Effect of Dopants on the Local Atomic Structure and Sintering Behavior of Bismuth Sodium Titanate

The most commonly used piezoceramic is lead zirconate titanate Pb(ZrxTi(1-x))O (PZT). It possesses outstanding piezoelectric properties which can be modified for numerous applications by the addition of dopants. However, because of environmental and health concerns regarding lead, lead-free alternatives are demanded by politics. One of the two most promising lead-free replacement materials is the ferroelectric bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT). Like PZT, it crystallizes in the perovskite structure. Since the dielectric and piezoelectric properties of pure BNT ceramics are insufficient for application, BNT is often modified by the addition of dopants. These influence a great variety of material properties to different degrees, e.g. the sintering behavior, the dielectric and piezoelectric properties and their respective temperature stabilities. Doping of BNT aims to decrease the sintering temperature in order to avoid Bi vaporization, to increase the depolarization temperature and to enhance the piezoelectric coefficient. The effects of numerous dopants on the resulting performance of BNT were studied extensively in the literature. However, so far little attention has been payed to the way in which dopants interact with the piezomaterial. Nevertheless, it is the understanding of these relationships that would make targeted modifications and improvements of BNT possible. The primary goal of this study was to investigate and explain the effects of a model dopant -cobalt- on the phase formation, sintering behavior and microstructure of BNT as well as on the resulting dielectric and piezoelectric properties. In this regard, a core issue was to determine the preferred lattice site of Co in BNT. BNT was synthesized from oxide powders using the classic solid-state route and sintered at temperatures ranging from 1000 °C to 1150 °C. Cobalt was added in concentrations between 0.1 mol% and 2.6 mol% Co prior to the calcination as Co3O4. About one third of the total cobalt amount was incorporated into the BNT lattice on the perovskite B-site, that is, it substituted for Ti. The cobalt in BNT appeared to be in equilibrium with the secondary phase Co2TiO4, which invariably formed at cobalt concentrations greater than 0.1 mol% Co. For charge balancing reasons, oxygen vacancies were created in the lattice of cobalt-doped BNT. These markedly enhanced the diffusivity. As a result, the sintering temperature of doped BNT decreased with increasing cobalt concentrations, and high final densities were achieved. However, in highly doped BNT sample swelling occurred at elevated temperatures of the sintering cycle. This phenomenon was attributed to evaporating oxygen caused by the valence transition of Co3+ to Co2+. Up to 950 °C, BNT was found to densify via solid state sintering mechanisms. Above this temperature, a small amount of liquid phase was present, which probably formed from decomposing BNT because of a slight Ti-deficiency due to doping. Increased Bi vaporization from the melt above 1000 °C appeared to have stabilized sodium cobalt titanate, an additional secondary phase. The rotation of the iso-lines in the kinetic field diagram of doped BNT was interpreted such that the activation energy for grain growth was higher than the activation energy for densification. Possible reasons are the solute-drag effect and the pinning of domain walls by secondary phase particles. Both the depolarization temperature and the piezoelectric coefficient d33 decreased with increasing cobalt concentrations. The dielectric properties deteriorated as well. This was attributed to the high electrical conductivity of the doped samples, which prevented full poling.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Schmitt, Veronika

Contributors dc:contributor

• Breu, Josef

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/77/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:77