Microfabricated thermoelectric materials and devices for waste heat harvesting [electronic resource]
Keywords: Mechanical Engineering.

Thermoelectric generators offer a unique opportunity to convert thermal gradients directly to electrical power in a reliable, solid-state device containing no moving parts. Devices are scalable to the kilowatt range and higher, and there has been great success implementing generators in niche applications like power supplies for deep space missions using bulk-machined materials. More recently, advances in fabrication technology have enabled the design of materials and devices with micro- and nano-scale features. These advances have opened new working domains for small form-factor generators to harvest near-ambient low-grade "waste" heat and convert it to electrical power in the μW-mW domain for use in low power consumption devices such as wireless sensors and wearable electronics. In particular, thin-film fabrication techniques like physical vapor deposition permit patterning of devices with very high thermocouple density resulting in voltages and power densities on the order of 1 V and 1 mW/cm2, respectively, driven by a temperature differential of only a few degrees. We begin with a study of fundamental thermal transport in a new class of nanostructured polysilicon thermoelectric materials that exhibit enhanced thermoelectric power factor. Through a combination of measurement techniques, we are able to demonstrate a positive correlation between material thermal conductivity and the size of nanoscale voids present within the material. These trends are validated conceptually using Matthiessen's rule scaling arguments, and numerically using Monte Carlo ray tracing to quantify the effects of geometric scattering on cross-plane thermal transport due to void and grain boundary contributions. The results of this study suggest the potential for process-dependent control of thermal conductivity in silicon with high power factor. Second, we shift perspective from nanoscale materials to microscale devices and discuss development and navigation of the design optimization space for microfabricated thermoelectric generators. Through conscientious design of factors such as the fill fraction and number of thermocouples, among others, a roadmap is developed for optimization of devices developed for near-ambient thermal energy harvesting. The applicability of the common thermoelectric "figure of merit" ZT is also called into question when evaluating devices fabricated at the micrometer scale. Third, we outline the fabrication process implemented to fabricate prototype devices using a thin film deposition approach and characterize their performance as electric power generators with an infrared microscopy technique. We demonstrate working generators delivering more than 1 mW actual load power from less than 10 °C temperature difference. Finally, we step back and review solutions for system-level thermomechanical packaging and integration with power conditioning circuitry for the robust, efficient implementation of small form factor thermoelectric generators as power supplies for low power electronics. Acknowledging the rapidly-growing interest in distributed-scale energy harvesters for autonomous sensor networks, wearable electronics, medical devices, and other low power electronics, the goal of this work is to demonstrate the connected nature of physics from the nanometer scale to macroscale systems and routes to optimization in each regime.