Ideal energy harvesting cycle using a phase transformation in ferroelectric crystals

Abstract

A near ideal mechanical-to-electrical energy harvesting cycle that takes advantage of a stress driven ferroelectric–ferroelectric phase transformation was demonstrated in oriented Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT). The cycle involves loading and unloading the material between two compressive stress loads under open-circuit conditions. The compressive stress loads exceed the coercive stresses required to drive the forward and reverse phase transformation; however, open-circuit conditions result in the surface charge on the electrodes producing an electric field that hinders the phase transformation. The crystal is then discharged through a shunt resistor at constant stress. The phase transformation takes place during the discharge and results in a charge output that is significantly greater than that of a linear piezoelectric material. An output electrical energy density of 6.22 kJ m−3 per cycle was demonstrated for a stress loading interval from −14 to −25 MPa and the peak efficiency was measured to be 36% for a stress loading interval of −16.5 to −22.5 MPa. Although electrical output increases with the stress loading interval, charge leakage at high electric fields occurred for large stress intervals. This placed a limit on the maximum energy density achievable.