全文获取类型
收费全文 | 1448篇 |
免费 | 57篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 52篇 |
化学工业 | 320篇 |
金属工艺 | 35篇 |
机械仪表 | 30篇 |
建筑科学 | 20篇 |
矿业工程 | 1篇 |
能源动力 | 129篇 |
轻工业 | 78篇 |
水利工程 | 9篇 |
石油天然气 | 6篇 |
无线电 | 146篇 |
一般工业技术 | 285篇 |
冶金工业 | 159篇 |
原子能技术 | 17篇 |
自动化技术 | 223篇 |
出版年
2024年 | 6篇 |
2023年 | 19篇 |
2022年 | 31篇 |
2021年 | 57篇 |
2020年 | 48篇 |
2019年 | 53篇 |
2018年 | 56篇 |
2017年 | 47篇 |
2016年 | 55篇 |
2015年 | 39篇 |
2014年 | 45篇 |
2013年 | 115篇 |
2012年 | 65篇 |
2011年 | 85篇 |
2010年 | 72篇 |
2009年 | 69篇 |
2008年 | 54篇 |
2007年 | 54篇 |
2006年 | 45篇 |
2005年 | 30篇 |
2004年 | 29篇 |
2003年 | 38篇 |
2002年 | 22篇 |
2001年 | 23篇 |
2000年 | 27篇 |
1999年 | 19篇 |
1998年 | 38篇 |
1997年 | 34篇 |
1996年 | 26篇 |
1995年 | 13篇 |
1994年 | 19篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 14篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 9篇 |
1978年 | 7篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1974年 | 3篇 |
排序方式: 共有1510条查询结果,搜索用时 15 毫秒
31.
Saha P.K. Udupa J.K. 《IEEE transactions on pattern analysis and machine intelligence》2001,23(7):689-706
Thresholding is a popular image segmentation method that converts a gray-level image into a binary image. The selection of optimum thresholds has remained a challenge over decades. Besides being a segmentation tool on its own, often it is also a step in many advanced image segmentation techniques in spaces other than the image space. We introduce a thresholding method that accounts for both intensity-based class uncertainty-a histogram-based property-and region homogeneity-an image morphology-based property. A scale-based formulation is used for region homogeneity computation. At any threshold, intensity-based class uncertainty is computed by fitting a Gaussian to the intensity distribution of each of the two regions segmented at that threshold. The theory of the optimum thresholding method is based on the postulate that objects manifest themselves with fuzzy boundaries in any digital image acquired by an imaging device. The main idea here is to select that threshold at which pixels with high class uncertainty accumulate mostly around object boundaries. To achieve this, a threshold energy criterion is formulated using class-uncertainty and region homogeneity such that, at any image location, a high energy is created when both class uncertainty and region homogeneity are high or both are low. Finally, the method selects that threshold which corresponds to the minimum overall energy. The method has been compared to a maximum segmented image information method. Superiority of the proposed method was observed both qualitatively on clinical medical images as well as quantitatively on 250 realistic phantom images generated by adding different degrees of blurring, noise, and background variation to real objects segmented from clinical images 相似文献
32.
A stability-based distributed routing mechanism to support unicast and multicast routing in ad hoc wireless network 总被引:6,自引:0,他引:6
An ad hoc network can be envisioned as a collection of mobile routers, each equipped with a wireless transceiver, which are free to move about arbitrarily. In ad hoc wireless networks, even if two nodes are outside the wireless transmission range of each other, they may still be able to communicate in multiple hops using other intermediate nodes. However, the dynamics of these networks, as a consequence of mobility and disconnection of mobile hosts, pose a number of problems in designing routing schemes for effective communication between any pair of source and destination. In this paper, a stability-based unicast routing mechanism, that considers both link affinity and path stability in order to find out a stable route from source to destination, is proposed. It is then extended to support multicast routing as well where only local state information (at source) is utilized for constructing a multicast tree. The performance of the proposed scheme is evaluated on a simulated environment to show that the stability-based scheme provides a unified framework for both unicast and multicast routing and reduces the probability of route error drastically in both the cases. 相似文献
33.
In human designer usage, symbols have a rich semantics, grounded on experience, which permits flexible usage — e.g. design ideation is improved by meanings triggered by contrastive words. In computational usage however, symbols are syntactic tokens whose semantics is mostly left to the implementation, resulting in brittle failures in many knowledge-based systems. Here we ask if one may define symbols in computational design as {label,meaning} pairs, as opposed to merely the label. We consider three questions that must be answered to bootstrap a symbol learning process: (a) which concepts are most relevant in a given domain, (b) how to define the semantics of such symbols, and (c) how to learn labels for these so as to form a grounded symbol. We propose that relevant symbols may be discovered by learning patterns of functional viability. The stable patterns are information-conserving codes, also called chunks in cognitive science, which relate to the process of acquiring expertise in humans. Regions of a design space that contain functionally superior designs can be mapped to a lower-dimensional manifold; the inter-relations of the design variables discovered thus constitute the chunks. Using these as the initial semantics for symbols, we show how the system can acquire labels for them by communicating with human designers. We demonstrate the first steps in this process in our baby designer approach, by learning two early grounded symbols, tight and loose. 相似文献
34.
Nanocomposites of polyfuran (PF) and polythiophene (PTP) with montmorillonite clay (MMT) were prepared and modified by loading
of polyaniline (PANI) and polypyrrole (PPY) moieties via polymerization of aniline (ANI) and pyrrole (PY) in aqueous dispersions
of PF-MMT and PTP-MMT nanocomposites. Formation of PANI and PPY and their subsequent incorporation in the PF-MMT and PTP-MMT
composites was confirmed by FTIR absorption studies. X-ray diffraction (XRD) patterns of PANI and PPY modified PF-MMT and
PTP-MMT composites showed that PF-MMT and PTP-MMT intercalates were still present in the modified composites. Scanning electron
microscopic analysis revealed distinctive morphological patterns of the various composite particles. The dc conductivity values
of PANI and PPY modified PF-MMT and PTP-MMT composites were in the order of 10−2 S/cm in either system – a value much improved compared to the same for both of the unmodified PF-MMT (10−7 S/cm) and PTP-MMT (10−5 S/cm) nanocomposites respectively. 相似文献
35.
O. Auciello S. Saha D.Y. Kaufman S.K. Streiffer W. Fan B. Kabius J. Im P. Baumann 《Journal of Electroceramics》2004,12(1-2):119-131
Thin films of Ba1?x Sr x Ti1+y O3+z (BST), were fabricated using both by RF-magnetron sputtering and MOCVD to demonstrate application to high frequency devices. Precise control of composition and microstructure is critical for the production of (Ba x Sr1?x )Ti1+y O3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high frequency devices. Here we review results on composition-microstructure-electrical property relationships of polycrystalline BST films produced by magnetron sputter deposition that are appropriate for microwave devices such as phase shifters. BST films with a multilayer structure were also developed with different Ti-elemental ratio in each layer to minimize losses and leakage current. Interfacial contamination from C and H species was studied and implications on electrical properties are highlighted. Finally, York's group at the University of California-Santa Barbara successfully integrated our BST films onto phase shifter arrays. The results show potential of BST films in such applications. Results from initial work on the integration of Cu-electrodes with BST films are also presented. 相似文献
36.
ABSTRACTClosed-form finite-element empirical models are available for elastic and elastic–plastic spherical and sinusoidal contact. However, some of these models do not consider the effect of interaction with adjacent asperities or require extensive numerical resources because they employ a full 3-D model. Therefore this work has analysed and quantified the behaviour of an elastic and elastic- perfectly plastic axisymmetric sinusoidal surface in contact with a rigid flat for a wide range of material properties and different values of the amplitude to wavelength ratio from initial to complete contact (high load). The numerical results agreed well with the Hertz model and the Jackson–Green elastic–plastic spherical contact model at low loads. Empirical equations for elastic and also elastic-perfectly plastic cases are formulated for the contact pressure, contact area and surface separation. From the current analysis, it is found that it is not any single parameter, but different combinations of material properties and surface roughness that govern the whole contact behaviour. The critical value of the amplitude of the sinusoidal asperity below which it will deform completely elastically from initial to complete contact is established. At low values of amplitude normalized by the critical amplitude, it was found that the contact behaved similar to a spherical contact, with the average pressure (hardness) always remaining lower than three times the yield strength. However, at higher values the average pressure increased toward a value as high as six times the yield strength at complete contact. All of these equations should be useful in rough surface contact modelling, lubrication analysis, electrical contact modelling and in many other applications. 相似文献
37.
The study of near-nanocrystalline cermet composite coating was performed by depositing near-nanocrystalline WC-17Co powder
using the high velocity oxy-fuel spraying technique. The WC-17Co powder consists of a core with an engineered near-nano-scale
WC dispersion with a mean grain size 427 nm. The powder particle contains 6 wt pct of the ductile phase Co matrix mixed into
the core to ensure that the reinforcing ceramic phase WC material is discontinuous to limit debridement during wear, while
the remainder of the binding phase (11 wt pct) is applied as a coating on the powder particle to improve the ductility. The
tribological properties of the coating, in terms of corrosion resistance, microhardness, and sliding abrasive wear, were studied
and compared with those of an industrially standard microcrystalline WC-10Co-4Cr coating with a WC mean grain size 3 μm. Results indicated that the WC-17Co coating had superior wear and corrosion resistance compared to the WC-10Co-4Cr coating.
The engineered WC-17Co powder with a duplex Co layer had prevented significant decarburization of the WC dispersion in the
coating, thereby reducing the intersplat microporosity necessary for initiating microgalvanic cells. The improved wear resistance
was attributed to the higher hardness value of the near-nanocrystalline WC-17Co coating. 相似文献
38.
Y. Hamamoto K.C.A. Alam B.B. Saha S. Koyama A. Akisawa T. Kashiwagi 《International Journal of Refrigeration》2006,29(2):315-327
Thermal heat driven adsorption systems have been gained considerable attention on the recent energy utilization trend. However, the drawbacks of these adsorption systems are their poor performance. It is urgently necessary to improve the system performance of the adsorption cycles. There are two major ways for the system performance improvement. One is to develop new adsorbent material well suited to low temperature heat regeneration. The other is to enhance heat and mass transfer in the adsorber/desorber heat exchanger. The objective of the paper is to investigate the system performance of an adsorption cycle. The cycle utilizes activated carbon fiber (ACF)/methanol as adsorbent/refrigerant pair. In this paper, specific cooling effect SCE and COP of the system are numerically evaluated from the adsorption equilibrium theory with different hot, cooling and chilled fluid inlet temperatures. It is confirmed that the influences of hot, cooling and chilled fluid inlet temperatures on the system performance are qualitatively similar to those of silica gel/water pair. Even though, the driving temperature levels of ACF/methanol and silica gel/water are different. There is an optimum condition for COP to reach at maximum for ACF/methanol pair. Particularly, the ACF/methanol system shows better performance with lower chilled fluid inlet temperature between −20 and 20 °C. 相似文献
39.
40.
Sensing coverage and network connectivity are two of the most fundamental problems in wireless sensor networks. Finding an optimal node deployment strategy that would minimize cost, reduce computation and communication overhead, be resilient to node failures, and provide a high degree of coverage with network connectivity is extremely challenging. Coverage and connectivity together can be treated as a measure of quality of service in a sensor network; it tells us how well each point in the region is covered and how accurate is the information gathered by the nodes. Therefore, maximizing coverage as well as maintaining network connectivity using the resource constrained nodes is a non-trivial problem. In this survey article, we present and compare several state-of-the-art algorithms and techniques that aim to address this coverage–connectivity issue. 相似文献