全文获取类型
收费全文 | 524698篇 |
免费 | 8058篇 |
国内免费 | 1471篇 |
专业分类
电工技术 | 9563篇 |
综合类 | 528篇 |
化学工业 | 78723篇 |
金属工艺 | 19963篇 |
机械仪表 | 15203篇 |
建筑科学 | 12668篇 |
矿业工程 | 2269篇 |
能源动力 | 14548篇 |
轻工业 | 47087篇 |
水利工程 | 5219篇 |
石油天然气 | 9338篇 |
武器工业 | 37篇 |
无线电 | 59955篇 |
一般工业技术 | 101839篇 |
冶金工业 | 103518篇 |
原子能技术 | 10562篇 |
自动化技术 | 43207篇 |
出版年
2022年 | 3187篇 |
2021年 | 4932篇 |
2020年 | 3514篇 |
2019年 | 4591篇 |
2018年 | 7841篇 |
2017年 | 7621篇 |
2016年 | 7952篇 |
2015年 | 5567篇 |
2014年 | 9252篇 |
2013年 | 24085篇 |
2012年 | 14641篇 |
2011年 | 20158篇 |
2010年 | 15803篇 |
2009年 | 17699篇 |
2008年 | 18098篇 |
2007年 | 17776篇 |
2006年 | 15505篇 |
2005年 | 14211篇 |
2004年 | 13603篇 |
2003年 | 13399篇 |
2002年 | 12634篇 |
2001年 | 12833篇 |
2000年 | 11868篇 |
1999年 | 12597篇 |
1998年 | 32823篇 |
1997年 | 22898篇 |
1996年 | 17449篇 |
1995年 | 13033篇 |
1994年 | 11284篇 |
1993年 | 10972篇 |
1992年 | 7914篇 |
1991年 | 7418篇 |
1990年 | 7261篇 |
1989年 | 7013篇 |
1988年 | 6468篇 |
1987年 | 5767篇 |
1986年 | 5656篇 |
1985年 | 6265篇 |
1984年 | 5901篇 |
1983年 | 5170篇 |
1982年 | 4900篇 |
1981年 | 4966篇 |
1980年 | 4705篇 |
1979年 | 4609篇 |
1978年 | 4408篇 |
1977年 | 5334篇 |
1976年 | 6971篇 |
1975年 | 3796篇 |
1974年 | 3550篇 |
1973年 | 3665篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Thin films on aluminum-tungsten alloys were prepared by co-deposition of pure aluminum and pure tungsten, each sputtered by an independently controlled magnetron source, on glass and sapphire substrates. Completely amorphous films were obtained in the Al80W20-Al67W33 composition range. Passivity and corrosion behavior of amorphous Al-W alloys were investigated in 1 M deaerated hydrochloric acid solution using polarization and impedance spectroscopy measurements and have been correlated with the properties of pure alloy components. Tungsten and sputter-deposited Al-W thin films are inherently passive materials while aluminum undergoes pitting corrosion in hydrochloric acid solution. The passive film formed at the OCP on each alloy possesses excellent electric and dielectric properties comparable to those of the isolating film on tungsten. The absolute impedance increases with increasing tungsten content in the alloy. According to electrochemical polarization measurements, alloying Al with W in solid solution significantly enhances the material's resistance to pitting corrosion by shifting the breakdown potential above 2000 mV (Al67W33) and lowering the corrosion rate at the OCP by more than two orders of magnitude. The most likely mechanism explaining the passivity of amorphous Al-W alloys, the Solute Vacancy Interaction Model (SVIM), involves the formation of complexes between highly oxidized solute atoms (W+6) and mobile cation vacancies, which restrict the transport of Cl− through the oxide film and inhibit its breakdown in hydrochloric acid solution. The role that film stress relaxation effects and microscopic defects in amorphous Al-W films, of the some composition, and deposited on various substrates play in their corrosion resistance is discussed. 相似文献
972.
In Malaysia, no long-term daylight data are measured. It was only recently that the need to measure the availability of daylight became urgent when the importance of daylighting in buildings was rediscovered. The hourly daylight availability has been simulated for the Malaysian sky using daylight modelling techniques based on empirical and measured solar irradiation and cloud cover data. This paper presents the techniques involved in producing exterior illuminance data. These data were then compared with measured illuminance at Shah Alam and Bangi, Malaysia. The global illuminance levels are generally high, with values exceeding 80,000 lux at noon during the months when solar irradiation is highest. Even during the months when the ground receives less solar irradiation, the peak illuminance can reach 60,000 lux. Applications and uses of such data are in daylighting design, both for visual and thermal comfort, task illuminance and energy-conscious design of buildings. Recommendations are made at the end of the paper on the various climatic data that are required to be measured for overall daylighting design applications. 相似文献
973.
TeMxMo1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N2. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb4Mo10O31 and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene. 相似文献
974.
In order to enhance product yields in selective oxidation reactions, numerous reaction engineering concepts are being studied worldwide. Periodic operation has been investigated for decades, yet its application is limited to a few examples, such as the butane oxidation after DuPont or reverse-flow reactors for VOC removal. The use of microchannel reactors is a younger field, but it has already yielded promising results for process optimization. Catalytic wall reactors have proved to be a helpful tool for kinetic studies. On the laboratory scale, membrane reactors have displayed favorable behavior in selective oxidation. The Na vapor-catalyzed dehydrogenation of methanol to formaldehyde is a final example of an unusual new concept for selective oxidation. 相似文献
975.
Metal Science and Heat Treatment - 相似文献
976.
We present a new algorithm for blind symbol synchronization in orthogonal frequency division multiplexing (OFDM) systems. The new algorithm declares symbol synchronization when a certain autocorrelation matrix, constructed from the received signal, achieves minimum rank. Unlike previously proposed blind algorithms, the new rank method guarantees correct symbol synchronization, even in the presence of intersymbol interference. Also, it does not assume that the OFDM time samples are i.i.d. In particular, the rank method works even with OFDM systems that employ pulse shaping. The increased complexity of the algorithm would be acceptable for systems, such as fixed-receiver broadcast systems, that require guaranteed synchronization under all conditions. 相似文献
977.
Chang-Ho Lee Sutono A. Sangwoo Han Kyutae Lim Pinel S. Tentzeris E.M. Laskar J. 《Advanced Packaging, IEEE Transactions on》2002,25(3):374-384
Presents design, implementation, and measurement of a three-dimensional (3-D)-deployed RF front-end system-on-package (SOP) in a standard multi-layer low temperature co-fired ceramic (LTCC) technology. A compact 14 GHz GaAs MESFET-based transmitter module integrated with an embedded bandpass filter was built on LTCC 951AT tapes. The up-converter MMIC integrated with a voltage controlled oscillator (VCO) exhibits a measured up-conversion gain of 15 dB and an IIP3 of 15 dBm, while the power amplifier (PA) MMIC shows a measured gain of 31 dB and a 1-dB compression output power of 26 dBm at 14 GHz. Both MMICs were integrated on a compact LTCC module where an embedded front-end band pass filter (BPF) with a measured insertion loss of 3 dB at 14.25 GHz was integrated. The transmitter module is compact in size (400 /spl times/ 310 /spl times/ 35.2 mil/sup 3/), however it demonstrated an overall up-conversion gain of 41 dB, and available data rate of 32 Mbps with adjacent channel power ratio (ACPR) of 42 dB. These results suggest the feasibility of building highly SOP integrated RF front ends for microwave and millimeter wave applications. 相似文献
978.
This letter further discusses the difference between different definitions of voltage unbalance. Contrary to an earlier letter (see P. Pillay et al., ibid., vol.5, p.50-1, 2001), it is concluded that different definitions may give significantly different results. The two IEEE definitions that were not discussed in the earlier letter give different results and both deviate significantly from the true value (ratio of negative, and positive-sequence voltage) when a zero-sequence component is present. 相似文献
979.
Small-signal and temperature noise model for MOSFETs 总被引:1,自引:0,他引:1
980.