Development and characterisation of electrospun calcium cobaltites for high temperature thermoelectric applications

Abstract

As demand for energy increases, there will be greater need for more efficient energy generating technologies. Much of the energy lost in current generation systems is through heat. Thermoelectricity provides one way to recover waste heat and convert it into useful energy, but the efficiency of commercially-available thermoelectric devices is relatively low, especially at temperatures approaching 800 C. Therefore, advanced materials for use in high-temperature thermoelectric devices are necessary. This thesis examines the thermoelectric properties of electrospun layered calcium cobaltites. Layered cobaltites have high Seebeck coefficients, good electrical conductivity, and poor thermal conductivity, leading to a good figure of merit, ZT (the key measure of thermoelectric efficiency). Cobalt oxides are safe and thermally robust compared to other thermoelectric materials, but ZT values for polycrystalline cobalt oxides are somewhat lower than the accepted benchmark of ZT {u0303} 1. Nanostructuring has the potential to improve the efficiency of thermoelectric materials, so calcium cobaltites were made using the electrospinning technique. Sol-gels of calcium and cobalt acetates dissolved in solutions of polyvinyl alcohol and deionized water were used for electrospinning. After electrospinning, the as-spun material was converted into a fibrous metal-oxide through a three-stage calcination process that heated the material up to 650 C. These samples were then pressed into pellets for thermoelectric characterization. Thermogravimetric analysis clarified that three-stage calcination with {u0303}10-20 C/min heating rates might help produce nanostructures. However, the three-stage calcination procedure was no more effective than single-stage calcination for producing fibrous calcium cobaltites at slower heating rates (e.g. 3 C/min). Calcination at 650 C produced CaxCoO2 with a layered structure similar to NaxCoO2. When calcined up to 800 C, misfit-layered Ca3Co4O9 was produced. The thermoelectric properties of calcium cobaltites made by bulk sol-gel processing and electrospun samples treated with H2O2 were compared to the standard electrospun CaxCoO2 and Ca3Co4O9. The effects of the helium testing environment were also investigated. H2O2 treatment and high-temperature helium appeared to reduce carrier concentration, which was not expected from H2O2 treatment. The Ca3Co4O9 samples exhibited superior ZT values between 0.08 (untreated electrospun) and 0.11 (H2O2-treated) with the sol-gel materials exhibiting ZT {u0303} 0.09, rivalling current polycrystalline Ca3Co4O9. The CaxCoO2 samples had consistent ZT values between 0.02 (sol-gel) and 0.03 (electrospun). Ca-substituted CaxCoO2 was investigated using lithium, sodium, strontium, barium, neodymium, and erbium as dopants. Each dopant improved the thermoelectric properties compared to undoped CaxCoO2. 10% neodymium and erbium substitution accelerated the calcination process, leading to possible Ca3Co4O9 formation at 650 C and improved ZT values of 0.064 and 0.072 at 690 K, respectively. Strontium at 10% concentration improved the ZT to 0.063 at 573 K, likely through lattice strain effects. These results suggest CaxCoO2 could be very efficient given the right composition and processing parameters. The thermal conductivity of every sample was relatively low (between 0.1 and 0.6 W/m/K), with rare-earth doped and peroxide-treated cobaltites exhibiting the lowest thermal conductivity. Electrospun calcium cobaltites are promising thermoelectric materials with potential for high efficiency, and will be crucial for producing safe, efficient energy generation technologies in the future. Show more