Anisotropy of Transport Properties Correlated to Grain Boundary Density and Quantified Texture in Thick Oriented Ca3Co4O9 Ceramics

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 rho(ab) reduction, leading to a marked enhancement in power factor PFab, which reached 520 mu W.m(-1).K-2 at 800 K for the HP sample. The anisotropy rho(c)/rho(ab) was substantially promoted as the texture was reinforced and the GBD(c)/GBD(ab) ratio increased, rho(c)/rho(ab) (HP) > rho(c)/rho(ab) (SPT) > rho(c)/rho(ab) with (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 PFc/PFab (HP) < PFc/PFab (SPT) < PFc/PFab(SPS) The PFab/PFc ratio attained 13.6 at 800 K for the thick HP sample, which is the largest ratio recorded thus far on undoped Ca3Co4O9 ceramics.