全文获取类型
收费全文 | 199505篇 |
免费 | 2501篇 |
国内免费 | 612篇 |
专业分类
电工技术 | 3468篇 |
综合类 | 149篇 |
化学工业 | 28968篇 |
金属工艺 | 8049篇 |
机械仪表 | 6068篇 |
建筑科学 | 4811篇 |
矿业工程 | 1303篇 |
能源动力 | 5127篇 |
轻工业 | 17123篇 |
水利工程 | 2297篇 |
石油天然气 | 4040篇 |
武器工业 | 8篇 |
无线电 | 23066篇 |
一般工业技术 | 40012篇 |
冶金工业 | 37241篇 |
原子能技术 | 5087篇 |
自动化技术 | 15801篇 |
出版年
2021年 | 1604篇 |
2019年 | 1569篇 |
2018年 | 2573篇 |
2017年 | 2645篇 |
2016年 | 2775篇 |
2015年 | 1794篇 |
2014年 | 3074篇 |
2013年 | 8941篇 |
2012年 | 5050篇 |
2011年 | 7021篇 |
2010年 | 5590篇 |
2009年 | 6353篇 |
2008年 | 6445篇 |
2007年 | 6511篇 |
2006年 | 5580篇 |
2005年 | 5228篇 |
2004年 | 5005篇 |
2003年 | 4887篇 |
2002年 | 4525篇 |
2001年 | 4656篇 |
2000年 | 4579篇 |
1999年 | 4729篇 |
1998年 | 11376篇 |
1997年 | 8129篇 |
1996年 | 6408篇 |
1995年 | 4674篇 |
1994年 | 4271篇 |
1993年 | 4075篇 |
1992年 | 3223篇 |
1991年 | 3076篇 |
1990年 | 2870篇 |
1989年 | 2951篇 |
1988年 | 2826篇 |
1987年 | 2398篇 |
1986年 | 2308篇 |
1985年 | 2715篇 |
1984年 | 2505篇 |
1983年 | 2345篇 |
1982年 | 2091篇 |
1981年 | 2203篇 |
1980年 | 2029篇 |
1979年 | 2124篇 |
1978年 | 2120篇 |
1977年 | 2366篇 |
1976年 | 3189篇 |
1975年 | 1833篇 |
1974年 | 1761篇 |
1973年 | 1772篇 |
1972年 | 1443篇 |
1971年 | 1339篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
An accurate printer model that is efficient enough to be used by halftoning algorithms is proposed. The proposed signal processing model (SPM) utilizes a physical model to train adaptive linear combiners (ALCs), after which the average exposure of each subpixel for any input pattern can be calculated using the optimized weight vector. The SPM can be used to model multi-level halftoning and resolution enhancement, as well as traditional halftoning. The SPM is comprised of a single ALC layer followed by a peak-to-average ratio (PAR) correction layer, which serves to produce a PAR of less than 1.5 in the modeled exposure. The PCN (PAR correction network) employs one ALC/pixel and exploits the physics governing the characteristics of exposure in small regions. A relatively small number of training patterns suffices to train the SPM. 相似文献
132.
Y.K. Su H.C. Wang C.L. Lin W.B. Chen S.M. Chen 《Photonics Technology Letters, IEEE》2003,15(10):1345-1347
The brightness of AlGaInP light emitting diodes (LEDs) has been raised by a factor of 1.12 at 20 mA by sulfide passivation. Meanwhile, the sulfide also can decrease leakage current of AlGaInP LEDs at -2 V to nearly one thousandth of that in the as-fabricated device. The possible causes for the brightness increase of AlGaInP LEDs after sulfide treatment including surface roughness, reduction of Fresnel loss, and effective injection of carriers were demonstrated. 相似文献
133.
134.
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
135.
The continuing progression of Moore's Law has enabled the miniaturisation and dramatic cost reduction in electronics over the last ten years. For a truly pervasive communications environment the challenges of hiding key hardware technologies from the user are rapidly being overcome. This paper reviews the status of these hardware technology developments in the pervasive space and briefly discusses other contributing factors that will enable the pervasive vision to be realised. 相似文献
136.
G. Murali Dhar B. N. Srinivas M. S. Rana Manoj Kumar S. K. Maity 《Catalysis Today》2003,86(1-4):45-60
Support effects form important aspect of hydrodesulfurization (HDS) studies and mixed oxide supports received maximum attention in the last two decades. This review will focus attention on studies on mixed oxide supported Mo and W catalysts. For convenience of discussion, these are divided into Al2O3 containing mixed oxide supports, TiO2 containing mixed oxide supports, ZrO2 containing mixed oxide supports and other mixed oxide supports containing all the rest. TiO2 containing mixed oxides received maximum attention, especially TiO2–Al2O3 supported catalysts. A brief discussion about their prospects for application to ultradeep desulfurization is also included. An overview of the available literature with emphasis on research carried out in our laboratory form the contents of this publication. 相似文献
137.
J. B. Quinn G. E. Schumacher L. W. Schultheis 《Journal of Failure Analysis and Prevention》2004,4(1):41-46
Several days after heart surgery, a patient discovered his upper right canine tooth had broken at the root. Such tooth damage,
recognized post-operatively, is usually assumed to be caused by blunt mechanical force from an instrument used by the anesthesiologist
during placement of a breathing tube at the start of surgery.
In this case, the patient had saved the crown portion of the broken tooth, and it was possible to examine the root fracture
characteristics. The curvature and direction of the crack path and natural tooth situation suggested that failure could be
described through a cantilever beam model. This was confirmed when a whole extracted sample tooth was embedded and broken
by a measured force in a manner consistent with the model. The resulting fracture surface matched that of the patient’s broken
canine tooth. However, the high load and force direction necessary to fracture the root was inconsistent with forces applied
during the anesthesia procedure. The failure analysis and further investigation indicated tooth clenching on the breathing
tube during recovery was the likely cause of fracture.
This paper presents an alternate explanation for intubation-related dental injury, demonstrates the practicality of fractographic
analysis of biological materials, and introduces a methodology for simulating in vitro tooth settings for mechanical testing. 相似文献
138.
Optimal space-time constellations from groups 总被引:10,自引:0,他引:10
Hughes B.L. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(2):401-410
We consider the design of space-time constellations based on group codes for fading channels with multiple transmit and receive antennas. These codes can be viewed as multiantenna extensions of phase-shift keying (PSK), in the sense that all codewords have equal energy, all are rotations of a fixed codeword, and there is a simple differential transmission rule that allows data to be sent without channel estimates at the transmitter or receiver. For coherent detection, we show that all optimal full-rank space-time group codes are unitary (each code matrix has equal-energy, orthogonal rows). This leads to a simpler code design criterion and suggests that unitary codes may play an important role in coherent as well as noncoherent communication. For any number of transmit antennas t, we then use the design criterion to characterize all full-rank unitary space-time group codes of minimum block length (also t) which have 2/sup p/ codewords. These results allow us to characterize all optimal 2/sup p/-ary unitary group codes with square code matrices. This restricted class of block codes matches the class proposed for differential modulation by Hughes (see IEEE Trans. Inform. Theory, vol.46, p.2567-78, Nov. 2000), and by Hochwald and Sweldens (see IEEE Trans. Commun., vol.48, p.2041-2052, Dec. 2000). 相似文献
139.
A fundamental problem for regulatory networks is to understand the relation between form and function: to uncover the underlying design principles of the network. Circadian clocks present a particularly interesting instance, as recent work has shown that they have complex structures involving multiple interconnected feedback loops with both positive and negative feedback. While several authors have speculated on the reasons for this, a convincing explanation is still lacking.We analyse both the flexibility of clock networks and the relationships between various desirable properties such as robust entrainment, temperature compensation, and stability to environmental variations and parameter fluctuations. We use this to argue that the complexity provides the flexibility necessary to simultaneously attain multiple key properties of circadian clocks. As part of our analysis we show how to quantify the key evolutionary aims using infinitesimal response curves, a tool that we believe will be of general utility in the analysis of regulatory networks. Our results suggest that regulatory and signalling networks might be much less flexible and of lower dimension than their apparent complexity would suggest. 相似文献
140.
This paper presents an analysis and design procedure for a buoyant cable array antenna used for satellite communications. The towed multi-element array is susceptible to washovers and fluctuations due to the ocean wave interaction with the floating hose where the elements are housed. A model of system performance for coherent multi-element combining is presented based on the dropouts and fades of the individual elements. The array design, including the number and spacing of elements, is shown to be highly dependent on the individual element exposure characteristics and the degree of correlation of the fades between channels. The joint statistics of the array are analyzed using a binary conditional element exposure model and a first-order Markov interelement correlation model. Using this analysis, a family of array designs meeting the communications requirements is produced. 相似文献