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101.
Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results. 相似文献
102.
Joana Lia Ferreira Maria João Melo Ana Maria Ramos 《Polymer Degradation and Stability》2010,95(4):453-461
The photochemistry of poly(vinyl acetate), PVAc, homopolymer, of PVAc mixtures with selected pigments and fillers, and of accurate historic reproductions based on the colours prepared by Portuguese artist Joaquim Rodrigo and on those supplied by the company A Favrel was studied. The systems, applied as films, were irradiated at λ ≥ 300 nm and the changes followed by size exclusion chromatography, infrared spectroscopy, and colorimetry.PVAc as homopolymer or as an emulsion paint proved to be very stable to light, and after 5000 h of irradiation no gel fraction was observed. ΦR (λirr = 313 nm) for chain scission for PVAc was determined to be 7 × 10−8. This value indicates that the mechanism/s operating when irradiating at λ ≥ 300 nm are different from those previously published with irradiation involving 254 nm. No side-group scission was observed, and main chain scission is the foremost photodegradation mechanism. Also, the metal ions present in the pigments do not affect the photochemical stability of the polymer. 相似文献
103.
A system of reaction–diffusion equations which governs the propagation of an ozone decomposition laminar flame in Lagrangian co-ordinates is analysed by means of compact operators and modified equation methods. It is shown that the use of fourth-order accurate compact operators yields very accurate solutions if sufficient numbers of grid points are located at the flame front, where very steep gradients of temperature and species concentrations exist. Modified equation methods are shown to impose a restriction on the time step under certain conditions. The solutions obtained by means of compact operators and modified equation methods are compared with solutions obtained by other methods; good agreement is obtained. 相似文献
104.
105.
Juliana Marques Ramos Thiago Martucci Artur Wilson Carbonari Messias de Souza Costa Rajendra Narain Saxena Reiner Vianden 《Hyperfine Interactions》2013,221(1-3):129-136
Electric quadrupole interactions were studied in pure and Mn-doped powder samples and thin films of SnO2 using perturbed γγ angular correlation spectroscopy (PAC). The powder samples were prepared by Sol gel method and the thin film were prepared on the Si (100) substrate by sputtering technique using Sn in the oxygen atmosphere. The samples were characterized by x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The thickness of the film was 100 nm. The average particle size of the SnO2 powder samples was determined to be smaller than 60 nm. The radioactive 111In and 181Hf tracers were introduced in the powder samples during the sol gel chemical process. Radioactive 111In was implanted on the SnO2 thin films using the University of Bonn ion implanter (BONIS). PAC measurements were carried out in a four BaF2 detector spectrometer in the temperature range of 77–973 K for samples annealed at different temperatures. The PAC results for both nuclear probes show the presence of two electric quadrupole interactions. The major fractions in both cases correspond to the substitutional sites in the rutile phase of SnO2. The results are compared with previous PAC measurements. 相似文献
106.
Brbara L. Almeida Joanna M. Ramos Otvio Versiani Marta Sousa Claudio Alberto Tllez Soto Ana Lúcia Ramalho Mercê Antnio Slvio Mangrich Judith Felcman 《Polyhedron》2008,27(18):3662-3668
A new complex of Cu(II) of stoichiometry [Cu(PCr)(H2O)] was prepared from aqueous solution at pH 6. Its synthesis, characterization, thermogravimetric, vibrational spectroscopy, and electron paramagnetic resonance analyses were described and suggest that phosphocreatine (PCr) in solid state is acting as a tridentate ligand (the nitrogen atom of the guanidine group and the oxygen atoms of the phosphate group and the carboxylate group being the donor atoms). The fourth position is occupied by a water molecule. These results were confirmed through computational calculations (DFT/B3LYP:6-311G theoretical procedure). The tetra coordination of Cu(II) arranged in a quadratic planar geometry was found to be more stable in the DFT calculations. The calculated vibrational spectrum agrees well with the FT-IR experimental spectrum. 相似文献
107.
108.
P. Beneš P. Lam Ramos R. Poliak 《Journal of Radioanalytical and Nuclear Chemistry》1989,133(2):359-376
The paper aims at the analysis of principal factors affecting the interaction of radiocesium with freshwater solids, important for the migration of radiocesium in rivers. Uptake of radiocesium by bottom sediments and suspended solids from small streams was studied as a function of pH and composition of aqueous phase, of the concentration of cesium in water and of the composition of freshwater solids, using laboratory model experiments. pH had negligible effect on the uptake in the pH range 5–9, the uptake decreased at pH values less than 3–5 depending on the nature and concentration of the solids. Addition of cations suppressed the uptake in the order K+>Na+>Ca2+, the suppression began at 0.001, 0.01 and 0.1 mol.dm?3 concentration, respectively. Increase in cesium concentration in water caused a decrease of radiocesium uptake, but at very low concentrations of cesium combined with higher concentration of sediment (2g·dm?3) the uptake was independent of cesium concentration. Removal of carbonates, oxidic coatings and organic matter from a sediment did not affect the sorption properties of the sediment. The nature of the effects found confirms that cesium is sorbed mainly by clay components of freshwater solids. Results obtained are compared with literature data and conclusions are drawn on the importance of the factors studied for modelling of radiocesium migration in rivers. 相似文献
109.
J. A. Ramos M. Larrañaga I. Mondragon W. Salgueiro A. Somoza S. Goyanes G. H. Rubiolo 《先进技术聚合物》2009,20(1):35-38
A set of diglycidylether of bisphenol‐A (DGEBA)/4,4′‐diaminodiphenylmethane (DDM) epoxy matrix modified with poly(ethylene oxide) (PEO), pre‐cured at two different temperatures, was examined by positron annihilation lifetime spectroscopy (PALS). The aim was to investigate the correlation between local free volume and mechanical properties. A negative deviation from the linear additivity rule of the local free volume is observed at both cure schedules. Using together the local free volume and mechanical results allows to conclude that the cure temperature makes small contribution to the flexural strength and modulus of blends but is responsible for the composition‐dependent rise of the fracture toughness. It is proposed that this behavior is a consequence of the nearest‐neighbor intrachain contacts or self‐association of the epoxy‐OH groups during cure leading to a non‐uniform space distribution of the DGEBA–PEO contacts, which causes the deflection of the crack path. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
110.
L. C. Mendes R. E. R. Abrigo V. D. Ramos P. S. C. Pereira 《Journal of Thermal Analysis and Calorimetry》2010,99(2):545-549
The influence of PC melt flow rate (MFR) on phase behavior, thermal and rheological properties of catalysed and non-catalysed
poly(ethylene terephthalate)/polycarbonate (PET/PC) (80/20 wt%) reactive blending were investigated. Two types of PC named
PC1 and PC2 with MFR 3.1 and 10.8 g/10 min, respectively, were used. Each PC and the catalyst showed significant influence
on calorimetric properties, thermal stability and WAXS patterns of the blends. Regarding to TG/DTG, the blends degraded in
two steps which were attributed to PET rich phase and PC one and permit to infer that a partially miscible blends were produced. 相似文献