Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4292−4301
www.acsaem.org
Anisotropic Thermoelectric Performance and Sustainable Thermal Stability in Textured Ca3Co4O9/Ag Nanocomposites Myung-Eun Song,*,†,‡ Heonjoong Lee,†,§ Min-Gyu Kang,∥ Wenjie Li,∥ Deepam Maurya,‡,⊥ Bed Poudel,∥ and Shashank Priya*,∥ †
Center for Energy Harvesting Materials and Systems (CEHMS), ‡Department of Materials Science and Engineering, and Department of Mechanical Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States ∥ Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania 16802, United States Downloaded via GUILFORD COLG on July 31, 2019 at 07:01:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: The Ca3Co4O9 (CCO) with layered structure has been considered as a potential candidate for high-temperature thermal energy harvesting application. However, CCO’s layered structure imparts anisotropy in transport properties, which results in anisotropic thermoelectric performance. So far, limited attempt has been made to understand the anisotropic thermoelectric performance of CCO, which often results in erroneous estimation of the thermoelectric response. Here, we fabricated highly textured CCO/x wt % Ag (x = 0, 1, 3, 5) nanoinclusion composites using the spark plasma sintering (SPS) technique and systematically investigated correlation between microstructure and anisotropic thermoelectric properties. The thermoelectric response was measured along both in-plane and out-of-plane directions (perpendicular and parallel to the pressure axis). We developed a twostep SPS method to achieve enhanced degree of texturing and increased electrical conductivity along ab-planes. The addition of Ag nanoinclusions was found to increase the overall electrical conductivity and thermoelectric power factor due to improved electrical connections among the grains. The peak ZT value for the CCO/3 wt % Ag composites, measured along both perpendicular and parallel directions, was found to be 0.14 and 0.06 at 640 °C, respectively. Almost the same values of resistivity, power factor, and ZT were maintained after repeated thermal cycling. These results reveal that CCO/3 wt % Ag composites have the desired thermal stability, which will make the thermoelectric module reliable for the intended period of operation. KEYWORDS: thermoelectric energy harvesting, Ca3Co4O9 (CCO), Ag nanoinclusions, anisotropy, spark plasma sintering (SPS)
1. INTRODUCTION High-temperature thermal energy harvesting has attracted much attention recently, which requires the development of efficient and robust thermoelectric materials. However, most of the well-known materials with high ZT values, Bi2Te3, PbTe, skutterudites, and half-Heusler alloys, are not thermally stable in air at high temperatures due to the presence of volatile elements and oxidation.1−4 As such, oxide thermoelectric materials are considered as potential candidates for hightemperature thermal energy harvesting due to their robust thermal and chemical stability in oxidizing atmosphere along with the reduced toxicity, easy fabrication, and low cost. Hightemperature thermoelectric generator (TEG) applications and representative materials’ operation temperature ranges in air are shown in Figure 1. Commercial materials such as Bi2Te3 (
