Etude de la localisation des excitons dans les alliages semiconducteurs II-VI: application a Zn_xHg_1_-_xTe

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 80626 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Cristallogenèse et caractérisations du diphosphate Na2ZnP2O7 pur et dopé et de la solution solide de type pérovskite Na(1x)BaxNb(1x)TixO3

Les propriétés physiques d un matériau sont intimement liées à sa structure cristalline et dans le cas d ions dopants aux sites qu ils occupent. La première partie de ce travail est dédiée au matériau diphosphate de sodium et de zinc Na2ZnP2O7, cristallisé out vitreux et ceci pour les ions dopants Co2+, Ni2+, Mn2+ et Eu3+. Les phases cristallisées ont été obtenues par la méthode Czochralski, les verres par trempe à partir de l état fondu. Un ensemble de caractéristiques physiques ont été mises en jeu (Raman, infrarouge, RPE, absorption optique, luminescence) pour déterminer les sites occupés par les ions dopants et l influence sur les propriétés optiques. La deuxième partie de cet travail consiste à une meilleure connaissance des matériaux diélectriques sans plomb appartenant à la famille pérovskite et plus particulièrement à la solution solide NaNbO3-BaTiO3. Des monocistaux ont été obtenus par la méthode des flux et caractérisés en utilisant plusieurs techniques : diffraction X, microanalyse, évolution thermique des domaines ferroélectriques-ferroélastiques, mesures diélectriques, piézoélectriques et pyroélectriques.Abstract The physical properties of a material are intimately related to its crystalline structure and in the case of doped ions they are dependent on to the sites they occupy. The first part of this work is dedicated to the diphosphate material of sodium and zinc Na2ZnP2O7, glassy and crystallized for doped ions of Co2+, Ni2+, Mn2+ and Eu3+. The crystallized phases were obtained by the Czochralski method whereas the glasses were obtained by quenching from the molten state. A set of physical characteristics were studied (Raman, infrared, EPR, optical absorption, luminescence) to determine the sites occupied by the doped ions and its influence on the optical properties. The second part of this work consists of a better knowledge of lead-free dielectric materials belonging to the family of Perovskite, more particularly to NaNbO3-BaTiO3 solid solution. Single crystals were obtained by the flux method and were characterized using several techniques: X-rays diffraction, microanalysis, thermal evolution of ferroelectric and ferroelastic domains; dielectric, piezoelectric and pyroelectric measurements.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Optical Properties of Blue Phosphor

LiIn1−Tm(WO4)2 (=0, 0.5, 1, 5, and 10 at.%) polycrystalline powders blue phosphors were prepared via the classical solid-state reaction method. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation, and emission spectra were used to characterize LiIn1−Tm(WO4)2 phosphors. By analyzing the excitation and emission spectra of LiIn1−Tm(WO4)2 samples, the result indicates that there exists the energy transfer only from the WO42− group to the 1G4 energy level of Tm3+ ion. On the other hand, the influence of the thulium concentration on the blue emission transition 1D2→3F4 and 1G4→3H6 and the emission of WO42− group are investigated

ESR and fluorescence studies of Mn-doped Na₂ZnP₂O₇ single crystal and glasses

X-band electron spin resonance (ESR) spectra and fluorescence measurements were performed on Mn-doped Na2ZnP2O7 (NZPO) single crystal synthesized by the Czochralski pulling method and glasses synthesized by the quenching process. For the single crystal, ESR angular dependences were measured in both the zx and xy plans of the NZPO lattice. The fine and hyperfine structure parameters and g-factor values were determined by modelling the experimental spectra. Using the Newman superposition model, the resonating centres in the single crystal and powder (crushed from crystals) samples are assigned to Mn2+ ions substituting for both zinc and sodium. For the glass sample the analysis of the ESR data shows that Mn2+ ions substitute for the Na+ ions. These interpretations are confirmed by the fluorescence measurements with a unique broad red band for the glassy compound and the presence of two emission bands (green and red) in the case of the crystal sample

Crystal growth and dielectric characterization of crystals derived from the solid-solution Ba_{(1−<i>x</i>)}Na_<i>x</i>Ti_{(1−<i>x</i>)}Nb_<i>x</i>O₃ (BTNN)

Single crystals of (1 − x)BaTiO3 + xNaNbO3 (BTNN) for x = 0.84 were obtained by high temperature solution growth using Na2B4O7 as solvent. The room temperature crystal structure of BTNN 16/84-phase was determined from X-ray single crystal diffraction data, in the tetragonal system with space group P4bm. The refinement from 246 independent reflections led to the following parameters: a = b = 5.5845(3) Å, c = 3.9453(2) Å, V = 123.041(11) Å3, Z = 2, with final cRwp = 0.150 and RB = 0.041. The structure of BTNN 16/84-phase can be described as a three-dimensional framework built up from (Nb–Ti)O6 octahedra with Na and Ba in the dodecahedral site of perovskite-like type. Some mm3-sized crystals have been selected and various dielectric measurements (ferroelectric, pyroelectric, and piezoelectric) have been performed. Transition from paraelectric to ferroelectric state at around 460 K has been observed to be in good agreement with ceramics of closer composition. Dielectric, piezoelectric and pyroelectric measurements on crystal confirm the ferroelectric behaviour of BTNN 16/84

Interference-aware congestion control protocol for wireless sensor networks

© 2014 The Authors. This paper deals with congestion and interference control in wireless sensor networks (WSN), which is essential for improving the throughput and saving the scarce energy in networks where nodes have different capacities and traffic patterns. A scheme called IACC (Interference-Aware Congestion Control) is proposed. It allows maximizing link capacity utilization for each node by controlling congestion and interference. This is achieved through fair maximum rate control of interfering nodes in inter and intra paths of hot spots. The proposed protocol has been evaluated by simulation, where the results rival the effectiveness of our scheme in terms of energy saving and throughput. In particular, the results demonstrate the protocol scalability and considerable reduction of packet loss that allow to achieve as high packet delivery ratio as 80% for large networks

DZ50: Energy-efficient wireless sensor mote platform for low data rate applications

© 2014 The Authors. A low cost and energy efficient wireless sensor mote platform for low data rate monitoring applications is presented. The new platform, named DZ50, is based on the ATmega328P micro-controller and the RFM12b transceiver, which consume very low energy in low-power mode. Considerable energy saving can be achieved by reducing the power consumption during inactive (sleep) mode, notably in low data rate applications featured by long inactive periods. Without loss of generality, spot monitoring in a Smart Parking System (SPS) and soil moisture in a Precision Irrigation System (PIS) are selected as typical representative of low data rate applications. The performance of the new platform is investigated for typical scenarios of the selected applications and compared with that of MicaZ and TelosB. Energy measurements have been carried out for different network operation states and settings, where the results reveal that the proposed platform allows to multiply the battery lifetime up to 7 times compared to MicaZ and TelosB motes in 10s sampling period scenarios

Emission spectrum and simulated laser parameters of Yb³⁺:LiLu(WO₄)₂ crystal

Crystal growth and spectroscopic study of the new compound Yb3+:LiLu(WO4)2 have been undertaken for the first time. Preliminary growth using flux method with Li2W2O7 as solvent allows us to obtain small single crystals. As a promising laser crystal, Yb3+:LiLu(WO4)2 have characteristics of a low concentration quenching effect and low laser threshold owing to the existence of covalent bands in WO4 group. Its absorption peak at 977 nm, corresponding to 2F7/2(1) → 2F5/2(5) transition, is suitable for laser diode pumping. Emission spectrum shows large bands extending from 900 to 1100 nm, five peaks centred respectively at 959, 978, 997, 1025 and 1056 nm are well resolved. The FWHM of the most intense emission band located at 997 nm is about 20 nm. Energy Stark levels are: 0, 201, 469 and 749 cm−1 for the 2F7/2 component and: 10,230, 10,427 and 10,890 cm−1 for the 2F5/2 component. The overall splitting of 2F7/2 ground-state manifold is 749 cm−1, which is larger than that observed in similar systems such as Yb3+:LiLa(WO4)2 (632 cm−1). Such large crystal-field splitting of the 2F7/2 ground-state will be advantageous to lower laser thermal effect and re-absorption losses. As we can see, the stark electronic levels are characterized by relatively large crystal-field splitting favourable to laser crystal applications in the near infrared spectral region