全文获取类型
收费全文 | 21719篇 |
免费 | 3770篇 |
国内免费 | 3539篇 |
专业分类
化学 | 10489篇 |
晶体学 | 406篇 |
力学 | 802篇 |
综合类 | 374篇 |
数学 | 1677篇 |
物理学 | 5792篇 |
无线电 | 9488篇 |
出版年
2024年 | 95篇 |
2023年 | 462篇 |
2022年 | 736篇 |
2021年 | 873篇 |
2020年 | 880篇 |
2019年 | 826篇 |
2018年 | 693篇 |
2017年 | 849篇 |
2016年 | 797篇 |
2015年 | 1071篇 |
2014年 | 1278篇 |
2013年 | 1676篇 |
2012年 | 1817篇 |
2011年 | 1865篇 |
2010年 | 1610篇 |
2009年 | 1586篇 |
2008年 | 1694篇 |
2007年 | 1585篇 |
2006年 | 1461篇 |
2005年 | 1208篇 |
2004年 | 980篇 |
2003年 | 698篇 |
2002年 | 734篇 |
2001年 | 614篇 |
2000年 | 671篇 |
1999年 | 380篇 |
1998年 | 230篇 |
1997年 | 194篇 |
1996年 | 188篇 |
1995年 | 190篇 |
1994年 | 180篇 |
1993年 | 160篇 |
1992年 | 128篇 |
1991年 | 97篇 |
1990年 | 104篇 |
1989年 | 79篇 |
1988年 | 61篇 |
1987年 | 41篇 |
1986年 | 54篇 |
1985年 | 44篇 |
1984年 | 27篇 |
1983年 | 28篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 8篇 |
1978年 | 6篇 |
1971年 | 2篇 |
1959年 | 4篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 678 毫秒
171.
172.
Xiangdong Xu Hockleong Kweh Zhengcao Zhang Zhihong Liu Wei Zhou Wei Zhang Peixin Qian 《Applied Surface Science》2006,252(20):7594-7598
Large-scale preparation of thin strain-relaxed SiGe is achieved by combining ion implantation and ultrahigh vacuum chemical vapor deposition. The resulting materials were analyzed by double crystal X-ray diffraction, micro-Raman spectroscopy, and tapping mode atomic force microscope. Results revealed that 100-nm-thick Si0.7Ge0.3 layers with the diameter of 125 mm and full strain relaxation are successfully prepared by pre-modifying the Si substrates using 50 keV Ar+ ions. The strain relaxation is also disclosed to change with both ion species and energy. However, post-modification of SiGe by ion implantation will cause serious damage to the crystal structures, and result in the formation of poly-crystal SiGe. 相似文献
173.
提出一种极近距离雷达概念,并导出其作用距离公式,在此基础上研制出一台3mm波段极近距离多普勒雷达样机.试验结果表明该雷达结构简单,成本低廉,在作用距离为零点几米至几米范围内可获得较高的定距精度. 相似文献
174.
高抗冲聚丙烯序列结构的综合表征 总被引:3,自引:0,他引:3
用高分辨^13C NMR、FTIR等手段获得了高抗冲聚丙烯的组成和序列分布等链结构方面的信息。用DSC、WAXD等方法研究了它们的结晶行为、熔融转变及晶体结构,多种方法相结合有效地表征了高抗冲聚丙烯的序列结构。 相似文献
175.
铁电致冷材料及其应用 总被引:4,自引:1,他引:3
利用铁电材料的电热效应(即逆热释电效应)可以制作新型致冷器。近年的研究进展表明,铁电致冷有可能发展成为一种新型的室温致冷技术。本文扼要介绍了铁电致冷的基本原理、可行性,以及铁电致冷材料和相关技术的发展,并展望了铁电致冷材料的发展前景。 相似文献
176.
177.
Chan C.C. Zheng Ming Zhao Qian C. Meng S. 《Industrial Electronics, IEEE Transactions on》2002,49(6):1342-1344
Based on the concepts of cascade multilevel converters and one-cycle technique, comparisons are conducted to characterize the distortion of the pulsewidth modulation (PWM) and one-cycle control methods that were applied to converters. Simulation results for the different control schemes are obtained in PSIM software initially to see the effect of one-cycle control different from that of PWM control. Through the comparisons, the advantages and disadvantages are identified for each method. The one-cycle scheme is better than PWM control in reducing undesirable harmonics and tracing dynamic waveforms. Simulation and experimental results are also provided to verify the conclusions. 相似文献
178.
179.
Leung V.C.M. Qian N. Malyan A.D. Donaldson R.W. 《Selected Areas in Communications, IEEE Journal on》1994,12(8):1376-1388
A distributed circuit-switched approach for supporting the deployment of high speed wireless personal communication services in urban areas through the interconnection of base stations via metropolitan area networks (MANs) is proposed. Broadband MANs minimize traffic congestion by dynamic sharing of link capacity and by serving as distributed switches for partitioning call control functions. While the DQDB protocol readily supports distributed packet-switching over the IEEE 802.6 MAN, isochronous traffic such as voice and video is best supported by circuit-switched connections. The authors present an enhanced bi-state pre-arbitrated (PA) transport mechanism, and associated call control and handoff management techniques, which enable distributed circuit-switching over the MAN. These capabilities are not currently addressed in the 802.6 standards. The bi-state PA transport mechanism facilitates statistical multiplexing of variable rate isochronous traffic sources. The network capacity is constrained by the call setup delay performance, and is analyzed by simulations. Alternative signaling architectures, involving different placements of call control network elements, are evaluated. The effects of erasure nodes, and close bus versus open bus architectures, are considered. The overlap inter-MAN call setup procedure is proposed to reduce delays. Different call handoff procedures are formulated according to the type of handoff and the resulting change in call connectivity. Most handoffs are intra-MAN, requiring simple procedures with short delays 相似文献
180.
关于RNP和SCS的若干等价条件 总被引:1,自引:0,他引:1
使用切片方法,讨论了Banach空间中有界闭凸子集的RNP和SCS的若干等价条件. 相似文献