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[en] Cs is one of the more problematic fission product radionuclides to immobilize due to its high volatility at elevated temperatures, ability to form water soluble compounds, and its mobility in many host materials. The hollandite structure is a promising crystalline host for Cs immobilization and has been traditionally fabricated by solid state sintering methods. This study presents the structure and performance of Ba1.0Cs0.3A2.3Ti5.7O16; A = Cr, Fe, Al hollandite fabricated by melt processing. Melt processing is considered advantageous given that melters are currently in use for High Level Waste (HLW) vitrification in several countries. This work details the impact of Cr additions that were demonstrated to i) promote the formation of a Cs containing hollandite phase and ii) maintain the stability of the hollandite phase in reducing conditions anticipated for multiphase waste form processing
[en] Crystalline and glass composite materials are currently being investigated for the immobilization of combined High Level Waste (HLW) streams resulting from potential commercial fuel reprocessing scenarios. Several of these potential waste streams contain elevated levels of transition metal elements such as molybdenum (Mo). Molybdenum has limited solubility in typical silicate glasses used for nuclear waste immobilization. Under certain chemical and controlled cooling conditions, a powellite (Ba,Ca)MoO4 crystalline structure can be formed by reaction with alkaline earth elements. In this study, single phase BaMoO4 and CaMoO4 were formed from carbonate and oxide precursors demonstrating the viability of Mo incorporation into glass, crystalline or glass composite materials by a melt and crystallization process. X-ray diffraction, photoluminescence, and Raman spectroscopy indicated a long range ordered crystalline structure. In-situ electron irradiation studies indicated that both CaMoO4 and BaMoO4 powellite phases exhibit radiation stability up to 1000 years at anticipated doses with a crystalline to amorphous transition observed after 1 X 1013 Gy. Aqueous durability determined from product consistency tests (PCT) showed low normalized release rates for Ba, Ca, and Mo (>0.05 g/m2)
[en] Here, we report the synthesis of the Ce-based quaternary compound CePtAl4Ge2 that crystallizes in the trigonal structure (space group R 3¯m, 166) with unit cell parameters, a = 4.1995(5) Å, c = 31.851(7) Å, and γ = 120°. Powder X-ray diffraction and energy dispersive X-ray spectroscopy show that CePtAl4Ge2 (LaPtAl4Ge2) is in a single, homogeneous phase. Magnetic susceptibility, electrical resistivity, and heat capacity measurements of CePtAl4Ge2 show that it exhibits antiferromagnetic behavior below 2.3 K. The magnetic susceptibility for the magnetic field applied perpendicular (χab) and parallel (χc) to the crystalline c-axis is very anisotropic, and the susceptibility ratio (χab/χc) reaches a maximum value of 10, indicating that the spin easy axis is within the Ce plane. The entropy recovered at TN is consistent with the doublet ground state of the crystal field split J = 5/2 multiplet of Ce3+ ions.
[en] An N-type Ti0.5Zr0.5NiSn0.994Sb0.006 powder, synthesized by levitation melting, has been mixed with 1.5 wt% of HfO2 particles and sintered by spark plasma sintering (grain size around 10 mm). Three kinds of nanocrystalline particles have been identified in the sintered microstructure. They are thought to promote scattering of lattice vibrations, leading to a 25% increase of the dimensionless-thermoelectric figure of merit (ZT, around 1.0 at 500 C) in comparison to the standard Zr0.25Hf0.25Ti0.5NiSn0.994Sb0.006 composition. In the same time, the replacement of Hf by HfO2 leads to a significant decrease (in the range 25-60%) of the cost per kilogram of raw material. (authors)
[en] The air stable uranyl and neptunyl phosphine oxide complexes act as good starting material to prepare the air and moisture sensitive phosphinimines species and show a preference for N-donor ligands. Calculations confirmed those observations and proved the U-N bond to be stronger than the U-O bond. To continue our investigations on trans-uranyl chemistry, the complexes PuO2Cl2(R3PO)2 have been synthesised as well as new amides species of NpO22+ and PuO22+ characterised by NMR spectroscopy. (authors)
[en] A new method involving the homogeneous dispersion of precursor compounds inside a methylcellulose matrix is used for the synthesis of a composite powder of Li2FeP2O7 and carbon. The properties of carbon-containing and carbon-free powders are studied by X-ray powder diffraction (XRD) including Rietveld refinement, Mössbauer spectroscopy, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). The structure of both powders is refined in a monoclinic framework (space group P21 /c). The structural refinement and Mössbauer spectroscopy reveal different degrees of partial occupancy of mixed-occupied sites by lithium. Electrochemical measurements show that the in situ formation of carbon improves capacity (90% of 1-electron theoretical capacity) through decreased charge-transfer resistance. © 2019 Elsevier B.V.
[en] Here we investigated the ambient temperature structural properties (thickness, width, microstructure, and lattice parameter), and the ambient and high temperature (up to 900 K) direct current (DC) magnetic properties—saturation magnetization (MS) and intrinsic coercivity (HCI)—of rapidly-solidified (melt-spun) Fe-x wt.% Si (x = 3, 5, & 8) alloys. The wheel surface speeds selected for the study were 30 m/s and 40 m/s. The ribbons produced at the lower wheel surface speed (30 m/s) were continuous having relatively uniform edges compared to the ribbons produced at the higher wheel surface speed. The thickness and the width of the melt-spun ribbons ranged between ~15 and 60 μm and 500–800 μm, respectively. The x-ray diffraction spectra of the melt-spun ribbons indicated the presence of disordered α-phase, irrespective of the composition, and the wheel surface speed. The lattice parameter decreased gradually as a function of increasing silicon content from ~0.2862 nm (Fe-3 wt.% Si) to ~0.2847 nm (Fe-8 wt.% Si). The wheel surface speed showed an insignificant effect on MS while increased silicon content resulted in a decreasing trend in MS. Elevated temperature evaluation of the magnetization (M-T curves at ~7.96 kA/m) in the case of Fe-3 & 5 wt.% Si alloy ribbons was distinctly different from that of the Fe-8 wt.% Si alloy ribbons. The curves of the as-prepared Fe-3 wt.% Si and Fe-5 wt.% Si alloy ribbons were irreversible while that of Fe-8 wt.% Si was reversible. The MS for any of the combinations of wheel surface speed and composition decreased monotonically with the increase in temperature (from 300 to 900 K). While HCI increased with the increase in temperature for all the wheel surface speed and composition combination, its nature of increase is distinct for Fe-8 wt.% Si alloy ribbons compared to Fe-3 & 5 wt.% Si alloys ribbons. Finally, it appears that rapidly-solidified Fe-3 wt.% Si and Fe-5 wt.% Si alloys ribbons are primarily comprised of the α phase (disordered phase) while the Fe-8 wt.% Si alloy ribbons are comprised primarily of disordered phase along with minor constituents of an ordered phase.
[en] We show that Li_2Se:Te is a potential neutron scintillator material based on density functional calculations. Li_2Se exhibits a number of properties favorable for efficient neutron detection, such as a high Li concentration for neutron absorption, a small effective atomic mass and a low density for reduced sensitivity to background gamma rays, and a small band gap for a high light yield. Our calculations show that Te doping should lead to the formation of deep acceptor complex V_L_i-Te_S_e, which can facilitate efficient light emission, similar to the emission activation in Te doped ZnSe
[en] The main goal of this research has been to investigate for the first time the effects of fluorination on the crystal structure, magnetic, and electrochemical properties of the P2-type NaxCoO2 powder. Sodium cobalt oxide with a P2-type structure is synthesized by a modified solid-state reaction consisting of alternating processes of rapid heating up to 750 °C and rapid cooling to the room temperature. The obtained powder is fluorinated using a gas-solid reaction with NH4HF2 as fluorinating agent. Fluorination causes a decrease of sodium content in the parent phase with the concurrent formation of the minor phases of Na2CO3 and NaF. The structure of NaxCoO2 in both powders is refined in P63/mmc space group. The results of the Rietveld refinement combined with the findings from the XPS measurements confirm the Na0.76CoO2 and Na0.44CoO1.96F0.04 stoichiometries for the pristine and fluorinated powders, respectively, which indicates that 4 at.% of fluorine ions per formula unit are incorporated in the structure. Preliminary electrochemical investigations have revealed an improved charge/discharge performance. The influence of fluorination on morphology and magnetic properties has also been examined. © 2018 Elsevier B.V.