Anisotropy of Transport Properties Correlated to Grain Boundary Density and Quantified Texture in Thick Oriented...

The misfit-layered Ca₃Co₄O₉ oxide is being seen as a potential thermoelectric (TE) candidate for high-temperature power generation in air. Given the very small size and low strength exhibited by single crystals, grain-oriented Ca₃Co₄O₉ ceramics are worth elaborating to capitalize on their anisotropy. However, the usual textured pellets are too thin to probe the TE properties along their principal crystallographic directions. In this paper, we report on the anisotropy of TE properties in the 350⁻860 K range within thick textured Ca₃Co₄O₉ ceramics fabricated by moderately pressing at 1173 K stacks of pellets primarily textured using spark plasma sintering (SPS), spark plasma texturing (SPT), and hot pressing (HP). The texture was quantitatively assessed, and the influent microstructural parameters were identified, particularly the grain boundary density parallel (GBD^c) and perpendicular (GBD^ab) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBD^ab and increasing GBD^c. This resulted in a resistivity ρ^ab reduction, leading to a marked enhancement in power factor PF^ab, which reached 520 μW·m^-1·K^-2 at 800 K for the HP sample. The anisotropy ρ^c/ρ^ab was substantially promoted as the texture was reinforced and the GBD^c/GBD^ab ratio increased, with ρ^c/ρ^ab (_HP) > ρ^c/ρ^ab (_SPT) > ρ^c/ρ^ab (_SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio S^c/S^ab >1 for the SPS-processed materials. This behavior was reversed (S^c/S^ab <1) for the more textured SPT and HP specimens. It therefore resulted in a PF anisotropy PF^c/PF^ab (_HP) < PF^c/PF^ab (_SPT) < PF^c/PF^ab (_SPS). The PF^ab/PF^c ratio attained 13.6 at 800 K for the thick HP sample, which is the largest ratio recorded thus far on undoped Ca₃Co₄O₉ ceramics.