Richard Donelson

Correlation of oxygen vacancy concentration and thermoelectric properties in Na0.73CoO2−δ

In this work, oxygen vacancies were precisely controlled by manipulating oxygen partial pressure and the oxygen partial pressure dependence of thermoelectric properties of polycrystalline Na0.73CoO2−δ was investigated. The results suggest that resistivity and thermopower augment simultaneously with increasing concentration of oxygen vacancies δ due to the recombination of electrons and holes and average Co valence state. The total thermal conductivity can be reduced by minimizing δ. The dimensionless figure of merit was strongly affected by δ at lower operating temperature. The experimental results provide guideline for the variation in performance of this material system under different processing and operating environments.

Tunable thermopower and thermal conductivity in Lu doped In2O3

Lu-doped polycrystalline (In1−xLux)2O3 (x = 0, 0.025, 0.05, 0.10, 0.15) materials were prepared by co-precipitation method followed by spark plasma sintering processing. The large and heavy Lu3+ was strategically selected to replace In3+ in order to lower the thermal conductivity while minimizing changes to the electronic structure of In2O3. The resulting samples had an average grain size of 200–400 nm at a density of ∼90% of the theoretical mass density. Lu doping was found to increase the thermopower of In2O3, however the electrical resistivity was also increased, primarily due to the remarkable decrease in carrier concentration. The thermal conductivity of In2O3 was significantly reduced by the substitution of In by Lu.

A Spherical Lens for the SKA

Spherical refracting lenses based upon the Luneburg lens offer unique capabilities for radioastronomy, but the large diameter of lens required for the Square Kilometre Array (SKA) means that traditional lens materials are either too dense or too lossy. We are investigating a composite dielectric that theoretically offers extremely low loss and low density, and is suitable for low-cost mass production. We describe our progress towards realising this material and demonstrating the manufacturing concept, via the manufacture and testing of a small (0.9 m) spherical lens.