全文获取类型
收费全文 | 591664篇 |
免费 | 7063篇 |
国内免费 | 2182篇 |
专业分类
电工技术 | 11320篇 |
综合类 | 535篇 |
化学工业 | 88251篇 |
金属工艺 | 24727篇 |
机械仪表 | 18746篇 |
建筑科学 | 13482篇 |
矿业工程 | 2560篇 |
能源动力 | 16170篇 |
轻工业 | 47837篇 |
水利工程 | 5573篇 |
石油天然气 | 8933篇 |
武器工业 | 20篇 |
无线电 | 71954篇 |
一般工业技术 | 116247篇 |
冶金工业 | 113892篇 |
原子能技术 | 11991篇 |
自动化技术 | 48671篇 |
出版年
2021年 | 5054篇 |
2020年 | 3779篇 |
2019年 | 4879篇 |
2018年 | 8199篇 |
2017年 | 8147篇 |
2016年 | 8735篇 |
2015年 | 5777篇 |
2014年 | 9414篇 |
2013年 | 28250篇 |
2012年 | 15314篇 |
2011年 | 20847篇 |
2010年 | 16658篇 |
2009年 | 18734篇 |
2008年 | 19487篇 |
2007年 | 19325篇 |
2006年 | 17492篇 |
2005年 | 15857篇 |
2004年 | 15016篇 |
2003年 | 14982篇 |
2002年 | 14491篇 |
2001年 | 14373篇 |
2000年 | 13351篇 |
1999年 | 14264篇 |
1998年 | 36315篇 |
1997年 | 25312篇 |
1996年 | 19646篇 |
1995年 | 14610篇 |
1994年 | 12883篇 |
1993年 | 12823篇 |
1992年 | 9244篇 |
1991年 | 8866篇 |
1990年 | 8523篇 |
1989年 | 8131篇 |
1988年 | 7832篇 |
1987年 | 6856篇 |
1986年 | 6709篇 |
1985年 | 7659篇 |
1984年 | 6976篇 |
1983年 | 6291篇 |
1982年 | 5861篇 |
1981年 | 5962篇 |
1980年 | 5560篇 |
1979年 | 5317篇 |
1978年 | 5277篇 |
1977年 | 6238篇 |
1976年 | 8556篇 |
1975年 | 4529篇 |
1974年 | 4283篇 |
1973年 | 4307篇 |
1972年 | 3625篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
151.
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
152.
In this letter, bandpass filters with one or two tunable transmission zeros are proposed. The reconfigurable transmission zeros are implemented through varactors in combination with tapped quarter-wavelength stubs. Based on mixed mode simulations including EM simulation and circuit simulation, the proposed filters were designed and fabricated on low-cost FR4 board. The measurement results verified the design concepts. 相似文献
153.
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
154.
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
155.
Towards intelligent dressing 总被引:1,自引:0,他引:1
Egana I. Mendikute A. Urionaguena X. Alberdi H. 《IEEE instrumentation & measurement magazine》2006,9(3):38-43
The aim of this article is to state the principles of an intelligent monitoring and control system for the grinding machine, comprising the dressing process as well as grinding stability. 相似文献
156.
157.
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
158.
159.
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
160.
Over the last five years, many activities have focused on the unexploited field of carrying out reactions on small scales. Due to the rapid development of new components, this paper deals with recent developments only in a compressed form. An important point is the analysis of possible plant concepts for microreactors and whether these are a sensible option. Due to the enormous difference in size between the microchannels and the fluid periphera of possible components this is not just a technical question. It touches on the microtechnology concept as a whole. The direction in which the field should be developed and which measures can be taken to influence its development are questions that are addressed here with respect to the big industrial interest in microreactors. 相似文献