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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Crescimento e caracterização de monocristais de BaY2F8:TR onde TR = Nd3+, Pr3+, Er3+, Tb3+, Dy3+ / Single crystals growth and characterization of BaY2F8:TR (TR= Nd3+; Pr3+; Er3+; Tb3+;Dy3+)

Cruz, Simone Ferreira de Almeida 31 July 2008 (has links)
Foram realizados estudos da síntese e do crescimento de cristais de BaY2F8:TR (TR= Nd3+; Pr3+; Er3+; Tb3+; Dy3+) simplesmente dopados e codopados (Nd3+:Dy3+ ; Nd3+:Pr3+) por meio do método de fusão por zona (FZ). Os cristais crescidos foram caracterizados quanto a sua estrutura por Difração de raios-x (DRX) e Difração de nêutrons (DN) com análise pelo método de Rietveld. Foi também estudado o comportamento na fusão deste material puro e dopado com elementos terras raras trivalentes através de: microscopia eletrônica de varredura (MEV) e espectroscopia dispersiva de raios-x (EDS). A concentração final de dopantes, bem como, sua distribuição nos cristais crescidos foi estimada e, quando possível, foi ainda calculado o coeficiente de segregação de cada dopante nesta matriz. Os cristais foram também analisados por espectroscopia de absorção e emissão no caso dos materiais duplamente dopados, sendo observada evidencias de transferência de energia Nd-Pr na região UV. Foram obtidos cristais com dimensões da ordem de 40 mm x 10mm com boa qualidade óptica e estrutural adequados para estudos espectroscópicos e testes de ação laser com bombeamento por laser de diodo. Experiências preliminares de crescimento de fibras monocristalinas desta matriz pura e dopada com Tb3+, pelo método de crescimento de micro-pulling down, foram também realizadas visando uma avaliação do método aplicado a este fluoreto. Foram obtidas apenas fibras translúcidas com diâmetro constante sendo necessária maior investigação sobre o crescimento de fibras deste material puro ou dopado. / Synthesis and Growth process of single crystals of BaY2F8:TR (TR= Nd3+; Pr3+; Er3+; Tb3+;Dy3+) single-doped and co-doped (Nd3+ :Dy3+ ; Nd3+:Pr3+) were performed by Zone Melting method (ZM). The grown crystals were characterized by X-ray diffraction (XRD) and Neutron Diffraction (ND) and analyzed by the Rietveld Method. The melting behavior of this pure and trivalent rare earth doped-fluoride was investigated also by scanning electron microscopy (MEV) and X-ray energy dispersive spectroscopy (EDS). The concentration and distribution of the dopants in the grown crystals were measured, and when feasible, the segregation coefficient was estimated. The single crystals were also analyzed by absorption spectroscopy and emission spectroscopy in the case of the co-doped crystals. Evidences of the occurrence of energy transfer process in the UV for Nd-Pr were detected. From the study of the growth process single crystals of 40mm x 10mm in size, with good optical and structural quality were obtained for spectroscopy studies and tests of laser emission. Initial experiments were also performed in the growth of single crystalline fibers of this host doped with Tb3+, by the micro-pulling down method aiming a preliminary evaluation of the use of this growth process. Only semi-transparent fibers were obtained. Additional studies are necessary to improve the growth of single crystalline fibers of this fluoride.
42

Crescimento e caracterização de monocristais de BaY2F8:TR onde TR = Nd3+, Pr3+, Er3+, Tb3+, Dy3+ / Single crystals growth and characterization of BaY2F8:TR (TR= Nd3+; Pr3+; Er3+; Tb3+;Dy3+)

Simone Ferreira de Almeida Cruz 31 July 2008 (has links)
Foram realizados estudos da síntese e do crescimento de cristais de BaY2F8:TR (TR= Nd3+; Pr3+; Er3+; Tb3+; Dy3+) simplesmente dopados e codopados (Nd3+:Dy3+ ; Nd3+:Pr3+) por meio do método de fusão por zona (FZ). Os cristais crescidos foram caracterizados quanto a sua estrutura por Difração de raios-x (DRX) e Difração de nêutrons (DN) com análise pelo método de Rietveld. Foi também estudado o comportamento na fusão deste material puro e dopado com elementos terras raras trivalentes através de: microscopia eletrônica de varredura (MEV) e espectroscopia dispersiva de raios-x (EDS). A concentração final de dopantes, bem como, sua distribuição nos cristais crescidos foi estimada e, quando possível, foi ainda calculado o coeficiente de segregação de cada dopante nesta matriz. Os cristais foram também analisados por espectroscopia de absorção e emissão no caso dos materiais duplamente dopados, sendo observada evidencias de transferência de energia Nd-Pr na região UV. Foram obtidos cristais com dimensões da ordem de 40 mm x 10mm com boa qualidade óptica e estrutural adequados para estudos espectroscópicos e testes de ação laser com bombeamento por laser de diodo. Experiências preliminares de crescimento de fibras monocristalinas desta matriz pura e dopada com Tb3+, pelo método de crescimento de micro-pulling down, foram também realizadas visando uma avaliação do método aplicado a este fluoreto. Foram obtidas apenas fibras translúcidas com diâmetro constante sendo necessária maior investigação sobre o crescimento de fibras deste material puro ou dopado. / Synthesis and Growth process of single crystals of BaY2F8:TR (TR= Nd3+; Pr3+; Er3+; Tb3+;Dy3+) single-doped and co-doped (Nd3+ :Dy3+ ; Nd3+:Pr3+) were performed by Zone Melting method (ZM). The grown crystals were characterized by X-ray diffraction (XRD) and Neutron Diffraction (ND) and analyzed by the Rietveld Method. The melting behavior of this pure and trivalent rare earth doped-fluoride was investigated also by scanning electron microscopy (MEV) and X-ray energy dispersive spectroscopy (EDS). The concentration and distribution of the dopants in the grown crystals were measured, and when feasible, the segregation coefficient was estimated. The single crystals were also analyzed by absorption spectroscopy and emission spectroscopy in the case of the co-doped crystals. Evidences of the occurrence of energy transfer process in the UV for Nd-Pr were detected. From the study of the growth process single crystals of 40mm x 10mm in size, with good optical and structural quality were obtained for spectroscopy studies and tests of laser emission. Initial experiments were also performed in the growth of single crystalline fibers of this host doped with Tb3+, by the micro-pulling down method aiming a preliminary evaluation of the use of this growth process. Only semi-transparent fibers were obtained. Additional studies are necessary to improve the growth of single crystalline fibers of this fluoride.
43

Critical Behavior of Thermal Expansion and Magnetostriction in the Vicinity of the First order transition at the Curie Point of Gd5(SixGe1-x)4

Mangui Han January 2004 (has links)
Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2309" Mangui Han. US Department of Energy 12/19/2004. Report is also available in paper and microfiche from NTIS.
44

Ultrafast processes in high temperature superconductors

Gay, Pierre January 2000 (has links)
Using time-resolved photo-induced reflectivity, we reported for the first time a systematic work on the ultrafast response of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+δ</sub> (BSCCO-2212) and Tl<sub>2</sub>Ba<sub>2</sub>CuO<sub>6+δ</sub> (TBCO-2201), measurements of detwinned YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> (YBCO-123) single crystal with the electric field ̲E parallel to the a and b-axis and high-resolution measurements of the rising edge dynamics of YBCO-123 thin films. We identified similar photo-induced responses for BSCCO, TBCO and for YBCO with ̲E ⊥ ̲b, which indicates that we observed a universal response of HTSC coming from the CuO2 superconducting planes. This latter dynamics is composed of three different components corresponding to the superconducting, pseudogap and normal state. A bi-molecular model has been put forward to explain the linear temperature dependence of the decay rate; the model implies that the re-formation of the condensate is limited by the rate at which quasiparticle interact. Moreover, we observed superconducting fluctuations up to 13K above T<sub>c</sub> and a divergence of the long-lived component magnitude at very low temperature, which is explained by a cw heating model. In the pseudogap state, we have several indications that the negative peak observed between T<sub>c</sub> and T* has a different origin from that of the superconducting signal below T<sub>c</sub>. We argued that the probe mechanism of the pseudogap signal is electronic excitations of the pseudogap correlations. In the normal state, the observed dynamics is similar to that of simple metals. In the second part of this thesis, the rising edge dynamics of YBCO has been resolved in time. The model developed to interpret the results implies that the hot quasiparticles relaxation time down to the Fermi energy is 55fs. In this context, we proved that the Mazin model cannot explain both the oscillatory and the non-oscillatory part of the dynamics in YBCO. Finally, in YBCO-123, a new response has been observed with ̲E ∥ ̲b. We argued that the origin of this component is intraband transitions. This dynamics is solely responsive to the pseudogap, coming from the difference in scattering rate between pre-formed pairs and quasiparticles in the Drude reflectivity. The response with ̲E ⊥ ̲b exhibits a strong a-b plane anisotropy in its long-lived component, which can be interpreted as a d-wave gap symmetry using the thermally-activated model.
45

Chemistry and physics of diamond surfaces

Domke, Andreas January 1999 (has links)
This thesis is concerned with the chemistry and physics of C(100) surfaces of diamond. The polished and cleaned C(100) surface is examined by surface microscopy (Atomic-force Microscopy), electron diffraction (Low-energy Electron Diffraction) and photoemission (X-ray Photoelectron Spectroscopy and Ultra-violet Photoelectron Spectroscopy). Results are presented on the presence of oxygen, nitrogen and hydrogen/deuterium on the C(100) surface. Finally, the valence band structure of diamond is probed by angle-resolved photoemission. We have confirmed by AFM that the grooves from the soft polishing process are present on a polished C(100) surface and found sporadic traces of hard polish on a surface polished in the soft polishing direction. XPS studies have verified heating cycles by electron beam bombardment as a suitable cleaning procedure for pure reconstructed C(100) surfaces. By allowing the crystal to cool slowly, the first experimental evidence of quarter-order LEED spots have been found, which suggest that buckled dimerisation might have occurred similar to those on Si(100) and Ge(100). We present the first experimental electron spectroscopy results for a nitrogen impurity in diamond by showing the N KLL Auger spectrum. An attempt to smooth a C(100) surface of diamond by an atomic hydrogen plasma did not succeed. AFM studies showed no evidence for the surface smoothing reported in other studies, but the results enable us to explain the different plasma published in the literature. The valence band of diamond is investigated by off-normal ARUPS. The features observed are consistent with possible transitions, which are determined using bulk band structure calculations and comparison with the experimental binding energies.
46

Surface studies of magnetic thin films

Zeybek, Orhan January 2000 (has links)
No description available.
47

Structure and properties of MTiOXO←4 crystals

Latham, Tina Joy January 2000 (has links)
No description available.
48

X-ray standing wave studies of surface adsorption structures

Kariapper, Mohamed Sirajudeen January 2000 (has links)
No description available.
49

Surface and sensor studies of doped titanium dioxide

Duncan, Morris January 2000 (has links)
No description available.
50

The magnetic properties of superconductors

Lloyd, Sion January 1999 (has links)
No description available.

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