If you're interested in any of our papers, please click the [download] or [request preprint] link after each abstract. If you request a reprint, it will be emailed to you immediately. If you prefer to email me to discuss any of our work, please do get in touch (see contact details in the sidebar).

Dynamic strain propagation in nanoparticulate zirconia refractory

Dynamic strain propagation in nanoparticulate zirconia refractory

ME Jones, S Fearn, R Winter, F Yuan, AR Lennie, JE Parker, SP Thompson, CC Tang
J Appl Cryst 48 (2015) 386

Residual and intrinsic strains in granular materials have been studied extensively. However, understanding the dynamic strains that cause these resultant residual strains is key to developing better strain-resistant materials. This investigation demonstrates a method for characterizing dynamic strain propagation in granular materials. The specimen is a zirconia based refractory composed of sol-gel-derived zirconia nanoparticles in a potassium silicate glass binder. In situ synchrotron X-ray powder diffraction in flat-plate geometry is used to characterize the sample structure on timescales of the order of 1ms. A 125W CO2 laser is used to strain the sample with a 25ms pulse length. To compensate for the poor flux on this timescale, a pump-probe method is repeated 1000 times and the resulting data are subsequently re-binned to improve statistics. A Gaussian weighting function is also used to introduce better contrast between strained and unstrained frames. TOPAS Academic is used for fitting with a Le Bail model in batch mode. Lattice parameters and sample height are refined during fitting, along with a Lorentzian line width for extracting microstrain broadening. Microstrains, epsilon, in the range of 1.01% < epsilon < 1.46% are reported on a 1ms timescale. [request reprint]

Yttria-zirconia coatings studied by grazing-incidence small-angle x-ray scattering during in situ heating

Yttria-zirconia coatings studied by grazing-incidence small-angle x-ray scattering during in situ heating

K Hoydalsvik, T Barnardo, R Winter, S Haas, D Tatchev, A Hoell
Phys Chem Chem Phys 12 (2010) 14492

The morphology of sol-gel derived dip-coated yttria-doped zirconia films containing variable amounts of yttria has been studied using in situ grazing-incidence small-angle X-ray scattering (GISAXS) whilst heated incrementally to 1000C. A procedure to analyse in situ GISAXS data has been devised which allows a quantitative analysis of time-dependent GISAXS data tracing processes such as chemical reactions or manufacturing procedures. To achieve this, the relative positions of the Yoneda peak and the through beam are used to fix the vertical q scale when the sample thickness is subject to fluctuations due to chemical reactions or deposition processes. A version of Beaucage\'s unified model with a structure factor from Hosemann\'s model for paracrystals describes the yttria-zirconia film data best. It is interpreted in terms of particles forming from a polymeric gel network and subsequently agglomerating into larger units subject to Ostwald ripening as both size and average separation distance of the scattering objects increase. The sample with the highest yttria content shows progressive surface roughening from 850C which may indicate the onset of chemical segregation. [request reprint]

In-situ double ASAXS of the sintering and calcination of sol-gel prepared YSZ ceramics

In-situ double ASAXS of the sintering and calcination of sol-gel prepared YSZ ceramics

T Barnardo, K Høydalsvik, R Winter, CM Martin, GF Clark
J Phys Chem C 113 (2009) 10021

Yttria stabilised zirconia xerogels are heat treated to temperatures of 1000oC, and the evolving structures are analysed using anomalous small-angle X-ray scattering (ASAXS) across two absorption edges. Homogeneous nanocrystals (~10nm in size) begin to grow at temperatures above 390oC for pure YSZ, whilst the presence of a silica matrix inhibits nucleation until 780oC. Anomalous effects haveshown the zirconia plays the dominant role in crystal growth, and varying concentrations of yttria do not affect the nucleation temperatures, or average size of the individual particles. The difference between scattering factors, necessary for accurate ASAXS analysis, is also investigated. Relations for this analysis were derived from X-ray absorption spectra - conducted as part of the small-angle scattering investigations. [request reprint]

Li conductivity of nanocrystalline Li4Ti5O12 prepared by a sol-gel method and high-energy ball milling

Li conductivity of nanocrystalline Li<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> prepared by a sol-gel method and high-energy ball milling

W Iwaniak, J Fritzsche, M Zukalová, R Winter, M Wilkening, P Heitjans
Defect and Diffusion Forum 289 (2009) 565

Spinel-type structured Li4+xTi5O12 (0<x<3) is actually one of the most promising anode materials for Li ion batteries. In its nanostructured form it is already used in some commercially available Li ion batteries. As was recently shown, Li diffusivity in microcrystalline Li4+xTi5O12 with x=0 is rather slow. In the present contribution the Li conductivity in nanocrystalline samples of the electronic insulator Li4Ti5O12 prepared by different routes is investigated using impedance spectroscopy. The mean crystallite size of the samples is about 20 nm. The ionic conductivity of nanocrystalline Li4Ti5O12 obtained by mechanical treatment is higher by about two orders of magnitude compared to that found for a material which was prepared following a sol-gel method. The latter resembles the behaviour of the microcrystalline sample with an average particle size in the μm range rather than that of a nano-crystalline ball milled one with a mean crystallite size of about than 20nm. The larger conductivity of the ball milled sample is ascribed to a much higher defect density generated when the particle size is reduced mechanically. [request reprint]

Determination of the optical band-gap energy of cubic and hexagonal boron nitride using luminescence excitation spectroscopy

Determination of the optical band-gap energy of cubic and hexagonal boron nitride using luminescence excitation spectroscopy

DA Evans, AG McGlynn, BM Towlson, M Gunn, D Jones, TE Jenkins, R Winter, NRJ Poolton
J Phys: Cond Mat 20 (2008) 075233

Using synchrotron-based luminescence excitation spectroscopy in the energy range 4-20eV at 8K, the indirect Γ-Χ optical band-gap transition in cubic boron nitride is determined as 6.36±0.03eV, and the quasi-direct band-gap energy of hexagonal boron nitride is determined as 5.96±0.04eV. The composition and structure of the materials are self-consistently established by optically detected x-ray absorption spectroscopy, and both x-ray diffraction and Raman measurements on the same samples give independent confirmation of their chemical and structural purity: together, the results are therefore considered as providing definitive measurements of the optical band-gap energies of the two materials. [request reprint]

Laser-heated high-temperature NMR: A time-resolution study

Laser-heated high-temperature NMR: A time-resolution study

R Winter, AR Jones, RN Shaw-West, M Wolff, P Florian, D Massiot
Appl Magn Reson 32 (2007) 635

The time resolution achievable in in-situ high-temperature nuclear magnetic resonance experiments is investigated using laser heating of refractory materials. Three case studies using 27Al in alumina nanoparticles, 29Si in silicon carbide and 23Na in a glass-forming mixture of sodium carbonate and quartz have been conducted to distinguish the cases of (a) a fast-relaxing, high natural abundance nucleus, (b) a probe nucleus with low abundance and low spin-lattice relaxation rate, and (c) a complex and changing system of industrial relevance. The most suitable nucleus for in situ high-temperature studies is one with high abundance but slow relaxation because the differential relaxation time between hot and cold parts of the sample effectively removes the signal from the cold material. There is no \'in situ penalty\' from the diminishing Boltzmann polarization at high temperature since this effect is balanced by a corresponding increase of the spin-lattice relaxation rate. [request reprint]

In situ SAXS studies of the morphological changes of an alumina-zirconia-silicate ceramic during its formation

In situ SAXS studies of the morphological changes of an alumina-zirconia-silicate ceramic during its formation

D Le Messurier, R Winter, CM Martin
J Appl Cryst 39 (2006) 589

Small-angle X-ray scattering is used at two energies, one either side of the zirconium K-edge, to probe the in situ formation of an alumina-zirconia-silicate ceramic. The use of energies either side of the edge allows the decomposition of information regarding the scattering from the zirconia particles from that of the glass matrix. Porod slope data show how the nanoparticles progress from being relatively isolated particles to becoming agglomerates as the pore network in the glass collapses. The shape of the agglomerates resembles the pore network of the glass at low temperature. The Guinier radii of the particles show the growth of the agglomerates past the Littleton softening point, whilst still resolving the primary particles. [download]

Energy-dependent in situ small-angle x-ray scattering study of nano-ceramics

Energy-dependent in situ small-angle x-ray scattering study of nano-ceramics

R Winter, D Le Messurier, CM Martin
Crystallography Reviews 12 (2006) 3

Energy-dependent small-angle X-ray scattering (SAXS) measurements near the Zr-K absorption edge of a nano-scale alumina-zirconia-silicate ceramic are reported. The in situ experiments cover the temperature range from 250 to 725oC, during which the sol-gel prepared nano-particles consolidate and the glass matrix phase begins to soften, leading to a compaction of the ceramic. The dominant scattering mechanism changes from a surface-fractal type to a mass-fractal type linked to direct contact areas between nano-particles and matrix. The energy-dependent data identify the zirconium-bearing phase as dominant in the high-q range. The experimental data are placed in the context of recent advances in in situ anomalous small-angle X-ray scattering (ASAXS) and some experimental problems and solutions of near-edge scattering are discussed. [request reprint]

23Na, 29Si, and 13C MAS NMR investigation of glass-forming reactions between Na2CO3 and SiO2

<sup>23</sup>Na, <sup>29</sup>Si, and <sup>13</sup>C MAS NMR investigation of glass-forming reactions between Na<sub>2</sub>CO<sub>3</sub> and SiO<sub>2</sub>

AR Jones, R Winter, GN Greaves, IH Smith
J Phys Chem B 109 (2005) 23154

The glass-forming reactions between sodium carbonate (Na2CO3) and silica (SiO2) have been investigated by 23Na, 29Si, and sup>13C magic-angle spinning (MAS) NMR spectroscopy. The multinuclear MAS NMR approach identifies and quantifies reaction products and intermediates, both glassy and crystalline. A series of powdered batches of initial composition Na2CO3·xSiO2 (x=1,2) corresponding to a sodium metasilicate (Na2SiO3) and sodium disilicate (Na2Si2O5) stoichiometry were investigated after periods of isothermal and nonisothermal heat treatments at different temperatures. Analysis of the 23Na quadrupolar coupling parameters has identified the early reaction product in all cases as crystalline Na2SiO3. In the non-isothermal experiment, this reaction is preceded by an early silica-rich melt phase formed around 850oC. The early reactions are controlled by solid-state Na+ diffusion across the reaction zone in the grain interface layer. Crystalline Na2SiO3 precipitates in the interface layer, increasing its thickness between the Na2CO3 and the SiO2 grains and slowing down the rate of Na+ migration. This creates a secondary phase, which is temperature dependent. At low temperatures, where Na+ migration is impaired, the production of Na2SiO3 ceases and silica-richer phases are precipitated. In the case of the sodium disilicate batch, where excess SiO2 is present, a secondary reaction of Na2SiO3 with SiO2 forming a glassy phase is observed. A transient carbon-bearing phase has been identified by 13C NMR as a NaCO3- complex loosely bound to bridging oxygens in the silicate network at the SiO2 grain surface. [request reprint]

Tracing the reactive melting of glass-forming silicate batches by in situ 23Na NMR

Tracing the reactive melting of glass-forming silicate batches by in situ <sup>23</sup>Na NMR

AR Jones, R Winter, P Florian, D Massiot
J Phys Chem B 109 (2005) 4324

The kinetics of the reaction of batches of powdered quartz and sodium carbonate was studied by in situ 23Na nuclear magnetic resonance (NMR) spectroscopy using a laser-heated probe. We show for the first time that the technique allows one to study solid-state reactions at high temperatures with good time resolution and without the risk of quenching artifacts. The reaction is controlled by solid-state Na+ diffusion across the grain interface. Independent of the batch composition, the first reaction product is crystalline sodium metasilicate, Na2SiO3, even if the temperature is high enough for much of the composition space between silica and metasilicate to be above the equilibrium liquidus. Fast Na+ diffusion allows the reaction front to cross the grain interface and form the solid product before liquid intermediate equilibrium products can be formed. This purely solid-state reaction slows down as the thickness of the interface increases; the reaction is more deceleratory than published models suggest. If excess quartz is present, it reacts in a second step involving a liquid film wetting the excess grains. Once this reaction has started, it pulls the reaction into the thermodynamic regime, which leads to an increase even in the rate of the first step leading to intermediate solid metasilicate. [request reprint]

Interfacial structure of annealed alumina-zirconia-silicate nanoceramics

Interfacial structure of annealed alumina-zirconia-silicate nanoceramics

D Le Messurier, N Sissouno, AR Vearey-Robert, S Evans, DA Evans, R Winter
Mat Sci Technol (London) 20 (2004) 975

An alumina-zirconia nanocomposite has been produced using the chloride sol-gel method and embedded into a silicate matrix by dispersing the nanocomposite into a powdered silica glass and subsequent annealing. The resultant nanoceramic was subjected to 27Al magic angle spinning (MAS) NMR, small angle X-ray scattering (SAXS), and x-ray photoelectron spectroscopy (XPS), leading to a core-shell type model of the interfacial region. Initially the particles are agglomerated with the shell containing mainly atoms of octahedral coordination and the core aluminium atoms of tetrahedral coordination. Upon annealing the agglomerates break up, causing a change in the coordination of the aluminium atoms. As the atoms diffuse into the matrix, the ones that were initially in the shell change to be tetrahedrally coordinated, and therefore increase the overall population of tetrahedrally coordinated aluminium atoms within the interface. [request reprint]