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排序方式: 共有264条查询结果,搜索用时 31 毫秒
21.
Doan Pham Minh Nathalie Lyczko Haroun Sebei Ange Nzihou Patrick Sharrock 《Materials Science and Engineering: B》2012,177(13):1080-1089
The synthesis of calcium hydroxyapatite (Ca-HA) starting from calcium carbonate and different orthophosphate sources, including orthophosphoric acid, potassium, sodium and ammonium dihydrogen orthophosphates, was investigated under ambient conditions. The reaction started with calcium carbonate dissolution in an acid medium, followed by rapid precipitation of calcium cations with orthophosphate species to form calcium phosphate based particles which were in the size range of 0.4–1 μm. These particles then agglomerated into much larger ones, up to 350 μm in diameter (aggregates). These aggregates possessed an unstable porous structure which was responsible for the porosity of the final products. The highest specific surface area and pore volume were obtained with potassium dihydrogen orthophosphate. On the other hand, orthophosphoric acid led to the highest dissolution of calcium carbonate and the complete precipitation of orthophosphate species. Under ambient conditions, calcium phosphate based solid products of low crystallinity were formed. Different intermediates were identified and a reaction pathway proposed. 相似文献
22.
In situ distortion of fibrous-cement slates is brought about by exposure to different temperature and hygrometric gradients
across the material. The aim of this research was to understand in detail the effect of each compositional change on the physical
properties, and to use this knowledge for the manufacture of slates which would exhibit improved mechanical properties. A
previous paper described the experimental measurements conducted to evaluate the impact of changing the composition of the
slates on the physical properties controlling hydrous transfer. In the present paper, the physical parameters obtained from
the experimental work are used in a numerical model. The numerical model, NSAT, employs a finite-element method to solve the
non-linear diffusion equation. The numerical model was validated and subsequent simulations are described. A theoretical model
has been obtained which shows good agreement between the results from practical tests at the experimental station and the
predictions of the numerical model. The results also confirm the previous hypotheses of the industrial partner. The theoretical
model obtained may be used for the prediction and specification of industrial materials having improved mechanical properties,
and in particular, exhibiting less distortion.
Editorial note A. Raoof and T. H. Doan are working at the Laboratoire des Matériaux et des Structures du Génie Civil of LCPC, France, which is a RILEM Titular Member. A. Sabouraud is working at Pont-à-Mousson SA, France, which is a RILEM Titular Member. 相似文献
Résumé Les déformations locales dans les ardoises de fibrociment peuvent être engendrées par l'existence de gradients de température et d'hygrométrie au sein du matériau. Le but de ce travail est de comprendre en détail l'effet des modifications de composition des ardoises sur leurs propriétés physiques et d'utiliser ces connaissances dans la fabrication d'ardoises présentant de meilleures propriétés mécaniques. Un article précédent a décrit l'effet des traitements subis par les ardoises sur les paramètres physiques influant sur les transferts d'humidité. Dans cet article, les paramètres physiques sont utilisés dans un modèle numérique, NSAT. Le modèle numérique emploie la méthode des éléments finis pour résoudre l'équation non-linéaire de diffusion. D'abord le modèle numérique a été validé. Ensuite d'autres simulations plus complexes sont présentées. Les calculs numériques ont clairement mis en évidence les différents types de comportement de séchage qui correspondent aux diverses observations expérimentales du partenaire industriel. Le modèle numérique peut être utilisé pour la prévision et l'élaboration des matériaux industriels avec un meilleur comportement mécanique. En particulier, une dimunition de la déformation est attendue.
Editorial note A. Raoof and T. H. Doan are working at the Laboratoire des Matériaux et des Structures du Génie Civil of LCPC, France, which is a RILEM Titular Member. A. Sabouraud is working at Pont-à-Mousson SA, France, which is a RILEM Titular Member. 相似文献
23.
Design considerations for 60 GHz CMOS radios 总被引:2,自引:0,他引:2
Doan C.H. Emami S. Sobel D.A. Niknejad A.M. Brodersen R.W. 《Communications Magazine, IEEE》2004,42(12):132-140
With the availability of 7 GHz of unlicensed spectrum around 60 GHz, there is a growing interest in using this resource for new consumer applications requiring very high-data-rate wireless transmission. Historically, the cost of the 60 GHz electronics, implemented in the compound semiconductor technology, has been prohibitively expensive. A fully integrated CMOS solution has the potential to drastically reduce costs enough to hit consumer price points. System, circuit, and device-level barriers to a low-cost 60 GHz CMOS implementation are described, potential solutions are explored, and remaining challenges are discussed. 相似文献
24.
The Musical Instrument Digital Interface (MIDI) specifies the standard hardware and data format that allow devices to communicate with one another. Developed by researchers in Japan, MIDI Specification 1.0 made its debut in 1983. Soon afterwards, Roland introduced the MPU-401 (MIDI Processor Unit model 401), which gave PC users the power of MIDI. By providing an affordable way to interconnect musical devices, MIDI opened the door to the music world for computer users 相似文献
25.
Gadolinium‐Doped Persistent Nanophosphors as Versatile Tool for Multimodal In Vivo Imaging 下载免费PDF全文
Thomas Maldiney Bich‐Thuy Doan Damien Alloyeau Michel Bessodes Daniel Scherman Cyrille Richard 《Advanced functional materials》2015,25(2):331-338
Recent breakthroughs in the rational development of multifunctional nanocarriers have highlightened the advantage of combining the complementary forces of several imaging modalities into one single nanotool fully dedicated to the biomedical field and diagnosis applications. A novel multimodal optical‐magnetic resonance imaging nanoprobe is introduced. Designed on the basis of a spinel zinc gallate structure doped with trivalent chromium and gadolinium, this nanocrystal bears the ability to serve as both a highly sensitive persistent luminescence nanoprobe for optical imaging, and a negative contrast agent for highly resolved magnetic resonance imaging (MRI). Additional proof is given that surface coverage can be modified in order to obtain stealth nanoparticles highly suitable for real‐time in vivo application in mice, showing delayed reticulo‐endothelial uptake and longer circulation time after systemic injection. 相似文献
26.
J. Gostick M. Pritzker A. Lohi H.D. Doan 《Chemical engineering journal (Lausanne, Switzerland : 1996)》2004,100(1-3):33-41
The limiting current method is applied to characterize the local mass transfer in the axial, radial and angular directions throughout a packed bed of Pall rings. This is expressed in terms of the particle-to-fluid mass transfer coefficient of ferricyanide ions obtained from the limiting currents measured during its reduction to ferrocyanide at different locations in the bed. Particular focus is placed on the variability of the mass transfer coefficient with position in terms of the quantity MTVAR, equivalent to the normalized deviation of the function Sh/Sc0.33. The results confirm those reported by other researchers using more traditional liquid collection and tracer methods. Mass transfer coefficients throughout the bed closely follow normal distributions for the range of Re values investigated. When a multipoint distributor is used, MTVAR is already low at the lowest flow rate (Re=95) considered and remains unchanged with any further increase in flow rate. In the case of a single-point distributor, on the other hand, MTVAR decreases with increasing flow rate, but always remains significantly higher than that observed with the multipoint distributor. Moreover, the MTVAR value depends on whether irrigated or full flow conditions prevail and on the cross-sectional area of the sampling electrodes, suggesting that local mass transfer variations are caused by local liquid velocity variations once the Pall rings are sufficiently wetted. 相似文献
27.
Although much research in machine learning has been carried out on acquiring knowledge for problem-solving in many problem domains, little effort has been focused on learning search-control knowledge for solving optimization problems. This paper reports on the development of SHAPES, a system that learns heuristic search guidance for solving optimization problems in intractable, under-constrained domains based on the Explanation-Based Learning (EBL) framework. The system embodies two new and novel approaches to machine learning. First, it makes use of explanations of varying levels of approximation as a mean for verifying heuristic-based decisions, allowing heuristic estimates to be revised and corrected during problem-solving. The provision of such a revision mechanism is particularly important when working in intractable and under-constrained domains, where heuristics tend to be highly over-generalized, and hence at times will give rise to incorrect results. Second, it employs a new linear and quadratic programming-based weight-assignment algorithm formulated to direct search toward optimal solutions under best-first search. The algorithm offers a direct method for assigning rule strengths and, in so doing, avoids the need to address the credit-assignment problem faced by other iterative weight-adjustment methods 相似文献
28.
Aly Farag El Sheikha Ana Condur Isabelle Métayer Doan Duy Le Nguyen Gérard Loiseau Didier Montet 《Yeast (Chichester, England)》2009,26(10):567-573
The determination of geographical origin is a demand of the traceability system of import–export food products. One hypothesis for tracing the source of a product is by global analysis of the microbial communities of the food and statistical linkage of this analysis to the geographical origin of the food. For this purpose, a molecular technique employing 26S rDNA profiles generated by PCR–DGGE was used to detect the variation in yeast community structures of three species of Physalis fruit (Physalis ixocarpa Brat, Physalis pubescens L, Physalis pruinosa L) from four Egyptian regions (Qalyoubia, Minufiya, Beheira and Alexandria Governments). When the 26S rDNA profiles were analysed by multivariate analysis, distinct microbial communities were detected. The band profiles of Physalis yeasts from different Governments were specific for each location and could be used as a bar code to discriminate the origin of the fruits. This method is a new traceability tool which provides fruit products with a unique biological bar code and makes it possible to trace back the fruits to their original location. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
29.
The melting of Fe-Cr-Ni alloys under 200 MPa nitrogen pressure has shown that nitrogen significantly improves the mechanical properties. Tensile strengths of these high-nitrogen alloys were found to be proportional to interstitial nitrogen concentration. The tensile strengths of nitrogen-alloyed steels could be significantly increased over comparable carbon-alloyed steels. The increase in tensile strength was found to be proportional to the square root of the interstitial nitrogen concentration indicating that the strengthening may be controlled by thermal effects. 相似文献
30.
BACKGROUND: Nonuniform attenuation in the thorax can generate artifacts in single-photon emission computed tomographic myocardial perfusion studies that mimic coronary artery disease. In this article we present both phantom and simulation data, as well as clinical data, in support of an emission-based method that provides reliable correction for attenuation effects without the need for a transmission measurement. METHODS AND RESULTS: The attenuation map is derived from the measured distribution of 99mTc-labeled macroaggregated albumin in the lungs and a radioactive binder wrapped about the thorax. This information is acquired as part of a dual-isotope acquisition during the rest 201Tl study. Segmentation is used to define the interiors of lung and body compartments, which are assigned a single attenuation coefficient for each of the two tissue types. The appropriateness of this approach was investigated by examining the measured attenuation coefficients in a group of 80 individuals (40 male, 40 female) from positron emission tomographic transmission studies. The correction technique was evaluated with computer simulations, a physical phantom, and clinical data acquired from 20 patients. Analysis of the positron emission tomographic data found a small SD in the mean attenuation coefficients for the body (<5%) and lungs (<15%). The application of emission-based attenuation-correction technique produced a substantial reduction in the magnitude of the attenuation artifact in images obtained from both the phantom and the simulation studies. The emission-based attenuation-correction technique was easily applied to myocardial perfusion studies, where it had a significant effect, resulting in changes in interpretation for nine of 20 patients. CONCLUSIONS: The results of this study provide strong support for the concept that an attenuation map can be generated with fixed attenuation values in place of those that are directly measured. Thus the emission-based attenuation-correction technique can be considered an inexpensive alternative to transmission-based correction methods. Because the emission-based correction technique does not require any additional hardware, it has the major advantage of being applicable to all single-photon emission computed tomographic systems. 相似文献