Colin Norman

Thermoelectric Power Generation from Lanthanum Strontium Titanium Oxide at Room Temperature through the Addition of Graphene.

The applications of strontium titanium oxide based thermoelectric materials are currently limited by their high operating temperatures of >700 °C. Herein, we show that the thermal operating window of lanthanum strontium titanium oxide (LSTO) can be reduced to room temperature by the addition of a small amount of graphene. This increase in operating performance will enable future applications such as generators in vehicles and other sectors. The LSTO composites incorporated one percent or less of graphene and were sintered under an argon/hydrogen atmosphere. The resultant materials were reduced and possessed a multiphase structure with nanosized grains. The thermal conductivity of the nanocomposites decreased upon the addition of graphene, whereas the electrical conductivity and power factor both increased significantly. These factors, together with a moderate Seebeck coefficient, meant that a high power factor of ∼2500 μWm(-1)K(-2) was reached at room temperature at a loading of 0.6 wt % graphene. The highest thermoelectric figure of merit (ZT) was achieved when 0.6 wt % graphene was added (ZT = 0.42 at room temperature and 0.36 at 750 °C), with >280% enhancement compared to that of pure LSTO. A preliminary 7-couple device was produced using bismuth strontium cobalt oxide/graphene-LSTO pucks. This device had a Seebeck coefficient of ∼1500 μV/K and an open voltage of 600 mV at a mean temperature of 219 °C.

A rationale for the development of thermally stable nanostructured CeO2-ZrO2-containing mixed oxides

CeO2-ZeO2 solid solutions are extensively used as oxygen storage promoters in the current automotive three-way catalysts. High thermal stability of the textural properties is one of the most important requirements for practical application since temperatures up to 1273 K are easily experienced by these materials under real working conditions. In the present paper, we investigated how hydrothermal treatments applied to cakes of doped and undoped ZrO2-rich CeO2-ZrO2 precursors might improve the thermal stability of the final CeO2-ZrO2 solid solution. A rationale was developed that allowed to correlate the morphology of the hydrothermally treated cake with the thermal stability at 1273 K of the final product, which did not depend on the composition of the mixed oxides.

Hydrothermal crystallisation of doped zirconia: An in situ X-ray diffraction study

Energy-dispersive (X-ray) diffraction has been used on a third generation synchrotron source to study, in situ, the hydrothermal crystallisation of zirconia from the pre-cursor hydroxide under autoclave conditions. Because the sample and enclosure are highly absorbing, it is necessary to collect data at high energy so that meaningful Avrami-kinetics parameters can be determined over the temperature range of 200–250 °C. A clear lowering of the activation energy for crystallisation is observed when using cerium-doped materials.

Tuning the thermoelectric properties of A-site deficient SrTiO3 ceramics by vacancies and carrier concentration

Ceramics based on Sr0.8La0.067Ti0.8Nb0.2O3-δ have been prepared by the mixed oxide route. The La1/3NbO3 component generates ∼13.4% A-site vacancies; this was fixed for all samples. Powders were sintered under air and reducing conditions at 1450 to 1700 K; products were of high density (>90% theoretical). Processing under reducing conditions led to the formation of a Ti1-xNbxO2-y second phase, core-shell structures and oxygen deficiency. X-ray diffraction (XRD) confirmed a simple cubic structure with space group Pm3[combining macron]m. Transmission electron microscopy revealed a high density of dislocations while analytical scanning transmission electron microscopy at atomic resolution demonstrated a uniform distribution of La, Nb and vacancies in the lattice. X-ray photoemission spectroscopy and thermogravimetry showed the oxygen deficiency (δ value) to be ∼0.08 in reduced samples with enhanced carrier concentrations ∼2 × 1021 cm-3. Both carrier concentration and carrier mobility increased with sintering time, giving a maximum figure of merit (ZT) of 0.25. Selective additional doping by La or Nb, with no additional A site vacancies, led to the creation of additional carriers and reduced electrical resistivity. Together these led to enhanced ZT values of 0.345 at 1000 K. The contributions from oxygen vacancies and charge carriers have been investigated independently.

‘Hydroxide’ precursor to zirconia: extended X-ray absorption fine structure study

The short-range structure of zirconium hydroxide, an amorphous precursor in the synthesis of zirconia-based ceramics, is considered. New EXAFS data are combined with existing chemical information on this hydroxide to produce a structural model based on two-dimensional linking of tetrameric units involving a combination of both hydroxo- and oxo-bonds and bound water molecules.

Chapter 3:Thermoelectric Oxides

Thermoelectric oxides are of increasing interest as they are produced from relatively abundant, low-cost and low-toxicity materials. The production of the precursors and the ceramic fabrication methods are discussed with reference to the effect on final properties. The different oxide systems that are in common use are then described. Finally, some of the future challenges for wide-scale commercialization of these materials are outlined.