WW-Colloquium: Prof. Dr. Julian Walker – Ionic Plastic Crystals and Their Ceramic Composites as Energy Conversion Systems

Prof. Dr. Julian Walker
Department of Materials Science and Engineering, NTNU, Norway
Ionic Plastic Crystals and Their Ceramic Composites as Energy Conversion Systems

Energy conversion using piezoelectrics, pyroelectrics, ferroelectrics and related phenomena are a key part of modern technology, allowing the conversion between mechanical, thermal and electrical energies. The field is dominated by metal oxide or metal nitride ceramics due to their robust functional properties, but interest in a more diverse combination of properties combined with the desire for lower cost, lower energy consumption and greater sustainability is driving exploration of novel material systems.
Ionic plastic crystals are crystalline solids consisting of charged molecular components that occupy the sites in the crystal lattice. The materials exhibit a high temperature phase where the molecules maintain fixed positions in a crystal lattice but gain orientational and rotational freedom, giving rise to soft mechanical properties and plastic deformation among other things. These materials can be non-centrosymmetric and thus may exhibit piezo, pyro and ferroelectric properties combined with the unique ability to be plastically deformed into various shapes and crystallized from aqueous solutions at low temperatures. Thie low temperature synthesis and plastic deformability provides some interesting avenues for device fabrication and composite formation.
In this lecture we will look at the fundamentals of ferroelectricity in these materials and the key properties of the orientationally disordered mesophases. We will discuss, using key examples, how the molecular composition influences the mechanistic contributions to polarization and ferroelectric switching, as well as evaluate various hypothesis as to how these materials may be engineered to enhance polarization and electromechanical responses to create high performance materials for future energy conversion devices.
As part of the lecture, we will recap the fundamentals of ferroelectric and piezoelectric properties, provide a concrete definition and understanding of ionic plastic crystals and describe the basics of some of the key experimental methods, proving useful for evaluating this novel class of materials. The audience will be left with many open questions and experimental hypothesis for this exciting class of materials, as there is much that is not understood about the fundamental composition-structure-property relationships in ionic plastic crystals.