全文获取类型
收费全文 | 748357篇 |
免费 | 8406篇 |
国内免费 | 2202篇 |
专业分类
电工技术 | 13595篇 |
综合类 | 956篇 |
化学工业 | 115681篇 |
金属工艺 | 30141篇 |
机械仪表 | 23450篇 |
建筑科学 | 17139篇 |
矿业工程 | 5086篇 |
能源动力 | 19390篇 |
轻工业 | 60449篇 |
水利工程 | 8848篇 |
石油天然气 | 17937篇 |
武器工业 | 76篇 |
无线电 | 81830篇 |
一般工业技术 | 151534篇 |
冶金工业 | 131806篇 |
原子能技术 | 18519篇 |
自动化技术 | 62528篇 |
出版年
2021年 | 6783篇 |
2019年 | 6442篇 |
2018年 | 11136篇 |
2017年 | 11369篇 |
2016年 | 11907篇 |
2015年 | 7571篇 |
2014年 | 12802篇 |
2013年 | 34123篇 |
2012年 | 19964篇 |
2011年 | 27159篇 |
2010年 | 21678篇 |
2009年 | 24381篇 |
2008年 | 24756篇 |
2007年 | 24474篇 |
2006年 | 21172篇 |
2005年 | 19376篇 |
2004年 | 18802篇 |
2003年 | 18330篇 |
2002年 | 17715篇 |
2001年 | 17297篇 |
2000年 | 16474篇 |
1999年 | 16636篇 |
1998年 | 39156篇 |
1997年 | 28306篇 |
1996年 | 21918篇 |
1995年 | 16890篇 |
1994年 | 15117篇 |
1993年 | 14935篇 |
1992年 | 11348篇 |
1991年 | 11004篇 |
1990年 | 10736篇 |
1989年 | 10486篇 |
1988年 | 9981篇 |
1987年 | 9038篇 |
1986年 | 8861篇 |
1985年 | 9935篇 |
1984年 | 9061篇 |
1983年 | 8582篇 |
1982年 | 7799篇 |
1981年 | 7915篇 |
1980年 | 7644篇 |
1979年 | 7575篇 |
1978年 | 7528篇 |
1977年 | 8292篇 |
1976年 | 10545篇 |
1975年 | 6661篇 |
1974年 | 6400篇 |
1973年 | 6474篇 |
1972年 | 5519篇 |
1971年 | 5073篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
63.
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
64.
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
65.
High-efficiency LEDs of 1.6–2.4 µm spectral range for medical diagnostics and environment monitoring
N. D. Stoyanov B. E. Zhurtanov A. P. Astakhova A. N. Imenkov Yu. P. Yakovlev 《Semiconductors》2003,37(8):971-984
High efficient LED structures covering the spectral range of 1.6–2.4 μm have been developed on the basis of GaSb and its solid
solutions. The electroluminescent characteristics and their temperature and current dependences have been studied. The radiative
and nonradiative recombination mechanisms and their effect on the quantum efficiency have been investigated. A quantum efficiency
of 40–60% has been obtained in the quasi-steady mode at room temperature. A short-pulse optical power of 170 mW was reached.
__________
Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 8, 2003, pp. 996–1009.
Original Russian Text Copyright ? 2003 by Stoyanov, Zhurtanov, Astakhova, Imenkov, Yakovlev. 相似文献
66.
67.
It is shown that, for the spectrum analysis of digital quasi-periodic signals, one must use a procedure based on approximating the sequence of data readouts by a first-order trigonometric polynomial with a varying frequency of its harmonic functions. 相似文献
68.
The characteristics of pulsation processes in dc plasmatrons with the cathode located in the channel center and the anode in the form of a cylindrical or a conical channel wall are investigated in plasmas of argon and nitrogen at atmospheric pressure. The fast Fourier transform of signals and their subsequent computer processing are used to obtain the dependences of the frequency of fluctuations of the arcing voltage on the working parameters of the plasmatron, namely, the electric arc current, the flow rate of plasma-forming gas, and the channel diameter. For plasmatrons with the self-adjusting length of the electric arc, analysis is performed of the mechanism of reclosing of the anode region of the arc, i.e., of the electric arc shunting associated with the stretching of the current filament by a flow of gas and with the electrodynamic interaction of different filament regions. A formula is derived which defines the dependence of the characteristic frequency of fluctuations of the arcing voltage on the external parameters of the problem, namely, the arc current, the flow rate of the working gas, and the characteristic channel diameter. It is demonstrated that the pattern of the dependence of the frequency of voltage fluctuations on the gas flow rate may vary with the values of the parameter of magnetohydrodynamic interaction. The formula generalizes the experimental results of numerous researchers obtained in a wide range of variation of the external parameters. 相似文献
69.
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
70.
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