W. McLean

Pulsed laser deposition of ZnO nanocluster films by Cu-vapor laser

The dependence of stoichiometry, grain size, cathodoluminescence colors, adhesion, and surface morphology of zinc oxide films, deposited by a Cu-vapor laser at room temperature, as a function of oxygen ambient pressure during synthesis were investigated. Auger electron spectroscopy showed that ZnO films with a Zn/O ratio close to 1 were obtained at oxygen pressures >10−1 Torr. X-ray diffraction revealed that pulsed laser deposited zinc oxide films were composed mainly of nanocrystals, the average grain size of which grew from 5 to 17.5 nm as the oxygen pressure was increased from 10−5 to 1 Torr. The surface morphology of the films, as determined by secondary electron microscopy, also exhibited increasing roughness as the grain size increased. Films grown in an oxygen pressure >1.5×10−1 Torr glowed blue under electron bombardment, while slightly substoichiometric films glowed white under similar excitation. Films deposited in an oxygen background pressure up to 1.5×10−1 Torr exhibited good adhesion to subs...

The nature and effects of the thermal stability of lithium hydroxide

Temperature programmed decomposition and complimentary microscopy/spectroscopy techniques were performed on lithium hydroxide with micron-sized grains. The lithium hydroxide grains thermally decomposed into Li2O, releasing H2O, following a three dimensional phase boundary moving from the surface inward. The energy barriers measured for the decomposition of surface and near-surface lithium hydroxide are noticeably smaller than those of bulk counterpart. The conversion of Li2O grains back to lithium hydroxide during moisture exposure was also found to proceed from the surface inward such that surface states are filled before bulk states. In a different set of experiments, nanometer-scale composite grains composed of LiD inner cores and LiOH outer layers were observed to form on top of pressed polycrystalline LiD upon moisture exposure. A diffusion coefficient on the order of 10−23 m2/s was measured for the diffusion controlled reaction of LiOH with LiD in the nanopowder at room temperature in a dry environment. The measured kinetics were used to model the evolution of the LiD/LiOH composite system in a dry environment.

Potential energy surfaces for the uranium hydriding reaction

We have computed the potential energy surfaces for the low-lying electronic states of uranium hydrides, UHn (n=1–3), which are important in the uranium hydriding reactions. We have employed a number of computational methods including the complete active space multiconfiguration self-consistent field followed by multireference relativistic configuration interaction computations with spin–orbit coupling that included up to 6 million configurations. We find that the activation barrier to insert uranium into H2 is reduced substantially by spin–orbit coupling, and the product species UH2 in its A1 spin–orbit ground state is substantially stable over U(5L)+H2 dissociated products. We have found two electronic states for UH to be quite close to each other, and depending on the level of theory the relative ordering of the 6Λ and 4I states changes, 4I state being the lowest at the highest second-order configuration interaction level. The UH2 species also exhibits a similar feature in that the triplet state is favo...

Measurement and prediction of H2O outgassing kinetics from silica-filled polydimethylsiloxane TR55 and S5370

The isoconversional technique was employed for the measurement and prediction of H2O outgassing kinetics from silica-filled polydimethylsiloxane TR55 and S5370 in a vacuum or dry environment. Isoconversional analysis indicates that the energy barrier for H2O release from TR55 and S5370 is an increasing function of the fractional H2O release. This can be interpreted as the release of H2O from physisorbed water and then chemisorbed water with decreasing OH density from the surfaces of the embedded silica particles. Model-independent predictions of H2O outgassing based on the measured kinetics follow the trend of actual isothermal outgassing at elevated temperatures and suggest gradual outgassing in dry/vacuum storage over many decades at low temperatures for both TR55 and S5370.

GaAs nanostructures and films deposited by a Cu-vapor laser

The properties of GaAs nanoclusters and films deposited on substrates by a Cu-vapor laser were investigated. Nanoclusters of GaAs were produced by laser ablating a GaAs target in an Ar background gas. X-ray diffraction and transmission electron microscopy revealed that these GaAs nanoclusters had randomly oriented crystalline cores and As-rich amorphous oxide outer shells. These clusters assembled, upon vacuum annealing, along step edges and at defects on substrates to form wire-like structures. Our results also showed that GaAs films, when deposited in vacuum, did not have crystalline cores and were rich in As. Postdeposition annealing in vacuum to between 400 and 500 °C drove off the excess As. The stoichiometry of the films was confirmed by both Auger electron spectroscopy and x-ray photoelectron spectroscopy.

Vacuum outgassing of high density polyethylene

A combination of thermogravimetric analysis and temperature programmed decomposition was employed to identify the outgassing species, the total amount of outgassing, and the outgassing kinetics of high density polyethylene (HDPE) in a vacuum environment. The isoconversional kinetic analysis was then used to analyze the outgassing kinetics and to predict the long-term outgassing of HDPE in vacuum applications at ambient temperature. H2O and CnHx, with n as high as 9 and x centering around 2n, are the major outgassing species from solid HDPE, but the quantities evolved can be significantly reduced by vacuum baking at 368K for a few hours prior to device assembly.

The moisture outgassing kinetics of a silica reinforced polydimethylsiloxane

A silica-filled polydimethylsiloxane (PDMS) composite M9787 was investigated for potential outgassing in a vacuum/dry environment with the temperature programmed desorption/reaction method. The outgassing kinetics of 463 K vacuum heat-treated samples, vacuum heat-treated samples which were subsequently re-exposed to moisture, and untreated samples were extracted using the isoconversional and constrained iterative regression methods in a complementary fashion. Density functional theory (DFT) calculations of water interactions with a silica surface were also performed to provide insight into the structural motifs leading to the obtained kinetic parameters. Kinetic analysis/model revealed that no outgassing occurs from the vacuum heat-treated samples in subsequent vacuum/dry environment applications at room temperature (∼300 K). The main effect of re-exposure of the vacuum heat-treated samples to a glove box condition (∼30 ppm by volume of H2O) for even a couple of days was the formation, on the silica surfa...

Industrial applications of high-power copper vapor lasers

A growing appreciation has developed in the last several years for the copper vapor laser because of its utility in ablating difficult materials at high rates. Laser ablation at high rates shows promise for numerous industrial applications such as thin film deposition, precision hole drilling, and machining of ceramics and other refractories.

High rate PLD of diamond-like-carbon utilizing copper vapor lasers

Pulsed Laser Deposition (PLD) has been shown to be an effective method for producing a wide variety of thin films of high-value-added materials. The high average powers and high pulse repetition frequencies of lasers under development at LLNL make it possible to scale-up PLD processes that have been demonstrated in small systems in a number of university, government, and private laboratories to industrially meaningful, economically feasible technologies. A copper vapor laser system at LLNL has been utilized to demonstrate high rate PLD of high quality diamond-like-carbon (DLC) from graphite targets. The deposition rates for PLD obtained with a 100 W laser were {approx} 2000 {mu}m{center_dot}cm{sup 2}/h, or roughly 100 times larger than those reported by chemical vapor deposition (CVD) or physical vapor deposition (PVD) methods. Good adhesion of thin (up to 2 pm) films has been achieved on a small number of substrates that include SiO{sub 2} and single crystal Si. Present results indicate that the best quality DLC films can be produced at optimum rates at power levels and wavelengths compatible with fiber optic delivery systems. If this is also true of other desirable coating systems, this PLD technology could become an extremely attractive industrial tool for high value added coatings.

X-ray excited Auger transitions of Pu compounds

X-ray excited Pu core–valence–valence and core–core–valence Auger line-shapes were used in combination with the Pu 4f photoelectron peaks to characterize differences in the oxidation state and local electronic structure for Pu compounds. The evolution of the Pu 4f core-level chemical shift as a function of sputtering depth profiling and hydrogen exposure at ambient temperature was quantified. The combination of the core–valence–valence Auger peak energies with the associated chemical shift of the Pu 4f photoelectron line defines the Auger parameter and results in a reliable method for definitively determining oxidation states independent of binding energy calibration. Results show that PuO2, Pu2O3, PuH2.7, and Pu have definitive Auger line-shapes. These data were used to produce a chemical state (Wagner) plot for select plutonium oxides. This Wagner plot allowed us to distinguish between the trivalent hydride and the trivalent oxide, which cannot be differentiated by the Pu 4f binding energy alone.