Study of thermal aging effect on space charge in poly(methyl methacrylate)

Charges evolution in poly(methyl methacrylate) (PMMA) samples under thermal aging effect has been studied by means of two complementary techniques, thermal step method (TSM) and thermally stimulated depolarization current (TSDC). For the first method, measurements reveal that injected charges, whose quantity is found depending on the number of applied temperature cycles, remain close to the surface sample. TSDC measurements have been carried out for different temperatures ranging from 25 °C to 140 °C. Three distinguishable dipolar relaxations (β₁, β₂ and α) have been highlighted. In the same way, the presence of polarization and injected charges has been confirmed. In support of electric characterization, X-ray reflectometry has been used. The obtained results equally emphasized the thermal aging effect on the material.     

Characterization of polylactic acid green composites and its biodegradation in a bacterial environment

The growing interest in the preservation of our environment is pushing for solutions to develop less impacting materials. Thus, the development of biocomposites and is recyclable and compostable end-of-life resources seem an interesting alternative. In this study, the characterization of Polylactic acid (PLA) reinforced with treated and untreated Olive husk flour (OHF) were investigated. More then, their biodegradation with a Bacillus sp. has been evaluated. The main results show that the bacteria degraded both the PLA and the composite. This degradation was confirmed by the release of reducing sugars as well as increasing weight loss of PLA matrix and composites.     

Enhancing techniques for learning decision trees from imbalanced data

Advances in Data Analysis and Classification - Several machine learning techniques assume that the number of objects in considered classes is approximately similar. Nevertheless, in real-world...     

Exploring the Limits of π-Acid Catalysis Using Strongly Electrophilic Main Group Metal Complexes: The Case of Zinc and Aluminium

The catalytic activity of cationic NHC-Zn^II and NHC-Al^III (NHC=N-heterocyclic carbene) complexes in reactions that require the electrophilic activation of soft C−C π bonds has been studied. The former proved able to act as a soft π-Lewis acid in a variety of transformations. The benefit of the bulky IPr NHC ligand was demonstrated by comparison with simple ZnX₂ salts. The tested NHC-Al^III catalyst is not able to activate C−C π bonds but simple AlX₂⁺ ions were found potent in some cases.     

Compound synthesis in TiO2 implanted with Rubidium

Rubidium ions, with energy in the range 0.1 MeV, 2.0 MeV have been implanted in TiO₂ single crystals at RT and LNT.

Defects induced by implantation have been studied by optical spectroscopy, X-ray diffraction, RBS, TEM and electrical conductivity.

During implantation, the implanted samples are blue colored after irradiation. This coloration is due to an optical absorption band localized at 900 nm which corresponds to optical transition of intrinsic defects identified as Ti³⁺. These defects are induced by a chemical reaction between the implanted ions and the oxygen of the lattice as in the case of D⁺, H⁺, Li⁺, Na⁺ and K⁺ implanted in rutile.^1-3

The synthesis of a new phase in heavily implanted rutile is exhibited by using a thermal treatment and by combining techniques such as RBS, TEM and X-ray diffraction at glancing angle in the temperature range 300°C-700°C.

This compound does not correspond to metallic precipitates of rubidium which are observed in MgO implanted with Rb ions.

Planar defects have been observed in the implanted area. A correlation is exhibited between these defects and the precipitates of the new phase. From X-ray diffraction measurements and TEM observations, the composition of the synthetized compound is likely Rb₂TiO₃.     

AC and DC conductivity study of LiH<Subscript>2</Subscript>PO<Subscript>4</Subscript> compound using impedance spectroscopy

The lithium dihydrogen phosphate LiH₂PO₄ has been investigated by X-ray powder diffraction, scanning electron microscopy (SEM), and electrical impedance spectroscopy. The Rietveld refinements based on the XRD patterns show that the compound is crystallized in the orthorhombic system with Pna2₁ space group, and the refined unit cell parameters are a = 6.2428 Å, b = 7.6445 Å, and c = 6.873 Å. The electrical properties were studied using complex impedance spectroscopy as a function of frequency (10⁴–10⁷ Hz) at various temperatures (300–400 K). The Nyquist plots are well fitted to an equivalent circuit consisting of a series of combination of grains and inhomogeneous electrode surface effect. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. Moreover, the near value of the activation energies obtained from the equivalent circuit and analysis of M″ confirms that the transport is through ion hopping mechanism dominated by the motion of the proton in the structure of the investigated material.     

Multi-stage IT project evaluation: The flexibility value obtained by implementing and resolving Berk, Green and Naik (2004) model

In this paper, we evaluate a multi-stage information technology investment project, by implementing and resolving Berk, Green and Naik’s (2004) model, which takes into account specific features of IT projects and considers the real option to suspend investment at each stage. We present a particular case of the model where the project value is the solution of an optimal control problem with a single state variable. In this case, the model is more intuitive and tractable. The case study confirms the practical potential of the model and highlights the importance of the real-option approach compared to classical discounted cash flow techniques in the valuation of IT projects.     

Synthesis and Characterization of New Fluorinated Tetrazines Displaying a High Fluorescence Yield

Following the case of tetrazines substituted with perfluorinated alkyl chains, separated by two methylene groups from the tetrazine core, a new series of analogous tetrazines, but featuring only one methylene group between the fluorescent core and the perfluorinated chain, have been synthesized, and their photo-physical properties investigated. Their fluorescence quantum yields in acetonitrile are in same range than chloroalkoxytetrazines, which make them interesting candidates for light emission. Surprisingly, the quantum yields are lower with one methylene group, rather than two methylene groups separating the fluorinated chain from the emitting core, in the case of unsymmetrical compounds, while they are superior in the case of symmetrical ones. This unusual observation is discussed in the article.     

Luminescent properties of GaN films grown on porous silicon substrate

GaN films have been grown on porous silicon at high temperatures (800-1050 °C) by metal organic vapor phase epitaxy. The optical properties of GaN layers were investigated by photoluminescence (PL) and cathodoluminescence (CL) spectroscopy. PL spectra recorded at 5 K exhibit excitonic emissions around 3.36-3.501 eV and a broad yellow luminescence at 2.2 eV. CL analysis at different electron excitation conditions shows spatial non-uniformity in-depth of the yellow and the band-edge emissions. These bands of luminescence are broadened and red- or blue-shifted as the electron beam penetrates in the sample. These behaviors are explained by a change of the fundamental band gap due to residual strain and the local thermal effect. It was found that the use of AlN buffer layer improves the crystalline quality and the luminescence property of GaN.