Spin–state transition in the layered barium cobaltite derivatives and their thermoelectric properties

A I Klyndyuk,S V Shevchenko,E A Chizhova

doi:10.15826/chimtech.2020.7.1.04

A I Klyndyuk, S V Shevchenko + Show 1 more

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https://doi.org/10.15826/chimtech.2020.7.1.04

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Journal: Chimica Techno Acta	Publication Date: Mar 25, 2020
Citations: 1	License type: CC BY 4.0

Affiliation: Belarusian State Technological University

Abstract

Ba1.9Me0.1Co9O14 (Me = Ba, Sr, Ca) (BCO) layered cobaltites were prepared by means of solid-state reactions method. Crystal structure, microstructure, thermal expansion, electrical conductivity, and thermo-EMF for the obtained oxides were studied; the values of their linear thermal expansion coefficient, activation energy of electrical transport, and power factor values were calculated. It was found that BCO are p-type semiconductors, in which the spin-state transition occurs within 460-700 K temperature interval due to change in spin state of cobalt ions, which accompanied the sharp increase in electrical conductivity, activation energy of electrical conductivity, and linear thermal expansion coefficient, while thermo-EMF coefficient decreased. Partial substitution of barium by strontium or calcium in BCO leads to the increase in spin-state transition temperature and electrical conductivity of the samples, and, at the same time, thermo-EMF coefficient; consequently, their power factor values decrease.

Highlights

Layered sodium, calcium, or bismuth– calcium cobaltites (NaxCoO2, Ca3Co4O9, Bi2Ca2Co1.7Ox) are prospective materials for producing of p-branches for high-temperature thermoelectric generators, since they possess high values of electrical conductivity (σ) and thermo-EMF coefficient (S) and low thermal conductivity (λ), as well as high stability at elevated temperatures in air [1,2,3,4]
The unit cell parameters for Ba1.9M0.1Co9O14 (M = Sr, Ca) solid solutions do not differ much from those reported for undoped Ba2Co9O14 (Table 1), despite of the large difference in ionic radii between dopants and barium, which is probably due to the small substitution degree of strontium or calcium (x = 0.1) in these solid solutions
Crystallites of BCO ceramics had a plate-like form, which is typical for ceramics of layered cobaltites; their sizes varied within 2–5 μm and the thickness changed within 0.5–1 μm (Fig. 1)

Summary

Introduction

Calcium, or bismuth– calcium cobaltites (NaxCoO2, Ca3Co4O9, Bi2Ca2Co1.7Ox) are prospective materials for producing of p-branches for high-temperature thermoelectric generators, since they possess high values of electrical conductivity (σ) and thermo-EMF coefficient (S) and low thermal conductivity (λ), as well as high stability at elevated temperatures in air [1,2,3,4]. The common elements of crystal structure, which are present in all of these compounds, are the conducting [CoO2] layers. Layered barium cobaltite Ba2Co9O14, which belongs to the Ba2n+1ConO3n+3(Co8O8) series [5], was tested recently as a possible thermoelectric oxide [6]. Crystal structure of Ba2Co9O14 consists of alternated [CoO2] layers (CdI2–type) and octahedral trimers Co3O12 that are interconnected by cornershared CoO4 tetrahedra [6]. Layered barium cobaltite is stable in air up to 1303 K; it decomposes to CoO and BaCoO2 at higher temperatures [7]

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