Mechanical properties of oxide materials

Thermoelectric modules composed of eight pairs of p-type Ca₃Co₄O₉ (Co-349) and n-type CaMnO₃ (Mn-113) or LaNiO₃ (Ni-113) bulks were constructed using Ag electrodes, paste, and alumina substrate. In these modules, it was reported that after power generation at 1273K of the hot side heater temperature the Mn-113 legs were broken near the alumina substrate. To make the mechanism of the destruction clear, coefficient of thermal expansion (CTE), three-point bending strength, and microstructure were investigated. The CTE of the Co-349 and Ni-113 bulks is closer than that of the Mn-113 one to the alumina substrate. The three-point bending strength of the Co-349 and Ni-113 bulks are higher than that of Mn-113 one. These are the reasons of the destruction of the Mn-113 legs near the alumina substrate in the modules by heating. Lower relative density of the Mn-113 bulks than the other oxide ones leads low bending strength. The Mn-113 grains, moreover, are grown much more than other oxides. This allows cracks to run easily in the Mn-113 bulks. The stronger bending strength, thermal expansion closer to the substrate, higher relative density, and smaller grains for the Ni-113 and Co-349 bulks than those for the Mn-113 bulks are the reasons of destruction of the latter bulks.