全文获取类型
收费全文 | 405410篇 |
免费 | 4027篇 |
国内免费 | 1434篇 |
专业分类
化学 | 180194篇 |
晶体学 | 5340篇 |
力学 | 15767篇 |
综合类 | 11篇 |
数学 | 38708篇 |
物理学 | 109732篇 |
无线电 | 61119篇 |
出版年
2021年 | 2810篇 |
2020年 | 3064篇 |
2019年 | 3350篇 |
2018年 | 4097篇 |
2017年 | 3964篇 |
2016年 | 6330篇 |
2015年 | 4171篇 |
2014年 | 6370篇 |
2013年 | 16769篇 |
2012年 | 13199篇 |
2011年 | 16096篇 |
2010年 | 11120篇 |
2009年 | 11850篇 |
2008年 | 16069篇 |
2007年 | 16821篇 |
2006年 | 16207篇 |
2005年 | 14863篇 |
2004年 | 13701篇 |
2003年 | 12266篇 |
2002年 | 11736篇 |
2001年 | 13395篇 |
2000年 | 10650篇 |
1999年 | 8664篇 |
1998年 | 7187篇 |
1997年 | 7165篇 |
1996年 | 6699篇 |
1995年 | 6244篇 |
1994年 | 6021篇 |
1993年 | 5834篇 |
1992年 | 6305篇 |
1991年 | 6248篇 |
1990年 | 5804篇 |
1989年 | 5488篇 |
1988年 | 5296篇 |
1987年 | 4721篇 |
1986年 | 4459篇 |
1985年 | 5915篇 |
1984年 | 6079篇 |
1983年 | 4985篇 |
1982年 | 5276篇 |
1981年 | 5143篇 |
1980年 | 4983篇 |
1979年 | 4930篇 |
1978年 | 5073篇 |
1977年 | 5021篇 |
1976年 | 5027篇 |
1975年 | 4807篇 |
1974年 | 4684篇 |
1973年 | 4883篇 |
1972年 | 3106篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
221.
A new frequency notched ultra-wideband microstrip slot antenna with a parasitic tuning stub is proposed. The antenna is similar in configuration to a conventional microstrip slot antenna; by introducing a parasitic open-circuit tuning stub, flexible frequency notched function is achieved. Several properties of the antenna, such as frequency notched function, antenna transfer function and gain, have been investigated. As is reported, the operation bandwidth of the antenna is 2.91-11.16 GHz, in which a frequency notched band of 5.10-5.85 GHz has been achieved. Furthermore, good ultra-wideband linear transmission performance over the entire operation frequency range has also been achieved. 相似文献
222.
Remotely powered addressable UHF RFID integrated system 总被引:3,自引:0,他引:3
Curty J.-P. Joehl N. Dehollain C. Declercq M.J. 《Solid-State Circuits, IEEE Journal of》2005,40(11):2193-2202
This paper presents a fully integrated remotely powered and addressable radio frequency identification (RFID) transponder working at 2.45 GHz. The achieved operating range at 4 W effective isotropically radiated power (EIRP) base-station transmit power is 12 m. The integrated circuit (IC) is implemented in a 0.5 /spl mu/m silicon-on-sapphire technology. A state-of-the-art rectifier design achieving 37% of global efficiency is embedded to supply energy to the transponder. The necessary input power to operate the transponder is about 2.7 /spl mu/W. Reader to transponder communication is obtained using on-off keying (OOK) modulation while transponder to reader communication is ensured using the amplitude shift keying (ASK) backscattering modulation technique. Inductive matching between the antenna and the transponder IC is used to further optimize the operating range. 相似文献
223.
A practical resource management method that can significantly reduce cochannel interference (CCI) and improve spectrum utilisation in FH-OFDMA packet-based cellular networks is presented. The proposed method seeks an effective combination of dynamic resource allocation with fractional coding and bit loading to respectively minimise CCI and maximise system throughput for a desired performance. 相似文献
224.
Emission of coherent light at 5.1 /spl mu/m wavelength from GaAs-based quantum cascade lasers is reported. This was achieved by integrating nonlinear cascades with large second-order susceptibility in the active regions of the laser. 相似文献
225.
A combination of concept (the product portfolio and the product life cycle) and empirical research (the PIMS study) can aid in clarifying the key issues involved in the selection of a firm's core strategies. Strategy control is multistage: periodic re-evaluation of strategy alternatives by use of the marketing audit; an analytical framework for control of chosen strategies by breaking down components and reporting their performance variances; and 'marketing segment' analysis for control of the detailed marketing plan.? 相似文献
226.
O. Gauthier-Lafaye D. Mulin S. Bonnefont X. Checoury J.-M. Lourtioz A. Talneau F. Lozes-Dupuy 《Photonics Technology Letters, IEEE》2005,17(8):1587-1589
Lasing of W1 square lattice-based photonic crystal lasers on the substrate approach is analyzed. A second-order distributed feedback highly monomode behavior is observed. A comprehensive two-dimensional and three-dimensional finite-difference time-domain computation analysis shows that this characteristic arises from the lattice geometry. Single-mode lasers can be obtained using this geometry, opening the way to the realization of monomode laser arrays. 相似文献
227.
Emerging large scale distributed networking systems, such as P2P file sharing systems, sensor networks, and ad hoc wireless networks, require replication of content, functionality, or configuration to enact or optimize communication tasks. The placement of these replicated resources can significantly impact performance. We present a novel self-stabilizing, fully distributed, asynchronous, scalable protocol that can be used to place replicated resources such that each node is "close" to some copy of any object. We describe our protocol in the context of a graph with colored nodes, where a node's color indicates the replica/task that it is assigned. Our combination of theoretical results and simulation prove stabilization of the protocol, and evaluate its performance in the context of convergence time, message transmissions, and color distance. Our results show that the protocol generates colorings that are close to the optimal under a set of metrics, making such a protocol ideal for emerging networking systems. 相似文献
228.
Zhang Z.G. Chan S.C. Tsui K.M. 《Circuits and Systems II: Express Briefs, IEEE Transactions on》2008,55(6):576-580
This paper proposes a new Kalman-filter-based recursive frequency estimator for discrete-time multicomponent sinusoidal signals whose frequencies may be time-varying. The frequency estimator is based on the linear prediction approach and it employs the Kalman filter to track the linear prediction coefficients (LPCs) recursively. Frequencies of the sinusoids can then be computed using the estimated LPCs. Due to the coloredness of the linear prediction error, an iterative algorithm is employed to estimate the covariance matrix of the prediction error and the LPCs alternately in the Kalman filter in order to improve the tracking performance. Simulation results show that the proposed Kalman-filter-based iterative frequency estimator can achieve better tracking results than the conventional recursive least-squares-based estimators. 相似文献
229.
In this letter, we propose a generalization of the progressive edge-growth (PEG) algorithm with the aim of designing LDPC code graphs with substantially improved approximated cycle extrinsic message degree (ACE) properties. The proposed realization of generalized PEG algorithm outperforms original PEG algorithm and its subsequent modification proposed by Xiao and Banihashemi. 相似文献
230.
Farzaneh Fayazpour Bart Lucas Roosmarijn E. Vandenbroucke Stefaan Derveaux Jan Tavernier Sam Lievens Joseph Demeester Stefaan C. De Smedt 《Advanced functional materials》2008,18(18):2716-2723
To obtain more biologically relevant data there is a growing interest in the use of living cells for assaying the biological activity of unknown chemical compounds. Density ‘multiplex’ cell‐based assays, where different cell types are mixed in one well and simultaneously investigated upon exposure to a certain compound are beginning to emerge. To be able to identify the cells they should be attached to microscopic carriers that are encoded. This paper investigates how digitally encoded microparticles can be loaded with cells while keeping the digital code in the microcarriers readable. It turns out that coating the surface of the encoded microcarriers with polyelectrolytes using the layer‐by‐layer (LbL) approach provides the microcarriers with a ‘highly functional’ surface. The polyelectrolyte layer allows the growth of the cells, allows the orientation of the cell loaded microcarriers in a magnetic field, and does not hamper the reading of the code. It has further been shown that the cells growing on the polyelectrolyte layer can become transduced by adenoviral particles hosted by the polyelectrolyte layer. It is concluded that the digitally encoded microparticles are promising materials for use in biomedical and pharmaceutical in‐vitro research where cells are used as tools. 相似文献