Entrada Departamento Notícias Palestras - 6 de maio de 2019

Palestras - 6 de maio de 2019

06 de Maio de 2019 às 14:30

Anfiteatro -120, FC3 - FCUP

 

Why do we keep investigating

titanates and niobates based ferroics?

Paula Maria Vilarinho, CICECO - University of Aveiro, Portugal

 

Abstract: Why, after so many years, do we keep investigating titanates and niobates based ferroics?

Ferroics are by definition materials that exhibit one of the four primary ferroic properties: ferromagnetism, ferroelectricity, ferroelasticity and ferrotoroidicity. The formation of domains upon a phase transition from a high temperature prototype phase to a low temperature ferroic phase, is the common feature of the four types of primary ferroics. In addition, switching of these domains is associated with a hysteresis loop. For specific symmetry cases, materials may exhibit two or three primary ferroic properties in the same phase; materials are then called multiferroics. With such a panoply of physical peculiarities in one single material, together with a multitude of possible external stimulus, it is not difficult to predict a huge area of relevant applications for ferroics. Some of the most important family of materials that exhibit ferroic behaviour are perovskite type titanates and niobates. Barium titanate (BaTiO3) is a prototype ferroelectric and strontium titanate (SrTiO3) is a prototype incipient ferroelectric. And both of these materials have been extensively used and studied; with us having contributed to their understating and development, as well. In spite of that, there is a current renewed interest on titanates and niobates. This is related to the increasing need for clean and sustainable energy sources and efficient ways of storing energy. The development of solid dielectrics with a high dielectric constant, high breakdown strength and low losses is still a key breakthrough in the realization of solid-state super capacitors. Here, polycrystalline Ba1-xSrxTiO3 (BST) with small grain size and defect free grain boundaries, will be relevant. The capture of unused heat is one possible source of energy that is still unexploited. A potential device to harvest the energy dissipated by heat is a thermoelectric generator. Optimised oxide thermoelectrics, as SrTiO3 are definitely playing a role. Finally, under the sustainability concepts, lead free ferroelectrics, as alkaline niobates, prepared at low temperatures via sustainable processes, are gaining relevance.

 

Simetria, relações constitutivas e efeitos

magnetoeléctricos em materiais multiferróicos

José Luís Ribeiro, Centro de Física da Universidade do Minho

 

Abstract: O recurso sistemático a considerações gerais de simetria permanece ainda relativamente marginal no estudo das propriedades magnéticas e na caracterização da resposta magnetoeléctrica de um material. As consequências deste facto são múltiplas. As relações constitutivas adequadas à descrição da resposta linear de um sistema são frequentemente procuradas por tentativa e erro, os constrangimentos nas propriedades físicas impostos pela sua simetria são por vezes confundidos com assinaturas de mecanismos microscópicos específicos, e o refinamento da estrutura de uma fase magnética é eventualmente feito recorrendo a restrições desnecessárias. Nesta palestra procurarei mostrar como a consideração do grupo espacial (superespacial) magnético de uma fase comensurável (incomensurável) permite ultrapassar estas limitações. A partir de exemplos concretos, tentarei ilustrar como a observação da simetria específica de uma fase permite racionalizar as suas propriedades, definir as relações constitutivas adequadas, explorar experimentalmente dos detalhes da sua resposta electromagnética e identificar as restrições a que as suas estruturas atómicas e magnéticas devem obedecer.

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