Graded ferroelectrics: a new class of steady-state thermal/electrical/mechanical energy interchange devices

The diffuse-phase-transition class of ferroelectric materials allows the fabrication of films graded in composition: the consequence of which is an attendant gradient in electric dipole moment density. Alternating electric-field excitation of these graded structures reveal an asymmetrical hysteresis characteristic, together with a static displacement of the hysteresis trace. This displacement bias is observed to be a function of excitation level and temperature, which leads to a giant effective pyroelectric coefficient. Said displacement, for example, provides a continuous open-circuit voltage, or short-circuit current, provided that the structure is held in an isothermal condition. All other combinations of heat, electrical and strain energies suggest a whole new class of devices, and invite ingenious concepts and/or contributions. Graded ferroelectrics, which operate on principles of asymmetrical bound-charge, are broadly the dielectric analogue of conductive asymmetrical free-charge devices. For example, metallic abrupt junctions (i.e., thermocouples), and semiconductor pn junctions, both operate on free-charge, to provide nonlinear or unidirectional behavior. The graded ferroelectrics, as for the thermocouple, can deliver a steady-state electrical energy in response to thermal or strain energy, whereas semiconductors generally lose any static response because of neutralization of the Fermi voltage by free-charge. Discovery of the "effect", first in potassium tantalate niobate (KTN), and subsequent studies of the barium strontium titanate (BST) series is presented. The method of achieving graded ferroelectrics is described, and the electrical/thermal characterization thereof is included. Finally, some theoretical explanations of the "effect" are offered.