Graduate Thesis Or Dissertation
 

The Local Structure of Functional Electroceramics

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mc87px63g

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  • This work investigates the relationship between the local structure and physical properties of a set of classical (MgTiO3, CaTiO3, SrTiO3, and BaTiO3) and complex ((1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3[BZT-BCT], Na0.5Bi0.5TiO3 [NBT]) perovskite electroceramics. The materials in this work were synthesized via standard solid state ceramic synthesis, the structure was investigated through a number of different scattering methods to probe different aspects of the unique atomic arrangements in these materials, and the electrical properties of the materials were characterized. Although the properties of MgTiO3, CaTiO3, SrTiO3, and BaTiO3 are well documented, there have been no comprehensive investigations of the local structure. These foundational materials are an important component in current electronic devices either as the host or a dopant, and it is therefore vital to understanding more complex materials. Neutron total scattering data was collected for these materials, and the local structure of MgTiO3 was well described by the reported ilmenite average structure with rhombohedral R3 ̅ symmetry. Similarly, the local structure of both CaTiO3 and SrTiO3 showed no deviation from the average structure (with orthorhombic Pbnm symmetry for CaTiO3, and cubic Pm3 ̅m symmetry for SrTiO3). However, the local structure of BaTiO3 was found to be rhombohedral (R3m), whereas the average structure was best fit as tetragonal (P4mm). This difference between the local and average structure is due to order-disorder behavior where the locally ordered rhombohedral unit cells are disordered relative to one another, and bulk probes reveal the average structure which is the vector sum of the ionic displacements. The major component of this work is focused on understanding both the origin of piezoelectricity and the local structure of the lead-free piezoelectric material (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT). For these studies, a range of compositions were synthesized and X-ray total scattering data was collected during the application of an electric field. From the pair distribution function (PDF) data, it was found that the electric field caused the peaks to shift towards higher-r, indicating a lattice expansion. The PDF peak shift was quantified and the intrinsic contribution to the piezoelectricity was calculated. Surprisingly, the intrinsic contributions to the macroscopic piezoelectric were found to be minimized at the morphotropic phase boundary composition (x = 0.50). Furthermore, the <200> diffraction reflections, on the BCT-rich tetragonal side, were investigated and it was found that domain wall motion (extrinsic contributions) is the dominating factor for the enhanced piezoelectricity observed. Neutron total scattering and X-ray absorption spectroscopy were also collected on the x = 0.50 sample at room temperature. The local structure was not orthorhombic (Amm2) as was expected. Instead, the titanium ions were found to be distorted along the [111] direction with rhombohedral symmetry, whereas the zirconium ions were found to be undistorted with cubic symmetry. Incorporating these findings into the small-box modeling yielded a two-phase BCT-BCZ (R3m-Pm3 ̅m). The final study in this work investigates the ionic conductivity and structure of Na0.5Bi0.5TiO3 (NBT). Stoichiometric, non-stoichiometric (bismuth deficient and excess), and magnesium-doped samples were synthesized. Synchrotron X-ray diffraction, synchrotron X-ray total scattering, neutron total scattering, and physical property measurements were performed. Both the average and local structure showed changes in symmetry with increasing temperature from monoclinic Cc to tetragonal P4bm to cubic Pm3 ̅m, with only minor variations between the different compositions. However, non-stoichiometry and magnesium doping did cause the cubic phase to appear at lower temperatures (~500 °C) as evidenced by diffraction. Although the structure did not show many changes, the physical properties from the dielectric and electrical impedance measurements showed dramatic differences. The excess bismuth decreased the high temperature leakage current and created a much better insulator, whereas the bismuth deficiency slightly increased the conductivity. The magnesium doped sample showed greatly enhanced conductivity at even lower temperatures. The slopes of the Arrhenius plots for all the compositions were very similar, yielding activation energies on the order of ~0.74 eV.
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