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CaCoO (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 and H ) and elastic modulus ( E and E ) values were shown to increase remarkably with the HP stress level and the anisotropy ratio between out-of-plane and in-plane resistivity values too. As ρ was considerably reduced and the Seebeck coefficient, S, remains constant when σ is raised, the power factor, PF, was greatly improved for the higher stress values. and PF are 64 and 595 μW.m.K, 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. [ABSTRACT FROM AUTHOR]

Bi2Te3‐based materials possess excellent thermoelectric (TE) properties near room temperature. The microstructure's orientations of these materials have a great influence on their properties owing to the lamellar structure. Herein, a process for the preparation of bismuth telluride, named hot‐pressing texture (HPT) where the hot‐pressed (HP) samples are preformed by powder molding, leads to the fine alignment of grains. The orientation factor of the HPT sample is improved by 20%, compared with that of the HP sample. After further carrier concentration optimization, the improved ZT of 1.1 at 350 K is achieved, with an average ZT of 0.9 between 300 and 500 K. Herein, it is shown that the proposed preparation process is an effective strategy for the fabrication of layered materials. [ABSTRACT FROM AUTHOR]

The misfit-layered Ca3Co4O9 oxide is being seen as a potential thermoelectric (TE) candidate for high-temperature power generation in air. Given the very small size and low strength exhibited by single crystals, grain-oriented Ca3Co4O9 ceramics are worth elaborating to capitalize on their anisotropy. However, the usual textured pellets are too thin to probe the TE properties along their principal crystallographic directions. In this paper, we report on the anisotropy of TE properties in the 350–860 K range within thick textured Ca3Co4O9 ceramics fabricated by moderately pressing at 1173 K stacks of pellets primarily textured using spark plasma sintering (SPS), spark plasma texturing (SPT), and hot pressing (HP). The texture was quantitatively assessed, and the influent microstructural parameters were identified, particularly the grain boundary density parallel (GBDc) and perpendicular (GBDab) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBDab and increasing GBDc. This resulted in a resistivity ρab reduction, leading to a marked enhancement in power factor PFab, which reached 520 μW·m−1·K−2 at 800 K for the HP sample. The anisotropy ρc/ρab was substantially promoted as the texture was reinforced and the GBDc/GBDab ratio increased, with ρc/ρab (HP) > ρc/ρab (SPT) > ρc/ρab (SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio Sc/Sab >1 for the SPS-processed materials. This behavior was reversed (Sc/Sab <1) for the more textured SPT and HP specimens. It therefore resulted in a PF anisotropy PFc/PFab (HP) < PFc/PFab (SPT) < PFc/PFab (SPS). The PFab/PFc ratio attained 13.6 at 800 K for the thick HP sample, which is the largest ratio recorded thus far on undoped Ca3Co4O9 ceramics. [ABSTRACT FROM AUTHOR]

Abstract: Multilayer Ca3Co4O9 (349) thick thermoelectric (TE) materials were fabricated by hot-pressing stacked dense and strongly textured single-layer samples. Microstructure and volume quantitative texture investigations were undertaken by using scanning electron microscopy and neutron diffraction techniques, respectively. The results show a bulk density similar to single-layer samples, but remarkable texture strength reinforcement. The electrical resistivity, ρ, and Seebeck coefficient, S, were reproducibly measured in directions parallel (ρ c and S c ) and perpendicular (ρ ab and S ab ) to the mean c-axis. ρ showed a high anisotropy ratio of 13.5 and 8.8 at 300 and 900K, respectively, and ρ ab kept the same values whereas ρ c decreased in the multilayer samples. S ab and S c unexpectedly revealed different values. The thermal conductivity also displayed a significant anisotropy, with ratio at 900K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio . ZT ab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication. [Copyright &y& Elsevier]