The pyroelectric energy harvesting capabilities of PMN--PT near the morphotropic phase boundary

This paper reports on direct thermal to electrical energy conversion by performing the Olsen cycle on pyroelectric materials. The energy harvesting capability of commercially available [001] oriented 68PbMg1/3Nb2/3O3–32PbTiO3 (PMN–32PT) single crystal capacitors was measured experimentally. An energy density of 100 mJ cm−3/cycle, corresponding to 4.92 mW cm−3, was obtained by successively dipping the material in oil baths at temperatures 80 and 170 ◦C and cycling the electric field between 2 and 9 kV cm−1. Similarly, an energy density of 55 mJ cm−3/cycle was obtained between 80 and 140 ◦C. An estimated 40% of this energy resulted from the strain polarization due to the rhombohedral to tetragonal phase transition. The strain from this transition disappeared when the maximum operating temperature exceeded the Curie temperature of about 150 ◦C. The optimal low electric field used in the Olsen cycle maximizing the energy harvested was found to be around 2 kV cm−1. In addition, the material suffered from (i) dielectric breakdown for electric fields larger than 9 kV cm−1 and (ii) cracking from thermal stress for operating temperature differences in excess of 90 ◦C. A physical model predicting the total amount of energy harvested was also derived, accounting for thermal expansion as well as temperature dependent dielectric constant and spontaneous polarization. The model predictions fell within 20% of the experimental results in the temperature range between 80 and 170 ◦C and electric fields ranging from 2 to 9 kV cm−1.