Fermi level Engineering Applied to Oxide Electroceramics

Our society is facing numerous challenges in reaching climate-neutrality, health-protection, and resource/energy efficiency. New technologies require high-performing, sustainable and reliable materials, to be used in energy conversion, energy storage, and electronics. Electroceramics exhibit a rich variety of functional properties that could tackle these issues and are thus being increasingly used in emerging technologies. However, the development of these materials requires a deep fundamental understanding of the underlying physical processes, which is often lacking. This knowledge is also needed to predict how the properties depend on the complex composition, structure, and processing. Within this project, which is a part of the large Collaborative research center FLAIR (Fermi Level Engineering Applied to Oxide Electroceramics), we will develop a new approach for the design of piezoceramics. Piezoceramics are materials that can convert electrical signals into mechanical force and vice versa, making them indispensable in sensors, actuators, and energy harvesters. In the future, these are envisaged to become important parts in autonomous vehicles, energy conversion, and medicine technology. The novelty of our approach lies in the extension of the established piezoceramic design principles, which consider materials chemical composition and structure, by a deeper understanding of the electronic structure. To this end, we will introduce a new design parameter, namely the Fermi energy, which describes the occupancy of electronic states within the material by electrons. Hereafter, this approach is called Fermi level engineering and is being widely used in the development of some other materials (for example, semiconductors); however, its role in piezoceramics is insufficiently understood and thus not exploited. Besides understanding and predicting materials functional properties, this parameter will also be used to forecast phase stability, to derive novel synthesis routes, and to control the evolution of materials microstructure. The novel approach will be investigated and developed on two emerging lead-free piezoceramic systems, namely (Ba,Ca)(Ti,Zr)O3 and Na1/2Bi1/2TiO3-BaTiO3. These systems will be modified by a series of selected doping elements, which are additives that deliberately change the Fermi level and thus influence the properties and processing. In particular, the induced changes of the materials global and local structure, resulting point defects, and electronic states will be investigated using advanced X-ray diffraction, electron microscopy, electrical/electro-mechanical measurements, photoelectron spectroscopy, and various modelling approaches (in collaboration with other projects of the collaborative research center). Besides increased understanding of functional electroceramics, one of the main outcomes will be a set of guidelines for Fermi level engineering of piezoceramics.