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81.
Cognitive Radio Networks (CRNs) have been receiving significant research attention recently due to their ability to solve issues associated with spectrum congestion and underutilization. In a CRN, unlicensed users (or Secondary Users, SUs) are able to exploit and use underutilized licensed channels, but they must evacuate the channels if any interference is caused to the licensed users (or Primary Users, PUs) who own the channels. Due to the dynamicity of spectrum availability in CRNs, design of protocols and schemes at different layers of the SU’s network stack has been challenging. In this article, we focus on routing and discuss the challenges and characteristics associated with it. Subsequently, we provide an extensive survey on existing routing schemes in CRNs. Generally speaking, there are three categories of challenges, namely channel-based, host-based, and network-based. The channel-based challenges are associated with the operating environment, the host-based with the SUs, and the network-based with the network-wide SUs. Furthermore, the existing routing schemes in the literature are segregated into three broad categories based on the relationship between PUs and SUs in their investigation, namely intra-system, inter-system, and hybrid-system; and within each category, they are further categorized based on their types, namely Proactive, Reactive, Hybrid, and Adaptive Per-hop. Additionally, we present performance enhancements achieved by the existing routing schemes in CRNs. Finally, we discuss various open issues related to routing in CRNs in order to establish a foundation and to spark new interests in this research area. 相似文献
82.
Musa S. van Weerden H.J. Yau T.H. Lambeck P.V. 《Quantum Electronics, IEEE Journal of》2000,36(9):1089-1097
Er-doped Al2O3 thin films have been deposited by reactive co-sputtering onto thermally oxidized Si-wafers. The deposition process has been optimized with respect to the requirements originating from the application of these multilayer structures as integrated optical amplifiers for the third telecom window, i.e., the wavelength range 1.52-1.55 μm. The films obtained at a substrate temperature of only 400°C are amorphous and show a homogenous structure, without columns or grains. For slabguides, background losses smaller than 0.25 dB/cm have been obtained, even without any annealing. A relatively broad luminescence band, having an FWHM of ~55 nm around the 1.533-μm wavelength, has been measured. From gain versus pumping power curves, an upconversion coefficient lower then 20·10-25 m3/s has been derived, being half of the values reported up to now in the literature. Simulations based on experimentally determined material parameters and assuming a channel attenuation of 0.5 dB/cm indicate, for 0.24 at.% Er channel devices with an optimal channel length of 7.7 cm, an amplification of 8 dB at 1.533 μm for a pump wavelength of 1.48 μm, and a pump power of only 8.7 mW 相似文献
83.
Jow-Lay Huang Yin-Tsan Jah Bao-Shun Yau Ching-Yun Chen Horng-Hwa Lu 《Thin solid films》2000,370(1-2):33-37
Indium tin oxide (ITO) films were deposited on acrylics by low temperature reactive magnetron sputtering. The effects of oxygen flow and bias voltage on the microstructure, surface morphology and bonding state of films were evaluated. In this investigation, X-ray photoelectron spectroscopy, X-ray diffraction, Atomic force microscope were used. It was found that the grain size of ITO films increased and surface roughness decreased with the increase of oxygen flow rate. The XPS spectra of In 3d and Sn 3d indicated that the oxygen flow had little effect on the binding energy of ITO films. The relative strength of O2−II increased, while that of O2−I decreased with increasing oxygen flow rate. The grain size increased with the bias voltage. However, at a maximum voltage of −90 V fine grains were detected due to the formation of numerous nuclei resulting from bombardment of high energy particles. The bias voltage had little effect on the bonding state of In, Sn and O ions. 相似文献
84.
Jren-Chit Chin Rautenberg J.M. Ma C.Y.T. Pujol S. Yau D.K.Y. 《IEEE sensors journal》2009,9(11):1361-1369
In this paper, we present the design, implementation, and experimental evaluation of a wireless sensor network for real-time structural ldquohealthrdquo monitoring. We use simple custom-built gages to detect cracks in critical structural elements. The main data reports require no structural analysis for interpretation, have a low data rate, and are naturally resilient to loss. We show how a variety of low-cost, off-the-shelf data acquisition/communication devices can be used to support remote monitoring by a control center. The heterogeneous hardware is accommodated by the use of open technology standards and a software architecture that is portable, modular, and highly configurable. We present an experimental evaluation of our structural-assessment network done using a full-scale three-story reinforced concrete building that was tested under lateral forces emulating forces induced by earthquakes. Our results show that a set of 12 strategically positioned sensors achieved a 100% detection rate for cracks crossing sensors and a zero false-alarm rate (in the sense that all signals exceeding a preset threshold were traced to cracks exceeding a specified total width). 相似文献
85.
Tasopoulos-Chan Marina; Smetana Judith G.; Yau Jenny P. 《Canadian Metallurgical Quarterly》2009,23(3):364
Strategies for managing information about activities to parents, including partial disclosure, avoidance, lying, and full disclosure, were examined in 479 American adolescents (M = 16.38 years, SD = 0.77) varying in generational status and from Mexican, Chinese, and European backgrounds. Information management strategies for personal, prudential, and overlapping (multifaceted) activities as defined within social domain theory were examined. With age, parental education, and generational status controlled, Chinese American adolescents partially disclosed more to mothers about personal and multifaceted activities than did Mexican American adolescents and more to fathers about personal activities than did European American teens. In contrast, European and Mexican American adolescents fully disclosed more to mothers about personal activities than did Chinese-origin adolescents. Strategies varied by generation among Chinese American youth; second-generation adolescents avoided discussing activities with parents more than did immigrants. Adolescents who fully disclosed about all activities and lied less about multifaceted and personal activities reported stronger endorsement of obligations to assist their families, more trust in parents, and less problem behavior. More depressed mood was associated with more lying about personal activities. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
86.
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89.
Function point analysis is a widely cited method for estimating software project size, which is an important activity of project management. At the beginning stage of planning, the top-down approach can be applied. Having obtained more systems specifications at later stages, the bottom-up approach might also be used to improve the accuracy of the estimation. However, the bottom-up approach is not a conventional way of function point analysis. There was no empirical evidence showing the difference between the fully informed top-down approach and the bottom-up approach. Through the implementation of a function point analysis system in an in-house software development department, this paper compares the results of the two approaches. This comparison study shows that the bottom-up approach does not contribute a significant added value to a fully-informed top-down approach. Therefore, the fully-informed top down approach has been chosen as a method for building a software metric database in the organization. More important, the observations and experience gained from this project may help in-house development organizations to establish their own function point analysis systems. 相似文献
90.
Reverse engineering of engine intake ports by digitization and surface approximation 总被引:2,自引:0,他引:2
Hong-Tzong Yau 《International Journal of Machine Tools and Manufacture》1997,37(6):855-871
This paper describes a research development for the design of engine intake ports using reverse engineering techniques. The background is typical of many automotive applications where digitization and reverse engineering become necessary when the design is subject to changes during production or testing. After careful analysis of the problem, CMM contact measurement was selected to digitize the complex intake ports. Part segmentation and semi-automatic scanning were applied to the digitization process. For the purpose of data reduction and surface approximation, a new approach to the fitting of rational B-splines was developed. This is because rational B-splines have more flexibility and can approximate complex geometry more accurately than traditional Bezier or B-splines. Furthermore, skinning—a cross-sectional design technique—was utilized to construct the surface to reduce the computation cost. Surface merging was also implemented to maintain the surface boundary continuity. Finally, the enclosed surface volume is produced and can be transferred to commercial CAD/CAM systems through IGES translation. Examples with complex intake ports were described to validate the approach. 相似文献