Abstract
Porous ferroelectric materials are often dismissed as mechanically fragile and electrically compromised. In this work, I challenge that assumption and show that, with careful design, porosity can be turned into an advantage—unlocking new levels of performance in piezoelectric sensors, particularly for underwater acoustic applications. This thesis presents an integrated approach combining analytical modeling, novel material development, and device-level testing, all focused on creating and understanding a new generation of lead-free porous piezocomposites.One of the central achievements of this research is the development of a new analytical model, designed specifically for freeze-cast porous piezoelectric structures. Built in MATLAB, this model captures how the alignment and geometry of pores affect the material’s anisotropic piezoelectric behavior. It shows, both numerically and conceptually, how directional porosity can decouple the longitudinal (d₃₃) and transverse (d₃₁) piezoelectric coefficients, which leads to a significant boost in the hydrostatic figure of merit (dₕ = d₃₃ − 2d₃₁). This is a key parameter for passive SONAR and hydrophone applications, where broadband and low-frequency sensitivity are essential. The model not only predicts trends seen in experiments but also serves as a practical design tool for future materials.
Building on this theoretical foundation, I developed a new class of porous piezoceramic/polymer composites based on lead-free Barium Calcium Zirconate Titanate (BCZT). These materials were engineered to explore how the properties of the polymeric phase—particularly its elasticity—affect the overall piezoelectric response. A key finding is that reducing the stiffness of the polymer matrix actually improves charge generation. By enhancing mechanical coupling between the polymer and the ceramic skeleton, the system delivers higher effective d₃₃ values, making it more responsive to mechanical stress. This counterintuitive result has important implications for composite design.
Mechanical performance was also a central focus of this research. Porous ceramics, while functionally promising, are notoriously brittle. Through a series of mechanical tests, I quantified how the addition of a compliant polymer phase improves structural resilience. The composites showed significant gains in fracture strain and toughness—demonstrating that it’s possible to combine functional performance with mechanical durability. This makes the materials much more viable for real-world use, especially in harsh underwater environments.
The final part of the work brought these materials into a practical context: the development of a working hydrophone demonstrator. Built entirely from the porous BCZT/polymer composite, the device was tested underwater to evaluate its acoustic sensitivity and directional response. The results were very promising. The hydrophone showed excellent sensitivity, good impedance matching with water, and clear directional behavior, all consistent with the anisotropic design of the material. This validates not only the material development but the entire design-to-device approach.
Key Contributions
1. Custom Analytical Model: A new model for freeze-cast porous piezoelectrics that explains and predicts hydrostatic performance with directional porosity.
2. Materials Innovation: Creation of novel BCZT/polymer composites that show improved piezoelectric behavior by tuning the elasticity of the secondary phase.
3. Mechanical Enhancement: Comprehensive testing demonstrates that these composites significantly improve mechanical robustness while maintaining strong electromechanical properties.
4. Device Integration: A functional lead-free hydrophone prototype built with the developed materials, tested successfully in an underwater setting.
Altogether, this research offers a new perspective on porous ferroelectrics—not as flawed materials, but as tunable systems that, when properly engineered, can outperform their dense counterparts. It opens new directions in the design of smart acoustic sensors that are both environmentally friendly and functionally advanced, with real potential for impact in SONAR, medical imaging, and beyond.
| Date of Award | 12 Nov 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chris Bowen (Supervisor), Hamideh Khanbareh (Supervisor), James Roscow (Supervisor) & Guylaine Poulin-Vittrant (Supervisor) |