LIMA, Cleânio da Luz. Estudo das propriedades vibracionais do molibidato de sódio hidratado por espectroscopia Raman em função da pressão hidrostática. 2008. 87 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2008. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-09T18:03:35Z No. of bitstreams: 1 2008_dis_cllima.pdf: 1735905 bytes, checksum: a38aca9b6ecaf707ab1b6f24cdf68c3d (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-06-11T17:49:18Z (GMT) No. of bitstreams: 1 2008_dis_cllima.pdf: 1735905 bytes, checksum: a38aca9b6ecaf707ab1b6f24cdf68c3d (MD5) / Made available in DSpace on 2015-06-11T17:49:18Z (GMT). No. of bitstreams: 1 2008_dis_cllima.pdf: 1735905 bytes, checksum: a38aca9b6ecaf707ab1b6f24cdf68c3d (MD5) Previous issue date: 2008 / The molibidatos up a large class of inorganic compounds and have been the subject of several investigations. Usually the members of this family have Y2XO4.nH2O general formula, where Y = (Na, Ca), X = (Mo, S, W, Mn) with n = (0, 1, 3, 4...). The Na2MoO4.2H2O was studied through experiments of Raman spectroscopy aimed at investigating the effects of hydrostatic pressure on their properties structural and vibrational. The material Na2MoO4.2H2O experiencing a transition phase when the compressor is half the mixture of alcohols, this transition is structural. When the compressor is the way nujol (mineral oil) Na2MoO4.2H2O try the two-phase transitions. The difference for these events is that the Na2MoO4.2H2O interacts with the environment compressor alcohol, since the pressure to relax the spectrum of the material presents itself as being semidesidratrado not going back to the original phase. When the compressor means is the nujol, as well as an additional step, the Na2MoO4.2H2O, in the process of relaxation, restores the original phase, where the interim stage is deleted due to the effect of hysteresis. Therefore, it identifies three distinct phases in Na2MoO4.2H2O, in a range of 0 to 3.0 GPa (phase α), another between 3.0 and 4.0 GPa (phase δ) and third over 4.0 GPa (phase β). The first transition from phase α to phase δ is a conformal transition, due to distortions in tetraedros MoO4, leaving the group of symmetry D2h 15. The second transition is a structural transition due to the collapse of two modes in a single event, especially in the modes of the network, which indicates a transition to a structure for greater symmetry than the previous (possibly of the orthorhombic for tetragonal). / Os molibidatos constituem uma grande classe de compostos inorgânicos e têm sido o objeto de várias investigações, devido o estudo ainda não realizado no mesmo usando espectroscopia Raman em função da pressão e as diversas aplicabilidades em diversos campos de estudos do Na2MoO4.2H2O nos motivamos a realiza este estudo. Geralmente os membros desta família possuem fórmula geral Y2XO4.nH2O, onde Y = (Na, Ca), X = (Mo, S, W, Mn) com n = (0, 1, 3, 4...). O Na2MoO4.2H2O foi estudado através de experimentos de espectroscopia Raman visando investigar os efeitos da aplicação de pressão hidrostática em suas propriedades estruturais e vibracionais. Na célula de pressão (DAC, do inglês Diamond Anvil Cell, que permite a obtenção de pressão de 0-10GPa) foram utilizados como meios compressores uma mistura de álcoois (metanol e etanol na proporção de 4:1) e óleo mineral (Nujol). O material Na2MoO4.2H2O experimenta uma transição de fase quando o meio compressor é a mistura de álcoois, sendo esta transição estrutural. Já quando o meio compressor é o nujol (óleo mineral) o Na2MoO4.2H2O experimenta duas transições de fase. A diferença para tais acontecimentos é que o Na2MoO4.2H2O interage com o meio compressor álcool, uma vez que ao relaxar-se a pressão o espectro do material apresenta-se como se estivesse semidesidratrado não voltando para a fase original (fase α). Quando o meio compressor é o nujol, além de uma fase adicional, o Na2MoO4.2H2O, no processo de relaxamento, recupera a fase original, sendo que a fase intermediária é suprimida devido ao efeito de histerese. Portanto, identificam-se três fases distintas no Na2MoO4.2H2O, uma no intervalo de 0 a 3,0 GPa (fase α), outra entre 3,0 e 4,0 GPa (fase δ) e a terceira acima de 4,0 GPa (fase β). A primeira transição da fase α para a fase δ é uma transição conformacional, devido a distorções nos tetraedros MoO4, saindo do grupo de simetria D2h15. A segunda transição é uma transição estrutural devido ao colapso de dois modos em um único modo, principalmente na região dos modos da rede, o que indica uma transição para uma estrutura de maior simetria que a anterior (possivelmente de ortorrômbica para tetragonal).

Made available in DSpace on 2014-06-11T19:32:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-02-17Bitstream added on 2014-06-13T19:42:33Z : No. of bitstreams: 1 delmonte_mrb_dr_araiq.pdf: 5366733 bytes, checksum: 51bba7b88f423a4eac52eb8d0154dc1f (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Cristais de PbMoO4 foram obtidos por intermédio do método coprecipitação (CP) a temperatura ambiente e processadas em diferentes temperaturas (60oC-120oC) por 10 minutos utilizando o método processamento hidrotérmico (PH). Os micros octaedros foram caracterizados estruturalmente por intermédio de técnica de difração de raios X (DRX), refinamento estrutural de Rietveld, espectroscopia de micro-Raman (MR) e de absorção no infravermelho (IVTF). Suas propriedades ópticas foram investigadas por intermédio das técnicas de espectroscopia de ultravioleta visível (UV-Vís) e de fotoluminescência (FL). Os padrões de difração de raios X e os espectros de micro-Raman indicaram que estes cristais possuem uma estrutura tipo scheelita com estrutura cristalina tetragonal sem a presença de fases secundárias. Os dados de refinamentos Rietveld foram empregados a fim de monitorar as diferenças estruturais nos pós de PbMoO4 preparados pelos dois métodos. Os espectros de MR mostraram um modo de vibração ν1 (Ag), situado na em torno de 876 cm-1, o qual está relacionado com as vibrações simétricas de alongamento da ligação Mo-O nos clusters [MoO4]. Espectros de IVTF apresentaram uma banda larga de absorção com dois modos ν3 (Eu e Au), localizada cerca de 749 e 756 cm-1, para PbMoO4 obtido pelo método de CP e 864 e 851 cm-1 para PbMoO4 obtidos pelo método de PH em diferentes temperaturas. Esses modos vibracionais estão associados ao alongamento antisimétrico O-Mo-O nos clusters [MoO4]. As imagens de microscopia eletrônica de varredura de campo induzido (MEV-FEG) foram empregadas para observar as morfologias dos materiais e acompanhar a evolução do crescimento e transformação desses cristais. Medidas de UV-Vís evidenciaram um aumento nos valores de band gap (3,14 eV a 3,19 eV) com o aumento da temperatura de processamento hidrotérmico. As propriedades PL foram explicadas... / PbMoO4 crystals have been obtained by means of coprecipitation (CP) method at room temperature and processed at different temperatures (60oC -120oC) for 10 minutes by using conventional hydrothermal method (CH). These micro-octahedrons were structurally characterized by means of X-ray diffraction (XRD), Rietveld refinements, micro-Raman (MR) and Fourier transformed infrared (FTIR) spectroscopies. Their optical properties were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. XRD patterns and micro-Raman spectra indicated that these crystals have a scheelite-type tetragonal structure without the presence of deleterious phases. Rietveld refinements data were employed in order to monitor the structural differences into PbMoO4 lattice of the crystals prepared by two methods. MR spectra showed a high mode ν1 (Ag) situated at around 876 cm-1, which is related to the Mo–O symmetric stretching vibrations into the [MoO4] clusters. FTIR spectra exhibited a large absorption band with two modes ν3 (Eu and Au) located at around 749 and 756 cm-1, for PbMoO4 obtained by CP method and 864 and 851cm-1 for PbMoO4 obtained by PH at different temperatures, which are associated to the O–Mo–O antisymmetric stretching vibrations into the [MoO4] clusters. Field emission gun scanning electron microscopy (FEG-SEM) micrographs were employed to observe the shape and monitor the evolution of growth and processing of these crystals. UV-vis absorption measurements evidenced a reduction in optical band gap values (from 3,14 eV to 3,19 eV) with the increase of CH processing temperature. PL properties of were explained through a model based in distortions on the tetrahedral [MoO4] clusters due to intrinsic defects at medium range and presence of intermediary energy levels (deep and shallow holes) within the band gap. Finally, the growth mechanisms were proposed for... (Complete abstract click electronic access below)

Neste trabalho, foi investigado o processo de crescimento de fibras monocristalinas (FMc) para aplicação em sistemas laser. Para o aperfeiçoamento na fabricação de FMc de fluoretos, foi projetada e construída uma nova câmara de crescimento para um forno comercial de micro-pulling down (μ-PD), permitindo um rígido controle da atmosfera de processamento, (sob vácuo, fluxo e atmosferas estáticas). Paralelamente, foi investigado o processo de crescimento de FMc de molibdatos duplos de terras raras, sendo crescidas e caracterizadas fibras puras e dopadas com Nd3+ do composto LiLa(MoO4)2 (LLM) com concentrações na faixa de 0,5 a 10mol%. A câmara de crescimento projetada para o controle de atmosfera foi avaliada em diversas condições de operação. Em particular, a mesma foi testada com o puxamento de fibras de LiF, sob atmosfera de CF4, para observação da sua eficiência no controle da atmosfera de crescimento. Foram obtidas FMc de LLM, homogêneas e transparentes; nas fibras puras foi observada a presença de facetas, entretanto, a adequação dos parâmetros de crescimento e a adição de dopantes permitiu a redução significativa destes defeitos. A difração de raios-x mostrou a cristalização no grupo espacial I41/a; medidas de tomografia por coerência ótica permitiram a detecção de centros de espalhamento em regiões onde ocorreram instabilidades no processo de crescimento. Foi possível observar que o coeficiente de segregação do íon Nd3+ é próximo à unidade nesta matriz. O ganho potencial para ação laser foi determinado utilizando-se uma solução numérica das equações de taxa do sistema em 805nm e regime CW. O ganho máximo da emissão laser em 1,064 μm foi previsto para a dopagem de 5% mol Nd3+. / In this work we investigated crystal growth procedures aiming the development of single crystal fiber (SCF) for laser applications. For quality optimization in the fabrication of fluorides SCF a new growth chamber for a micro-pulling down furnace (μ-PD) was constructed targeting the fibers fabrication with strict atmosphere control (high vacuum, gas flux and static atmospheres). Simultaneously, the SCF growth process of rare earth double molybdates was studied. The growth of pure and Nd3+-doped SCF of LiLa(MoO4)2 (LLM) was studied in the range of 0,5 - 10mol% doping. The designed furnace growth chamber with controlled atmosphere was successfully constructed and tested under different conditions. Specially, it was tested with the growth of LiF SFC under CF4 atmosphere showing the expected results. Transparent and homogeneous SCF of Nd:LLM were grown. In the pure fibers was observed facets formation, however, these defects were minimized after tuning of the growth parameters and additionally with the fibers doping. X-ray analysis showed the crystallization of a single phase (space group I41/a); the optical coherence tomography showed the presence of scattering centers only in regions were some growth stability occurred due to the manual process control. The measured Nd3+ distribution showed uniform incorporation, indicative of a segregation coefficient close to unity in LLM. The potential laser gain of the system was determined using a numerical solution of the rate equations system for the 805nm, CW pumping regime, showing the maximum laser emission gain at 1.064 μm for a Nd3+-doping of 5mol%.

Abstract This work presents a method for the fabrication of Li₂MoO₄ ceramics at room-temperature based on utilizing a small amount of water with Li₂MoO₄ powder. The densification of the ceramic takes place during pressing. Thus the shape and size of the final ceramic compact can easily be managed by controlling the mould dimensions and the amount of material. Post-processing at 120 °C is applied to remove residual water from the compact. This post-processing temperature can be chosen to be suitable to the other materials integrated, such as the substrate or electrodes, as long as the post-processing time is adequate to remove the residual water. The dielectric properties (relative permittivity of 5.1 and a loss tangent value of 0.00035 at 9.6 GHz) after optimization of the powder particle size, sample pressing pressure, and post-processing time were similar to those achieved for Li₂MoO₄ ceramics fabricated by sintering at 540 °C. The dielectric properties of Li₂MoO₄ ceramics were also modified using composite methods. For example, an addition of 10 volume-% of BaTiO₃ increased the relative permittivity from 6.4 to 9.7 and the loss tangent value from 0.0006 to 0.011 at 1 GHz. To investigate the thermal dependence of the permittivity, different amounts of rutile TiO₂ were incorporated into a Li₂MoO₄ ceramic matrix fabricated with the method described in this work. As the amount of TiO₂ increased from 10 to 30 volume-%, the thermal coefficient of permittivity decreased from 180 ppm/°C to -170 ppm/°C. The low processing temperature made the fabrication approach introduced here feasible for silver electrode integration without the formation of extra phases, which were observed in sintered samples with similar compositions in another study. A patch antenna was realized utilizing a Li₂MoO₄ ceramic disk fabricated by the room-temperature method. The antenna operating at ~4 GHz showed reasonably good performance. A relative humidity of 80% lowered the resonant frequency by 3.25% from the initial value, and reduced the total and radiation efficiencies of the antenna by ~2 dB. The changes were slowly reversible. Use of a silicone conformal coating reduced the shift of the resonant frequency to 1.26% from the initial value and also reduced the effect on efficiencies to ~1 dB. The use of the coating also speeded up the reversibility of the changes when the humidity was decreased. / Tiivistelmä Tässä työssä esitellään menetelmä, jolla Li₂MoO₄-keraameja voidaan valmistaa huoneenlämpötilassa. Menetelmä hyödyntää pientä määrää Li₂MoO₄-vesiliuosta ja sen kiteytymistä. Keraami tiivistyy kappaletta puristettaessa, joten sen koko ja muoto ovat sama kuin muotilla riippuen vain keraamin määrästä. Kappaleeseen jäänyt vesi poistetaan lämpökäsittelyllä yleensä 120 celsiusasteessa. Jälkikäsittelylämpötila voidaan valita muiden integroitavien materiaalien mukaan, kuten alusta- tai elektrodimateriaalin, kunhan jälkikäsittelyaikaa muokataan vastaavasti, jotta kaikki vesi poistuu. Optimoimalla Li₂MoO₄-jauheen partikkelikokoa, puristuspainetta ja jälkikäsittelyaikaa saavutettiin samankaltaiset dielektriset ominaisuudet taajuudella 9,6 GHz (suhteellinen permittiivisyys 5,1 ja häviötangentti 0,00035) kuin Li₂MoO₄-keraameilla, jotka on sintrattu 540 celsiusasteessa. Li₂MoO₄-keraamien dielektrisiä ominaisuuksia muokattiin myös lisäaineilla. Esimerkiksi 10 tilavuus-% BaTiO₃-jauhetta kasvatti suhteellista permittiivisyyttä taajuudella 1 GHz arvosta 6,4 arvoon 9,7 ja häviötangenttia arvosta 0,0006 arvoon 0,011. Myös eri määriä TiO₂-jauhetta (rutiili) lisättiin Li₂MoO₄-matriisiin permittiivisyyden lämpötilariippuvuuden tutkimiseksi. TiO₂-jauheen määrän kasvaessa 10 tilavuusprosentista 30 tilavuusprosenttiin laski permittiivisyyden lämpötilariippuvuus arvosta 180 ppm/°C arvoon -170 ppm/°C. Matalan käsittelylämpötilan ansiosta työssä esitelty valmistusmenetelmä soveltui käytettäväksi hopeaelektrodien kanssa. Aiemman tutkimuksen mukaan nämä komposiittimateriaalit muodostivat ei-toivottuja faaseja sintrattaessa hopean kanssa. Menetelmällä valmistettua Li₂MoO₄-keraamikiekkoa käytettiin mikroliuska-antennin valmistuksessa. Taajuudella 4 GHz toimivan antennin suorituskyky oli suunnitellun kaltainen. 80 prosentin suhteellinen ilmankosteus laski resonanssitaajuutta 3,25 % alkuperäisestä arvosta ja vähensi antennin kokonais- ja säteilytehokkuutta noin 2 dB. Muutokset palautuivat hitaasti. Silikonisuojalakan käyttö vähensi taajuuden laskua 1,26 prosenttiin alkuperäisestä arvosta ja tehokkuudet laskivat vain noin 1 dB. Suojalakan käyttö nopeutti muutosten palautuvuutta ilmankosteuden laskiessa.

An Investigation into Red Emitting Phosphors for Display and Lightings Two very different classes of red phosphors were evaluated in this thesis; sulphide based phosphors (CaS:Eu2+) and phosphors based on the formulae:- LiEu1- xMx(MoO4)2-y(WO4)y, [M=Al3+, Y3+ and Gd3+]. CaS:Eu2+ was coated with Al2O3 using atomic layer deposition technique and placed into a humidity chamber at high temperature and high humidity for a period of time. The emission spectra were measured and evaluated against uncoated CaS:Eu2+. The results from the humidity tests revealed that Al2O3 coated CaS:Eu2+ increased the life span of the phosphor and therefore can be the potential use for the purpose of application at moderate humidity and temperature. For the phosphors based on LiEu1-xMx(MoO4)2-y(WO4), all the findings reported herein showed that, depending upon the types of cations introduced to the host lattices, incorporation of M at some point offered very much the same or better luminous efficacy when compared to the parent compound with 100 mol% of Eu3+. This work also attempted to examine the correlative relationship between the crystal structures and the luminous efficacies although some attempts were unsuccessful. The findings discovered within this research are beneficial for solid state lighting industries where the cost of using rare earth metals has become significant and recycling is difficult.

The redox cycling of iron is intimately linked to the cycling of C, S, N, P as well as the speciation, mobility, and bioavailability of various toxic contaminants in soils and sediments. Within these environments, the cycling of iron is catalytically driven by iron-oxidizing (FeOB) and iron reducing bacteria (FeRB) which mediate the formation, transformation, and dissolution of various iron-bearing minerals. Under oxic conditions, FeOB promote the formation of iron oxides on or in close proximity of their cell walls and extracellular polymeric substances, and such composite, termed biogenic iron oxides (BIOS), offers highly reactive heterogenous sites that efficiently immobilize trace metals and contaminants alike. However, under reducing conditions, FeRB mediate the reductive dissolution of BIOS and in turn lead to the remobilization of associated contaminants. Conversely, contaminants may become immobilized by secondary iron minerals that form from the metabolic activity of FeRB. Therefore, determining the factors that influence the reactivity of BIOS, as well as the formation of secondary iron minerals is of critical importance to develop a better understanding of the geochemical cycling of iron and in turn the transport of contaminants in the environment. This thesis investigated (1) the impact of simulated diagenesis (ageing for ~5 years at 4ºC) on the mineral stability and reactivity of BIOS towards reduction by Shewanella putrefaciens CN32, (2) the effects of phosphate at an environmentally relevant (10µM) and excess (3.9mM) concentration on the rates and extent of microbial reduction of synthetic 2-line ferrihydrite and BIOS, as well as the formation of secondary iron minerals, and (3) the impact of sterilization by γ-irradiation on the mineral stability and reactivity of BIOS. It was found that simulated diagenesis did not affect the mineralogical composition of BIOS but significantly lowered the reactivity of BIOS towards microbial reduction. The concentration of phosphate was found to have contrasting effects on the rates of reduction of ferrihydrite and BIOS, but in general, excess concentration of phosphate enhanced the extent of Fe(III) reduction. The formation of a specific secondary iron mineral was also found to depend on the concentration of phosphate, as well as, in the case for BIOS, the presence of intermixed cell derived organic matter. γ-irradiation did not alter the mineralogy and reactivity of BIOS towards microbial reduction, and it was concluded to be a suitable technique to sterilize BIOS.

Philosophiae Doctor - PhD / The ideal conditions for the use of hydrazinium sulphate as a reducing agent to determine phosphorus in dental plaque were investigated. When this method was compared to the tin (II) chloride (Kuttner and Cohen, 1927) and ascorbic acid method (Chen, loribara and Warner, 1956),which is generally in use for the measurement of phosphorus in plaque, it was found to have certain advantages. Different ashing techniques were compared, as well as the influence of different acids on wet ashing. The hydrazinium sulphate method could determine as little as 1,8 ~g phosphorus per 50 ml and is recommended for the routine analysis of phosphorus in dental plaque samples with wet ashing. Today phosphorus can be determined, among other methods gravimetrically, titrimetically and spectroscopically (Williams, 1979). Through spectroscopic methods phosphate might be analysed spectrophotometrically by 3 main methods, as molybdophosphoric acid, its reducing product molybdenum blue and the yellow vanadomolybdophosphoric complex (Stuart and Duff, 1980). In addition indirect methods derived from molybdophosphoric acid might also be used (Williams, 1979). When phosphorus is spectrophotometrically determined during the reduction of a phosphomolybdate complex by means of different reducing agents (Kuttner and Cohen, 1927; Chen et aI, 1956; Fiske and Subbarow, 1925 and 1929, Taylor and Miller, 1914; Martin and Doty, 1949; Eibl and Lands, 1969; Vogel, 1961; Boltz and Mellong, 1947; Lazarus and Chou, 1972; Laws and Webley, 1959; Burton and Riley, 1955; Dickman and Bray, 1940), it is necessary to establish the optimum conditions under which the reducing agent, hydrazinium sulphate, can be used. In spite of this, many details about exact experimental procedures are not known (Taylor and Miller, 1914; Vogel, 1961; Boltz and Mellon, 1947). The present investigation is an attempt to establish the ideal conditions for the use of hydrazinium sulphate as a reducing agent during the formation of molybdenum blue (Schirmer et al, 1942) and the effectiveness of the method for the determination of phosphorus in dental plaque by different ashing techniques. The hydrazine method is also compared to that of Chen et al (1956) which is generally in use for plaque phosphate determinations by many authors (Ashley, 1975; Ashley and Wilson, 1976; Kleinberg et al, 1971; Zuniga et al, 1973), as well as to the sometimes used (Dawes and Jenkins, 1962) tin (II) chloride method of Kuttner and Cohen (1927). For the review of many other accepted methods, see Lindberg and Ernster (1956), and Williams (1979). The results are compared to the modified ascorbic acid method (Chen et al, 1956) as well as to the tin (II) chloride method (Kuttner and Cohen, 1927).

Humankind’s tremendous industrial and technological progress over the last two centuries has been driven by the natural abundance and availability of fossil fuels. As those reserves deplete, the prudent course of action would be to develop other readily available fuel sources. Some research efforts using biomolecules involve the hydrogenases and nitrogenases with the goal of evolving dihydrogen. At the nitrogenase active site, the iron-molybdenum cofactor (FeMo-co) catalyzes the reduction of dinitrogen and protons to form ammonia and dihydrogen. Toward the goal of producing dihydrogen passively as an alternative fuel, a novel advanced material has been developed. CdSe nanoparticles complexed with FeMo-co, in both aqueous and organic solvent systems showed complex formation. When the system was interrogated by EPR spectroscopy, evidence of electron transfer was observed. The CdSeMSA●NafY●FeMo-co system when illuminated with visible light evolved dihydrogen consistently in four different experimental sets under the same reaction conditions. NifEN protein plays an important role in the biosynthesis of FeMo-co in addition to the involvement of NifU, NifS, NifB, NifX, NifH and NafY. After NifB synthesizes a FeMo-co precursor, 6-Fe NifB-co, NifEN further incorporates additional Fe, S, Mo, and (R)-homocitrate to complete the synthesis of FeMo-co. Molybdenum is provided to NifEN as its oxoanion, Mo(VI)O4 2- ; however, in FeMo-co molybdenum is in the oxidation state of Mo(IV). EPR spectroscopic investigation of NifEN turnover samples showed a signal at g = 2.00 that was dependent on molybdate concentration. Power and temperature profiles gave evidence that the g iv = 2.00 EPR signal was distinct from the Fe-S clusters in NifEN. The species observed at g = 2.00 was assigned to the reduction of Mo(VI) to Mo(V). How to utilize the effectiveness of FeMo-co and complex it to photoactive materials for the purpose of evolving dihyrogen upon illumination, thus providing a sustainable alternative energy source is one subject of this dissertation. A related subject is to gain an understanding of the biosynthetic pathway of FeMo-co by investigation of NifEN turnover experiments. This understanding should contribute towards the development of improved catalysts for meeting future energy demands.

No description available.

L'étude de la double désintégration bêta sans neutrinos joue un rôle important dans plusieurs questions en physique des particules et cosmologie. Ce processus nucléaire hypothétique viole la conservation du nombre leptonique par deux unités et jusqu'à présent il est le seule moyen pratique pour dévoiler la nature du neutrino : sa détection implique forcement que neutrino et antineutrino sont la même particule. En outre, le taux de décroissance est sensible à la masse efficace de Majorana du neutrino, du coup à l'échelle absolue des valeurs propres de la masse et leur hiérarchie. La marque expérimentale de la DDB0ν est un pic monochromatique dans le spectre énergétique de la somme des deux électrons émis. Le but des expériences de prochaine génération est une sensibilité sur la masse efficace du neutrino de l'ordre de dizaines de meV, c'est-à-dire demi-vies de l'ordre de 10²⁷-10²⁸ années : en pratique, il s'agit de construire des sources de quelques centaines de kg d'isotope candidat, au moins, et les sonder par des détecteurs très efficients, tout en gardant le bruit de fond dans la région énergétique d'intérêt au niveau d'un coup/tonne/an. Les bolomètres luminescents sont une technique prometteuse vu leur excellentes résolutions énergétiques, haute efficacité de détection, ample choix pour les matériaux et extensibilité modulaire à grande échelle; de plus, grâce à la détection simultanée de chaleur et lumière produites par l'interaction des particules, il est possible de discriminer les contaminations α, dangereuse source de bruit aux énergies d'intérêt pour plusieurs noyaux candidats à la DDB0ν. Cette thèse a été effectuée dans le contexte de l'expérience LUMINEU : une expérience pilote qui a pour but la construction d'une expérience de prochaine génération basée sur les bolomètres scintillants en molybdate de zinc, pour l'étude de l'isotope candidat ¹⁰⁰Mo. En vue de la construction d'une expérience à grande échelle, il est nécessaire d'effectuer des caractérisations systématiques pour s'assurer des performances et de la reproductibilité des détecteurs et leurs composantes. La disponibilité d'installations expérimentales en surface, facilement accessibles, est souhaitable pour des tests routiniers : j'ai mené la plupart des expériences au CSNSM, où j'ai aussi travaillé à l'installation d'un nouvel cryostat à dilution basé sur la technologie du Pulse-Tube. Une partie de ma thèse a concerné l'étude de détecteurs bolométriques de lumière aux absorbeurs en germanium et thermomètres NTD (thermistors Neutron Transmutation Doped) : une structure standard pour LUMINEU a été établie et on a mesuré les performances des bolomètres telles que sensibilité, résolution énergétique, bruit de base et reproductibilité. Les résultats sont satisfaisants en vue d'un emploi dans une expérience avec bolomètres scintillants, bien que la configuration soit très sensible à la microphonie. En outre, j'ai testé avec succès des détecteurs bolométriques de lumière exploitants l'effet Neganov-Luke, qui augmente le rapport signal-bruit au niveau compatible avec la détection de la lumière Cherenkov pour la discrimination des événements. Une autre partie a vu la caractérisation des bolomètres scintillants en molybdate de zinc avec masse allant jusqu'à environ 300 g : couplés aux capteurs de lumière susmentionnés et lus par thermistors NTD, ils ont été refroidis en surface au CSNSM et en endroit souterrain à Modane, dans l'installation de l'expérience EDELWEISS pour la Matière Noire. Grâce à la détection simultanée de lumière et chaleur, les interactions des particules β/γ sont séparées des contaminations α. Les résultats démontrent que la caractérisation de bolomètres massifs, presque la taille cible des détecteurs finaux, est possible même en surface, en dépit du rayonnement cosmique. En outre, ces tests ont permis d'optimiser la compatibilité de l'installation de Modane avec les exigences des bolomètres scintillants pour la recherche de la DDB0ν. / Neutrinoless Double Beta Decay (0νDBD) is regarded as an important key in the decryption of some hot astroparticle and cosmological enigmas: it violates lepton number by two units and it is currently the only known practical way to shed light on the neutrino nature, being possible only in case of a Majorana neutrino, identical to its antiparticle. Moreover, the 0νDBD rate is sensitive to the effective neutrino mass, so it would be useful to define the absolute neutrino mass scale and hierarchy. The experimental footprint of 0νDBD is a monochromatic peak in the sum energy spectrum of the two emitted electrons. Next-generation experiments aim at reaching a sensitivity on the effective neutrino mass of the order of ten meV, corresponding to half lives in the range 10²⁷-10²⁸ years: this means to be able to gather, at least, a few hundred kilograms of 0νDBD candidate isotope source and to efficiently scrutinize it with very sensitive detectors. Meanwhile, background levels in the energy region of interest of the 0νDBD signal should be lowered to less than one count/ton/y. Cryogenic luminescent bolometers are a promising technique for 0νDBD search, as they feature excellent energy resolutions, high detection efficiency, flexibility in the material choice and easy scalability to large modular experiments; furthermore, the simultaneous read-out of heat and light signals produced by particle interactions provides an active discrimination method against the dangerous α contaminations, populating the 0νDBD energy region of several interesting candidate isotopes. The work presented in this dissertation was carried out in the context of the LUMINEU project: a pilot experiment focused on zinc molybdate scintillating bolometers, to define the strategies for the construction of a next-generation experiment based on the 0νDBD candidate ¹⁰⁰Mo. In view of the construction of a large 0νDBD experiment, involving hundreds of modules, systematic cryogenic measurements have to be performed to ensure good performance and reproducibility of the detectors and their components. Aboveground facilities are preferred for routinary tests because of their easier accessibility: most of the tests were carried out at CSNSM, where I also worked on the setup of a new cryogenic apparatus, based on the Pulse-Tube technology. One part of my thesis work saw the study of bolometric light detectors based on germanium absorbers and Neutron Transmutation Doped (NTD) thermometers: a proper design was developed in view of LUMINEU and the devices were characterized in terms of sensitivity, energy resolution, baseline noise and reproducibility. The results are compatible with a 0νDBD search final experiment, though this detector configuration is very sensitive to vibrational noise. In addition, the feasibility of bolometric light detectors based on NTD thermometers and Neganov-Luke amplification was investigated, demonstrating that this technique can actually boost the signal-to-noise gain to a level compatible with event discrimination based on Cherenkov light detection. Another part of my work dealt with the test of scintillating zinc molybdate bolometers of mass up to ~300 g, coupled to the aforementioned light detectors and operated both in the aboveground facilities at CSNSM and underground at Modane, in the cryostat of the EDELWEISS Dark Matter search experiment. Good event discrimination capability was achieved: thanks to double read-out of heat and light, it is possible to identify α particles, the threatening background for 0νDBD interests, against β/γ interactions. The results proved the possibility to pre-characterize aboveground detectors of mass close to the one of a final experiment module, despite the high cosmic rays rates. Besides, the measurements opened the way to the mutual compatibility of the underground setup, conceived for another kind of experiment, and LUMINEU 0νDBD search detectors.