全文获取类型
收费全文 | 1704599篇 |
免费 | 27287篇 |
国内免费 | 7228篇 |
专业分类
电工技术 | 35755篇 |
综合类 | 6590篇 |
化学工业 | 282394篇 |
金属工艺 | 67758篇 |
机械仪表 | 49686篇 |
建筑科学 | 50414篇 |
矿业工程 | 12109篇 |
能源动力 | 50754篇 |
轻工业 | 135289篇 |
水利工程 | 17422篇 |
石油天然气 | 38605篇 |
武器工业 | 171篇 |
无线电 | 203275篇 |
一般工业技术 | 316529篇 |
冶金工业 | 257771篇 |
原子能技术 | 34911篇 |
自动化技术 | 179681篇 |
出版年
2021年 | 15849篇 |
2020年 | 12215篇 |
2019年 | 14997篇 |
2018年 | 20840篇 |
2017年 | 20641篇 |
2016年 | 24163篇 |
2015年 | 18023篇 |
2014年 | 29123篇 |
2013年 | 88920篇 |
2012年 | 40989篇 |
2011年 | 57882篇 |
2010年 | 48179篇 |
2009年 | 55373篇 |
2008年 | 52615篇 |
2007年 | 51075篇 |
2006年 | 48999篇 |
2005年 | 45255篇 |
2004年 | 46029篇 |
2003年 | 45560篇 |
2002年 | 43831篇 |
2001年 | 41034篇 |
2000年 | 38855篇 |
1999年 | 39464篇 |
1998年 | 73875篇 |
1997年 | 56913篇 |
1996年 | 46844篇 |
1995年 | 37669篇 |
1994年 | 34328篇 |
1993年 | 34194篇 |
1992年 | 27872篇 |
1991年 | 25283篇 |
1990年 | 25466篇 |
1989年 | 24600篇 |
1988年 | 23103篇 |
1987年 | 21186篇 |
1986年 | 20640篇 |
1985年 | 23981篇 |
1984年 | 23190篇 |
1983年 | 21167篇 |
1982年 | 19938篇 |
1981年 | 20160篇 |
1980年 | 18839篇 |
1979年 | 19101篇 |
1978年 | 18351篇 |
1977年 | 19689篇 |
1976年 | 23044篇 |
1975年 | 16444篇 |
1974年 | 15829篇 |
1973年 | 16002篇 |
1972年 | 13447篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Gas/liquid mass transfer has been investigated using a stirred vessel gas/liquid contactor using non‐Newtonian media and carbon dioxide as absorbent and gas phase, respectively. The volumetric mass transfer coefficients at different operational variables have been determined. Non‐Newtonian media (liquid phase) were prepared as aqueous solutions of sodium carboxymethyl cellulose salt. The influence of the rheological properties, polymer concentration, stirring rate, and gas flow rate on mass transfer was studied for these liquid phases. Kinematic viscosity and density experimental data were used to calculate the average molecular weight corresponding to the polymer employed. The Ostwald model has been used to fit the rheological behavior of aqueous solutions of the polymer employed as absorbent phase. Reasonably good agreement was found between the predictions of the proposed models and the experimental data of mass transfer coefficients. 相似文献
992.
993.
The vapour phase compositions of a series of pack powder mixtures containing elemental Al and Hf or W powders as depositing sources and CrCl3·6H2O or AlF3or CrF3as activators were analysed in an attempt to further develop the pack cementation process to codeposit Al and Hf or W to form diffusion coatings on nickel base superalloys. The results suggested that Al could be codeposited with Hf, but not with W, from the vapour phase. Compared with both AlF3and CrF3, CrCl3·6H2O has been shown to be a more suitable activator for codepositing Al with Hf. The optimum coating temperature was identified to be in the range of 1050°C to 1150°C. Based on the thermochemical analysis, a series of coating deposition studies were undertaken, which confirmed that codeposition of Al and Hf could be achieved at a deposition temperature of 1100°C in the CrCl3·6H2O activated packs containing elemental Al and Hf powders. The coating obtained had a multilayer structure consisting of a Ni7Hf6Al16top layer and a NiAl layer underneath, followed by a diffusion zone, which revealed that the coating was formed by the outward Ni diffusion. It is suggested that the compositions suitable for codeposition of Al and Hf could be effectively identified by comparing the vapour pressures of HfCl4and HfCl3with that of AlCl in the packs activated by chloride salts. It has also been experimentally demonstrated that, although W could not be deposited from the vapour phase, a high volume of fine W particles can be entrapped into the outer NiAl coating layer formed by the outward Ni diffusion using a modified pack configuration. This leads to the formation of a composite coating layer with W particles evenly distributed in a matrix of NiAl. It is suggested that this modified pack process could be similarly applied to develop nickel aluminide coatings containing other refractory metals that may not be codeposited with Al from the vapour phase. 相似文献
994.
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
995.
996.
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
997.
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
998.
A Method to Measure Emissions from Dryers with Diffuse Leakages, Using Evaporated Water as a Tracer 总被引:2,自引:0,他引:2
This paper describes a novel method to measure emission from dryers. The method resolves the known difficulties caused by diffuse emissions, and also solves the problems associated with high moisture content of the drying medium. The basic idea is to use water vapor to determine the exhaust flow, while a dry ice trap is used both to preconcentrate emitted VOCs and to determine the moisture content of the drying medium. 相似文献
999.
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
1000.
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