全文获取类型
收费全文 | 529894篇 |
免费 | 9488篇 |
国内免费 | 2445篇 |
专业分类
电工技术 | 9920篇 |
技术理论 | 5篇 |
综合类 | 1332篇 |
化学工业 | 85193篇 |
金属工艺 | 21163篇 |
机械仪表 | 15563篇 |
建筑科学 | 13416篇 |
矿业工程 | 3284篇 |
能源动力 | 12025篇 |
轻工业 | 50683篇 |
水利工程 | 5816篇 |
石油天然气 | 11679篇 |
武器工业 | 262篇 |
无线电 | 55835篇 |
一般工业技术 | 101869篇 |
冶金工业 | 97678篇 |
原子能技术 | 11680篇 |
自动化技术 | 44424篇 |
出版年
2021年 | 4934篇 |
2019年 | 4290篇 |
2018年 | 7605篇 |
2017年 | 7565篇 |
2016年 | 8157篇 |
2015年 | 5730篇 |
2014年 | 9187篇 |
2013年 | 23788篇 |
2012年 | 15071篇 |
2011年 | 19431篇 |
2010年 | 15710篇 |
2009年 | 17031篇 |
2008年 | 18216篇 |
2007年 | 18049篇 |
2006年 | 15733篇 |
2005年 | 14142篇 |
2004年 | 12942篇 |
2003年 | 12293篇 |
2002年 | 12303篇 |
2001年 | 12005篇 |
2000年 | 11288篇 |
1999年 | 11378篇 |
1998年 | 27419篇 |
1997年 | 20013篇 |
1996年 | 15333篇 |
1995年 | 11589篇 |
1994年 | 10509篇 |
1993年 | 10231篇 |
1992年 | 7825篇 |
1991年 | 7592篇 |
1990年 | 7460篇 |
1989年 | 7495篇 |
1988年 | 7145篇 |
1987年 | 6166篇 |
1986年 | 6161篇 |
1985年 | 7018篇 |
1984年 | 6753篇 |
1983年 | 6203篇 |
1982年 | 5677篇 |
1981年 | 5993篇 |
1980年 | 5637篇 |
1979年 | 5737篇 |
1978年 | 5696篇 |
1977年 | 6336篇 |
1976年 | 8271篇 |
1975年 | 5120篇 |
1974年 | 4865篇 |
1973年 | 4910篇 |
1972年 | 4204篇 |
1971年 | 3851篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Waveband-switchable SOA ring laser constructed with a phase modulator loop mirror filter 总被引:1,自引:0,他引:1
M.P. Fok K.L. Lee C. Shu 《Photonics Technology Letters, IEEE》2005,17(7):1393-1395
An electrically tunable multiwavelength source has been developed using a birefringence-based optical comb filter in a semiconductor optical amplifier ring laser. The filter is constructed with an electrooptic phase modulator placed inside a fiber loop mirror. By controlling the birefringence of the modulator through the applied bias, the radio-frequency power, or the modulation frequency, we achieve a continuous shift of the spectral comb to access different interleaved wavebands. Electrical waveband switching has been successfully demonstrated for 21 wavelengths at 100-GHz grid spacing with an optical signal-to-noise ratio over 40 dB. 相似文献
992.
Widely tunable bottom-emitting vertical-cavity SOAs 总被引:1,自引:0,他引:1
Cole G.D. Bjorlin E.S. Wang C.S. MacDonald N.C. Bowers J.E. 《Photonics Technology Letters, IEEE》2005,17(12):2526-2528
We present bottom-emitting tunable vertical-cavity semiconductor optical amplifiers (VCSOAs) with an effective wavelength tuning range of >20 nm. These devices utilize a high reflectivity micromechanically tunable Bragg mirror as the back reflector. Compared with our first generation tunable VCSOAs, the bottom-emitting devices exhibit a two-fold increase in the effective tuning range as well as a five-fold reduction in the required tuning voltage. 相似文献
993.
In asynchronous transfer mode (ATM) networks, fixed length cells of 53 bytes are transmitted. A cell may be discarded during transmission due to buffer overflow or a detection of errors. Cell discarding seriously degrades transmission quality. The quality degradation can be reduced by employing efficient forward error control (FEC) to recover discarded cells. In this paper, we present the design and implementation of decoding equipment for FEC in ATM networks based on a single parity check (SPC) product code using very‐large‐scale integration (VLSI) technology. FEC allows the destination to reconstruct missing data cells by using redundant parity cells that the source adds to each block of data cells. The functionality of the design has been tested using the Model Sim 5.7cXE Simulation Package. The design has been implemented for a 5 ° 5 matrix of data cells in a Virtex‐E XCV 3200E FG1156 device. The simulation and synthesis results show that the decoding function can be completed in 81 clock cycles with an optimum clock of 56.8 MHz. A test bench was written to study the performance of the decoder, and the results are presented. 相似文献
994.
V. V. Andrikanis B. V. Andreev K. B. Rudyak F. V. Karpeko A. V. Ivanov N. N. Parsent’ev V. S. Nikitchenko G. V. Danilov I. A. Sinel’nikov 《Chemistry and Technology of Fuels and Oils》2006,42(3):229-234
Increasing attention at oil refineries is being focused on solving environmental problems caused by stiffening of the requirements
for emissions of harmful substances into air and water. Advanced technologies that reduce the effect of industrial enterprises
on the environment to the minimum are becoming increasingly in demand. Treatment of process wastewaters to remove hydrogen
sulfide and ammonia is one such problem of the modern refinery with cat crackers.
__________
Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 3, pp. 52–55, May–June, 2006. 相似文献
995.
Several coats, such as gray cast iron, cast iron alloyed with nickel and copper, nickel-aluminum, Nichrome (nickel-chromium
alloy), bronze, etc., have been investigated. Corrosion resistance of coats in a medium based on drilling mud and sodium chloride
and sulfate solutions has been checked. Alloyed cast iron admixed with tin-based compounds showed the best results.
__________
Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 3, pp. 43–44, March, 2006. 相似文献
996.
With the creation of balancing authorities by the North American Reliability Council that span large portions of the North American interconnection, and stringent requirements for real time monitoring of power system evolution, faster and more accurate state estimation algorithms that can efficiently handle systems of very large sizes are needed in the present environment. This paper presents a distributed state estimation algorithm suitable for large-scale power systems. Synchronized phasor measurements are applied to aggregate the voltage phase angles of each decomposed subsystem in the distributed state estimation solution. The aggregated state estimation solution is obtained from the distributed solution using a sensitivity analysis based update at chosen boundary buses. Placement of synchronized phasor measurements in the decomposed subsystems is also investigated in this paper. Test results on the IEEE 118-bus test bed are provided 相似文献
997.
Su M.F. Reda Taha M.M. Christodoulou C.G. El Kady I. 《Photonics Technology Letters, IEEE》2008,20(10):761-763
Processing methods used in photonics and nanotechnology possess many limitations restricting their application areas such as high cost, inability to produce fine details, problems with scalability, and long processing time. Proximity field nanopatterning is a lithography method which surpasses these limitations. By using interference patterns produced by a two-dimensional phase mask, the technique is able to generate a submicron detailed exposure on a millimeter-size slab of light sensitive photopolymer, which is then developed like a photographic plate to reveal three-dimensional interference patterns from the phase mask. While it is possible to use simulations to obtain the interference patterns produced by a phase mask, realizing the mask dimensions necessary for producing a desired interference pattern is analytically challenging due to the intricacies of light interactions involved in producing the final interference pattern. An alternative method is to iteratively optimize the phase mask until the interference patterns obtained converge to the desired pattern. However, depending on the optimization technique used, one either risks a significant probability of failure or requires a prohibitive number of iterations. We argue that an optimization technique that is to take advantage of the physics of the problem using machine learning methods (here fuzzy learning) can lead to competent mask design. This technique is described in this letter. 相似文献
998.
A 0.9 V 92 dB Double-Sampled Switched-RC Delta-Sigma Audio ADC 总被引:1,自引:0,他引:1
Min Gyu Kim Gil-Cho Ahn Hanumolu P.K. Sang-Hyeon Lee Sang-Ho Kim Seung-Bin You Jae-Whui Kim Temes G.C. Un-Ku Moon 《Solid-State Circuits, IEEE Journal of》2008,43(5):1195-1206
A 0.9 V third-order double-sampled delta-sigma audio ADC is presented. A new method using a combination of a switched-RC technique and a floating switched-capacitor double-sampling configuration enabled low-voltage operation without clock boosting or bootstrapping. A three-level quantizer with simple dynamic element matching was used to improve linearity. The prototype IC implemented in a 0.13 CMOS process achieves 92 dB DR, 91 dB SNR and 89 dB SNDR in a 24 kHz audio signal bandwidth, while consuming 1.5 mW from a 0.9 V supply. The prototype operates from 0.65 V to 1.5 V supply with minimal performance degradation. 相似文献
999.
Sol–gel films are actively investigated during the last decade as possible candidates for environmentally friendly pre-treatments. However, the important drawback in this case is the lack of active corrosion protection and self-healing ability. 相似文献
1000.
Heat resistant coatings are required primarily for stacks, exhaust pipes, reactors, space crafts and similar equipments that are permanently or occasionally exposed to elevated temperatures. High-temperature coatings are generally based on silicone resin with ceramic and metallic pigments. In this study, iron oxide, cobalt oxide (thermo chromic compound) and aluminum oxide are used for the preparation of four new types of coloured pigments. The thermal resistant characteristics of these ceramic pigments were studied by differential thermal analysis, thermo gravimetric analysis and differential scanning calorimetric analysis. These ceramic pigments are found to be thermally stable up to 400 °C. 相似文献