1,721,032 research outputs found
Élaboration, caractérisation structurale et mesures électriques de matériaux oxynitrures semiconducteurs pour application comme capteurs chimiques de gaz
No full textLes travaux réalisés au cours de cette thèse ont, dans un premier temps, été consacrés à la synthèse et à la caractérisation de matériaux (oxy)nitrures semiconducteurs: le nitrure de gallium (GaN), l oxynitrure de gallium (GaON) et l oxynitrure de zinc et germanium (ZnGeON). Ces matériaux ont été obtenus par nitruration d oxydes précurseurs commerciaux, obtenus par des procédés de chimie douce ou par réaction à haute température. Une étude comparative par diffraction des rayons X et spectroscopie Raman sur GaON et GaN a permis de déterminer la structure de GaON, jusque là encore controversée. GaON, obtenu à partir de l oxyde NiGa O , cristallise dans une variété cristallographique originale: la structure carborundum II, également appelée 6H-SiC. La technologie du dip coating, méthode de dépôt alternative à la sérigraphie, a permis l obtention de capteurs chimiques à base de ZnGeON. Le comportement de capteurs ZnGeON dip coatés, d une épaisseur de 35 m, a finalement été étudié vis-à-vis du monoxyde de carbone et du dioxyde de carbone. Les mesures électriques effectuées ont montré que les capteurs ZnGeON sont sensibles à ces gaz, à des concentrations de 100 ppm. Les propriétés de détection de ces capteurs sont intéressantes, particulièrement vis-à-vis du monoxyde de carbone pour lequel on observe une sensibilité très importante entre 150 et 200C.Gallium nitride (GaN), gallium oxynitride (GaON) and zinc and germanium oxynitride (ZnGeON) have been prepared by ammonolysis of various oxides precursors : commercial powder and precursors prepared by soft chemistry process or solid state reaction. These (oxy)nitride semiconductors have been prepared and then characterized. A comparative structural study has been carried out on GaN and GaON using XRD and Raman spectroscopy to determine GaON crystallographic structure. We found that GaON prepared from nickel gallate NiGa O crystalises in a carborundum II structure (6H-SiC). Dip coating technology has been studied as an alternative to screen printing technology, and allowed the preparation of ZnGeON based chemical sensors. Dip coated ZnGeON sensors of 35 m thickness have been studied for detection of carbon monoxide and carbon dioxide at a concentration of 100 ppm. Electrical characterizations showed that ZnGeON sensors have interesting detection properties, especially in the case of carbon monoxide where we observed a very high sensitivity between 150 and 200C.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF
Characterization of fluorite-type oxynitride phases in the R2Tasingle bondOsingle bondN system (R = rare-earth element)
No full textInternational audienceThe synthesis of reactive nitridation precursors prepared using citrate complexation/calcination route has shown interest in the study of the system R2Tasingle bondOsingle bondN. Novel fluorite-type phases have been evidenced in the oxide series R2TaO5.5 (R = Ndsingle bondYb, Y) and oxynitride series R2.67Ta1.33(O,N,□)8. Chemical and optical analyses were performed to characterize these phases, as well as structural analogies with R2WO6 and R2MoO5 oxide families
Nano-scaled TiO(OD)2: a time resolved 1H/2D isotope exchange study observed in situ with neutron scattering at 20°C and 40°C
No full textInternational audienceIn situ neutron diffraction measurements of the nanocrystalline deuterated oxyhydroxide TiO(OD)(2) compound were performed as a function of time and temperature under NH(3) gas flow in order to study the hydrogen-deuterium exchange mechanism. Data were collected on the instrument D20 at the ILL (France) and the analysis of the kinetics was directly based on the contrast variation of the incoherent neutron cross section of hydrogen and deuterium. The time evolution of the hydrogenated phase fraction was described using the well-known Kolmogorov-Johnson-Mehl-Avrami (KJMA) expression. The H/D exchange reaction is complete within 140s at 20°C and within 120s at 40°C. The activation energy for the H/D exchange reaction is estimated to be 37kJ/mol
Preparation and characterization of the (Cd,Zn)<sub>2-x</sub>GeO<sub>4-x-3y</sub>N<sub>2y</sub>oxynitride phase. Utilization as sensitive material for H<sub>2</sub>S gas sensor
No full textInternational audienc
Gas-sensing properties of Ta-doped MoO<sub>3-x</sub>
No full textInternational audienceDuring a search for alternative materials to replace tin(IV) oxide in semiconducting oxide gas sensors it has been found that sensors comprising sub-stoichiometric molybdenum trioxide exhibit promising characteristics
Barium lanthanum sulfide ceramics as new far-infrared transparent materials
No full textInternational audienceBarium lanthanum ternary sulfide (compositions within the BaLa2S4- γ-La2S3 solid solution of cubic Th3P4-type structure, referred to as BLS) was investigated as a novel infrared transparent ceramic for applications as external structural window material. This research marks the initial efforts to consolidate BLS powders into dense ceramics and reports, for the first time, the processing of polycrystalline BLS infrared transparent ceramics. The powders were produced through a combustion synthesis method followed by a sulfurization treatment in pure H2S. Sintering was carried out using hot-pressing and natural sintering. The characteristics of the powders and ceramics (purity, morphology, microstructure and optical transmission) are detailed and compared to evaluate the effectiveness of the two sintering techniques. A peak transmission of 20% at 16.5 μm was achieved for 1.4 mm-thick ceramics that were hot-pressed for 2 h at 1250 °C and subsequently annealed for 12 h at 1250 °C in pure H2S. With an extended transparency window (up to 20 μm, compared with 12 μm for ZnS), BLS polycrystalline ceramics are uniquely suited for use in harsh environments for space applications in both civil and military fields
Préparation et caractérisation de poudres et céramiques (oxy) sulfures pour applications en optique active et passive
No full textCe travail de recherche porte sur la synthèse, la mise en forme et la caractérisation de matériaux à base de sulfures ou oxysulfure pour des applications optiques passives ou actives. Différentes techniques de synthèse, notamment la précipitation en milieu aqueux ou la synthèse par combustion, ont été utilisées pour préparer les nanopoudres précurseurs, ZnS, CaLa S , BaLa S ou La O S. Il a été constaté qu'un traitement sous H S/N permet de purifier ces précurseurs tout en conservant la phase cubique (ZnS) nécessaires pour la préparation de céramiques transparentes. Des céramiques ZnS, avec une transmission maximale d'environ 70% à la longueur d'onde de 10 m, ont été élaborées par compaction à chaud en utilisant les techniques Hot Pressing ou Spark Plasma Sintering pour des applications optiques dans la fenêtre 8-12 m. Des poudres de ZnS dopées au Fe ont été synthétisées et étudiées avec une bande d'émission très large entre 3 m et 4,5 m. Des effets laser ont été obtenus à des longueurs d'ondes entre 3,4 m et 3,6 m en fonction de la température. Des poudres d'oxysulfure de lanthane co-dopées à l'Er et l'Yb ont été étudiées comme matériaux fluorescents pour modifier le spectre solaire avec l'objectif d'augmenter le rendement des cellules solaires photovoltaïques. Un rendement quantique supérieur à 100% a été obtenu avec un pompage à 523 nm pour des émissions dans le visible et le proche infrarouge. Ces résultats montrent que ces matériaux sont des matrices très efficaces pour le dopage des terres rares.This work focuses on the preparation and characterization of sulfides and oxysulfide materials for passive or active optical applications. Precipitation and combustion synthesis were used as synthesis methods for the preparation of the precursor nanopowders such as ZnS, CaLa S , BaLa S or La O S. Additional treatment under H S/N was performed to purify the powders without modifying the cubic structure (ZnS) which is necessary for obtaining transparent ceramics. ZnS ceramics, with a maximum transmission of about 70% at the wavelength of 10 m, were elaborated by Hot Pressing or Spark Plasma Sintering techniques for optical applications in the 8-12 m. Fe doped ZnS powders were synthesized and studied with a large emission band between 3 m and 4.5 m. Lasing has been obtained between 3.4 m and 3.6 m depending on the temperature. Er and Yb co-doped lanthanum oxysulfide powders were studied as fluorescent materials for solar spectrum in order to increase the efficiency of photovoltaic solar cells. Quantum yield higher than 100% was obtained with pumping at 523 nm for emission in the visible and the near-infrared. These results show that the lanthanum oxysulfide materials are efficient hosts for rare earths doping.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF
Study of the Thermal Nitridation of Nanocrystalline Ti(OH)4 by X-ray and in Situ Neutron Powder Diffraction
No full textInternational audienceIn situ neutron diffraction measurements of nanocrystalline titanium oxynitrides were performed, as a function of temperature and time, to explore the nitrogen/oxygen substitution mechanism occurring during their synthesis by reaction of gaseous ammonia with nanocrystalline Ti(OH)4. These neutron diffraction experiments are supported by chemical analysis and X-ray diffraction, allowing the description of the structural variations and ordering process between the Ti(O/N)2 anatase and the Ti(O/N) rock-salt phases. Our results show that the formation of the Ti(O/N) rock-salt phase goes along with the creation of vacancies on the Ti sites and that the N/O substitution proceeds but without N/O ordering
Nitridation under ammonia of high surface area vanadium aerogels
No full textInternational audienceVanadium pentoxide gels have been obtained from decavanadic acid prepared by ion exchange on a resin from ammonium metavanadate solution. The progressive removal of water by solvent exchange in supercritical conditions led to the formation of high surface area V 2 O 5 , 1.6H 2 O aerogels. Heat treatment under ammonia has been performed on these aerogels in the 450–900 1C temperature range. The oxide precursors and oxynitrides have been characterized by XRD, SEM, TGA, BET. Nitridation leads to divided oxynitride powders in which the fibrous structure of the aerogel is maintained. The use of both very low heating rates and high surface area aerogel precursors allows a higher rate and a lower threshold of nitridation than those reported in previous works. By adjusting the nitridation temperature, it has been possible to prepare oxynitrides with various nitrogen enrichment and vanadium valency states. Whatever the V(O,N) composition, the oxidation of the oxynitrides in air starts between 250 and 300 1C. This determines their potential use as chemical gas sensors at a maximum working temperature of 250 1C.
Synthesis of crystallized TaON and Ta3N5 by nitridation of Ta2O5 thin films grown by pulsed laser deposition
No full textInternational audienceTaON and Ta 3 N 5 thin films of different thicknesses were prepared by pulsed laser deposition of tantalum oxide followed by ex situ thermal nitridation under ammonia. The nitridation was carried out in flowing gas in the 600–800 • C temperature range. The dependence of tantalum oxynitride and nitride crystalline phases formation on nitridation reaction parameters was investigated. Structural and microstructural characteristics were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM)
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