Research Article
Enhanced thermoelectric performance of spark plasma sintered p-type Ca3−xYxCo4O9+δ systems

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Abstract

Highly textured dense Ca3−xYxCo4O9+δ (0 ≤ x ≤ 0.3) samples were fabricated by combining sol−gel process with spark plasma sintering (SPS). Y3+ substitution for Ca2+ simultaneously increased the Seebeck coefficient and reduced the thermal conductivity. The latter was attributed to the increase in lattice anharmonicity, structural distortion, and grain boundary area, which enhanced the phonon scattering. Ca2.7Y0.3Co4O9+δ showed the largest dimensionless figure-of-merit (ZT = 0.194) at 1073 K because it had the largest Seebeck coefficient and the lowest thermal conductivity. This ZT value was 55 % larger than that of undoped Ca3Co4O9 (0.125 at 1073 K). Therefore, Y3+ substitution, sol−gel powder synthesis, and SPS are highly effective for enhancing the thermoelectric properties of Ca3Co4O9.

Introduction

Thermoelectric technology can directly convert waste heat into electricity without moving parts and releasing greenhouse gases, and thus is a promising route of harvesting electricity from waste heat [[1], [2], [3]]. To date, thermoelectric devices still have limited applications mainly because of their low energy-conversion efficiency. The energy-conversion efficiency of thermoelectric materials is determined in terms of a dimensionless figure-of-merit, defined as ZT = (σα2/κ)T, where σ, α, κ, and T are the electrical conductivity, Seebeck coefficient, thermal conductivity, and absolute temperature, respectively [4,5]. Thermoelectric materials should possess high σ, high α, and low κ to obtain a high ZT. However, obtaining a high ZT is extremely difficult because σ, α, and κ values are strongly interdependent, that is, improving one parameter adversely affects another parameter. For example, increasing the carrier concentration to enhance the electrical conductivity leads to a low Seebeck coefficient and high thermal conductivity.

Ca3Co4O9 has recently attracted significant attention because of its high σ, high α, and low κ [6], which are indispensable for thermoelectric materials. Partial substitution of cations in Ca3Co4O9 is highly effective in enhancing ZT [[7], [8], [9]]. Prasoetsopha et al. [7] fabricated a highly dense Ca3Co4−xCrxO9+δ (0 ≤ x ≤ 0.2) through spark plasma sintering (SPS) using thermal hydrodecomposition-processed powders. Cr substitution increased the electrical resistivity and Seebeck coefficient and reduced the thermal conductivity. The highest ZT (0.19) was obtained for Ca3Co3.85Cr0.15O9+δ at 1073 K. Constantinescu et al. [8] fabricated Ca3−xSrxCo4O9+δ (0 ≤ x ≤ 0.1) through conventional solid-state reaction. The electrical conductivity and Seebeck coefficient increased when the Sr content increased up to 0.07. The highest power factor (3.0 × 10−4 W m−1 K−2) was obtained for Ca2.93Sr0.07Co4O9+δ at 1073 K, which was ∼50 % higher than that for undoped Ca3Co4O9. Porokhin et al. [9] fabricated Ca3−xNaxCo4O9−xFx (0 ≤ x ≤ 0.6) through conventional two-step solid-state reaction followed by SPS. The ZT values of Ca3Co4O9 and Ca2.55Na0.45Co4O8.55F0.45 at 873 K were ∼0.11 and ∼0.13, respectively. Liu et al. [10,11] prepared Y-doped Ca3−xYxCo4O9+δ (x = 0, 0.15, and 0.3) and Y/Gd co-doped Ca3−xyYxGdyCo4O9+δ by combining the polyacrylamide gel method with SPS. Y substitution enhanced the Seebeck coefficient and electrical resistivity due to the decrease in carrier concentration and reduced the thermal conductivity due to the increase in impurity scattering. Among the prepared samples, the largest ZT (0.26) was obtained for Ca2.7Y0.15Gd0.15Co4O9+δ at 973 K.

To extend the applications of Ca3Co4O9 as high temperature thermoelectric materials and to investigate the detailed structural and thermoelectric properties of Ca3−xYxCo4O9+δ thermoelectric materials, we performed a differential thermal analysis (DTA)/thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), UV‒visible‒near infrared (UV‒Vis‒NIR) spectroscopy, X-ray diffraction (XRD), XRD Rietveld refinement, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Hall effect measurement, and thermoelectric property measurements. Furthermore, to understand the dependence of powder synthesis process on the thermoelectric properties, the ZT values of the Ca2.7Y0.3Co4O9+δ fabricated using sol−gel processed powders were compared with those fabricated using commercial powders. Here, Ca3−xYxCo4O9+δ (0 ≤ x ≤ 0.3) was fabricated by combining sol−gel process and SPS. Subsequently, the structural and thermoelectric properties of Ca3−xYxCo4O9+δ as a function of Y3+ content were investigated, and the ZT values of the Ca2.7Y0.3Co4O9+δ fabricated using sol−gel processed powders and commercial powders were also compared.

Section snippets

Experimental

Sol−gel method was utilized to prepare Ca3−xYxCo4O9+δ (x = 0, 0.1, 0.2, and 0.3) powders. Ca(NO3)2·xH2O (99.9 %, High Purity Chemicals Co., Japan), Co(NO3)2·6H2O (99.9 %, High Purity Chemicals Co., Japan), Y(NO3)3·6H2O (99.9 %, Alfa Aesar Co., Russia), citric acid (C6H8O7; 99.5 %, Duksan Pure Chemicals Co., Korea), and polyethylene glycol (PEG) 400 (99 %, Duksan Pure Chemicals Co., Korea) were used as starting materials. The starting materials were weighed according to the designed composition.

Structural properties

The simultaneous DTA/TGA curves of mixed Ca3−xYxCo4O9+δ (0 ≤ x ≤ 0.3) powders are shown in Fig. 1. The TGA curve of undoped Ca3Co4O9 powders shows a small weight loss in the temperature range of 298 − 878 K, which is attributed to the dehydration of the powders [14]. The temperature corresponding to the dehydration of the powders slightly increases with increasing Y3+ content, that is, 878, 881, 882, and 886 K for x = 0, 0.1, 0.2, and 0.3 powders, respectively. Upon further heating for the

Conclusion

The substitution of Y3+ for Ca2+ in nominal composition Ca3Co4O9 increased the solid-state reaction temperature, thereby leading to the decrease in the grain size of sintered Ca3−xYxCo4O9+δ samples with increasing Y3+ content. The sintered Ca3−xYxCo4O9+δ formed a single monoclinic phase and showed high densities and textured plate-like grains. High Y3+ content yielded low electrical conductivity and large Seebeck coefficient mainly because of the decrease in hole concentration. The Seebeck

Acknowledgement

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1B03031196).

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