AVS2010 Session AC-TuA: Science and Technology of Actinides and Rare Earths

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).

Acknowledgement:

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.

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 pC_H+, 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 pC_H+ 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.

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 ²F_7/2 and ²F_5/2 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.