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Submitted by ANA KARLA PEREIRA RODRIGUES (anakarla_@hotmail.com) on 2017-07-28T14:27:54Z No. of bitstreams: 2 arquivototal.pdf: 2804573 bytes, checksum: 3d7ab3b438b73e5c56ea35d7e5a75d78 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2017-07-28T14:27:54Z (GMT). No. of bitstreams: 2 arquivototal.pdf: 2804573 bytes, checksum: 3d7ab3b438b73e5c56ea35d7e5a75d78 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2016-05-27 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The ceramic oxides that exhibit ferromagnetic behavior are important commercial products for electronics industries and are commonly known as ferrites. In this work, we synthesized different compositions of CuxNi1-xFe2O4 ferrite with x = 0,0, 0,2, 0,4, 0,6, 0,8 and 1,0 by using the combustion method starting from the iron nitrate, nickel and copper and urea as a reducing agent. The powders were characterized by determination of specific surface area by the method (BET), X-ray diffraction (XRD), refinement of XRD data by the Rietveld method and vibrating sample magnetometry (MAV). The obtained copper nickel ferrites, referring x = 0,0, 0,2, 0,4, 0,6 synthesized had the same number of peaks corresponding to the single phase-forming powders that have been indexed to the spinel ferrite phase, forming a cubic phase and x = 0,8 to 1,0 tetragonal phase formed enhancing the efficiency of the applied method of synthesis. The refinement by the Rietveld method resulted in a determination of cell parameter of the samples, the cubic phase the value 0.8359 to 0.8340 nm and the crystallite size whose values occur between 239.62 to 187.24 nm. While in the tetragonal phase the cell parameter a = b was 5.8253 to 5,8230nm c = 8.5693 to 8.6941 nm. The hysteresis curves showed magnetic materials profile. The nickel ferrite - synthesized copper proved as materials with high potential for applicability in different areas of high-frequency devices such as phones and components for computers. / Os óxidos cerâmicos que exibem comportamento ferrimagnético representam importantes produtos comerciais para indústrias de eletrônica e são comumente conhecidos por ferritas. Neste trabalho, foram sintetizadas diferentes composições da ferrita CuxNi1-xFe2O4 com X = 0,0, 0,2, 0,4, 0,6, 0,8 e 1,0 pelo uso do método de combustão partindo-se dos nitratos de ferro, níquel e cobre e ureia como agente redutor. Os pós obtidos foram caracterizados por determinação de área superficial específica pelo método (BET), difração de raio X (DRX), refinamento dos dados de DRX pelo método Rietveld e magnetometria de amostra vibrante (MAV). As ferritas de níquel cobre obtidas, referentes x = 0,0, 0,2, 0,4, 0,6 sintetizadas apresentaram o mesmo conjunto de picos correspondentes à formação de pós monofásicos que foram indexados à fase espinélio de ferrita, formando uma fase cubica e para x = 0,8, 1,0 formou fase tetragonal predominante reforçando a eficiência do método de síntese aplicado. O refinamento pelo método de Rietveld resultou na determinação de parâmetro de cela das amostras, da fase cúbica o valor de 0,8359 a 0,8340 nm e do tamanho de cristalito cujos valores ocorrem entre 239,62 a 187,24 nm. Enquanto na fase tetragonal o parâmetro de cela a=b foi 5,8253 a 5,8230nm e c= 8,5693 a 8,6941 nm. As curvas de histerese mostraram perfil de materiais magnéticos. As ferritas de níquel - cobre sintetizadas se mostraram como materiais com alto potencial de aplicabilidade em diferentes áreas de equipamentos de alta frequência como telefonia e de componentes para de computadores.

Made available in DSpace on 2015-03-03T15:43:03Z (GMT). No. of bitstreams: 1 PauloRCS_DISSERT.pdf: 5184228 bytes, checksum: d3e31e6fed97c72238a19b42897cf9b0 (MD5) Previous issue date: 2014-05-27 / Universidade Federal do Rio Grande do Norte / In this work we obtain nickel ferrite by the combustion synthesis method whcih involves synthesising in an oven at temperatures of 750oC, 950oC and 125oC. The precursors oxidizing used were nickel nitrate, ferric as an oxidizing and reducing urea (fuel). After obtaining the mixture, the product was deagglomerated and past through a 270 mesh sieve. To assess the structure, morphology, particle size, magnetic and electrical properties of nanoparticles obtained the samples were sintered and characterized by x-ray distraction (XRD), x-ray fluorescence spectroscopy (FRX); scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), vibrating sample magnetometer (MAV ) and electrical permittivity. The results indicated the majority of phase inverse spinel ferrite and Hematite secondary phase nickel and nickel oxide. Through the intensity of the distraction, the average size of the crystallization peaks were half-height width which was calculated using the Scherrer equation. From observing the peaks of all the reflections, it appears that samples are crystal clear with the formation of nanoparticles. Morphologically, the nanoferritas sintered nickel pellet formation was observed with three systems of particle size below 100mn, which favored the formation of soft pellets. The average size of the grains in their micrometric scale. FRX and EDS showed qualitatively the presence of iron elements nickel and oxygen, where through quantitative data we can observe the presence of the secondary phase. The magnetic properties and the saturation magnetization and the coercive field are in accordance with the nickel, ferrite where the curve of hysteresis has aspects of a soft material. Dielectric constant values are below 10 and low tangent loss / Este trabalho descreve a obten??o de ferrita de n?quel pelo m?todo de s?ntese de combust?o, seguida pela sinteriza??o em forno, ?s temperatura de 750oC, 950oC e 1250oC. Os precursores oxidantes utilizados foram: nitratos de n?quel e ferro III como redutor, foi usada a ur?ia (combust?vel). Depois da obten??o do p?s-misturas, o produto foi desaglomerado e passado em uma peneira de mesch 270. A estrutura, a morfologia, o tamanho das part?culas e as propriedades magn?ticas e el?tricas das nanopart ?culas obtidas das amostras p?s sinterizadas foram caracterizadas por difra??o de raios-X (DRX), espectroscopia de uoresc?ncia de raios-X (FRX), microscopia eletr?nica de varredura (MEV ), espectroscopia de energia dispersiva (EDS), magnet ?metro de amostra vibrante (MAV ) e an?lise el?trica. Os resultados indicaram a fase majorit?ria do espin?lio inverso ferrita de n?quel e a fase secund?ria de hematita e ?xido de n?quel. Atrav?s da intensidade dos picos de difra??o de raios-X e da largura de meia altura no espectro, foi calculado o tamanho m?dio do cristalito pela equa??o Scherrer, observando-se os picos de todas as re ex?es, veri cando a cristalinidade das amostras, e com forma??o de nanopart?culas. Morfologicamente, para as nanoferritas de n?quel sinterizadas, observou-se a forma??o de aglomerados moles nos tr?s sistemas com part?culas de tamanho inferior a 100nm. O tamanho m?dio dos gr?os est? em escala microm?trica. Os espectros de FRX e EDS mostraram qualitativamente a presen?a dos elementos ferro, n?quel e oxig?nio. Atrav?s dos dados quantitativos, pode-se observar a presen?a da fase secund?ria. As propriedades magn?ticas, como a magnetiza??o de satura??o e o campo coercitivo, est?o de acordo com a ferrita de n?quel, em que a curva de histerese tem aspectos de um material mole. Os valores de constante diel?trica est?o abaixo de 10 e baixa tangente de perdas.

Increased demand of power, limited fuel resources and environmental concerns have recently prompted a huge thrust on research areas of alternative energy and photovoltaics have been hailed as energy source for future. Particularly, third generation solar cell configurations like dye-sensitized solar cells and quantum dot Schottky barrier solar cells have drawn more attention because of their ease of processability, cheap cost with decent performance, low payback time and portability. Quantum dots are very attractive materials as sensitizers because of their size dependent bandgap tunability, increased oscillator strength and hence higher absorption coefficient, possibility of multiple exciton generation and photochemical robustness. Hence syntheses of quantum dot based hybrid nanostructures have received huge attention among researchers for using it quantum dot sensitized solar cell configuration. This dissertation can be divided in two parts. In the first part two different methods have been reported to prepare interconnected mesoporous nanostructures of wide band gap semiconductors like TiO2 and ZnO which is very important in providing high surface area for absorption or attachment of the sensitizers. In the second part, methods have been developed to establish direct contacts between quantum dots and wide bandgap substrates without molecular linkers which are expected to increase the electron injection rate from quantum dots to TiO2/ZnO. The entire thesis based on the results and findings obtained from the present investigation is organised as follows: Chapter-I provides a general introduction on the working principle of different type of solar cells and then gives a detailed description of the structure and electronic process of dye sensitised solar cells. Then, benefits of quantum dots as sensitizer over dye molecules has been discussed followed by the modification needed in case of quantum dot sensitized solar cells. Chapter-II deals with the materials and methods which essentially gives the information about the materials used for the synthesis and the techniques utilized to characterize the materials chosen for the investigation. Chapter-III describes a hybrid sol-gel combustion technique to synthesize large quantities of highly crystalline and phase-pure anatase powder in a single step. Titanium isopropoxide reacts with oleic acid to form a viscous liquid (oxocarboxoalkoxide) which undergoes non-hydrolytic polycondensation to form TiO2 during combustion. Oleylamine takes part in formation of reverse micelle which expands during combustion giving rise to porous interconnected membrane like microstructure of pore size ~5 nm, BET surface area of ~100 m2/g and porosity of ~ 48%. More importantly, this porous powder having a pre-existing network can be used to form thicker film by doctor blade technique from its paste and at the same time is expected to have better transport properties due to its less particulate nature. Chapter-IV presents a general method to prepare mesoporous structure from rod-like morphologies by partial sintering of a green pellet. Material having inherent anisotropy in their crystal structure tends to grow in a particular direction rather undergoing equiaxial growth. For instance, hexagonal ZnO and tetragonal rutile usually grow as rod-shaped particles. A loose compact of these nanorods give nanoporous morphology upon heating. Advantage of this method is the tunability of pore size by tuning the aspect ratio of the nanorods. Preparation of porous TiO2, ZnO and hydroxyapatite has been demonstrated from their corresponding nanorods. Chapter-V deals with a solvothermal based technique that has been developed for in-situ deposition of nanoparticles on any plane or curved surfaces conformally. This has been demonstrated for nanoparticles of FeCo, Au, Co, CdS on substrates like glass, mica, Si, NaCl, Al2O3 M-plane and also conformal coating of Au nanoparticles on polystyrene latex spheres. CdSe on rutile nanorods, ZnO nanorods and CNTs are promising hybrid nanostructures for third generation photovoltaics and their successful preparation has been detailed in the chapter. The mechanism proposed is based on dominant attractive sphere-plate interaction under high temperature and high autogeneous pressure condition which at reduced density and surface tension of the solvent reduces the dispersibility of the nanoparticle and allow effective spreading of the nanoparticles on the substrate. This method is also advantageous for coating of complicated geometry like inner walls of porous structures. Chapter-VI presents a method to coat chalcogenide nanoparticles on mesoporous TiO2 without any molecular linker which can enhance the electron injection rate from the chalcogenide quantum dots to TiO2. CdS, PbS can be easily synthesized through aqueous chemistry. For deposition of these sulfides, the ion layer gas absorption and reaction (IGLAR) method was modified to form uniform dense nanoparticles on anatase and ZnO surfaces. Nitrate salts of corresponding metal ions are dried directly on the semiconductor surface and instead of exposing it to H2S gas, it was treated with a concentrated sulfide solution. This introduces two competitive process i) dissolution of nitrate salt ii) formation of the metal sulfide. This dissolution step was absent when treated with H2S gas (IGLAR) and hence lead to a continuous coating. We have successfully produced CdS-TiO2 and PbS-TiO2 composites using this approach. Photoelectrochemical measurements on CdSTiO2 composites show an overall efficiency of 2.8% which is among the highest values obtained for this system demonstrating the applicability of the method to engineer interfaces to achieve high efficiency solar cells. Chapter-VI explores the combination of strategies of nanocrystal conversion chemistry with previously described sol-gel combustion technique to create dense and uniformly coated QD sensitized TiO2 electrode without compromising heat-treatment routines which is essential for better adhesion and to enhance performance with reduced leakage. Intimate biphasic oxide mixtures of PbO and CdO with TiO2 are first synthesized by nonhydrolytic solgel process with is followed by combustion to produce porous morphology. This powder can be coated as electrode and can sustain high temperature heat treatment routines and finally can be selectively converted to sulfides with Na2S treatment as TiO2 is immune to sulfidation under this condition. Materials at different stages are characterised by XRD, TEM, EDS, UV-Vis and XPS.

Σκοπός της παρούσας διδακτορικής διατριβής ήταν η ανάπτυξη ενός αποτελεσματικού καταλυτικού συστήματος με βάση το χαλκό, για την αναμόρφωση της μεθανόλης. Για το σκοπό αυτό εξετάστηκαν οι καταλυτικές ιδιότητες τριών συστημάτων βασιζόμενων σε καταλύτες χαλκού και παρασκευασμένων με τη μη συμβατική μέθοδο της καύσης: CuO-CeO2, τροποποιημένων καταλυτών CuO-CeO2 και Cu-Mn-O για την προαναφερθείσα διεργασία, καθώς και τα βέλτιστα δείγματα των καταλυτών CuO-CeO2 και Cu-Mn-O υποστηριγμένων σε μεταλλικούς αφρούς Al. Τα φυσικοχημικά χαρακτηριστικά των καταλυτών CuO-CeO2, βρέθηκαν να εξαρτώνται από τις παραμέτρους σύνθεσης. Ο βέλτιστος καταλύτης παρασκευάστηκε με λόγο Cu/(Cu+Ce)= 0.15. Στους τροποποιημένους καταλύτες CuO-CeO2, ένα μέρος του τροποποιητή εισχωρεί στο πλέγμα της δημήτριας, οδηγώντας στο σχηματισμό στερεού διαλύματος. Αυτό είχε ως αποτέλεσμα να επηρεαστούν τα φυσικοχημικά χαρακτηριστικά των δειγμάτων, αλλά και η καταλυτική συμπεριφορά τους. Οι σπινελικοί καταλύτες Cu-Mn-O είναι πολύ ενεργοί παρά τη μικρή ειδική επιφάνειά τους. Η ενεργότητά τους είναι συγκρίσιμη με αυτή των εμπορικών καταλυτών Cu-Zn-Al. Ο βέλτιστος καταλύτης ήταν αυτός με λόγο Cu/(Cu+Mn)= 0.30. Εξίσου αποδοτικοί για την παραγωγή υδρογόνου μέσω αναμόρφωσης της μεθανόλης, μονολιθικοί καταλύτες Cu-Ce/Al foam και Cu-Mn/Al foam παρασκευάστηκαν με τη μέθοδο της καύσης. Με βάση τα ευρήματα της ισοτοπικής μελέτης, προτείνεται για τον καταλύτη Cu-Mn-O ότι η αναμόρφωση πραγματοποιείται αποκλειστικά μέσω μηχανισμού που περιλαμβάνει τον ενδιάμεσο σχηματισμό μυρμηκικού μεθυλεστέρα. Για τους καταλύτες Cu-Ce-O και Cu-Zn-Al πραγματοποιείται ταυτόχρονα και μηχανισμός που περιλαμβάνει ως ενδιάμεσο είδος το διοξομεθυλένιο. / The scope of the present thesis was the development of an effective catalytic copper-based system for methanol reforming. The catalytic properties of three different copper-based systems prepared via the non conventional combustion method, were investigated for the aforementioned process: CuO-CeO2, modified CuO-CeO2 and Cu-Mn-O, as well as the optimal CuO-CeO2 and Cu-Mn-O oxide cata¬lysts supported on Al metal foam. The physicochemical characteristics of CuO-CeO2 catalysts were found to be influenced by the parameters of the synthesis. The optimal catalyst was prepared with Cu/(Cu+Ce) ratio equal to 0.15. In the case of modified CuO-CeO2 catalysts, at least part of dopant cations gets incorporated into the CeO2 lattice leading to solid solution formation. As a result, the physicochemical characteris¬tics of the samples were influenced, as well as their catalytic performance. Cu-Mn spinel oxide catalysts were found to be highly active despite their low surface area. Their activity is comparable to that of commercial Cu-Zn-Al catalysts. The optimal catalyst was prepared with a Cu/(Cu+Mn) ratio equal to 0.30. Structured Cu-Ce/Al foam and Cu-Mn/Al foam catalysts prepared via in situ combustion method were equally effective for hydrogen production via methanol reforming. Based on the findings of an isotopic study, a mechanism has been proposed for the reforming reaction over Cu-Mn-O, where methyl formate is formed as a reaction intermediate. An additional reaction mechanism is taking place over Cu-Ce-O and commercial Cu/ZnO/Al2O3 catalysts, resulting in the intermediate dioxomethylene.

In this work, alkaline earth aluminate phosphors doped with rare-earth ions and manganese were synthesized using combustion method. Several characterization techniques were used to study the structural and luminescent properties of the as-synthesized phosphors, namely X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray energy Dispersive Spectroscopy (EDS), Ultraviolet-Visible (UV-Vis) Spectroscopy, Photoluminescence (PL), and Thermoluminescence (TL). The structural properties were studied by collecting the XRD patterns of the samples using an X'Pert PRO PANalytical diffractometer with CuKα at λ = 0.15405 nm. The particle morphologies of the as-synthesized powder phosphors were investigated using a JEOL JSM-7500F field-emission scanning electron microscope (FE-SEM). The optical properties of the phosphors were studied using Perkin-Elmer Lambda 750s UV-Vis spectrometer, Jobin Yvon/SPEX FluoroLog spectrofluorometer (Model FL-1040) and Riso TL/OSL reader (Model DA-20). The as-prepared SrAl2O4:Eu 2+ ; SrAl2O4:Dy 3+; SrAl2O4:Mn 2+; phosphors were synthesized at an initiating temperature of 600 oC. The XRD patterns were consistent with the low temperature monoclinic structure of SrAl2O4 for all the as-synthesized phosphor powders. SEM measurements showed nano-rod like particles. The SrAl2O4:Eu 2+ ; SrAl2O4:Dy 3+; SrAl2O4:Mn 2+ samples were excited using a 450 W Xenon light source at 364 nm, 390 nm, and 426 nm respectively. A broad blue emission peak at 500 nm shown by the SrAl2O4:Eu 2+ sample is attributed to the 6 1 7 4f 5d 4f transition of the Eu 2+ ion. Also, the red sharp emission lines due to the 4f-4f transition of the Eu 3+ were observed. SrAl2O4:Dy3+ samples exhibited blue, green, and red emissions which can be atributed to the 4 6 9/2 15/2 F  H ,4 6 9/2 13/2 F  H , and 4 6 9 11 2 2 F  H transitions of Dy 3+ ions respectively. The two broad emissions (green at 513 nm and red at 650 nm) shown by 2+ 0.98 2 4 0.02 Sr Al O :Mn sample can be atributed to the 4 4 6 6 1 1 T ( G)  A ( S) transition of the Mn 2+ ion in the sample. The SrAl2O4:Eu 2+ , Dy 3+ ; SrAl2O4:Eu 2+, Mn 2+ ; SrAl2O4:Dy 3+, Mn 2+; and SrAl2O4:Eu 2+ ,Mn 2+, Dy 3+ phosphors were synthesized by combustion method at an initiating temperature of 600 oC. The blue emissions were observed in all the samples except SrAl2O4:Eu 2+ ,Mn 2+, Dy 3+ sample. The SrAl2O4:Eu 2+ ,Mn 2+, Dy 3+ phosphor showed the longest afterglow intensity. The BaAl2O4 doped with Eu 2+ , Mn 2+ and Dy 3+ phosphors synthesized at an initiating temperature of 600 oC using combustion method. The XRD patterns confirmed the hexagonal structure of BaAl2O4 in all the as-synthesized samples. A broad blue emission of the BaAl2O4:Eu 2+ sample at 490 nm is attributed to the 6 1 7 4f 5d 4f transition of the Eu 2+ ion in the sample. A red emission peak observed at 611 nm is due to the 4f - 4f transition of un-reduced Eu 3+ ions during the combustion reaction. A blue emission at 482 nm, a green emission at 575 nm, and a red emission at 663 nm of the BaAl2O4:Dy 3+ sample can be associated with 4 6 9/2 15/2 F  H ,4 6 9/2 13/2 F  H , and 4 6 9 11 2 2 F  H transitions of the Dy 3+ ions respectively. The green emission peaks exhibited by BaAl2O4:Mn 2+ sample at 512 nm is due to the 4 4 6 6 1 1 T ( G)  A ( S) transitions of the Mn 2+ ions. Barium aluminate phosphors doped with different concentrations of Dy 3+ ion were synthesized by combustion method at an initiating temperature of 600 oC. The XRD patterns confirmed the hexagonal structure of BaAl2O4. The emission peaks observed at 482 nm, 575 nm, and 663 nm are due to4 6 9/2 15/2 F  H ,4 6 9/2 13/2 F  H and 4 6 9 11 2 2 F  H transitions of Dy 3+ ion respectively. The PL measurements also confirmed the quenching of luminescence at higher concentrations of the Dy 3+ ion. The UV-Vis measurements has confirmed the increase in the band-gap of the BaAl2O4 sample followed by a decrease and an increase again as doping concentration of the Dy 3+ increased. The X-ray diffraction patterns of the Ca0.97M0.3Al2O4:Eu 2+ , Dy 3+ (M = Ba, Mg, and Sr) powder samples prepared by combustion method confirms the monoclinic structure of CaAl2O4 in all samples. A broad emission peak at 490 nm for both Ba 2+ and Mg 2+ substituted samples and the one for Sr 2+ substituted sample at 485nm are attributed to the 6 1 7 4f 5d 4f transition of the Eu 2+ . The decay curves confirmed that the Mg 2+ substituted sample has a longer persistence (phosphorescence) than all the other samples. / Physics / M. Sc. (Physics)