Mitchell Trkula

Possible behavior of a diamond (111) surface in methane/hydrogen systems

A combined numerical and experimental investigation into the behavior of diamond (111) surfaces in plasma CVD reactors is presented. Numerically, semiempirical molecular orbital methods are used as a model of diamond (111) surfaces represented by a 20-atom carbon cluster plus surface species. The abstraction of hydrogen atoms by gas-phase hydrogen atoms, the coverage dependence of the heat of formation for submonolayers of CH{sub 3} and C{sub 2}H groups coadsorbed with H, and the energy change for abstraction of H atoms from the surface by various radicals in the gas-phase are examined. No barrier to abstraction is found, steric effects in achieving clusters of CH{sub 3} groups are large, and C{sub 2}H and atomic oxygen are found to be the most energetically favored for removal of adsorbed H. Experimentally, relative concentrations of atomic H in the near-surface region as a function of added O{sub 2} mole fraction were measured. A weak dependence on O{sub 2} concentration is observed, but does not appear to be significant enough to account for observed changes in growth rate. This suggests that other radical species be investigated for their contribution to diamond film growth.

The use of plasma immersion ion processing in the synthesis of protective coatings for Al die casting

Abstract Plasma immersion ion processing (PIIP) has been used to synthesize adherent coatings on steel substrates that are wear-resistant and provide wetting resistance to molten aluminum. Synthesis procedures were developed for two coatings, boron carbide and a chrome-carbi-oxide. The coatings were produced using a plasma generated by the pulsed glow discharge method with various gas mixtures. Boron carbide coatings were produced using mixtures of diborane (B 2 H 6 ) diluted with He and mixed with acetylene (C 2 H 2 ) gas, and the chrome-carbi-oxide coating was produced using Cr(CO) 6 . The boron carbide coatings were found to be amorphous with hardness values in range of 12–13 GPa. The Cr-based coatings were somewhat harder with values in the range of 14–20 GPa. Both coatings show some resistance to reaction with molten Al; the boron carbide coating showed limited reaction with Al while the Cr–C–O coating showed no reaction. The non-wetting properties of the chrome-carbi-oxide coating have been ascribed to the formation of a thin Al 2 O 3 membrane around the molten Al that kinetically and thermodynamically inhibits a reaction between the Al and the Cr–C–O film.

Hydrogen uptake on film surfaces produced by a unique codeposition process

Abstract Hydrogen uptake on several different film surfaces has been achieved by deposition of a conventional hydrogen gettering system via a novel combination of physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes. We decided to use a conventional hydrogen gettering system, developed by Smith and Schicker [J.R. Schicker, AS/KCD Project No. EPN-047620, May 1994], that uses an acetylenic organic compound mixed with carbon supported palladium metal. The organic material, 1,4-bis-(phenylethynyl) benzene (DEB), is mixed with palladium and carbon by employing conventional solid state ceramic preparative techniques. Our novel codeposition process combines PVD and CVD techniques for fabricating thin-film coatings of the palladium-catalyzed DEB hydrogen gettering system. Hydrogen uptake was confirmed by 1 H NMR and our novel process lends itself well to placing hydrogen getter onto complex shapes and substrates of various compositions.

Progress Update on CVD of Beryllium

Abstract Capsules with beryllium ablators are very important targets for the DOE National Ignition Facility (NIF) laser in the Inertial Confinement Fusion Program. Two leading candidates for fabricating beryllium capsules are the machining and bonding of hemispheres, and physical vapor deposition of beryllium onto plastic or other shells. An attractive possibility would be to coat a spherical mandrel with a thin layer of beryllium by a non-line-of-sight process. This coating could be applied via the chemical vapor deposition (CVD) of beryllium. Our first attempt at coating beryllium via CVD was done by using bis(cyclopentadienyl)beryllium, (C5H5)2Be, as the precursor material. Results obtained by use of (C5H5)2Be as the precursor material is discussed. However, difficulties we encountered with use of the (C5H5)2Be precursor material led us to examine a relatively unexplored area of beryllium chemistry, namely that of its amines. This redirection also led us to change surrogate material for use in the developmental work.

An Approach To Single Molecule Detection By Laser-Induced Fluorescence

A sheath flow cuvette was evaluated in laser-induced fluorescence determination of aqueous rhodamine 6G. A detection limit of 18 attograms was obtained within a one-second signal integration time. The concentration detection limit was 8.9 x 10-14 mole per liter. An average of one-half rhodamine 6G molecule was present within the 11 pL excitation volume. However, dUring the signal integration time a total of 22,000 analyte molecules passed through the excitation region in a 0.42 microliter volume. The biomedical technique of flow cytometry has been used to study the fluorescence and light scatter properties of biological cells and cellular components.1 The hydrodynamic focusing property of the sheath flow cuvette employed in flow cytometry provides a well designed flow chamber for laser-induced fluorescence analysis of small volume samples. The sheath flow cuvette has been applied as a laser-induced fluorescence detector in high performance liquid chromatography and flow injection analysis.2-4 A tightly focused laser beam was used in those experiments to define an excitation volume of several nanoliters. In the present report, the performance of the sheath flow cuvette is considered for fluorescence analysis in excitation volumes of several picoliters.5 The sample cell is shown in Figure 1.

Annealing of chromium oxycarbide coatings deposited by plasma immersion ion processing (PIIP) for aluminum die casting

Abstract Chromium oxycarbide coatings have been investigated for use as non-wetting coatings for aluminum die casting. This paper examines Cr–C–O coating stability and non-wetability at elevated temperatures for extended periods. Coatings were deposited onto 304 stainless steel from chromium carbonyl [Cr(CO)6] by plasma immersion ion processing. The coatings were annealed in air at an aluminum die casting temperature of 700°C up to 8 h. Coatings were analyzed using resonant ion backscattering spectroscopy, nanoindentation and pin-on-disk tribometry. Molten aluminum was used to determine coating wetting and contact angle. Results indicate that the surface oxide layer reaches a maximum thickness of 900 nm. Oxygen concentrations in the coatings increased from 24% to 34%, while the surface concentration rose to almost 45%. Hardness values ranged from 22.1 to 6.7 GPa, wear coefficients ranged from 21 to 8×10 −6 mm 3 / Nm and contact angles ranged from 156° to 127°.

Laser melting of photoluminescent (Y0.92Eu0.08)2O3 films

Fluorescent red-emitting (Y0.92Eu0.08)2O3 films were deposited on sapphire substrates by the metallorganic chemical vapor deposition technique. The films were weakly luminescent in the as-deposited condition. The as-deposited films were composed of nanocrystals embedded in columnar grains. A KrF laser with ultraviolet (λ=248 nm) pulses at a fluence level between 0.9 and 2.3 J/cm2 was applied to different regions of the film. Increasing the energy fluence density initially increased the photoluminescence intensity but decreased it at the highest level. Transmission and scanning electron microscopy verified that surface melting and ablation occurred at all fluence levels. Computational modeling of the laser melting and ablation process predicted that a significant fraction of the film is removed by ablation at the highest fluence levels, thereby decreasing the photoluminescence intensity of the films due to the significant amount of material removed.

Thermal Stability and Wetting Properties of Cr–O–C Coatingssynthesized by Plasma Immersion Ion Processing

Microcrystalline Cr-O-C films, synthesized by the plasma immersion ion processing technique, were annealed in vacuum and air at 700 °C for different times so their thermal stability could be evaluated. The study showed that annealing does not greatly change the chemical composition of the Cr-O-C films but induces a transformation of the film structure from the mixed phases of Cr 2 O 3 , CrO 2 , and CrC 1 . 1 3 O 0 . 1 2 2 to the dual phases of Cr 3 C 2 , and Cr 2 O 3 . In addition, the annealed Cr-O-C films exhibit excellent nonwetting qualities to liquid aluminum despite the presence of pin holes. The contact angle of resolidified molten aluminum against the Cr-O-C films was found to be as high as 120-150°. The aluminum contact angle was found not to vary with thermal aging of the Cr-O--C films. The Cr-O-C films showed high hardness (16-24 GPa) and good wear resistance as compared to the steel substrates (H 13 and 304 stainless steels) used in this study.

Improvement of luminescent properties of thin-film phosphors by excimer laser processing

Thin-films of europium doped yttrium oxide, (Y1-xEux)2O3, were deposited on sapphire substrates by metallorganic chemical vapor deposition. The films, -400 nm thick, were weakly luminescent in the as-deposited condition. A KrF laser was pulsed once on the surface of the films at a fluence level between 0.9 - 2.3 J/cm2. One pulse was sufficient to melt the film, which increased the photoluminescent emission intensity. Melting of a rough surface resulted in smoothing of the surface. The highest energy pulse resulted in a decrease in luminous intensity, presumably due to material removal. Computational modeling of the laser melting and ablation process predicted that a significant fraction of the film is removed by ablation at the highest fluence levels.

INTERNAL ENERGY DISTRIBUTIONS IN A SHIELDED PLASMA DEVICE

A shielded plasma device has been used extensively in plasma synthesis and plasma processing. In order to optimize these processes, optical diagnostics were used to study the device operated at 545 kHz and pressures between 2.5 and 580 Torr. Plasma‐induced emission for Ar, N2, and N+2 suggested that a local thermodynamic equilibrium point of view was inappropriate. Spatially resolved laser‐induced fluorescence and laser absorption measurements were performed on the metastable argon state 3P2 and the radiative state 3P1. The metastable profiles were described in terms of a kinetic model dominated by electron‐atom kinetics. The model was used to determine the electron temperature Te as a function of axial position and total gas pressure. These temperatures were found to lie in the range of 5000 K at 580 Torr to 11 000 K at 25 Torr.