全文获取类型
收费全文 | 21757篇 |
免费 | 1322篇 |
国内免费 | 57篇 |
专业分类
电工技术 | 311篇 |
综合类 | 25篇 |
化学工业 | 4895篇 |
金属工艺 | 879篇 |
机械仪表 | 1298篇 |
建筑科学 | 427篇 |
矿业工程 | 6篇 |
能源动力 | 865篇 |
轻工业 | 1850篇 |
水利工程 | 91篇 |
石油天然气 | 16篇 |
无线电 | 3656篇 |
一般工业技术 | 4627篇 |
冶金工业 | 1541篇 |
原子能技术 | 291篇 |
自动化技术 | 2358篇 |
出版年
2024年 | 22篇 |
2023年 | 275篇 |
2022年 | 399篇 |
2021年 | 684篇 |
2020年 | 470篇 |
2019年 | 497篇 |
2018年 | 699篇 |
2017年 | 661篇 |
2016年 | 768篇 |
2015年 | 602篇 |
2014年 | 938篇 |
2013年 | 1389篇 |
2012年 | 1456篇 |
2011年 | 1790篇 |
2010年 | 1299篇 |
2009年 | 1331篇 |
2008年 | 1211篇 |
2007年 | 938篇 |
2006年 | 813篇 |
2005年 | 708篇 |
2004年 | 638篇 |
2003年 | 586篇 |
2002年 | 593篇 |
2001年 | 517篇 |
2000年 | 437篇 |
1999年 | 423篇 |
1998年 | 710篇 |
1997年 | 420篇 |
1996年 | 395篇 |
1995年 | 257篇 |
1994年 | 169篇 |
1993年 | 151篇 |
1992年 | 112篇 |
1991年 | 100篇 |
1990年 | 75篇 |
1989年 | 88篇 |
1988年 | 74篇 |
1987年 | 63篇 |
1986年 | 51篇 |
1985年 | 43篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 31篇 |
1981年 | 26篇 |
1980年 | 29篇 |
1979年 | 13篇 |
1977年 | 22篇 |
1976年 | 33篇 |
1975年 | 17篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
151.
152.
The photonic quantum ring (PQR) laser is a three dimensional whispering gallery (WG) mode laser and has anomalous quantum wire properties, such as microampere to nanoampere range threshold currents and √T‐dependent thermal red shifts. We observed uniform bottom emissions from a 1‐kb smart pixel chip of a 32×32 InGaAs PQR laser array flip‐chip bonded to a 0.35 µm CMOS‐based PQR laser driver. The PQR‐CMOS smart pixel array, now operating at 30 MHz, will be improved to the GHz frequency range through device and circuit optimization. 相似文献
153.
M.H. Song B. Park S. Nishimura T. Toyooka I.J. Chung Y. Takanishi K. Ishikawa H. Takezoe 《Advanced functional materials》2006,16(14):1793-1798
A liquid crystal (LC) photonic device with an anisotropic optical heterojunction structure has been fabricated. The device has a phase‐retarding nematic LC (NLC) layer sandwiched between two polymer cholesteric LC films with right‐handed helices of different pitches. Electrotunable non‐reciprocal light transmittance and unidirectional circularly polarized (CP) lasing emission have been successfully demonstrated for this device structure. Two left CP (LCP) lasing emission peaks are observed at the edges of the overlapping region between the two photonic bands in the structure and are shifted upon the application of a voltage. In contrast, a non‐reciprocal right CP (RCP) lasing emission peak emerges at one of the band edges and diminishes upon the application of a voltage. These phenomena are interpreted based on the selective reflection of RCP light and the reorientation of the NLC molecules by the application of a voltage. 相似文献
154.
Ilyong Yoon Byoungho Lee Soojin Park 《Photonics Technology Letters, IEEE》2006,18(6):776-778
A new fiber depolarizer employing a polarization beam splitter loop structure is proposed and demonstrated. The depolarizer is devised for broad-band operation and the depolarization of narrow linewidth light source without any help of polarization controllers or Faraday rotator mirrors. A polarizing method is developed that shows good performance without polarization control unit. Therefore, the proposed depolarizer can be cost-effective and easily configured. From experiments, low output degree of polarization less than 10% is obtained for a narrow linewidth light source. 相似文献
155.
Jinho Park Kiyong Choi Allstot D.J. 《IEEE transactions on circuits and systems. I, Regular papers》2004,51(10):1953-1966
RF circuit synthesis techniques based on particle swarm optimization and adaptive simulated annealing with tunneling are described, and comparisons of parasitic-aware designs of an RF distributed amplifier and a nonlinear power amplifier are presented. Synthesized in 0.35-/spl mu/m digital CMOS using a single 3.3-V power supply, the designs provide an 8-dB gain and 8-GHz bandwidth for a four-stage distributed amplifier, and 1.2-W output power with 55% drain efficiency at 900 MHz for a three-stage power amplifier. A standard circuit simulator, HSPICE or SPECTRE, embedded in an optimization loop is used to evaluate cost functions. The proposed design and optimization methodology is computationally efficient and robust in searching complex multidimensional design spaces. 相似文献
156.
157.
Fatigue growth of multiple-cracks near a row of fastener-holes in a fuselage lap-joint 总被引:4,自引:0,他引:4
The fatigue growth of multiple cracks, of arbitrary lengths, emanating from a row of fastener holes in a bonded, riveted, lap joint in a pressurized aircraft fuselage is studied. The effects of residual stresses due to a rivet misfit, and of plastic deformation near the hole, are included. A Schwartz-Neumann alternating method which uses the analytical solution for a row of multiple colinear cracks in an infinite sheet (the crack-faces being subject to arbitrary tractions), is developed to analyze this MSD problem on a personal computer. It is found that for a range of crack lengths, a phenomena wherein the shorter cracks may grow faster than longer cracks may exist. 相似文献
158.
Microwave induced plasma processing was used to sinter synthetic Idaho Chemical Processing Plant (ICPP) alumina and zirconia based high level nuclear waste calcines in a nitrogen atmosphere. The microwave densification behaviour of these nuclear waste calcines was observed parallel with identification of the phases formed after sintering. Sintered densities of > 3.20 g cm–3 were obtained within 10 min of microwave sintering of pure calcines. Glass frit containing calcines showed lower sintering densities (< 2.0 g cm–3) due to reactions between the frit and volatile substances in both zirconia based and alumina based calcines; prior removal of frit volatiles increased the sintered density. Phases formed in the microwave sintered calcines were identified by X-ray diffraction. 相似文献
159.
Seoijin Park R. Leavitt R. Enck V. Luciani Y. Hu P.J.S. Heim D. Bowler M. Dagenais 《Photonics Technology Letters, IEEE》2005,17(5):980-982
A semiconductor optical amplifier was developed for coarse wavelength-division-multiplexing (CWDM) operating over 1540-1620 nm (C-L band). A unique quantum-well structure was designed to meet the requirements for the CWDM operation such as wide bandwidth, low polarization-dependent gain, and high-saturation power at the short wavelength end of the band (1540 nm). Over the band, 24-dB maximum chip gain was obtained with less than 4.3-dB gain flatness and more than 14.6-dBm saturation power. 相似文献
160.
Park Y.K. Granlund S.W. Cline T.W. Tzeng L.D. French J.S. Delavaux J.-M.P. Tench R.E. Korotky S.K. Veselka J.J. DiGiovanni D.J. 《Photonics Technology Letters, IEEE》1992,4(2):179-182
The authors have achieved a 2.488 Gb/s, 318 km repeaterless transmission without any fiber dispersion penalty through a nondispersion-shifted fiber in a direct detection system. The system was loss limited with a T-R power budget of 57 dB. Three key components enabled the authors to achieve this result: (1) a Ti:LiNbO3 external amplitude modulator enabling a dispersion-free transmission, (2) erbium-doped fiber amplifiers increasing the transmitting power to +16 dBm, and (3) an erbium-doped fiber preamplifier enabling a high-receiver sensitivity of -4.1 dBm for 10-9 BER. To the author's knowledge, this result is the longest repeaterless transmission span length ever reported for direct detection at this bit rate. From the experimental results and a theoretical model, the authors identified the sources of the receiver sensitivity degradation from the quantum limit (-48.6 dBm) and estimated the practically achievable receiver sensitivity of ~-44 dBm (~-124 photons/bit) for 2.5 Gb/s optical preamplifier detection 相似文献