Thermomechanical Energy Conversion Using Ferroelectric Materials

This study is concerned with the direct conversion of thermal and mechanical energy into electricity using ferroelectric materials. These materials possess a spontaneous polarization and can undergo solid-state phase transitions as a result of a change in applied electric field. All ferroelectric materials are pyroelectric, i.e., phase transitions can also be induced by a change in temperature. The Olsen cycle takes advantage of the thermally-induced phase transitions to directly convert thermal energy into electrical energy. It consists of two isoelectric field and two isothermal processes in the electric displacement versus electric field diagram. This study aims to improve the understanding and performance of the Olsen cycle by exploring the use of relaxor ferroelectric lead zirconate niobate-lead titanate (PZN-5.5PT) and lead magnesium niobate-lead titanate (PMN-28PT) single crystals. These materials were chosen for the fact that their phase transitions take place at low temperatures. The performance of the cycle was improved by varying the operating frequency to maximize changes in polarization due to phase transition taking place during the cycle.Moreover, all pyroelectric materials are also piezoelectric, i.e., phase transitions can occur as a result of a change in applied compressive stress. This study aims to explore new methods to simultaneously convert mechanical and thermal energy directly into electrical energy by combining electric field, compressive stress, and/or thermal cycling to induce phase transitions. Two new energy conversion cycles were conceived and demonstrated on PMN-28PT for its outstanding piezoelectric response over a broad temperature range. First, a thermomechanical cycle converting thermal and mechanical energy into electricity was performed. It proved to be more versatile than the Olsen cycle by generating electrical energy at all temperatures. It also achieved larger power densities thanks to larger operating frequency. In addition, it improved upon the material efficiency of the Olsen cycle by reducing the heat input during the cycle. Second, a thermally-biased mechanical cycle converting mechanical energy into electricity under a thermal bias was performed. It improved upon the power generation and material efficiency of the thermomechanical cycle by substituting the thermal cycling with a thermal bias to approach the desirable phases. In addition, it was capable of generating larger power per unit volume of material than existing direct mechanical to electrical energy conversion methods