Development of nanostructured cobaltite ceramic fibres for thermoelectric applications

Abstract

The aim of this work has been to develop a better understanding of the structure-property-processing relationships associated with calcium cobaltite, when produced as nanostructured fibres via the sol gel electrospinning process. Nanostructured metal oxides are becoming increasingly studied because of their potential as energy conversion or energy storage materials. In particular, calcium cobaltite is of interest due to its promising thermoelectric capabilities that are believed to be enhanced when engineered in the nanoscale. Good thermoelectric properties are obtained through a combination of high thermopower and electrical conductivity, and low thermal conductivity. This work details various strategies adopted to enhance thermoelectric properties through structural modification on the nanolevel. Thermal analysis has been used to establish the calcination mechanisms that are required when converting ceramic doped polymer nanofibres into ceramic oxide structures, and a multi-stage calcination regime has been developed to ensure that the resulting material is fibrous. Also, when the calcination process is starved of atmospheric oxygen, the conversion mechanisms are retarded, but not changed. Varying sol parameters such as mixing regime, sol viscosity, acetate-to-polymer ratio and polymer molecular weight have all revealed significant influence on fibre diameter, grain size, fibre integrity and fibre yield. In this study, the sol gel consists of cobalt acetate, calcium acetate and polyvinyl alcohol (PVA) dissolved in ethanol and deionized water. The effect of varying processing parameters such as applied field voltage, flow rate and collector distance on fibre morphology has been studied with little variation in fibre diameter with applied voltage, but a strong positive correlation between diameter and flow rate. A degree of fibre alignment has been achieved through the use of non-conducting Teflon strips adhered to the collector mechanism. Initial thermopower measurements have shown that in nanofibres form, higher thermopower is achieved than in the bulk and the room temperature value of 160{u00B5}V/K is the highest so far reported with this system. When the sol is modified through the addition of hydrogen peroxide, several interesting features and changes have occurred in the material. Firstly, the degree of crystallinity of the PVA has markedly increased, and this has led to better crystallinity and better grain definition in the calcined fibres. The average oxidation state of the cobalt ions has increased, and this has led to improved thermopower values. However, this improvement is somewhat negated by lower electrical conductivity caused by better grain definition. The characteristic layered structure seen in bulk material that is so critical to thermoelectric performance is seen in the nanofibres, but only if calcination temperature is around 650{u00B0}C. This study has shown that there are myriad processing parameters that can influence the structure-property relationships in this material, but when in nanofibres form, and when peroxide treated, thermoelectric performance is better than in bulk equivalents and is a very promising avenue for further study. Show more