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Thermoelectric Properties of Silicon Germanium Clemson University TigerPrints All Dissertations Dissertations 5-2015 Thermoelectric Properties of Silicon Germanium: An Investigation of the Reduction of Lattice Thermal Conductivity and Enhancement of Power Factor Ali Sadek Lahwal Clemson University Follow this and additional works at: https://tigerprints.clemson.edu/all_dissertations Recommended Citation Lahwal, Ali Sadek, "Thermoelectric Properties of Silicon Germanium: An Investigation of the Reduction of Lattice Thermal Conductivity and Enhancement of Power Factor" (2015). All Dissertations. 1482. https://tigerprints.clemson.edu/all_dissertations/1482 This Dissertation is brought to you for free and open access by the Dissertations at TigerPrints. It has been accepted for inclusion in All Dissertations by an authorized administrator of TigerPrints. For more information, please contact [email protected]. Thermoelectric Properties of Silicon Germanium: An Investigation of the Reduction of Lattice Thermal Conductivity and Enhancement of Power Factor ___________________________________________________________________ A Dissertation Presented to the Graduate School of Clemson University ___________________________________________________________________ In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Physics ____________________________________________________________________ by Ali Sadek Lahwal May 2015 ____________________________________________________________________ Accepted by: Dr. Terry Tritt, Committee Chair Dr. Jian He Dr. Apparao Rao Dr. Catalina Marinescu Abstract: The imminent oil crisis and global warming has revived research on sustainable energy resources. The researchers are seeking for alternative, clean, cheap, and safe resources of energy, such as solar energy, wind, sea waves, and heat. To this end thermoelectric materials are of technological interest owing to their ability of direct thermal-to-electrical energy conversion. In thermoelectricity, thermal gradients can be used to generate an electrical power output. Recent efforts in thermoelectrics are focused on developing higher efficient power generation materials. These materials can open many new horizons of applications, such as converting solar thermal energy to electricity, waste heat recovery, and as power generators for deep space exploration of our solar system when coupled with a radioactive heat source. In this dissertation, the overall goal is to investigate both the n-type and p-type of the state of the art thermoelectric material, silicon germanium (SiGe ), for high temperature power generation. Further improvement of thermoelectric performance of Si -Ge alloys hinges upon how to significantly reduce the as yet large lattice thermal conductivity, and optimizing the thermoelectric power factor PF . Our methods, in this thesis, will be into two different approaches as follow: The first approach is manipulating the lattice thermal conductivity of n and p- type SiGe alloys via direct nanoparticle inclusion into the n-type SiGe matrix and, in a different process, using a core shell method for the p-type SiGe . This approach is in line with the process of in-situ nanocomposites . Nanocomposites have become a new paradigm for thermoelectric research in recent years and have resulted in the reduction of thermal conductivity via the nano-inclusion and grain boundary ii scattering of heat-carrying phonons. To this end, a promising choice of nano-particle to include by direct mixing into a SiGe matrix would be Yttria Stabilized Zirconia (YSZ ). In this work we report the preparation and thermoelectric study of n-type SiGe + YSZ nanocomposites prepared by direct mechanical mixing followed by Spark Plasma Sintering (SPS) processing. Specifically, we experimentally investigated the reduction of lattice thermal conductivity ( ) in the temperature range (30-800K) of n-type alloys with the incorporation of YSZ nanoparticles (20 40 nm ~ diameter) into the Si-Ge matrix. These samples synthesized by SPS were found to have densities > 95% of the theoretical density. At room temperature, we observed approximately a 50% reduction in the lattice thermal conductivity as result of adding 10 volume % YSZ to the host matrix. A phenomenological Callaway model was used to corroborate both the temperature dependence and the reduction of over the measured temperature range (30-800K) of both and samples. The observed is discussed and interpreted in terms of + various phonon scattering mechanisms including alloy disorder, the Umklapp process, and boundary scattering. Specifically, a contribution from the phonon scattering by YSZ nanoparticles was further included to account for the of + samples. In addition, a core shell treatment was applied onto p-type SiGe . Ball milled Si 80 Ge 20 B1.7 alloys were coated with YSZ with different thicknesses and characterized upon their thermoelectric properties. The results show that YSZ coatings are capable of greatly reducing the thermal conductivity especially the lattice thermal conductivity. These coatings are applied directly onto mechanical alloyed (MA), p- iii type SiGe . The only concern about the YSZ core shelling is that these coatings turned out to be too thick degrading the electrical conductivity of the material. Our second approach, in a parallel work, is to enhance the thermoelectric power factor as well as the dimensionless figure of merit ZT of: ( i) single element spark plasma sintered (SE SPS) SiGe alloys. (ii ) ball milled (BM) SiGe , via sodium boron hydrate ( NaBH 4) alkali-metal-salt treatment. Sodium boron hydrate alkali-metal-salt thermally decomposes (decompose temperature 600 ~ 700 K) to elemental solid sodium, solid boron, and hydrogen gas, as binary phases, e.g., Na-B or Na-H, or as a ternary phase, Na-B-H. Upon SPS at 1020 K, it is inferred that Na dopes SiGe while forming Na 2B29 phase, leading to a reduction in the electrical resistivity without much degrading the Seebeck coefficient, consequently enhancement of the power factor. Both Hall and Seebeck coefficient showed that all the samples are p-type. Data analysis shows that the reduction of the electrical resistivity can be attributed to the increased carrier concentration. While the reduction of the thermal conductivity, in the ball milled samples, is mainly due to the enhanced phonon scattering at the increased grain boundaries in addition to contribution of scattering by the Na 2B29 phases, consequently resulting in a very significant 80% improvement of the ZT figure of merit. iv Dedication As much as we seek and gain more knowledge it shows us the greatness of Allah (God) and how he (Allah) has created this universe by and in this accuracy. There are many who I can count as worthy to receive the dedication of this work. There is my loving mother who raised me and covered me with her tenderness. There is my father's pure soul who taught me the morals and decency before he passed away. There is my loving family, wife and children, who struggled with me and suffered of the pain of homesickness. There are great teachers who taught me the respectful manners before teaching me what was written in books. There are my brothers and sisters who supported me. To all of them and all of those who know me I would like to dedicate this work. v Acknowledgments Stumbling during walking in our path is expected, but we have to grasp in the rope of hope to stand up and keep going. During my PhD journey I had stumbled many times and was about to fall, the only mercy hand after Allah (God) that I had grasped with and guarded me was my supervisor Dr. Tritt’s hand. Therefore, I would like to begin by thanking my advisor and friend Dr. Terry M. Tritt, who took me under his umbrella. The words generous, kind, gracious, patient, wise, and beneficent are not enough to describe Dr. Tritt. I am forever and ever grateful, thankful and appreciative of him for the blessings he has bestowed upon me. Even if I am forgetful, I will never forget my friend Dr. Jian He and how much my relationship with him has meant to me. I always appreciated Dr. He’s valuable advice and directions. I would like to express my thankfulness and gratitude to Dr. Jian He. From the bottom of my heart, thank you both! I would like to express my gratitude to my other Ph.D. committee members, Dr. Apparao Rao, Dr. Catalina Marinescu and Dr. Ramakrishna Podila. You all have spent so many of your precious time providing counsel for my studies. Thank you for sharing your wisdom and knowledge to push me to enhance my work, and helping me to have a deep understanding of physics. I would like to express my gratefulness to my sister, friend and colleague Dr. Sriparna Bhattacharya for her advice and helping me in working on the modeling, measurements and papers. Thank you so much Dr. Sriparna Bhattacharya and I appreciated your time. Also, to my fellow group members: those before me, those after me, and those here with me, thank you for making Clemson and CAML a vi wonderful place in my heart and sharing with me your ideas. Thank you Xiaoyu Zeng “Bella” for your patience and time helping me on measuring some samples. Thank you Dale Hitchcock, Menghan Zhou, Yufei Liu, Arash Mehdizadeh, Jennifer Hubbard, Pooja Puneet, Song Zhu, Tim Holgate, Jared Williams and Gezhou Zhang. Daniel Thompson old buddy, thank you! This work could have never been without the learnedness you passed on. I still remember first sample I had prepared under your supervision. Your friendship has been invaluable to me. I wish you all the best and I know that you will do well. To my many collaborators it has been amazing and stupendous, and I look forward to another opportunity to work together again. To Dr. Joe Poon, Di Wu and Alex Petersen from the University of Virginia, thank you for your consistent support and timely feedback on SiGe . Also I would like to thank the Nanosonic Inc. and Lee Williams for synthesizing the YSZ and core shell materials used in this research. I would like to acknowledge all the professors at Physics and astronomy department at Clemson University who taught me all the courses and classes required for the PhD degree.
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