Abstract
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 (GBDc) and perpendicular (GBDab) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBDab and increasing GBDc. This resulted in a resistivity ρab reduction, leading to a marked enhancement in power factor PFab, which reached 520 μW·m−1·K−2 at 800 K for the HP sample. The anisotropy ρc/ρab was substantially promoted as the texture was reinforced and the GBDc/GBDab ratio increased, with ρc/ρab (HP) > ρc/ρab (SPT) > ρc/ρab (SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio Sc/Sab >1 for the SPS-processed materials. This behavior was reversed (Sc/Sab <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.
Highlights
Thermoelectric (TE) materials, which have the ability to directly convert between thermal and electrical energy, offer a unique solution to sustainable power generation from various waste heat sources [1,2,3]
We have explored in recent works [25,26,27,28] the spark plasma sintering (SPS) method with the aim of processing textured Ca3 Co4 O9 ceramic pellets toward the promotion of current flow in their in-plane (i.e., (a,b) planes) and, subsequently, reducing the corresponding electrical resistivity ρab
The achieved PFab values remained significantly lower than the ones we reported for Ca3 Co4 O9 ceramics textured by the hot pressing (HP) process, which was mainly derived from a stronger texture and a larger (a,b) plane mean grain size that yielded much more ρab reduction [24,28]
Summary
Thermoelectric (TE) materials, which have the ability to directly convert between thermal and electrical energy, offer a unique solution to sustainable power generation from various waste heat sources [1,2,3]. Some or all of its TE properties are anisotropic, thereby fostering TE performance following a specific crystallographic direction To capitalize on such anisotropic character, grain-oriented Ca3 Co4 O9 ceramics are worth elaborating [24], especially as single crystals would be too expensive to grow and shape. We have explored in recent works [25,26,27,28] the spark plasma sintering (SPS) method with the aim of processing textured Ca3 Co4 O9 ceramic pellets toward the promotion of current flow in their in-plane (i.e., (a,b) planes) and, subsequently, reducing the corresponding electrical resistivity ρab This was shown to result in an enhancement of the power factor PFab = Sab2 /ρab compared to the naturally sintered (NS) ceramics. The induced anisotropy of transport properties as well as of power factor was correlated to the texture strength and the grain boundary density and compared to that of the reported textured materials
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