Stephen C. Glade

Electric current enhanced defect mobility in Ni3Ti intermetallics

The effect of the application of a dc current on the annealing of point defects in Ni3Ti was investigated by positron annihilation spectroscopy. An increased defect annealing rate was observed under the influence of the current and was attributed to a 24% decrease in the activation energy of mobility. The results are interpreted in terms of the electron wind effect and the complex nature of diffusion in ordered intermetallic phases. They provide direct evidence for an increase in defect mobility in ordered intermetallics under the influence of a current.The effect of the application of a dc current on the annealing of point defects in Ni3Ti was investigated by positron annihilation spectroscopy. An increased defect annealing rate was observed under the influence of the current and was attributed to a 24% decrease in the activation energy of mobility. The results are interpreted in terms of the electron wind effect and the complex nature of diffusion in ordered intermetallic phases. They provide direct evidence for an increase in defect mobility in ordered intermetallics under the influence of a current.

Positron annihilation spectroscopy and small-angle neutron scattering characterization of the effect of Mn on the nanostructural features formed in irradiated Fe–Cu–Mn alloys

The size, number density, and composition of the nanometre-sized defects responsible for the hardening and embrittlement in irradiated Fe–0.9 wt% Cu and Fe–0.9 wt% Cu–1.0 wt% Mn model reactor pressure vessel alloys were measured using small-angle neutron scattering and positron annihilation spectroscopy. These alloys were irradiated at 290°C to relatively low neutron fluences (E > 1 MeV, 6.0 × 1020 to 4.0 × 1021 n m−2) in order to study the effect of manganese on the nucleation and growth of copper-rich precipitates and secondary defect features. Copper-rich precipitates were present in both alloys following irradiation. The effect of Mn was to reduce the size and increase the number density of precipitates in the Fe–Cu–Mn alloy relative to the Fe–Cu alloy. Vacancy clusters were observed in the Fe–Cu alloy, but not in the Fe–Cu–Mn alloy. These results suggest a strong effect of Mn on vacancy diffusion and clustering.

Characterization of densified fully stabilized nanometric zirconia by positron annihilation spectroscopy

Fully stabilized nanometric zirconia samples with varying degrees of porosity and grain sizes were analyzed using the coincidence Doppler broadening mode of the positron annihilation spectroscopy (PAS). A decrease in the low-momentum fraction was observed and coincided with a decrease in porosity. In addition to pores, it is proposed that defects in the negatively charged grain-boundary space region act as positron trapping centers; their effectiveness decreases with an increase in grain size. It is shown that PAS is sensitive to small grain-size differences within the nanometric regime in these oxide materials.

Instrument for the investigation of the nanostructure of Pu and other actinides

An instrument for the synthesis and analysis of nanoparticles and ultrathin films of Pu and other actinides has been constructed. To facilitate the production of nanoscale specimens of these materials, pulsed laser ablation was chosen for the deposition process. The highly toxic and radioactive nature of these materials created a challenging safety environment that needed to be addressed before any work could begin. Particular attention has been paid in this respect towards our future work with plutonium, though the design structure presented here would work equally well with other nonradioactive toxic or reactive materials. The analytical capabilities of the instrument include in situ photoelectron spectroscopy, low energy electron diffraction, scanning tunneling microscopy, and scanning tunneling spectroscopy. The instrument design and first results will be discussed.

Positron Annihilation Spectroscopy of Nanostructural Features in Model Reactor Pressure Vessel Steels

Irradiation embrittlement in nuclear reactor pressure vessel steels results from the formation of a high number density of nanometer sized copper rich precipitates and sub-nanometer defect-solute clusters. We present positron annihilation spectroscopy (PAS) results to characterize the compositions and magnetic character of these defects in model A533B reactor pressure vessel steels. The results confirm the presence of copper-rich precipitates after irradiation. The measured orbital electron momentum spectra indicate the precipitates are alloyed with Mn and Ni. The copper precipitates larger than R {approx} 1.2 nm (from SANS measurements) are non-magnetic, which limits the possible Fe content of the precipitates to at most a few %. Notably, large vacancy clusters observed in neutron irradiated Fe-Cu alloys were not observed in the steels after irradiation.

Characterization of Uranium Particles Produced via Pulsed Laser Deposition

We have constructed an experimental apparatus for the synthesis (via pulsed laser deposition) and analysis of nanoparticles and thin films of plutonium and other actinides. In-situ analysis techniques include x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). Also, the oxidation kinetics and the reaction kinetics of actinides with other gaseous species can be studied with this experimental apparatus. Preliminary results on depleted uranium are presented.

Bulk materials analysis using high-energy positron beams

This article reviews some recent materials analysis results using high‐energy positron beams at Lawrence Livermore National Laboratory. We are combining positron lifetime and orbital electron momentum spectroscopic methods to provide electron number densities and electron momentum distributions around positron annihilation sites. Topics covered include the correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glasses and compositional studies of embrittling features in nuclear reactor pressure vessel steel.

Oxidation of uranium nanoparticles produced via pulsed laser ablation

An experimental apparatus designed for the synthesis, via pulsed laser deposition, and analysis of metallic nanoparticles and thin films of plutonium and other actinides was tested on depleted uranium samples. Five nanosecond pulses from a Nd:YAG laser produced films of {approx}1600 {angstrom} thickness that were deposited showing an angular distribution typical thermal ablation. The films remained contiguous for many months in vacuum but blistered due to induced tensile stresses several days after exposure to air. The films were allowed to oxidize from the residual water vapor within the chamber (2 x 10{sup -10} Torr base pressure). The oxidation was monitored by in-situ analysis techniques including x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and scanning tunneling microscopy (STM) and followed Langmuir kinetics.