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Aydemir, Umut

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Umut

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    High temperature thermoelectric properties of the type-I clathrate BA(8)NI(X)GE46(-X-Y)square(Y)
    (Institute of Physics (IOP) Publishing, 2014) Candolfi, Christophe; Örmeci, Alim; Baitinger, Michael; Oeschler, Niels; Steglich, Frank; Grin, Yu; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    Polycrystalline samples of the type-I clathrate Ba8NixGe46-x-y square(y) were synthesized for 0.2 <= x <= 3.5 by melt quenching and for 3.5 < x <= 6.0 by melting with subsequent annealing at 700 degrees C. The maximum Ni content in the clathrate framework at this temperature was found to be x approximate to 4.2 atoms per unit cell. Thermoelectric and thermodynamic properties of the type-I clathrate were investigated from 300 to 700 K by means of electrical resistivity, thermopower, thermal conductivity and specific heat measurements. As the Ni content increases, the electronic properties gradually evolve from a metallic character (x < 3.5) towards a highly doped semiconducting state (x >= 3.5). Below x approximate to 4.0 transport is dominated by electrons, while further addition of Ni (x approximate to 4.2) switches the electrical conduction to p-type. Maximum value of the dimensionless thermoelectric figure of merit ZT approximate to 0.2 was achieved at 500K and 650K for x approximate to 2.0 and x approximate to 3.8, respectively.
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    Melt-centrifuged (BI,SB)(2)TE-3: engineering microstructure toward high thermoelectric efficiency
    (Wiley-V C H Verlag Gmbh, 2018) Pan, Yu; Grovogui, Jann A.; Witting, Ian T.; Hanus, Riley; Xu, Yaobin; Wu, Jinsong; Wu, Chao-Feng; Sun, Fu-Hua; Zhuang, Hua-Lu; Dong, Jin-Feng; Li, Jing-Feng; Dravid, Vinayak P.; Snyder, G. Jeffrey; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (kappa(l)) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt-centrifugation to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a approximate to 60% reduction of kappa(l) compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid-fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)(2)Te-3 alloys. A segmented leg of melt-centrifuged Bi0.5Sb1.5Te3 and Bi0.3Sb1.7Te3 could produce a high device ZT exceeding 1.0 over the whole temperature range of 323-523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high-efficiency porous thermoelectric materials through an unconventional melt-centrifugation technique.
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    Metal-substituted zirconium diboride (Zr= Ni, Co, and Fe) as low-cost and high-performance bifunctional electrocatalyst for water splitting
    (Elsevier, 2021) N/A; N/A; N/A; N/A; Department of Chemistry; Mete, Büşra; Peighambardoust, Naeimeh Sadat; Aydın, Samet; Sadeghi, Ebrahim; Aydemir, Umut; Master Student; Researcher; Other; 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; N/A; N/A; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 58403
    Recent years have witnessed an unprecedented surge in research on earth-abundant and efficient electrocatalysts for the water splitting process. Among those, the development of boron-based advanced catalysts is subject to designing the active and durable compounds, working as bifunctional materials under alkaline medium. In this study, a series of ZrB2-based catalysts with a general formula of Zr(1-x)TMxB2 (x = 0.05, 0.1, and 0.2) (TM = Fe, Co, and Ni) were prepared through a straightforward route and employed as bifunctional electrocatalysts in hydrogen and oxygen evolution reactions (HER and OER). The electrochemical measurements confirmed that the incorporation of Ni into the crystal structure of ZrB2 in the Zr0.8Ni0.2B2 sample led to an onset potential of 1.58 V in OER at a current density of 10 mA cm(-2), indicating a remarkable performance with a very low overpotential of 350 mV. Besides, Zr0.8Ni0.2B2 displayed an infinitesimal value of 56.6 mV dec(-1) regarding the Tafel slope, which was lesser as compared to the commercial RuO2 (66.2 mV dec(-1)). For the case of HER, Zr0.8Co0.2B2 showed the best performance compared to other samples with an overpotential of 420 mV and a Tafel slope of 101.6 mV dec(-1), following the Volmer mechanism. Both catalysts were examined for their long-term stability, manifesting excellent catalytic durability even after 12 h. Surprisingly, Zr0.8Co0.2B2 exhibited a drop from 420 to 380 mV in the overpotential value after 1000 CV sweeps, providing a promising performance in terms of HER. As-prepared metal-substituted ZrB2 electrocatalysts have great potential to be implemented in various green energy system applications. (C) 2021 Elsevier Ltd. All rights reserved.
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    Thermoelectric properties and electronic structure of the zintl phase SR5IN2SB6 and the CA5-XSRXIN2SB6 solid solution
    (Institute of Physics (IOP) Publishing, 2015) Zevalkink, Alex; Chanakian, Sevan; Ormeci, Alim; Pomrehn, Gregory; Bux, Sabah; Fleurial, Jean-Pierre; Snyder, G. Jeffrey; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    The Zintl phase Sr5In2Sb6 is isostructural with Ca5In2Sb6-a promising thermoelectric material with a peak zT of 0.7 when the carrier concentration is optimized by doping. Density functional calculations for Sr5In2Sb6 reveal a decreased energy gap and decreased valence band effective mass relative to the Ca analog. Chemical bonding analysis using the electron localizability indicator was found to support the Zintl bonding scheme for this structure type. High temperature transport measurements of the complete Ca5-xSrxIn2Sb6 solid solution were used to investigate the influence of the cation site on the electronic and thermal properties of A(5)In(2)Sb(6) compounds. Sr was shown to be fully miscible on the Ca site. The higher density of the Sr analog leads to a slight reduction in lattice thermal conductivity relative to Ca5In2Sb6, and, as expected, the solid solution samples have significantly reduced lattice thermal conductivities relative to the end member compounds.
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    Electronic band structure and low-temperature transport properties of the type-I clathrate BA8NIXGE46-X-Y
    (Royal Soc Chemistry, 2015) Candolfi, C.; Ormeci, A.; Baitinger, M.; Burkhardt, U.; Oeschler, N.; Steglich, F.; Grin, Yu.; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    We present the evolution of the low-temperature thermodynamic, galvanomagnetic and thermoelectric properties of the type-I clathrate Ba8NixGe46-x-y square(y) with the Ni concentration studied on polycrystalline samples with 0.0 <= x <= 6.0 by means of specific heat, Hall effect, electrical resistivity, thermopower and thermal conductivity measurements in the 2-350 K temperature range and supported by first-principles calculations. The experimental results evidence a 2a x 2a x 2a supercell described in the space group Ia (3) over bard for x <= 1.0 and a primitive unit cell a x a x a (space group Pm (3) over barn) above this Ni content. This concentration also marks the limit between a regime where both electrons and holes contribute to the electrical conduction (x <= 1.0) and a conventional, single-carrier regime (x > 1.0). This evolution is traced by the variations in the thermopower and Hall effect with x. In agreement with band structure calculations, increasing the Ni content drives the system from a nearly-compensated semimetallic state (x = 0.0) towards a narrow-band-gap semiconducting state (x = 4.0). A crossover from an n-type to a p-type conduction occurs when crossing the x = 4.0 concentration i.e. for x = 4.1. The solid solution Ba8NixGe46-x-y square(y) therefore provides an excellent experimental platform to probe the evolution of the peculiar properties of the parent type-I clathrate Ba8Ge43 square(3) upon Ge/Ni substitution and filling up of the vacancies, which might be universal among the ternary systems at low substitution levels.
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    Vibrational spectra of cluster anions. 2 [1] - Vibrational spectra of compounds with the cluster anions [E-4](4-) : M4E4 (M = K, Rb, Cs; E = Ge, Sn) and beta-Na4Sn4
    (Wiley-V C H Verlag Gmbh, 2006) Baitinger, Michael; von Schnering, Hans Georg; Department of Chemistry; Department of Chemistry; Somer, Mehmet Suat; Aydemir, Umut; Faculty Member; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 178882; 58403
    Vibrational spectra of the compounds M4E4 (M = K, Rb, Cs; E = Ge, Sn) and of beta-Na4Sn4 with the cluster anions [E-4](4-) were analysed based on the point group (4) over bar 3m-T-d of isolated tetrahedranide units. The lower individual (4) over bar 2m-D-2d symmetry of the anions in the real structure being more patterned and complex primarily affects the spectra of the tetrahedro-tetragermanides. nu(3)(F-2) clearly splits both in Raman and IR and in the case of K4Sn4 only in JR. Rb4Sn4 and Cs4Sn4 exhibit very simple spectra with three bands in Raman and one band in JR. The breathing mode nu(1)(A(1)) for the quasi isolated [E-4](4-) cluster appears only in the Raman spectrum and is hardly influenced by the structural environment and by the nature of the alkali metal cations: nu(1)(A(1)) = 274 cm(-1) ([Ge-4](4-)) and 183-187 cm(-1) ([Sn-4](4-)), respectively. The calculated valence force constants fd(E-E) are: [Ge-4](4-): f(d) = 0.89 Ncm(-1) (K), 0.87 Ncm(-1) (Rb), 0.86 Ncm(-1) (Cs) and [Sn-4](4-): 0.67 Ncm(-1) (Na), 0.66 Ncm(-1) (K), 0.67 Ncm(-1) (Rb), 0.68 Ncm(-1) (Cs). Both, the frequencies and the force constants fit well into the range previously reported.
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    High temperature transport properties of BAZN2SN2
    (Elsevier Science Sa, 2015) Zevalkink, Alex; Bux, Sabah; Snyder, G. Jeffrey; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    BaZn2Sn2 (space group P4/nmm, a = 4.7459(5) angstrom, c = 11.330(2) angstrom, Z = 2) crystallizes in the CaBe2Ge2 structure type with a polyanionic framework comprising alternately stacked PbO-like {ZnSn4/4} and anti-PbO-like {SnZn4/4} layers along the c-axis. BaZn2Sn2 samples were obtained by either direct solid state reaction of the elements or from a Sn-flux method in very high yield with very small amount of b-Sn as the secondary phase. The samples were characterized by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). The chemical compositions were determined to be off-stoichiometric with Zn/Sn ratio lower than 1.0 and Sn-2 atoms in the crystal structure were found to be either loosely bonded or not bonded which might lead to an incomplete charge balance. Electrical and thermal transport measurements have been performed in the temperature range 300-773 K. BaZn2Sn2 displays the electrical resistivity of a metal (or semimetal) along with very low Seebeck coefficients and relatively high thermal conductivity. (C) 2014 Elsevier B.V. All rights reserved.
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    CS4GE9 center dot en: a novel compound with [GE-9](4-) clusters - synthesis, crystal structure and vibrational spectra
    (Wiley-V C H Verlag Gmbh, 2007) Carrillo-Cabrera, Wilder; Kırcalı, Aslıhan; Faessler, Thomas F.; Hoffmann, Stephan D.; Department of Chemistry; Department of Chemistry; Aydemir, Umut; Somer, Mehmet Suat; Faculty Member; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 58403; 178882
    Red-orange prismatic crystals of CS4Ge9.en (en = ethyl-enediamine) were obtained via metathesis reaction of an en solution of K4Ge9 with a large excess of CsI. The compound is very sensitive to air and moisture. CS4Ge9.en is not isotypic to Rb4Ge9.en and crystallizes in a new structure type (monoclinic, space group P2(1)/c (No. 14); a = 16.757(3), h = 16.462(3), c = 15.657(2) angstrom, beta = 90.37(1)degrees; Z = 8; Pearson code mP200). The crystal structure comprises two types (A and B) of discrete [Ge-9](4-)units arranged in the motif of a distorted hexagonal close-packing (h.c.p.). In this h.c.p. framework, each tetrahedral hole is filled by one Cs atom and each larger octahedral hole is tilled by a group of one en and two Cs atoms. The topology of the two crystallographically distinct [Ge-9](4-) clusters corresponds to distorted, monocapped tetragonal antiprisms with bond lengths varying in the range 2.551-2.848 angstrom (type A cluster) or in the range 2.538-2.975 angstrom (less regular type B cluster). According to the DTA and TG measurements, CS4Ge9.en decomposes at 378 K under liberation of gaseous ethylenediamine yielding CS4Ge9. The Raman spectra of the title compound were interpreted based on the idealized C-4v symmetry of an isolated [Geg](4-) cluster. The characteristic breathing mode was localized at 222 cm(-1). The results are analyzed and discussed in context with those of the previously reported compounds CS4Ge9, K4Ge9 and Rb4Ge9.en.
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    Mechanical properties in thermoelectric oxides: ideal strength, deformation mechanism, and fracture toughness
    (Elsevier, 2018) Li, Guodong; Morozov, Sergey I.; Miller, Samuel A.; An, Qi; Goddard, William A., III; Zhai, Pengcheng; Zhang, Qingjie; Snyder, G. Jeffrey; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    The recent dramatic improvements in high-performance thermoelectric (TE) oxides provide new exciting applications in the TE field, but the mechanical properties so important for engineering applications remain largely unexplored. Based on density functional theory (DFT) calculations, we report the ideal strength, deformation mechanism, and fracture toughness of such TE oxides as n-type ZnO and SrTiO3 and p-type BiCuSeO and NaCo2O4. The Zn-O and Ti-O bonds forming the 3D Zn-O and Ti-O frameworks dominate the deformation and failure mechanisms of ZnO and SrTiO3, respectively. Due to the higher stiffness of Ti-O octahedra compared with that of Zn-O tetrahedra, SrTiO3 exhibits more robust macro-mechanical properties such as elastic modulus and fracture toughness than ZnO. The Bi-Se and Na-O bonds, which couple the different 2D substructures, are responsible for the relative slip in BiCuSeO and NaCo2O4, respectively. Since the Zn-O and Ti-O bonds are much stronger than the Bi-Se and Na-O bonds, we find that n-type ZnO and SrTiO3 have a higher ideal strength and fracture toughness compared with p-type BiCuSeO and NaCo2O4. This work reveals that for TE module applications of oxides, it is most important to significantly improve the mechanical properties of the p-leg.
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    Isotropic zero thermal expansion and local vibrational dynamics in (SC,FE)F-3
    (American Chemical Society (ACS), 2017) Qin, Feiyu; Chen, Jun; Sanson, Andrea; Wang, Lu; Pan, Zhao; Xu, Jiale; Sun, Chengjun; Ren, Yang; Deng, Jinxia; Yu, Ranbo; Hu, Lei; Snyder, G. Jeffrey; Xing, Xianran; Department of Chemistry; Aydemir, Umut; Faculty Member; Department of Chemistry; College of Sciences; 58403
    Scandium fluoride (ScF3) exhibits a pronounced negative thermal expansion (NTE), which can be suppressed and ultimately transformed into an isotropic zero thermal expansion (ZTE) by partially substituting Sc with Fe in (Sc0.8Fe0.2)F-3 (Fe20). The latter displays a rather small coefficient of thermal expansion of -0.17 X 10(-6)/K from 300 to 700 K. Synchrotron X-ray and neutron pair distribution functions confirm that the Sc/ Fe-F bond has positive thermal expansion (PTE). Local vibrational dynamics based on extended X-ray absorption fine structure indicates a decreased anisotropy of relative vibration in the Sc/Fe-F bond. Combined analysis proposes a delicate balance between the counteracting effects of the chemical bond PTE and NTE from transverse vibration. The present study extends the scope of isotropic ZTE compounds and, more significantly, provides a complete local vibrational dynamics to shed light on the ZTE mechanism in chemically tailored NTE compounds.