A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in the Department of Chemical Engineering, University of the Witwatersrand. / This work was undertaken to study the growth rate of APT crystals and hence obtain some information on the nucleation rate [Abbreviated Abstract. Open document to view full version] / AC2017

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 112-120).

No description available.

Molybdates and tungstates belonging to the scheelite family constitute an important class of materials, which have advantages as a relatively low cost and being non-polluting. Barium molybdate (BaMoO4), barium tungstate (BaWO4), calcium molybdate (CaMoO4) and calcium tungstate (CaWO4) have been extensively studied due their photoluminescent properties, besides that they also present catalysis and photocatalysis applications. However, to the best of our knowledge there are no structural characterizations of BaMoO4, BaWO4 and CaMoO4 by x-ray absorption spectroscopy (XAS) in the literature. In this work, powders of these 4 compounds were prepared by microwave-assisted hydrothermal (MAH) method and polymeric precursor method (PPM) and their structural properties were characterized by X-ray diffraction (XRD), X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The morphology and particle size of these crystalline powders were observed by field emission scanning electron microscopy (FE-SEM). Furthermore, BaMoO4, BaWO4 and CaWO4 were employed as solid catalysts towards gas phase toluene oxidation reactions and their optical properties were investigated by ultraviolet visible (UV-Vis) absorption and photoluminescence (PL) measurements. XRD patterns confirm the phase purity of materials from both preparation methods and reveal a preferential growth when the powders are prepared by MAH due polymeric agents and processing using microwave, which was confirmed by FE-SEM. XANES and EXAFS results show that the preparation method did not introduce high disorders into the structure, however the H2 Temperature-Programmed Reduction (H2-TPR) measurements indicated that the catalyst reducibility is affected by the preparation method of the samples. PL emissions were attributed to the charge-transfer transitions within the [WO4]2- and [MoO4]2- complexes. / Os óxidos molibdatos e tungstatos, pertencentes a família das scheelitas, constituem urna importante classe de materiais que apresentam a vantagem de possuem relativo baixo custo e não serem poluentes. Molibdato de bário (BaMoO4), tungstato de bário (BaWO4), molibdato de cálcio (CaMoO4) e o tungstato de cálcio (CaWO4) tern sido extensivamente estudados devido as suas propriedades fotoluminescentes, além de apresentarem aplicações em catálise e fotocatálise. No entanto, não foi encontrada na literatura caracterizações estruturais de BaMoO4, BaWO4 e CaMoO4 por espectroscopia de absorção de raios X (XAS). Neste trabalho, partículas destes quatro compostos foram preparados pelo método hidrotermal assistido por micro-ondas (MAH) e método dos precursores poliméricos (PPM). Suas propriedades estruturais foram caracterizadas por difração de raios X (XRD) e espectroscopia de absorção de raios X na região XANES (do inglês X-Ray Absorption Near Edge Structure) e região EXAFS (do inglês Extended X-Ray Absorption Fine Structure). A morfologia e o tamanho de partícula desses pós cristalinos foram observados por microscopia eletrônica de varredura por emissão de campo (FE-SEM). Além disso, BaMoO4, BaWO4 e CaWO4 foram empregados como catalisadores sólidos para as reações de oxidação de tolueno em fase gasosa e as suas propriedades ópticas foram investigadas por medidas de absorção no ultravioleta/visível (UV-Vis) e fotoluminescência (PL). Os padrões XRD confirmam a pureza de fase dos materiais obtidos em ambos os métodos de preparação e revelam um crescimento preferencial dos pós preparados por MAH devido aos agentes poliméricos e ao processamento usando micro-ondas, esse crescimento foi confirmado pelas micrografias obtidas por FE-SEM. Os resultados de XANES e EXAFS mostram que o método de preparação não introduz desordens elevadas na estrutura, no entanto, as medidas de redução à temperatura programada (H2-TPR) indicaram que a redução do catalisador e afetada pelo método de preparação das amostras. As emissões de PL foram atribuídas às transições de transferência de carga dentro dos complexos [WO4]2- e [MoO4]2-.

<p>The Large Hadron Collider (LHC) at CERN have a capacity to produce protonproton collisions with an energy of 14 TeV. Four particle detectors are included in the LHC with the purpose to detect all the particles that are created in the collisions. In one of these detectors are scintillating lead tungstate crystals used, to detect the energy of photons and electrons created in the collisions. The energy is detected by measuring of the emitted light from the scintillating crystals. As much knowledge as possible about the optical properties of the crystals are desired to be able to analyze the acquired data from the crystals.</p><p><p>This thesis work presents some techniques used for the characterization of the optical properties for the crystals. It also presents measurements done on the decay time of lead tungstate crystals and on the temperature influence to the light yield from the crystals. These measurement results are in addition used in an attempt to estimate how big influence the Cherenkov radiation has to the total amount of emitted light from the scintillating crystals.</p><p>The influence from the temperature to the light yield is around <em>−</em>2<em>.</em>02 %<em>/◦C </em>for BTCP and around <em>−</em>1<em>.</em>75 %<em>/◦C </em>for SIC¹. No conclusions could been drawn concerning the influence from the Cherenkov radiation to the total amount of emitted light from the temperature measurements.The decay time measurements showed an influence from the Cherenkov radiation to the total amount of emitted light of; 8 % for crystal 1003, 47 % for crystal 1002 and 19 % for crystal 1001.</p></p><p><p><p>¹BTCP (Bogoroditsk Technical Chemical Plant) and SIC (Shanghai Institute of Ceramics) are the two different crystal production facilities used for the production of the crystals.</p></p></p>

The Large Hadron Collider (LHC) at CERN have a capacity to produce protonproton collisions with an energy of 14 TeV. Four particle detectors are included in the LHC with the purpose to detect all the particles that are created in the collisions. In one of these detectors are scintillating lead tungstate crystals used, to detect the energy of photons and electrons created in the collisions. The energy is detected by measuring of the emitted light from the scintillating crystals. As much knowledge as possible about the optical properties of the crystals are desired to be able to analyze the acquired data from the crystals. This thesis work presents some techniques used for the characterization of the optical properties for the crystals. It also presents measurements done on the decay time of lead tungstate crystals and on the temperature influence to the light yield from the crystals. These measurement results are in addition used in an attempt to estimate how big influence the Cherenkov radiation has to the total amount of emitted light from the scintillating crystals. The influence from the temperature to the light yield is around −2.02 %/◦C for BTCP and around −1.75 %/◦C for SIC1. No conclusions could been drawn concerning the influence from the Cherenkov radiation to the total amount of emitted light from the temperature measurements.The decay time measurements showed an influence from the Cherenkov radiation to the total amount of emitted light of; 8 % for crystal 1003, 47 % for crystal 1002 and 19 % for crystal 1001. 1BTCP (Bogoroditsk Technical Chemical Plant) and SIC (Shanghai Institute of Ceramics) are the two different crystal production facilities used for the production of the crystals.

No description available.

Thesis (M.S.)--West Virginia University, 2000. / Title from document title page. Document formatted into pages; contains viii, 71 p. : ill. Includes abstract. Includes bibliographical references (p. 70-71).

Orientador: Elson Longo da Silva / Banca: Fenelon Martinho Lima Pontes / Banca: Ieda Lucia Viana Rosa / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: Tungstatos em geral têm despertado um grande interesse tecnológico devido às suas propriedades eletro-ópticas, como material para as mais variadas aplicações dentro de faixa específica do espectro fotoluminescente. Neste trabalho, pós de tungstatos de bário (BaW 'O IND. 4') foram preparados pela síntese química de co-precipitação na presença de hidróxido de amônio e processados nos sistemas de hidrotermal convencional e em hidrotermal assistido por irradiação de microondas. Após síntese por co-precipitação, as amostras foram separadas em três grupos, onde no primeiro o material foi deixado em repouxo ao resguardo da luz na temperatura de 28ºC por 48 horas, para verificar se há formação da fase tetragonal. No segundo e terceiro grupo o material foi encaminhado para processamento em hidrotermal convencional e hidrotermal assistido por microondas na temperatura de 140°C em diferentes tempos. Cada material obtido nos três processos foi lavado com água destilada e seco com acetona. Posteriormente aos tratamentos técnicos os pós foram caracterizados utilizando as técnicas de difração de raios X (DRX), espectroscopia Raman com transformadas de Fourier, espectroscopia na região do ultravioleta visível (UV-Vis), Fotoluminescência (FL) e microscopia eletrônica de varredura por canhão de emissão de campo de alta resolução (MEV-FEG). / Abstract: Tungstates, in a general way, have attracted a great technological interest due to their electro-optical properties, as materials for the most diversified applications within the especific range of the photoluminecent spectra. In this work, powders of barium tungstates were prepared by coprecipitation in the presence of ammonium hydroxide. The materials were processed by conventional hydrothermal and also by microwave-assisted hydrothermal technique. After the synthesis by coprecipitation, the samples were aged in a light sensitive storage device at 28°C and 48°C, in order to investigate the formation of a tetragonal phase. In the second and third groups, the samples were heat-treated in the conventional and the microwave-assisted hydrothermal systems at 140°C for different times. Each sample of the three processes was washed with distilled water and drid with acetone. Subsequently to the heat treatments, the powders were characterized by X-Ray Diffraction (XRD), Fourier-transform Raman Spectrocopy, UV-Visible spectrocopy (UV-Vis), Photoluminescence (PL) and Field Emission Gun Scanning Electronic Microscopy (FEG). / Mestre

As estruturas cristalinas e algumas propriedades elásticas das fases a e g do tungstato de zircônio, ZrW2O8, foram calculadas de acordo com a Teoria do Funcional da Densidade Eletrônica usando o funcional B3LYP (DFT/B3LYP). Para a fase a-ZrW2O8, a estrutura foi otimizada em diferentes pressões e suas constantes elásticas foram estimadas. As interações interatômicas, ordenadas em termos das compressibilidades das ligações, diminuem de acordo com a seqüência W-O > Zr· · ·W > Zr-O. O tetraedro em torno dos átomos do tungstênio é muito mais rígido do que os octaedros de ZrO6. Estes últimos são, de fato, mais compressíveis que a cela unitária da fase a-ZrW2O8. As constantes elásticas calculadas no limite atérmico estão em excelente acordo com os recentes resultados experimentais obtidos próximo de 0 K. O mecanismo de compressão em torno dos átomos W1 e W2 é completamente diferente. Enquanto o primeiro é descrito essencialmente em termos de uma rotação correlacionada dos poliedros, o último envolve a rotação correlacionada dos poliedros de primeira coordenação e a translação das unidades WO4 para baixo, ao longo dos eixos < 111>. Na medida em que estes modos de deformação são semelhantes aos modos de baixa energia responsáveis pela expansão térmica negativa do tungstato de zircônio, este resultado pode auxiliar na elaboração do mecanismo microscópico responsável por este fenômeno. Para a fase g-ZrW2O8 foram otimizadas as estruturas a pressão ambiente e para V/V0 = 0,97 (limiar da transição de fase g ® amorfa) com o objetivo de estudar a evolução da estrutura desta fase com a pressão e obter indícios sobre o mecanismo de amorfização induzida por altas pressões. Com a redução de 3% no volume da cela unitária ocorre uma variação maior que 1,5% nas distâncias interatômicas entre o oxigênio terminal de um determinado poliedro com o átomo de tungstênio do poliedro vizinho. Portanto, é esperado que a redução de 15% no volume molar com a transição g ® amorfa promova a formação de novas ligações W-O, responsáveis pela retenção metaestável da fase amorfa após alívio da pressão. / Submitted by Ana Guimarães Pereira (agpereir@ucs.br) on 2015-10-06T17:15:41Z No. of bitstreams: 1 Dissertacao Camila Araujo de Figueiredo.pdf: 3160462 bytes, checksum: 8f241d48581adf9fde2fe1a3ef81dafc (MD5) / Made available in DSpace on 2015-10-06T17:15:41Z (GMT). No. of bitstreams: 1 Dissertacao Camila Araujo de Figueiredo.pdf: 3160462 bytes, checksum: 8f241d48581adf9fde2fe1a3ef81dafc (MD5) / Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul. / The crystalline structure and some elastic properties of the a and g phases of zirconium tungstate, ZrW2O8, were calculated according to Density Functional Theory using the B3LYP functional (DFT/B3LYP). The structure of a-ZrW2O8, was optimized at different pressures and its elastic constants were estimates. The interatomic iterations, ranked in terms of bond compressibilities, decrease according to the sequence W-O > Zr· · ·W > Zr-O. The tetrahedra around tungsten atoms are found to be much stiffer than the ZrO6 octahedra. These latter are, in fact, more compressible than the a-ZrW2O8 unit cell. The elastic constants calculated in the athermal limit are in excellent agreement with recent experimental results obtained near 0 K. The compression mechanism around W1 and W2 atoms is quite different. While the former can be described essentially in terms of a correlated polyhedral rotation, the latter involves correlated rotation of the first coordination polyhedra and translation of WO4 units downward along the < 111> axis. As far as these modes of deformation should bear some resemblance to the low-energy modes responsible for the negative thermal expansion in zirconium tungstate, this result can shed some light on the microscopic mechanism behind this phenomenon. The structure of g-ZrW2O8 was optimized at ambient pressure and for V/V0 = 0.97 (threshold of the g ® amorphous phase transition) aiming to study the evolution of the structure of this phase with the pressure and the pressure induced amorphization mechanism. A reduction of 3 % in the volume of the unit cell leads to a variation greater than 1,5 % in the interatomic distances between the terminal atoms and the tungsten of the nearest tetrahedron. Therefore, it is expected that the reduction of 15 % in the molar volume with the g ® amorphous transition promotes the formation of new W-O bonds, which would be responsible for the metaestable retention of the amorphous phase upon pressure release.