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Abstract
Piezoelectric single crystals provide a large electro-mechanical response that is desired for sensor and actuator applications. However, the complexity and cost of single crystal growth inhibits their widespread use. Alternatively, polycrystalline piezoelectrics can be easily fabricated at low cost. Due to the anisotropic piezoelectric response, the electro-mechanical response of polycrystals is largely limited by the elastic coupling of randomly aligned grains. Microstructural engineering via crystallographic texturing of polycrystalline ceramics offers an alternative method that both enhances the electro-mechanical response while maintaining an economical fabrication process. There is currently limited understanding on how the changes in the microstructure from the texturing process, such as the addition of heterogenous templates and changes in grain orientation distribution, impacts the electro-mechanical response mechanisms. Understanding the structural and electro-mechanical mechanism that enhances the piezoelectric response will benefit sensor and actuator applications such as sonar systems and medical imaging instruments.
In this thesis, a range of techniques were used to study the microstructural changes and the strain mechanisms from crystallographic texturing, and its impact on the electro-mechanical response. The local response of electro-mechanical phase-change ceramics was investigated by theoretical calculations to see how texturing changed the strain heterogeneity at the grain-scale. Textured ceramics of Pb(Mg1/3Nb2/3)O3− x%PbTiO3 (PMN-PT) were fabricated by tape casting and templated grain growth, using BaTiO3 platelet templates. The structure of these textured ceramics was characterised using synchrotron x-ray diffraction to gain a more comprehensive understanding on the texture that develops. In situ electric-field dependent diffraction measurements were then used to study how texture affects the electro-mechanical response mechanism. And lastly, the piezoelectric non-linearity and response stability over time was analysed in the direct piezoelectric response mode.
Changes in electric-field-induced phase transformation local strain response in polycrystalline piezoelectrics of varying grain orientation distribution was studied through theoretical calculations. The March-Dollase function was used to generate texture distributions in a model polycrystal, and strains associated with phase transitions from pseudo-cubic to tetragonal, rhombohedral and orthorhombic symmetries were calculated. In the tetragonal system, the overall strain response was improved by 60% with crystallographic texture, at very strong texture (of March Dollase distribution r = 0.05); and the local strain heterogeneity was similar to the random system. However, moderate increases in texture had a negative impact on the strain heterogeneity. In crystallographic symmetries with higher numbers of possible polarization (and thus spontaneous strain) directions, the magnitude of the strain response increased, and the heterogeneity of the system decreased.
Using high energy x-ray diffraction and in situ electric-field dependent measurements, the microstructural modifications due to crystallographic texturing and the resulting electro-mechanical response mechanisms were investigated. Structural analysis of textured ceramics, by calculating the orientation distribution function (ODF), showed that sheet-like texture developed in the piezoelectric. Texture characterisation, by calculating the ODF, was able to provide a more comprehensive quantification of texture than the Lotgering factor, where multiple samples with a 95% Lotgering factor showed different ODFs and maximum Multiples of Random Distribution (MRD) values of 12.8 and 16.54. In addition to the crystallographic texture observed in the diffraction pattern, a ferroelastic texture was also seen. Termed the ‘self-poling effect’, this unique structural distortion reduced the remanent strain of the textured piezoelectric to 0.13% after poling to 2 kV/mm, while the random (untextured) ceramic produced a remanent strain of 0.18% after poling to 2 kV/mm. On a subsequent unipolar strain cycle, however, the textured ceramic achieved a larger response of 0.16% compared to 0.096% of random ceramic. A new strain mechanism model was proposed to explain the relationship between the self-poling effect and the observed strain behaviour.
Finally, the direct piezoelectric response in textured ceramics was measured to understand the piezoelectric non-linearity and response stability under application-like conditions. By applying a static offset and dynamic sinusoidal compressive loading, the d33 and d32 response were measured. The piezoelectric non-linear behaviour increased with crystallographic texturing. In particular, the piezoelectric property in the d33 mode, deteriorated faster and was more unstable, losing around 60% of its original d33 value once the offset load was released. On the other hand, the d32 response of texture ceramics was able to recover 90% of its original response after loading. Furthermore, the stability of the piezoelectric response in textured piezoelectric was observed to be compositionally dependent, where the d32 response decayed over time faster in PMN-31PT than in PMN-32PT.
The results from this thesis shows that crystallographic texturing alters the local strain environment, affecting the electro-mechanical response mechanisms, both enhancing the response but also potentially degrading the expected lifetime and performance of these ceramics. Further work in understanding the microstructural interaction with the electro-mechanical response will help to optimise texture fabrication of piezoelectric materials for industrial applicability.
