Microscale Thermoelectric Property Characterization and Performance-Driven Thermoelectric System Design

Thermoelectric (TE) materials have promising energy-related applications, including waste heat recovery to improve energy efficiency in automobiles and industrial processes, energy harvesting to power remote sensors and devices, and power generation in radioactive environments like nuclear plants and space. Despite tremendous progress in TE material efficiency over the past few decades, the lack of effective TE property characterization methods and TE system design models remain two outstanding challenges in TE research. This thesis focuses on two important topics to address these challenges: (1) microscale characterization of TE material properties and (2) design and simulation of TE generators for waste heat recovery. Scanning thermal microscopy (SThM) is a powerful tool for materials characterization. High-throughput SThM can greatly augment the value of combinatorial film studies by quickly mapping TE properties and identifying the optimal composition of a TE material system. The power of SThM is further amplified when multiple measurements, such as thermal conductivity and Seebeck coefficient, are obtained at the same time and same location with a single probe. While several probes for separate thermal and electrical measurements are available, unique challenges arise for combined microscale thermal and electrical measurements. Most TE materials develop nonconductive native oxide layers. Commercially available thermal AFM and SThM probes cannot establish electrical contact through these oxide layers, making combined electrical and thermal measurements impossible with conventional techniques. In this work, a novel thermal microprobe capable of simultaneous thermal conductivity and Seebeck coefficient measurements with minimal sample preparation is developed. New methods to measure the effective thermal contact radius and calculate the effective heat transfer coefficient in contact mode are introduced. The new probe can measure a wider range of thermal conductivity than its commercial counterpart with superior sensitivity. The probe is demonstrated with co-registered Seebeck coefficient and thermal conductivity on combinatorial films and materials with microscale radiation damage and native oxide layers. The second part of the thesis addresses the thermal design and simulation of thermoelectric generators (TEGs). A high-temperature TEG that converts engine exhaust waste heat into electricity is modeled based on a light-duty passenger vehicle with a 4-cylinder gasoline engine. The model is validated by comparing simulation results to the experimental results of a TEG prototype tested on a diesel engine, and shows agreement within 3% for temperature distribution and 6% for pressure drop, respectively. Strategies to optimize TEG configuration and heat exchanger design for maximum fuel efficiency improvement are provided. Accounting for major parasitic losses, a maximum fuel efficiency increase of 2.5% is achievable using state-of-the-art nanostructured bulk half-Heusler thermoelectric modules