Browsing by Author "Thomas, BS"

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    Atomistic simulation of cation ordering and radiation damage in Sr1-3x/2LaxTiO3 defect perovskites
    (Australian Institute of Physics, 2005-01-31) Thomas, BS; Marks, NA; Begg, BD; Corrales, LR; Devanathan, RL
    Sr-1.3x/2LaxTiO3 perovskites are known to contain charge-compensating cation vacancies, which display one-dimensional ordering at high La concentrations. Recently, the radiation resistance of these perovskites has been measured, revealing an anomalously high radiation resistance at around x = 0.2. We use atomistic computer simulation techniques to study short-range cation and vacancy ordering as a function of La concentration and thermal history. Long-range electrostatic effects dominate the interactions, and ordering in one- and two-dimensions is observed. We also give preliminary results on the effects of La concentration and ordering on radiation resistance, including both primary damage creation and defect annealing. © 2005 Australian Institute of Physics
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    Evolution of functional group of lignocellulosic biomass and its delignified form during thermal conversion using synchrotron-based THz and laboratory-based in-situ DRIFT spectroscopy
    (Elsevier, 2023-05-05) Kundu, C; Biswas, S; Thomas, BS; Appadoo, D; Duan, A; Bhattacharya, S
    In the industry, platform chemicals are traditionally synthesized from non-renewable resources. In order to determine the ideal temperature range and associated emissions for the production of platform chemicals from biomass, which is a renewable resource, mechanistic insights into the thermochemical conversion of biomass are needed. Here, the high-resolution synchrotron-based gas-phase THz (Far-IR) spectra and lab-based in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of both raw and delignified biomass have been studied to identify real-time evolution of the functional groups and any gas-phase secondary reactions during the thermochemical conversion as a function of temperature; these are linked to weight loss measurements through differential thermogravimetry. The high-resolution synchrotron-based technique and DRIFTS were used to acquire the spectra in two different wavenumber ranges of 50–600 cm−1 and 500–4500 cm−1, respectively. The synchrotron-based spectra were used to identify the major gaseous components between 300 and 500 °C of methane, ethane, acetylene and formaldehyde, and their generation followed the order 300 > 400 > 500 °C. The DRIFTS spectra showed that the covalent hydrogen bonds of both raw and delignified biomass was cleaved below 250 °C, between 250 and 300 °C the decarboxylation reaction took place, whereas between 300 and 400 °C platform chemicals (furan, levoglucosan, levoglucosenone) and aromatic compounds were formed from the dehydration of the cellulosic part of the biomass. No changes in the DRIFTS spectra were observed above 400 °C. These results suggest that 300–400 °C is the ideal temperature range for the thermochemical conversion of biomass to platform chemicals. Pyrolysis-gas chromatography/mass spectrometry (Py-GCMS) results demonstrated that the identification of platform chemicals laid the groundwork for large-scale operation. © 2023 The Author(s). Published by Elsevier Ltd.
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    Mechanisms of radiation damage and properties of nuclear materials
    (Materials Research Society, 2009-11-30) Lumpkin, GR; Smith, KL; Whittle, KR; Thomas, BS; Marks, NA
    A wide range of materials are currently under consideration for use in advanced nuclear fuel cycle applications. The effects of radiation on these materials by exposure to external neutron irradiation and internal alpha and beta decay processes may have significant effects on the physical and chemical properties. This is especially true for materials that are subject to hundreds of displacements per atom during their service life. In this paper, we explore some of the radiation damage mechanisms prevalent in oxide based materials, including mathematical models and other concepts of amorphization (e.g., percolation), the role of defects on picosecond time scales, and longer term effects such as diffusion and recrystallization. As radiation "tolerance" or the ability of a material to maintain crystallinity under intense irradiation is a key issue for many fuel cycle applications, we will briefly review and comment on some of the underlying factors that have been identified as important in driving the short-term damage recovery. These include aspects of the structure (e.g., connectivity, polyhedral distortion), bonding, energetics of defect formation and migration, and melting point and similar criteria. The primary materials of interest here are those under development as special purpose nuclear waste forms, novel materials for separations, inert matrix fuels, and transmutation targets. In this context, we will illustrate the behavior of simple oxides and several more complex oxides such as perovskite, multicomponent fluorite systems, and related derivative structures (e.g., pyrochlore and zirconolite). The damage mechanisms in these materials are briefly compared with those of intermetallic and metallic materials.
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    Radiation damage response of ceramics in extreme environments
    (Sociedad Nuclear Mexicana (Mexican Nuclear Society), 2010-10-24) Lumpkin, GR; Smith, KL; Whittle, KR; Thomas, BS; Marks, NA
    Oxide-based and inter-metallic compounds have great potential as new materials for clean and renewable energy production. Many of these materials, especially those designed for operation in Generation IV fission reactors or in fusion reactors, must exhibit robust performance under extreme conditions of temperature, irradiation, and chemical attack. Others, such as nuclear waste forms, may be required to retain radioactive elements for long periods of time in geological repositories. The mechanisms of radiation damage production and recovery in these materials may vary considerably as a function of the damage source, e.g., energetic neutrons in reactor systems versus alpha decay in nuclear waste forms. Furthermore, the kinetics of damage recovery are complicated by multiply activated processes and in certain cases, longer-term diffusion may modify the structural state left by irradiation in the short term. Here, we review some basic concepts regarding the mechanisms of radiation damage in selected ceramic materials, including mathematical models, fluence-temperature relationships, and predictive methodologies. A major consideration for materials performance is the ability of a given compound to resist amorphization. Historically, there are a number of general criteria for radiation resistance, including those involving melting point (thermodynamics), structural freedom, bonding, and energetics of defect formation. These are discussed using specific examples.