Mechanical energy can be converted to electrical energy through the so-called direct piezoelectric effect. Correspondingly, a mechanical movement can be obtained by application of an electric field due to the converse piezoelectric effect. The direct and converse piezoelectric effect can be utilized in sensors and actuators, which become increasingly more important due to internet-of-things. The state-of-the-art piezoelectric materials and the closely related ferroelectric materials are mainly based on lead oxide in combination with zirconium and titanium oxide. The environmental concerns with lead oxide during materials processing and in electronic waste have resulted in a considerable effort the last decades to explore lead-free alternatives to the current lead-based materials. This project aims to develop a new class of lead-free piezoelectric materials, which also have the potential to be applied at relatively high temperatures relevant for process and automotive industry. The main principle idea of the project is to take advantage of a well-established mechanism for enhancing the performance of lead-based system and apply the same principle idea in a lead-free system. This methodology has been pursued by many researchers worldwide, but only with materials with the so-called perovskite crystal structure. In this project the idea will be explored in a system with a completely different crystal structure. Finally, the project also aims to investigate if it is possible to incorporate magnetic elements into the system. If successful this open the possibility to develop multiferroic materials, materials which are both ferroelectric and ferromagnetic at the same time.
Ferroelectric materials have unique electrical properties utilized in modern electronics such as sensors and actuators. State-of-the-art ferroelectric/piezoelectric components and devices are based on lead-containing PbZr1-xTixO3 (PZT). The search for lead-free alternatives to PZT has become a major topic in functional materials research due to legislation in many countries that restricts the use of lead alloys and compounds in commercial products. Although there are positive signs that lead-free ferroelectric materials start to penetrate the marked, there is still a great need for research in this field. So far the tremendous effort worldwide to develop lead-free alternatives to PZT has mainly focused perovskite oxides and surprisingly far less attention have been spent on tungsten bronzes and other types of polar materials. The unexplored potential to utilize tungsten bronze solid solutions as lead-free alternatives to PZT constitute the basis for this project. A morphtropic phase boundary (MPB) is proposed in tungsten bronze solid solutions due to the presence of the two fundamental different polarization mechanisms in tungsten bronzes, which give rise to both in-plane and out-of-plane polarization with respect to c-axis of crystal structure. The two compounds K4Bi2Nb10O30 and Ba2Na4Nb10O30 are proposed as two possible endmembers of the solid solution with the potential to demonstrate a MPB in lead-free tungsten bronze system. The two endmember have both a high Curie temperature, which open up for high temperature applications. A compositional approach, accompanied by first principles calculations, will be applied to tailor the ferroelectric/piezoelectric properties of the two endmembers. The ferroelectric/piezoelectric properties of tailor-made compositions in the vicinity of the MPB will be explored, particularly at elevated temperatures, in order to demonstrate the potential of the lead-free tungsten bronze materials for high temperature applications.