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51.
Verbeek X.A.A.M. Ledoux L.A.F. Brands P.J. Hoeks A.P.G. 《IEEE transactions on bio-medical engineering》1998,45(10):1217-1226
All Doppler systems, whether conventional Doppler domain or radio frequency (RF) processing is employed, relate the temporal frequency characteristics of the signal at a certain point in depth as function of time to the spatial frequency characteristics of the received signal as function of depth. The mean frequency of the latter may change as a result of depth-dependent attenuation, nonlinear scattering mechanisms, as in harmonic imaging of ultrasound contrast agents, or RF signal demodulation. For all these cases, the relationship between spatial and temporal mean frequency and target velocity is still governed by the familiar Doppler expression if the signal modifications have been properly accounted for. A major drawback of RF signal processing to extract the target velocity is the large number of data points to consider. The computational complexity increases further for harmonic imaging. It is shown conceptually, and demonstrated by signal simulations, that prior to velocity estimation RF demodulation followed by decimation (1) does not affect the Doppler equation, (2) enhances the information content of the samples, (3) reduces the computational load by a factor of four and for harmonic signals by a higher factor, and (4) while demodulation does not have to be actually performed, but can be accounted for by a scaling factor in the cross-correlation function. It is concluded that decimation hardly affects the precision of the velocity estimate if possible frequency aliasing is maintained within bounds, suggesting that the decimation factor is not critical 相似文献
52.
F. Huisken G. Ledoux O. Guillois C. Reynaud 《Advanced materials (Deerfield Beach, Fla.)》2002,14(24):1861-1865
Crystalline Si nanoparticles with diameters between 2.5 and 20 nm are prepared by CO2‐laser‐induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone is extracted through a nozzle into a high‐acuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles that can be deposited on suitable substrates. The strong visible photoluminescence (PL) of the Si nanocrystals is studied as a function of their size, and as a function of the time for which they are exposed to air. All observations can be explained on the basis of quantum confinement as the only origin of the PL. Chemical methods are exploited to modify the surface of the Si nanoparticles and to reduce their size, thus shifting their PL to shorter wavelengths. With this technique, the Si nanoparticles, collected in much larger quantities in the filter of the flow reactor, can be made strongly luminescent so that they may be used for various applications. 相似文献
53.
Transient response data for the cyclohexane dehydrogenation reaction were collected on a surface coated Pt-Al2O3 catalyst. Mathematical analysis with the surface coated catalyst is considerably simpler due to the absence of intraparticle gradients. The data, which often displayed a characteristic overshoot type of response, were correlated by a surface coverage model where the π/σa-bonded benzene transition surface complex can reversibly orient itself to a σ-bonded hydrogen deficient surface species which cannot desorb. As in previous work cyclohexane adsorption was found to be slow and surface reactions involving the adsorbed hydrocarbon species were found to be rapid. 相似文献
54.
High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions
P. Nguyen D. Edouard J.-M. Nhut M.J. Ledoux Ch. Pham C. Pham-Huu 《Applied catalysis. B, Environmental》2007,76(3-4):300-310
This study aims at synthesizing a new by substituting 1 atom% Pd2+ in ionic state in TiO2 in the form of Ti0.99Pd0.01O1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O2 and NO reduction by CO was proposed. The model based on CO adsorption on Pd2+ and dissociative chemisorption of O2 in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd2+, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N2, defined as the ratio of the formation of N2 and formation of N2 and N2O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N2O + CO reaction is an intermediate reaction for NO + CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction. 相似文献
55.
Ledoux P. Auge J. Boniort L.Y. Dupont P. Goudeau J. Saugrain J.M. Rousseau J.C. Mohanna Y. 《Lightwave Technology, Journal of》1989,7(8):1270-1274
The authors point out the formation mechanism of axial stress, describes the method and the apparatus used for measurements, and presents the results obtained during a study of loss reduction on trapezoidal core 1.55-μm dispersion-shifted fibers manufactured by the MCVD technique. It is concluded that the stress measurements on fiber are less accurate than those on preform (experimental dispersion reaches 15% instead of 5%). The various observations made from measurements on preform show that the axial stress is related to the index profile and that the core axial stress increases with the GeO2 content. The measurements performed on fiber show that the level of stress is linearly dependent on the drawing tension and that the stress profile is related to the glass-transition temperature of the different materials. Furthermore, this method of stress measurement is complementary to the more standard index profile measurement on fiber or on preform 相似文献
56.
S Ledoux JC Dussaule PL Michel A Vahanian JP Wolf J Acar R Ardaillou 《Canadian Metallurgical Quarterly》1993,72(12):932-938
The role of left atrial and aortic pressures on the secretion of the main hormones controlling blood volume is still subject to debate in humans. Because of increased mean left atrial pressure and decreased mean aortic pressure produced by balloon inflation in patients with mitral stenosis treated with balloon valvulotomy, the hormonal changes occurring acutely (group II of patients) were measured. The same studies (group I patients) were also performed 48 hours after this treatment, a period at which left atrial pressure permanently diminished. Inflation of the balloon resulted in a decrease in plasma renin activity and increases in plasma atrial natriuretic factor (ANF) and plasma arginine vasopressin (AVP). Forty-eight hours after balloon valvulotomy, which had produced a decrease in left atrial pressure, plasma ANF was lower (58.9 +/- 7.9 vs 95.3 +/- 11.9 pg/ml; p < 0.001), and plasma renin activity (2,575 +/- 533 vs 960 +/- 113 pg/ml/hour; p < 0.01), plasma angiotensin II (25.0 +/- 4.1 vs 9.3 +/- 1.3 pg/ml; p < 0.001) and plasma aldosterone (181.7 +/- 36.7 vs 139.9 +/- 19.8 pg/ml; p < 0.05) were higher than their respective control levels 24 hours before treatment of the stenosis. In contrast, plasma AVP (3.7 +/- 0.25 vs 4.4 +/- 0.31 pg/ml; p = 0.001) diminished moderately along with plasma osmolality (282.4 +/- 0.1 vs 286.2 +/- 0.6 mOsm/kg; p < 0.001). Urinary sodium excretion was also examined before and after balloon valvulotomy.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
57.
58.
Bru J. Ledoux J. L. Menard J. Waschkowski E. 《Bulletin of Engineering Geology and the Environment》1982,25(1):25-31
Bulletin of Engineering Geology and the Environment - La présente communication rapporte les développements et les mises au point de matériels effectués par les Laboratoires des... 相似文献
59.
Stéphane Bouchard Stéphanie Dumoulin Jeanne Talbot André-Anne Ledoux Jennifer Phillips Johana Monthuy-Blanc Geneviève Labonté-Chartrand Geneviève Robillard Matteo Cantamesse Patrice Renaud 《Interacting with computers》2012,24(4):227-236
The feeling of presence has been shown to be an important concept in several clinical applications of virtual reality. Among the factors influencing presence, realism factors have been examined extensively from the angle of objective realism. Objective realism has been manipulated by altering numerous technological characteristics such as pictorial quality, texture and shading, or by adding more sensory information (i.e., smell, touch). Much less studied is the subjective (or perceived) realism, the focus of the two pilot studies reported in this article. In Study 1, subjective realism was manipulated in order to assess the impact on the feeling of presence. Method: Presence was measured in 31 adults after two immersions in virtual reality. Participants were immersed in a neutral/irrelevant virtual environment and subsequently subjected to the experimental manipulation. Participants in the experimental condition were falsely led to believe that they were immersed live in real time in a “real” room with a “real” mouse in a cage. In the control condition, participants believed they were immersed in a replica of the nearby room. All participants were actually immersed in the exact same virtual environment. Results: A manipulation check revealed that 80% of the participants believed in the deception. A 2 Times by 2 Conditions repeated measure ANOVA revealed that leading people to believe they were seeing a real environment digitized live in virtual reality increased their feeling of presence compared to the control condition. In Study 2, the same experimental design was used but with simultaneous functional magnetic resonance imaging (fMRI) in order to assess brain areas potentially related to the feeling of presence. fMRI data from five participants were subjected to a within subject fixed effect analysis to verify differences between the experimental immersion (higher presence) and the control immersion (lower presence). Results revealed a statistically significant difference in left and right parahippocampus areas. Conclusion: Results are discussed according to layers of presence and consciousness and the meaning given to experiences occurring in virtual reality. Some suggestions are formulated to target core presence and extended presence. 相似文献
60.
An ATSC DTV receiver with improved robustness to multipath and distributed transmission environments
Wu Y. Xianbin Wang Citta R. Ledoux B. Lafleche S. Caron B. 《Broadcasting, IEEE Transactions on》2004,50(1):32-41
This paper presents the design and implementation of an 8-VSB DTV receiver for indoor and distributed transmission environments. The receiver is designed to handle severe multipath distortion from indoor and Single Frequency Network (SFN) transmission conditions. The architecture of the receiver is first introduced. The adaptive equalizer structure and design are then discussed in detail. A channel-matched filter is employed as a pre-filter such that the signal energies from different echoes are combined optimally and the signal to noise ratio of the equalizer input is maximized. Feedforward and feedback equalizers are used to handle the pre-echo (pre-cursor) and post-echo (post-cursor), respectively. The feedforward filter is designed to minimize the pre-cursors or convert them into post-cursors, while the feedback equalizer is used to eliminate the post-cursors. Initial tap coefficients are computed to speed up the convergence of these two filters based on the channel estimation. Laboratory tests show that the new prototype DTV receiver has very robust performance in multipath environments. 0 dB echoes can be handled with this receiver due to the enhanced design of the equalizer. It can withstand a -10 dB single echo within a -29.5 to +38.5 microsecond range and a 0 dB echo within a 12 microsecond range. 相似文献