全文获取类型
收费全文 | 270232篇 |
免费 | 4436篇 |
国内免费 | 1832篇 |
专业分类
电工技术 | 5519篇 |
技术理论 | 5篇 |
综合类 | 1167篇 |
化学工业 | 43206篇 |
金属工艺 | 11123篇 |
机械仪表 | 8018篇 |
建筑科学 | 6910篇 |
矿业工程 | 1690篇 |
能源动力 | 5768篇 |
轻工业 | 26606篇 |
水利工程 | 2987篇 |
石油天然气 | 6024篇 |
武器工业 | 243篇 |
无线电 | 28628篇 |
一般工业技术 | 50336篇 |
冶金工业 | 50649篇 |
原子能技术 | 5717篇 |
自动化技术 | 21904篇 |
出版年
2021年 | 2554篇 |
2019年 | 2199篇 |
2018年 | 3481篇 |
2017年 | 3461篇 |
2016年 | 3784篇 |
2015年 | 2875篇 |
2014年 | 4709篇 |
2013年 | 11723篇 |
2012年 | 7909篇 |
2011年 | 10515篇 |
2010年 | 8366篇 |
2009年 | 8948篇 |
2008年 | 9783篇 |
2007年 | 9889篇 |
2006年 | 8717篇 |
2005年 | 7660篇 |
2004年 | 6862篇 |
2003年 | 6417篇 |
2002年 | 6427篇 |
2001年 | 6486篇 |
2000年 | 6061篇 |
1999年 | 6075篇 |
1998年 | 13639篇 |
1997年 | 10021篇 |
1996年 | 7710篇 |
1995年 | 5813篇 |
1994年 | 5343篇 |
1993年 | 5195篇 |
1992年 | 4071篇 |
1991年 | 3883篇 |
1990年 | 3963篇 |
1989年 | 3905篇 |
1988年 | 3675篇 |
1987年 | 3151篇 |
1986年 | 3165篇 |
1985年 | 3526篇 |
1984年 | 3410篇 |
1983年 | 3178篇 |
1982年 | 2764篇 |
1981年 | 2992篇 |
1980年 | 2726篇 |
1979年 | 2920篇 |
1978年 | 2817篇 |
1977年 | 2957篇 |
1976年 | 3912篇 |
1975年 | 2530篇 |
1974年 | 2357篇 |
1973年 | 2387篇 |
1972年 | 2038篇 |
1971年 | 1831篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
In self-organizing ad hoc networks, all the networking functions rely on the contribution of the participants. As a basic example, nodes have to forward packets for each other in order to enable multihop communication. In recent years, incentive mechanisms have been proposed to give nodes incentive to cooperate, especially in packet forwarding. However, the need for these mechanisms was not formally justified. In this paper, we address the problem of whether cooperation can exist without incentive mechanisms. We propose a model,based on game theory and graph theory to investigate equilibrium conditions of packet forwarding strategies. We prove theorems about the equilibrium conditions for both cooperative and noncooperative strategies. We perform simulations to estimate the probability that the conditions for a cooperative equilibrium hold in randomly generated network scenarios.. As the problem is involved, we deliberately restrict ourselves to a static configuration. We conclude that in static ad hoc networks where the relationships between the nodes are likely to be stab le-cooperation needs to be encouraged. 相似文献
52.
Peter V. Sushko Alexander L. ShlugerKatsuro Hayashi Masahiro HiranoHideo Hosono 《Thin solid films》2003,445(2):161-167
Recently it has been discovered that a nano-porous main group oxide 12CaO·7Al2O3 (C12A7) can be converted from a wide-gap insulator to a good transparent conductor. Using ab initio modelling we explain good conductivity of this material by very small barriers for hopping of localised electrons between neighbouring positive cages. We show that optical absorption of C12A7 in infrared region and at energies higher than 2.7 eV is due to inter-cage and intra-cage electron transitions, respectively. The proposed mechanisms can be useful in further search for conducting transparent media. 相似文献
53.
Staker S.W. Holloway C.L. Bhobe A.U. Piket-May M. 《Electromagnetic Compatibility, IEEE Transactions on》2003,45(2):156-166
The alternating-direction implicit finite-difference time-domain (ADI-FDTD) technique is an unconditionally stable time-domain numerical scheme, allowing the /spl Delta/t time step to be increased beyond the Courant-Friedrichs-Lewy limit. Execution time of a simulation is inversely proportional to /spl Delta/t, and as such, increasing /spl Delta/t results in a decrease of execution time. The ADI-FDTD technique greatly increases the utility of the FDTD technique for electromagnetic compatibility problems. Once the basics of the ADI-FDTD technique are presented and the differences of the relative accuracy of ADI-FDTD and standard FDTD are discussed, the problems that benefit greatly from ADI-FDTD are described. A discussion is given on the true time savings of applying the ADI-FDTD technique. The feasibility of using higher order spatial and temporal techniques with ADI-FDTD is presented. The incorporation of frequency dependent material properties (material dispersion) into ADI-FDTD is also presented. The material dispersion scheme is implemented into a one-dimensional and three-dimensional problem space. The scheme is shown to be both accurate and unconditionally stable. 相似文献
54.
Examination of the wreckage of a light aircraft revealed that approximately 20 cm was missing from one tip of the aluminum
alloy propeller. Fractographic and metallographic examination of the remaining portion of the propeller revealed extensive
grain-boundary separation in the vicinity of the fracture, and grain edges and corners rounded by corrosion on the fracture
surface. Energy dispersive X-ray analysis (EDXA) revealed fluorine on, and in the vicinity of, the fracture surface. In the
ensuing litigation, it was asserted that the crash occurred because the propeller fractured in flight as the result of intergranular
attack caused by the use of a fluorine-bearing cleaner. 相似文献
55.
Codecasa L. D'Amore D. Maffezzoni P. Batty W. 《Components and Packaging Technologies, IEEE Transactions on》2004,27(1):87-95
In this paper, the multipoint moment matching method for model order reduction of discretized linear thermal networks is extended to distributed linear thermal networks. As a result, from the analytical canonical forms of distributed linear thermal networks, reduced thermal networks are derived analytically. This direct construction of the reduced network, from the exact analytical solutions, avoids the inevitable inaccuracies inherent in conventional surface and volume meshing. It allows nearly exact reduced thermal network construction by domain decomposition for arbitrarily complicated structures. 相似文献
56.
J. B. Quinn G. E. Schumacher L. W. Schultheis 《Journal of Failure Analysis and Prevention》2004,4(1):41-46
Several days after heart surgery, a patient discovered his upper right canine tooth had broken at the root. Such tooth damage,
recognized post-operatively, is usually assumed to be caused by blunt mechanical force from an instrument used by the anesthesiologist
during placement of a breathing tube at the start of surgery.
In this case, the patient had saved the crown portion of the broken tooth, and it was possible to examine the root fracture
characteristics. The curvature and direction of the crack path and natural tooth situation suggested that failure could be
described through a cantilever beam model. This was confirmed when a whole extracted sample tooth was embedded and broken
by a measured force in a manner consistent with the model. The resulting fracture surface matched that of the patient’s broken
canine tooth. However, the high load and force direction necessary to fracture the root was inconsistent with forces applied
during the anesthesia procedure. The failure analysis and further investigation indicated tooth clenching on the breathing
tube during recovery was the likely cause of fracture.
This paper presents an alternate explanation for intubation-related dental injury, demonstrates the practicality of fractographic
analysis of biological materials, and introduces a methodology for simulating in vitro tooth settings for mechanical testing. 相似文献
57.
L. A. Dombrovskii 《High Temperature》2003,41(6):819-824
Theoretical models of heat transfer by radiation through a vapor gap under conditions of film boiling of liquid are treated, namely, a general wave model for a gap of arbitrary thickness and an approximate model of geometrical optics. Calculations are performed of heat transfer by radiation under conditions of film boiling of water on hot surfaces of refractory metal and molten oxide. Practical recommendations are given to calculate the distribution of the power of absorbed radiation in a water layer. 相似文献
58.
59.
A new dual-quantisation sigma-delta modulator is proposed, which introduces an additional feedback path in the input of the second integrator. In this way, unlike other dual-quantisation architectures, larger signal-to-noise ratios can be obtained by means of aggressive noise-shaping, like in a conventional multibit modulator. The proposed modulator is also shown to be more robust against non-idealities than other dual-quantisation architectures. 相似文献
60.
Optimal space-time constellations from groups 总被引:10,自引:0,他引:10
Hughes B.L. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(2):401-410
We consider the design of space-time constellations based on group codes for fading channels with multiple transmit and receive antennas. These codes can be viewed as multiantenna extensions of phase-shift keying (PSK), in the sense that all codewords have equal energy, all are rotations of a fixed codeword, and there is a simple differential transmission rule that allows data to be sent without channel estimates at the transmitter or receiver. For coherent detection, we show that all optimal full-rank space-time group codes are unitary (each code matrix has equal-energy, orthogonal rows). This leads to a simpler code design criterion and suggests that unitary codes may play an important role in coherent as well as noncoherent communication. For any number of transmit antennas t, we then use the design criterion to characterize all full-rank unitary space-time group codes of minimum block length (also t) which have 2/sup p/ codewords. These results allow us to characterize all optimal 2/sup p/-ary unitary group codes with square code matrices. This restricted class of block codes matches the class proposed for differential modulation by Hughes (see IEEE Trans. Inform. Theory, vol.46, p.2567-78, Nov. 2000), and by Hochwald and Sweldens (see IEEE Trans. Commun., vol.48, p.2041-2052, Dec. 2000). 相似文献