UC Berkeley

The growing interest in autonomous wireless sensor networks (WSNs) for industrial applications, whereby wireless sensors are distributed to target sites without considering to replace their power sources, spurs an obvious need for substituting conventional lifespan-limited batteries with sustainable energy-harvesting based power sources. Particularly, such power solutions become critical for wireless sensors deployed in harsh industrial environments, e.g., automotive engines, gas turbines, and geothermal/oil wells, where elevated temperatures (> 250°C) largely limit battery usage and wiring and maintenance for these sensors appears cost-ineffective.

This research describes the development of micro electromechanical system (MEMS) piezoelectric energy harvesters via aluminum nitride/silicon carbide (AlN/SiC) composite diaphragms, which enable harvesting energy at elevated temperatures from ambient vibrations/pressure to power harsh environment wireless sensors for industrial condition monitoring.

The dissertation first presents a design approach for optimizing top-level harvester attributes such as resonance frequencies and quality factors based on field vibration data obtained from machinery, followed by device-level parametric optimization via genetic algorithm for a cantilever energy harvester considering the defined harvester attributes and other physical constraints on size and mechanical limits.

A micromachined AlN/SiC diaphragm energy harvester (DEH) for harvesting periodic pressure fluctuation existed in industrial gas turbines is realized by a modular fabrication platform that adopts AlN/SiC thin film processing, which potentially enables further system integration. The fabricated AlN/SiC DEH based on a bossed-diaphragm with clamped edge demonstrates an output power density of 87 μW/cm2 at room temperature. Design considerations towards further enhancing power generation from the DEHs are discussed. In particular, first, with a focus on harvester dynamic responses to various pressure pulse profiles, the effects of pulse shapes and durations on output voltage are experimentally investigated, showing that an optimal resonance frequency of DEH for specific pressure pulse profile can lead to higher power generation. In addition, an improved diaphragm design that utilizes elastic supports rather than clamped edge is proposed to increase output power. The elastic support formed by introducing annular slits on the diaphragm edge, which are designed for decoupling process-induced residual stresses from diaphragm mechanical behaviors, achieving higher sensitivity and better electromechanical coupling, successfully yields a 2.5× improvement in harvester output power compared with a clamped-edge diaphragm design.

In addition to room temperature characterizations, by utilizing on-chip microheaters, the AlN/SiC DEHs are demonstrated to stably operate at elevated temperatures up to 320°C, while higher temperature measurements are beyond the microheater capacity. In fact, the DEH developed in this research represents the first piezoelectric energy harvester with experimentally verified operation at elevated temperatures > 300°C. Investigation on resonance characteristics of the DEHs are presented for a wider temperature range from 25°C to 600°C on a heated-chuck probe station--showing the temperature dependence of diaphragm mechanical properties and more importantly, verifying the potential functionality of DEH at such extreme temperature of 600°C.