Fabrication and Characterization of Ba1/3CoO2 Epitaxial Films Exhibiting Thermoelectric ZT = 0.12 at Room Temperature

Yugo Takashima,Bin Feng,Hiromichi Ohta,Yuichi Ikuhara,Jiake Wei,Yu-Qiao Zhang,Hai Jun Cho

doi:10.23919/am-fpd49417.2020.9224485

Abstract

Among many thermoelectric materials, oxide-based materials are attractive in terms of environmental compatibility. Layered cobaltite represented Na <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</inf> CoO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> shows a rather large thermoelectric power factor along to the layers. However, the thermal conductivity ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$K$</tex> ) is rather high (~7 W m −1 K −1 ), and therefore, the thermoelectric figure of merit ZT is small (~0.03) at room temperature. Here we show that Ba 2+ ion substitution with Na + ion in Na <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</inf> CoO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> significantly reduces the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$K$</tex> while keeping the large power factor, resulting in the large enhancement of ZT. We fabricated the epitaxial films of Na <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</inf> CoO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> by the reactive solid-phase epitaxy method and performed the ion-exchange treatment from Na + to Ba 2+ . The room temperature electrical conductivity (σ), thermopower ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$S$</tex> ), and power factor of the c-axis oriented Ba <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</inf> CoO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> films along the in-plane were 2500 S cm −1 , +72 µV K −1 , and 1.3 mW m −1 K- 2 , respectively. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$K$</tex> along the in-plane, which was clarified by measuring the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$K$</tex> of the c-axis inclined (55°) film, was 3.3 W m −1 K −1 . The ZT (= S 2 ·σ· T·K −1 ) along the in-plane was 0.12 at room temperature, which is highest among the oxide thermoelectric materials ever reported.

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