Advancing thermoelectric research with new measurement systems and thermoelectric modeling for AC electrical measurements

Bulk thermoelectric materials are created and studied at Michigan State University (MSU) for use in high temperature power generation devices. These new materials exhibit figures of merit, ZT, which approach values of 2 near 800K, and are being used to develop power generators that convert heat flow to electricity. To help with the development of the thermoelectric materials, new measurement systems are needed to evaluate the components of the figure of merit (electrical conductivity, thermal conductivity, and thermopower).; In this research work, two measurement systems are developed for the simultaneous measurements of electrical conductivity, thermal conductivity, and thermopower. Both systems use a new "drifting in temperature" technique where measurements are made as the overall sample temperature slowly drifts in time. The first system called the "Drift System", is for low temperature measurements (100K to 300K) and the second system, "Ultra High Temp System", is for high temperature measurements (300K to 900K). The accuracy of these systems was verified using reference materials. The Ultra High Temp System has become a very useful tool in the investigation of new thermoelectric materials as doping concentrations and material synthesis parameters are changed.; In addition to this research, the AC impedance characteristics of thermoelectric modules and single pellets were investigated. This research led to the development of a new AC model for thermoelectric modules using AC electrical measurements by comparing the impedance behavior of thermoelectric modules to a simple RC circuit. The modeling of thermoelectric materials was then expanded upon by the application of transmission line theory to describe the thermal dynamic behavior in a module. This new model could be applied to commercially available modules as well as unicouples, and single thermoelectric pellets. Infrared imaging was used to verify the existence of a thermal wave developing along the module legs due to an AC electrical signal at the module's inputs. This thermal wave can be modeled with the transmission line theory, but not with the simple RC one-port model.; Finally, this research discusses a new ZT measurement system for rapid figure of merit measurements of new materials. This system is capable of measuring the figure of merit on more than 30 samples per week at room temperature. For the first time, large amounts of data can be collected for statistical analysis on doping concentrations and thermal processing to determine the optimal sample preparation conditions.