全文获取类型
收费全文 | 306292篇 |
免费 | 3839篇 |
国内免费 | 839篇 |
专业分类
电工技术 | 5233篇 |
综合类 | 180篇 |
化学工业 | 49366篇 |
金属工艺 | 12363篇 |
机械仪表 | 8939篇 |
建筑科学 | 7700篇 |
矿业工程 | 1914篇 |
能源动力 | 7224篇 |
轻工业 | 27135篇 |
水利工程 | 3423篇 |
石油天然气 | 7200篇 |
武器工业 | 17篇 |
无线电 | 32706篇 |
一般工业技术 | 59996篇 |
冶金工业 | 55490篇 |
原子能技术 | 7682篇 |
自动化技术 | 24402篇 |
出版年
2021年 | 2313篇 |
2019年 | 2197篇 |
2018年 | 3946篇 |
2017年 | 3884篇 |
2016年 | 4242篇 |
2015年 | 2743篇 |
2014年 | 4651篇 |
2013年 | 13120篇 |
2012年 | 7506篇 |
2011年 | 10074篇 |
2010年 | 8245篇 |
2009年 | 9335篇 |
2008年 | 9732篇 |
2007年 | 9661篇 |
2006年 | 8538篇 |
2005年 | 7986篇 |
2004年 | 7395篇 |
2003年 | 7100篇 |
2002年 | 7161篇 |
2001年 | 6832篇 |
2000年 | 6505篇 |
1999年 | 6567篇 |
1998年 | 15144篇 |
1997年 | 11391篇 |
1996年 | 8903篇 |
1995年 | 6860篇 |
1994年 | 6185篇 |
1993年 | 6115篇 |
1992年 | 4762篇 |
1991年 | 4646篇 |
1990年 | 4540篇 |
1989年 | 4554篇 |
1988年 | 4494篇 |
1987年 | 3836篇 |
1986年 | 3852篇 |
1985年 | 4431篇 |
1984年 | 4236篇 |
1983年 | 3883篇 |
1982年 | 3635篇 |
1981年 | 3789篇 |
1980年 | 3650篇 |
1979年 | 3600篇 |
1978年 | 3641篇 |
1977年 | 4126篇 |
1976年 | 5218篇 |
1975年 | 3331篇 |
1974年 | 3166篇 |
1973年 | 3186篇 |
1972年 | 2752篇 |
1971年 | 2556篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
142.
Limited-trial Chase decoding 总被引:1,自引:0,他引:1
Arico G. Weber J.H. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(11):2972-2975
Chase decoders permit flexible use of reliability information in algebraic decoding algorithms for error-correcting block codes of Hamming distance d. The least complex version of the original Chase algorithms uses roughly d/2 trials of a conventional binary decoder, after which the best decoding result is selected as the final output. On certain channels, this approach achieves asymptotically the same performance as maximum-likelihood (ML) decoding. In this correspondence, the performance of Chase-like decoders with even less trials is studied. Most strikingly, it turns out that asymptotically optimal performance can be achieved by a version which uses only about d/4 trials. 相似文献
143.
M. Ganschow C. Hellriegel E. Kneuper M. Wark C. Thiel G. Schulz‐Ekloff C. Bruchle D. Whrle 《Advanced functional materials》2004,14(3):269-276
Dye‐loaded AlPO4‐5 single crystals were prepared by microwave‐assisted hydrothermal synthesis from a batch, containing a mixture of three chromophores (Coumarin 40, Rhodamine BE50, and Oxazine 1) differing in their absorption range, molecular dimensions, and solubilities. Confocal fluorescence images reveal a spatial separation effect of the dye molecules, where the slimmer, more‐soluble dye molecule (Coumarin 40) is uniformly distributed in the body of the single crystal, and the bulky and/or less‐soluble ones (Rhodamine BE50, Oxazine 1) are situated in distinct domains. Visible spectra show good panchromatic absorption of visible light. Fluorescence lifetime measurements indicate the presence of an energy transfer cascade of the entirely fixed dye molecules from Coumarin 40 to Oxazine 1. The transfer mechanism is predominantly radiative. 相似文献
144.
Polycrystalline Cu(In,Ga)Se2 (CIGS) films with various ratios of Cu, In, and Ga were grown by codeposition of all elements in vacuum. The X-ray diffraction
study showed that the films are single-phase and possess a chalcopyrite structure with predominant [112] orientation. The
films exhibited a mirror smooth surface and had a close-packed structure composed of crystallites with clear faceting and
a transverse size of 0.1–0.3 μm. Related surface barrier structures of the (In,Ag)/Cu(In,Ga)Se2 type were obtained and their spectra of the quantum efficiency of photoconversion were studied. The obtained structures can
be used for optimization of the CIGS film technology. 相似文献
145.
The effect of thermal annealing in the temperature range T a=300–600°C of films of microcrystal-line hydrogenated silicon (μc-Si:H) lightly doped with boron on the spectral dependences of the absorption coefficient (α) at photon energies hν=0.8–2.0 eV, dark conductivity (σd), and photoconductivity (Δσph) was studied at room temperature. With increasing annealing temperature, a nonmonotonic variation of α (at hν<1.2 eV), σd, and Δσph was observed. The data obtained are attributed to a change in the concentration of electrically active impurities and formation of defects, caused by hydrogen effusion and bond restructuring at high annealing temperatures. 相似文献
146.
Previously unknown compounds AI
I(BUO5)2·nH2O (AI
I = Mg, Ca, Sr, Ba; n = 0-7) were synthesized. Their structure and thermal decomposition were studied by X-ray diffraction, IR spectroscopy, and thermal analysis. 相似文献
147.
V. S. Butylkin G. A. Kraftmakher 《Journal of Communications Technology and Electronics》2006,51(5):484-497
A new composite medium based on waveguiding structures loaded with bianisotropic gratings of planar double split rings (PDSRs) and planar helices is considered. The microwave transmission spectra of this medium are studied experimentally. The effective parameters of the medium are determined theoretically, and its passbands are investigated. The characteristics of the medium are compared to those of infinite homogeneous bianisotropic media. A relationship between the spectral transmission characteristics and the type of resonance excitation is established. The transparency bands and overforbidden bands of the composite medium are revealed. The combinations of the medium’s effective parameters that correspond to these bands are determined. It is shown that the transparency bands where the medium is characterized simultaneously by negative permittivity and negative permeability can be identified. 相似文献
148.
Solar cells based on SnO2/Cd0.4Zn0.6S/CdTe heterostructures are fabricated by electrochemical deposition, and the dependences of their electrical and photoelectric properties on the thermal annealing conditions are studied. It is shown that thermal annealing reduces the tunnel currents by almost two orders of magnitude. The best conditions of thermal annealing are determined (t = 300°C and τ = 9 min). These conditions provide the highest photosensitivity of the heterostructures under study (I sc ≈ 21.2 mA/cm2, U oc ≈ 813 mV, and η = 14.7%). 相似文献
149.
L. G. Gerchikov Yu. A. Mamaev A. V. Subashiev Yu. P. Yashin D. A. Vasil’ev V. V. Kuz’michev A. E. Zhukov E. S. Semenova A. P. Vasil’ev V. M. Ustinov 《Semiconductors》2006,40(11):1326-1332
Photoemission of polarized electrons from heterostructures based on InAlGaAs/GaAs superlattices with minimum conduction-band offsets is investigated. The comparison of the excitation energy dependence of the photoemission polarization degree with the calculated spectra make it possible to determine the polarization losses at different stages of the photoemission. A maximum polarization of P = 91% and a quantum yield of 0.14% are close to the best results obtained for photocathodes on the basis of strained semiconductor superlattices. 相似文献
150.
In this work it is presented a study on the residence time distribution (RTD) of particles in a co-current pilot-plant spray dryer operated with a rotary atomization system. A nuclear technique is applied to investigate the RTD responses of spray dryers. The methodology is based on the injection of a radioisotope tracer in the feed stream followed by the monitoring of its concentration at the outlet stream. The experiments were performed during the drying of aqueous suspensions of gadolinium oxide. The RTD responses obtained experimentally presented good reproducibility, indicating that the technique applied is well suited to investigating fluid-dynamics of spray dryers. In addition to the experimental investigation, a mathematical model was used to describe the RTD experimental curves. 相似文献