1. Αποθετήριο Παραδοτέων Υποτρόφων Δράσεων ΕΣΠΑ Τριτοβάθμιας Εκπαίδευσης 2017-2023
2. Όλο το Αποθετήριο
3. Υποψήφιοι διδάκτορες

Τύπος:	Διδακτορική διατριβή
Τίτλος:	Synthesis of silicides for thermoelectric applications by innovative methods
Εναλλακτικός τίτλος:	Ανάπτυξη πυριτιδίων για θερμοηλεκτρικές εφαρμογές με καινοτόμες μεθόδους
Συγγραφέας:	[EL] Τεκνετζή, Αικατερίνη[EN] Teknetzi, Aikaterini
Επιβλέπων διατριβής:	[EL] Βουρλιάς, Γεώργιος[EN] Vourlias, Georgios
Συμβουλευτική επιτροπή:	[EL] Χρυσάφης, Κωνσταντίνος[EN] Chrissafis, Konstantinos [EL] Παρασκευόπουλος, Κωνσταντίνος[EN] Paraskevopoulos, Konstantinos
Μέλος εξεταστικής επιτροπής:	[EL] Παυλίδου, Ελένη[EN] Pavlidou, Eleni [EL] Πατσαλάς, Παναγιώτης[EN] Patsalas, Panos [EL] Χατζηκρανιώτης, Ευριπίδης[EN] Hatzikraniotis, Euripides [EL] Στεργιούδη, Φανή[EN] Stergioudi, Fani
Ημερομηνία:	21/11/2022
Περίληψη:	Thermoelectric devices are green technology that directly converts heat to electrical power and vice versa. The last decades the thermoelectric power generators (TEGs) have received renewed interest as they can be exploited for waste heat recovery and, consequently, for a better management of the available energy. Among the different families of thermoelectric materials being investigated for TEGs, metal silicides stand out greatly due to their low cost and non-toxicity. Higher manganese silicides (HMS) and Mg2Si-based compounds are the most promising p- and ntype materials of this category, respectively. Apart from the development of cost-effective and safe thermoelectric materials, developing economic, simple and eco-friendly synthesis routes of thermoelectric materials for mass production is equally important for the market. The method of pack cementation falls into this category and could be easily adapted by the industry for the industrial-scale preparation of thermoelectric silicides. The purpose of this dissertation is to explore the prospects of preparing high-performance and high-quality HMS-based and Mg2Si-based thermoelectric materials in powder form by employing pack cementation. First, undoped HMS powder has been synthesized by pack cementation under different experimental conditions, aiming to determine the optimum ones combining good-quality powder product with the minimum possible energy consumption. The optimum synthesized HMS powder contains only a trace of metallic MnSi and exhibits a figure of merit ZTmax=0.47 at 777 K. Its performance competes that reported for high-efficiency polycrystalline HMS produced by other, conventional or more expensive techniques. A detailed study of the high-temperature oxidation process of HMS powder is presented for the first time too, in order to gain an insight of its degradation process. The as-synthesized HMS powder contains a very limited amount of oxides and impurities. Upon heating in air, HMS powder has demonstrated high oxidation resistance up to at least 793 K. Furthermore, the material is generally sufficiently stable at cyclic procedures in the region of thermoelectric interest (up to 823 K). Exposure to relatively more hostile conditions can have an appreciable impact on its performance. Subsequently, pack cementation has been applied to the synthesis of enhanced HMS-based powders with different doping elements. Both Cr- and Fe-substituted HMS powders have been prepared. This is the very first time the method is employed to directly prepare any doped material in powder form. The final products are obtained by a single-step procedure. Different synthesis conditions have been investigated leading to those that result in the least amount of secondary phases and are energy-saving simultaneously. Mn1-xCrxSi1.80 powders contain considerable amount of secondary phase CrSi2 for x≥0.07, having a negative impact on the thermoelectric properties. The best thermoelectric performance is achieved for Cr 4% substitution of Mn reaching the peak figure of merit ZTmax=0.59 at 814 K. The efficiency is comparable to those reported for Cr-doped HMS prepared by conventional techniques. The enhanced powder possesses remarkable oxidation resistance and good thermal stability up to at least 823 K. Mn1-xFexSi1.765 powders develop various solid-solved secondary phases as Fe addition increases. Si impurity occurs as a result of incorporated Fe in the HMS matrix that rejects Si to maintain the hole concentration. This mechanism also leads to MnSi and FeSi formation. For x≥0.20, FeSi2 regions grow too. It has been proved that this system is more complex inducing a greater compositional and structural inhomogeneity. The highest thermoelectric efficiency is obtained for Fe 3% substitution at Mn site reaching ZTmax=0.62 at 820 K. This is the highest efficiency reported so far for Fe-substituted HMS material. The Fe-doped powder exhibited improved oxidation resistance and possibly sufficient stability to cyclic procedures up to 833 K so far. Finally, the feasibility of extending further pack cementation in fabricating enhanced Mg2Si powder has been preliminarily examined. Mg2.18Si1-xSbx powders have been prepared under different experimental conditions, aiming to reach completion in thermochemical reactions and the following processes, as well as to diminish any impurity and secondary phases. Although unreacted Mg and Si have been effectively eliminated, it is speculated that at least a percentage of nominal Sb forms oxides or Mg3Sb2 secondary phase rather than being introduced in Mg2Si matrix. Traces of MgO and SiO2 are also present in the powders, the latter for x>0.015. Overall, the total amount of impurities and secondary phases is negligible for x<0.02. Pack cementation has been successfully employed for preparing high-performance thermoelectric HMS-based powder materials. The method may be promising for synthesis of Mg2Si-based powders too or even other enhanced silicide powders. Pack cementation could potentially stand out as a cost-effective synthesis route of industrial scale for the direct production of high-performance thermoelectric materials in powder form.
Γλώσσα:	Αγγλικά
Τόπος δημοσίευσης:	Thessaloniki, Greece
Σελίδες:	317
DOI:	10.26262/heal.auth.ir.343290
Θεματική κατηγορία:	[EL] Εφαρμοσμένη φυσική[EN] Applied Physics [EL] Φυσική συμπυκνωμένης ύλης[EN] Condensed Matter Physics [EL] Επιστήμη υλικών, γενικά[EN] Materials Science, general
Λέξεις-κλειδιά:	Θερμοηλεκτρισμός; Thermoelectrics; Χημική εναπόθεση ατμών στερεάς κλίνης; Pack cementation; Ανώτερα πυριτίδια μαγγανίου; Higher manganese silicides; Προσμίξεις; Doping
Κάτοχος πνευματικών δικαιωμάτων:	© Αικατερίνη Τεκνετζή 2022
Όροι και προϋποθέσεις δικαιωμάτων:	Η παρούσα διατριβή υποβλήθηκε στο Τμήμα Φυσικής του Αριστοτελείου Πανεπιστημίου Θεσσαλονίκης (ΑΠΘ) προς ολοκλήρωση των απαιτήσεων για την απονομή του Διδακτορικού Τίτλου στη Φυσική και εγκρίθηκε από την Εξεταστική Επιτροπή. Είναι προϊόν πρωτότυπης εργασίας αποκλειστικά δικής μου, εκτός των περιπτώσεων που ρητώς αναφέρονται μέσω βιβλιογραφικών αναφορών, σημειώσεων ή άλλων δηλώσεων. Τεκνετζή Αικατερίνη, κάτοχος πνευματικών δικαιωμάτων. This doctoral dissertation was submitted in the School of Physics at the Aristotle University of Thessaloniki (AUTH) in fulfillment of the requirements for the degree of Doctor of Philosophy in Physics. The dissertation was accepted by the Examination Committee. I am the sole author of this dissertation and it is the product of original work solely mine, except where expressly indicated by bibliographic references, notes or other statements. Teknetzi Aikaterini, copyright holder.
Σημειώσεις:	«This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme «Human Resources Development, Education and Lifelong Learning» in the context of the project “Strengthening Human Resources Research Potential via Doctorate Research – 2nd Cycle” (MIS-5000432), implemented by the State Scholarships Foundation (ΙΚΤ)»
Εμφανίζεται στις συλλογές:	Υποψήφιοι διδάκτορες