Structural and thermal characterization of calcium cobaltite electrospun nanostructured fibers

Electrospinning is a well-established method for synthesizing microdimensional and one-dimensional (1D) fibers from a large variety of precursor solutions. Initially, research focused on the production of polymer nanofibers, but in recent years, a large variety of oxide ceramics have been produced through electrospinning of sol-gel systems. In this study, polycrystalline calcium cobaltite (Ca₃Co₄O₉) fibers of diameter 30-100 nm have been electrospun from sol-gels based on a novel combination of polyvinyl alcohol, cobalt acetate and calcium acetate precursors. X-ray diffraction data have showed that at calcination temperatures of between 250° and 500°C, metastable CoO and CaCO₃ exist as transition phases and that subsequent heating at 650°C converts these phases to calcium cobaltite. Thermal analysis has confirmed this staged calcination mechanism and has also revealed the effect of oxygen starvation on the final structures. Microscopic analysis has confirmed the highly crystalline nature of the oxide fibers, and the presence of highly faceted grains, around 20-40 nm in thickness, as the primary building blocks in these fibers. In addition, the material will only exist in fibrous form if calcination is a staged process, rather than a single, high-temperature process. Bulk calcium cobalt oxide is regarded as a competitive thermoelectric material due to structural complexities associated with a layered structure. Microscopic analysis has shown that this layered structure is preserved when in electrospun nanostructured form and the measured thermopower in these nanostructures is at least 30% higher than that reported from bulk material at 300 K.