Jonathan D'Angelo

Temperature dependent thermoelectric material power factor measurement system

Thermoelectric materials can be used for cooling/heating applications, or converting waste heat into electricity. Novel thermoelectric materials have been discovered in recent years. Characterization of an electrical conductivity and thermopower of a sample from room temperature to > or = 900 K is often necessary for thermoelectric materials. This paper describes a system built for measurement of the power factor of thermoelectric materials from 300 to 1273 K. Characterization results of the system are also presented.

Prospects for implementation of thermoelectric generators as waste heat recovery systems in class 8 truck applications

With the rising cost of fuel and increasing demand for clean energy, solid-state thermoelectric (TE) devices are an attractive option for reducing fuel consumption and CO2 emissions. Although they are reliable energy converters, there are several barriers that have limited their implementation into wide market acceptance for automotive applications. These barriers include: the unsuitability of conventional thermoelectric materials for the automotive waste heat recovery temperature range; the rarity and toxicity of some otherwise suitable materials; and the limited ability to mass-manufacture thermoelectric devices from certain materials. One class of material that has demonstrated significant promise in the waste heat recovery temperature range is skutterudites. These materials have little toxicity, are relatively abundant, and have been investigated by NASA-JPL for the past twenty years as possible thermoelectric materials for space applications. In a recent collaboration between Michigan State University (MSU) and NASA-JPL, the first skutterudite-based 100 W thermoelectric generator (TEG) was constructed. In this paper, we will describe the efforts that have been directed towards: (a) enhancing the technology-readiness level of skutterudites to facilitate mass manufacturing similar to that of Bi2Te3, (b) optimizing skutterudites to improve thermal-to-electric conversion efficiencies for class 8 truck applications, and (c) describing how temperature cycling, oxidation, sublimation, and other barriers to wide market acceptance must be managed. To obtain the maximum performance from these devices, effective heat transfer systems need to be developed for integration of thermoelectric modules into practical generators. [DOI: 10.1115/1.4023097]

Long term thermoelectric module testing system

Thermoelectric generators can be used for converting waste heat into electric power. Significant interest in developing new materials in recent years has led to the discovery of several promising thermoelectrics, however, there can be considerable challenges in developing the materials into working devices. Testing and feedback is needed at each step to gain valuable information for identification of difficulties, quality of the materials and modules, repeatability in fabrication, and longevity of the devices. This paper describes a long-term module testing system for monitoring the output power of a module over extended testing times. To evaluate the system, we have tested commercially available thermoelectric modules over a one month time period.

Transport behavior and thermal conductivity reduction in the composite system PbTe-Pb-Sb

We report the synthesis of nanostructured composite PbTe with excess Pb and Sb metal inclusions. Scanning and transmission electron microscopy reveal these inclusions in both the nano- and macroscales. The electrical conductivity and Seebeck coefficient dependence on temperature show unusual trends which depend on the inclusion Pb/Sb ratio. Several ratios showed marked enhancements in power factor at 700 K. The thermal conductivity of these composites is reported.

Electrical contact fabrication and measurements of metals and alloys to thermoelectric materials

Low electrical contact resistance is essential for the fabrication of high efficiency thermoelectric generators in order to convert heat to electricity. These contacts must be stable to high temperatures and through thermal cycling. A ratio of the contact resistance to the leg resistance below 0.1 is the goal for fabrication of a high efficiency thermoelectric power generation device. Here we present the fabrication procedures and characterization of contacts of metal alloys to Pb-Sb-Ag-Te (LAST) and Pb-Sb-Ag-Sn-Te (LASTT) compounds. Contacts were fabricated and measured for both ingot and hot pressed materials. Stainless steel 316 has shown a low resistance contact to these thermoelectric materials when the proper bonding conditions are used. Different time-temperature-pressure conditions for bonding to n-type and to p-type legs are presented. Contact resistances below 10μΩcm 2 have been measured. In addition, break tests have shown bond strengths exceeding the semiconductor fracture strength. One of the considerations used in selecting iron alloys for electrical interconnects is the similarity in the coefficient of thermal expansion to the LAST and LASTT materials which is 18 ppm/°C and relatively temperature insensitive. Contacts to the thermoelectric materials were accomplished by diffusion bonding in a furnace developed in our lab at Michigan State University. The furnace is capable of reaching temperatures of up to 1000°C with a controlled atmosphere of a reducing gas. Fabrication procedures and contact data are presented in this paper.

Nanostructuring and its Influence on the Thermoelectric Properties of the AgSbTe 2 -SnTe Quaternary System

The structural and thermoelectric properties of the AgSbTe2-SnTe quaternary system were studied. Powder averaged x-ray diffraction of Ag0.85SnSb1.15Te3 indicates a cubic NaCltype structure in contrast with the single crystal refinements, which point towards tetragonal symmetry. Furthermore, high-resolution electron microscopy imaging revealed the system to be a nano-composite formed by thermodynamically driven compositional fluctuations rather than a solid solution as it was viewed in the past. The lattice thermal conductivity attains very low values, which is in accord with recent theories on thermal transport in heterogeneous systems. The charge transport properties of the system exhibit a rich physical behavior highlighted in the coexistence of an almost metallic carrier concentration (~5×10 21 cm -3 ) with a large thermoelectric power response of ~160 μV/K at 650 K. This is attributed to a heavy hole effective mass that is almost six times that of the electron rest mass.

Progress on the Fabrication and Characterization of High Efficiency Thermoelectric Generators

Timothy P. Hogan, Adam D. Downey, Jarrod Short, Jonathan D’Angelo, Eric Quarez, John Androulakis, Pierre F. P. Poudeu, Mercouri G. Kanatzidis, Ed Timm, Kim Sarbo, Harold Schock Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48824, U.S.A. Chemistry Department, Michigan State University, East Lansing, MI 48824, U.S.A. Department of Mechanical Engineering, Michigan State University, East Lansing, MI 48824, U.S.A.

Effects of Antimony on the Thermoelectric Properties of the Cubic Pb 9.6 Sb y Te 10−x Se x Materials

The thermoelectric properties of Pb9.6SbyTe10-xSex were investigated in the intermediate temperature range of 300 – 700 K. The effect of the variation of Sb content (y) on the electronic properties of the materials is remarkable. Samples with compositions Pb9.6Sb0.2Te10xSex (y = 0.2) show the best combination of low thermal conductivity with moderate electrical conductivity and thermopower. For Pb9.6Sb0.2Te8Se2 (x = 2) a maximum figure of merit of ZT~ 1.1 was obtained around 700 K. This value is nearly 1.4 times higher than that of PbTe at 700 K. This enhancement of the figure of merit of Pb9.6Sb0.2Te8Se2 derives from its extremely low thermal conductivity (~0.7 at W/m.K at 700 K). High resolution transmission electron microscopy of Pb9.6Sb0.2Te10-xSex samples shows broadly distributed Sb-rich nanocrystals, which may be the key feature responsible for the suppression of the thermal conductivity.

Investigation of Low Resistance Contacts to Pb-Sb-Ag-Te (LAST) Materials for Module Fabrication

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 a 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 (TLM), and ohmic behavior was verified through current vs. voltage measurements. The best contact resistivities of less than 20 µΩ·cm 2 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 contact materials toward the fabrication of high efficiency thermoelectric generator modules.

Characterization of Thermoelectric Power Generation Modules Made from New Materials

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 [1]. These materials are of great interest for power generation applications with hot side temperatures in the range of 600800 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.