Abstract
Thermoelectric (TE) performance of Ca3Co4O9 (CCO) has been investigated extensively via a doping strategy in the past decades. However, the doping sites of different sublayers in CCO and their contributions to the TE performance remain unrevealed because of its strong correlated electronic system. In this work, Sr and Ti are chosen to realize doping at the [Ca2CoO3] and [CoO2] sublayers in CCO. It was found that figure of merit (ZT) at 957 K of Ti-doped CCO was improved 30% than that of undoped CCO whereas 1 at% Sr doping brought about a 150% increase in ZT as compared to undoped CCO. The significant increase in electronic conductivity and the Seebeck coefficient are attributed to the enhanced carrier concentration and spin-entropy of Co4+ originating from the Sr doping effects in [Ca2CoO3] sublayer, which are evidenced by the scanning electron microscope (SEM), Raman, Hall, and X-ray photoelectron spectroscopy (XPS) analysis. Furthermore, the reduced thermal conductivity is attributed to the improved phonon scattering from heavier Sr doped Ca site in [Ca2CoO3] sublayer. Our findings demonstrate that doping at Ca sites of [Ca2CoO3] layer is a feasible pathway to boost TE performance of CCO material through promoting the electronic conductivity and the Seebeck coefficient, and reducing the thermal conductivity simultaneously. This work provides a deep understanding of the current limited ZT enhancement on CCO material and provides an approach to enhance the TE performance of other layered structure materials.
Highlights
IntroductionJ Adv Ceram 2020, 9(6): 769–781 respectively. The most well-studied commercial TE materials with high ZT values, like Bi–Te [2], Pb–Te [3], and half-heusler alloys [4], are unstable at higher temperatures due to their volatile/toxic elements and oxidation, resulting in the degradation of TE performance at high temperatures
The need for sustainable and clean energy is moreJ Adv Ceram 2020, 9(6): 769–781 respectively
Refs. [14,15] suggested that doping at Ca site largely decreases the concentration of Co4+ and improves the spin-entropy enhancement, which further increases the Seebeck coefficient
Summary
J Adv Ceram 2020, 9(6): 769–781 respectively. The most well-studied commercial TE materials with high ZT values, like Bi–Te [2], Pb–Te [3], and half-heusler alloys [4], are unstable at higher temperatures due to their volatile/toxic elements and oxidation, resulting in the degradation of TE performance at high temperatures. Doping is one of the most effective strategies to enhance the TE performance by manipulation of carrier concentration. Doping at [Ca2CoO3] or [CoO2] layer will affect the concentration of Co3+/Co4+ to change the spin-entropy contribution. [14,15] suggested that doping at Ca site largely decreases the concentration of Co4+ and improves the spin-entropy enhancement, which further increases the Seebeck coefficient. The combined TE performance and spin-entropy contribution effects by doping at Co site in [CoO2] layer of CCO system have been rarely reported so far. We chose two typical elements Sr and Ti to realize the manipulation of doping at the desired site in [Ca2CoO3] and [CoO2] sublayers and investigated the enhancement of TE performance through the induced Co spin-entropy via the doping effects of Sr and Ti within two sublayers. The highest ZT value of 0.38 at 957 K was obtained in a CCO sample with Sr doping level of 0.1, which increased ZT of ~150% than the pristine CCO sample with ZT of 0.15 at 957 K
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