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

The lamellar thermoelectric (TE) oxides Ca3Co4O9 (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 (σR) and fracture toughness (KIc) were shown to drastically enhance versus the pressures PHP and PSPS 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 PHP=30 MPa which depicted (nHxy=2.1±0.4 GPa, Exy=56±4 GPa) and (nHz=2.3±0.2 GPa, Ez=85±5 GPa) in its respective planes perpendicular and parallel to the pressing axis, revealing an anisotropy of the elastic modulus. It presented σR=251±12 MPa and KIc=2.3±0.4 MPa m1/2. Although the lower size of the intrinsic flaws was found for the HP materials, the largest mechanical characteristics were achieved by SPS under PSPS≥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.