Correlation between (di)electrical properties and (micro)structural characteristics in PTCR-type Ba1-x(RE)xTiO3 thermistor materials

Abstract

The unusual properties of ferroelectrics make them attractive materials for the manufacture of various devices. These properties include ferro-, piro- and piezo-electric responses. BaTiO3 belongs to this group of materials, and presents, moreover, the PTCR (Positive Temperature Coefficient of Resistivity) thermistor effect when doped with tri- or penta-valent cations. It is well-known that this effect involves the formation of p-n barriers at the grain-surface micro-regions. Despites the various studies to date performed on this issue, where several factors have been postulated or identified to contribute to this effect, recent research in literature reveals some controversies when trying to account for the main factor to be considered as real driving force of these barriers, leaving open questions about the phenomenology of these materials. The present project will be aimed at studying Ba1-x(RE)xTiO3-like thermistors, where RE(=Er3+, Sm3+) are rare earths with, particularly, intermediate ionic radii between Ba2+ and Ti4+. Accordingly, these foreign cations have, from the stress energy viewpoint, an amphoteric character, as they might substitute any of both host cations, depending only on their concentration. The main focus of this project will be the study of the (di)electrical responses of these materials in terms of bulk and internal interface properties, followed by the establishment, if any, of the correlations applying between these responses, occurrence and magnitude of the PTCR effect, and the (micro)structural characteristics of these thermistors. The materials will be produced via conventional sintering and flash sintering, the last one being a novelty in this kind of study. By combining several characterization techniques, including dielectric/impedance spectroscopy (without and with an applied DC bias), current-voltage curves from bulk and interfaces, chemical stoichiometry (micro)analysis and defects chemistry analysis, it is the objective of this project to contribute significantly to a better understanding of this effect at the (micro)structural level and, therefore, a better understanding of the basic science of BaTiO3-related thermistor materials. (AU)