Structure and properties of sodium bismuth titanate ferroelectric ceramics
Abstract
Piezoelectric materials are commonly used in sensor and actuator technologies due to their unique ability to couple electrical and mechanical displacements. Applications of piezoelectric materials range from diesel engine fuel injectors, sonar, ultrasound, and nanopositioners in scanning microscopes. Changing environmental regulations and policies have led to a recent surge in the research of lead-free piezoelectric materials. One such system currently under investigation is sodium bismuth titanate (Na0.5Bi0.5 TiO3) or NBT. It has recently been investigated with the addition of chemical modifiers as well as part of various solid solutions with other compounds. However, research into the structure and properties of NBT is still in its infancy. The aim of this dissertation was to develop a comprehensive understanding of the crystal structure and property relationships in NBT. First, the formation of the NBT phase during solid state processing was examined using in situ X-ray diffraction. It was determined that NBT forms through a particle conversion mechanism of the Bi2O 3 particle. The average and local room temperature structure of calcined and sintered NBT were examined using both high resolution synchrotron X-ray diffraction and neutron diffraction techniques. It was determined that the room temperature average structure of this material is best modeled using the monoclinic Cc space group rather than the previously accepted rhombohedral R3c space group. A combined high resolution XRD and neutron diffraction Rietveld refinement provided refined lattice parameters, atomic positions, and displacement parameters. The departure of the local structure of NBT from the average structure was examined through the Pair Distribution Function analysis. It was determined that Na+ and Bi3+, which share the A-site, have differing bonding environments with their surrounding O2- ions. In order to understand the origin of the piezoelectric depolarization behavior of NBT, crystal structure refinements were undertaken at elevated temperatures. It was determined that a fraction of the material is present in short range regions and could not be described using the average Cc structure. It was therefore modeled using the prototype cubic phase of the Pm3¯m space group. It was proposed that thermal depoling in NBT occurs due to the loss of long-range ferroelectric order along with a decreasing volume fraction of the major Cc phase and the associated percolation of a nanoscale disordered phase. Finally, effect of a donor ( La•A , where A denotes the A-site) and an acceptor ( Fe/B , where B denotes the B-site) chemical modification on the temperature dependent structure and properties was explored. It was shown that La substitution decreased the depolarization temperature with an associated higher fraction of the Pm3¯m phase, while Fe modification led to an increase in the depolarization temperature and a decrease in the fraction of the Pm3¯m phase. The correlation between the changes in depolarization temperature and the Pm3¯ m phase fraction supports the hypothesis that the local scale disordered portion of the material influences the depolarization of NBT.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2012
- Bibcode:
- 2012PhDT.......248A
- Keywords:
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- Physics, Solid State;Engineering, Materials Science