Mechanical properties of Ca3Co4O9 bulk oxides intended to be used in thermoelectric generators

Abstract The lamellar thermoelectric (TE) oxides Ca 3 Co 4 O 9 (Ca349) are promising candidates for energy conversion in a temperature range of 300–1200 K in air. To be well-suited for being integrated in TE devices, Ca349 bulk materials must show high mechanical reliability to withstand the assembly constrains and in-service conditions. In the aim of optimizing TE performances of these materials, specimens were elaborated by using Hot-Pressing (HP) and Spark Plasma Sintering (SPS). Indentation measurements were operated on these ceramics using both micro-hardness testing and depth-sensing nano-indentation. Fracture characteristics were assessed by 3 point bending tests. Nano-hardness ( nH ), elastic modulus ( E ), strength ( σ R ) and fracture toughness ( K Ic ) were shown to drastically enhance versus the pressures P HP and P SPS applied during HP and SPS treatments, respectively, which is ascribed to higher densification and, to a lesser extent, to the texture strengthening and grain boundary density decrease in the direction perpendicular to the pressing axis. The contribution to micro-hardness ( µH ) of both later factors was estimated to ~30% for the hot-pressed sample under P HP =30 MPa which depicted ( nH xy =2.1±0.4 GPa, E xy =56±4 GPa) and ( nH z =2.3±0.2 GPa, E z =85±5 GPa) in its respective planes perpendicular and parallel to the pressing axis, revealing an anisotropy of the elastic modulus. It presented σ R =251±12 MPa and K Ic = 2.3±0.4 MPa m 1/2 . Although the lower size of the intrinsic flaws was found for the HP materials, the largest mechanical characteristics were achieved by SPS under P SPS ≥50 MPa. The elastic recovery H / E of Ca349 ceramics was found among the highest ratios reported for oxides and one order of magnitude larger compared to the half-Heusler or skutterudites potential TE materials.