Enhanced p-type thermoelectric ceramics by combined doping and asymmetric structuring using electrospun nanoribbons and spark plasma sintering

• Applicant: Professor Dr. Armin Feldhoff
• Subject Area: Synthesis and Properties of Functional Materials ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 536531583

The misfit layered cobaltite Ca3Co4O9 has emerged as a promising p-type thermoelectric material for high-temperature energy harvesting from heat sources. The grains of the material exhibit strong anisotropic properties, making texturing and nanostructuring mostly favoured to improve thermoelectric performance. To unfold the full potential of Ca3Co4O9, it is aimed to utilize its orientation-dependent thermoelectric properties in highly textured, nanostructured ceramics combined with selective doping on the calcium site. The approach involves electrospinning of flat nanoribbons to achieve nanostructure and preferred texturing. To improve the electrical conductivity of the thermoelectric ceramic in a preferred direction, primary nanoparticles are aligned in nanoribbons prior to sintering. Perfect stacking of the nanoribbons is achieved by controlled drying and collapse of the fibers into ribbons on the collector. This ensures good contact between the ribbons, thus improving the electrical conductivity. Subsequent spark plasma sintering (SPS) is used to obtain ceramics with high-density and small, nanosized, oriented grains, which is favourable to decreasing the thermal conductivity. During the SPS process, metallic spacers are used to control carbon contamination and preserve oxygen content and thermoelectric properties. Selective doping on the Ca site is done with two to three elements simultaneously (co-doping) to improve the power factor and electrical conductivity while decreasing the thermal conductivity. Obtained microstructures of nanoribbons and sintered ceramics are characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy combined with electron diffraction and elemental analysis via X-ray spectroscopy and electron energy-loss spectroscopy. Functional characterization covers the thermoelectric figure of merit zT from room-temperature to 1173 K in air based on measurements of direction-dependent Seebeck coefficient, electrical conductivity, and thermal conductivity. It is expected to obtain Ca3Co4O9 ceramics with anisotropically improved thermoelectric properties at high temperatures in air.

• DFG Programme: Research Grants
• International Connection: Israel
• International Co-Applicant: Professor Dr. Gideon Grader