全文获取类型
收费全文 | 1594460篇 |
免费 | 32490篇 |
国内免费 | 8397篇 |
专业分类
化学 | 703086篇 |
晶体学 | 20726篇 |
力学 | 76073篇 |
综合类 | 112篇 |
数学 | 247053篇 |
物理学 | 388592篇 |
无线电 | 199705篇 |
出版年
2021年 | 14891篇 |
2020年 | 17503篇 |
2019年 | 17622篇 |
2018年 | 16321篇 |
2017年 | 14613篇 |
2016年 | 31170篇 |
2015年 | 22259篇 |
2014年 | 33126篇 |
2013年 | 79318篇 |
2012年 | 43503篇 |
2011年 | 43331篇 |
2010年 | 42492篇 |
2009年 | 46943篇 |
2008年 | 45410篇 |
2007年 | 43504篇 |
2006年 | 46946篇 |
2005年 | 40714篇 |
2004年 | 40702篇 |
2003年 | 37754篇 |
2002年 | 38222篇 |
2001年 | 38253篇 |
2000年 | 33246篇 |
1999年 | 29744篇 |
1998年 | 27838篇 |
1997年 | 27719篇 |
1996年 | 27120篇 |
1995年 | 24847篇 |
1994年 | 24350篇 |
1993年 | 23745篇 |
1992年 | 23784篇 |
1991年 | 23974篇 |
1990年 | 22728篇 |
1989年 | 22307篇 |
1988年 | 21495篇 |
1987年 | 20237篇 |
1986年 | 19096篇 |
1985年 | 25578篇 |
1984年 | 26677篇 |
1983年 | 22504篇 |
1982年 | 23860篇 |
1981年 | 23032篇 |
1980年 | 22330篇 |
1979年 | 22338篇 |
1978年 | 23512篇 |
1977年 | 23093篇 |
1976年 | 22735篇 |
1975年 | 21361篇 |
1974年 | 21023篇 |
1973年 | 21490篇 |
1972年 | 15643篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Hranilovic S. Kschischang F.R. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(6):1385-1399
Traditional approaches to constructing constellations for electrical channels cannot be applied directly to the optical intensity channel. This work presents a structured signal space model for optical intensity channels where the nonnegativity and average amplitude constraints are represented geometrically. Lattice codes satisfying channel constraints are defined and coding and shaping gain relative to a baseline are computed. An effective signal space dimension is defined to represent the precise impact of coding and shaping on bandwidth. Average optical power minimizing shaping regions are derived in some special cases. Example lattice codes are constructed and their performance on an idealized point-to-point wireless optical link is computed. Bandwidth-efficient schemes are shown to have promise for high data-rate applications, but require greater average optical power. 相似文献
942.
Vales-Alonso J. Gonzalez-Castano F.J. Pousada-Carballo J.M. 《Communications Letters, IEEE》2003,7(3):148-149
The authors present an experimental real-time GSM terminal detector, to be installed in a restricted area. The detector triggers terminal signaling, which can be captured. 相似文献
943.
A fuzzy logic based-method for prognostic decision making in breast and prostate cancers 总被引:1,自引:0,他引:1
Seker H. Odetayo M.O. Petrovic D. Naguib R.N.G. 《IEEE transactions on information technology in biomedicine》2003,7(2):114-122
Accurate and reliable decision making in oncological prognosis can help in the planning of suitable surgery and therapy, and generally, improve patient management through the different stages of the disease. In recent years, several prognostic markers have been used as indicators of disease progression in oncology. However, the rapid increase in the discovery of novel prognostic markers resulting from the development in medical technology, has dictated the need for developing reliable methods for extracting clinically significant markers where complex and nonlinear interactions between these markers naturally exist. The aim of this paper is to investigate the fuzzy k-nearest neighbor (FK-NN) classifier as a fuzzy logic method that provides a certainty degree for prognostic decision and assessment of the markers, and to compare it with: 1) logistic regression as a statistical method and 2) multilayer feedforward backpropagation neural networks an artificial neural-network tool, the latter two techniques having been widely used for oncological prognosis. In order to achieve this aim, breast and prostate cancer data sets are considered as benchmarks for this analysis. The overall results obtained indicate that the FK-NN-based method yields the highest predictive accuracy, and that it has produced a more reliable prognostic marker model than both the statistical and artificial neural-network-based methods. 相似文献
944.
Several kinds of waveguide components such as curved waveguide bends, arbitrary angle waveguide bends and T-junctions have been analyzed with boundary element method in this paper. A new discretization method for the boundary element method to solve the waveguide discontinuities has been given. The numerical results obtained agree well with the experimental results and numerical results in other literature. Especially, the scattering characteristics of Forded E-, H-plane T-junctions in 3mm band have been analyzed using boundary element method and the calculation results are presented. 相似文献
945.
946.
A. A. Andronov Yu. N. Nozdrin A. V. Okomel’kov V. S. Varavin R. N. Smirnov D. G. Ikusov 《Semiconductors》2006,40(11):1266-1274
The experimental data on observation of spontaneous and stimulated emission from thin epitaxial CdxHg1?x Te films optically pumped by Nd: YAG laser radiation are reported. A simple theoretical model is suggested to describe the initiation of population inversion under these conditions. The parameters realized under the experimental conditions are theoretically estimated. 相似文献
947.
Chong D. Y. R. Lim B. K. Rebibis K. J. Pan S. J. Sivalingam K. Kapoor R. Sun A. Y. S. Tan H. B. 《Advanced Packaging, IEEE Transactions on》2006,29(4):674-682
The recent advancement in high- performance semiconductor packages has been driven by the need for higher pin count and superior heat dissipation. A one-piece cavity lid flip chip ball grid array (BGA) package with high pin count and targeted reliability has emerged as a popular choice. The flip chip technology can accommodate an I/O count of more than five hundreds500, and the die junction temperature can be reduced to a minimum level by a metal heat spreader attachment. None the less, greater expectations on these high-performance packages arose such as better substrate real estate utilization for multiple chips, ease in handling for thinner core substrates, and improved board- level solder joint reliability. A new design of the flip chip BGA package has been looked into for meeting such requirements. By encapsulating the flip chip with molding compound leaving the die top exposed, a planar top surface can be formed. A, and a flat lid can then be mounted on the planar mold/die top surface. In this manner the direct interaction of the metal lid with the substrate can be removed. The new package is thus less rigid under thermal loading and solder joint reliability enhancement is expected. This paper discusses the process development of the new package and its advantages for improved solder joint fatigue life, and being a multichip package and thin core substrate options. Finite-element simulations have been employed for the study of its structural integrity, thermal, and electrical performances. Detailed package and board-level reliability test results will also be reported 相似文献
948.
Kontogiannopoulos N. Psychalinos C. 《Circuits and Systems II: Express Briefs, IEEE Transactions on》2006,53(12):1373-1377
In this brief, the well-known switched-current (SI) filtering technique is revisited using the concept of the square-root domain (SRD) filtering. It is proved that SI filters are a subclass of the SRD filters, where sampled-data signal processing is performed. This is achieved by considering typical lossless and lossy SRD sampled-data integrator configurations, using a set of complementary SRD operators which are based on the quadratic I-V relationship of MOS transistor operated in the saturation. Circuit examples are given, where linear-domain integrator and third-order filter configurations were derived using appropriate SRD sampled-data building blocks 相似文献
949.
Why does it pay to be selfish in a MANET? 总被引:2,自引:0,他引:2
Younghwan Yoo Dharma P. Agrawal 《Wireless Communications, IEEE》2006,13(6):87-97
Routing protocols for a mobile ad hoc network have assumed that all mobile nodes voluntarily participate in forwarding others' packets. This was a reasonable assumption because all MNs in a MANET belonged to a single authority. In the near future, however, a MANET may consist of MNs that belong to many different organizations since numerous civilian applications are expected to crop up. In this situation, some MNs may run independently and purposely decide not to forward packets so as to save their own energy. This could potentially lead to network partitioning and corresponding performance degradation. To minimize such situations in MANETs, many studies have explored the use of both the carrot and the stick approaches by having reputation-based, credit-payment, and game theory schemes. This article summarizes existing schemes, identifies their relative advantages, and projects future directions 相似文献
950.
S. P. Skobelev 《Journal of Communications Technology and Electronics》2006,51(7):748-758
A recent approach to solution of 2D scattering problems for electromagnetic waves scattered by thin screens is analyzed. With the use of examples of scattering by a strip and an unclosed cylindrical surface, it is shown that the proposed approach has no advantages in terms of the efficiency of numerical solution over a well-known approach based on exact integral equations for currents that have singular kernels and that are solved with the Krylov-Bogoliubov method. 相似文献