Browsing by Author "Tewari, Girish C."

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Efficacies of dopants in thermoelectric BiOCuSe
(2016-07-01)
Chou, Ta-Lei
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Tewari, Girish C.
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Srivastava, Divya
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Yamamoto, Atsushi
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Chan, Ting-Shan
Moderately-doped BiOCuSe-based materials are actively investigated worldwide as new promising candidates for intermediate-temperature thermoelectric power generation. Here we explain for two differently doped BiOCuSe materials, i.e. Cu-deficient BiOCu1-xSe and Ca-for-Bi substituted (Bi1-yCay) OCuSe systems, the differences in their efficacies to enhance the thermoelectric performance of the pristine BiOCuSe material using theoretical calculations and x-ray absorption spectroscopy (XAS) at various edges. In the Cu-deficient system the doping dominantly induces an oxide layer-to-selenide layer charge transfer process. On the other hand, the Ca-for-Bi substitution not only gives lower electrical resistivity but also better thermal stability thanks to the increased ionic bonding character in the selenide block. Density of states nearby the Fermi level suggests that even though the Bi-Se distance is much longer than the Bi-O bond, its contribution cannot be ruled out. The Se-Bi-O chain contributes to the hole states at the VBM with the Cu 3d - Se 4p antibonding state. (C) 2016 Elsevier B.V. All rights reserved.
EXAFS study of thermoelectric BiCuOSe: Effects of Cu vacancies
(2015-03-01)
Chou, Ta-Lei
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Tewari, Girish C.
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Chan, Ting-Shan
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Hsu, Ying-Ya
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Yamauchi, Hisao
Excellent thermoelectric performance has been reported for the ZrCuSiAs-type BiCuOSe derivatives; for these materials the transport properties can be effectively adjusted by chemical substitutions or by introducing Cu vacancies. In this work we compare stoichiometric BiCuSeO and its Cu-deficient derivative BiCu0.95SeO by utilizing temperature dependent extended X-ray absorption fine structure (EXAFS) measurements. Such a local probe evidences that Cu vacancies make the lattice more ductile and susceptible to temperature variation. (C) 2015 Elsevier Ltd. All rights reserved.
Semiconducting BiOCuSe Thermoelectrics and Its Metallic Derivative Bi2YO4Cu2Se2
(2015-05-01)
Chou, Ta-Lei
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Tewari, Girish C.
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Chan, Ting-Shan
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Hsu, Ying-Ya
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Chen, Jin-Ming
ZrCuSiAs-type mixed-anion oxychalcogenides and oxypnictides form a massive family of functional materials readily tailored to host diverse transport properties. Here we investigate semiconducting BiOCuSe, which has been considered as a promising candidate for intermediate-temperature thermoelectric applications, and its intergrowth derivative Bi2YO4Cu2Se2, by using X-ray absorption spectroscopy. Stronger anisotropy and bond covalency are revealed for the latter phase, and in contrast to the simple charge-balance scenario of [Bi3+O2-](+)[Cu+Se2-](-) in the former phase, mixed-valence Cu+/Cu2+ with a single coordination sphere is seen in the selenide block of Bi2YO4Cu2Se2, which gives rise to a metallic electrical conductivity.
Thermoelectric misfit-layered cobalt oxides with interlayers of hydroxide and peroxide species
(2013-12-01)
Chou, Ta-Lei
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Lybeck, Jenni
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Chan, Ting-Shan
;
Hsu, Ying-Ya
;
Tewari, Girish C.
Among the thermoelectric misfit-layered cobalt oxides, [M(m)A(2)O(m+2)]qCoO(2), the parent m=0 phases exhibit divergent chemical features but are less understood than the more common m>0 members of the series. Here we synthesize Sr-for-Ca substituted [(Ca(1-x)Sr(x)c)(z)(O,OH)(2)](q)CoO2 zero phases up to x=0.2 through low-temperature hydrothermal conversion of precursor powders of the m=1 misfit system, [Co(Ca1-xSrx)(2)O-3](q)CoO2. In the zero-phase [(Ca1-xSrx)(z)(O,OH)(2)](q)CoO2 system, as the Sr content x increases the lattice expands anisotropically along the c axis such that the ab-plane dimension and the misfit parameter q remain essentially constant. X-ray absorption spectroscopy data suggest the presence of peroxide-type oxygen species in the (Ca1-xSrx)(z)(O,OH)(2) rock-salt block and together with infrared spectroscopy, thermogravimetric and low-temperature resistivity and thermopower measurements evidence that the isovalent Sr-for-Ca substitution controls the balance between the peroxide and hydroxide species in the (Ca1-x,Sr-x)(z)(O,OH)(2) block but leaves the valence of Co essentially intact in the CoO2 block. The higher electrical conductivity of the Sr-substituted phases is explained as a consequence of increased carrier mobility. (C) 2013 Elsevier Inc. All rights reserved.