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71.
An approach for Raman measurements of highly radioactive samples is presented here. The innovative part of this approach lies in the fact that no single part of the Raman equipment is in direct contact with the radioactive sample, as the sample is sealed in an alpha‐tight capsule. Raman analysis is effectively performed through the optical‐grade quartz window closing the capsule. This allows performing micro‐Raman measurements on radioactive samples with no limitations on the laser source wavelength, polarisation mode, spectrometer mode and microscope mode (provided the focal length of the microscope objective is greater than the thickness of the quartz window and with sub mg samples). Some example results are shown and discussed. In particular, some spectral features of americium‐containing oxide nuclear fuel specimens are presented. Raman spectra clearly reveal in these specimens the presence of abundant oxygen defects induced in the fcc fluorite lattice by trivalent americium. In order to complete the analysis the Raman spectrum of pure americium dioxide was also measured with a lower energy excitation source compared with previous research. The current results seem to be consistent with the possible occurrence of a photolysis process induced by the Raman laser, resulting in the formation of hyperstoichiometric americium sesquioxide Am2O3 + z. Such a photolytic process is deemed to be unavoidable when visible lasers are used as excitation sources for the Raman analysis of americium dioxide. © 2015 The Authors Journal of Raman Spectroscopy Published by John Wiley & Sons, Ltd.  相似文献   
72.
The superiority of silver‐doped titania for photocatalytic oxidation (PCO) of organic compounds inspired us to investigate PCO of 1,2‐cyclohexanediol. Ag/TiO2 was prepared, characterized (nanosize 19–24 nm) and used for oxidation of 1,2‐cyclohexanediol (1) in acetonitrile. The photolysate was analyzed using GC and GC/MS techniques. The PCO products are 2‐hydroxylcyclohexanone (2), 1,2‐cyclohexandione (3), 2‐cyclohexenone(4), cyclohexanone (5), and adipic acid (6).The formation of electron–hole pair at the surface of the catalyst followed by oxidation reactions was the suggested mechanism. Kinetic studies revealed first‐order mechanism for PCO of 1 and rate constant (k) = ?0.145 h–1. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
73.
Single-phase polycrystalline samples of La0.7Sr0.3Mn1-xCrxO3 with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature TC decreases with increasing x and the maximum magnetic entropy change (−ΔSM) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.  相似文献   
74.
SrBi2-xHoxNb2O9 (0?≤?x?≤?0.5) ceramics were prepared through the conventional solid state route. The materials have been characterized by XRD, FTIR and SEM. All compounds were pure and well crystallized. In FTIR spectroscopy, the spectral patterns of the crystal structures of these polyphases are unique and smoothly different from each other. The grains were prone to become spherical with increasing x. Dielectric and electrical properties of these materials as a function of temperature at different frequencies have been carried out. The dielectric constant and dielectric loss were found to decrease with an increase of the holmium concentration at room temperature. Reaching up to x?=?0.4 and x?=?0.5, the diffusivity increases, leading to the occurrence of relaxor behavior. 20–30 at.% Ho doping on the Bi-site can fill up the oxygen vacancies and decrease the AC conductivity. However, at higher doping levels greater than 40% holmium oxide, the AC conductivity is found to increase.  相似文献   
75.
In this study, lead sulphide (PbS) was prepared by the chemical bath deposition technique. The sample was characterized by X-ray diffraction (XRD), Energy Dispersive Analysis of X-rays (EDAX) and cyclic voltammetry. EDAX spectrum shows peaks attributable to lead and sulphur. The EDAX analysis also shows that the prepared sample is stoichiometric. Cyclic voltammetry experiments were recorded at 100 mV·s−1 and 400 mV·s−1 scan rates. Results show that the rate controlling electrochemical reaction is electron transfer. The presence of redox waves shows that the lithium intercalation and deintercalation can occur as a result of lattice expansion in PbS. There were no differences in the PbS XRD data before and after the cyclic voltammetry experiments indicating that the PbS structure is not modified upon lithium ion intercalation and deintercalation in PbS. The discharge characteristics for 35 cycles of the cell using the LiCoO2/PbS couple is presented indicating the possible development of such materials as anode in lithium ion cells.  相似文献   
76.
The dc and ac conductivities as well as the dielectric constant () were measured for different zeolites encapsulated gold (AuCl3) samples at different temperatures (300-500 K) and various frequencies (5 kHz-1 MHz). The conductivity was found to change in the order Au/FSM-27>Au/NaY>Au/FSM-47. Sorbed water contained inside zeolites assists greatly the proton mobility (zeolite protons) and the ion mobility (Na+ and Au+) and hence enhance the electric conduction in the temperature range 300-373 K. Raising the temperature over 373 K induces dehydration effect that assists the metallic gold formation and thus a dramatic loss in conductivity was revealed. The conduction mechanism was expected to be partially ionic and partially electronic. The IR study showed that the exposure of Au zeolites to CO gas produced a characteristic band of Au+-CO at 2180 cm−1 that tends to decrease with temperatures and even vanishes at 376 K in favor of Au0-CO at 2128 cm−1. Similarly, a phase transition at 338 K, that occurs in the range 300-376 K, was confirmed by DTA to further emphasize the temperature regions of either Au+ cations (338 K) or Au0 (376 K) formation.  相似文献   
77.
78.
We introduce a metamaterial design composed of square plasmonic loops loaded by Kerr nonlinearities that combines enhanced nonlinear response with strong artificial magnetism, ensuring a negative refractive index with bistable and self-tunable response. We verify with full-wave simulations that positive-to-negative switching of refractive index may be obtained with moderate loss. The design of a finite-size metamaterial prism is also presented, supporting at the same frequency, and for the same light intensity, positive or inverted Snell refraction as a function of its previous excitation history.  相似文献   
79.
80.

Abstract  

As effective devices to extend the fuel residence time in supersonic flow and prolong the duration time for hypersonic vehicles cruising in the near-space with power, the backward-facing step and the cavity are widely employed in hypersonic airbreathing propulsive systems as flameholders. The two-dimensional coupled implicit RANS equations, the standard k-ε turbulence model, and the finite-rate/eddy-dissipation reaction model have been used to generate the flow field structures in the scramjet combustors with the backward-facing step and the cavity flameholders. The flameholding mechanism in the combustor has been investigated by comparing the flow field in the corner region of the backward-facing step with that around the cavity flameholder. The obtained results show that the numerical simulation results are in good agreement with the experimental data, and the different grid scales make only a slight difference to the numerical results. The vortices formed in the corner region of the backward-facing step, in the cavity and upstream of the fuel injector make a large difference to the enhancement of the mixing between the fuel and the free airstream, and they can prolong the residence time of the mixture and improve the combustion efficiency in the supersonic flow. The size of the recirculation zone in the scramjet combustor partially depends on the distance between the injection and the leading edge of the cavity. Further, the shock waves in the scramjet combustor with the cavity flameholder are much stronger than those that occur in the scramjet combustor with the backward-facing step, and this causes a large increase in the static pressure along the walls of the combustor.  相似文献   
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