Upside-down composites for recycling piezoelectric ceramics: A systematic study on the interplay between fillers and binders

This thesis examines the feasibility of recycling Pb based piezoceramics using the upside down composite method. It also evaluates the dielectric, piezoelectric, and ferroelectric properties of the recycled materials through combined experimental and theoretical approaches.

Oxide-halide perovskite composites based on recycled Pb(Zr,Ti)O3 (PZT) fillers and Me3NCH2ClCdCl3 and C6H5N(CH3)3CdCl3 binders exhibit good densification and d33 and g33 values of ≈90–100 pC/N and 37–38 mV·m N-1, respectively, enabled by excellent wetting of binders on fillers. It is found that overlap between the filler phase transition and binder decomposition can induce microstructural frustration, reducing permittivity and piezoelectric response in recycled materials made from the PZT filler and the Me3NCH2ClCdCl3 binder. In contrast, composites made from the Sm doped Pb(Mg,Nb)O3–PbTiO3 filler avoid this overlap, thus achieving high relative permittivity (≈650 at 1 kHz).

To overcome the limitations of piezoelectric response, a high permittivity polymer, polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene, is introduced as the binder. In the composites, the polymer binder solidifies into a continuous phase, producing a more uniform microstructure and reducing permittivity mismatch between filler and binder. As a result, the d33 values are significantly improved to 160–170 pC N-1, with a theoretical cap of 200 pC N-1 predicted in modelling. These d33 values correspond to about 40 % retention of the piezoelectric response in pristine piezoceramics. The g33 values of ≈30–40 mV·m N-1 are also achieved in these recycled materials, fully preserving the performance of pristine ceramics.

This thesis suggests a critical threshold for the binder’s relative permittivity, which is approximately 30. Below this threshold, both d33 and g33 decrease sharply. The recycling process in this thesis requires only minimal energy, hence offering a favorable balance between environmental impact and functional performance. The inferior electromechanical coupling strength remains as a constraint for the recycled materials, highlighting the need for binders with improved dielectric, piezoelectric, and mechanical properties. Overall, the findings demonstrate that waste piezoceramics can be successfully recycled and reused. Future research should focus on developing high permittivity binders with low processing temperatures and evaluating mechanical properties of the recycled materials.