Compositionally-graded ferroelectric ceramics and multilayers for electronic and sensing applications

Abstract The concept of graded ferroelectrics became of significant interest for researchers working in the field of ferroelectric materials and related applications during the 1990s, when a number of published papers reported a series of intriguing phenomena observed in this type of structure. Among these are the shift of the hysteresis loop along the polarization axis, the presence of a giant effective pyroelectric coefficient, and the possibility of using the graded structures as ferroelectrics similar to semiconductor p–n junctions. Some pioneering experimental work was performed on simple structures composed of two layers with different values of polarization, called also ferroelectric bimorph. During the years, the interest for graded structures remained significant both from the point of view of the basic research as well as considering the potential applications in electronics, sensing, and energy. The concept of graded ferroelectric structures includes all structures in which a polarization gradient is present. This gradient can be obtained in several ways, such as (1) producing a concentration gradient along the direction normal to the electrodes; (2) producing a temperature gradient between the opposite electrodes of the capacitor; and (3) producing a strain gradient along the direction normal to the electrodes.