Abstract
This thesis investigates piezoelectric materials for their applications as energy harvesting materials in water environments. Through a combination of modelling, material fabrication and characterisation, and demonstration testing, this thesis comprehensively examines how the properties of piezoelectric materials influence their capabilities in energy harvesting in water flow for self-powered systems. These insights guides the development of a novel high-performance, lead-free piezoelectric material: porous Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT).Through a literature review, it is highlighted that the performance of water flow piezoelectric harvester is dominantly limited by the energy density of the piezoelectric materials, which has shown slow progress since the year 2000, as indicated by the piezoelectric energy harvesting figure-of-merit (FoM33). To better understand the roles of piezoelectric material properties in energy harvesting performance, an analytical energy flow model is developed. This model establishes a direct connection between material’s FoM33 to the practical power output of the harvester. In addition, it provides a clear target for optimizing piezoelectric materials for energy harvesting applications, primarily emphasizing the need for a high piezoelectric coefficient (d33) and low permittivity (ε33).
For this target, an aligned porous structure is intentionally introduced into BCZT ceramics using a freeze-casting technique. Compared to its dense, pore-free counterpart, porous BCZT exhibits notable advantages, including a higher piezoelectric coefficient (d33), lower permittivity (ε33), lower Young’s modulus (Y), and lower bulk density (ρ). Remarkably, the enhancement of d_33 caused by the aligned porous structure - contrasting the typical d33 reduction with introducing porosity - is for the first time reported. This enhancement leads to breakthroughs in the piezoelectric voltage constant (g33), piezoelectric energy harvesting figure-of-merit (FoM33) and electromechanical coupling coefficient (k33^2). A combination of experiments and finite element simulation reveals two mechanisms underlying the abnormal d33 enhancement. First, the porous structure increases the oxidation degree of BCZT during sintering, reducing oxygen vacancy concentration and improving ferroelectric domain mobility, thereby enhancing the material’s polarisation. Second, the aligned porous structure induces an inhomogeneous dipole arrangement, generating an internal bias electric field which is in the same direction of the material’s polarization, further enhancing the polarisation.
In water flow energy harvesting applications, a 50 vol% porous BCZT is tested and exhibits significantly improved energy harvesting performance. Compared to a harvester based on dense BCZT, the porous BCZT harvester achieved a 3.5-times increase in open-circuit voltage. During a capacitor charging test, the porous BCZT harvester also exhibited a 2.3-times increase in the saturated charging voltage (Vcharge) and a 5.1-fold increase in the charging power compared to the dense BCZT harvester. This benefit was attributed to the higher piezoelectric voltage coefficient (g33) and piezoelectric harvesting figure-of-merit (FoM33) of the porous BCZT, enabling a direct current (DC) power supply for devices requiring voltage input below 2.1 V, providing a novel strategy to overcome the limited performance of water flow piezoelectric harvesters in turbulent, low-velocity flow conditions.
The primary contributions of this thesis include the insight of piezoelectric material properties in their transducer performance, primarily in energy harvesting, which promotes the development of porous BCZT ceramics as a high-performance lead-free piezoelectric material for water flow energy harvesting and acoustic propulsion systems. Novel enhancement mechanisms of porous BCZT are identified and linked to porous structure to material properties. Moreover, the advantage of porous BCZT in water flow energy harvesters is practically demonstrated. Future work is expected to further exploit the d33 enhancement mechanisms, design novel porous structure for breakthroughs in material properties, and broaden the application fields of porous piezoelectric ceramics in transducer applications.
| Date of Award | 10 Sept 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chris Bowen (Supervisor), James Roscow (Supervisor), Hamideh Khanbareh (Supervisor) & Min Pan (Supervisor) |