全文获取类型
收费全文 | 541474篇 |
免费 | 6361篇 |
国内免费 | 1917篇 |
专业分类
电工技术 | 10166篇 |
综合类 | 404篇 |
化学工业 | 81613篇 |
金属工艺 | 22450篇 |
机械仪表 | 17284篇 |
建筑科学 | 11980篇 |
矿业工程 | 2841篇 |
能源动力 | 15168篇 |
轻工业 | 41335篇 |
水利工程 | 5555篇 |
石油天然气 | 10126篇 |
武器工业 | 19篇 |
无线电 | 64264篇 |
一般工业技术 | 108185篇 |
冶金工业 | 101539篇 |
原子能技术 | 12178篇 |
自动化技术 | 44645篇 |
出版年
2021年 | 4833篇 |
2020年 | 3606篇 |
2019年 | 4677篇 |
2018年 | 7936篇 |
2017年 | 7919篇 |
2016年 | 8464篇 |
2015年 | 5496篇 |
2014年 | 9066篇 |
2013年 | 26017篇 |
2012年 | 14687篇 |
2011年 | 19894篇 |
2010年 | 15907篇 |
2009年 | 18122篇 |
2008年 | 18391篇 |
2007年 | 18221篇 |
2006年 | 16091篇 |
2005年 | 14643篇 |
2004年 | 13877篇 |
2003年 | 13555篇 |
2002年 | 13012篇 |
2001年 | 12844篇 |
2000年 | 12124篇 |
1999年 | 12686篇 |
1998年 | 32448篇 |
1997年 | 22479篇 |
1996年 | 17262篇 |
1995年 | 12896篇 |
1994年 | 11238篇 |
1993年 | 11261篇 |
1992年 | 8133篇 |
1991年 | 7704篇 |
1990年 | 7630篇 |
1989年 | 7281篇 |
1988年 | 6950篇 |
1987年 | 6173篇 |
1986年 | 5982篇 |
1985年 | 6749篇 |
1984年 | 6252篇 |
1983年 | 5673篇 |
1982年 | 5242篇 |
1981年 | 5382篇 |
1980年 | 4926篇 |
1979年 | 4840篇 |
1978年 | 4812篇 |
1977年 | 5481篇 |
1976年 | 7172篇 |
1975年 | 4140篇 |
1974年 | 3948篇 |
1973年 | 4063篇 |
1972年 | 3393篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The emerging software defined radio technologies will be an enabler for a new generation of dynamic wireless systems. It will also open up the possibility of allocating frequencies in a, more dynamic way than today. From an intersystem-interference point of view, this can cause unforeseen problems to occur due to the increased complexity in such applications. In such applications, a measure indicating whether or not a frequency band is possible to use from an electromagnetic interference point of view, must be found. A simple approach is to use the measured total average interference power within the receiver band. Since the interference impact on modern digital communication systems from an interference signal does not only depend on the power but also on the actual waveform of the interference signal, some kind of quality measure of the average-power approach would be convenient to use. In this paper, we introduce a simple quality measure of the average-power approach so that a rough adjustment for the interference-waveform properties can be done. 相似文献
132.
Reekmans S. De Maeyer J. Rombouts P. Weyten L. 《IEEE transactions on circuits and systems. I, Regular papers》2006,53(12):2529-2538
Quadrature sigma-delta analog-to-digital converters require a feedback path for both the I and the Q parts of the complex feedback signal. If two separate multibit feedback digital-to-analog converters (DACs) are used, mismatch among the unit DAC elements leads to additional mismatch noise in the output spectrum as well as an I/Q imbalance. This paper proposes new quadrature bandpass (QBP) mismatch shaping techniques. In our approach, the I and Q DACs are merged into one complex DAC, which leads to near-perfect I/Q balance. To select the unit DAC elements of the complex multibit DAC, the well-known butterfly shuffler and tree structure are generalized towards a complex structure, and necessary constraints for their correct functioning are derived. Next, a very efficient first-order QBP shaper implementation is proposed. Finally, the newly presented complex structures are simulated to prove their effectiveness and are compared with each other with respect to performance 相似文献
133.
Fifty-eight commercial yogurt products of seven brands (which claimed to include bifidobacteria) were obtained from local stores in Greensboro, North Carolina, USA. These products were examined at 0, 1, 2, 3 and 4 weeks for the viability of bifidobacteria and yogurt starter culture during refrigerated storage at 4°C. Our results showed that bifidobacteria counts were variable, ranging from 0 to 5.5 log cfu/mL. The average yogurt starter culture counts ranged from 5.20 to 8.87 log cfu/mL and 7.51–8.94 log cfu/mL for Lactobacillus delbrueckii spp. bulgaricus and Streptococcus thermophilus , respectively. Of the 58 products tested, only 44 products (76%) contained viable cultures. Viability of bifidobacteria in yogurt samples remained within the same levels during 3 weeks of storage at 4°C; however, the bacterial count started to decline during the fourth week. These results suggest optimal beneficial consumption of yogurt foods with live bifidobacteria should occur within 3 weeks of production. Results obtained from this research could be used by the industry to develop new technologies to ensure consumers receive high-quality products. 相似文献
134.
Solar cells based on SnO2/Cd0.4Zn0.6S/CdTe heterostructures are fabricated by electrochemical deposition, and the dependences of their electrical and photoelectric properties on the thermal annealing conditions are studied. It is shown that thermal annealing reduces the tunnel currents by almost two orders of magnitude. The best conditions of thermal annealing are determined (t = 300°C and τ = 9 min). These conditions provide the highest photosensitivity of the heterostructures under study (I sc ≈ 21.2 mA/cm2, U oc ≈ 813 mV, and η = 14.7%). 相似文献
135.
This paper provides an overview of the main aspects of modern fluorescence microscopy. It covers the principles of fluorescence and highlights the key discoveries in the history of fluorescence microscopy. The paper also discusses the optics of fluorescence microscopes and examines the various types of detectors. It also discusses the signal and image processing challenges in fluorescence microscopy and highlights some of the present developments and future trends in the field. 相似文献
136.
137.
Finite element piezothermoelasticity analysis and the active control of FGM plates with integrated piezoelectric sensors and actuators 总被引:3,自引:0,他引:3
An efficient finite element model is presented for the static and dynamic piezothermoelastic analysis and control of FGM
plates under temperature gradient environments using integrated piezoelectric sensor/actuator layers. The properties of an FGM plate are functionally graded in the thickness
direction according to a volume fraction power law distribution. A constant displacement-cum-velocity feedback control algorithm that couples the direct and inverse piezoelectric effects is applied to provide active feedback
control of the integrated FGM plate in a closed loop system. Numerical results for the static and dynamic control are presented
for the FGM plate, which consists of zirconia and aluminum. The effects of the constituent volume fractions and the influence
of feedback control gain on the static and dynamic responses of the FGM plates are examined.
Received: 13 March 2002 / Accepted: 5 March 2003
The work described in this paper was supported by a grant awarded by the Research Grants Council of the Hong Kong Special
Administrative Region, China (Project No. CityU 1024/01E). 相似文献
138.
139.
Seidel A.R. Bisogno F.E. Pinheiro H. do Prado R.N. 《Industrial Electronics, IEEE Transactions on》2003,50(6):1267-1274
This paper presents a simple alternative for an electronic ballast operating in self-sustained oscillating mode with dimming capability for fluorescent lamps. A simple modification in one of the gate drivers side circuit allows the lamp to dim without compromising the simplicity, reliability, and low cost which characterize the self-oscillating electronic ballast (SOEB). A qualitative analysis is presented to explain the behavior of the proposed self-oscillating electronic ballast with dimming feature. In addition, the stability and the key equations for the design are derived using the extended Nyquist criterion and describing function method. Experimental results from two 40-W electronic ballasts are presented to demonstrate the performance and to validate the analysis carried out. 相似文献
140.
In this work it is presented a study on the residence time distribution (RTD) of particles in a co-current pilot-plant spray dryer operated with a rotary atomization system. A nuclear technique is applied to investigate the RTD responses of spray dryers. The methodology is based on the injection of a radioisotope tracer in the feed stream followed by the monitoring of its concentration at the outlet stream. The experiments were performed during the drying of aqueous suspensions of gadolinium oxide. The RTD responses obtained experimentally presented good reproducibility, indicating that the technique applied is well suited to investigating fluid-dynamics of spray dryers. In addition to the experimental investigation, a mathematical model was used to describe the RTD experimental curves. 相似文献