Daniel Jorge Rodriguez

An α-Pu sample was studied in its "as-received" and "sputtered" state by both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Analysis of the "as-received" surface indicated significant carbon and oxygen, with oxygen signal pertaining to plutonium dioxide (PuO₂) and sesquioxide (Pu₂O₃). The "sputtered" clean metal surface was then dosed with hydrogen (H₂) gas at 300 K and found to nucleate hydride species (PuH_x). Interestingly, H₂ was observed to be critical for nucleation of carbides (Pu_xC_y) by acting as a catalyst for Pu's reaction with background gases. Because Pu metal has a high affinity for oxygen, reoxidation occurred under UHV, forming Pu_xO_y and disrupting both PuH_x and Pu_xC_y growth. Observations from the experiments were modeled for the α-Pu(020) surface, illustrating the ability of H₂ to readily dissociate onto sputtered and speciated surfaces at 300 K. The projected density of states from these calculations were compared to experiments, showing good agreement between the Pu 5f, 6d, and 7s states and the H 1s, O 2p, and C 2p valence states. Overall, the above results indicated that the formed hydride layer is unstable at 300 K in UHV conditions.

Alloying plutonium with gallium affords a fundamental change in the electronic behaviour at the atomic level. The charge transfer between plutonium and gallium in dilute gallium concentration alloys has been investigated using the initial state Auger parameter determined from x-ray photoelectron spectroscopy measurements. These experimental results have been compared to Bader valence charge analysis theoretical calculations. The agreement between experimental and theoretical results is excellent, both showing approximately −0.6 e ⁻ increase in charge on gallium which is invariant over the range of alloys investigated in this study.

Designs for two automated atomic layer deposition (ALD) flow reactors are presented, and their capabilities for coating additively manufactured (AM) metal prints are described. One instrument allows the coating of several AM parts in batches, while the other is useful for single part experiments. To demonstrate reactor capabilities, alumina (Al2O3) was deposited onto AM 316L stainless steel by dosing with water (H2O) vapor and trimethylaluminum (TMA) and purging with nitrogen gas (N2). Both instruments are controlled by custom-programmed LabVIEW software that enables in situ logging of temperature, total pressure, and film thickness using a quartz crystal microbalance. An initial result shows that 150 ALD cycles led to a film thickness of ∼55 nm, which was verified with Rutherford backscattering spectroscopy. This indicates that the reactors were indeed depositing single atomic layers of Al2O3 per ALD cycle, as intended.

Chemical solution deposition (CSD) methods involvingthe thermaldecomposition of 5.0 M Er-(NO3)(3)center dot 5H(2)O and Y-(NO3)(3)center dot 6H(2)O precursorsolutions were employed to fabricate protective erbia and yttria coatingsonto stainless steel (SS304/SS316) coupons. The two techniques testedwere dip and spray coating, which were then compared to a commercialyttria spray (ZYP Coatings). It was determined that solution concentration,solvent choice, injection of Er2O3 and Y2O3 micropowder, and the annealing temperature/rampprofile were critical to the coatings' physical properties.For dip coatings, thicknesses were 1-20 mu m after twodipping/annealing cycles, and adhesion strength was similar to 1000 psi,increasing up to similar to 1300 psi if the SS coupons had preliminarysandblasting. Spray coatings from precursor solutions were reportedto have thicknesses of 20-80 mu m and adhesion strengthless than 500 psi (regardless of the coupon surface finish). Cross-sectionalviews of the coatings confirmed subsurface porosity, and XRD resultsindicated that the coatings were polycrystalline, with patterns typicalto that of cubic Er2O3 and Y2O3.

Absolute kinetics for reactions of C2H2 with a series of ∼60 individual carbon nanoparticles (NPs) from graphite, graphene, graphene oxide, carbon black, diamond, and nano-onion feedstocks were measured for temperatures (TNP) ranging from 1200 to 1700 K. All the NPs were observed to gain mass by carbon addition under conditions that varied with feedstock but with large variations in initial growth rate. Long reaction periods were studied to allow the evolution of growth rates over time to be observed. Diamond NPs were found to passivate against C2H2 addition if heated above ∼1400 K, and the highly variable initial reactivity for carbon nano-onions was found to depend on the presence of non-onion-structure surface carbon. For graphitic and carbon black NPs, three distinct growth modes were observed, correlated with the initial NP mass (Minitial). Smallest graphitic and carbon black NPs, with masses <∼25 MDa, initially grew rapidly but also passivated quickly after adding 50 MDa, grew rapidly and continuously, adding up to ∼300% of Minitial with no sign of rate slowing as long as C2H2 was present. The efficiencies for C2H2 addition and etching by O2 are strongly correlated, but the correlation changes as the NPs passivate. Growth and passivation mechanisms are discussed.

O₂ oxidation and sublimation kinetics for >30 individual nanoparticles (NPs) of five different feedstocks (graphite, graphene oxide, carbon black, diamond, and nano-onion) were measured using single-NP mass spectrometry at temperatures (T_NP) in the 1100–2900 K range. It was found that oxidation, studied in the 1200–1600 K range, is highly sensitive to the NP surface structure, with etching efficiencies (EEO₂) varying by up to 4 orders of magnitude, whereas sublimation rates, significant only for T_NP ≥ ∼1700 K, varied by only a factor of ∼3. Its sensitivity to the NP surface structure makes O₂ etching a good real-time structure probe, which was used to follow the evolution of the NP surface structures over time as they were either etched or annealed at high T_NP. All types of carbon NPs were found to have initial EEO₂ values in the range near 10^–3 Da/O₂ collision, and all eventually evolved to become essentially inert to O₂ (EEO₂ < 10^–6 Da/O₂ collision); however, the dependence of EEO₂ on time and mass loss was very different for NPs from different feedstocks. For example, diamond NPs evolved rapidly and monotonically toward inertness, and evolution occurred in both oxidizing and inert atmospheres. In contrast, graphite NPs evolved only under oxidizing conditions and were etched with complex time dependence, with multiple waves of fast but non-monotonic etching separated by periods of near-inertness. Possible mechanisms to account for the complex etching behavior are proposed.