Volume Texture and Anisotropic Thermoelectric Properties in Ca3Co4O9 Bulk Materials

Abstract

Ca3Co4O9lamellar thermoelectric (TE) oxides are potentially suitablefor energy conversion applications at high temperature in air. To be used in TE devices, bulk Ca3Co4O9 materialsmust possess large texture strengths to attain the optimal TE performances, and sufficient material size to fabricate TE ceramic elements (legs). This paper reports the development of thick Ca3Co4O9 multilayer samples by hot-pressing a stack of dense and strongly textured Ca3Co4O9bulk single-layer samples along their mean c*-axis. The volume texture was investigated by using the neutron diffraction method, and revealed the largest strength achieved hitherto in such materials. The Seebeck coefficient, S, electrical resistivity,ρ, and thermal conductivity,κ, were determinedparallel (Sc,ρc, κc) and perpendicular (Sab,ρab, κab) to the mean c*-axis. ρ and κ were found to be highly anisotropic, with ratios ρc/ρab = 8.8 and κ ab/ κ c = 2.7 at 900K, respectively.S unexpectedly depicted an anisotropic character as well, but it remains less prominent. The power factor, PF, and figure-of-merit, ZT, werehence anisotropic, exhibiting respective ratios PFab/PFc = 12 and ZTab/ZTc= 4.6. The ZT abvalue reached 0.16 at 900K which is 2 fold the one of conventionally sintered materials.