A comprehensive investigation was reported into the synthesis, characterization and photoluminescence properties of calcium metaniobates and associated alkali earth alloy systems. Previous studies have shown that calcium metaniobate exhibits a strong self-activated blue luminescence at room temperature in stark contrast to the pyroniobates which are known to exhibit a temperature dependent luminescence that quenches above 100 K. The mechanism of this behavior has been studied by measuring the spectral characteristics of the photoluminescence and photoluminescence excitation spectra on the crystalline and morphological properties of the powders as determined from x-ray diffraction and scanning electron microscopy. By correlating the synthesis parameters with the physical, chemical and optical properties of calcium metaniobate, the optimum conditions for efficient blue-visible emission and chemical stability under vacuum ultraviolate (VUV) radiation has been determined. These materials have the potential to replace Barium Magnesium Aluminate, which is currently used as the blue phosphor in plasma displays.

A novel solid state illumination source has been developed. A two terminal dual LED has been created with the ability to control the relative intensities of the two emission peaks by varying drive current. Doping profiles have been used to extend the dynamic range of the dual LED over other reported devices. Operation of the dual LEDs is explained as a function of drive current. In addition, novel use of phosphor mixtures allows the creation of a broadband spectral power distribution that can be varied using a dual LED as an excitation source. Combinations of phosphors that have varied excitation spectra provide the ability to selectively excite different phosphors with the different LED emission peaks. First and second generations of the two terminal dual LED and the phosphor combination are discussed. The final source has the ability to mimic the light of a blackbody radiator over a range of 3200 K - 5300 K. The development of a three terminal dual LED as a pump source was prohibited by the need for a III-nitride tunnel junction, that proved unattainable in the scope of this work. However, several novel doping schemes were investigated toward this end. Finally, a circadian light source has also been developed that can affect physiological changes in humans, and a light box for entrainment of circadian rhythms in rats has been built.

The thesis is devoted to the frequency-resolved investigation of the relaxation processes in optoelectronic materials and devices. Conventional fluorescence decay time measurement technique in the frequency domain was adapted for the use with light-emitting diode (LED) excitation and signal registration by a lock-in amplifier. Inorganic phosphors synthesized by aqueous sol-gel combustion method for using as wavelength converters in white LEDs as well as advanced organic semiconducting materials were investigated by photoluminescence decay time and quantum yield measurement techniques. The photoluminescence decay time measurement technique with extremely low quasi-continuous UV LED excitation was applied for the carrier dynamics research in GaN epitaxial layers. The investigation under such a low excitation conditions revealed the contribution of donor-acceptor recombination in the yellow luminescence of GaN. The techniques for in-situ thermal characterization of encapsulated LEDs, including the measurements of phosphors converter temperature and heat relaxation time constants inside a LED, were developed and demonstrated for the investigation of commercial low- and high- power LEDs. / Disertacija yra skirta relaksacijos procesų, vykstančių optoelektronikos medžiagose ir prietaisuose, tyrimui dažninės skyros metodu. Įprastas fluorescencijos gesimo trukmės tyrimo metodas, veikiantis harmoniškai moduliuoto žadinimo režimu, buvo adaptuotas žadinimui puslaidininkiniais šviestukais ir signalų registravimui radijo dažnių faziniu detektoriumi. Neorganiniai vandeniniu zolių-gelių metodu susintetinti fosforai, skirti baltų puslaidininkinių šviestukų gamybai, ir modernios organinės optoelektronikos medžiagos buvo ištirtos fotoliuminescencijos gesimo dėsnio ir kvantinės išeigos matavimo metodikomis, siekiant nustatyti krūvininkų rekombinacijos savybes ir optimizuoti sintezės parametrus. Realizuotas GaN epitaksinių sluoksnių liuminescencijos kinetikos tyrimas labai žemo kvazitolydinio sužadinimo atveju patvirtino rekombinacijos per priemaišas svarbą šio tipo medžiagose. Buvo sukurtos naujos prekinių šviestukų šiluminių savybių in-situ tyrimo metodikos, veikiančios dažninės skyros režimu. Bangos ilgio keitiklių temperatūra baltuose šviestukuose ir šilumos relaksacijos konstantos įvairaus tipo šviestukų konstrukciniuose elementuose buvo išmatuotos esant vardiniam šviestukų veikimo režimui.

Disertacija yra skirta relaksacijos procesų, vykstančių optoelektronikos medžiagose ir prietaisuose, tyrimui dažninės skyros metodu. Įprastas fluorescencijos gesimo trukmės tyrimo metodas, veikiantis harmoniškai moduliuoto žadinimo režimu, buvo adaptuotas žadinimui puslaidininkiniais šviestukais ir signalų registravimui radijo dažnių faziniu detektoriumi. Neorganiniai vandeniniu zolių-gelių metodu susintetinti fosforai, skirti baltų puslaidininkinių šviestukų gamybai, ir modernios organinės optoelektronikos medžiagos buvo ištirtos fotoliuminescencijos gesimo dėsnio ir kvantinės išeigos matavimo metodikomis, siekiant nustatyti krūvininkų rekombinacijos savybes ir optimizuoti sintezės parametrus. Realizuotas GaN epitaksinių sluoksnių liuminescencijos kinetikos tyrimas labai žemo kvazitolydinio sužadinimo atveju patvirtino rekombinacijos per priemaišas svarbą šio tipo medžiagose. Buvo sukurtos naujos prekinių šviestukų šiluminių savybių in-situ tyrimo metodikos, veikiančios dažninės skyros režimu. Bangos ilgio keitiklių temperatūra baltuose šviestukuose ir šilumos relaksacijos konstantos įvairaus tipo šviestukų konstrukciniuose elementuose buvo išmatuotos esant vardiniam šviestukų veikimo režimui. / The thesis is devoted to the frequency-resolved investigation of the relaxation processes in optoelectronic materials and devices. Conventional fluorescence decay time measurement technique in the frequency domain was adapted for the use with light-emitting diode (LED) excitation and signal registration by a lock-in amplifier. Inorganic phosphors synthesized by aqueous sol-gel combustion method for using as wavelength converters in white LEDs as well as advanced organic semiconducting materials were investigated by photoluminescence decay time and quantum yield measurement techniques. The photoluminescence decay time measurement technique with extremely low quasi-continuous UV LED excitation was applied for the carrier dynamics research in GaN epitaxial layers. The investigation under such a low excitation conditions revealed the contribution of donor-acceptor recombination in the yellow luminescence of GaN. The techniques for in-situ thermal characterization of encapsulated LEDs, including the measurements of phosphors converter temperature and heat relaxation time constants inside a LED, were developed and demonstrated for the investigation of commercial low- and high- power LEDs.

L’éclairage par « LEDs blanches » est devenu un enjeu majeur afin d’élaborer des dispositifs à bas coût, produisant une lumière confortable pour l’œil en évitant de forte composante bleue nocive pour la vue et la santé. A l’Institut Néel, nous développons un nouveau type de luminophores à base de poudres d’aluminoborate d’yttrium. Le caractère innovant de ces poudres est de produire une large bande d’émission dans l’ensemble du spectre visible, à partir de défauts structuraux présents dans la matrice amorphe. Ainsi ces luminophores permettraient de générer un éclairage blanc sous l'excitation de LED émettant dans le proche UV (370-390 nm). De plus, ces matériaux n’ont pas de lanthanides, ce qui réduit leur coût.Les luminophores d’aluminoborate d’yttrium ont dans un premier temps été préparés par la méthode des précurseurs polymériques, impliquant plusieurs étapes de recuits sous atmosphères contrôlées à des températures relativement élevées (700-740°C). Le travail de thèse a porté dans un premier temps sur la mise au point de la synthèse par voie sol-gel. Ces travaux ont permis de réduire simultanément les durées du procédé et les températures des traitements thermiques (450-650°C), ainsi que la perte de masse globale par décomposition des fonctions organiques des précurseurs.Par analyses thermiques (ATD, ATG) couplée à la spectrométrie de masse et RMN 13C, nous avons caractérisé la présence de résidus carbonés dans les poudres lorsqu’elles présentent de la luminescence. Néanmoins, une partie de ces résidus carbonés se trouvent sous forme de carbone pyrolytique (carbones aromatiques) qui entrainent une réabsorption partielle de la luminescence émise dans le visible, entrainant ainsi une diminution de l’intensité d’émission. Les caractérisations structurales (DRX, FTIR, RMN) menées sur les poudres ont montré que la première phase cristalline apparaissant pour les deux méthodes de synthèse est une phase d’aluminoborate Al4B2O9. L’étude par Fonction de Distribution de Paires (PDF) montre que la matrice aluminoborate amorphe présente un ordre local proche de la phase Al4B2O9, à savoir une organisation tridimensionnelle cyclique de métaux pontés par des ligands oxo ou hydroxo de 6 à 10 chaînons. D’autre part, sur la base des résultats RMN 13C, l’yttrium semble garder dans sa sphère de coordination des ligands propionates jusqu’à hautes températures. Par ailleurs, la présence d’espèces radicalaires dans les poudres luminescentes a été mise en évidence par résonnance paramagnétique électronique. Un ensemble de mesures à différentes fréquences, en mode continu et pulsé, ont permis d’attribuer ces espèces à des radicaux carbonés. Les études de photoluminescence non-résolues et résolues en temps, couplées à des analyses de thermoluminescence ont mis en évidence la présence de plusieurs espèces luminescentes présentant essentiellement des propriétés de fluorescence (durée de vie de quelques ns) mais aussi une très faible part de phosphorescence (durée de vie de la ms à plusieurs s). Les poudres synthétisées par voie sol-gel présentent un rendement quantique interne d’environ 30 %.Aux vues des différentes analyses, les poudres luminescentes synthétisées par voie sol-gel semblent contenir deux types de résidus carbonés : l’un à l’origine des propriétés de photoluminescence et l’autre défavorable (carbone pyrolytique piégé) car absorbant partiellement l’émission de luminescence. En conclusion, nous proposons un mécanisme de photoluminescence extrinsèque, basé sur des centres carbonés dispersés au sein de la matrice minérale, favorisant majoritairement la fluorescence dans le bleu (bande centrée vers 430 nm) et dans le vert (bande centrée vers 500 nm) mais aussi une faible proportion de phosphorescence émettant dans la même gamme de longueurs d’onde. / White solid state lighting is recognized as a major disruptive technology with an urgent need of low coast prices, associated to good color quality, confortable for eyes, by reducing the bluish harmful contribution of “cold” lighting. At Néel Institute, we develop a new type of phosphors based on yttrium aluminoborate powders. These innovating powders exhibit a large emission band on the whole visible range, arising from structural defects in the amorphous matrix. Thus, with a single phosphor, one can generate warm white lighting through the excitation of LEDs emitting in the near UV (370-390 nm). Moreover, these phosphors don’t possess lanthanides, making them less expensive.The powders synthesized by chimie douce routes, are annealed under controlled atmosphere. The yttrium alominoborate phosphors were first prepared by the polymercic precursor method. This synthesis road involved several steps and relatively high annealing temperatures (700-740°C). This thesis was focused on the sol-gel synthesis method. By this work, the duration process, the annealing temperature (450°C-650°C), and the global mass loss incoming from the organic precursors decomposition were significantly reduced.Thermal analysis (TDA-TG) coupled with mass spectrometry and 13C RNM show residual carbon groups in luminescent powders. Nevertheless, one part of the residual carbons is pyrolytic carbon (aromatic carbon), which leads to partial re-absorption of the visible emitted luminescence, and thus induces a decrease of the emission intensity.The structural characterizations of yttrium aluminoborate powders (XRD, FTIR, NMR) show that Al4B2O9 aluminoborate phase, is the first appearing crystalline phase during the increase of calcination temperature. The Pair Distribution Function (PDF) study demonstrates that amorphous aluminoborate matrix exhibit a short range ordering close to Al4B2O9 phase: a cyclic tridimensional organization of metal bridges by oxo or hydroxo ligands. On the other hand, based on 13C NMR results, yttrium seems to conserve propionate ligands in its coordination sphere until high temperature. Otherwise, the presence of radical species was evidenced in luminescent powders by electronic paramagnetic resonance. A set of measurements performed at different frequencies, in continuous and pulsed modes, allows attributing that species to carbon radicals. The presence of several luminescence species exhibiting essentially fluorescence properties (ns life time) and very weak phosphorescence emission (ms and s lifetime) was shown by the means of photoluminescence studies steady-state and time resolved coupled to thermoluminescence analysis. The powders synthesized by sol-method exhibit a 30 % internal quantum yield.Thanks to the different characterizations, luminescent powders synthesized by the sol-gel route seem to contain two types of residual carbons: one at the origin of the luminescence properties (carbon-related radicals) while the other, pyrolytic carbon, is damaging as it absorbs partially the luminescence emission. To conclude, we suggest an extrinsic mechanism for the photoluminescence, which is based on carbon centers dispersed inside the mineral matrix, favoring mainly fluorescence in blue (430 nm) and green emissions (500 nm) associated with a weak phosphorescence emission in the same emission range.

This work specifies the synthesis and the characterization of amorphous powders belonging to the Y2O3 - Al2O3 - B2O3 system. The main objective of this work was to develop amorphous powders near the YAl3(BO3)4 composition without any rare earth as doping for the development of a new family of phosphors for solid state lighting systems excited by near ultra-violet light. The sol-gel and polymeric precursor methods were applied in order to produce these powders. After these syntheses we optimized several parameters such as thermal treatments: two different routes were tested: direct calcination and pyrolytic decomposition followed by calcination. The temperature, annealing time, heating rate and the effects of impurities on the photoluminescence (PL) were studied. A comparison between these two synthesis methods was done. The powder samples were characterized by thermal analysis technique, X-ray diffraction, photoluminescence and infra-red spectroscopies, transmission and scanning electron microscopy, elemental analysis and electron probe microanalyses, nuclear magnetic resonance, cathodoluminescence, electronic paramagnetic resonance. From the thermal analyzes, it was observed that the powder prepared by polymeric precursor method shows a glass transition temperature (Tg) around 740 °C and crystallization temperatures (Tx) at 815, 850 and 900 °C. Amorphous powder showing high photoluminescence emission (between 400 and 750 nm) and quantum yields higher than 90% at 365 nm excitation, without any phase segregation were obtained when the samples are annealed at temperatures at around Tg, above this temperatures the powder start to crystallize decreasing their PL properties. Other compositions were studied by increasing the relative amounts of Y2O3, Al2O3 and B2O3, by removing the yttrium, addition of SiO2. Finally, the first measurements of color coordinates and the preliminary tests on the thermal and photo stability have been done. / Ce travail de these porte sur la synthèse et la caractérization de poudres amorphes en appartenant système Y2O3 - Al2O3 - B2O3. L'objectif principal du travail a été de préparer des poudres amorphes dont la composition est proche de YAl3(BO3)4 sans terre rare en vue la réalisation de phosphores pour des dispositif d'éclairage solide à base de LED emmetant dans le proche UV. Pour la synthèse des poudres les methodes sol-gel et celles des precurseurs polymériques ont été utilisées. Nous avons ensuite optimizé les paramètres thermiques, en utilisant deux stratégies: la calcination directe et la pyrolyse suivrie d'une seconde étape de calcination. La température, le temps de recuit, vitesse de chauffage et les effect des impurities sur la photoluminescence ont été étudié. Les échantillons en poudre ont été caractérisés par les techniques d'analyse thermiques, diffraction des rayons X, la spectroscopie de photoluminescence et la spectroscopie infra-rouge, microscopies électronique à balayage et en transmission, les méthodes des analyses élémentaire et microsonde électronique, la résonance magnétique nucléaire, cathodoluminescence et le résonance paramagnétique électronique. Par l'analyse thermique, on a observé que la poudre préparée par la méthode de précurseur polymère a une température de transition vitreuse (Tg) autour de 740 ° C et des températures de cristallisation (Tx) à 815, 850 et 900 ° C. Les poudres amorphes presentent de larges bandes d'émission de photoluminescence (entre 400 et 750 nm) avec des rendements quantiques supérieurs à 90% pour une excitation de 365 nm. De plus, les poudres microscopiques obtenu sont chimiquement homogene avec des composition très proche de celle initialmente visée YAl3(BO3)4 lorsque les échantillons sont recuits à des températures voisine du Tg. Au-dessus de cette température, la poudre commence à se cristalliser conduisant à réduction de l'intensité de PL. D'autres compositions ont été étudiées en augmentant la quantité relative de Y2O3, Al2O3 et B2O3, en éliminant complètement l'yttrium, ou en ajutant SiO2. Finalement, les premiers mesures de coordonnées de couleur et les essais préliminaires sur la stabilité thermique et photométrique sont très prometeur. En effet, outres les rendemente specifiques de luminescence très elevés ces luminophores émitent de lumiére très chaudes.

Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro / O objetivo deste trabalho é a síntese e investigação estrutural e óptica de amostras SrGa2O4 dopados com 1% de íons Ni2+. Estas amostras foram sintetizados por reação do estado sólido convencional, utilizando como materiais de partida de alta pureza Ga2O3, SrCO3 e NiO em quantidades estequiométricas. As amostras foram caracterizadas estruturalmente pelo método de difração de raios - X( XRD ) e as medições de difração mostraram que as amostras têm uma única fase monoclínica. Os padrões de XRD também foram refinados pelo método de Rietveld, que permitiu a determinação dos parâmetros de célula unitária. A Caracterização óptica das amostras puras e dopadas SrGa2O4 foram realizadas as medições a partir de fotoluminescência, de excitação e de absorção fotoacústica, à temperatura ambiente. Os espectros de emissão mostraram três bandas de emissão localizadas em 557 nm, 661 nm e 844 nm e foram identificadas essas bandas, respectivamente, com as seguintes transições eletrônicas :1T2 (1D) → 3A2 (3F), 3T1 (3F)→ 3A2 (3F) e 1T2 (1D) → 3T2 (3F). Os espectros de excitação mostraram seis bandas de absorção associadas às transições electrônicas do nível 3A2 (3F) para o 3T1 (3P) , T1 (3P), 1A1 (1G), 1T2 (1D), 3T1 (3F), 1E (1D) e 1T2, 1E (1G). Medidas de absorção fotoacústica também foram realizados com o fim de verificar as transições ópticas observadas nos espectros de excitação e de identificar novas bandas de absorção óptica. Os resultados demonstraram que os íons de Ni2+ ocupam dois locais octaédricos diferentes na amostra SrGa2O4 dopado. A partir das transições ópticas observadas nos espectros de excitação e fotoacústica, determinou-se o parâmetro de cristal de campo, dq, e parâmetros Racah, B e C. A proporção Dq / B ≈ 1.2 para ambos os locais são típicos para Ni2+ íons inseridos em redes de óxido e em coordenação octaédrica. / The aim of this work is the synthesis and structural and optical investigation of SrGa2O4 samples doped with 1% of Ni2+ ions. These samples were synthesized by conventional solid-state reaction using as starting materials high purity SrCO3, Ga2O3 and NiO in stoichiometric quantities. Samples were structurally characterized by X-ray diffraction (XRD) method and the diffraction measurements showed that the samples have a single monoclinic phase. The XRD patterns were also refined by the Rietveld method which allowed the determination of the unit cell parameters. Optical characterization of the pure and doped SrGa2O4 samples were performed from photoluminescence, excitation and photoacoustic absorption measurements at room temperature. Emission spectra showed three emission bands localized at 557 nm, 661 nm and 844 nm and these bands were identified, respectively, with the following electronic transitions: 1T2 (1D) → 3A2 (3F), 3T1 (3F)→ 3A2 (3F) e 1T2 (1D) → 3T1 (3F). Excitation spectra showed six absorption bands associated to the electronic transitions from the level 3A2 (3F) to the 3T1 (3P), 1A1 (1G), 1T2 (1D), 3T1 (3F), 1E (1D) e 1T2, 1E (1G) levels. Photoacoustic absorption measurements were also carried out in order to verify the optical transitions observed in the excitation spectra and to identify new optical absorption bands. The results showed that Ni2+ ions occupy two different octahedral sites in the SrGa2O4 doped sample. From the optical transitions observed in the excitation and photoacoustic spectra, we determined the crystal-field parameter, Dq, and Racah parameters, B and C. The ratio Dq/B ≈ 1.2 for both sites are typical for Ni2+ ions inserted in oxide lattices and in octahedral coordination.

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2016-11-11T17:17:08Z No. of bitstreams: 0 / Made available in DSpace on 2016-11-11T17:17:08Z (GMT). No. of bitstreams: 0 / Neste trabalho o carbono comercial Vulcan XC72 foi modificado com heteroátomos de N, S e P por meio do tratamento térmico a 800 °C com ureia, ácido sulfúrico e ácido fosfórico, respectivamente. Os carbonos modificados foram utilizados na preparação de eletrocatalisadores Pt/C e PtRu/C e aplicados na oxidação eletroquímica do metanol. Os materiais obtidos foram caracterizados por espectroscopia dispersiva de raios X, difração de raios X, espectroscopia Raman, microscopia eletrônica de transmissão e voltametria cíclica. A oxidação eletroquímica do metanol foi estudada por voltametria cíclica e cronoamperometria. Os espectros Raman mostraram que as razões de intensidades das bandas D e G dos suportes de carbono modificados com heteroátomos foram maiores que a observada para o carbono Vulcan XC72 sugerindo a incorporação na estrutura do carbono. As análises por difração de raios X mostraram para todos os eletrocatalisadores Pt/C a estrutura cúbica de face centrada (cfc) e as micrografias eletrônicas de transmissão que todos materiais apresentaram tamanhos de partícula na faixa 4-5 nm. Para os eletrocatalisadores PtRu/C preparados com os suportes de carbono modificados com N e S, os difratogramas de raios-X apresentaram apenas a fase Pt (cfc) não havendo deslocamento dos picos desta fase. Nestes materiais os picos referentes a fases de Ru não foram identificados sugerindo que o Ru apresenta-se como uma fase amorfa. Por outro lado, para o material modificado com P, observou-se um deslocamento dos picos da fase Pt(cfc) para ângulos maiores, sugerindo a formação de liga Pt-Ru e também foi observado a presença da fase Ru hexagonal compacta. Os eletrocatalisadores Pt/C e PtRu/C preparados com os suportes contendo heteroátomos apresentaram uma melhora significativa na atividade eletrocatalítica quando comparados aos materiais preparados com o suporte não modificado. A melhora na atividade pode estar relacionada a modificações na interação metal-suporte, bem como, as diferentes espécies Pt e Ru presentes no caso do eletrocatalisador PtRu/C preparado utilizando o suporte modificado com P. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Ce travail de thèse porte sur l'élaboration de luminophores (La₂O₂S:Er,Yb, La₂O₂S:Nd, Lu₂O₂S:Nd, CaLa₂S₄:Nd) et céramiques optiques infra-rouges (ZnS et CaLa₂S₄). Il a consisté, en premier lieu, à développer des méthodes de synthèse de poudres de très grande pureté, principalement par méthode de combustion suivie de traitement sous H₂S(/N₂). Des nanopoudres de La₂O₂S:Er,Yb (50-200nm) se sont révélées être des luminophores très efficaces pour l'up‐conversion. Des nanopoudres de La₂O₂S: Nd (~250 nm) présentent un effet laser avec une énergie émise de 2,5 mJ et un rendement supérieur à 15%. L'étude de la densification (par hot pressing, HP) de poudres de ZnS préparées selon trois voies de synthèse (précipitation, synthèse par combustion et réaction en bain fondu) a permis d'élaborer des céramiques transparentes ZnS de qualité CVD‐HIP, avec des transmissions maximales supérieures à 73% à 12μm (précipitation) et la transmission maximale théorique de 75% à 10μm (combustion). Le frittage par HP de poudres de CaLa₂S₄, entre 1100°C et 1300°C, conduit, à cause d'une contamination par le carbone de la matrice de frittage et/ou d'une perte de soufre, à des céramiques noires et opaques en IR. CaLa₂S₄ comme nouveau matériau de matrice tolère des taux de dopage élevés en Nd. Après excitation à 815nm, il présente deux émissions intenses à 900nm et 1060nm, ainsi qu'une émission plus faible à 1350nm avec une intensité de luminescence qui augmente jusqu'à 10% molaire en Nd. / This thesis deals with the elaboration of phosphors (La₂O₂S:Er,Yb, La₂O₂S:Nd, Lu₂O₂S:Nd, CaLa₂S₄:Nd) and optical IR ceramics (ZnS and CaLa₂S₄). First, the routes for the synthesis of high purity powders have been developed, mainly by combustion method followed by H₂S(/N₂) treatment. La₂O₂S:Er, Yb nanopowders (50-200 nm) proved to be very efficient phosphors for up-conversion. La₂O₂S: Nd nanopowders (~250 nm) have a laser effect with a 2.5 mJ output energy and a yield higher than 15%. The densification by hot pressing of ZnS powders prepared following three synthesis routes (precipitation, combustion synthesis and reaction melt) has developed ZnS transparent ceramic of CVD‐HIP quality, with maximum transmission greater than 73% at 12μm (from powders obtained by precipitation) and with the theoretical maximum transmission of 75% at 10μm (from powders obtained by combustion). Hot pressing of CaLa₂S₄ powders, between 1100°C and 1300°C, leads to IR opaque ceramics with black color, due to carbon diffusion from the sintering matrix and/or to sulfur loss. CaLa₂S₄ as new phosphor matrix material tolerates high Nd doping level. After excitation at 815 nm, it shows two intense emissions at 900 nm and 1060 nm, and a lower transmission at 1350nm with a luminescence intensity that increases up to 10% (mol) Nd.

The aim of the present thesis is to evaluate fast scintillator materials, in both single-crystal and powder form, for possible usage in dedicated gamma ray imaging applications as well as in X-ray imaging techniques, requiring high frame rates. Powder scintillators are traditionally used in conventional X-ray imaging due to their property to produce high resolution images. This is because laterally directed optical photons, originating from the point of X-ray interaction, are strongly attenuated by light scattering effects on powder grains. This property however is their principal drawback for routine Nuclear Medicine applications. In these applications, photon counting accuracy rather than spatial resolution is required and to this aim high transparency crystals are used. In the present study we have tried to estimate whether the use of powder phosphors can improve the image quality in a dedicated gamma-ray system where spatial resolution than sensitivity is of primary significance. Evaluation was performed in thin and thick phosphor layers easily produced in the laboratory. In addition we present a low cost solution – consisting of a thick continuous powder scintillator screen – for use in dedicated high resolution small gamma imager. The advantages and disadvantages of proposed powder detector performance were compared to two standard 3 x 3 x 5mm3 and 2 x 2 x 3mm3 pixellated CsI:Tl scintillator detector configurations. System performance in terms of system sensitivity, system spatial resolution, energy resolution and linear energy response were measured at energy of 140 keV for close-proximity nuclear emission imaging. All measurements were carried out in photon counting mode in planar imaging configuration. The investigation was divided into two parts: Fast powder scintillators: In this part, powder scintillator screens of LSO:Ce, YAG:Ce and GOS:Pr were prepared in various coating thicknesses. Measurements concerning determination of emission spectra and absolute luminescence efficiency were carried out under X-ray excitation from 22 to 140 kV. Related parameters giving informations on luminescence and intrinsic properties of the phosphors such as X-ray luminescence efficiency, quantum detection efficiency, energy absorption efficiency and intrinsic conversion efficiency were also examined. Low cost and high resolution detector module: The goal of this part was to propose and evaluate a low cost solution for detector module – consisting of a thick continuous powder scintillator screen – for use in dedicated high resolution small gamma imagers. For the latter purpose, we examined the performance of the aforementioned fast powder scintillators in the form of thick screens easily produced in the laboratory. System performance in terms of system sensitivity, system spatial resolution, energy resolution and linear energy response were measured and compared for two standard 3 x 3 x 5mm3 and 2 x 2 x 3mm3 pixellated CsI:Tl. / Ο σκοπός της παρούσας διατριβής είναι η αποτίμηση φθοριζόντων υλικών υψηλής απόκρισης, τόσο σε κρυσταλλική όσο και σε κοκκώδη μορφή για πιθανή χρησιμοποίησή τους σε συγκεκριμένους τύπους ανιχνευτικών συστημάτων Πυρηνικής Ιατρικής όπως επίσης σε συστήματα απεικόνισης με ακτίνες Χ που απαιτούν πολύ γρήγορες λήψεις ιατρικής εικόνας. Τα φθορίζοντα υλικά κοκκώδους μορφής χρησιμοποιούνται ευρύτατα στην απεικόνιση με ακτίνες-Χ λόγω της υψηλής διακριτικής ικανότητας που μπορεί να επιτευχθεί. Στην παρούσα διατριβή μελετήθηκε κατά πόσο η χρήση νέων, γρήγορων φθοριζόντων υλικών κοκκώδους μορφής μπορεί να βελτιώσει την απόδοση συγκεκριμένων τύπων ανιχνευτικών συστημάτων Πυρηνικής Ιατρικής (π.χ. dedicated small nuclear imagers), στα οποία η διακριτική ικανότητα του συστήματος είναι πιο σημαντική από την ευαισθησία. Η αποτίμηση έγινε σε φθορίζοντα μεγάλου πάχους που παρασκευάστηκαν στο εργαστήριο. Επιπρόσθετα στην παρούσα διατριβή παρουσιάζεται η εφαρμογή ενός χαμηλού κόστους συμπαγούς ανιχνευτικού υλικού κοκκώδους μορφής σε ένα εξειδικευμένο σύστημα Πυρηνικής Ιατρικής. Γίνεται συστηματική μελέτη και εκτενής αναφορά στα πλεονεκτήματα και τα μειονεκτήματα αυτού του συστήματος. Τα αποτελέσματα συγκρίνονται με αντίστοιχα αποτελέσματα που ελήφθησαν με χρήση διακριτοποιημένων σπινθηριστών τύπου CsI:Tl, μεγέθους 3 x 3 x 5mm3 και 2 x 2 x 3 mm3. Η απόδοση του συστήματος ως προς την ευαισθησία (sensitivity), τη χωρική διακριτική ικανότητα (spatial resolution) και την ενεργειακή διακριτική ικανότητα (energy resolution) αποτιμήθηκε για ενέργεια 140 keV, που αντιστοιχεί στην ενέργεια του ισοτόπου 99mTc που χρησιμοποιείται ευρύτατα σε εξετάσεις Πυρηνικής Ιατρικής. Η πειραματική μελέτη χωρίστηκε σε δύο μέρη: Φθορίζοντα υλικά κοκκώδους μορφής: Στο πρώτο μέρος της παρούσας διδακτορικής διατριβής μελετήθηκαν τα φθορίζοντα υλικά κοκκώδους μορφής LSO:Ce, YAG:Ce και GOS:Pr σε διάφορα πάχη και για μεγάλη κλίμακα ενεργειών (Υψηλή τάση λυχνίας ακτίνων-Χ από 22 kV έως 140 kV). Ανιχνευτής χαμηλού κόστους και υψηλής διακριτικής ικανότητας: Ο τελικός στόχος της παρούσας διατριβής ήταν η κατασκευή ενός ενιαίου ανιχνευτή, βασισμένου σε σπινθηριστή κοκκώδους μορφής, χαμηλού κόστους και υψηλής διακριτικής ικανότητας, κατάλληλου για χρήση σε εξειδικευμένα συστήματα Πυρηνικής Ιατρικής. Για το σκοπό αυτό μελετήθηκε η συμπεριφορά των υλικών κοκκώδους μορφής LSO:Ce, YAG:Ce και GOS:Pr υπό διέγερση ακτίνων γάμμα με ισότοπο Τεχνητίου (99mTc), ενέργειας 140 keV, που χρησιμοποιείται ευρύτατα στην Πυρηνική Ιατρική. Τα υλικά αυτά υπό την μορφή ενιαίου, μεγάλου πάχους ( ≥2mm) και διαμέτρου (9 cm), συμπαγούς ανιχνευτή αξιολογήθηκαν με τεχνικές απεικόνισης μονού φωτονίου (single photon counting mode).