用的帽层掺杂浓度约1019cm-3.器件制备采用了ICP刻蚀等技术.微波测试结果表明,1mm栅宽功率器件封装后在2GHz下输出功率达到了2W. 相似文献
999.
1000.