Layered thermoelectric materials : misfit cobalt oxides and oxyselenides

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School of Chemical Technology | Doctoral thesis (article-based) | Defence date: 2015-12-08
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Authors

Date

2015

Department

Kemian laitos
Department of Chemistry

Major/Subject

Mcode

Degree programme

Language

en

Pages

76 + app. 82

Series

Aalto University publication series DOCTORAL DISSERTATIONS, 181/2015

Abstract

Because of the growing awareness of environmental and energy-related issues, harvesting waste heat by thermoelectric materials is getting increasing attention. A decent thermoelectric material is typically a heavily doped semiconductor with high thermopower and low thermal conductivity. To simultaneously achieve these characteristics, a complex structure is an essence. Inspired by the artificial superlattices various multi-layered compounds with a repetitive stacking of conducting and blocking layers which enables to decouple electrical and thermal transport are currently investigated as candidates to approach the ideal "phonon glass, electron crystal" scenario. Metal oxides feature advantages like availability and thermal stability in air at elevated temperatures yet poor carrier mobility without appropriate doping. Replacing oxygen with less electronegative anions, for instance chalcogens, would turn the framework more covalent and lead to readily improved electrical conduction. This dissertation presents several layered misfit cobalt oxides and oxyselenides as thermoelectrics. To begin with, double-interlayered phases [Sr2O2]0.52CoO2 and [Ca1.7(OH)2]0.58CoO2 were successfully made under compression and investigated by x-ray absorption spectroscopy (XAS) and thermogravimetry (TG) which unveil subtle local geometry, electronic structure and defects embedded in the lattice. Given that a mixed-anion oxide-chalcogenide framework might effectively benefit the electrical transport, some layered oxyselenides, such as BiOCuSe, Bi2MO4Cu2Se2 (M = Y or early rare earth element) and AE2CoO2Cu2Se2 (AE = alkaline earth element) which in common possess an anti-fluorite-type [Cu2Se2] sub-layer, were then investigated. Temperature-dependent extended x-ray absorption fine structure (EXAFS) is found to be a powerful probe to resolve the lattice characteristics of BiOCuSe upon finite doping. Meanwhile, Cu vacancies and alkaline-earth dopants are found to affect distinctively in the BiOCuSe matrix. Bi2MO4Cu2Se2 compounds, unfortunately, are metals with poor thermopower. The origin of metallicity is readily evidenced in the XAS analysis. Lastly the Sr2CoO2Cu2Se2 p-type semiconductor was visited. Computational work suggests there is no way to overlook the contribution from the oxide sub-layer at the valence band maximum (VBM). Replacing Sr with Ba, in contrast to the Ca-for-Sr substitution case, would more efficiently raise the thermoelectric power factor. It is quite convincing once an appropriate doping scheme is employed, either in the oxide or the selenide layers, the Sr2CoO2Cu2Se2 oxyselenides could be a promising option for intermediate-temperature thermoelectrics.

Description

Julkaistu vain painettuna, saatavuus katso Bibid. Published only in printed form, availability see Bibid

Supervising professor

Karppinen, Maarit, Prof., Aalto University, Department of Chemistry, Finland

Keywords

thermoelectrics, misfit cobalt oxides, oxyselenides, x-ray absorption spectroscopy

Other note

Parts

  • [Publication 1]: Ta-Lei Chou, Ting-Shan Chan, Jin-Ming Chen, Hisao Yamauchi & Maarit Karppinen, X-ray absorption spectroscopy study of parent misfit-layered cobalt oxide [Sr2O2]qCoO2, J. Solid State Chem. 202 (2013) 27-32.
    DOI: 10.1016/j.jssc.2013.03.016 View at publisher
  • [Publication 2]: Ta-Lei Chou, Jenni Lybeck, Ting-Shan Chan, Ying-Ya Hsu, Girish C. Tewari, Eeva-Leena Rautama, Hisao Yamauchi & Maarit Karppinen, Thermoelectric misfit-layered cobalt oxides with interlayers of hydroxide and peroxide species, J. Solid State Chem. 208 (2013) 109-115.
    DOI: 10.1016/j.jssc.2013.10.012 View at publisher
  • [Publication 3]: Ta-Lei Chou, Girish C. Tewari, Ting-Shan Chan, Ying-Ya Hsu, Hisao Yamauchi & Maarit Karppinen, EXAFS study of thermoelectric BiCuOSe: Effects of Cu vacancies, Solid State Commun. 206 (2015) 12–16.
    DOI: 10.1016/j.ssc.2015.01.007 View at publisher
  • [Publication 4]: Ta-Lei Chou, Girish C. Tewari, Divya Srivastava, Atsushi Yamamoto, Ting-Shan Chan, Ying-Ya Hsu, Jin-Ming Chen, Hisao Yamauchi & Maarit Karppinen, Efficacies of dopants in thermoelectric BiOCuSe, submitted
  • [Publication 5]: Ta-Lei Chou, Girish C. Tewari, Ting-Shan Chan, Ying-Ya Hsu, Jin-Ming Chen, Hisao Yamauchi & Maarit Karppinen, Semiconducting BiOCuSe thermoelectrics and its metallic derivative Bi2YO4Cu2Se2, Eur. J. Inorg. Chem. (2015) 2574-2578.
  • [Publication 6]: Ta-Lei Chou, Otto Mustonen, Tripurari S. Tripathi & Maarit Karppinen. Isovalent Ca and Ba substitutions in thermoelectric layer-structured oxyselenide Sr2CoO2Cu2Se2, submitted.
    DOI: 10.1088/0953-8984/28/3/035802 View at publisher

Citation

Permanent link to this item

https://urn.fi/URN:ISBN:978-952-60-6506-9

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[diss] Kemian tekniikan korkeakoulu / CHEM