UC Berkeley

Increasing global energy demands continually heighten the need for novel, alternative energy conversion technologies, especially those that recover waste heat generated by traditional energy conversion processes. The so-called pyroelectric effect is one intrinsic material phenomenon that can be leveraged for waste heat energy scavenging. The pyroelectric effect is characterized by the pyroelectric coefficient (p), which describes the magnitude of the change in a material’s spontaneous polarization in response to a change in its temperature. The future of pyroelectric energy harvesting is dependent upon reliable experimental measurement technique, and an understanding of how to design harvesting systems to maximize their performance. In this thesis, we both describe measurement techniques for quantifying thin film pyroelectric materials and provide guidance for maximizing the power output of pyroelectric harvesting systems.

We adapt an electrothermal technique commonly used for measuring thermal conductivity, the 3ω method, to measure p of mechanically clamped PZT thin films. Using the 3ω method combined with a 1D heat transport model, we simultaneously collect the thermally-generated pyroelectric current and measure the average temperature of the metallic heater line atop the thin film, allowing us to calculate p. Frequency sweeps across several decades allow us to identify the secondary (piezoelectric) contributions to the pyroelectric coefficient, which arise due to mismatched thermal expansion coefficients between the thin film and the bulk substrate. We develop and extend these techniques using the same physical system to directly measure the electrocaloric effect in thin films as a supplement to these pyroelectric measurements.

Next, we experimentally quantify the performance of pyroelectric thin films as energy converters. Using the same experimental architecture as with the PZT measurements, we synchronize the temperature of PMN-0.32PT thin films with an externally-applied electric field to complete Ericsson cycles in the thermodynamic electric field-electric displacement space. The large pyroelectric coefficient of the PMN-0.32PT and fast cycle frequencies result in large energy and power densities. This combination of large energy and power density is a first in pyroelectric energy conversion, demonstrating the benefits of the thin film architecture and showing promise for pyroelectrics as a viable waste heat scavenging technology.

We lastly analyze the heat transport in idealized, 1D pyroelectric energy harvesters to identify the physical constants (e.g. thermal conductivity and volumetric heat capacity) and system properties (e.g. length and heating frequency) that determine power harvesting performance. Modeling the pyroelectric as a current source and capacitor in parallel, we relate the spatially-averaged temperature amplitude of the pyroelectric material to the power delivered to a purely resistive external load. We consider both a temperature and a heat flux source as the driving thermal energy source for the pyroelectric harvester, and through dimensional analysis we isolate the effects of different system parameters on the power delivered to the external load. Finally, we identify figures of merit (FOM’s) for various operating regimes and provide general guidance for optimizing the power output of pyroelectric energy harvesters given freedom over different system variables.