Researcher:
Özen, Melis

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PhD Student

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Melis

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Özen

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Özen, Melis

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2019 - 201912020 - 20224

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Researcher Of Publications: search.filters.isAuthorOfPublication.20392334-0455-4966-ac35-589a6b24a9cb

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    Stress/pressure-stabilized cubic polymorph of Li3Sb with improved thermoelectric performance
    (Royal Society of Chemistry (RSC), 2021) Soldi, Thomas; Candolfi, Christophe; Snyder, G. Jeffrey; N/A; N/A; Department of Chemistry; Yahyaoğlu, Müjde; Özen, Melis; Aydemir, Umut; PhD Student; Master Student; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 58403
    Li3Sb has two polymorphs crystallizing in a face-centered cubic cell (c-Li3Sb; BiF3 structure type; space group Fm3m) and in a hexagonal unit cell (h-Li3Sb; Na3As structure type; space group P6(3)/mmc). c-Li3Sb was predicted to be a promising thermoelectric material based on recent first-principles studies; however, the experimental transport characteristics have remained unknown so far. Herein, successful preparation of c-Li3Sb is reported by stress-induced mechanochemical synthesis (high-energy ball milling) along with its high-temperature thermoelectric properties. Hexagonal Li3Sb (h-Li3Sb) was revealed to be the stable phase at ambient conditions, while it starts unexpectedly transforming to c-Li3Sb by ball milling or under 60 MPa applied pressure at room temperature. The transport properties measurements performed on two polycrystalline specimens evidence that c-Li3Sb behaves as a p-type degenerate semiconductor due to the formation of Li vacancies. In agreement with lattice dynamics calculations, c-Li3Sb exhibits very low lattice thermal conductivity despite the lightweight of Li. A zT value of around 0.3 was obtained at 550 K. Modelling suggests that the hole concentration should be reduced through aliovalent substitutions or under Li-rich conditions for further optimization. Although the strong air sensitivity of Li3Sb makes its use in thermoelectric applications challenging, this simple superionic binary provides an attractive experimental platform to elucidate the effect of stress/pressure on phase transitions as well as that of Fermi surface complexity on thermoelectric properties.
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    Phase-transition-enhanced thermoelectric transport in rickardite mineral Cu3-xTe2
    (American Chemical Society (ACS), 2021) Prots, Yurii; El Hamouli, Oussama; Tshitoyan, Vahe; Ji, Huiwen; Burkhardt, Ulrich; Lenoir, Bertrand; Snyder, G. Jeffrey; Jain, Anubhav; Candolfi, Christophe; N/A; N/A; Department of Chemistry; Yahyaoğlu, Müjde; Özen, Melis; Aydemir, Umut; PhD Student; Master Student; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 58403
    The binary copper chalcogenides Cu2-delta X (X = S, Se, and Te) have recently gained significant interest due to their high thermoelectric performance at moderate temperatures. In an effort to unveil new Cu-based compounds with promising thermoelectric potential, Cu3-xTe2 rickardite mineral emerged as a candidate based on a purely text mining approach applied by a machine learning method. Polycrystalline samples of Cu3-xTe2 within the homogeneity range (x = 0.1, 0.2) were successfully synthesized from the raw elements by a solid-state method. High-temperature powder Xray diffraction combined with differential scanning calorimetry and specific heat measurements showed several reversible phase transitions at around 458, 640, and 647 K. Signatures of these transitions were observed on the electronic and thermal transport properties, measured over a broad range of temperatures (5-733 K). The transition undergone by this compound at 647 K results in a crossover from metallic-like to semiconducting-like properties. The combination of high power factor and low thermal conductivity in the high-temperature phase results in improved thermoelectric performances with a peak dimensionless thermoelectric figure-of-merit zT of similar to 0.14 at 733 K. The synthetic rickardite mineral is an exciting candidate to be used as a phase change material in broad application areas such as in waste heat harvesting and photovoltaic systems.
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    Enhanced thermoelectric performance in Mg3+xSb1.5Bi0.49Te0.01 via engineering microstructure through melt-centrifugation
    (Royal Society of Chemistry (RSC), 2021) Candolfi, Christophe; Veremchuk, Igor; Kaiser, Felix; Burkhardt, Ulrich; Snyder, G. Jeffrey; Grin, Yuri; N/A; N/A; Department of Chemistry; Özen, Melis; Yahyaoğlu, Müjde; Aydemir, Umut; Master Student; PhD Student; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials & High-technology Chemicals Research & Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 58403
    N-type Zintl phases with earth-abundant and non-toxic constituent elements have attracted intense research interest thanks to their high thermoelectric efficiencies in the mid-temperature range, exemplified by the recently discovered Mg3Sb2 material. In this study, the liquid phase is expelled from the microstructure of the optimized n-type phase Mg3+xSb1.5Bi0.49Te0.01 by applying a meltcentrifugation technique leading to the formation of lattice dislocations, grain boundary dislocations and increasing porosity. Additional phonon scattering mechanisms were introduced in the microstructure through this manufacturing method, resulting in a significant 50% reduction in the total thermal conductivity from similar to 1 W m(-1) K-1 to similar to 0.5 W m(-1) K-1 at 723 K. Combined with high power factors, this reduced heat transport leads to a dimensionless thermoelectric figure of merit, zT, value of similar to 1.64 at 723 K, 43% higher than the value obtained in untreated Mg3+xSb1.5Bi0.49Te0.01 (zT similar to 1.14 at 723 K). This peak zT value yields a predicted device ZT of 0.95, and a promising theoretical thermoelectric efficiency of about 12%. These results further underline the great potential of the lightweight Mg3Sb2 material for midtemperature energy harvesting via thermoelectric effects.
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    TiB2–SiC-based ceramics as alternative efficient micro heat exchangers
    (Elsevier Sci Ltd, 2019) Nekahi, Sahar; Vajdi, Mohammad; Moghanlou, Farhad Sadegh; Vaferi, Kourosh; Motallebzadeh, Amir; Sha, Jianjun; Asl, Mehdi Shahedi; N/A; Department of Chemistry; Özen, Melis; Aydemir, Umut; PhD Student; Faculty Member; Department of Chemistry; Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 58403
    Ceramics as functional materials durable at corrosive media or high temperatures are suitable candidates for manufacturing heat exchangers. In the present work, a series of numerical simulations was carried out to investigate the heat transfer in micro heat exchanger made by TiB2-SiC and TiB2-SiC doped with 2 wt% carbon fiber. Experimental thermal diffusivity data of materials were obtained by laser flash method and used for numerical simulations. This article demonstrates the feasibility of better heat transfer and performance for ceramic heat exchangers. Conduction heat transfer equation in solid walls of heat exchanger along with fluid flow governing equation was used for solid and fluid domains. The governing equations were discretized by the finite element method and solved by COMSOL Multiphysics software. The obtained results showed that the thermal conductivity of both TiB2-based composites are approximately the same and small amount of carbon fiber had no considerable effect on the thermal properties of the composites. For the ceramic heat exchanger, a higher heat transfer with 15.5% enhancement occurs using TiB2 composites compared to utilization of Al2O3.
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    PublicationOpen Access
    Key properties of inorganic thermoelectric materials - tables (version 1)
    (Institute of Physics (IOP) Publishing, 2022) Freer, R.; Ekren, D.; Ghosh, T.; Biswas, K.; Qiu, P.; Wan, S.; Chen, L.; Han, S.; Fu, C.; Zhu, T.; Ashiquzzaman Shawon, A.K.M.; Zevalkink, A.; Imasato, K.; Snyder, G.J.; Cardoso-Gil, R.; Svanidze, E.; Funahashi, R.; Powell, A.V.; Mukherjee, S.; Tippireddy, S.; Vaqueiro, P.; Gascoin, F.; Kyratsi, T.; Sauerschnig, P.; Mori, T.; N/A; Department of Chemistry; Aydemir, Umut; Sağlık, Kıvanç; Özen, Melis; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences; 58403; N/A; N/A
    This paper presents tables of key thermoelectric properties, which define thermoelectric conversion efficiency, for a wide range of inorganic materials. The twelve families of materials included in these tables are primarily selected on the basis of well established, internationally-recognized performance and promise for current and future applications: tellurides, skutterudites, half Heuslers, Zintls, Mg-Sb antimonides, clathrates, FeGa3-type materials, actinides and lanthanides, oxides, sulfides, selenides, silicides, borides and carbides. As thermoelectric properties vary with temperature, data are presented at room temperature to enable ready comparison, and also at a higher temperature appropriate to peak performance. An individual table of data and commentary are provided for each family of materials plus source references for all the data.