Textured Ca3Co4O9 thermoelectric oxides by thermoforging process

Abstract

Using thermoforging process, dense Ca3Co4O9 (Co349) thermoelectric oxides have been successfully textured. The various parameters influencing the formation of the Co349-textured material have been investigated. The electrical transport measurements show an anisotropy of the resistivity in good agreement with scanning electron microscopy observations. Texture is quantified by neutron-diffraction measurements and correlated to anisotropic resistivity measurements and Seebeck coefficients.

Thermoelectric Ca3Co4O9 ceramics consolidated by Spark Plasma sintering

“…The maximum of the distribution near 4.9 m.r.d. on the {001} pole figure for both samples is comparable to the average orientations observed so far in Co349 ceramics [18,19,21,35]. The minimum of density is nearly 0, indicating that no other orientation component is present in the samples.…”

“…However one cannot exclude a priori an effect of its larger crystallite distribution, as seen in the broader centre of its {001} pole figure compared to SPS-1123K. The positive Seebeck coefficient S value is an indication [3,11,21] of the hole mediated conduction in Ca 3 Co 4 O 9 .…”

“…Single crystal composite templating [18], reactive template grain growth [19], hot pressing [20], sinter-forging [11,21], and magnetic field [22] are also used to densify and/or to induce an orientation between grains. In addition, the novel Spark Plasma Sintering (SPS) technique has been developed and commonly used nowadays to prepare various materials [23][24][25][26][27].…”

Thermoelectric properties of Ca3Co4O9 and Ca2.8Bi0.2Co4O9 thin films in their island formation mode

“…Owing to this fact, a large amount of research has been carried out on improving the TE properties of randomly oriented polycrystalline Ca 3 Co 4 O 9 ceramics, but with very limited improvement. [2][3][4][5] Therefore, several efforts have been made to improve the texture of Ca 3 Co 4 O 9 ceramic by various methods, such as spark plasma sintering, 6 magnetic alignment, 7 and hot-forging processes, 8 owing to a general belief that the grain boundaries negatively affect the electrical conductivity and hence the TE properties of this system. The epitaxial growth and c-axis-oriented films fabricated using the pulsed laser deposition (PLD), 9,10 radio-frequency magnetron sputtering, 11 and the topotactic ion exchange method 12 have proven to be successful in improving the TE properties of this system.…”

Synthesis and characterization of <i>n</i>-type and <i>p</i>-type thermoelectric oxides

“…3a) gives evidence that to increase the electrical conductivity and the power factor an improvement of the connectivity and orientation of the grains is required. One of the possible approaches to reduce the porosity of the sintered materials and align the grains is the hot-forging, and in general it was shown that the performance of piezoelectric [14], ferroelectric [15] and thermoelectric [16] oxide ceramics is greatly enhanced by the hot-forging technique. Another possible approach to obtain aligned grains was widely applied to superconducting ceramics and it is called "melt textured growth" (MTG) [17].…”