Hybrid 3D printing of Ferroelectric Super-structures for Electromechanical Energy Systems

Functional ceramics have gained significant attention in advanced biomedical, catalysis, and piezoelectric applications. Ferroelectric ceramics are particularly interesting because of their unique intrinsic properties that can convert mechanical stress to electrical output and vice versa. However, their application as actuators, electroacoustic transducers, and energy harvesting devices has been limited by simple geometries restricting their full potential.

The geometrical complexity offered by the development of Additive Manufacturing (AM) played a crucial role in improving the properties and performance of engineering ceramics. Particularly, periodic structures with reduced volume and gradient porosity are possible with AM and have been proven beneficial for biomedical, catalysis, and thermal management applications. Concerning piezoelectric ceramics, periodic structures and Triply Periodic Minimal Surface (TPMS) structures have exhibited an immense potential for multi-directional response and control of the electromechanical coupling factor, respectively. Controlling the structure’s geometrical features (e.g., thickness and node design) is a considerable advantage for tuning the resulting properties.

Despite several advanced ceramic AM methods available today, the material and process development stage can be cumbersome, especially when several material formulations and precursors are required for research or prototyping. To overcome these limitations with a more sustainable approach during different development stages, the present thesis explores the novel Freeform Injection Moulding (FIM) method. FIM combines the geometrical benefits of Digital Light Processing (DLP), with the proven performance of standardised, materials used for Ceramic Injection Moulding (CIM). The unique approach of FIM involves 3D printing of a robust thermoset mould, as a single-cavity tool, that is injection moulded with CIM feedstocks and can be dissolved. As a result, freeform features printed as negative geometry allow periodic structures to be injection moulded. Apart from the geometrical benefits of FIM, the cost and lead time required to fabricate 3D-printed mould inserts are significantly lower than conventional mould machining of metallic blocks. As a result, lower investment costs are necessary for research and development and subsequent prototyping stages or customised applications.

This thesis aims to elucidate how the increased geometrical complexity of ferroelectric ceramics will affect their electromechanical response. Specifically, it examines how the micro-strain developed upon polarisation and domain switching is translated to macroscopic displacement and piezoelectric properties on periodic 3D structures. The ferroelectric materials selected are barium titanate, BaTiO₃ (BTO), bismuth sodium titanate such as (1-x)Bi_1/2Na_1/2TiO₃ – xBaTiO₃ (BNT-BT), and potassium sodium niobate, K_1-XNa_XNbO₃ (KNN). The above were chosen as alternative formulations that do not contain lead (Pb), commonly referred as lead-free (Pb-free), as in the case of the most widely known and used is several application lead zirconate titanate, Pb(Zr_xTi_1-x) (PZT).

The objectives were identified both in process and material development. The thesis focuses on industrial applications of engineering ceramics as a benchmark for the proposed manufacturing process. Further research was carried out in developing ferroelectric feedstocks and periodic structures, focusing on Triply Periodic Minimal Surface (TPMS) structures exhibiting great potential for metamaterial response.

The first part of the thesis focuses on the shape evolution of functional ceramics with FIM, such as alumina and zirconia. The evolution of geometrical complexity is demonstrated through the fabrication of simple bone implants, partially consolidated lenses, and a fully consolidated cylindrical lattice with internal double-helix features. Digital volumetric metrology was used to visualize the resulting shrinkage in 3D geometries. Injection moulding simulations elucidated the advanced requirements introduced by the freeform geometries. Freeform features achieved only with AM methods were fabricated with the conventional CIM process and standardised materials. The second part of the thesis shifts the focus to ceramic material development of ferroelectric feedstock formulations. The ferroelectric powders were procured and compounded into feedstocks using different binder systems targeting partially and fully consolidated geometries. Process optimisation was also achieved by comprehensive simulation analysis and injection parameter adjustment. The electromechanical performance exhibited by simple, curved, and periodic 3D geometries confirmed that the geometrical freedom of FIM applies to state-of-the-art ferroelectric ceramics without loss of functionality. The poling process employed on the 3D lattice produced with FIM, improved the ferroelectric response. In the final part, periodic structures of TPMS with ferroelectric ceramics were investigated by simulations and electromechanical characterisation. The unique surface characteristics and reduced volume of the TPMS gyroid structures are exploited by a new electrode placement approach. The dynamic arrangement of the geometry and the micro-strain developed upon switching resulted in a unique metamaterial actuation behaviour.

Using FIM as the prototyping platform for a new ferroelectric metamaterial concept provided significant design and quality considerations for such a niche ferroelectric performance. The ability to produce various shapes from different ferroelectric formulations showcased the inherent sustainability advantage of FIM for research and development. Integrating periodic and TPMS structures in the FIM process opens new possibilities for state-of-the-art ferroelectric applications such as bone implant osseointegration, catalysis applications (e.g., water purification), and improved resonance response for energy harvesting.