Calcium cobaltite Ca₃Co_4-xO_9+δ (CCO) is a promising p-type thermoelectric material with good functional properties for high-temperature applications in air. The material exhibits strong anisotropic properties, making texturing and nanostructuring mostly favored to improve thermoelectric performance. Electrospinning is a suitable and cost-effective method to fulfill these demands. In this work, flat-shaped CCO nanofibers designated as nanoribbons were electrospun and the resulting nanoribbons mats were further processed into a textured Ca₃Co_4?xO_9+δ ceramic. In previous work, we have shown that mats composed of a mixture of nanoribbons and cylindrical nanofibers exhibit good thermoelectric properties [1]. However, it is expected that the thermoelectric properties of the compacted sample can be further improved if mats consisting only of flat nanoribbons are used. Primarily because nanoribbons are considered to provide more efficient packing due to their flatness compared to cylindrical nanofibers, allowing higher densification in the green body and ceramic, but also because of their potential contribution to sample texturing since the primary particles in the nanoribbons are assumed to be oriented flat. Therefore, we investigated the influence of electrospinning conditions and precursor composition on the microstructure of the electrospun material to obtain pure nanoribbons mats. We found a strong dependence of nanoribbon formation on the polymer concentration in the electrospinning precursor and discuss the possible formation mechanisms. We also point out an important step in the calcination process of the nanoribbons to achieve texturization of the primary particles. Finally, we have fabricated green bodies and ceramics from nanoribbons mats and investigated their texturing using X-ray diffraction with measurement of pole figures of the (0002) lattice planes, referring to the 4-dimensonal superspace group of the C-face centered monoclinic aperiodic crystal structure, and scanning electron microscopy cross-sectional images. In addition, we measured the Seebeck coefficient and electrical conductivity of the nanoribbon-based samples to evaluate their thermoelectric abilities.

Reference
[1] Kruppa K, Maor II, Steinbach F, et al. Electrospun Ca₃Co_4−xO_9+δ nanofibers and nanoribbons: Microstructure and thermoelectric properties. J Am Ceram Soc. 106 (2023) 1170-1181. https://doi.org/10.1111/jace.18842