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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.
1

Synthesis and characterisation of a novel oxygen- deficient manganese-based perovskite series

Kruth, Angela January 1999 (has links)
A family of oxygen-deficient perovskite phases with compositions, Ca<sub>2</sub>Mn<sub>2-x</sub>Nb<sub>x</sub>O<sub>γ</sub> has been synthesised and characterised using X-ray Powder Diffraction and Thermogravimetry. Property characterisation has included the study of electrical properties by Impedance Spectroscopy and the investigation of magnetic properties for one composition, x = 1.0. Some compositions were tested for possible application as electrode materials in gas sensors. The Ca<sub>2</sub>Mn<sub>2-x</sub>Nb<sub>x</sub>O<sub>γ</sub> system 0 ≤ x ≤ 1.2, with variable oxygen content, γ, can accommodate up to 20% vacancies at oxygen sites and Mn occurs in valence states ranging from +2 to +4. Depending on the B-cation ratio and oxygen content, two solid solution form: an extensive GdFeO<sub>3</sub>-type solid solution over the entire range of cation content, 0 ≤ x ≤ 1.2, with zero or small oxygen deficiencies and a closely-related, grossly oxygen-deficient solid solution over the range 0.3 ≤ x ≤ 0.8 with a simple cubic perovskite structure. Unit cell volume and orthorhombic GdFeO<sub>3</sub>-type distortion vary greatly with Mn valency, oxygen content and B-cation content. The orthorhombic GdFeO<sub>3</sub>-type structure of composition x = 1.0 was refined by the Rietveld method. Mn and Nb are disordered over the B-sites. Jahn-Teller activity of Mn<sup>3+</sup> does not result in cooperative distortions of (Mn,Nb)O<sub>6</sub> octahedra, but it was observed to effect the oxygen stoichiometry. The Nb-rich composition, x = 1.0, can accommodate large amounts of Ca-vacancies (up to 10%). Electrical conductivity varies greatly with the composition. Results suggest that t<sub>2g</sub> electrons are responsible for a variation of conductivity and activation energy rather than e<sub>g</sub> electrons. Conductivities are believed to depend mainly on the degree of π-orbital overlap between Mn and oxygen and hence, on interatomic distances. At low temperatures, the material exhibits spin glass-like behaviour.
2

On Delocalization Effects in Multidimensional Lattices

Bystrik, Anna 05 1900 (has links)
A cubic lattice with random parameters is reduced to a linear chain by the means of the projection technique. The continued fraction expansion (c.f.e.) approach is herein applied to the density of states. Coefficients of the c.f.e. are obtained numerically by the recursion procedure. Properties of the non-stationary second moments (correlations and dispersions) of their distribution are studied in a connection with the other evidences of transport in a one-dimensional Mori chain. The second moments and the spectral density are computed for the various degrees of disorder in the prototype lattice. The possible directions of the further development are outlined. The physical problem that is addressed in the dissertation is the possibility of the existence of a non-Anderson disorder of a specific type. More precisely, this type of a disorder in the one-dimensional case would result in a positive localization threshold. A specific type of such non-Anderson disorder was obtained by adopting a transformation procedure which assigns to the matrix expressing the physics of the multidimensional crystal a tridiagonal Hamiltonian. This Hamiltonian is then assigned to an equivalent one-dimensional tight-binding model. One of the benefits of this approach is that we are guaranteed to obtain a linear crystal with a positive localization threshold. The reason for this is the existence of a threshold in a prototype sample. The resulting linear model is found to be characterized by a correlated and a nonstationary disorder. The existence of such special disorder is associated with the absence of Anderson localization in specially constructed one-dimensional lattices, when the noise intensity is below the non-zero critical value. This work is an important step towards isolating the general properties of a non-Anderson noise. This gives a basis for understanding of the insulator to metal transition in a linear crystal with a subcritical noise.
3

Inner elasticity and the higher-order elasticity of some diamond and graphite allotropes

Cousins, Christopher Stanley George January 2001 (has links)
No description available.
4

Optical characterisation of cubic silicon carbide

Jackson, Stacey Michael January 1998 (has links)
The varied properties of Silicon Carbide (SiC) are helping to launch the material into many new applications, particularly in the field of novel semiconductor devices. In this work, the cubic form of SiC is of interest as a basis for developing integrated optical components. Here, the formation of a suitable SiO2 buried cladding layer has been achieved by high dose oxygen ion implantation. This layer is necessary for the optical confinement of propagating light, and hence optical waveguide fabrication. Results have shown that optical propagation losses of the order of 20 dB/cm are obtainable. Much of this loss can be attributed to mode leakage and volume scattering. Mode leakage is a function of the effective oxide thickness, and volume scattering related to the surface layer damage. These parameters have been shown to be controllable and so suggests that further reduction in the waveguide loss is feasible. Analysis of the layer growth mechanism by RBS, XTEM and XPS proves that SiO2 is formed, and that the extent of formation depends on implant dose and temperature. The excess carbon generated is believed to exit the oxide layer by a number of varying mechanisms. The result of this appears to be a number of stable Si-C-O intermediaries that form regions to either depth extreme of the SiO2 layer. Early furnace tests suggest a need to anneal at temperatures approaching the melting point of the silicon substrate, and that the quality of the virgin material is crutial in controlling the resulting oxide growth.
5

Structure-microwave dielectric property relations in Sr and Ca titanates

Wise, Peter Leonard January 2001 (has links)
No description available.
6

Hydrogen diffusion in concentrated metal hydrides

Lam, James January 1998 (has links)
No description available.
7

A study of structure-property relationships in layered copper oxides

Hyatt, Neil January 2000 (has links)
No description available.
8

Estudo de interacoes hiperfinas eletromagneticas em oxidos perovskitas do tipo RCrOsub(3)(R=Gd, Tb e Dy) / Hyperfine interaction studies of the perovskite oxides of the type RCrO3 (R = Gd, Tb e Dy)

SILVA, RENILSON A. da 09 October 2014 (has links)
Made available in DSpace on 2014-10-09T12:27:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:37Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP
9

Estudo de interacoes hiperfinas eletromagneticas em oxidos perovskitas do tipo RCrOsub(3)(R=Gd, Tb e Dy) / Hyperfine interaction studies of the perovskite oxides of the type RCrO3 (R = Gd, Tb e Dy)

SILVA, RENILSON A. da 09 October 2014 (has links)
Made available in DSpace on 2014-10-09T12:27:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:37Z (GMT). No. of bitstreams: 0 / Óxidos perovskita do tipo ABO3 têm estrutura ideal cúbica, mas algumas distorções nesse tipo de estrutura podem produzir mudança da simetria para ortorrômbica ou romboédrica. Na estrutura cúbica, o cátion A (de maior raio atômico) ocupa o centro de cada cubo, os cátions B ocupam os 8 vértices e os ânions situamse na metade de cada uma das 12 arestas de cada cubo. Distorções ocorridas nessas estruturas produzem o surgimento de novas propriedades elétricas, estruturais e magnéticas, que instigam grande interesse ciêntifico e tecnologico. Neste trabalho, foram estudados os compostos RCrO3 (R = Gd, Tb, Dy), conhecidas como ortocromitas. As amostras foram confeccionadas através do processo químico Sol Gel. Após a confecção as amostras foram analisadas pela difração de raios-x na qual foi concluida que as amostras tiveram uma única estrutura pertencente ao grupo espacial Pbnm. Logo foram realizadas medidas de correlação angular gama-gama perturbada utilizando para isso os nucleos de prova 181Hf(181Ta) e 111In(111Cd) que substituam os átomos A ou B. Os núcleos de prova foram introduzidos durante a fase de preparo das amostras. Um dos objetivos desse trabalho foi o estudo sobre variação do campo magnético hiperfino nos sítios A e B como função da temperatura, estrutura cristalográfica e a transição de fase antiferromagnética. As medidas foram feitas nas a temperaturas entre 20 e 300K para R = Gd, Tb e 20 a 800 K para R = Dy. Também foi medido o Gradiente de campo elétrico em função da temperatura. Foi possivel observar a transição e comprovar o alinhamento dos Spins do Cr, conforme consta na literatura. As temperaturas de Néel (TN ) para as amostras são ~170 K, ~164 K, e ~148 K, respectivamente para os compostos GdCrO3, TbCrO3 e DyCrO3. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP
10

Mass transport in mixed conducting perovskite related oxides

Shaw, Cynthia Kit Man January 2001 (has links)
No description available.

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