全文获取类型
收费全文 | 60118篇 |
免费 | 14351篇 |
国内免费 | 4344篇 |
专业分类
化学 | 50969篇 |
晶体学 | 637篇 |
力学 | 1129篇 |
综合类 | 233篇 |
数学 | 3529篇 |
物理学 | 7311篇 |
无线电 | 15005篇 |
出版年
2024年 | 62篇 |
2023年 | 387篇 |
2022年 | 588篇 |
2021年 | 864篇 |
2020年 | 2409篇 |
2019年 | 3745篇 |
2018年 | 2041篇 |
2017年 | 1677篇 |
2016年 | 4613篇 |
2015年 | 4914篇 |
2014年 | 5020篇 |
2013年 | 5948篇 |
2012年 | 4950篇 |
2011年 | 4281篇 |
2010年 | 4605篇 |
2009年 | 4586篇 |
2008年 | 4162篇 |
2007年 | 3506篇 |
2006年 | 2914篇 |
2005年 | 2870篇 |
2004年 | 2429篇 |
2003年 | 2173篇 |
2002年 | 2875篇 |
2001年 | 2077篇 |
2000年 | 1877篇 |
1999年 | 766篇 |
1998年 | 360篇 |
1997年 | 273篇 |
1996年 | 266篇 |
1995年 | 225篇 |
1994年 | 203篇 |
1993年 | 146篇 |
1992年 | 101篇 |
1991年 | 109篇 |
1990年 | 89篇 |
1989年 | 77篇 |
1988年 | 51篇 |
1987年 | 60篇 |
1986年 | 49篇 |
1985年 | 60篇 |
1984年 | 51篇 |
1983年 | 45篇 |
1982年 | 38篇 |
1981年 | 35篇 |
1980年 | 22篇 |
1979年 | 28篇 |
1978年 | 23篇 |
1977年 | 24篇 |
1976年 | 28篇 |
1975年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
61.
Dong‐Wook Kim 《ETRI Journal》2006,28(1):84-86
This letter presents a small‐sized, high‐power single‐pole double‐throw (SPDT) switch with defected ground structure (DGS) for wireless broadband Internet application. To reduce the circuit size by using a slow‐wave characteristic, the DGS is used for the quarter‐wave (°/4) transmission line of the switch. To secure a high degree of isolation, the switch with DGS is composed of shunt‐connected PIN diodes. It shows an insertion loss of 0.8 dB, an isolation of 50 dB or more, and power capability of at least 50 W at 2.3 GHz. The switch shows very similar performance to the conventional shunt‐type switch, but the circuit size is reduced by about 50% simply with the use of DGS patterns. 相似文献
62.
由于AD HOC网络自身的特点,其网络管理很多方面与固定网络不同,如移动性管理和体系结构管理等。讨论了AD HOC网络管理的各个方面,并列举了各个方面的设计思路和已有的相关算法的简介。 相似文献
63.
用CPLD实现SRAM工艺FPGA的安全应用 总被引:1,自引:0,他引:1
提出了一种利用CPLD产生的伪随机码来加密SRAM工艺FPGA的方法,并详细介绍了具体的电路和VHDL代码。 相似文献
64.
65.
66.
W. Shieh R. Hui X. Yi 《Photonics Technology Letters, IEEE》2006,18(10):1122-1124
We perform a systematic measurement of the degree-of-polarization (DOP) and eye-closure penalty for optical signals with orthogonal polarizations. We find that the symmetry of DOP is maintained for the orthogonal polarizations under both first and higher order polarization-mode dispersion (PMD), whereas the symmetry of eye-closure penalty is broken under second-order PMD. An orthogonal polarization pair can have large disparity of eye-closure penalty despite an identical DOP. We also demonstrate a novel approach to estimate the maximum eye-closure penalty asymmetry with three orthogonal polarizations on the Poincare/spl acute/ sphere. 相似文献
67.
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
68.
Hui Luo Wei Hao David H Foos Craig W Cornelius 《IEEE transactions on information technology in biomedicine》2006,10(2):302-311
Chest radiography is one of the most widely used techniques in diagnostic imaging. It comprises at least one-third of all diagnostic radiographic procedures in hospitals. However, in the picture archive and communication system, images are often stored with the projection and orientation unknown or mislabeled, which causes inefficiency for radiologists' interpretation. To address this problem, an automatic hanging protocol for chest radiographs is presented. The method targets the most effective region in a chest radiograph, and extracts a set of size-, rotation-, and translation-invariant features from it. Then, a well-trained classifier is used to recognize the projection. The orientation of the radiograph is later identified by locating the neck, heart, and abdomen positions in the radiographs. Initial experiments are performed on the radiographs collected from daily routine chest exams in hospitals and show promising results. Using the presented protocol, 98.2% of all cases could be hung correctly on projection view (without protocol, 62%), and 96.1% had correct orientation (without protocol, 75%). A workflow study on the protocol also demonstrates a significant improvement in efficiency for image display. 相似文献
69.
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
70.
The current Internet and wireless networks are harsh environments for transporting high-bandwidth multimedia data. We examine the technical issues involved, and describe an end-to-end solution to support a Web-based learn-on-demand system that operates in a wireless campus environment. 相似文献