Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph. D., Electrical Engineering)--University of Idaho, July 2007. / Major professor: Jeffrey L. Young. Includes bibliographical references (leaves 133-137). Also available online (PDF file) by subscription or by purchasing the individual file.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2009. / Committee Chair: Z. John Zhang; Committee Member: Angus Wilkinson; Committee Member: C P Wong; Committee Member: E. Kent Barefield; Committee Member: Mostafa El-Sayed. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph. D.)--University of Texas at El Paso, 2009. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (M.M.S.E.)--University of Delaware, 2008. / Principal faculty advisor: John W. Gillespie, Dept. of Materials Science. Includes bibliographical references.

Made available in DSpace on 2014-10-09T12:37:45Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:55:58Z (GMT). No. of bitstreams: 1 04760.pdf: 5699610 bytes, checksum: c39af246787ef81d5c48ece3cf888efc (MD5) / Thesis (Ph.D) / IPEN/T / University of Birmingham, Birmingham, England

Made available in DSpace on 2014-06-11T19:32:11Z (GMT). No. of bitstreams: 0 Previous issue date: 2003Bitstream added on 2014-06-13T19:42:35Z : No. of bitstreams: 1 varanda_lc_dr_araiq.pdf: 7417295 bytes, checksum: ab47f79b7f3fc0d50c2f050195357f2a (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Neste trabalho, nanopartículas metálicas (NPM) aciculares e monodispersas de FeCo, FeTR (TR = Nd, Eu, Sm e Tb) e FeCoTR (TR = La-Tb), com ~60 nm de comprimento, razão axial de ~6 e estáveis frente à oxidação e sinterização foram obtidas pela redução em atmosfera de hidrogênio de NP de goetita contendo Co ou Co/TR e recobertas com alumina. O precursor na forma de NP de goetita monodispersas (60±5 nm de comprimento e razão axial de ~6) foi obtido por uma nova rota sintética baseado na hidrólise alcalina de íons Fe(III) em solução na presença de íons carbonato em três estágios: (i) formação de ferrihidrita, (ii) dissolução da ferrihidrita e nucleação de goetita, e (iii) crescimento dos núcleos de goetita. Duas temperaturas distintas foram utilizadas para a nucleação e o crescimento das partículas de goetita e o mecanismo de formação das NP monodispersas de goetita pode ser proposto. NP aciculares e monodispersas de hematita (210±20 nm de comprimento e razão axial de ~5) também foram sintetizadas por reações de hidrólise ácida forçada, e o mecanismo de transformação de -Fe2O3 a -Fe foi avaliado. NP de hematita foram recobertas com agentes antisinterizantes (sílica ou alumina), reduzidas a metal em atmosfera de hidrogênio (450 °C/12 h) e passivadas utilizando uma nova metodologia de passivação: uma mistura de vapores de etanol e gás nitrogênio passada pela amostra durante 15-20 min (processo comercial: nitrogênio:oxigênio (1:1) durante 4-6 h). NP recobertas com sílica apresentaram pequenas partículas esféricas metálicas encapsuladas em um molde acicular de sílica, conduzindo a perda da forma elipsoidal da fase metálica. NP recobertas com alumina resultaram em partículas aciculares metálicas recobertas com uma fina camada do agente anti-sinterizante (alumina) e estabilizadas pela formação... / Acicular monodispersed, metallic nanoparticles (MNP) of FeCo, FeRE (RE = Nd, Eu, Sm and Tb) and FeCoRE (RE = La-Tb), ~60 nm in the length and axial ratio ~6, stabilized against oxidation and sintering, obtained by reduction of alumina-coated goethite NP containing either Co or Co/RE under hydrogen flow were reported. Monodispersed goethite NP precursor (60±5 nm in length, axial ratio ~6) was obtained by a new synthetic route based on the hydrolysis of alkaline Fe(III) solutions in the presence of carbonate ions by a three step-procedure: (i) ferrihydrite formation, (ii) ferrihydrite dissolution and goethite nucleation, and (iii) goethite nuclei growth. Two different temperatures to promote goethite nucleation and growth were used and the formation mechanism of monodispersed goethite NP was proposed. Monodispersed and acicular hematite NP (210±20 nm in length, axial ratio ~5) were also synthesized by forced acid hydrolysis reaction, and the transformation mechanism from -Fe2O3 to -Fe was evaluated. Hematite was covered with anti-sintering agents (silica or alumina), reduced to metal in hydrogen atmosphere (450 °C/12 h) and passivated using a new methodology: ethanol vapors and nitrogen gas mixture for 15-20 min (commercial procedure: nitrogen:oxygen (1:1) gas mixture for 4-6 h). Silica-coated NP showed small metallic spherical particles encapsulated in an acicular silica template, leading to ellipsoidal-shape metallic phase loss. Alumina-coated particle showed an acicular metallic particle with thin layer anti-sintering alumina agent stabilized by maghemite thin layer on particle surface. The influence of the temperature, hydrogen flows and reduction time on the magnetic properties, crystallite size, crystallographic structure and phases were also investigated. A topotatic ...(Complete abstract, click electronic access below)

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<p> An investigation of domain walls in some uniaxial magnetic materials is reported in this thesis. Firstly, a method for measuring the wall energy anisotropy in orthoferrites, which causes cylindrical magnetic (bubble) domains to be elliptical is described. In Sm0.55Tb0.45FeO3 a measured anisotropy energy of 1.7% of the wall-energy density at room temperature is responsible for eccentricities as large as 0.4 at average bubble radii equal to 85% of the bubble strip-domain transition radius. The relationship between material parameters and wall-energy anisotropy is discussed. The hypothesis that in orthoferrites walls parallel to the a axis are Bloch walls while walls parallel to the b axis are Néel walls is investigated by measuring the wall anisotropy as a function of the quality factor of the material by varying the temperature of the sample. The measurements seem to verify the predicted dependence of wall anisotropy on the quality factor and thus the hypothesis.</p> <p> A method for measuring the temperature dependence of the wall-energy density in orthoferrites and the saturation magnetization in garnets is described. The advantage of the method is that it uses a single isolated bubble domain without the need to destroy the bubble in order to obtain the measurements. This method led to the derivation of the temperature sensitivities of bubble domains in orthoferrites and garnets in terms of the material parameters. Optimum plate thicknesses to minimize the variation of bubble diameter with temperature are considered. Also, the condition for zero temperature sensitivity of bubbles in some uniaxial materials is derived in terms of the material parameters.</p> <p> Finally, a study of the current requirement to cut a bubble domain from a strip domain or another bubble in uniaxial plates is reported in this thesis.</p> / Thesis / Doctor of Philosophy (PhD)