Influence of the permittivity between fillers and binders on the properties of upside-down composites for recycling purposes.

Upside-down composites have recently advanced towards recycling piezoceramics. However, the recycled piezoceramics retain only about 10-30% of the pristine piezoelectric properties. To date, there has been no systematic study on the origin of this limitation and on potential routes for improving these recycled materials. This work addresses this issue by combining empirical and modelling evidence. The phenomenon of the influence of disparate permittivity between the fillers and binders is explained by fitting experimental parameters from sets of lead-based and lead-free upside-down composite samples into the Lichtenecker and Yamada models. Results suggest that for high filler contents, the biasing field permeation caused by the binders that leads to lower piezoelectric properties can be experimentally confirmed and correctly modelled. For lower filler contents, the models significantly deviate from the experimental data due to the distinctive shaping method. This issue indicates the necessity of developing a new theoretical methodology for upside-down composites.