Lumin Wang

The radiation-induced crystalline-to-amorphous transition in zircon

A comprehensive understanding of radiation effects in zircon, ZrSiO[sub 4], over a broad range of time scales (0.5 h to 570 million years) has been obtained by a study of natural zircon, Pu-doped zircon, and ion-beam irradiated zircon. Radiation damage in zircon results in the simultaneous accumulation of both point defects and amorphous regions. The amorphization process is consistent with models based on the multiple overlap of particle tracks, suggesting that amorphization occurs as a result of a critical defect concentration. The amorphization dose increases with temperature in two stages (below 300 K and above 473 K) and is nearly independent of the damage source ([alpha]-decay events or heavy-ion beams) at 300 K. Recrystallization of completely amorphous zircon occurs above 1300 K and is a two-step process that involves the initial formation of pseudo-cubic ZrO[sub 2].

Solid silver particle production by spray pyrolysis

Abstract Solid, spherical, micron-sized silver metal particles were produced by spray pyrolysis from a silver nitrate solution. The effects of reaction temperature, carrier gas type, solution concentration, and aerosol droplet size on the characteristics of the resultant silver particles were examined. Pure, dense, unagglomerated particles were produced with an ultrasonic generator at and above 600° C using N 2 carrier gas, and at and above 900°C using air as the carrier gas. Solid particle formation at temperatures below the melting point of silver (962°C) was attributed to sufficiently long residence times (3.5–54 s) which allowed aerosol-phase densification of the porous silver particles resulting from reaction of the precursor.

Palladium metal and palladium oxide particle production by spray pyrolysis

Abstract Spray pyrolysis was used to produce dense, spherical palladium metal particles at and above 900 °C in air and 800 °C in nitrogen, well below the melting point of palladium (1554 °C). Palladium oxide particles were produced at lower temperatures. At 500 °C the PdO particles were composed of nanocrystalline grains 5 to 15 nm in diameter and had surface areas of 30.2 to 32.8 m2/g. The particles became less porous and less polycrystalline as temperature increased. At 800 °C the PdO particles were polycrystalline with grains of 20 to 50 nm and a surface area of 3.23 m2/g. The Pd particles produced at 900 °C by decomposition of the oxide were single-crystalline and fully-dense. These observations are consistent with the formation of porous but not hollow aggregates of PdO at lower temperatures, which can be densified in the gas phase to form solid particles of Pd above 900 °C.

Nanophase oxide formation by intraparticle reaction

Abstract We have generated 0.1–0.5 μm aggregates that consist of nanocrystalline P d O and V 2 O 5 . The particles were generated by reacting aerosol particles that are composed of metal nitrates in a hot flowing gas stream. The microstructure of the particles was controlled by varying the temperature. Nanophase materials were formed at temperatures low enough to avoid grain growth in each particle, but sufficiently high to fully react the precursors. This process allows reasonable production rates of nanophase materials. Generation of multicomponent materials is also possible.

Plasma Coating and Enhanced Dispersion of Carbon Nanotubes

Ultrathin polymer films have been deposited on both multi-wall and aligned carbon nanotubes using a plasma polymerization treatment. TEM experimental results showed that a thin film of polystyrene layer (several nanometers) was uniformly deposited on the surfaces of the nanotubes including inner wall surfaces of the multi-wall nanotubes. The coated multi-wall nanotubes were mixed in polymer solutions for studying the effects of plasma coating on dispersion. It was found that the dispersion of multi-wall carbon nanotubes in polystyrene composite was significantly improved. The deposition mechanisms and the effects of plasma treatment parameters are discussed.

Gas-phase particle size distributions during vapor condensation of fullerenes

Abstract Nanometer-size fullerene particles were generated via vapor condensation in a continuous flow system starting from pure C60 and mixed fullerene extract (C60/C70). Vapor condensation was carried out by cooling a nitrogen gas stream containing fullerene vapors from 400–650°C to room temperature. Particles were partly amorphous at 400°C, and became crystalline C60 when processed above 500°C. The average particle diameter and particle number concentration increased with increasing processing temperature. The particles had average sizes of 30, 35 and 40 nm and total number concentrations of 2.2 × 106, 3.5 × 106, and 5.0 × 106 #/cm3, respectively at 500, 525 and 550°C.

Synthesis and characterization of Group 6 metal silicide–silicon carbide composites via solid-state displacement reactions of nanosize grains of M2C and silicon

Individual reactions of nanosize grains of Group 6 metal carbides, M2C (M = Cr, Mo and W)(1 equivalent) with 5 equivalents of silicon powder under the relatively mild conditions of 1000 °C for 4 h have given nanosize grains of metal silicide–silicon carbide composite materials consisting of the intermetallic silicide alloys CrSi2, MoSi2–Mo5Si3 and WSi2–W5Si3.

Radiation Effects in Nuclear Waste Materials

The objective of this project is to develop a fundamental understanding of radiation effects in glasses and ceramics, as well as the influence of solid-state radiation effects on aqueous dissolution kinetics, which may impact the performance of nuclear waste forms and stabilized nuclear materials.

INERT-MATRIX FUEL: ACTINIDE ''BURINGIN'' AND DIRECT DISPOSAL

Excess actinides result from the dismantlement of nuclear weapons (Pu) and the reprocessing of commercial spent nuclear fuel (mainly 241 Am, 244 Cm and 237 Np). In Europe, Canada and Japan studies have determined much improved efficiencies for burnup of actinides using inert-matrix fuels. This innovative approach also considers the properties of the inert-matrix fuel as a nuclear waste form for direct disposal after one-cycle of burn-up. Direct disposal can considerably reduce cost, processing requirements, and radiation exposure to workers.