Anisotropy of the Mechanical and Thermoelectric Properties of Hot-Pressed Single-Layer and Multilayer Thick Ca_3Co_4O_9 Ceramics

摘要

Ca_3Co_4O_9 (349) thermoelectric (TE) ceramics were prepared by hot-pressing (HP) process under various stress levels up to 30 MPa. Microstructure investigations have revealed strong enhancements of the bulk density and the texture strength, and a remarkable decrease of the in-plane grain boundary density as the HP stress, σ, is increased. The mechanical properties obtained from nanoindentation and three-point bending tests, and the TE properties were correlated to the microstructure. The influence of the HP stress level on these properties was examined in the parallel (c) and perpendicular (ab) directions to the pressing axis. Hardness (H~(ab) and H~c) and elastic modulus (E~(ab) and E~c) values were shown to increase remarkably with the HP stress level and the anisotropy ratio between out-of-plane and in-plane resistivity values (ρ~c/ρ~(ab)) too. As ρ~(ab) was considerably reduced and the Seebeck coefficient, S~(ab) , remains constant when a is raised, the power factor, PF~(ab) , was greatly improved for the higher stress values. (ρ~c/ρ~(ab)) and PF~(ab) 900K are 64 and 595 μW.m~(-1).K~(-2), respectively, for the HP samples processed under 30 MPa. Thick specimens usable in practical devices were obtained by HP stacked single layers. Their microstructure was investigated and correlated to the TE and mechanical properties.