Browsing by Author "Koshy, P"

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    Candidate glass–ceramic wasteforms for the immobilisation of Cs-loaded IONSIV® wastes: a scoping study
    (Springer Nature, 2024-03-28) Bahmanrokh, G; Whitelock, E; Dayal, P; Farzana, R; Koshy, P; Gregg, DJ
    In some cases, nuclear wastes can be treated with ion exchange materials to remove specific radionuclides from solution via cationic exchange. A promising inorganic ion exchange material, crystalline silicotitanate (CST) or IONSIV®, has been previously employed to remove Cs-137 from contaminated aqueous systems with high specificity. Once the radioactive Cs-137 has been incorporated within the IONSIV® structure, the ion exchange material itself becomes radioactive waste and requires immobilisation within a nuclear wasteform. The current scoping study investigated design and development of advanced glass–ceramic wasteforms for the immobilisation of Cs-loaded IONSIV®. Two well-established Cs-bearing ceramic phases, hollandite, and pollucite, were considered as the ceramic component of the novel glass–ceramic design. Hollandite appeared to react with the borosilicate glass-component to form celsian and rutile. The pollucite system produced a phase assemblage of pollucite, rutile, srilankite, and glass, as targeted, and is therefore considered a promising wasteform design for Cs-loaded IONSIV® material. © 2024 Springer Nature.
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    Characterisation of hot isostatically pressed (HIPed) hollandite wasteform-canister interaction zone
    (Elsevier, 2024-02) Mann, J; Farzana, R; Aughterson, RD; Dayal, P; Sorrell, CC; Koshy, P; Gregg, DJ
    A potential hollandite wasteform for immobilising waste containing Cs, Ba, Sr, and Rb, projected from a solvent extraction process that separates Cs/Sr from spent nuclear fuel, was fabricated via hot isostatic pressing (HIPing) within a stainless-steel (SS) canister at 1250 °C / 30 MPa / 2 h. Before HIPing, 2 wt.% Ti metal was added to the precursor, as a redox control additive. Detailed elemental profiling and microstructural analysis at the interaction zone between the wasteform and the SS HIP canister were thoroughly investigated with transmission electron microscopy (TEM) using a lamella extracted by focused ion beam (FIB) milling, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The interaction zone towards the wasteform was ∼20–30 µm in distance and in this region, a hollandite composition with varying chemistry was observed relative to the bulk wasteform. Moreover, the regular Cr-oxide layer, often observed previously for HIPed Synroc-type materials, was not present due to the achievement of reducing condition by adding Ti-metal as redox additive and simultaneous diffusion of canister material towards the ceramic. Predominant Cr-diffusion was observed with incorporation in the hollandite phase along with minor Fe, Mn and Co from the SS canister. This study provides a detailed understanding of the HIP canister – wasteform interaction zone for a hollandite-rich wasteform design for the first time. Importantly, no deleterious phases were formed that may otherwise reduce the performance of the wasteform. This study further demonstrates the flexibility of HIPing as a consolidation process for the treatment of radioactive wastes. © 2023 Elsevier B.V.
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    Effect of Ti‐metal addition on hot‐isostatically pressed (HIPed) Synroc‐C
    (Wiley, 2023-11) Farzana, R; Dayal, P; Peristyy, A; Sutton, P; Aly, Z; Aughterson, RD; Nguyen, TH; Yeoh, M; Koshy, P; Gregg, DJ
    Synroc, a candidate nuclear wasteform and Synroc technology, a waste treatment solution utilizing hot‐isostatic pressing (HIPing) have significant potential for the immobilisation of challenging nuclear wastes from both current and innovative reactors and fuel cycles. Hot isostatic press (HIP) consolidation is undertaken within sealed metal HIP canisters, where metal buffers (e.g., Ti, Fe and Ni) can be incorporated to control the redox environment within the canister. This study, for the first time, reports the effect of varying Ti‐metal addition (0, 2, 4, and 8 wt.%) on phase formation, microstructural characteristics, and wasteform performance for HIP consolidated Synroc‐C containing 20 wt.% simulated PUREX type (PW‐4b) high level waste. Quantitative X‐ray diffraction analysis, scanning electron microscopy‐energy dispersive X‐ray spectroscopy (EDS) and transmission electron microscopy‐EDS analyses were undertaken for analytical investigations. The chemical durability of the samples was assessed using ASTM C1220‐21 standard test. Hot‐isostatically pressed (HIPed) samples with 0 and 8 wt.% Ti added for redox control produced unfavourable phase formation. However, the HIPed samples with Ti additions of 2 and 4 wt.% as a redox buffer showed the desired phase formation of Synroc‐C without any significant change to the partitioning of waste elements among the phases along with compatible durability results, when compared to previous literature for hot uniaxial pressing (HUPed) or sintered materials. © 2023 Commonwealth of Australia and The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society. - Open Access CC-BY-NC.
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    Mechanistic impacts of long-term gamma irradiation on physicochemical, structural, and mechanical stabilities of radiation-responsive geopolymer pastes
    (Elsevier, 2021-04-05) Yeoh, MLY; Ukritnukun, S; Rawal, A; Davies, JB; Kang, BJ; Burrough, K; Aly, Z; Dayal, P; Vance, ER; Gregg, DJ; Koshy, P; Sorrell, CC
    The mechanistic effects of long-term γ irradiation on the mineralogical, microstructural, structural, physical, and chemical properties of 40 wt% blast furnace slag + 60 wt% fly ash geopolymer pastes have been examined. Ambient curing for 28 days during normal equilibration was followed by exposure to 60Co irradiation (1574, 4822, 10,214 kGy). The material characteristics are controlled largely through the competing mechanisms of beneficial equilibration at initial lower dosages, which enhances gelation and crosslinking, and detrimental equilibration at subsequent higher dosages, which causes structural and microstructural destabilisation. Irradiation for 2 months (1574 kGy) increases the compressive strength ~45% (~57 to ~83 MPa) through conversion of less-crosslinked (Q0/Q1/Q1′) to more-crosslinked (Q2/Q3/Q4) silicate species. The transition between these regimes occurs after ~5 months of irradiation (~4000 kGy). Beyond this, the rates of beneficial equilibration and detrimental equilibration equalise upon completion of normal geopolymerisation. Additional geopolymerisation from γ irradiation is controlled by the rate-limiting release of Si4+ from the unreacted aluminosilicates and silicates and their rapid incorporation in the geopolymer network. The aqueous leaching of the geopolymer pastes is not affected significantly by γ irradiation. These data reveal the potential for these materials as intermediate-level wasteforms that can outperform Portland cement-based materials. © 2020 Elsevier B.V.
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    Mo-doped, Cr-doped, and Mo–Cr codoped TiO2 thin-film photocatalysts by comparative sol-gel spin coating and ion implantation
    (Elsevier, 2021-02-18) Chen, A; Chen, WF; Majidi, T; Pudadera, B; Atanacio, AJ; Manohar, M; Sheppard, LR; Liu, R; Sorrell, CC; Koshy, P
    Uniformly codoped anatase TiO2 thin films of varying (equal) Mo and Cr concentrations (≤1.00 mol% for each dopant) were fabricated using sol-gel spin coating and deposited on fused silica substrates. All films were annealed at 450 °C for 2 h to recrystallise anatase. Undoped anatase films have been subjected to dual ion implantation for the first time, using Mo, Cr, and sequential Mo + Cr at 1 × 1014 atoms/cm2. The films were characterised by GAXRD, AFM, SIMS, XPS, and UV–Vis and the performance was assessed by dye degradation. Despite the volumetric doping by sol-gel and the directional doping by ion implantation, neither method resulted in homogeneous dopant distributions. Both methods caused decreasing crystallinities and associated partial amorphisation. The XPS signal of the uniformly codoped films is dominated by undissolved dopant ions, which is not the case for the ion-implanted films. Increasing Ti valences are attributed to the fully oxidised condition of the Ti4+ ions that diffuse to the surface from Ti vacancy formation compared to the Ti valence of the bulk lattice, which contains Ti3+. Increasing O valence is attributed to the electronegativity of O2−, which is higher than that of Ti4+. Detailed structural mechanisms for the solubility and energetics mechanisms involve the initial formation of Mo and Cr interstitials that fill the two voids adjacent to the central Ti ion in the TiO6 octahedron, followed by integrated solid solubility (ISS) and intervalence/multivalence charge transfer (IVCT/MVCT). The sequential order of the last two is reversed for the two different doping methods. These two effects are likely to be the source of synergy, if any, between the two dopant ions. The photocatalytic performances of the uniformly codoped films are relatively poor and correlate well with the band gap (Eg). The performances of the ion-implanted films do not correlate with the Eg, where TiO2–Mo performs poorly but TiO2–Cr and TiO2–Mo–Cr outperform the undoped film. These results are interpreted in terms of the competition between the effects of Mo doping, which causes partial amorphisation and/or blockage of active sites, and Cr doping, which may cause Mo–Cr synergism, Cr-based heterojunction formation, and/or improved charge-carrier separation owing to the surface-deposition nature of ion implantation. © 2021 Hydrogen Energy Publications LLC.
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    Phase assemblage and microstructures of Gd2Ti2-xZrxO7 (x = 0.1–0.3) pyrochlore glass-ceramics as potential waste forms for actinide immobilization
    (Elsevier, 2021-11-15) Bhuiyan, A; Wong, V; Abraham, JL; Aughterson, RD; Kong, L; Farzana, R; Gregg, DJ; Sorrell, CC; Zhang, YJ; Koshy, P
    Glass-ceramics (GCs) based on titanate pyrochlores have attracted recent attention as candidate waste forms for actinide immobilization. As zirconate pyrochlore has a superior radiation resistance, it is anticipated that Zr substitution of Ti in titanate pyrochlore GCs would increase their potential for waste form applications. The concept was primarily addressed via the preparation of Gd2Ti2-xZrxO7 (x = 0.1–0.3) GCs to determine the effects of sintering temperature, Zr substitution, and pyrochlore content (50–70 wt%) on the properties of the resultant GCs. XRD and SEM analyses were used to reveal the phase assemblages and microstructures while TEM and Raman spectroscopy were used to analyze the local structures. XRD results confirmed the formation of the targeted pyrochlore as the major phase, with Gd9.33(SiO4)6O2 oxyapatite present as a minor phase. SEM analyses revealed that up to 0.2 formula units of Ti could be substituted by Zr under the processing conditions. The pyrochlore crystallite sizes were largely controlled by the sintering temperature and cooling rate and showed little sensitivity to the glass contents. This work has demonstrated successful substitutions of Ti with Zr in Gd2Ti2O7 GCs as potential waste forms for actinide wastes owing to their superior radiation resistance. © 2021 Elsevier B.V.
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    Role of oxygen vacancy ordering and channel formation in tuning intercalation pseudocapacitance in Mo single-ion-implanted CeO2–x nanoflakes
    (American Chemical Society, 2021-12-07) Zheng, XR; Mofarah, SS; Cen, A; Cazorla, C; Haque, E; Chen, EY; Atanacio, AJ; Manohar, M; Vutukuri, C; Abraham, JL; Koshy, P; Sorrell, CC
    Metal oxide pseudocapacitors are limited by low electrical and ionic conductivities. The present work integrates defect engineering and architectural design to exhibit, for the first time, intercalation pseudocapacitance in CeO2–x. An engineered chronoamperometric electrochemical deposition is used to synthesize 2D CeO2–x nanoflakes as thin as ∼12 nm. Through simultaneous regulation of intrinsic and extrinsic defect concentrations, charge transfer and charge–discharge kinetics with redox and intercalation capacitances together are optimized, where reduction increases the gravimetric capacitance by 77% to 583 F g–1, exceeding the theoretical capacitance (562 F g–1). Mo ion implantation and reduction processes increase the specific capacitance by 133%, while the capacitance retention increases from 89 to 95%. The role of ion-implanted Mo6+ is critical through its interstitial solid solubility, which is not to alter the energy band diagram but to facilitate the generation of electrons and to establish the midgap states for color centers, which facilitate electron transfer across the band gap, thus enhancing n-type semiconductivity. Critically, density functional theory simulations reveal, for the first time, that the reduction causes the formation of ordered oxygen vacancies that provide an atomic channel for ion intercalation. These channels enable intercalation pseudocapacitance but also increase electrical and ionic conductivities. In addition, the associated increased active site density enhances the redox such that the 10% of the Ce3+ available for redox (surface only) increases to 35% by oxygen vacancy channels. These findings are critical for any oxide system used for energy storage systems, as they offer both architectural design and structural engineering of materials to maximize the capacitance performance by achieving accumulative surface redox and intercalation-based redox reactions during the charge/discharge process. © 2021 American Chemical Society