Influência dos defeitos cristalinos nas propriedades dielétricas e não-ôhmicas do 'CA' 'CU IND.3' 'TI IND.4' 'O IND.12'(CCTO)
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2010-08-13
Autores
Tararam, Ronald [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
A descoberta da alta constante dielétrica () no CaCu3Ti4O12 (CCTO) despertou grande interesse nesse material com estrutura tipo perovskita, principalmente por não apresentar uma transição de fase ferroelétrica ou comportamento relaxor comparado aos demais titanatos. O CCTO, sem adição de dopantes, apresenta também forte característica não-linear de corrente-tensão com possibilidade de aplicações em varistores. O comportamento elétrico do CCTO tem sido intensivamente discutido na literatura e tem sido atribuído a defeitos intrínsicos ou extrínsicos. Entretanto, não existe concordância na literatura em relação aos mecanismos que permitem explicar a origem dessas propriedades. O objetivo principal deste trabalho foi sinterizar cerâmicas de CCTO a 1100 ºC por tempo variável e avaliar por Espectroscopia Fotoelétrica de Raios X (XPS), Difração de Raios X (XRD), Análise Termogravimétrica (TGA), Microscopia de Força Elétrica (EFM), e Microscopia de Varredura por Potencial de Superfície (SSPM) os defeitos cristalinos de grão e contorno de grão. Mecanismos de defeitos puntiformes do tipo substitucional e defeitos planares com falhas de empilhamento (stacking faults) foram discutidos para concordar com os resultados experimentais. Foi introduzindo o conceito de polarons na construção de um modelo para explicar a alta constante dielétrica do CCTO. Medidas elétricas em campo alternado, utilizando espectroscopia de impedância, enfatizaram os mecanismos de relaxação dos polarons no modelo proposto. No estudo de fenômenos de transporte em campo elétrico contínuo, para avaliar o comportamento não-ôhmico do CCTO, foram realizadas medidas da densidade de corrente (J) em função do campo elétrico aplicado (E). Essas medidas macroscópicas relacionam-se com medidas elétricas locais usando técnicas de EFM e SSPM. Com os resultados dessas técnicas, utilizadas...
The discovery of high dielectric constant () in the CaCu3Ti4O12 (CCTO) generates a great scientific interest in this perovskite type of material mainly because it does not show ferroelectric phase transition or relaxor behavior compared with other titanates. The CCTO, with no dopants, displays also strong current-tension nonlinear behavior with possibility to be used as varistor. The CCTO electrical behavior has been intensively discussed in the literature and has been attributed to intrinsic and extrinsic defects. However, there is no agreement in the literature about the mechanisms which allow explaining the origin of these properties. The main objective of this work was sintering CCTO ceramics at 1100 ºC for different times and to evaluate by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA), Electrostatic Force Microscopy (EFM), and Scanning Surface Potential Microscopy (SSPM) the grain and grain boundary crystalline defects. Substitution point defects and planar stacking faults defects were discussed to agree with experimental results. The polaron concept was introduced in building a physical model to explain the high dielectric constant of CCTO. AC electrical measurements using impedance spectroscopy reinforced the polaron relaxation mechanisms of the proposed model. In the transport phenomena study in continuous electric field, to evaluate the CCTO non-ohmic behavior, measurements of current density (J) as function of electric field were carried out. These macroscopic measurements are related to the electric local measurements using EFM and SSPM. The obtained results based on these techniques, for mapping space charges and local surface potential, allowed to propose a model for potential barrier in the CCTO grain boundaries due to the presence of crystalline defects and polarons. Other objective of this work... (Complete abstract click electronic access below)
The discovery of high dielectric constant () in the CaCu3Ti4O12 (CCTO) generates a great scientific interest in this perovskite type of material mainly because it does not show ferroelectric phase transition or relaxor behavior compared with other titanates. The CCTO, with no dopants, displays also strong current-tension nonlinear behavior with possibility to be used as varistor. The CCTO electrical behavior has been intensively discussed in the literature and has been attributed to intrinsic and extrinsic defects. However, there is no agreement in the literature about the mechanisms which allow explaining the origin of these properties. The main objective of this work was sintering CCTO ceramics at 1100 ºC for different times and to evaluate by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA), Electrostatic Force Microscopy (EFM), and Scanning Surface Potential Microscopy (SSPM) the grain and grain boundary crystalline defects. Substitution point defects and planar stacking faults defects were discussed to agree with experimental results. The polaron concept was introduced in building a physical model to explain the high dielectric constant of CCTO. AC electrical measurements using impedance spectroscopy reinforced the polaron relaxation mechanisms of the proposed model. In the transport phenomena study in continuous electric field, to evaluate the CCTO non-ohmic behavior, measurements of current density (J) as function of electric field were carried out. These macroscopic measurements are related to the electric local measurements using EFM and SSPM. The obtained results based on these techniques, for mapping space charges and local surface potential, allowed to propose a model for potential barrier in the CCTO grain boundaries due to the presence of crystalline defects and polarons. Other objective of this work... (Complete abstract click electronic access below)
Descrição
Palavras-chave
Físico-química, Dieletricos, Nanofios, Dielectric, Varistor
Como citar
TARARAM, Ronald. Influência dos defeitos cristalinos nas propriedades dielétricas e não-ôhmicas do 'CA' 'CU IND.3' 'TI IND.4' 'O IND.12'(CCTO). 2010. 120 f. Tese (doutorado) - Universidade Estadual Paulista, Instituto de Química de Araraquara, 2010.