全文获取类型
收费全文 | 335329篇 |
免费 | 3502篇 |
国内免费 | 1470篇 |
专业分类
化学 | 158391篇 |
晶体学 | 4744篇 |
力学 | 13513篇 |
综合类 | 34篇 |
数学 | 33287篇 |
物理学 | 94097篇 |
无线电 | 36235篇 |
出版年
2020年 | 2709篇 |
2019年 | 2943篇 |
2018年 | 3998篇 |
2017年 | 3925篇 |
2016年 | 5619篇 |
2015年 | 3385篇 |
2014年 | 5545篇 |
2013年 | 13032篇 |
2012年 | 10070篇 |
2011年 | 12240篇 |
2010年 | 9012篇 |
2009年 | 9298篇 |
2008年 | 12118篇 |
2007年 | 12700篇 |
2006年 | 11802篇 |
2005年 | 10740篇 |
2004年 | 9882篇 |
2003年 | 8963篇 |
2002年 | 8857篇 |
2001年 | 9769篇 |
2000年 | 7833篇 |
1999年 | 6265篇 |
1998年 | 5602篇 |
1997年 | 5521篇 |
1996年 | 5082篇 |
1995年 | 4752篇 |
1994年 | 4816篇 |
1993年 | 4498篇 |
1992年 | 4938篇 |
1991年 | 4990篇 |
1990年 | 4831篇 |
1989年 | 4533篇 |
1988年 | 4555篇 |
1987年 | 4171篇 |
1986年 | 3937篇 |
1985年 | 5052篇 |
1984年 | 5353篇 |
1983年 | 4446篇 |
1982年 | 4570篇 |
1981年 | 4505篇 |
1980年 | 4394篇 |
1979年 | 4394篇 |
1978年 | 4677篇 |
1977年 | 4632篇 |
1976年 | 4675篇 |
1975年 | 4279篇 |
1974年 | 4359篇 |
1973年 | 4507篇 |
1972年 | 3216篇 |
1971年 | 2675篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
An expression is derived for the power spectral density of a coded mark inversion (CMI) TV signal transmitted through a fibre-optic heterodyne system. An illustrative example is given, and its power spectral density has been computed numerically. 相似文献
52.
H. Moers H. Klewe-Nebenius G. Pfennig und H. J. Ache 《Fresenius' Journal of Analytical Chemistry》1987,329(2-3):361-366
Summary The adsorption of gaseous molecular iodine on clean silver surfaces proceeds in the first stage by dissociative chemisorption. After the formation of half a monolayer of chemisorbed iodine atoms the formation of silver iodide begins, which forms a very regular overlayer on top of the substrate. Both iodine species can be distinguished due to a difference in chemical shift of the M4N4, 5N4, 5-Auger transitions. After completion of this overlayer further iodine uptake is slowed down significantly. Above an overlayer thickness of about 10 nm a further progress of the reaction cannot be followed anymore due to the limited information depth of XPS. 相似文献
53.
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
54.
In this article we survey a number of predeployed secure key distribution (PSKD) schemes proposed in the technical literature. We also propose a new time-based PSKD (TPSKD), which operates under the assumption of loose time synchronization, and discuss the performance of the scheme. Since the TPSKD scheme uses time information, which would typically already be available in sensor nodes, the cost of the scheme?s implementation is low. 相似文献
55.
Steensgaard J. Zhiqing Zhang Wenhuan Yu Sarhegyi A. Lucchese L. Dae-Ik Kim Temes G.C. 《IEEE transactions on circuits and systems. I, Regular papers》2008,55(5):1289-1296
Incremental data converters (IDCs) are useful in instrumentation and measurement applications, where low-frequency analog signals need to be converted into digital form with high accuracy and low power dissipation. They are particularly well suited for applications where a single analog-digital converter is multiplexed between many channels. This paper proposes an exact design methodology for IDCs, which optimizes the signal-to-noise ratio of the converter under practical design constraints. The process also allows the designer to apportion the noise budget in an arbitrary manner between thermal and quantization noise. The design process is illustrated by an example which describes the optimization of a third-order multiplexed IDC. 相似文献
56.
Chaves R. Kuzmanov G. Sousa L. Vassiliadis S. 《Very Large Scale Integration (VLSI) Systems, IEEE Transactions on》2008,16(8):999-1008
57.
It is shown that, at thermodynamic equilibrium, the release of charge carriers from the localized states of bistable amphoteric centers into quasi-free states depends on the degree of compensation. This brings about different functional dependences of the concentration of free charge carriers on temperature. It is found that, in uncompensated semiconductors, the concentration of free charge carriers follows the same dependence in the case of bistable amphoteric centers and bistable amphoteric U ? centers, although the distributions of charge carriers over the charge states and configurations are different for these types of centers. The results can be used for interpreting various experimental data insufficiently explained in the context of the traditional approach. 相似文献
58.
V. N. Jmerik A. M. Mizerov T. V. Shubina A. V. Sakharov A. A. Sitnikova P. S. Kop’ev S. V. Ivanov E. V. Lutsenko A. V. Danilchyk N. V. Rzheutskii G. P. Yablonskii 《Semiconductors》2008,42(12):1420-1426
Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the Al x Ga1 ? x N/Al y Ga1 ? y N quantum wells. Structural and optical properties of the Al x Ga1 ? x N layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al2O3 substrates, heterostructures for light-emitting diodes with Al x Ga1 ? x N/Al y Ga1 ? y N quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm. 相似文献
59.
Distributed Transmit Antenna Selection (DTAS) Under Performance or Energy Consumption Constraints 总被引:1,自引:0,他引:1
Michalopoulos D.S. Karagiannidis G.K. Tsiftsis T.A. Mallik R.K. 《Wireless Communications, IEEE Transactions on》2008,7(4):1168-1173
Motivated by the transmit antenna selection (TAS) concept, used in Multiple-Input-Multiple-Output systems, we argue for distributed transmit antenna selection (DTAS), which corresponds to a method of selecting a subset of available relays in cooperative diversity systems. Assuming amplify and forward relays, the proposed selection method represents a low-complexity tool for determining the optimum relaying set. Two optimization problems are studied: the error probability minimization subject to total energy consumption constraints, and the dual one, the total energy consumption minimization under error performance constraints. Numerical examples verify the advantage of the proposed method in adapting the number of relaying terminals to the desired performance-consumption tradeoff. 相似文献
60.