Effect of precursor calcination temperature on the microstructure and thermoelectric properties of Ca3Co4O9 ceramics |
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Authors: | Song Chen Xueyan Song Xueqin Chen Yun Chen Ever J Barbero Evan L Thomas Paul N Barnes |
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Institution: | 1. Department of Mechanical and Aerospace Engineering, West Virginia University, Evansdale Drive, Morgantown, WV, 26506, USA 2. Air Force Research Laboratory-WPAFB, Energy Technologies and Materials Division, University of Dayton Research Institute, 300 College Park, Dayton, OH, 45469, USA 3. Army Research Laboratory, 2800 Powder Mill Rd., Adelphi, MD, 20783, USA
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Abstract: | Ca3Co4O9 (CCO) powder precursors were prepared by the chemical sol–gel route and calcined at various temperatures between 923?K (CCO-923?K) and 1,073?K (CCO-1,073?K). The calcination temperature was found to be a critical factor affecting the microstructure and thermoelectric properties of CCO ceramic bulk samples. The grain size increases with calcination temperature. The nano-crystals with size about 100?nm in the powders calcined at 923?K promote large crystal growth and texture development during sintering. Bulk pellets made from CCO-923?K powder have large crystal grains, uniform grain size distribution, and a high degree of crystal alignment. By contrast, pellets made from CCO powders at higher calcination temperatures have a bimodal distribution of large and small grains and a large amount of randomly oriented grains. Transmission electron microscopy analysis shows that each crystal grain (identified in SEM images) consists of bundles of CCO nano-lamellas. The nano-lamellas within one bundle share the same c-axis orientation and have fiber texture. The electrical resistivity of CCO-923?K is weakly dependent on operating temperature. Compared to the CCO-1,073?K sample, the CCO-923?K sample has the highest power factor, a lower thermal conductivity, and higher electrical conductivity. |
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