Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.778812
Title: Structure-property relations in Sodium-Bismuth Titanate related materials
Author: Pradal Velazquez, Emilio
ISNI:       0000 0004 7964 5409
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2019
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Abstract:
The electrical properties of the ferroelectric perovskite Na0.5Bi0.5TiO3 (NBT) are sensitive to low levels of Na and Bi non-stoichiometry. Previous work established that NBT can possess high oxide-ion conduction which makes it a potential electrolyte for solid oxide fuel cells or it can be electrically insulating making it useful as a dielectric material for high temperature capacitor or Pb-free piezoelectric applications. Although acceptor-doping on the A- or B-site can create oxygen deficiency in the NBT lattice, the increase in oxide-ion conductivity has been modest. The limited solid solutions for single dopants and structural complexity of NBT have hindered the understanding of the origin of its high ionic conductivity. As a consequence, the study of NBT with other perovskites, which have shown higher solid solution limits with NBT, is of interest to obtain more information on the behaviour of this challenging system. In this work, solid solutions of NBT with BiMg2/3Nb1/3O3 (BMN), BiZn2/3Nb1/3O3 (BZN) and NaNbO3 (NN) are explored. In both the NBT-BMN and NBT-BZN cases there is little effect on the permittivity profiles and a gradual decrease in bulk (oxide-ion) conductivity until the system becomes insulating at ~ 6 mol% BMN and 4 mol% BZN. NBT-BMN has a solid solution limit between 7 and 8 mol% and the conductivity behaviour is similar to that reported in the literature for NBT-BiAlO3 and explained as trapping of oxygen vacancies by the B-site dopant. For the NBT-BZN system, the solid solution limit is below 4 mol%, making it inconclusive whether the same trapping mechanism occurs in NBT-BZN. NBT-NN forms an extensive solid solution with the limit exceeding 30 mol% NN. Laboratory XRD and variable temperature neutron diffraction results indicate stabilisation of the orthorhombic polymorph to lower temperatures with increasing NN content. Similar to NBT, the bulk conductivity of NBT-NN compositions display a strong dependence on small variations in the nominal Na and Bi content in the starting composition. It has previously been reported that this system produces permittivity-temperature profiles with remarkable stability. In this work, data extracted from impedance spectroscopy are compared against data from fixed frequency permittivity measurements and demonstrate that the attractive flattening of the permittivity profiles at 1 MHz involves a mixed response from more than one electroactive element. Powder and ceramic processing conditions are shown to have a strong influence on the electrical properties, impacting upon the reproducibility of these materials and raising concerns about their applicability. The electrical heterogeneity in these samples is sensitive to sample history and in conductive samples can dominate the conductivity at high temperatures (>600 °C), leading to unreliable measurements of oxide-ion transport numbers normally associated with a bulk response. Finally, a previously reported but relatively unstudied Sodium-Bismuth Titanate Pyrochlore solid solution (Pss) is explored; however, it forms via an entirely different mechanism from that originally reported. In both cases, A-site vacancies are involved, similar to that reported for undoped Bismuth Titanate Pyrochlore. In contrast to Bismuth Titanate Pyrochlore, this Pss can be prepared and ceramics sintered using conventional methods; however, the processing remains challenging. Although many processing issues are addressed here and a method developed to obtain dense and phase-pure Pss ceramics, other potential issues occur such as redox activity which makes control of the pO2 an important processing parameter when preparing and characterising Pss materials. This cubic Pss has low electrical conductivity (and does not exhibit any evidence of oxide-ion conduction), a broad permittivity maximum of ~100 near room temperature and exhibits relaxor ferroelectric behaviour. Variable temperature neutron diffraction data does not provide any conclusive evidence for a phase transition in the Pss between 4.2 and 873.2 K.
Supervisor: Sinclair, Derek ; Reaney, Ian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.778812  DOI: Not available
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