N151-068 TITLE: Ultra-High Temperature Thermoelectrics

TECHNOLOGY AREAS: Air Platform, Materials/Processes, Weapons

ACQUISITION PROGRAM: Navy Conventional Prompt Global Strike, DARPA Tactical Boost Glide Demo

OBJECTIVE: Develop thermoelectric technology which converts aerodynamic heating into electricity via thermoelectric generators. Thermoelectrics could provide power for Hypersonic/Long Range vehicles which require significant electrical power while surviving temperatures greater than 1250 degrees C.

DESCRIPTION: Hypersonic vehicles need compact, high temperature capable power sources. Batteries are not sufficiently compact and require insulation. A thermoelectric (TE) energy-harvesting system takes advantage of the temperature difference between two surfaces converting thermal energy into electricity. The objective of this effort would be to push the temperature limits of current TE materials from approximately 600 degrees C to 1250 degrees C while achieving an effectiveness figure of merit (ZT) above 1. Currently, efficiencies are limited by the interdependence of thermal and electrical properties. Due to the lack of space available for coolant, TE concepts will need to be integrated with low thermal diffusivity insulators or with high temperature phase change materials. As the output of TE generators is a function of the temperature difference between hot and cold sides, the output will be dependent on the generators� ability to separate the sides with insulating or phase change materials.

To date, thermoelectric generators have been designed for operation up to 600 degrees C. Many materials have an upper temperature limit of operation, above which the material is unstable. Theoretical and experimental studies have shown that low-dimensional TE materials, such as super-lattices and nano-wires, can enhance the Thermoelectric Effectiveness (ZT).

Currently, material science include bulk materials, low-dimensional materials, nano-crystalline materials, doping, molecular rattling, multiphase nano-composites, silicon-germanium alloys, high temperature capable clathrates, homologous oxide compounds, Skutterudite materials, and Half Heusler alloys. This list refers to current approaches and is not prescriptive for proposed approach.

PHASE I: Develop thermoelectric generator concepts using high melt temperature materials. Develop thermoelectric material-to-insulation or phase change material integrated configurations. Perform measurements of candidate material electrical conductivity, thermal conductivity, and Seebeck coefficient as a function of temperature up to the expected maximum use temperature. Perform imaging of candidate material grain and lattice structure at temperatures spanning the range of interest. Develop predictions of expected TE figure of merit and thermoelectric efficiency. If awarded a Phase I Option, perform imaging of candidate material grain and lattice structure at temperatures spanning the range of interest. Develop morphology and structure of the TE devices from the imaging data.

PHASE II: Using the data developed in the Phase I option, identify material modifications to improve generator performance. A prototype thermoelectric generator will be fabricated and integrated with insulation. Laboratory tests will be conducted to measure the electrical output of the integrated thermoelectric generator-insulator circuit at temperatures spanning the range of interest. Sizing of 100 and 250 W devices based on the results of prototype test will be projected. Designs capable of meeting expected missile form factors and combined mechanical and thermal environments will be developed and demonstrated in relevant mechanical and thermal environments. Key cost, size and performance attributes will be developed for commercial application. Designs for commercial application will be developed and demonstrated.

PHASE III: Develop revisions to the single unit fabrication methods to meet quality requirements. Identify revisions to the prototype to meet quality requirements, leading to fabrication of additional 20 final prototypes which will be subjected to quality inspection, electrical performance testing over the temperature span, and combined thermal/mechanical loads testing. Identify large scale production alternatives. Develop a cost model of expected large scale production to provide estimates of non-recurring and recurring unit production costs. Production concept for commercial application will be developed addressing commercial cost and quality targets.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Commercial and dual applications of this technology include electrical power supplies for satellites, fuel cells and combustion driven engines such as for aircraft and ground transportation. By harvesting combustion engine waste heat, the overall efficiency of these engines is improved. A further use is to provide back up to solar photovoltaic cells.

REFERENCES:
1. Yang, Jihui and Caillat, Thierry, MRS Bulletin, Volume 31 March 2006. "Thermoelectric Materials for Space and Automotive Power Generation."

2. Snyder, G. Jeffrey, The Electrochemical Society Interface, Fall 2008. "Small Thermoelectric Generators."

3. Tritt, Terry M. and Subramanian, M. A., MRS Bulletin, Volume 31 March 2006. "Thermoelectric Materials, Phenomena, and Applications: A Bird's Eye View."

4. Nolas, George S., Poon, Joe, and Kanatzidis, Mercouri, MRS Bulletin, Volume 31 March 2006. "Recent Developments in Bulk Thermoelectric Materials."

5. Koumoto, Kunihito, Terasaki, Ichiro and Funahashi, Ryoji, MRS Bulletin, Volume 31 March 2006. "Complex Oxide Materials for Potential Thermoelectric Applications."

6. Rao, Apparao, M., Ji, Xiahua and Tritt, Terry, M., MRS Bulletin, Volume 31 March 2006. "Properties of Nanostructured One-Dimensional and Composite Thermoelectric Materials."

7. Bottner, Harald, Chen, Gang and Venkatasubramanian, Rama, MRS Bulletin, Volume 31 March 2006. "Aspects of Thin-Film Superlattice Thermoelectric Materials, Devices, and Applications."

KEYWORDS: thermoelectric efficiency; Seebeck effect; nanostructured materials; molecular rattling; multi-phase nano-composites; complex crystals; thin film superlattices; clathrates

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