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排序方式: 共有52条查询结果,搜索用时 31 毫秒
1.
J. Mañes R. Farré A. Llopis C. Cerveró 《Fresenius' Journal of Analytical Chemistry》1991,339(9):658-660
Summary A method for the determination of lead in airborne particulates by graphite furnace atomic absorption spectrometry after extraction with HCl-HNO3 (120–150°C) is described. Particulate lead levels in the atmosphere have been measured in samples collected throughout a year, sampling has been made at eleven sites within the city of Valencia. The results obtained in this study show that annual means of lead levels are within the range of 0.526 to 7.033 g/m3 depending upon the sampling site. Lead levels are higher in winter and autumn. Lead and nitrogen oxide levels are well correlated. 相似文献
2.
Summary A polarimetric study of the tungsten (VI)-gluconic acid system in an excess of metal reveals the formation of four stable complexes: two monomers with 1 : 2 and 2: 1 stoichiometries and two dimers of 2:2 composition. The pH ranges of these species, the amount of acid equivalents their formation requires, and their conditional stability constants have also been calculated.The probable coordination of the organic ligand to the metallic centre in solutions containing different metal : ligand ratios has been investigated by13C-n.m.r. spectroscopy. The results confirm the formation of different complexes depending on the reagent which is in excess. The behaviour of the system when the metal is in excess is related to that of other polyhydroxylic ligands such as mannitol and sorbitol. 相似文献
3.
Polarimetric and spectrophotometric studies of the mannitol-tungsten(IV) system show the formation of three table complexes. Their stoichiometries, degrees of condensation and interconversion equilibria have been studied. The structure of these compounds has been investigated by 13C NMR spectroscopy in aqueous solution and by IR spectroscopy in the solid state for the two species stable at pH < 7. 相似文献
4.
The cathodoluminescence (CL) from indented Fe doped MgO single crystals has been studied with the scanning electron microscope and the results have been compared with the CL from undoped crystals. As in the undoped crystals, the iron doped crystals show in the deformed regions an enhanced luminescence emission but it has been found that iron acts as a quencher of luminescence. The results indicate that in the blue emission of the deformed crystals there is not a noticeable contribution of iron. From the evolution of CL with the irradiation time in the electron microscope it is concluded that the total Fe content influences the CL emission more than the Fe3+ content. 相似文献
5.
We analyze the hydrodynamic coupling of pairs of squirmers and the impact it has on their short and long-time behavior. The study combines an analytic analysis of the hydrodynamic interactions between pairs of squirmers with computer simulations to elucidate the quantitative capabilities of the theoretical approach. The numerical study allows us to address the motion of simple geometries of squirmers on long times and perform a complete discussion of the effective repulsive interactions in squirmer ensembles. The contrast between analytic and numerical results identifies the features of active motion responsible for such effective interactions. The framework developed also allows for an analysis of the hydrodynamic coupling between a squirmer and a solid wall and shows the possibility of bounded motion next to a solid wall. 相似文献
6.
Temperature dependence of photoluminescence (PL) spectra and time decay ranging from 90 to 330 K are investigated in magnesia-stabilized zirconia single crystals. The emission PL spectra can be decomposed into two bands. The prominent one is centered in the blue-green region of the spectrum whereas the secondary one is centered in the yellow-orange region. The temperature dependence of these bands are analyzed in terms of the so-called configuration coordinate model. The Huang-Rhys parameter for the prominent band is found near 40 and the effective phonon at about 0.030 eV. Thermal quenching energy is determined to be 0.24 eV from the decreasing part of the I(T) curve. Luminescent decays were satisfactorily fitted by two exponentials over the whole temperature range investigated. Total lifetime temperature dependence can be accounted for by assuming a radiative decay from two metastable levels with a separation energy of 0.073 eV. Results are discussed on the basis of the major defects, oxygen vacancies and complex defects. 相似文献
7.
The effect of electron beam on the cathodoluminescence (CL) from MgO single crystals has been studied in the scanning electron microscope. It has been found that CL intensity and spectrum vary with irradiation time. The behaviour is discussed in terms of impurities and vacancy defects. 相似文献
8.
S.E. Paje J. Llopis M.A. Villegas M.A. García J.M. Fernández Navarro 《Applied Physics A: Materials Science & Processing》1998,67(4):429-433
2 O·10 CaO·74 SiO2 mol%) glass doped with 0.11 and 0.35 wt.% silver are investigated. Heating treatments are carried out in a temperature range
between 400 and 575 °C for times ranging from 30 to 300 min in different atmospheres and cooling rates. The starting glasses
show a colourless and transparent appearance, but after thermal treatments under a reducing atmosphere become coloured, confirming
the presence of silver colloids related to the 410-nm absorption band. On the other hand, the main effects of thermal treatments
on the PL spectra concern those from samples treated in a reducing atmosphere. Thus, the intensity of both excitation and
emission spectra chiefly diminishes in the 220–230 nm and 325–350 nm ranges, respectively. In addition, time-resolved spectra
show the main ultraviolet (UV) emission centred above 330 nm upon excitation with 228-nm light. In contrast to the starting
glass, we notice no shift or even slight shifts of the peak position to longer wavelengths with increasing delay time after
pulse excitation, even for delay times as short as 0.01 ms. The results are discussed on the basis of transitions in which
Ag+ ions are involved.
Received: 6 February 1998/Accepted: 9 March 1998 相似文献
9.
Soda-lime-silicate glass containing arsenic oxide and undoped soda-lime-silicate glass (blank) are prepared by melting from pure sand (iron concentration lower than 0.01 wt%). The effect of arsenic on the optical properties of the glass with and without silver ion exchange at 325 °C for various times is investigated by optical absorption and photoluminescence spectroscopy. Emission/excitation spectra of silver ion exchanged glass allow differentiation of three stages in the silver incorporation into the glass network. First and second stages are only observed in the undoped glass ion exchanged for short times. Such stages are associated with the presence of isolated Ag+-ions and Ag+-Ag+ pairs, respectively. The third stage appears in the undoped glass ion exchanged for times longer than 10 min and in the arsenic-doped glass even for exchange times as short as 1 min. Then, this stage is characterised by molecular mixed species formed with Ag+ and Ag0, which coexist with nanoparticles of metallic silver. The presence of those Ag0-aggregates gives a yellow colour to the glasses, which show the well-know absorption band at about 400 nm due to surface plasmon resonance. 相似文献
10.
In many generic combustion models, one finds that a combustionwave will develop with a specific wave speed. However, thereare possible initial temperature profiles which do not evolveinto such waves, but rather die out to the ambient temperature.There can exist, in some models, a clear distinction betweenthose initial conditions that do evolve into combustion wavesand those that do not; this is sometimes referred to as thewatershed initial condition. When fuel consumption is consideredto be negligible, analytical methods can be used to obtain theexact watershed. In this paper, we consider the problem of determiningpseudo-watersheds and ascertaining the relationship betweenthese pseudo-watersheds and the exact watersheds. In the processa novel weight-function approach for infinite spatial domainsis developed. 相似文献