Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2017, Tutor: Elena Cerdeiras Montero
Piezoelectric materials based on lead zirconate titanate (PZT) are widely used in a large number of electronic devices due to their excellent properties. This improvement occurs in a region of the phase diagram related to a morphotropic phase boundary (MPB). However, the major drawback is the environmental and health problems arising from the presence of lead due to its high toxicity. Therefore, nowadays and during last years, the scientific community has focused on the search for lead-free piezoelectric materials.
(Bi0.5Na0.5)TiO3-BaTiO3 based ceramics (BNT-BT) has attracted considerable interest attributable to their similar characteristics to PZT. This system has a morphotropic phase boundary where a great piezoelectric response is obtained and, for this reason, BNT-BT system is considered a promising candidate to replace lead-based piezoelectric materials. However, even with the large number of studies carried out there is no exact composition around MPB where the best properties are obtained. So, it is necessary to continue doing research and try to improve the functional properties.
Hence, the goal of this work has been the study of different aspects associated with (Bi0.5Na0.5)TiO3-BaTiO3 system based ceramic materials.In this work, the preparation of BNT-BT based ceramics has been made by solid state reaction and their characterization has been carried out with the different techniques used on solid state chemistry. The planetary ball mill has been stablished as an important part of the synthesis pathway and optimizing the sintering conditions has been done. High density ceramics have been obtained in all compositions with a relative density around 98%. In all cases, perovskite unique phase ceramics have been obtained with coexistence of rhombohedric and tetragonal phase, confirming that they are on the morphotropic phase boundary. Using Scanning Electron Microscopy, the presence of a BNT matrix with grains of BT on the obtained ceramics has been observed.
In order to improve the functional properties of these materials, the effect of lanthanide donor dopants has been studied, focusing on the influence of different radius and their concentration. Through X-Ray Diffraction the introduction of dopants on the BNT-BT structure has been confirmed since the diffraction maximums have been shifted to higher angles, meaning the incorporation of lower radius cations and the creation of cationic vacancies. The introduction of Lanthanum has provoked a morphologic variation of the barium titanate grains, which leads us to believe that Lanthanum has been introduced on the BaTiO3 structure, as it was expected due to the stoichiometry of the compound.
The dielectric properties of BNT-BT based ceramics have been studied on the morphotropic phase transition, obtaining high values of relative permittivity for the non-doped ceramic and low dielectric losses in all compositions. The Praseodymium doped BNT-BT shows a dielectric behavior dependent on frequency at room temperature, both related to relative permittivity and dielectric losses, better than the non-doped BNT-BT.