TECHNOLOGY AREAS: Materials/Processes

ACQUISITION PROGRAM: MARCOR SYSCOM - Marine Corps PM Expeditionary Power Systems

OBJECTIVE: Develop a high conversion efficiency 100W thermoelectric power generator module that exploits recent advances in bandgap engineered nanostructured thermoelectric materials. Develop thick film and/or bulk materials growth processes for and demonstrate intrinsic conversion efficiency of thermoelectric materials with a thermoelectric figure-of-merit, ZT, > 2, through designed control of electronic energy band structures (Seebeck enhancement) and nanostructure (thermal conductivity reduction). Demonstrate extrinsic systems conversion efficiency through reduction of parasitics (interface and systems engineering). This technology development is directed at direct primary power generation for DoD platforms, vastly enhanced fuel efficiencies through cogeneration of electricity via waste heat utilization and auxiliary power units for DoD transport systems (ground, sea, and air), power for remote or unattended stations, energy harvesting for self-powered sensor systems, and solid-state cooling and refrigeration.

DESCRIPTION: Significant improvements in thermoelectric performance of semiconductor systems have recently been realized in thin film and bulk materials through the incorporation of nanometer scale structures that significantly increase phonon scattering, leading to record low thermal conductivities. Such performance enhancements have been demonstrated in n-type PbSeTe-based quantum dot superlattice systems prepared by molecular beam epitaxy (1,2), p-type BiTe-SbTe and n-type BiTe-BiTeSe quantum well superlattices deposited by metal-organic chemical vapor deposition (3), and bulk n- and p-type LAST (Pb-Sb-Ag-Te) chalcogenides (4). Realization of further improvements in the thermoelectric figure-of-merit, ZT, will require increases in the power factor (Seebeck coefficient and/or electrical conductivity) while maintaining low thermal conductivity. Several approaches for accomplishing this have been proposed, including, among others, quantum confinement (5), heterostructure thermionic emission (6), and diffusive electron transport (7) and experimental validation is being actively pursued.

Realization in bulk or thick film form of the performance-enhancement concepts demonstrated in thin film form and further optimization of thermoelectric performance of these thicker materials will greatly expand the technological utility, cost-effectiveness, and intrinsic efficiency of the thermoelectric materials.

To maximize system-level conversion efficiency, modules must be designed and materials selected that minimize parasitic losses and maintain mechanical robustness at operating temperature and through repeated temperature cycling.

The development of an advanced high efficiency thermoelectric power generator will require major advances in the growth of thick film and/or bulk materials incorporating advanced concepts for high efficiency thermoelectric performance and integration of high ZT thermoelectric materials (by definition resulting from enhanced power factors and reduced thermal conductivities), with advanced module engineering to optimize electrical, thermal, and mechanical properties of the interfaces and the module. This program seeks to identify new approaches to accomplish these goals.

PHASE I: Develop detailed plan for constructing prototype high efficiency (20%) 50W thermoelectric generator incorporating advanced thermoelectric materials with average ZT > 2 between 300K � 700K that derive significant enhancements in thermoelectric performance through bandgap engineering and incorporation of nanoscale elements within a semiconductor composite. Develop and demonstrate feasibility of bulk crystal growth process for advanced thermoelectric materials with average ZT > 2 between 300K � 700K that derive significant enhancements in thermoelectric performance through bandgap engineering and incorporation of nanoscale elements within a semiconductor composite. Demonstrate initial module proof-of-concept of intrinsic thermoelectric materials conversion efficiency and extrinsic low parasitics at the 100 mW-level.

PHASE II: Optimize and scale up bulk and/or thick film crystal growth process for advanced thermoelectric materials with average ZT > 2 between 300K � 700K that derive significant enhancements in thermoelectric performance through bandgap engineering and incorporation of nanoscale elements within a semiconductor composite. Fabricate and test a 100W prototype thermoelectric device incorporating the bandgap and nanoscale-enhanced thermoelectric materials to demonstrate the overall system efficiency. Analyze manufacturability, reliability, scalability, and cost issues for producing commercially viable power generation system.

PHASE III: The integration of advanced thermoelectric materials with optimized materials and module engineering that minimize parasitic losses and provide mechanical robustness will enable the development of commercially viable thermoelectric systems for power generation, waste heat recovery, and cooling.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The integration of advanced thermoelectric materials with optimized materials and module engineering that minimize parasitic losses and provide mechanical robustness will enable the development of commercially viable thermoelectric systems for power generation, waste heat recovery, and cooling.

REFERENCES:
1. T. C. Harman, P.J. Taylor, D.L. Spears and M.P. Walsh, "Thermoelectric quantum-dot superlattices with high ZT", J. Electron Mater. Lett. 29, L1-4 (2000).

2. R. Venkatasubramanian, E. Siivola, T. Colpitts and B. O�Quinn, "Thin-film thermoelectric devices with high room-temperature figures of merit", Nature 413, 597-602 (2001).

3. W. Kim, J. Zide, A. Gossard, D. Klenov, S. Stemmer, A. Shakouri, A. Majumdar, "Thermal Conductivity Reduction and Thermoelectric Figure of Merit Increase by Embedding Nanoparticles in Crystalline Semiconductors," Phys. Rev. Lett. 96, 045901 (2006).

4. M.G. Kanatzidis, K.-F. Hsu, J. Do, T.P. Hogan, F. Guo, S. Loo, "Thermoelectric Properties of Cubic AgPbnSbTe2+n", Abstract S6.3, Fall 2003 Materials Research Society Meeting, Boston, MA (Dec. 2003).

5. L.D. Hicks and M.S. Dresselhaus, "Effect of quantum-well structures on the thermoelectric figure of merit," Phys. Rev. B47, 12727-31 (1993).

6. D. Vashaee and A. Shakouri, "Improved Thermoelectric Power Factor in Metal-Based Superlattices," Phys. Rev. Lett. 92, 106103 (2004).

7. T.E. Humphrey and H. Linke, "Reversible Thermoelectric Materials," Phys. Rev. Lett. 94, 096601 (2004).

KEYWORDS: Thermoelectrics, bandgap engineering, Seebeck, nanostructures, power generator

** TOPIC AUTHOR (TPOC) **

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