NONLINEAR PYROELECTRIC MATERIALS FOR ELECTRO-THERMAL ENERGY HARVESTING

Nearly two-thirds of the global energy derived from primary resources is dissipated as low-grade waste heat, which remains a largely untapped source of recoverable energy. As global energy demand continues to increase, it is imperative to adopt alternative and sustainable energy conversion technologies that focus on reducing energy losses. One such technology is pyroelectric energy harvesting, which converts the heat directly into electrical energy by utilizing the intrinsic pyroelectric effect in certain polar dielectric materials. Moreover, this technology serves niche application areas where thermoelectric energy harvesting becomes ineffective, especially in harnessing low-grade heat. Unlike thermoelectric materials, which require a steady-state temperature difference, pyroelectric devices operate under temperature fluctuations, making them especially suitable for applications involving temperature variations over time. Although the concept of pyroelectric energy harvesting has been known for several decades, there is limited understanding of how nonlinear behaviour of pyroelectric materials, as a function of temperature and electric field, influences energy conversion performance. This work investigates the nonlinear pyroelectric conversion potential of a prototypical pyroelectric material, lead scandium tantalate (PST) in different device geometries, from thin films to bulk samples. Despite its excellent electrocaloric and pyroelectric properties, PST thin films were not widely studied due to the challenges associated with high processing temperatures. In this work, the processing conditions were carefully optimized to yield high quality PST thin films. Indeed, thin films can withstand high electric fields which directly enhance the pyroelectric energy output and are also suitable for integration in microelectronic devices. High-quality PST thin films developed in this work achieved a pyroelectric energy density of up to 9 J∙cm−3 under optimized thermal and electrical conditions. Furthermore, to expand the operating temperature range, the transition temperature of PST thin films was shifted to higher temperatures by systematically doping with Ti4+ ions. While thin films offer numerous advantages, their limited active volume restricts their applicability in macroscopic energy harvesting systems. To overcome this challenge, the study expanded to include both PST bulk ceramics and PST multilayer capacitors (MLCs). These geometries benefit from high B-site cation ordering, resulting in a first-order phase transition and high pyroelectric coefficient. In addition to the detailed electrical characterizations, direct pyroelectric energy conversion cycles were implemented on PST MLCs using a dedicated experimental setup. The results indicate that PST MLCs can achieve a maximum of 50% Carnot efficiency for a 5 K temperature span near their phase transition temperature, compared to 22% achieved by PST bulk ceramics for a ΔT of 10 K at their transition. These results were obtained under the Olsen pyroelectric conversion cycle without any heat regeneration. Based on these results, PST MLCs were selected to demonstrate two proofs of concept to highlight the practical feasibility of non-linear pyroelectric energy harvesting devices. First, a standalone autonomous pyroelectric energy harvester was developed using only two PST MLCs. The device automatically initiates the Stirling pyroelectric conversion cycle based on the temperature profile of the material obtained from a thermocouple and the energy harvested by the materials is reused to initiate successive cycles without drawing energy from external power sources. Following the same device concept, a macroscopic self-powered pyroelectric energy harvester consisting of 60 PST MLCs was developed. To successfully implement this system, a closed-loop fluidic control system was introduced for the first time, enabling nonlinear pyroelectric energy harvesting in a macroscopic device without relying on heat regeneration or large volumes of heat transfer fluid. The energy extracted from the device was used not only to sustain the autonomous operation of the device but also to continuously power an external Bluetooth communication module for more than 30 minutes, thereby demonstrating a fully self-sustaining pyroelectric energy harvesting device. The results presented in this dissertation highlight the practical feasibility of nonlinear pyroelectric energy conversion, showing that high electrical output and efficiency can be achieved by carefully tuning both the material properties and the device architecture. Furthermore, the macroscopic self-powered pyroelectric energy harvester indicate that this technology can be used to develop autonomous devices or serve as a supplementary energy source to extend device lifetime by harnessing energy from ambient sources such as the ubiquitous waste heat. These outcomes not only open new avenues for real-world applications but also suggest that future efforts in nonlinear pyroelectric energy harvesting should shift from purely material-focused improvements toward system-level design and integration.