University of Groningen
Design of Advanced Thermoelectric Materials
Shaabani, Laaya
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Publication date: 2018
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Shaabani, L. (2018). Design of Advanced Thermoelectric Materials. Rijksuniversiteit Groningen.
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Summary
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Summary
The world’s increasing demand for sources of energy other than fossil fuels, as well as concern over the environmental impact of global climate change, implies the need for significant research towards alternative energy generation techniques, including renewable sources and the exploration of different energy conversion techniques to increase the net power output of existing resources. Thermoelectric (TE) technology, which is based on transforming heat directly to electricity, is one type of energy conversion technology that has the potential to help overcome the energy crisis and provide cleaner forms of energy. A significant barrier to wide-scale use and adoption of thermoelectric generators is the relatively low conversion efficiency, arising from the inter-dependence of the three key properties that determine the figure of merit: the Seebeck coefficient, electrical conductivity and thermal conductivity. Modifying existing materials to improve the thermoelectric efficiency is the main approach considered in this dissertation, which covers a series of experimental studies on several IV–VI based thermoelectric materials. The thermoelectric properties have been investigated in GeSe and (PbTe)0.55(PbS)0.1(PbSe)0.35 compounds doped with Na. Furthermore, we have studied the thermoelectric properties of PbSe doped with Ce along with the magnetic properties.
Chapter 1 gives an introduction to the fundamental background of thermoelectric properties and briefly discusses the strategies that have been proposed to enhance the thermoelectric conversion efficiency. Chapter 2 describes the synthesis methods used to prepare the samples and the different experimental characterization techniques used in this research. In chapter 3, the effect of Na doping on the thermoelectric properties of GeSe is studied. The electrical resistivity is large for all samples, resulting in low power factors. The lattice thermal conductivity and carrier mobility of GeSe are similar to those of polycrystalline samples of the well-known thermoelectric material SnSe, leading to quality factors of comparable magnitude. Powder x-ray diffraction in combination with scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS) results confirm the presence of Na-rich precipitates within the GeSe matrix, suggesting that the optimal carrier concentration cannot be reached by doping with Na. However, we expect that
Summary
the combination of these results with further experiments on different dopants can result in significant improvement in the thermoelectric figure of merit of GeSe-based materials, as well as the efficiency of thermoelectric devices.
Lead chalcogenide quaternary systems have been shown to provide thermoelectric efficiency superior to that of binary and ternary lead chalcogenides, arising from both altered electronic band structures and a reduction in lattice thermal conductivity. In chapter 4, we have synthesized p-type single-phase quaternary Lead chalcogenide compounds in which 45% of Tellurium is substituted by Sulfur and Selenium, and we have investigated the thermoelectric performance of these compounds with different Na-dopant concentrations. A high thermoelectric figure of merit of approximately 1.5 is reached at 825 K in heavily-doped samples and it is greater than 1.0 over a wide temperature range above 675 K. This enhancement in the figure of merit can be mainly attributed to a low lattice thermal conductivity that is of comparable magnitude to multiphase nanostructured lead chalcogenides. This is due to the phonon scattering that takes place at point defects associated with solute atoms with a large atomic mass contrast.
In chapter 5, we study the effect of Ce doping on the thermoelectric properties of PbSe. The substitution of 1% Ce for Pb in PbSe results in an enhanced Seebeck coefficient and a highest zT value of 0.27 at 775 K due to the enhanced electrical transport properties, which is still rather low compared to other high performance thermoelectric materials.
Chapter 6 is focused on a detailed study on the effect of Ce doping in PbSe with respect to the magnetic, structural and thermoelectric properties. A high temperature ferromagnetic signal is observed in samples with low dopant concentration (x ≤ 0.03). The observed ferromagnetism is thought to arise probably from CeO2 nanoparticles embedded in the PbSe matrix.
In summary, in this thesis research we have experimentally explored the thermoelectric performance of GeSe-based materials, which according to previous theoretical studies have promising potential for high performance thermoelectric applications. We have also focused on identifying complex lead chalcogenide systems that allow the content of scarce tellurium to be reduced without sacrificing the performance. In addition, our research provides new insight regarding high
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temperature ferromagnetic properties of Ce-doped PbSe. The outcome of this research provides a great starting point for modifying the presented materials and obtaining further improvements in their thermoelectric properties.
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Samenvatting
De toenemende vraag in de wereld naar andere energiebronnen dan fossiele brandstoffen, alsook bezorgdheid over de milieueffecten van de wereldwijde klimaatverandering, impliceert de noodzaak van grootschalig onderzoek naar alternatieve energietechnieken, waaronder hernieuwbare energiebronnen en de verkenning van verschillende energieconversietechnieken om het nettovermogen van bestaande bronnen te verhogen. Thermo-elektrische (TE) technologie, die gebaseerd is op directe omzetting van warmte in elektriciteit, is een type energieconversietechnologie die het potentieel heeft om de energiecrisis te helpen overwinnen en schonere vormen van energie te bieden. Een belangrijke barrière voor grootschalig gebruik en toepassing van thermo-elektrische generatoren is de relatief lage conversie-efficiëntie, die voortkomt uit de onderlinge afhankelijkheid van de drie belangrijkste eigenschappen die de zT-waarde bepalen: de Seebeck-coëfficiënt, elektrische geleidbaarheid en thermische geleidbaarheid. Het modificeren van bestaande materialen om de thermo-elektrische efficiëntie te verbeteren is de belangrijkste strategie die in dit proefschrift wordt behandeld, die een reeks experimentele studies over verschillende op IV-VI gebaseerde thermo-elektrische materialen omvat. De thermo-elektrische eigenschappen zijn onderzocht in GeSe en (PbTe)0.55(PbS)0.1(PbSe)0.35 verbindingen gedoteerd met Na. Verder hebben we de thermo-elektrische en magnetische eigenschappen van met PbCe gedoteerd PbSe bestudeerd.
Hoofdstuk 1 geeft een inleiding tot de fundamentele achtergrond van thermo-elektrische eigenschappen en bespreekt in het kort de strategieën die zijn voorgesteld om de thermo-elektrische conversie-efficiëntie te verbeteren. Hoofdstuk 2 beschrijft de synthesewerkwijzen die zijn gebruikt om de monsters te prepareren en de verschillende experimentele karakterisatietechnieken die in dit onderzoek worden gebruikt. In hoofdstuk 3 wordt het effect van Na-dotering op de thermo-elektrische eigenschappen van GeSe bestudeerd. De elektrische weerstand is groot voor alle monsters, wat resulteert in lage vermogensfactoren. De thermische geleiding van het kristalrooster en de mobiliteit van de ladingsdrager van GeSe zijn vergelijkbaar met die van polykristallijne monsters van het zeer bekende
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elektrische materiaal SnSe, wat leidt tot kwaliteitsfactoren van vergelijkbare omvang. Poederröntgendiffractie in combinatie met scanning-elektronenmicroscopie (SEM) en energiedispersieve spectroscopie (EDS) bevestigt de aanwezigheid van Na-rijke precipitaten in de GeSe-matrix, wat suggereert dat de optimale ladingsdragerconcentratie niet kan worden bereikt door te doteren met Na. We verwachten echter dat de combinatie van deze resultaten met verdere experimenten met verschillende doteerstoffen kan resulteren in een significante verbetering van de thermo-elektrische waarde van de op GeSe gebaseerde materialen, evenals de efficiëntie van thermo-elektrische apparaten.
Quartaire loodchalcogenidesystemen hebben bewezen een thermo-elektrische efficiëntie te bieden die beter is dan die van binaire en ternaire loodchalcogeniden, als gevolg van zowel veranderde elektronische bandstructuren als een vermindering van thermische geleidbaarheid van het kristalrooster. In hoofdstuk 4 hebben we enkelfasige p-type loodchalcogenideverbindingen gesynthetiseerd waarvan 45% van het telluur is gesubstitueerd door zwavel en selenium en hebben we de thermo-elektrische prestaties van deze (PbTe)0.55(PbS)0.1(PbSe)0.35 verbindingen onderzocht, die gedoteerd zijn met verschillende concentraties Na. Een hoge thermo-elektrische zT-waarde van ongeveer 1,5 wordt bereikt bij 825 K in zwaar gedoteerde monsters en deze is groter dan 1,0 over een breed temperatuurbereik van meer dan 675 K. Deze verbetering in de zT-waarde kan voornamelijk worden toegeschreven aan een lage thermische geleidbaarheid van het kristalrooster die van vergelijkbare grootte is als multi- fase nanogestructureerde loodchalcogeniden. Dit komt door de fononverstrooiing die plaatsvindt bij puntdefecten die samenhangen met opgeloste atomen met een sterk afwijkende atomaire massa.
In hoofdstuk 5 bestuderen we het effect van Ce-doping op de thermo-elektrische eigenschappen van PbSe. De substitutie van 1% Ce voor Pb in PbSe resulteert in een verbeterde Seebeck-coëfficiënt en een hoogste zT-waarde van 0,27 bij 775 K vanwege de verbeterde elektrische transporteigenschappen, die nog steeds vrij laag is in vergelijking met andere hoogwaardige thermo-elektrische materialen.
Hoofdstuk 6 is gericht op een gedetailleerde studie naar het effect van Ce-doping in PbSe met betrekking tot de magnetische, structurele en thermo-elektrische eigenschappen. Een ferromagnetisch signaal bij hoge temperatuur
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wordt waargenomen in monsters met een lage concentratie doteringsstof(x ≤ 0,03). Het waargenomen ferromagnetisme wordt vermoedelijk veroorzaakt door CeO2-nanodeeltjes die ingebed zijn in de PbSe-matrix.
Samenvattend hebben we in dit proefschrift onderzoek gedaan naar de thermo-elektrische prestaties van op GeSe gebaseerde materialen, die volgens eerdere theoretische studies veelbelovend zijn voor thermo-elektrische toepassingen met hoge prestaties. We hebben ons ook gericht op het identificeren van complexe loodchalcogenidesystemen die het mogelijk maken het gehalte aan het zeldzame materiaal telluur te verlagen zonder daarvoor de goede prestaties op te offeren. Bovendien biedt ons onderzoek nieuw inzicht met betrekking tot ferromagnetische eigenschappen bij hoge temperaturen van Ce-gedoteerd PbSe. De resultaten van dit onderzoek vormen een goed startpunt voor het aanpassen van de gepresenteerde materialen en het verkrijgen van verdere verbeteringen in hun thermo-elektrische eigenschappen.
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Acknowledgments
Coming to the end of my PhD journey is of course a joyful moment. I would like to thank all those who have accompanied and supported me during this journey, and apologize in advance if I have forgotten to mention those who have helped and supported me along this way.
First, I would like to express my gratitude to my promoter, Prof. Thom Palstra and my co-promoter Dr. Graeme Blake for giving me the opportunity to be part of the wonderful research group, SSME. Thom, I am very inspired by your marvelous scientific knowledge and by your perception on both experimental and fundamental aspects of the research. I always appreciated your style and the friendly way you shared knowledge during group meetings. I am very happy to have had you as my promoter.
Graeme, you are a talented crystallographer and precise scientist. It was nice to have frequent and regular access to you. I always felt motivated after talking to you. I truly admire your extreme patience and kindness and really appreciate the degree of creative freedom you gave me during my PhD research. Thank you for all your help and support during my PhD.
I am very grateful to my reading committee members: Prof. dr. Oliver Oeckler at Leipzig University, Prof. dr. M. Huijben at University of Twente and Prof. dr. Bart Kooi at University of Groningen. Thank you for taking the time to read my thesis, valuable comments and suggestions to improve my thesis.
I would like to express my thanks to all our collaborators for their fruitful contributions: Dr. Srinivasa Popuri and Prof. Jan-Willem Bos at School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, United Kingdom for the hot pressing on undoped GeSe sample, Amin Afshar and Dr. Ali Akbar Nejhad at School of Civil and Environmental Engineering, University of New South Wales, Sydney, Australia for the high temperature thermal conductivity measurements on PbSe samples, Prof. Bart Kooi and Paul Vermeulen at Zernike Institute for Advanced Materials, University of Groningen, The Netherlands, for the great TEM measurements on Ce doped PbSe samples, Prof. YanZhong Pei at Tongji University, Shanghai, China for measuring the carrier concentration of undoped GeSe sample, Dr. Sima Aminorroaya at Australian
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Institute for Innovative Materials (AIIM), Wollongong, Australia for the successful collaboration on different projects.
Dear Sima, I want to express my special gratitude to you for inviting me to AIIM and for a very kind hospitality at the institute. Sima, we first met in a conference in Madrid and we continued with a great collaboration. Thanks for your guidance and help during my stay in Australia and later. You are very knowledgeable and full of energy. I was impressed by your ability of remembering all the details and results of the experiments that we have discussed before, even those from long time ago. I really enjoyed our discussions at UOW or via Skype. I am also very thankful to Jacob Byrnes for the great SEM studies on my samples, Andrew Manettas for preparing and high temperature thermoelectric measurements on the 2% Na doped PbSeSTe sample, Shokat Keshavarzi for measuring the carrier concentration of PbSeSTe samples, Rafael Santos for the high temperature thermal conductivity measurements on PbSe samples, Xavier, Lijuan, Sanyam, Vaughan and all other members of Australian Institute for Innovative Materials for being so cooperative.
Dear Prof. Beatriz Noheda, thank you for your useful suggestions and creative discussions during the weekly FUN meetings. After Thom moved to Twente, you put a lot of effort on organizing the group. I really appreciate your kindness and optimism.
My special thanks go to Jacob Baas for his unconditional technical assistance in the lab. Jacob, you are always so kind, patient and a very caring person for everyone. I really appreciate that you could appear anytime in the lab whenever someone needs you, even at the weekend. Your help was not just limited to the lab, even outside the lab you are a good friend. I feel lucky to have you in our lab. Bedankt voor alles! Henk Bonder, I am really grateful to you for always being kind and supportive. Thank you for designing a customized sample holder for high temperature thermal diffusivity measurements and thanks for arranging the group sailing. Henriet, thank you for all your endless help with all types of official matters and paper works. I really appreciate that you always make sure that everything works properly. Thank you for everything.
I would also like to express my thanks to all my colleagues in the SSME group. It has been great pleasure to work with brilliant and supportive people in a friendly
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environment: Anil, Aisha, Mart, Saeedeh, Machteld, Arnoud, Yingfen, Silang, Monica, Jin, Qikai, Jordi, Silvia, Liany, Hong, Joshua and former members: Guowei, Mai, Ping, Lei, Arijit, Geert, Reena, Oleksiy, Alex, Jeroen, Resti, Vidushi, Nadya, Silvia Matzen, Justin, Shiva, Daniel, Oleksandr, Safdar, Romel, Prawistin and Naureen.
Anil, Thank you for your kindness to help me in my early PhD days and for sharing your knowledge and experience with me. Even though, it was a totally new field for me, I was able to ramp up many works in the lab perfectly well. You are very kind, supportive and full of enthusiasm. We attended many conferences together and I really enjoyed your positive attitude and the scientific discussions with you. All the best for your future career and your family. Aisha, my top master colleague, officemate and friend, you are very kind and full of energy. We had a lot of fun together not only within the institute but also at ACLO, FOM conferences, WIRE activities and home visits. Guowei, thanks so much for being approachable for any kind of questions and for the great time in the lab and office. I wish you great success at the Max Planck Institute. Saeedeh, I enjoyed very much our discussions about many things. Your suggestions and comments are always very precise and useful. I appreciate Mart and Arnoud for organizing wonderful group activities. Mart, I am also very grateful to you for spending time to translate my thesis summary into Dutch. Anil and Mart thank you for accepting to be my paranymphs. Arthur, it was nice sharing office with you at the final stage of my thesis writing. Other PhD’s of our institute: Jing, Julius, Tashfeen, Juan, Lam and many others thanks for your sweet company.
A sincere THANK YOU to my friends in Groningen and far. Somayeh, Atieh, Solmaz, Azadeh, Samaneh, Talieh and Soheil, we had so nice and joyful moments and events together, I am so grateful for having you in my life. My dear Shadzi friends you are all amazing and I appreciate the wonderful moments with you. Mahdieh, Sara Zarghami, Farzad, Saeedeh Naeimi, Ehsan and Pakhshan, I appreciate the moments we shared and thanks for everything! Farshad, Thank you for your valuable comments on cover design.
Aida, Mehdi, Zahra, Hesam, Zohreh, Davood, Fatemeh, Mehran, Susan, Omid, Marziyeh, Yazdan, thank you for all beautiful memories you left for me.
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Ahmadreza, Reyhaneh, Fahimeh, Hadi and Pouriya, thank you for all you have done for me.
I owe a great deal to Dick and Anneke. Dick, You were a kind hearted person and always available for help whenever someone needed your help. I will remember you as one of the few nicest people I have ever met. Anneke, Thank you for all your kind support and unconditional help. You are a true friend. I thank you for all you have done for me.
At the end, a heartfelt thanks goes out to my family who provided support, inspiration, mentoring, and motivation along the way. Thank you for encouraging me in all of my pursuits and inspiring me to follow my dreams. My dear father, I always knew that you believed in me and wanted the best for me. Thank you for showing me that the key to life is enjoyment. Your advices are always the best hints for life, they are always encouraging and inspiring. Although words cannot express the appreciation, but my promotion will definitely make your soul happy. My dear mother, without your unconditional love and support, it would have been impossible for me to stand where I am today. It is beyond words to thank you, my lovely mother. My dear brother, I owe you a lot. You always fulfill me with energy and motivate me to conquer new and new heights. Thank you for believing in me and reminding me of my capabilities. Thank you for always supporting me, at any time, in any case. My dear brothers, sister and my extended family members, thank you for your love, unwavering encouragement and moral support throughout my life and during this study. Love you!
Laaya Shaabani Groningen February, 2018
