全文获取类型
收费全文 | 157498篇 |
免费 | 18798篇 |
国内免费 | 7993篇 |
专业分类
化学 | 96965篇 |
晶体学 | 1715篇 |
力学 | 6270篇 |
综合类 | 413篇 |
数学 | 14375篇 |
物理学 | 37616篇 |
无线电 | 26935篇 |
出版年
2023年 | 1382篇 |
2022年 | 2221篇 |
2021年 | 3090篇 |
2020年 | 4545篇 |
2019年 | 5988篇 |
2018年 | 4806篇 |
2017年 | 4522篇 |
2016年 | 7862篇 |
2015年 | 7019篇 |
2014年 | 8302篇 |
2013年 | 10406篇 |
2012年 | 9633篇 |
2011年 | 9194篇 |
2010年 | 8428篇 |
2009年 | 8557篇 |
2008年 | 8331篇 |
2007年 | 7391篇 |
2006年 | 6646篇 |
2005年 | 6262篇 |
2004年 | 5328篇 |
2003年 | 4922篇 |
2002年 | 5681篇 |
2001年 | 4601篇 |
2000年 | 3865篇 |
1999年 | 2521篇 |
1998年 | 1995篇 |
1997年 | 1728篇 |
1996年 | 1564篇 |
1995年 | 1284篇 |
1994年 | 1306篇 |
1993年 | 1249篇 |
1992年 | 1212篇 |
1991年 | 1238篇 |
1990年 | 1198篇 |
1989年 | 1097篇 |
1988年 | 960篇 |
1987年 | 953篇 |
1986年 | 910篇 |
1985年 | 952篇 |
1984年 | 934篇 |
1983年 | 839篇 |
1982年 | 821篇 |
1979年 | 815篇 |
1978年 | 819篇 |
1977年 | 818篇 |
1976年 | 924篇 |
1975年 | 820篇 |
1974年 | 849篇 |
1973年 | 849篇 |
1972年 | 757篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Hyun‐Cheol Kim Charalampos Z. Patrikakis Nikos Minogiannis Pantelis N. Karamolegkos Alex Lambiris Kyuheon Kim 《ETRI Journal》2006,28(4):411-424
This paper presents a multimedia streaming platform for efficiently transmitting MPEG‐4 content over IP networks. The platform includes an MPEG‐4 compliant streaming server and client, supporting object‐based representation of multimedia scenes, interactivity, and advanced encoding profiles defined by the ISO standard. For scalability purposes, we employ an application‐layer multicast scheme for media transmission using overlay networks. The overlay network, governed by the central entity of the network distribution manager, is dynamically deployed according to a set of pre‐defined criteria. The overlay network supports both broadcast delivery and video‐on‐demand content. The multimedia streaming platform is standards‐compliant and utilizes widespread multimedia protocols such as MPEG‐4, real‐time transport protocol, real‐time transport control protocol, and real‐time streaming protocol. The design of the overlay network was architected with the goal of transparency to both the streaming server and the client. As a result, many commercial implementations that use industry‐standard protocols can be plugged into the architecture relatively painlessly and can enjoy the benefits of the platform. 相似文献
143.
B.‐J. Fang Y.‐J. Shan H.‐Q. Xu H.‐S. Luo Z.‐W. Yin 《Advanced functional materials》2004,14(2):169-173
Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 (PZNT91/9) single crystals were grown by two methods: from solution using PbO as a self‐fluxing agent (SC method) and directly from the melt without fluxing (MC method). In both growth methods, an allomeric Pb[(Mg1/3‐Nb2/3)0.69Ti0.31]O3 (PMNT69/31) single crystal was used as a seed. X‐ray diffraction patterns of ground crystals showed that phase‐pure perovskite PZNT91/9 single crystals were successfully fabricated by the above two methods. The composition of the crystals obtained by both the SC and MC methods was analyzed using X‐ray fluorescence, which confirmed that the crystal composition is close to the nominal value, although volatilization of PbO and segregation during crystal growth are inevitable. The MC PZNT91/9 crystals exhibit excellent piezoelectric properties, with the piezoelectric constant, d33, in the range of 1800–2200 pC N–1. This value is comparable to that of the SC crystals. However, the MC crystals show an abnormal dielectric behavior. In contrast with the SC crystals, in the MC crystals a much broader dielectric peak appears in the dielectric response curves, accompanied by a much lower peak temperature of around 105 °C. Furthermore, frequency dispersion is apparent over a much wider temperature range (even more apparent than in pure relaxors), where a large, i.e., about 70 °C, full width at half maximum (FWHM) for the dielectric peaks is observed in the dielectric response. It is speculated that such an unusual phenomenon correlates with defects, microinhomogeneities, and polar regions in the as‐grown MC crystals. The origins of this abnormality have not been interpreted in detail until now. However, optical observation of the domain structure confirms that both the SC and MC crystals possess complex structural states. 相似文献
144.
连续波半主动雷达制导空空导弹截获跟踪距离计算 总被引:1,自引:1,他引:0
分析了连续波半主动雷达制导导弹导引头工作电磁环境和对武器系统攻击区的影响,建立了热噪声和杂波干扰环境下导引头截获距离方程,分析推导了直波泄漏干扰,地杂波干扰的计算公式,并在此基础上给出了导弹允许攻击区计算公式。 相似文献
145.
基于多相滤波器的信道化接收机及其应用研究 总被引:16,自引:0,他引:16
本文主要研究了应用多相滤波技术的信道化接收机建模问题。在给定信道频谱划分方案下,推导了基于多相 滤波器的信道化接收机数学模型。并由此模型设计了一个四信道模拟系统。最后用仿真实验结果验证了模型的正确性。 相似文献
146.
147.
主要对各国3G许可证与使用技术标准的抽绑政策.各国对2G/3G网络漫游的管制规定。以及3G网络之间共享的种类及实施方式等方面做了详尽的讲解,并进行比较和分析。 相似文献
148.
High-efficiency LEDs of 1.6–2.4 µm spectral range for medical diagnostics and environment monitoring
N. D. Stoyanov B. E. Zhurtanov A. P. Astakhova A. N. Imenkov Yu. P. Yakovlev 《Semiconductors》2003,37(8):971-984
High efficient LED structures covering the spectral range of 1.6–2.4 μm have been developed on the basis of GaSb and its solid
solutions. The electroluminescent characteristics and their temperature and current dependences have been studied. The radiative
and nonradiative recombination mechanisms and their effect on the quantum efficiency have been investigated. A quantum efficiency
of 40–60% has been obtained in the quasi-steady mode at room temperature. A short-pulse optical power of 170 mW was reached.
__________
Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 8, 2003, pp. 996–1009.
Original Russian Text Copyright ? 2003 by Stoyanov, Zhurtanov, Astakhova, Imenkov, Yakovlev. 相似文献
149.
150.