Gordon J. Thorogood

Zn-doped TiO2 electrodes in dye-sensitized solar cells for enhanced photocurrent

Porous nanocrystalline Zn/TiO2 materials were synthesized employing agarose gel as a template, achieving a maximum Zn-doping concentration of around 2 at.%. Low Zn-doping concentrations (e.g. <1 at.%) improved the photocurrent of the dye-sensitized solar cell when these materials were applied as the working electrode, as n-type doping favors electron transportation. Compared to the efficiency of the undoped sample, 6.7%, the 0.5 at.% sample showed a much higher efficiency of 7.6%. Further doping decreases the performance of the device as it introduces deficiencies that act as electron–hole recombination centers.

Structural phase transitions and magnetic order in SrTcO3

The structure of the perovskite SrTcO(3) has been investigated using both synchrotron X-ray and neutron powder diffraction. At room temperature SrTcO(3) is orthorhombic as a consequence of cooperative tilting of the corner sharing TcO(6) octahedra. The tilts are sequentially removed as the sample is heated with the oxide displaying the sequence of structres Pnma→Imma→I4/mcm→Pm ̅3m. Neutron powder diffraction data collected in the temperature range 4-1023 K indicate that SrTcO(3) has G-type antiferromagnetic structure, in which each Tc moment is antiparallel to its six nearest neighbours, below ∼1000 K. The magnetic structure is collinear antiferromagnetic with the technetium moments parallel to c-axis and can be described by the propagation vector k = [0,0,0] and the basis vector (0,0,A(z)). The same magnetic structure is observed in each of the four crystal structures.

Formation and stability of Pb-, Zn- & Cu-PO₄ phases at low temperatures : implications for heavy metal fixation in polar environments

Low temperatures and frequent soil freeze-thaw in polar environments present challenges for the immobilisation of metals. To address these challenges we investigated the chemical forms of Pb, Zn and Cu in an Antarctic landfill, examined in vitro reaction kinetics of these metals and orthophosphate at 2 and 22 °C for up to 185 days, and subjected the products to freeze-thaw. Reaction products at both temperatures were similar, but the rate of production varied, with Cu-PO(4) phases forming faster, and the Zn- and Pb-PO(4) phases slower at 2 °C. All metal-orthophosphate phases produced were stable during a 2.5 h freeze-thaw cycle to -30 °C. Metal immobilisation using orthophosphate can be successful in polar regions, but treatments will need to consider differing mineral stabilities and reaction rates at low temperatures.

High-temperature study of CaZrTi2O7

Abstract Previously reported anomalous thermal expansion effects in the 1200–1500°C range for hot-pressed CaZrTi 2 O 7 were shown to be due to irreversible bloating effects, from occluded gases. X-ray diffraction and differential thermal analysis of ordered CaZrTi 2 O 7 did not reveal evidence of a solid-state transformation at temperatures up to 1450°C.

Antiferromagnetism in a Technetium Oxide. Structure of CaTcO3

The technetium perovskite CaTcO(3) has been synthesized. Combining synchrotron X-ray and neutron diffraction, we found that CaTcO(3) is an antiferromagnetic with a surprisingly high Neel temperature of ∼800 K. The transition to the magnetic state does not involve a structural change, but there is obvious magnetostriction. Electronic structure calculations confirm the experimental results.

Thermal expansion coefficients of zirconolite (CaZrTi2O7) and perovskite (CaTiO3) from X-ray powder diffraction analysis

Abstract The thermal expansion coefficients of zirconolite, both ordered and disordered, and perovskite, were measured by X-ray powder diffraction analysis in the temperature range 25–1200°C. The principal results are: (1) The mean thermal expansion coefficients ( K −1 ) of ordered zirconolite are:α a = (11.35±0.25) × 10 −6 ;α b = (8.72± 0.22) × 10 −6 ;α c = (10.0 ±0.5) × 10 −6 . (2) The mean thermal expansion coefficients ( K −1 ) of disordered zirconolite areα ∥ = (9.89±0.15) × 10 −6 in the (001) plane and α ⊥ = (9.37±0.14) × 10 −6 normal to (001). (3) The mean thermal expansion coefficients ( K −1 ) of perovskite are:α a = (7.86±0.30) × 10 −6 ;α b = (13.46±0.17) × 10 −6 ;α c = (16.55±0.26) × 10 −6 .

Novel K rattling: A new route to thermoelectric materials?

We have performed ab initio molecular dynamics simulations to study the alkali-metal dynamics in the Al-doped (KAl0.33W1.67O6 and RbAl0.33W1.67O6) and undoped (KW2O6 and RbW2O6) defect pyrochlore tungstates. The K atoms exhibit novel rattling dynamics in both the doped and undoped tungstates while the Rb atoms do not. The KAl0.33W1.67O6 experimental thermal conductivity curve shows an unusual depression between ∼50 K and ∼250 K, coinciding with two crossovers in the K dynamics: the first at ∼50 K, from oscillatory to diffusive, and the second at ∼250 K, from diffusive back to oscillatory. We found that the low-temperature crossover is a result of the system transitioning below the activation energy of the diffusive dynamics, whereas the high-temperature crossover is driven by a complex reconstruction of the local potential around the K atoms due to the cage dynamics. This leads to a hardening of the K potential with increasing temperature. This unusual reconstruction of the potential may have important im...

Synthesis and Characterisation of Ln 2 TiO 5 Compounds

Bulk samples of six Ln 2 TiO 5 compounds with Ln = La, Pr, Nd, Eu, Gd and Tb were prepared and characterised. Most of the samples have a phase purity of ∼95% (based on BEI and EDS) with the predominant secondary phase primarily being Ln 2 Ti 2 O 7 . Using XRD, TEM selected area diffraction and high resolution imaging techniques, we have confirmed the results of previous studies which showed that at room temperature Pr 2 TiO 5 , Nd 2 TiO 5 , Eu 2 TiO 5 and Tb 2 TiO 5 have orthorhombic structures with Pnma symmetry. The structure of Tb 2 TiO 5 was further monitored as a function of temperature. The relevance of Ln 2 TiO 5 compounds to advanced nuclear fuel cycles is discussed.

The Effect of a Rapid Heating Rate, Mechanical Vibration and Surfactant Chemistry on the Structure–Property Relationships of Epoxy/Clay Nanocomposites

The role of processing conditions and intercalant chemistry in montmorillonite clays on the dispersion, morphology and mechanical properties of two epoxy/clay nanocomposite systems was investigated in this paper. This work highlights the importance of employing complementary techniques (X-ray diffraction, small angle X-ray scattering, optical microscopy and transmission electron microscopy) to correlate nanomorphology to macroscale properties. Materials were prepared using an out of autoclave manufacturing process equipped to generate rapid heating rates and mechanical vibration. The results suggested that the quaternary ammonium surfactant on C30B clay reacted with the epoxy during cure, while the primary ammonium surfactant (I.30E) catalysed the polymerisation reaction. These effects led to important differences in nanocomposite clay morphologies. The use of mechanical vibration at 4 Hz prior to matrix gelation was found to facilitate clay dispersion and to reduce the area fraction of I.30E clay agglomerates in addition to increasing flexural strength by over 40%.

Novel rattling of K atoms in aluminium-doped defect pyrochlore tungstate

Rattling dynamics have been identified as fundamental to superconductivity in defect pyrochlore osmates and aluminium vanadium intermetallics, as well as low thermal conductivity in clathrates and filled skutterudites. Combining inelastic neutron scattering (INS) measurements and ab initio molecular dynamics (MD) simulations, we use a new approach to investigate rattling in the Al-doped defect pyrochlore tungstates: AAl0.33W1.67O6 (A = K, Rb, Cs). We find that although all the alkali metals rattle, the rattling of the K atoms is unique, not only among the tungstates but also among the analogous defect osmates, KOs2O6 and RbOs2O6. Detailed analysis of the MD trajectories reveals that two unique features set the K dynamics apart from the rest, namely, (1) quasi one-dimensional local diffusion within a cage, and (2) vibration at a range of frequencies. The local diffusion is driven by strongly anharmonic local potentials around the K atoms exhibiting a double-well structure in the direction of maximum displacement, which is also the direction of local diffusion. On the other hand, vibration at a range of frequencies is a consequence of the strong anisotropy in the local potentials around the K atoms as revealed by directional magnitude spectra. We present evidence to show that it is the smaller size rather than the smaller mass of the K rattler which leads to the unusual dynamics. Finally, we suggest that the occurrence of local diffusion and vibration at a range of frequencies in the dynamics of a single rattler, as found here for the K atoms, may open new possibilities for phonon engineering in thermoelectric materials.