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排序方式: 共有109条查询结果,搜索用时 15 毫秒
91.
A. Fernández Timón M. Jurado Vargas J. F. Ziegler 《Journal of Radioanalytical and Nuclear Chemistry》2014,302(1):297-302
The dependence of efficiency of a gas-flow proportional counter with respect to the source thickness and α-particle energy was evaluated using the code SRIM. Results for the mass-efficiency curves were evaluated for 209Po and 241Am in a substrate of NaNO3. The results revealed good agreement with a linear and a hyperbolic dependence for low and high thickness sources, respectively. However, some deviations were found for very thin thicknesses, which can be explained by taking into account scattering effects. A function is proposed for predicting the dependence of efficiency with α-particle energy in the range from 4 to 8 MeV. 相似文献
92.
J. MEDINA YU. G. GUREVICH G. N. LOGVINOV P. RODRÍGUEZ G. GONZÁLEZ DE LA CRUZ 《Molecular physics》2013,111(19):3133-3138
The problem of the effective thermal diffusivity of two-layer systems is investigated using the photoacoustic spectroscopy. The experimental results are examined in terms of the effective thermal parameters of the composite system determined from a homogeneous material that produces the same physical response under an external perturbation in the detector device. It is shown that the effective thermal conductivity is not symmetric under exchange of the two layers of the composite, i.e. the effective thermal parameters depend upon which layer is illuminated in the photoacoustic experiments. Particular emphasis is given to the characterization of the interface thermal conductivity between the layer system. 相似文献
93.
B. Caro Marroyo A. Martín Sánchez M. Jurado Vargas 《Journal of Radioanalytical and Nuclear Chemistry》2013,296(3):1247-1252
A new code for the deconvolution of complex alpha-particle spectra has been developed. The ALFITeX code is written in Visual Basic for Microsoft Office Excel 2010 spreadsheets, incorporating several features aimed at making it a fast, robust and useful tool with a user-friendly interface. The deconvolution procedure is based on the Levenberg–Marquardt algorithm, with the curve fitting the experimental data being the mathematical function formed by the convolution of a Gaussian with two left-handed exponentials in the low-energy-tail region. The code also includes the capability of fitting a possible constant background contribution. The application of the singular value decomposition method for matrix inversion permits the fit of any kind of alpha-particle spectra, even those presenting singularities or an ill-conditioned curvature matrix. ALFITeX has been checked with its application to the deconvolution and the calculation of the alpha-particle emission probabilities of 239Pu, 241Am and 235U. 相似文献
94.
Multivariate calibration methods (partial least squares calibration, back propagation multilayer perceptrons networks, radial basis functions and generalized regression neural networks) were applied to the simultaneous fluorometric quantification of levofloxacin, garenoxacin and grepafloxacin, without previous separation steps. A data matrix was obtained by registering the emission spectra of mixtures of the three quinolones in urine (with concentrations ranging over 0.00-0.40 microg mL-1 for each quinolone) with a 283 nm excitation at pH 4.0. The generalized regression neural network model proved to be the most adequate model for simultaneous quantification of the three quinolones in urine samples. 相似文献
95.
96.
Almeida JA Marques EF Jurado AS Pais AA 《Physical chemistry chemical physics : PCCP》2010,12(43):14462-14476
Gemini surfactants possess interesting interfacial and aggregation properties that have prompted comprehensive studies and successful applications in a wide variety of fields. However, a systematic study on the effect of gemini tail and spacer length upon the organization of lipid membranes has not been presented so far. In this study, we analyze the action of dicationic alkylammonium bromide gemini surfactants on DPPC liposomes, the latter employed as a model of lipid membranes. Differential scanning calorimetry results indicate that the surfactants presenting shorter tails (12 carbons) induce a decrease in the overall order of the bilayer, while those with longer tails (16 and 18 carbons) lead to the formation of more ordered structures. The respective influence on the degree of lipid order transverse to the bilayer was additionally studied resorting to a detailed fluorescence anisotropy study. In this case, it is observed that among the shorter tail surfactants, those with longer spacers (6 and 10 carbons) are responsible for a more pronounced disrupting effect upon the membrane, especially close to the lipid polar heads. Molecular dynamics simulation supports the most important findings and provides insight into the mechanism that governs this interaction. Accordingly, the interplay between tail and spacer length accounts for the differential vertical positioning of the gemini molecules and atom-density in the core of the bilayer, that provide a rationale for the experimental observations. 相似文献
97.
Beatriz Jurado‐Sánchez Evaristo Ballesteros Mercedes Gallego 《Journal of separation science》2010,33(21):3365-3373
For the first time, a systematic overview deals with the advantages and disadvantages of several stationary phases (polar and non‐polar) and gas chromatographic detectors (flame ionization detector, nitrogen–phosphorus detector and MS) for the determination of 27 amines (aliphatic and aromatic amines and N‐nitrosamines) in water samples. To increase sensitivity (250 mL of sample was eluted with 150 μL of solvent) and matrix elimination, an automatic SPE system was employed prior to GC determination. The best results in terms of resolution and retention times were achieved using a column coated with 5% phenyl‐dimethylpolysiloxane (DB‐5). Capacity factor (k) values for the 27 amines increased with the rise in the polarity of the stationary phase, ranging from 3.0–27.7 and 2.2–14.4 for polar (polyethylene glycol) and non‐polar (DB‐5) columns, respectively. The detection limits of the method were 0.9–9 μg/L for flame ionization detector, 8–95 ng/L for nitrogen–phosphorus detector and 0.2–6.3 ng/L for MS. The precision was similar for the three detectors (RSD, 3.7–6.0%). The GC‐MS method was applied with a high degree of accuracy and precision to determine amines in real samples including tap, river, pond, well, swimming pool and wastewaters. 相似文献
98.
99.
The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric
membranes in Proton Exchange Membrane Fuel Cells (PEMFCs). For that purpose, we need to obtain micro or micro plus mesoporous
silica xerogels with a high porosity.
The preparation of micro (<2 nm) and micro plus mesoporous silica xerogels (2<dpore size ≤10 nm) from particulate as opposed to polymeric suspensions of silica using TEOS as precursor is employed in this work.
Mixing of sols with different average particle size (particles formed under acid and base catalyzed reactions) allows silica
xerogels with a high porosity and a controlled average pore size to be obtained. Proton conductivity of the xerogels was measured
as a function of temperature and relative humidity (RH).
The calcined xerogels showed microporosity or micro plus mesoporosity in the whole range of calcination temperatures. By mixing
sols with a different average particle size (molar ratio: acid/base =1.2) porosities up to 48.0 %±0.1 are achieved after calcination
at 300 °C for 1 hour. According to EMF measurements, electrical transport is due to protons in this kind of material. The
proton conductivity of the studied xerogels increased linearly with temperature. An S-shaped dependence of the conductivity
with the RH was observed with the greatest increase noted between 58 and 81% RH. Xerogels with a low porosity (40.8%±0.1)
and an average pore size less than 2.0 nm showed lower values of proton conductivity than that of xerogels with a higher porosity
and a higher average pore size in the whole range of temperature and RH. When silica xerogels with the highest conductivity
are treated at pH 1.5, the proton conductivity increased from 2.84 × 10−3±5.11 × 10−5 S/cm to 4.0 × 10−3±7.2×10−5 S/cm, at 81% RH and 80 °C, indicating that the surface site-density of these materials has a strong effect on conductivity.
Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002. 相似文献
100.
This paper presents a robust control algorithm for plants involvingboth internal (i.e. in the state) and external (i.e. in theoutput or input) known point delays. Several stabilizing controllerstructures are given and analysed for the case of perfectlymodelled plants with known parameters. The parametrized partsof two of the controller structures involve delays, while thoseof the two remaining controllers are delay-free. However, auxiliarycompensating signals which weight the plant input and outputintegrals are incorporated in all the controller structuresfor stabilization and model matching purposes. 相似文献