全文获取类型
收费全文 | 243412篇 |
免费 | 3328篇 |
国内免费 | 1240篇 |
专业分类
电工技术 | 4831篇 |
综合类 | 264篇 |
化学工业 | 35487篇 |
金属工艺 | 10861篇 |
机械仪表 | 7074篇 |
建筑科学 | 6247篇 |
矿业工程 | 483篇 |
能源动力 | 6277篇 |
轻工业 | 24212篇 |
水利工程 | 1839篇 |
石油天然气 | 1336篇 |
武器工业 | 3篇 |
无线电 | 31057篇 |
一般工业技术 | 44921篇 |
冶金工业 | 50150篇 |
原子能技术 | 3863篇 |
自动化技术 | 19075篇 |
出版年
2021年 | 1571篇 |
2019年 | 1435篇 |
2018年 | 2329篇 |
2017年 | 2230篇 |
2016年 | 2450篇 |
2015年 | 1914篇 |
2014年 | 3166篇 |
2013年 | 10816篇 |
2012年 | 5459篇 |
2011年 | 7807篇 |
2010年 | 6300篇 |
2009年 | 6978篇 |
2008年 | 7584篇 |
2007年 | 7663篇 |
2006年 | 7042篇 |
2005年 | 6532篇 |
2004年 | 6250篇 |
2003年 | 6268篇 |
2002年 | 6247篇 |
2001年 | 6381篇 |
2000年 | 5804篇 |
1999年 | 6420篇 |
1998年 | 16571篇 |
1997年 | 11615篇 |
1996年 | 8954篇 |
1995年 | 6696篇 |
1994年 | 5912篇 |
1993年 | 5727篇 |
1992年 | 4057篇 |
1991年 | 3995篇 |
1990年 | 3583篇 |
1989年 | 3509篇 |
1988年 | 3441篇 |
1987年 | 2910篇 |
1986年 | 2842篇 |
1985年 | 3413篇 |
1984年 | 3050篇 |
1983年 | 2744篇 |
1982年 | 2547篇 |
1981年 | 2569篇 |
1980年 | 2430篇 |
1979年 | 2271篇 |
1978年 | 2235篇 |
1977年 | 2810篇 |
1976年 | 4172篇 |
1975年 | 1899篇 |
1974年 | 1795篇 |
1973年 | 1783篇 |
1972年 | 1460篇 |
1971年 | 1317篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
201.
Mohsen Ghazel George H Freeman Edward R Vrscay 《IEEE transactions on image processing》2006,15(9):2669-2675
The essence of fractal image denoising is to predict the fractal code of a noiseless image from its noisy observation. From the predicted fractal code, one can generate an estimate of the original image. We show how well fractal-wavelet denoising predicts parent wavelet subtress of the noiseless image. The performance of various fractal-wavelet denoising schemes (e.g., fixed partitioning, quadtree partitioning) is compared to that of some standard wavelet thresholding methods. We also examine the use of cycle spinning in fractal-based image denoising for the purpose enhancing the denoised estimates. Our experimental results show that these fractal-based image denoising methods are quite competitive with standard wavelet thresholding methods for image denoising. Finally, we compare the performance of the pixel- and wavelet-based fractal denoising schemes. 相似文献
202.
Yang C.W. Fang Y.K. Lin C.S. Tsair Y.S. Chen S.M. Wang W.D. Wang M.F. Cheng J.Y. Chen C.H. Yao L.G. Chen S.C. Liang M.S. 《Electronics letters》2003,39(21):1499-1501
A novel technique to form high-K dielectric of HfSiON by doping base oxide with Hf and nitridation with NH/sub 3/, sequentially, is proposed. The HfSiON gate dielectric demonstrates excellent device performances such as only 10% degradation of saturation drain current and almost 45 times of magnitude reduction in gate leakage compared with conventional SiO/sub 2/ gate at the approximately same equivalent oxide thickness. Additionally, negligible flatband voltage shift is achieved with this technique. Time-dependent dielectric breakdown tests indicate that the lifetime of HfSiON is longer than 10 years at V/sub dd/=2 V. 相似文献
203.
H.S. Seo Y.G. Choi B.J. Park D.H. Cho K.H. Kim 《Photonics Technology Letters, IEEE》2003,15(9):1198-1200
A flat signal gain over in the entire C- and L-bands by erbium (Er) ions' radiative transition and stimulated Raman scattering in an Er-doped germano-silica fiber can be obtained if proper values of the concentration of Er and background loss in a fiber core are obtained during the fiber fabrication process. The optimized conditions for the flat C- and L-band gain are analyzed as functions of Er concentrations. Even for a low-gain value provided by a germano-silica core fiber with a low Er concentration and an optimum fiber length, a relatively low pump is required to obtain the flat gain band. 相似文献
204.
N. Jongen M. Donnet P. Bowen J. Lemaître H. Hofmann R. Schenk C. Hofmann M. Aoun‐Habbache S. Guillemet‐Fritsch J. Sarrias A. Rousset M. Viviani M.T. Buscaglia V. Buscaglia P. Nanni A. Testino J.R. Herguijuela 《化学工程与技术》2003,26(3):303-305
The synthesis of powders with controlled shape and narrow particle size distributions is still a major challenge for many industries. A continuous Segmented Flow Tubular Reactor (SFTR) has been developed to overcome homogeneity and scale‐up problems encountered when using batch reactors. Supersaturation is created by mixing the co‐reactants in a micromixer inducing precipitation; the suspension is then segmented into identical micro‐volumes by a non‐miscible fluid and sent through a tube. These micro‐volumes are more homogeneous when compared to large batch reactors leading to narrower size distributions, better particle morphology, polymorph selectivity and stoichiometry. All these features have been demonstrated on single tube SFTR for different chemical systems. To increase productivity for commercial application the SFTR is being “scaled‐out” by multiplying the number of tubes running in parallel instead of scaling‐up by increasing their size. The versatility of the multi‐tube unit will allow changes in type of precipitate with a minimum of new investment as new chemistry can be researched, developed and optimised in a single tube SFTR and then transferred to the multi‐tube unit for powder production. 相似文献
205.
Chou Y.C. Leung D. Lai R. Grundbacher R. Barsky M. Kan Q. Tsai R. Wojtowicz M. Eng D. Tran L. Block T. Liu P.H. Nishimoto M. Oki A. 《Electron Device Letters, IEEE》2003,24(6):378-380
The authors have investigated the reliability performance of G-band (183 GHz) monolithic microwave integrated circuit (MMIC) amplifiers fabricated using 0.07-/spl mu/m T-gate InGaAs-InAlAs-InP HEMTs with pseudomorphic In/sub 0.75/Ga/sub 0.25/As channel on 3-in wafers. Life test was performed at two temperatures (T/sub 1/ = 200 /spl deg/C and T/sub 2/ = 215 /spl deg/C), and the amplifiers were stressed at V/sub ds/ of 1 V and I/sub ds/ of 250 mA/mm in a N/sub 2/ ambient. The activation energy is as high as 1.7 eV, achieving a projected median-time-to-failure (MTTF) /spl ap/ 2 /spl times/ 10/sup 6/ h at a junction temperature of 125 /spl deg/C. MTTF was determined by 2-temperature constant current stress using /spl Delta/G/sub mp/ = -20% as the failure criteria. The difference of reliability performance between 0.07-/spl mu/m InGaAs-InAlAs-InP HEMT MMICs with pseudomorphic In/sub 0.75/Ga/sub 0.25/As channel and 0.1-/spl mu/m InGaAs-InAlAs-InP HEMT MMICs with In/sub 0.6/Ga/sub 0.4/As channel is also discussed. The achieved high-reliability result demonstrates a robust 0.07-/spl mu/m pseudomorphic InGaAs-InAlAs-InP HEMT MMICs production technology for G-band applications. 相似文献
206.
Super-orthogonal space-time trellis codes 总被引:3,自引:0,他引:3
Jafarkhani H. Seshadri N. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(4):937-950
We introduce a new class of space-time codes called super-orthogonal space-time trellis codes. These codes combine set partitioning and a super set of orthogonal space-time block codes in a systematic way to provide full diversity and improved coding gain over earlier space-time trellis code constructions. We also study the optimality of our set partitioning and provide coding gain analysis. Codes operating at different rates, up to the highest theoretically possible rate, for different number of states can be designed by using our optimal set partitioning. Super-orthogonal space-time trellis codes can provide a tradeoff between rate and coding gain. Simulation results show more than 2-dB improvements over the codes presented in the literature while providing a systematic design methodology. 相似文献
207.
Y.K. Su H.C. Wang C.L. Lin W.B. Chen S.M. Chen 《Photonics Technology Letters, IEEE》2003,15(10):1345-1347
The brightness of AlGaInP light emitting diodes (LEDs) has been raised by a factor of 1.12 at 20 mA by sulfide passivation. Meanwhile, the sulfide also can decrease leakage current of AlGaInP LEDs at -2 V to nearly one thousandth of that in the as-fabricated device. The possible causes for the brightness increase of AlGaInP LEDs after sulfide treatment including surface roughness, reduction of Fresnel loss, and effective injection of carriers were demonstrated. 相似文献
208.
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
209.
We have analyzed degradation of N-channel thin-film-transistor (TFT) under dynamic stress using a pico-second time-resolved emission microscope. We have successfully detected emission at pulse fall edge for the first time. Emission intensity increased with the decrease of pulse fall time. As the degradation depended on the pulse fall time, this dependence clearly illustrates that hot electrons are the dominant cause of the degradation under dynamic stress. Based on these dependences, we proposed a model considering electron traps in the poly-Si. 相似文献
210.
VCOs fully integrated in an SiGe preproduction technology are presented. By cutting interconnection lines a wide frequency range can be covered, up to 88.4 GHz. Special effort has been made to meet the demands for 77 GHz automotive radar. In the corresponding tuning range from 75.3 to 79.6 GHz, an output power of about 11 dBm (with -2.2 dBm potential measurement uncertainty) was achieved for each of the two complementary outputs, i.e. in total 14 dBm. The phase noise is about -94 dBc/Hz at 1 MHz offset frequency. 相似文献