Browsing by Author "Chea, K"
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- ItemNew insights into colloidal phase transitions using neutron scattering techniques(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Chea, K; Bryant, G; Garvey, CJ; Van Megan, BThe fundamentals of crystallisation and glass formation in atomic systems are not yet fully understood. Hardsphere colloidal nanoparticles have been shown to be promising model systems for understanding crystallisation and glass formation in atomic systems: As colloidal motion is Brownian, rather than ballistic, kinetics and dynamics are orders of magnitude slower than in atomic systems and can be studied in real-time. However, despite previous work, key elements are still missing from our understanding of phase transition in colloidal suspensions especially regarding metastability, supercooling and the glass transition. In particular, there is still no clear understanding of the effects of polydispersity: although studies of both polydisperse and binary mixtures of hard sphere colloids have been performed, a systematic study of the effects of polydispersity on structure, crystallisation kinetics and particle dynamics is still lacking. One of the reasons for this is the relatively limited types of suspensions which have be studied - most particles used for such studies need to be suspended in mixed solvents for refractive index matching for light scattering studies, which introduces potential problems such as selective solvation and evaporation. In this work we explore the possibility of using ionic liquids (ILs) and deep eutectic solvents (DESs) as the suspending solvent, as these can be tuned to match the refractive index of the particles, and don’t suffer from evaporation. We will then develop suitable binary colloidal suspensions consisting of deuterated & non-deuterated nanoparticles suspended in the solvent. With a combination of lab techniques and beam time allocations at the Australian Synchrotron, ANSTO and overseas neutron facilities, we will expansively investigate the nature of metastability, crystallisation and the glass transition, and provide a significant advance on our current understanding of these processes.
- ItemStructure and dynamics in photovoltaic metal hydrides(Australian Institute of Physics, 2018-01-30) Chea, K; Greaves, TL; Le, T; Rule, KC; Mole, RA; Wang, P; Shrestha, SK; Conibeer, G; Iles, GNSolar cell technology is an active area of research with the quest to improve the efficiency of solar cells to above the current value of 44%. Hot carrier solar cells are particular types of cells which may enable higher efficiencies to be obtained. However, these are only feasible where there is a sufficiently large band gap in the phonon dispersion of the bulk material to minimise energy loss to thermalisation, thus keeping the electrons ‘hot’. Binary compounds with a large mass difference between the two constituent atoms, and high level of crystal symmetry such as titanium hydride, can have such a gap in their phonon dispersion. Titanium hydride is an interesting photovoltaic material with a broad range of properties, which vary depending on the proportion of hydride present. Theoretical studies show TiH2 has a phonon band gap of 95 meV in the bulk phase, however, experimentally this compound exists as a powder because the hydrogenation process causes large stresses in the lattice which are strong enough to crack the bulk sample. For solar cell absorber materials, a bulk sample is preferred and these can be manufactured by hydrogenating very pure Ti metal. We have previously studied TiH1.65 using X-ray powder diffraction and inelastic neutron scattering and found that while the width of the acoustic and optical phonon bands is different from those of TiH2, it did have a phonon band gap of 65 meV i.e. large enough to block Klemens’ decay. We present here an extension of this work with Fourier Transform Infra-red (FTIR) and Raman spectroscopy, along with X-Ray Diffraction (XRD) data from the photovoltaic materials, TiH2 and ZrH2.
- ItemStructure and dynamics in photovoltaic metal hydrides(Australian Institute of Nuclear Science and Engineering (AINSE), 2018-11-18) Iles, GN; Chea, K; Rule, KC; Mole, RA; Elcombe, MMSolar cell technology is an active area of research with the quest to improve the efficiency of solar cells to above the current value of 44% [1]. Hot carrier solar cells are particular types of cells which may enable higher efficiencies to be obtained. However, these are only feasible where there is a sufficiently large band gap in the phonon dispersion of the bulk material to minimise energy loss to from thermalisation, thus keeping the electrons ‘hot’. Binary compounds with a large mass difference between the two constituent atoms, and high level of crystal symmetry such as titanium hydride, can have such a gap in their phonon dispersion. Titanium hydride is an interesting photovoltaic material with a broad range of properties, which vary depending on the proportion of hydride present. Theoretical studies show TiH2 has a phonon band gap of 95 meV in the bulk phase [2], however, there is little experimental data to confirm this. TiH1.65 has been measured using X-ray powder diffraction and inelastic neutron scattering whereby it was found that this sample had a phonon band gap of 65 meV [3]. We present here further X-ray powder diffraction and ineleastic neutron scattering data on powder samples of TiH2 and TiH1.5 whereby we show the correlation of phonon band gap with hydrogen content. © The authors.