Synthesis and thermoelectric properties of Bi_2.5Ca_2.5Co₂O_x layered cobaltites

Abstract

Bi-Ca-Co-O polycrystalline materials with a layered structure were prepared. The synthesis of the sintered specimens from two starting compositions, Bi 1.8 Ca 2 Co 2 O x and Bi 2.5 Ca 2.5 Co 2 O x , revealed the latter is preferentially formed at high temperature (850°C). The increase in sintering time was shown by growth of large platelike grains (up to 50 m in diameter and several micrometers in thickness). The reaction mechanisms during the heat treatment and the preferential formation of the Bi 2.5 Ca 2.5 Co 2 O x phase were observed by x-ray diffraction, thermogravimetrydifferential thermal analysis, and scanning electron microscopy. These techniques supposed the presence of a liquid phase at high temperature, origin of a highly kinetic phase formation, and the growth of large grains. Interestingly the liquid phase reaction promotes an efficient stacking and sliding of grains during hot-forging treatment, and highly (00l) oriented materials were prepared. A relationship between thermoelectric performance, texture strength, and microstructure is clarified.

Cerámicas termoeléctricas Bi₂Ca₂Co_1.7O_x texturadas mediante fusión zonal flotante inducida por láser

“…As it can be easily seen, the ρ(T) curve exhibits a semiconducting behavior from high (400 K) to low temperature (5 K) independently of the growth rate. This semiconducting behavior is in agreement with earlier reports on this cobaltite system (8,14). The resistivity values decrease when lower growth speeds are used, as a consequence of the better grain alignment and the larger grain sizes.…”

“…at 15 mm/h, is about 60 mΩ.cm, which is lower than the resistivities reported for the sintered specimens (6,8), which show resistivities around 80-100 mΩ.cm. Figure 5 shows the variation of the thermopower with the temperature, of the as-processed rods.…”

“…This application can be achieved using thermoelectric oxide ceramics. This is the main reason of the intense research work performed, after the discovery of large thermoelectricity in Na x CoO 2 (1) The crystal structure of these layered cobaltites is composed of an alternate stacking of a common conductive CdI 2 -type CoO 2 layer with a two-dimensional triangular lattice and a block layer, composed of insulating rock-salt-type (RS) layers (6)(7)(8). The two RS and CoO 2 layers have common a and c axes, while the b-axis lengths of the two layers are different.…”

Mejora de las propiedades termoeléctricas de Bi₂Sr₂Co_1.8O_x por métodos de síntesis en disolución

“…Crystallographically, these cobaltites are composed of two different layers, with an alternate stacking of a common conductive CdI 2 -type CoO 2 layer, with a two-dimensional triangular lattice, and an insulating block layer, with the rocksalt-type (RS) structure. The two sublattices (RS block and CdI 2 -type CoO 2 layer) possess common a-and c-axis lattice parameters and b angles but different b-axis length, causing a misfit along the b-direction [6][7][8] that gives their common name: misfit cobaltites.…”

Neutron diffraction texture analysis and thermoelectric properties of BiCaCoO misfit compounds

“…This misfit system exhibits a large thermopower TEP 300 ~150 µV/K and a rather low resistivity value [5][6][7]. Due to its high structural anisotropy, the alignment of plate-like grains by mechanical and/or chemical processes is necessary to attain macroscopic properties comparable to the intrinsic crystallographic ones.…”