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171.
172.
Vos HJ Frijlink ME Droog E Goertz DE Blacquière G Gisolf A de Jong N van der Steen AF 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2005,52(12):2418-2422
A recent study has shown the feasibility of tissue harmonic imaging (THI) using an intravascular ultrasound (IVUS) transducer. This correspondence describes the design, fabrication, and characterization of a THI-optimized piezoelectric transducer with oval aperture of 0.75 mm by 1 mm. The transducer operated at 20 MHz and 40 MHz, and was comprised of a single piezoelectric layer with additional passive layers. The Krimholtz-Leedom-Matthaei (KLM) model was used to iteratively find optimal material properties of the different layers. The transducer characterization showed -6 dB fractional bandwidths of 30% and 25%, and two-way insertion losses of -20 dB and -36 dB, respectively. 相似文献
173.
Reviews computer programs that simulate human long-term memory. A model of memory is described that may be constructed from the various programming efforts. Memory may be conceptualized as a large network with labeled links, where the nodes refer to ideas, and the links to the relations between those ideas. A model of this kind appears capable of giving meaningful answers to factual questions. The model may be so specified as to manifest spontaneous activity and local activity variations based upon excitation of associated contents; this explains context effects and other features. Retrieval, in the model, is effected by means of organized retrieval cues and complex information-retrieval strategies, which later permit the model to respond to cues differing from the original stimuli. (4 P. Ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
174.
We present a method to approximate a simple, regular C2 surface W in by a (tangent continuous) skin surface S. The input of our algorithm is a set of approximate W-maximal balls, where the boundary of the union of these balls is homeomorphic to W. By generating patches of spheres and hyperboloids over the intersection curves of the balls the algorithm determines a one-parameter family of skin surfaces, where a parameter controls the size of the patches. The skin surface S is homeomorphic to W, and the approximate W-maximal balls in the input set are also S-maximal. The Hausdorff distance between the regions enclosed by the input surface W and the approximating skin surface S depends linearly on a parameter related to the sampling density of the approximate W-maximal balls. 相似文献
175.
We present a series of numerically calculated plate height and flow resistance data obtained for an idealized chromatographic support mimic with variable bed porosity (0.3 = epsilon = 0.9), yielding a unique insight into how the main chromatographic performance parameters can be expected to vary with the external bed porosity, unbiased by any differences in molecular diffusivity or retention factor. The influence of pore heterogeneity effects is considered as well. It is found that the product h(min)nu(opt) depends only very weakly on epsilon and on the degree of pore heterogeneity. It is also found that the minimal separation impedance E(min) decreases monotonically with epsilon. This shows that the minimal plate height increase that can be expected for large porosity systems is always more than compensated by their reduced flow resistance, in agreement with the current observations in real silica monolith columns. Using the computed plate height data in an optimization analysis, it is found that large porosity supports can always potentially yield shorter analysis times or larger plate numbers than small porosity supports but need submicrometer feature sizes to actually achieve this. Assuming a lower limit on the producible or useable structural feature sizes, it is found that small N separations can best be performed with a small porosity packing, whereas large N separations require a large porosity packing if the column length (L) is left free. A plot yielding epsilon(opt) as a function of the required plate number has been established, showing that roughly epsilon(opt) approximately log(N) in both the ordered and the disordered support cases. It is also shown that the maximal increase in peak capacity ever to be expected from the use of high porosity supports is a factor of 2 (if the mobile-phase viscosity can be kept constant), potentially to be increased by a factor of 1.5 by increasing the homogeneity of the packing. 相似文献
176.
Hubig M Suchandt S Adam N 《Journal of the Optical Society of America. A, Optics, image science, and vision》2004,21(10):1975-1987
Phase unwrapping (PU) represents an important step in synthetic aperture radar interferometry (InSAR) and other interferometric applications. Among the different PU methods, the so called branch-cut approaches play an important role. In 1996 M. Costantini [Proceedings of the Fringe '96 Workshop ERS SAR Interferometry (European Space Agency, Munich, 1996), pp. 261-272] proposed to transform the problem of correctly placing branch cuts into a minimum cost flow (MCF) problem. The crucial point of this new approach is to generate cost functions that represent the a priori knowledge necessary for PU. Since cost functions are derived from measured data, they are random variables. This leads to the question of MCF solution stability: How much can the cost functions be varied without changing the cheapest flow that represents the correct branch cuts? This question is partially answered: The existence of a whole linear subspace in the space of cost functions is shown; this subspace contains all cost differences by which a cost function can be changed without changing the cost difference between any two flows that are discharging any residue configuration. These cost differences are called strictly stable cost differences. For quadrangular nonclosed networks (the most important type of MCF networks for interferometric purposes) a complete classification of strictly stable cost differences is presented. Further, the role of the well-known class of node potentials in the framework of strictly stable cost differences is investigated, and information on the vector-space structure representing the MCF environment is provided. 相似文献
177.
S Easter Selvan C Cecil Xavier Nico Karssemeijer Jean Sequeira Rekha A Cherian Bharathi Y Dhala 《IEEE transactions on information technology in biomedicine》2006,10(4):685-695
The appearance of disproportionately large amounts of high-density breast parenchyma in mammograms has been found to be a strong indicator of the risk of developing breast cancer. Hence, the breast density model is popular for risk estimation or for monitoring breast density change in prevention or intervention programs. However, the efficiency of such a stochastic model depends on the accuracy of estimation of the model's parameter set. We propose a new approach-heuristic optimization-to estimate more accurately the model parameter set as compared to the conventional and popular expectation-maximization (EM) algorithm. After initial segmentation of a given mammogram, the finite generalized Gaussian mixture (FGGM) model is constructed by computing the statistics associated with different image regions. The model parameter set thus obtained is estimated by particle swarm optimization (PSO) and evolutionary programming (EP) techniques, where the objective function to be minimized is the relative entropy between the image histogram and the estimated density distributions. When our heuristic approach was applied to different categories of mammograms from the Mini-MIAS database, it yielded lower floor of estimation error in 109 out of 112 cases (97.3 %), and 101 out of 102 cases (99.0%), for the number of image regions being five and eight, respectively, with the added advantage of faster convergence rate, when compared to the EM approach. Besides, the estimated density model preserves the number of regions specified by the information-theoretic criteria in all the test cases, and the assessment of the segmentation results by radiologists is promising. 相似文献
178.
Karst groundwater protection: First application of a Pan-European Approach to vulnerability, hazard and risk mapping in the Sierra de Líbar (Southern Spain) 总被引:11,自引:0,他引:11
Andreo B Goldscheider N Vadillo I Vías JM Neukum C Sinreich M Jiménez P Brechenmacher J Carrasco F Hötzl H Perles MJ Zwahlen F 《The Science of the total environment》2006,357(1-3):54-73
The European COST action 620 proposed a comprehensive approach to karst groundwater protection, comprising methods of intrinsic and specific vulnerability mapping, validation of vulnerability maps, hazard and risk mapping. This paper presents the first application of all components of this Pan-European Approach to the Sierra de Líbar, a karst hydrogeology system in Andalusia, Spain. The intrinsic vulnerability maps take into account the hydrogeological characteristics of the area but are independent from specific contaminant properties. Two specific vulnerability maps were prepared for faecal coliforms and BTEX. These maps take into account the specific properties of these two groups of contaminants and their interaction with the karst hydrogeological system. The vulnerability assessment was validated by means of tracing tests, hydrological, hydrochemical and isotope methods. The hazard map shows the localization of potential contamination sources resulting from human activities, and evaluates those according to their dangerousness. The risk of groundwater contamination depends on the hazards and the vulnerability of the aquifer system. The risk map for the Sierra de Líbar was thus created by overlaying the hazard and vulnerability maps. 相似文献
179.
Frans Nieuwenhout Nico van der Borg Wilfried van Sark Wim Turkenburg 《Progress in Photovoltaics: Research and Applications》2007,15(5):387-404
In order to evaluate the performance of solar home systems (SHSs), data on local insolation is a prerequisite. We present a new method to estimate insolation if direct measurements are unavailable. This method comprises estimation of daily irradiation by correlating photovoltaic (PV) module currents from a number of SHSs, located a few kilometres apart. The method was tested with a 3‐year time series for nine SHS in a remote area in Indonesia. Verification with reference cell measurements over a 2‐month period showed that our method could determine average daily irradiation with a mean bias error of 1·3%. Daily irradiation figures showed a standard error of 5%. The systematic error in this method is estimated to be around 10%. Especially if calibration with measurements during a short period is possible, the proposed method provides more accurate monthly insolation figures compared with the readily available satellite data from the NASA SSE database. An advantage of the proposed method over satellite data is that irradiation figures can be calculated on a daily basis, while the SSE database only provides monthly averages. It is concluded that the new method is a valuable tool to obtain information on insolation when long‐term measurements are absent. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
180.