全文获取类型
收费全文 | 1620813篇 |
免费 | 32528篇 |
国内免费 | 8493篇 |
专业分类
化学 | 723397篇 |
晶体学 | 20745篇 |
力学 | 76588篇 |
综合类 | 112篇 |
数学 | 248560篇 |
物理学 | 392237篇 |
无线电 | 200195篇 |
出版年
2021年 | 14925篇 |
2020年 | 17561篇 |
2019年 | 17677篇 |
2018年 | 16491篇 |
2017年 | 14665篇 |
2016年 | 31295篇 |
2015年 | 22341篇 |
2014年 | 33190篇 |
2013年 | 79390篇 |
2012年 | 45044篇 |
2011年 | 45886篇 |
2010年 | 43170篇 |
2009年 | 47225篇 |
2008年 | 47650篇 |
2007年 | 45879篇 |
2006年 | 49112篇 |
2005年 | 42993篇 |
2004年 | 42512篇 |
2003年 | 39262篇 |
2002年 | 39538篇 |
2001年 | 39660篇 |
2000年 | 34258篇 |
1999年 | 30211篇 |
1998年 | 27980篇 |
1997年 | 27762篇 |
1996年 | 27198篇 |
1995年 | 24928篇 |
1994年 | 24433篇 |
1993年 | 23872篇 |
1992年 | 24101篇 |
1991年 | 24199篇 |
1990年 | 22962篇 |
1989年 | 22551篇 |
1988年 | 21775篇 |
1987年 | 20441篇 |
1986年 | 19297篇 |
1985年 | 25740篇 |
1984年 | 26758篇 |
1983年 | 22552篇 |
1982年 | 23879篇 |
1981年 | 23068篇 |
1980年 | 22343篇 |
1979年 | 22415篇 |
1978年 | 23542篇 |
1977年 | 23132篇 |
1976年 | 22808篇 |
1975年 | 21468篇 |
1974年 | 21141篇 |
1973年 | 21573篇 |
1972年 | 15752篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
931.
Open Shortest Path First (OSPF) traffic engineering (TE) is intended to bring long-awaited traffic management capabilities into IP networks, which still rely on today's prevailing routing protocols: OSPF or IS-IS. In OSPF, traffic is forwarded along, and split equally between, equal cost shortest paths. In this letter, we formulate the basic requirements placed on a practical TE architecture built on top of OSPF and present a theoretical framework meeting these requirements of practicality. The main contribution of our work comes from the recognition that coupled with an instance of the maximum throughput problem there exists a related inverse shortest-path problem yielding optimal OSPF link weights. 相似文献
932.
An efficient method for preparation of semiconductor quantum rod films for robust lasing in a cylindrical microcavity is reported. A capillary tube, serving as the laser cavity, is filled with a solution of nanocrystals and irradiated with a series of intense nanosecond laser pulses to produce a nanocrystal film on the capillary surface. The films exhibit intense room‐temperature lasing in whispering‐gallery modes that develop at the film–capillary interface as corroborated from the spacing detected for the lasing modes. Good lasing stability is observed at moderate pump powers. The method was applied successfully to several quantum‐rod samples of various sizes. 相似文献
933.
Dongsu Kim Yoonsu Choi Ahn M. Allen M.G. Stevenson Kenney J. Marry P. 《Microwave and Wireless Components Letters, IEEE》2003,13(10):434-436
Continuously variable ferroelectric (BST on sapphire) phase shifters based on all-pass networks are presented. An all-pass network phase shifter consists of only lumped LC elements, and thus the total size of the phase shifter is kept to less than 2.2 mm /spl times/ 2.6 mm at 2.4 GHz. The tunability (C/sub max//C/sub min/) of a BST interdigital capacitor is over 2.9 with a bias voltage of 140 V. The phase shifter provides more than 121/spl deg/ phase shift with the maximum insertion loss of 1.8 dB and the worst case return loss of 12.5 dB from 2.4 GHz to 2.5 GHz. By cascading two identical phase shifters, more than 255/spl deg/ phase shift is obtained with the maximum insertion loss of 3.75 dB. The loss figure-of-merit of both the single- and double-section phase shifters is over 65/spl deg//dB from 2.4 GHz to 2.5 GHz. 相似文献
934.
A discussion about the quantum mechanical effects on noise properties of ballistic (phase-coherent) nanoscale devices is presented. It is shown that quantum noise can be understood in terms of quantum trajectories. This interpretation provides a simple and intuitive explanation of the origin of quantum noise that can be very salutary for nanoelectronic engineers. In particular, an injection model is presented that, coupled with a standard Monte Carlo algorithm, provides an accurate modeling of quantum noise. As a test, the standard results of noise in tunneling junction devices are reproduced within this approach. 相似文献
935.
Relevance of coupling effects on DRA array design 总被引:1,自引:0,他引:1
Gentili G.B. Morini M. Selleri S. 《Antennas and Propagation, IEEE Transactions on》2003,51(3):399-404
A finite-difference time-domain (FDTD) approach is proposed in order to obtain suitable criteria for the design of a four-element dielectric resonator antenna (DRA) array. The numerical analysis is focused on the study of the electromagnetic coupling phenomena arising between DRA fed by a microstrip-slot transition. Scattering parameters behavior and radiation patterns are determined by taking advantage of the geometrical symmetry of the structure, analyzing only one half or one quarter of the entire domain through two or four separate FDTD runs. 相似文献
936.
937.
Rankin D.M. Gulliver T.A. Taylor D.P. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(9):2230-2235
This article investigates the asymptotic performance of single parity-check (SPC) product codes (PCs) from a decoding point of view. Specifically, the probability of bit error is bounded before and after the decoding of each dimension, similar to the analysis of "iterated codes" by Elias (1954). It is shown that the asymptotic probability of bit error can be driven to zero as the number of dimensions, and hence the block length, increases at signal-to-noise ratios (SNRs) within 2 dB of capacity over the additive white Gaussian noise (AWGN) channel. 相似文献
938.
Sethuraman B.A. Rajan B.S. Shashidhar V. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(10):2596-2616
We present some general techniques for constructing full-rank, minimal-delay, rate at least one space-time block codes (STBCs) over a variety of signal sets for arbitrary number of transmit antennas using commutative division algebras (field extensions) as well as using noncommutative division algebras of the rational field /spl Qopf/ embedded in matrix rings. The first half of the paper deals with constructions using field extensions of /spl Qopf/. Working with cyclotomic field extensions, we construct several families of STBCs over a wide range of signal sets that are of full rank, minimal delay, and rate at least one appropriate for any number of transmit antennas. We study the coding gain and capacity of these codes. Using transcendental extensions we construct arbitrary rate codes that are full rank for arbitrary number of antennas. We also present a method of constructing STBCs using noncyclotomic field extensions. In the later half of the paper, we discuss two ways of embedding noncommutative division algebras into matrices: left regular representation, and representation over maximal cyclic subfields. The 4/spl times/4 real orthogonal design is obtained by the left regular representation of quaternions. Alamouti's (1998) code is just a special case of the construction using representation over maximal cyclic subfields and we observe certain algebraic uniqueness characteristics of it. Also, we discuss a general principle for constructing cyclic division algebras using the nth root of a transcendental element and study the capacity of the STBCs obtained from this construction. Another family of cyclic division algebras discovered by Brauer (1933) is discussed and several examples of STBCs derived from each of these constructions are presented. 相似文献
939.
Long-range prediction of the mobile-radio fading envelope is an enabling technology for many fading compensation approaches. Because the fading envelope is well modeled as a bandlimited process, it has special predictability properties. In this paper, we find a linear predictor that is optimal in the mean-square sense when the predictor impulse response is energy constrained. This solution may be used to determine the minimum mean squared error of a prediction based on past values that are corrupted with estimation errors. 相似文献
940.
A distributed scheduling for agro-food manufacturing problems 总被引:2,自引:0,他引:2
E. Gargouri S. Hammadi 《IEEE transactions on systems, man and cybernetics. Part C, Applications and reviews》2003,33(2):176-185
Workshop scheduling problems can be considered as one of the main factors to improve the productivity and efficiency of a manufacturing system. The continuous evolution and the dynamic characteristics of industrial workshops, particularly those of agro-food industries, impose the generation of a real time decision scheduling process. In the agro-food industries, the products to be processed and the used primary products are characterized by their limit validity dates which generate some particularly and especially antagonist criteria. A distributed decision support system for real time scheduling is described in this paper. It is based on an original cooperative approach aimed to elaborate robust decisions that propose to the "best action" to the decision maker. The decision process is distributed all along the production chain. At each decision time, in order to ensure coherency, we have to take into account the neighborhood constraints in order to simultaneously satisfy different criteria which do not have the same importance. An aggregation criteria system is defined to manage the criteria among their importance degrees. We define a decisional model based on the evaluation and the comparison of a set of proposed actions. 相似文献