全文获取类型
收费全文 | 420796篇 |
免费 | 6421篇 |
国内免费 | 1499篇 |
专业分类
电工技术 | 7260篇 |
综合类 | 340篇 |
化学工业 | 61851篇 |
金属工艺 | 15096篇 |
机械仪表 | 12326篇 |
建筑科学 | 10888篇 |
矿业工程 | 1790篇 |
能源动力 | 11515篇 |
轻工业 | 41935篇 |
水利工程 | 4108篇 |
石油天然气 | 5741篇 |
武器工业 | 13篇 |
无线电 | 52136篇 |
一般工业技术 | 80203篇 |
冶金工业 | 78711篇 |
原子能技术 | 7901篇 |
自动化技术 | 36902篇 |
出版年
2021年 | 3160篇 |
2020年 | 2344篇 |
2019年 | 2987篇 |
2018年 | 4754篇 |
2017年 | 4881篇 |
2016年 | 5306篇 |
2015年 | 3700篇 |
2014年 | 6212篇 |
2013年 | 19477篇 |
2012年 | 10869篇 |
2011年 | 15425篇 |
2010年 | 12123篇 |
2009年 | 13657篇 |
2008年 | 14292篇 |
2007年 | 14254篇 |
2006年 | 12833篇 |
2005年 | 11753篇 |
2004年 | 11317篇 |
2003年 | 11087篇 |
2002年 | 10308篇 |
2001年 | 10755篇 |
2000年 | 10047篇 |
1999年 | 10659篇 |
1998年 | 25715篇 |
1997年 | 18084篇 |
1996年 | 13919篇 |
1995年 | 10455篇 |
1994年 | 9281篇 |
1993年 | 9005篇 |
1992年 | 6691篇 |
1991年 | 6395篇 |
1990年 | 6008篇 |
1989年 | 5926篇 |
1988年 | 5690篇 |
1987年 | 4717篇 |
1986年 | 4536篇 |
1985年 | 5570篇 |
1984年 | 5035篇 |
1983年 | 4654篇 |
1982年 | 4233篇 |
1981年 | 4372篇 |
1980年 | 3993篇 |
1979年 | 3962篇 |
1978年 | 3817篇 |
1977年 | 4354篇 |
1976年 | 5676篇 |
1975年 | 3270篇 |
1974年 | 3111篇 |
1973年 | 3188篇 |
1972年 | 2486篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Peak-to-average power reduction in space division multiplexing based OFDM systems through spatial shifting 总被引:2,自引:0,他引:2
A new method to reduce the peak-to-average power ratio (PAPR) in space division multiplexing systems applying orthogonal frequency division multiplexing is proposed. The method applies spatial shifting to partial transmit sequences to achieve a decreased PAPR on all transmit branches. 相似文献
992.
A new frequency notched ultra-wideband microstrip slot antenna with a parasitic tuning stub is proposed. The antenna is similar in configuration to a conventional microstrip slot antenna; by introducing a parasitic open-circuit tuning stub, flexible frequency notched function is achieved. Several properties of the antenna, such as frequency notched function, antenna transfer function and gain, have been investigated. As is reported, the operation bandwidth of the antenna is 2.91-11.16 GHz, in which a frequency notched band of 5.10-5.85 GHz has been achieved. Furthermore, good ultra-wideband linear transmission performance over the entire operation frequency range has also been achieved. 相似文献
993.
O Bernard B Chachuat A Hélias B Le Dantec B Sialve J-P Steyer L Lardon P Neveu S Lambert J Gallop M Dixon P Ratini A Quintabà S Frattesi J M Lema E Roca G Ruiz J Rodriguez A Franco P Vanrolleghem U Zaher D J W De Pauw K De Neve K Lievens D Dochaine O Schoefs H Fibrianto R Farina V Alcaraz Gonzalez V Gonzalez Alvarez P Lemaire J A Martinez F Esandi O Duclaud J F Lavigne 《Water science and technology》2005,52(1-2):457-464
The TELEMAC project brings new methodologies from the Information and Science Technologies field to the world of water treatment. TELEMAC offers an advanced remote management system which adapts to most of the anaerobic wastewater treatment plants that do not benefit from a local expert in wastewater treatment. The TELEMAC system takes advantage of new sensors to better monitor the process dynamics and to run automatic controllers that stabilise the treatment plant, meet the depollution requirements and provide a biogas quality suitable for cogeneration. If the automatic system detects a failure which cannot be solved automatically or locally by a technician, then an expert from the TELEMAC Control Centre is contacted via the internet and manages the problem. 相似文献
994.
J. Chen J. B. Xu K. Xue J. An N. Ke W. Cao H. B. Xia J. Shi D. C. Tian 《Microelectronics Reliability》2005,45(1):137-142
Transition-metal compound TiC60 thin films were grown by co-deposition from two separated sources of fullerene C60 powder and titanium. Study of structural properties of the films, by Raman spectroscopy, atomic force microscopy, and scanning tunneling spectroscopy reveals that the films have a deformed C60 structure with certain amount of sp3 bonds and a rough surface with a large number of nanoclusters. z–V tunnelling spectroscopic measurements suggest that several charge transport mechanisms are involved in as the tip penetrates into the thin film. Conventional field electron emission (FEE) measurements show a high emission current density of 10 mA/cm2 and a low turn-on field less than 8 V/μm, with the field enhancement factors being 659 and 1947 for low-field region and high-field region, respectively. By exploiting STM tunneling spectroscopy, local FEE on nanometer scale has also been characterized in comparison with the conventional FEE. The respective field enhancement factors are estimated to be 99–355 for a gap varying from 36 to 6 nm. The enhanced FEE of TiC60 thin films can be ascribed to structural variation of C60 in the films and the electrical conducting paths formed by titanium nanocrystallites embedded in C60 matrix. 相似文献
995.
Remotely powered addressable UHF RFID integrated system 总被引:3,自引:0,他引:3
Curty J.-P. Joehl N. Dehollain C. Declercq M.J. 《Solid-State Circuits, IEEE Journal of》2005,40(11):2193-2202
This paper presents a fully integrated remotely powered and addressable radio frequency identification (RFID) transponder working at 2.45 GHz. The achieved operating range at 4 W effective isotropically radiated power (EIRP) base-station transmit power is 12 m. The integrated circuit (IC) is implemented in a 0.5 /spl mu/m silicon-on-sapphire technology. A state-of-the-art rectifier design achieving 37% of global efficiency is embedded to supply energy to the transponder. The necessary input power to operate the transponder is about 2.7 /spl mu/W. Reader to transponder communication is obtained using on-off keying (OOK) modulation while transponder to reader communication is ensured using the amplitude shift keying (ASK) backscattering modulation technique. Inductive matching between the antenna and the transponder IC is used to further optimize the operating range. 相似文献
996.
A practical resource management method that can significantly reduce cochannel interference (CCI) and improve spectrum utilisation in FH-OFDMA packet-based cellular networks is presented. The proposed method seeks an effective combination of dynamic resource allocation with fractional coding and bit loading to respectively minimise CCI and maximise system throughput for a desired performance. 相似文献
997.
Role of chain symmetry and hydrogen bonding in segmented copolymers with monodisperse hard segments 总被引:3,自引:0,他引:3
Thermoplastic segmented polyurethane and polyurea copolymers whose monodisperse hard segments are based on only a single diisocyanate molecule are discussed. The solid-state structure-property behavior of these materials demonstrates that a proper selection of the level of symmetry and/or cohesiveness of the hard microdomains may allow elimination of the traditional requirement of chain extension to obtain melt processable segmented urethanes, and more specifically, urea copolymers with useful structural properties. 相似文献
998.
Summary Electropolymerization of o-(CH3O)2C6H4 (Veratrole) in dry acetonitrile/tetrabutylammonium tetrafluoroborate at platinum electrode, yields a green polymeric conducting solid. As the pressure is increased, the material becomes more conductive, but sharp changes towards lower conductivity occur at 4.2 and 6.2 metric tons pressure, probably due to phase transitions. The photoconductivity shows an exponential increase as temperature is increased, but it shows reversible changes at 200 K, 225 K and 250 K during slow warming, also attributable to phase transitions. Photoconductivity of Polyveratrole increases with the radiation frequency, being higher under blue light than under red of infrared radiation and there is a linear increase of photoconductivity with radiation density. Conductivity transients show that the conductivity increases exponentially with time to the saturation point within few minutes, and it also decreases exponentially when the light is turned off and the circuit is open. This behaviour can be explained by formation and encounter of electron and hole as current carriers. A preliminary study indicates a piezoelectric effect for the material. absorption spectroscopy allowed the measurements of the material band gap. 相似文献
999.
Emission of coherent light at 5.1 /spl mu/m wavelength from GaAs-based quantum cascade lasers is reported. This was achieved by integrating nonlinear cascades with large second-order susceptibility in the active regions of the laser. 相似文献
1000.
Aqueous solutions of five selected non-ionic surfactants: Triton (i-octylphenolethoxylates), Tergitol (2,6,8-trimethyl-4-nonanoloxethylates), Symperonic (n-nonylphenol-oxethylates) and Brij (fatty alcohol ethoxylates) were investigated in this study. Using the bioluminescent bacteria Vibrio fischeri the toxicity of the surfactants solutions were determined. An attempt was made to relate rather low biodegradability of nonionic surfactant solutions measured by the BOD5/COD ratio (ranging from 1 to 17%) to their toxicity. The ozonation process was carried out in a 1.5 dm3 stirred cell reactor equipped with two ozone detectors. The following parameters were analyzed: pH, COD, BOD5, DOC, TOC, polarography as well as UV spectrum. The positive effect of ozonation, represented by decay of UV absorption, was visible in almost complete destruction of the surfactants, with exception of Triton X-705 (only 65% degradation after absorption of 2000 mgO3/dm3). The most striking results were obtained in toxicity tests for ozonated solutions of the non-ionic surfactants – an increase of the bacterial growth inhibition (1.5 to 4 times increase in toxicity due to ozonation). The obtained results were discussed taking into account the literature and our own experimental data on mechanisms of ozonation and biodegradation of non-ionic surfactants of the Triton-type and similar chemical structures. 相似文献