Thermoelectric modules are of great interest for power generation applications where temperature gradients of approximately 500K exist, and hot side temperatures near 800K. The fabrication of such modules requires optimization of the material compositions, low contact resistivities, and low thermal loss.

AgPbmSbTe2+m (LAST) and Ag(Pb1-xSnx)m SbTe2+m (LASTT) compounds are among the best known materials appropriate for this temperature range. Various measurement systems have been developed and used to characterize bulk samples in the LAST and LASTT systems within this operating temperature range. From the characterized data, modeling of modules based on these materials and segmented legs using LAST(T) with Bi2Te3 have been used to identify the optimal geometry for the individual legs, and the length of the Bi2Te3 segments. We have segmented LAST(T) with Bi2Te3 and achieved contact resistivities of less than 10 μΩ•cm2.

Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on LAST, LASTT, and segmented with Bi2Te3 materials. We also present the output data on these generators.

Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators. These contacts must be stable to high temperatures and through thermal cycling. Here we present the fabrication procedure and characterization of several contacts to Pb-Sb-Ag-Te (LAST) compounds. Contact materials investigated include tungsten, antimony, tin, nickel, and bismuth antimony based solder. The contacts were typically deposited by an electron beam evaporation method after careful preparation of the sample surface. The resistances were measured by using the transmission line model, and ohmic behavior was verified through current vs. voltage measurements. The best contact resistivities of less than 20 µΩ·cm2 have been measured for annealed antimony to n-type LAST samples. We present these procedures for fabricating low resistance contacts and the use of these procedures towards the fabrication of high efficiency thermoelectric generator modules.

Measurements of assembled thermoelectric modules commonly include investigations of the module output power versus load resistance. Such measurements include non-ideal effects such as electrical and thermal contact resistances. Using an AC electrical measurement, two models for a thermoelectric module have been developed utilizing electrical circuits for both the thermal and electrical characteristics of the module.

Measurements were taken over the frequency range of 1mHz to 500Hz using lock-in amplifiers. We present data showing the extraction of ZT from such measurements on commercially available modules utilizing both the magnitude and phase of the measured impedance. Here we extend upon a simple RC equivalent circuit model by utilizing transmission line theory in electrical circuits to explain the thermal activity in a thermoelectric module. This model includes all components of a module such as nickel traces and ceramic end caps, and makes use of their corresponding thermal conductivities, thermal capacitance, and density. This model can then be applied to pn unicouples in either a standard or inline configuration, and to individual p or n legs of the module. Data is presented showing the advantages of both models. Measurements on new thermoelectric materials and modules are also presented.

Lead-Antimony-Silver-Tellurium (L-A-S-T) materials, synthesized at Michigan State University, show promising thermoelectric properties at high temperatures for use in power generation applications. Recent scaled-up quantities of L-A-S-T show a ZT=1.4 at 700 K approaching the figure of merit for samples made in small quantities. These materials are of great interest for power generation applications with hot side temperatures in the range of 600-800 K. Developing these materials into working devices requires minimization of the thermal and electrical parasitic contact resistances, so various fabrication methods are under investigation. To examine each method, a new measurement system has been developed to characterize these devices under various load and temperature gradients. An introduction to the system will be presented, as well as results for devices made of the L-A-S-T materials.

High efficiency thermoelectric modules are of great interest for power generation applications where hot side temperatures of approximately 800K exist. The fabrication of such modules requires a multidisciplinary effort for the optimization of the material compositions, the engineering of the module systems, modeling and fabrication of the devices, and constant feedback from characterization. Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds are among the best known materials for this temperature range. Modeling of these materials and possible cascaded structures shows efficiencies of 14% can be achieved for low resistance contacts. Using antimony we have achieved contact resistivities less than 20 µΩ·cm2. Here we give a detailed presentation on the procedures used in the fabrication of thermoelectric generators based on these new materials. We also present the characterization systems and measurements on these generators.