AVS2010 Session AC-TuA: Science and Technology of Actinides and Rare Earths
Tuesday, October 19, 2010 2:00 PM in Isleta
AC-TuA-1 Enhanced Photoluminescence from Europium-Doped Gadolinium-Based Nanocrystal Scintillators
Teng-Kuan Tseng, Jihun Choi, Mark R. Davidson, Paul H. Holloway (University of Florida)
Scintillator crystals have traditionally been grown with complex single crystal methods such as Czochralski and Bridgman techniques, which frequently result in high costs and small crystal size. Therefore, development of processes for larger area, polycrystalline ceramic scintillators with high luminescence is of great interest due to their potential for mass production, versatility in shape and size, and low cost. In this study, spherical Gd2O3:Eu3+ and almond-like GdVO4:Eu3+ nanocrystals were synthesized using a water-based solution precipitation method at low reaction temperatures (<90 ℃) in short times (3 min~1 h). Core/Shell nanostructures with mono-dispersed 220 nm SiO2 cores and an ~13 nm Gd2O3 shell, i.e. SiO2/Gd2O3:Eu3+, were prepared. With an additional un-doped Gd2O3 shell to form a SiO2/Gd2O3:Eu3+/Gd2O3 nanostructure, the quantum yield was 28% higher than that of SiO2/Gd2O3:Eu3+. This enhanced photoluminescence (PL) is attributed to a Gd2O3 surface shell serving (i) as a sensitizer with energy transfer to the Eu3+ in the Gd2O3:Eu3+ shell, plus (ii) passivation of non-radiative surface quenching sites. Enhanced PL was also demonstrated from polyol-synthesized Gd2O3:Eu3+/Y2O3 nanocrystals. Increased PL can also be achieved by incorporating Bi3+ sensitizer ions into colloidal GdVO4:Eu3+ nanocrystals which were self-assembled into almond-like clusters composed of ~60 nm nanorods. With 2% Bi3+ co-doped in GdVO4:Eu3+ nanocrystals, PL was enhanced by 45%, 90% and 570% when excited by 280, 323 and 347 nm photons, respectively. This enhancement is attributed to increased absorption from Bi-O bonds, plus extension of the excitation band edge to longer wavelength. For Bi3+ ion concentrations >10%, PL from co-doped nanocrystals decreased due to non-radiative decay from Bi3+-induced trapping centers, as well as increased Bi3+-Bi3+ energy transfer instead of Bi3+-Eu3+ transfer.
AC-TuA-2 Memory Effects of UF6 Adsorption and Reaction at Metallic Surfaces
Mark Paffett, David P. Moore, Doug Farr, Roland K. Schulze, Kiril Ianakiev (Los Alamos National Laboratory)
In this study we explore memory effects that arise from the reactivity of UF6 with surface hydroxyls (and other entities with a reactive H bond) that may be present at metallic surfaces. These chemical interactions are noted to leave behind low but measurable quantities of uranium oxy fluorides (UO2F2, UOF4 and related extended solids). Uranium re-depositions are noted to occur following sequential exposures to varying isotopic content in the UF6 gas stream. We explore the role that additional fluorinating agents (HF, ClF3) play in promoting uranium surface re-fluorination and memory effects in these deposits. The primary surface and radiochemical characterization techniques utilized in this study and include x-ray photoelectron spectroscopy, Auger electron spectroscopy depth profiling, static SIMS and alpha emission spectroscopy. The importance of these memory effects in enabling higher accuracy isotopic determinations and in forensic knowledge
AC-TuA-3 Nanocomposites for Thermoelectrics : Erbium Mono-Antimonide Nano Crystals Embedded in Group III -AsSb Host Materials
Takehiro Onishi (University of California, Santa Cruz and NASA Ames Research Center); Tela Favaloro (University of California, Santa Cruz); Ali Shakouri (University of California, Santa Cruz and NASA Ames Research Center); Elane Coleman, Gary S. Tompa (Structured Materials Industries Inc.); Stephan Kraemer, Hong Lu, Art Gossard (University of California, Santa Barbara); Nobuhiko P. Kobayashi (University of California, Santa Cruz and NASA Ames Research Center)
The increasing demand for efficiency in energy generation and use has increased interest in thermoelectrics (the direct conversion of heat to electricity), which has the promise to increase energy efficiency – if certain cost-performance metrics are met. However, in the continuing quest of the efficient bulk thermoelectrics material for more than 50 years, the improvement of thermoelectric properties has not been sufficient to widely replace other established power sources.
One of the promising lines of new material development is based on the use of nanostructures to dramatically change the heat transport properties of thermoelectrics while largely leaving the electrical properties in tact. In this effort we have focused on developing nanocomposites comprised of thin films containing semi-metallic nanocrystals. We herein report on the growth of nanocomposites that consist of erbium monoantimonide (semi-metal) in the form of nano crystals or nanocolumns self-assembled with in thin film group III- arsenide/antimonide alloys doped with acceptors. The nano composites are optimized in terms of three factors, electrical conductivity, and thermal conductivity, and Seebeck coefficient to maximize thermoelectric figure of merit.
Using low-pressure metal organic chemical vapor deposition (MOCVD), we have developed the growth processes of the nanocomposites that consist of indium gallium (arsenic) antimonide (InGa(As)Sb) host materials with embedded erbium antimonide (ErSb) nanocrystals. The size of ErSb nano crystals, carrier density and alloy composition of the InGa(As)Sb host materials are tuned by controlling of various growth parameters. The following techniques were used to obtain information on the growth of ErSb nanocomposites embedded InGa(As)Sb film on n-type InSb (100) substrate: Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red-absorption (FTIR), Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), and Transmission Electron Microscopy (TEM).
This research is funded by DARPA and DOE ; supported by Structured Materials Industries Inc. [www.structuredmaterials.com] for the growth; Advanced Studies Laboratories in NASA Ames research Center, and Materials Department, University of California, Santa Barbara for the characterization.
AC-TuA-4 TRU Waste Disposal in Waste Isolation Pilot Plant, WIPP
Marian Borkowski, Hnin Khaing, Jean-Francois Lucchini, Donald Reed, Michael Richmann, Julie Swanson, David Ams (Los Alamos National Laboratory)
The mobility and potential release of actinides into the accessible environment continues to be the key performance assessment concern of nuclear repositories. Actinide, in particular plutonium speciation under the wide range of conditions that can exist in the subsurface is complex and depends strongly on the coupled effects of redox conditions, inorganic/organic complexation, and the extent/nature of aggregation. Understanding the key factors that define the potential for actinide migration is, in this context, an essential and critical part of making and sustaining a licensing case for a nuclear repository. Herein we report on recent progress in a concurrent modeling and experimental study to determine the speciation of plutonium, uranium and americium in high ionic strength Na-Cl-Mg brines. This is being done as part of the ongoing recertification effort in the Waste Isolation Pilot Plant (WIPP).
A key feature of salt-based repositories is the relatively rapid self-sealing nature of the salt. This feature leads to geologic isolation of the waste form, and when reduced metals are present (e.g., iron containers), the system is driven anoxic by corrosion leading to strongly reducing environments. The consequence of this is that the combination of anaerobic microbial activity, reactions of reduced metals, and, when present, reactions of organics leads to the reduction of higher valent Pu(V) and Pu(VI) species to the lower valent Pu(III) and Pu(IV) species. The reduction of Pu(V/VI) species has been studied extensively. Less is known about microbial effects with halophiles although there is no question that bioreduction of higher valent plutonium occurs readily by soil bacteria under anoxic conditions. These lower valent oxidation states have lower solubilities and correspondingly lead to lower solubility and mobility of the plutonium.
The oxidation-state specific solubility of actinides were established in brine as function of pCH+, brine composition and the presence and absence of organic chelating agents and carbonate. An oxidation-state invariant analog approach using Nd3+ and Th4+ was used for An3+ and An4+ respectively. These results show that carbonate and hydrolysis predominate at pCH+ above 8. Organic complexation is more important for An3+. Carbonates are the key factor for U(VI) solubility. Modeling efforts are focused on the use of Pitzer parameters to correct for high-ionic strength effects and show that there is still some uncertainty about the predominant carbonate and hydrolytic species, particularly when longer-term timeframes are considered.
AC-TuA-7 Elastic Moduli of Pure Alpha, Beta, Gamma Plutonium—Three Different Metals
Albert Migliori (Los Alamos National Laboratory)
From 10 K to 580K plutonium changes phase from monoclinic alpha to body centered monoclinic beta to orthorhombic gamma structures. Each crystal structure is rare or unique for an elemental metal. Measurements presented here provide the first high-accuracy values for a single high-purity specimen of the elastic moduli of unalloyed polycrystal plutonium as a function of temperature throughout the entire range of existence of the alpha, beta, and gamma phases. The bulk and shear moduli, essential thermodynamic material properties, reflect important and huge changes with temperature, such that these phases present as three different metals. Unlike phase transformations in many other elements where the bonding, nearest-neighbor distances, and physical properties are closely related among phases, in the three lowest-temperature phases of plutonium, the relationships are missing, and support the extreme sensitivity of plutonium properties to phase, temperature, and almost-certainly, electronic structure. We describe here the characteristics and implications of these newly-observed properties.
AC-TuA-9 Synthesis and Characterization of Scintillating Gd2SiO5:Ce Nanoparticles using Hot-Solution Growth
Jihun Choi, Teng-Kuan Tseng, Mark R. Davidson, Paul H. Holloway (University of Florida)
Scintillation detectors are commonly used for measuring radiation from nuclear materials. To date the scintillating material has been a single crystal, commonly doped with a rare earth ion that controls the wavelength and intensity of radioluminescence. Scintillating nanoparticles have the potential to replace the expensive, energy-intensive, limited volume single crystal detectors. In this study, scintillating Gd2SiO5:Ce3+ (GSO) nanoparticles with 5~10 nm diameters were prepared by a two-pot hot-solution growth (HSG at 200~300 oC) method. The Ce dopant concentration was varied between 0.2%~5% and concentration quenching was examined by photoluminescence (PL). Low (0.5% Ce) doped GSO nanoparticles exhibited good PL from both as-synthesized and calcined (1100˚C for 2 h in air) nanoparticles. Concentration quenching for nanoparticles occurred at higher Ce concentrations than for bulk samples; this will be discussed. The PL emission was from the 5d to two 4f levels (2T2 to 2F7/2 and 2F5/2 transitions) of Ce3+ at 420~450 nm. Photoluminescent excitation (PLE) spectra showed that the emission resulted from the direct excitation of the 4f–5d transition of Ce3+ excited between 270~375 nm. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data showed that the GSO nanoparticles were amorphous as grown, but well crystallized after calcining. Quantum yield and radioluminescence data will be presented and discussed.