Recycling piezoelectric ceramics using upside-down composite fabrication method. A collaborative experimental and theoretical study

Piezoelectric ceramics play an essential role in modern technology. They convert mechanical motion into electrical signals and are found in devices such as vehicle sensors, wearable sensors, industrial monitors and small energy harvesters. Despite their usefulness, these materials are traditionally produced at very high temperatures (> 1000 °C) and often contain lead, making their manufacturing energy‑intensive and their disposal environmentally problematic.

To reduce these impacts, a low‑temperature recycling method known as the upside‑down composite technique has recently been developed. This method allows old piezoelectric ceramics to be crushed and formed into composites at far lower temperatures (< 150 °C) using a small amount of binder. Although the technique is found to be feasible for recycling in practice, the scientific reasons behind the behavior of the recycled materials have not been fully understood, particularly why some electrical properties weaken while others remain unexpectedly strong after recycling.

This thesis fills such a knowledge gap. Through detailed experiments, it examines how electric fields distribute within recycled composites and how the binder and ceramic particles interact. A key observation is the composite effect, where differences in the electrical properties of the components cause the electric field to concentrate in the binder rather than in the ceramic particles. This explains the reduction in charge‑generation capability while clarifying why the voltage response stays high enough for many sensing applications.

To turn these insights into practical tools, the thesis firstly validates experiments using analytical models and extends them into an AI‑compatible modelling framework. These models quantify how microstructure, particle shape, and poling conditions influence the final performance of recycled materials. The methodology is expanded to include other potential recycling techniques and makes it possible to predict how a given recycled mixture will behave, aiding the optimization of future materials without relying solely on experimental trial‑and‑error.

By uncovering the mechanisms behind recycled piezoelectric composites and providing a modelling framework to guide their design, the thesis strengthens the scientific foundation of a new recycling method. It enables more scalable and application‑oriented reuse of piezoelectric ceramics, supporting the broader goal of sustainable electronics manufacturing.