Karl Whittle

Investigation of the conditions required for quantitative isotopic enrichment and the acquisition of quantitative¹⁷O NMR spectra of ceramic oxides.

Nanoparticle (NP) aggregation is typically investigated in either quiescent or turbulent mixing conditions; neither is fully representative of dynamic natural environments.

The current generation of nuclear reactors are evolutionary in design, mostly based on the technology originally designed to power submarines, and dominated by light water reactors. The aims of the Generation IV consortium are driven by sustainability, safety and reliability, economics, and proliferation resistance. The aims are extended here to encompass the ultimate and universal vision for strategic development of energy production, the “perpetuum mobile”—at least as close as possible. We propose to rethink nuclear reactor design with the mission to develop an innovative system which uses no fresh resources and produces no fresh waste during operation as well as generates power safe and reliably in economic way. The results of the innovative simulations presented here demonstrate that, from a theoretical perspective, it is feasible to fulfil the mission through the direct reuse of spent nuclear fuel from currently operating reactors as the fuel for a proposed new reactor. The produced waste is less burdensome than current spent nuclear fuel which is used as feed to the system. However, safety, reliability and operational economics will need to be demonstrated to create the basis for the long term success of nuclear reactors as a major carbon free, sustainable, and applied highly reliable energy source.

The use of solid-state NMR spectroscopy to provide new insight into the composition and atomic-scale disorder in pyrochlore-based ceramics produces a very different picture to that obtained using other techniques.

Pyrochlores based on the general composition CaLnZrNbO₇ (where Ln = La, Nd, Sm, Gd and Ho) have been prepared, and irradiated through the crystalline-amorphous transition, with 1 MeV Kr ions at the IVEM-TANDEM user facility. The obtained critical temperatures show a decrease from La to Gd (∼680 K to ∼230 K), with Ho being resistant to amorphisation at 50K. The results suggest that the amorphisation cross section for these materials is directly related to the Ln component.

Ceramics based on the general compositions CaLnXNbO7 (where Ln = La, Nd and Sm, and X=Zr and Sn) have been prepared, and irradiated with 1 MeV Kr ions at the IVEM-TANDEM user facility. The radiation tolerance of these materials has been found to be less than Zr and Hf equivalents. The results also suggest that the amorphisation cross section for these materials is related to the Ln component, and is similar to those observed for Zr and Hf equivalents.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Synthesis of Pb(Zr_1–xTi_x)O₃ (PZT) on a single substrate using a high-throughput molecular-beam epitaxy technique was demonstrated. In situ synthesis of crystalline PZT at elevated substrate temperatures could not be achieved, as reevaporation of Pb (PbO) occurred and the partial pressure of O₂ was insufficient to prevent formation of a PbPt_x phase during deposition. Instead, ex situ postdeposition annealing was performed on PZT deposited at room temperature. Dense single phase PZT was prepared with a compositional range of 0.1 > x > 0.9, for film thicknesses up to 800 nm. Transmission electron microscopy revealed the grain size increased from 50 nm to ∼0.5 μm with increasing Zr-concentration and became more columnar. Raman, x-ray diffraction, and scanning electron microscopy/energy dispersive spectroscopy results revealed a morphotropic phase boundary between rhombohedral and tetragonal phases occurred at x ∼0.4 rather than at x = 0.47 in bulk ceramics. This was attributed to clamping arising from mismatch in thermal expansion between the film and substrate.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Raman spectroscopy has been used to study the ferroelectric tetragonal to paraelectric cubic phase transition Tc in undoped BaTiO3 (BT) and (Ba0.92A0.08)TiO3 [where A=Ca (BCT), Sr (BST), and Pb (BPT)]. In BT and BPT, mode characteristic of the tetragonal phase at 303 and 710cm−1 persisted until >50°C above Tc, indicating the presence of local dynamic polar clusters well above the displacive phase transitions, ∼120 and ∼149°C, respectively. However, in BCT and BST, the Raman spectra exhibited only broad second order bands above Tc, suggesting that Ca and Sr, A-site dopants supress local dynamic polar clusters.

High-throughput synthesis of the ferroelectric solid solution Pb(Zr1−xTix)O3 (PZT) on single Pt∕Ti∕SiO2∕Si substrates was demonstrated using a modified molecular beam epitaxy (MBE) system. The PZT films exhibited a phase transition from rhombohehdral to tetragonal symmetry as a function of Zr:Ti ratio, across the substrate diagonal. This was consistent with the presence of a morphotropic phase boundary at Zr:Ti ratio of 0.64:0.36, different from the value of 0.53:0.47 observed for bulk ceramics. All points on the films exhibited ferroelectric hysteresis. Results demonstrate the feasibility of high-throughput MBE for deposition of complex ferroelectric oxides, and pave the way for further materials discovery.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Pollucite, CsAlSi₂O₆, a Cs polymorph of leucite (KAlSi₂O₆), has been proposed for ceramic immobilization of ¹³⁵Cs and ¹³⁷Cs fission products. ¹³³Cs NMR of both low‐ (tetragonal) and high‐temperature (cubic) forms of pollucite exhibit a considerable distribution of local Cs environments. ²⁹Si and ²⁷Al NMR data from directly prepared pollucite show greater Al/Si disorder than either leucite, or pollucite produced by ion exchange. Little evidence for Cs motion is observed in tetragonal or cubic pollucite, and only at high temperatures (∼850°C) is any substantial dynamic behavior detected. Dynamic NMR lineshape calculations allow a determination of the frequency of Cs motion and diffusivity.

Nanoscale structural features of amorphous zircon and pyrochlore produced by self-irradiation induced amorphization have been characterized by small-angle X-ray scattering (SAXS). Electron density fluctuations were observed in the untreated samples and studied as a function of annealing temperature (up to 1300 °C). In untreated zircon, density fluctuations were found to have a characteristic length-scale of approximately 1 nanometer diameter. A clear scattering maximum develops at ∼3 nm^–1 as a function of the degree of damage, but only for amorphous fractions higher than 70%. For the untreated pyrochlore sample such a strong signal was not observed in the scattering function. As annealing temperature increases, phase segregation is observed in zircon with a clear nucleation at 850 °C of zirconia grains. The SAXS contribution, however, points towards a lower nucleation temperature, ca. 700 °C. In the pyrochlore structure, at temperatures as low as 300 °C the appearance of a first crystalline phase, Ca₂Nb₂O₇, is observed, with the subsequent crystallization of TiO₂ at 800 °C. Observed electron density variations are compared between the two structures and discussed in terms of nanoscale phase segregation phenomena.

The sequence of ferroelectric phase transitions (Pm3̅m—P4mm—Amm2) in BaTiO₃ has been studied as a function of temperature (298 K–130 K) at a fixed pressure (3.2 GPa). The same sequence of transitions is observed as for ambient pressures, but the transition temperatures are reduced. The spontaneous strains of the non-cubic phases are determined. Whilst the structure of the tetragonal phase at high pressure is qualitatively similar to the ambient structure, both the spontaneous strain and the Ti displacement are smaller in the high pressure structure than the ambient structure.