Microstructural engineering of piezoelectric composites

This project will create novel fabrication approaches, using the freeze-casting method combined with slip- and tape-casting, to produce piezoelectric composites with microstructures tailored to yield piezoelectric properties that exceed the performance of off-the-shelf materials, whilst providing advantages over traditional manufacturing methods. The global market for piezoelectric ceramics was valued at $19.6 billion in 2019 and is expected to grow in the areas of energy harvesting, IoT-related sensors and piezoelectric composites in the next decade. Piezoelectric composites are critical to the UK's defence (SONAR), and public health (medical ultrasound) sectors, as well as being used widely in the transport and energy industries. Developing new methods for producing high performance piezoelectric composites represents a significant benefit in terms of materials cost and manufacture, as well as device performance, by enabling low-cost fabrication of bespoke piezoelectric materials with properties tuned depending on the desired application.

Freeze casting is an effective method for controlling the microstructures of porous materials, whereby pores are templated on solvent crystals whose growth and morphology depends on temperature gradients and freezing behaviour during processing. These porous microstructures, e.g. porous piezoelectric ceramics, can then be infiltrated with polymer second phases to improve mechanical and electrical properties. The properties of piezoelectric composites depend strongly on local interactions between electric- and mechanical fields and the material structure over a range of length scales, from ferroelectric domains (sub-micron) through to macro-structure (on the order of millimetres) of the composites. In this project, the aim is to increase the understanding of these electromechanical field/material interactions in piezoelectric composites and design microstructures to exploit beneficial effects accordingly. This will be underpinned by developing advanced numerical models to both aid with microstructural/fabrication process design, and provide insight into experimental observations of the properties of materials fabricated during the project.

The methods that will be investigated offer several advantages over current techniques used to produce commerically available piezoelectric composites. Firstly, the materials can be produced at near-net shape, reducing post-machining processes or manual fibre lay up common for macro-fibre composites fabricated by dice-/arrange-and-fill processes. Secondly, the level of control that is theoretically possible, although not yet realised, by utilising freezing processes to template microstructures, provides the potential to fabricate materials with bespoke properties tuned to specific applications, yielding an optimised combination of piezoelectric, dielectric and mechanical properties to promote enhanced electromechanical coupling between the active piezoelectric and the wider device. Thirdly, the reduced length scale of microstructural features introduced using freeze casting, compared to dice-and-fill composites for example, may provide a route to engineering the inherent properties of the piezoelectric ceramic matrix. Using water as a freezing agent means these processes have a low environmental impact, and near-net shape, optimised composite microstructures with comparable performance to dense piezoceramics will reduce the volume of raw material required in the first place.