ORNL/TM-2015/227 Mechanical Response of Thermoelectric Materials A. A. Wereszczak E. D. Case Approved for public release; distribution is unlimited. May 2015 DOCUMENT AVAILABILITY Reports produced after January 1, 1996, are generally available free via US Department of Energy (DOE) SciTech Connect. Website http://www.osti.gov/scitech/ Reports produced before January 1, 1996, may be purchased by members of the public from the following source: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone 703-605-6000 (1-800-553-6847) TDD 703-487-4639 Fax 703-605-6900 E-mail [email protected] Website http://www.ntis.gov/help/ordermethods.aspx Reports are available to DOE employees, DOE contractors, Energy Technology Data Exchange representatives, and International Nuclear Information System representatives from the following source: Office of Scientific and Technical Information PO Box 62 Oak Ridge, TN 37831 Telephone 865-576-8401 Fax 865-576-5728 E-mail [email protected] Website http://www.osti.gov/contact.html This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ORNL/TM-2015/227 Materials Science and Technology Division MECHANICAL RESPONSE OF THERMOELECTRIC MATERIALS Andrew. A. Wereszczak Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge, TN 37831 [email protected] Eldon. D. Case Chemical Engineering and Materials Science (retired) Michigan State University Lansing, MI 48824 Date Published: May 2015 Prepared by OAK RIDGE NATIONAL LABORATORY Oak Ridge, TN 37831-6283 managed by UT-BATTELLE, LLC for the US DEPARTMENT OF ENERGY under contract DE-AC05-00OR22725 CONTENTS LIST OF FIGURES .......................................................................................................................................v LIST OF TABLES .........................................................................................................................................v ACRONYMS .............................................................................................................................................. vii ACKNOWLEDGMENTS ........................................................................................................................... ix ABSTRACT ...................................................................................................................................................1 1. INTRODUCTION .................................................................................................................................1 2. MECHANICAL CONSIDERATIONS OF TEMATS ..........................................................................3 2.1 TENSILE STRESS LOCATION AND FLAW TYPES ..............................................................4 2.2 WEIBULL DISTRIBUTIONS AND EFFECTIVE SIZE ...........................................................6 3. RELEVANT MECHANICAL PROPERTIES, CHARACTERISTICS, AND TEST METHODS ............................................................................................................................................7 3.1 ELASTIC MODULUS AND POISSON'S RATIO .....................................................................7 3.2 TENSILE OR FLEXURE STRENGTH ......................................................................................8 3.2.1 Uniaxial flexure testing ...................................................................................................8 3.2.2 Biaxial flexure testing .....................................................................................................9 3.3 OTHER MECHANICAL CHARACTERISTICS .....................................................................10 3.3.1 Fracture toughness ........................................................................................................10 3.3.2 Compressive strength ....................................................................................................11 3.3.3 Hardness ........................................................................................................................11 3.4 THERMAL SHOCK AND GRADIENT ...................................................................................11 4. MICROSTRUCTURAL ISSUES ........................................................................................................14 4.1 RELATIONSHIP OF PROCESSING AND MICROSTRUCTURE ........................................14 4.2 POROSITY IN THERMOELECTRIC MATERIALS ..............................................................15 5. MATERIAL NONEQUILIBRIUM .....................................................................................................16 5.1 BLOATING ...............................................................................................................................17 5.2 MICROCRACKING ..................................................................................................................19 6. REFERENCES ....................................................................................................................................24 APPENDIX A. ELASTIC PROPERTIES OF VARIOUS THERMOELECTRIC MATERIALS ........................................................................................................... A-1 APPENDIX B. UNAXIAL FLEXURE STRENGTH OF VARIOUS THERMOELECTRIC MATERIALS .......................................................................B-1 APPENDIX C. BIAXIAL FLEXURE STRENGTH OF VARIOUS THERMOELECTRIC MATERIALS .......................................................................C-1 iii LIST OF FIGURES 1. Schematic of a thermoelectric leg unit cell of a thermoelectric device that is subjected to a temperature gradient ............................................................................................................................1 2. Directionality and the sign (i.e., tension vs. compression) of stress as a function of position within a thermoelectric leg subjected to an axial temperature gradient ...................................3 3. Examples of one-dimensional (edge), two-dimensional (surface), and three-dimensional (volume) flaws that could be operative in thermoelectric legs ..............................................................5 4. A flaw-type triangle may be used to illustrate the different potential strength-limiting flaw classifications for brittle materials .................................................................................................5 5. Examples of uniaxial and biaxial flexure test figures used with thermoelectric strength testing .....................................................................................................................................................8 6. Dilatometry can be an effective way to identify at what temperature a material may exhibit nonequilibrium and a change in mechanical response .............................................................17 7. For a Co0.95Pd0.05Te0.05Sb3 specimen hot pressed at 793 K at 74.4 MPa maximum pressure, (a) a fracture surface of the specimen as a hot-pressed specimen and (b) a fracture surface of the same specimen, annealed at 973 K for 4 h in an argon atmosphere. ..........................................................................................................................................18 8. For a pulsed electric current SnTe specimen sintered at 673 K with a maximum pressure of 60 MPa, (a) a fracture surface of the as-densified specimen and (b) a fracture surface of the same specimen following annealing in argon at 873 K ...........................19 LIST OF TABLES 1. Example of effective size calculations for three-point bend testing for a skutterudite ..........................7 v ACRONYMS CARES ceramic analysis and reliability evaluation of structures CTE coefficient of thermal expansion FEA finite-element analysis LAST lead-antimony-silver-tellurium LASTT lead-antimony-silver-tellurium-tin PECS pulsed electric current sintering RUS resonant ultrasound spectroscopy SEM scanning electron microscope TAGS tellurium-antimony-germanium-silver TE thermoelectric TEA thermal expansion anisotropy TEMat thermoelectric material vii ACKNOWLEDGMENTS The authors wish to thank the following for their contributions and support: J. Gibbs and J. Fairbanks [US Department of Energy (DOE)]; R. Johnson, D. Stinton, A. Haynes, H. Wang, M. Ferber, H. -T Lin, E. Fox, S. Waters, T. Kirkland, E. Lara-Curzio, R. Trejo, and W. Cai [Oak Ridge National Laboratory (ORNL)]; J. Sharp, R. McCarty, and A. Thompson (Marlow Industries); J. Salvador (General Motors); J. Yang (University of Washington); F. Ren (Temple University); J. Ni (Jet Propulsion Laboratory); R. Schmidt (Michigan State University); and O.