Thermoelectric Exploration of Silver Antimony Telluride and Removal of the Second Phase Silver Telluride A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Masters of Science in the Graduate School of The Ohio State University By Michele D. Nielsen, B.S. Graduate Program in Mechanical Engineering The Ohio State University 2010 Thesis Committee: Joseph Heremans, Advisor Walter Lempert © Copyright by Michele D. Nielsen 2010 Abstract As demands for energy increase throughout the world, the desire to create energy efficient technologies has emerged. While the field thermoelectricity has been around for well over a century, it is becoming increasingly popular, especially for automotive applications, as new and more efficient materials are discovered. Thermoelectricity is a technology in which a temperature difference can be applied to create a potential difference for the application of waste heat recovery or a potential difference can be used to create a temperature difference for heating and cooling applications. Materials used in thermoelectric devices are semiconductors with high Figure of Merit, zT. The dimensionless thermoelectric Figure of Merit is a function of Seebeck coefficient, S, electrical resistivity, ρ, and thermal conductivity, κ. Experimental testing is used to determine the properties of these materials for optimized zT. This thesis covers a new class of thermoelectric semiconductors based on rocksalt I-V- VI 2 compounds, which intrinsically possess a lattice thermal conductivity at the amorphous limit. It has been shown experimentally that AgSbTe 2, when optimally doped, reaches a zT =1.2 at 410 K. 3 Unfortunately, there is a metallurgical phase transition at 417 K (144 °C). The phase transition at 417 K was identified to be a potential problem in thermal cycling, and is expected with all heavy chalcogenides of group Ib elements. This issue has to be resolved before I-V-VI 2 can be considered practical. After examination, two routes to avoid the phase transition were planned: (1) alloying AgSbTe 2 with Na, and (2) using off-stoichiometry formulations Ag 1-xSb 1+y Te 2. Experimental results show that with increased sodium concentration, resistivity increases substantially, causing this method to be impractical for optimizing zT, however it did ii eliminate the phase transition at 417K. Using off-stoichiometric silver antimony telluride shows promising results with high S, on the order of 250-400 µV/K, and maintains the low thermal conductivity on the order of 0.6-0.7 W/mK. iii Dedication This is dedicated to my parents, sister, and grandparents for their continual support and encouragement throughout my entire life and especially during my time in graduate school. iv Acknowledgments I would like to acknowledge Dr. Heremans for the guidance that he has given me during my time as a graduate student. His teaching in lab and encouragement in conferences and presentations has genuinely helped expand my knowledge and confidence in the field. I would also like to thank Vladimir Jovovic for teaching me the fundamentals of thermoelectricity and measurement methods. Additionally, I would like to thank Christopher Jaworski for helping me to develop an in depth understanding of the physics and chemistries of the material systems I was working on and for being a part of the daily bagel break and brainstorming session. v Vita April 23, 1985……………………….Born – Mansfield, Ohio 2003…………………………………Monroeville High School 2008…………………………………B.S. Mechanical Engineering, The Ohio State University 2008 to present……………………...Graduate Research Associate, Department of Mechanical Engineering, The Ohio State University Fields of Study Major Field: Mechanical Engineering vi Table of Contents Abstract.......................................................................................................................... ii Dedication......................................................................................................................iv Acknowledgments ...........................................................................................................v Vita ................................................................................................................................vi List of Figures ............................................................................................................. viii Chapter 1: Introduction to Thermoelectricity ...................................................................1 Seebeck coefficient, Peltier, & Thomson Effects and Joule Heating.............................3 Transverse and Magnetically Induced Thermoelectric Effects......................................5 Hall Effect ...............................................................................................................6 Thermoelectric Material and Device Efficiency ...........................................................7 Device Setup & Efficiency.......................................................................................7 Material Figure of Merit.........................................................................................10 Thermal Conductivity ............................................................................................11 Thermoelectric Material Overview ............................................................................12 Experimental Methods...............................................................................................16 Sample Preparation................................................................................................16 Experimental Setup................................................................................................16 Error Considerations ..............................................................................................18 Chapter 2: Introduction to Silver Antimony Telluride ....................................................20 Chapter 3: Sodium Substitution in Silver Antimony Telluride........................................22 Background ...............................................................................................................22 Conclusions ...............................................................................................................31 Chapter 4: Off-Stoichiometric Silver Antimony Telluride..............................................32 Intrinsic Doping.........................................................................................................36 Extrinsic Doping........................................................................................................44 Conclusions ...............................................................................................................49 References.....................................................................................................................50 vii List of Figures Figure 1. Simplified thermoelectric circuit.......................................................................4 Figure 2. Hall Effect ........................................................................................................6 Figure 3. Typical thermoelectric device setup ..................................................................8 Figure 4. Relationship between electric field and direction of carrier velocity................15 Figure 5. Cryostat experimental setup...........................................................................17 Figure 6. Ternary phase diagram for Na-Sb-Te system. 5 ...............................................22 Figure 7. X-Ray diffraction of Ag (1-x) Na xSbTe 2. ── ── : AgSbTe 2 , ─ ─ ─: Ag 0.5 Na 0.5 SbTe 2 , ─── : NaSbTe 2 ..................................................................................23 Figure 8. Lattice constant calculation of 2 nd peak from X-ray diffraction demonstrates a linear relationship with increasing amounts of Sodium. .................................................24 Figure 9. 2 nd peak analysis of X-ray diffraction shows leftward shift of peaks with increasing Sodium concentrations..................................................................................24 Figure 10. Differential Scanning Calorimetry (DSC) analysis was conducted to determine phase transition temperatures. (bottom to top) Red: Ag0 .97 Na 0.03 SbTe 2 , Blue: Ag 0.9 Na 0.1 SbTe 2 , Light Green: Ag 0.8 Na 0.2 SbTe 2 , Purple: Ag 0.7 Na 0.3 SbTe 2 , Pink: Ag 0.5 Na 0.5 SbTe 2 , Brown: Ag 0.25 Na 0.75 , Dark Green: NaSbTe 2.......................................25 Figure 11. Resistivity as a function of Temperature. + AgSbTe 2 , ●Ag 0.97 Na 0.03 SbTe 2 , ▲Ag 0.9 Na 0.1 SbTe 2 , ♦Ag 0.8 Na 0.2 SbTe 2 , * Ag 0.7 Na 0.3 SbTe 2 , ˟ Ag 0.5 Na 0.5 SbTe 2 , ■ Ag 0.25 Na 0.75 SbTe 2 , ▼ NaSbTe 2.....................................................................................26 Figure 12. Seebeck coefficient through the entire range of samples. (left), ●Ag 0.97 Na 0.03 SbTe 2 , ▲Ag 0.9 Na 0.1 SbTe 2 , ♦Ag 0.8 Na 0.2 SbTe 2 , * Ag 0.7 Na 0.3 SbTe 2 , ■ Ag 0.25 Na 0.75 SbTe 2. (right) NaSbTe 2...............................................................................27 Figure 13. Seebeck coefficient (left) and resistivity (right) as a function of sodium concentration at 300K....................................................................................................28 Figure 14. Thermal conductivity. The symbols are & AgSbTe 2 ▲ Ag 0.97 Na 0.03 SbTe 2 ● Ag 0.8 Na 0.2 SbTe 2 ▪ Ag 0.5 Na 0.5 SbTe 2.................................................................................29