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[en] The electron beam irradiation technique has been utilized to prepare hydrogel wound dressings. The composition of the dressings is based on polyvinylpyrrolidone (PVP), poly (ethylene glycol) (PEG), and agar. Increasing the irradiation dose leads to an increase in the gel fraction; this increase is due to increased crosslink density. The gel fraction% decreases as the PEG concentration increases. The maximum swelling% decreases with increasing the irradiation dose, but increases with increasing the PEG concentration. This relationship can be utilized to modify the gel properties as fel fraction% and maximum swelling of the hydrogel. The prepared dressings could be considered as a good barrier against microbes. (author)
[en] We present a method for time-resolving the coherent precession of the spin of a single neutral exciton confined to an InAs/GaAs quantum dot. A pair of circularly polarized π pulses are used to pump and probe the exciton spin, rather than a pair of interferometrically stable π/2-pulses in a quantum interference type experiment, faster, technically simpler, measurements of the small 15 μeV exciton fine-structure can be made in the photocurrent regime. A small change in the fine-structure splitting with vertically applied electric-field is observed.
[en] This paper addresses the selective maintenance optimization problem in a multi-component system, carrying out several missions with scheduled inter-mission breaks. To improve the probability of the system successfully completing the next mission, maintenance is performed on the system's components during the break. Each component is assigned a list of eligible maintenance actions ranging from minimal repair, through intermediate imperfect maintenance actions, to replacement. The quality of a maintenance action is assumed to be stochastic, reflecting the degree of expertise of the repairman and the tools used to perform the maintenance action. This quality is thus treated as a random variable with an identified probability distribution. The selective maintenance problem aims thus at finding a cost-optimal subset of maintenance actions, to be performed on the system during the limited duration of the break, which guarantees that the pre-set minimum probability of successfully completing the next mission is attained. The fundamental constructs and the relevant parameters of this nonlinear and stochastic optimization problem are developed and thoroughly discussed. It is then put into practice for a series–parallel system and the added value of solving it as a stochastic problem is demonstrated on some test cases. - Highlights: • A stochastic selective maintenance optimization problem is modeled and investigated. • Each component of the system may be maintained at different maintenance levels. • Variability of maintenance actions' effects are accounted for in the optimization process. • The selective maintenance problem is solved as a nonlinear stochastic optimization program. • Cost-effective maintenance solutions and accurate system's reliability are obtained.
[en] This paper investigates the optimal age replacement policy for used systems, such as second-hand products, which start their second life-cycle in a more severe environment with an initial age that is uncertain. This uncertain age is modelled as a random variable following continuous probability distributions. A mathematical model is developed to minimize the total expected cost per unit of time for these systems on an infinite time horizon. Optimality and existence conditions for a unique optimal solution are derived and used in a numerical procedure to solve the problem. Numerical experiments are provided to demonstrate the added value and the impacts of the random initial age on the optimal replacement policy.
[en] This paper investigates the preventive age replacement policy (ARP) for a system subject to random failures. Unlike most maintenance models in the literature, our model considers a system that is exploited under different operating environments each characterized by its own degree of severity. The system lifetimes follow a different distribution depending on the environment it is operating under. Furthermore, the system lifetimes distribution is assumed unknown and therefore estimated from field reliability data. The reliability of the system is calculated using two kernel estimators. This method offers the advantage of non-parametric estimation methods and completely determined by two parameters, namely the smoothing parameter and the kernel function. First, a probability maintenance cost model is derived and conditions under which an optimal preventive maintenance age exists are provided. Then, a statistical maintenance cost model is developed using two kernel estimators. The impact of the variability of the kernel smoothing parameter on the cost model is also investigated. Numerical experiments are provided to illustrate the proposed approach. Results obtained demonstrate the accuracy of the proposed statistical maintenance cost model. - Highlights: • A stochastically deteriorating system under different operating environments is considered. • Probability and statistical maintenance optimization models are proposed. • Theoretical maintenance optimality conditions are derived. • The assumption according to which time to failure distribution is known is relaxed. • Two kernel estimators are implemented. Results demonstrate the accuracy of the approach.
[en] In this paper, a K-out-of-N:G system with N categories of components is studied. Each component category is characterized by its own failure and repair rates. There are R repair facilities, and repair priorities are specified between the N non-identical components. An algorithm for automatic construction of the system state transition diagram is presented. The stationary availability of each component and that of the system are evaluated by using a multi-dimensional Markov model. We show how this model can be represented as a network of stochastic automata with state-dependent transitions that can be implemented via generalized tensor (or Kronecker) algebra. For the efficiency assessment, an analog Monte Carlo simulation model is developed. Experiments are then conducted and simulation results are compared to those obtained by the proposed approach
[en] Dilute nitride GaAs1-xNx layers have been grown by molecular beam epitaxy with nitrogen concentration ranging from 0.2% to 0.8%. These samples have been studied before and after hydrogen irradiation by using standard deep level transient spectroscopy (DLTS) and high resolution Laplace DLTS techniques. The activation energy, capture cross section and density of the electron traps have been estimated and compared with results obtained in N-free as-grown and H-irradiated bulk GaAs.
[en] The optical properties of alkaline earth aluminates doped with rare earth ions have received much attention in the last years and this is due to their chemical stability, long-afterglow (LAG) phosphorescence and high quantum efficiency. However, there is a lack of understanding about the nature of the rare earth ion trapping sites and the mechanisms which could activate and improve the emission centers in these materials. Therefore a new phosphor material composition, SrAl2O4:Mn2+, co-doped with Nd3+ was synthesized by a traditional solid-state reaction method. The influence of transition metal and rare earth doping on crystal structure and its luminescence properties have been investigated by using X-ray diffraction (XRD), Raman scattering, Photoluminescence (PL) and Radioluminescence (RL). Analysis of the related diffraction patterns has revealed a major phase characteristic of the monoclinic SrAl2O4 compound. Small amounts of the dopants MnCO3 and Nd2O3 have almost no effect on the crsytalline phase composition. Characteristic absorption bands from Nd3+ 4f–4f transitions in the spectra can be assigned to the transitions from the ground state 4I9/2 to the excited states. The luminescence of Mn2+ activated SrAl2O4 exhibits a broad green emission band from the synthesized phosphor particles under different excitation sources. This corresponds to the spin-forbidden transition of the d-orbital electron associated with the Mn2+ ion. In photo- and radio-luminescence spectra, Nd3+ 4f–4f transition peaks were observed. The emitted radiations for different luminescence techniques were dominated by 560, 870, 1057 and 1335 nm peaks in the visible and NIR regions as a result of 4I9/2→4G7/2 and 4F3/2→4IJ (J=9/2, 11/2 and 13/2) transitions of Nd3+ ions, respectively. Multiple emission lines observed at each of these techniques are due to the crystal field splitting of the ground state of the emitting ions. The nature of the emission lines is discussed. -- Highlights: • A new stannate phosphor, SrAl2O4:Mn2+,Nd3 has been synthesized by solid state reaction method. • The role of Nd3+ co-doped into SrAl2O4:Mn2+ was discussed. • Nd3+ 4f–4f transition peaks were observed
[en] We investigated effects of localization and strain on the optical and magneto-optical properties of diluted nitrogen III–V quantum wells theoretically and experimentally. High-resolution x-ray diffraction, photoluminescence (PL), and magneto-PL measurements under high magnetic fields up to 15 T were performed at low temperatures. Bir-Pikus Hamiltonian formalism was used to study the influence of strain, confinement, and localization effects. The circularly polarized magneto-PL was interpreted considering localization aspects in the valence band ground state. An anomalous behavior of the electron-hole pair magnetic shift was observed at low magnetic fields, ascribed to the increase in the exciton reduced mass due to the negative effective mass of the valence band ground state
[en] Nanocrystalline zinc ferrite (ZnFe2O4) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe2O3) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Moessbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 oC, the lattice parameter reduces from 8.448 to 8.427 A and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Moessbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.