Tabbetha Dobbins

A novel laser-liquid-solid interaction technique for synthesis of silver, nickel and immiscible silver-nickel alloys from liquid precursors

Silver, nickel, nickel oxide and silver-nickel alloys have been produced from their inexpensive liquid precursors using CO2 and Nd-YAG lasers. Ethylene glycol, diethylene glycol and 2-ethoxyethanol were used as reductants in the synthesis reactions. Spherical and faceted silver particles of high purity were formed by laser interaction between the precursor solution and a rotating substrate, while porous dual phase nickel and nickel oxide particles were produced when nickel nitrate was used as a precursor. The composition and morphology of the alloy particles was dependent on laser parameters and chemical composition of the precursor solution. The product composition was dependent only upon the chemistry of the precursors used. The mean particle size was dependent upon the temperature generated by irradition and the duration of exposure to the laser beam. The synthesis of nano-particles and metastable alloys is proposed to occur primarily at the laser-liquid-solid interface by a nucleation and growth mechanism.

HVOF thermal spray deposited Y2O3-stabilized ZrO2 coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (Po) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures Tp that are high enough for partial melting of small (∼10 µm) zirconia particles, Tp=(1.10−0.95)Tm. Further analysis shows that for larger particles (38 µm), adherent coatings are produced when the particle temperature, Tp=0.59−0.60 Tm, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.

Deformation twinning at low temperatures in a Hf–V–Nb cubic laves phase

Deformation twinning in the C15 cubic laves phase of the Hf-V-Nb alloy system is analyzed at temperatures between 77 K and room temperature by conventional and high-resolution TEM. In order to reduce the stacking fault energy (SFE), alloy compositions are chosen such that the cubic laves phase composition is close to a region of C14 phase stability. A high density of stacking defects of intrinsic character in the as-homogenized alloy confirms that the SFE has been reduced. Twinning is observed at all temperatures placing a low upper limit on any thermally activated deformation process. Twin clusters are observed which are narrow and have a similar distribution to the stacking defects in the undeformed alloy. The structure of twins and stacking defects is solved via HREM combined with image calculations. Finally, the mechanism of twinning is discussed and the use of alloying to control the SFE or to refine the microstructure in order to increase the low temperature ductility in these intermetallic compounds with complex cubic crystal structures is proposed.

An x-ray photoemission electron microscopy study of the formation of Ti-Al phases in 4 mol% TiCl3 catalyzed NaAlH4 during high energy ball milling.

This study reports reaction pathways to form TiAlx metallic complexes during the high energy ball milling of 4 mol% TiCl3 with NaAlH4 powders determined using local structure analysis of Tix+ and Alx+ species. Using x-ray photoemission electron microscopy (XPEEM) and x-ray diffraction (XRD), the oxidation state of Alx+ and Tix+ and the crystalline compounds existing in equilibrium with NaAlH4 were tracked for samples milled for times of 0 (i.e. mixing), 5, and 25 min. XPEEM analysis of the Al K edge after 5 min of milling reveals that Al remains in the 3+ oxidation state (i.e. in NaAlH4) around Ti0-rich regions of the sample. After 25 min of high energy milling, Ti0 has reacted with Al3+ (in nearby NaAlH4) to form TiAlx complexes. This study reports the pathway for TiAlx complex formation during milling of 4 mol% TiCl3catalyzed NaAlH4 to be as follows: (1) Ti3+ reduces to Ti0 (with Al3+ near Ti0 regions) and (2) Ti0 reacts with Al3+ in NaAlH4 to form TiAlx complexes.

Synthesis of Micron and Submicron Nickel and Nickel Oxide Particles by a Novel Laser–Liquid Interaction Process

The laser–liquid–solid interaction is a new technique for synthesis of nickel and nickel oxide particles. The process uses a continuous-wave CO2 laser beam as the source of thermal energy required to induce precipitation reactions in solution. The uniqueness of the process is the synthesis reaction taking place in a localized region, which allows better control of the chemical reaction. Porous nickel and nickel oxide powders have been synthesized by laser-induced reactions between a nickel nitrate hexahydrate [Ni(NO3)2·6H2O] precursor and 2-ethoxyethanol-based mixtures. Nickel powders were produced after irradiating a solution of the precursor salt and a 2-ethoxyethanol and d-sorbitol mixture. Crystalline nickel oxide (NiO) powders were isolated after irradiating a solution containing the precursor salt and a 2-ethoxyethanol and water mixture. Powders containing both nickel and nickel oxide crystalline phases were produced after irradiating a solution of the precursor salt and 2-ethoxyethanol. The mean particle diameter is found to be sensitive to irradiation time, substrate thermal conductivity, irradiation power density, and solution concentration. It is hypothesized that nucleation and growth of crystalline phases occurring in irradiated solutions are thermal driven.

Phase distribution in, and origin of, interfacial protrusions in Ni–Cr–Al–Y/ZrO2 thermal barrier coatings☆

Interfacial morphology and reaction products in thermal barrier coating systems were investigated by scanning and transmission electron microscopy (SEM and TEM). The samples consist of yttria-stabilized zirconia (YSZ; 6–8 wt.% yttria) deposited by air plasma spraying onto either of two types of bond coats: a layer consisting of Ni–15.9Cr–5.3Al–0.6Y with 5 wt.% of alumina particulate added, or one that was only the base Ni–Cr–Al–Y composition. In samples thermally cycled to failure in a burner rig, numerous interfacial protrusions of several microns or more in size are observed. These have a complex microstructure and contain elemental Ni intermixed with Ni(Al,Cr)2O4 spinel, (Al,Cr)2O3, and other oxides. Unlike some prior studies, nickel oxide (NiO) was not detected. Protrusion microstructures were similar for the two bond coat systems, but interfacial protrusions for the case of the base composition (i.e. no added alumina particulate) did not contain any spinel phase. Comparison of cross-sectional samples before and after oxidation indicates that the protrusions arise from the encapsulation of isolated segments of the bond coat. The intermixing of metallic Ni grains with oxides in the reaction zone may contribute to failure by affecting local stresses during thermal cycling.

Novel Nanomaterials Synthesis by Laser-Liquid-Solid Interaction

A novel laser-liquid-solid interaction (LLSI) technique has been developed that is highly flexible and allows engineering nanomaterials in form of particles, rods, and tubes from liquid precursors. Synthesis reaction takes place in a molecular level and reaction rate is controlled by laser energy input, precursor solution chemistry, and other processing parameters including interaction time, and thermal conductivity of solid that is spinning in the solution. By the proper selection of liquid precursor, mono-dispersed silver nanoparticles were produced with average particle size 7–10 nm. Alloy composed Ag and Ni were produced in the form of nanoparticles and nanotubes with an average diameter of 40 nm. HRTEM of nanorod exhibited that Ag acted as seed for the synthesis of immiscible of Ag–Ni alloy. Laser writing of silver exhibited 50% lower electrical resistivity and eliminated many intermediate steps involved as compared to conventional silver patterning process.

Decomposition Behavior of Ti-doped NaAlH 4 Studied using X-ray Absorption Spectroscopy at the Titanium K-edge

The local bonding environment of Ti 3+ -dopant atoms in NaAlH 4 after decomposition to release H 2 has been studied using x-ray absorption fine structure (XAFS). The titanium K-edge spectra from doped hydride samples and the standard materials TiCl 3 and TiO 2 were collected in ambient atmosphere at the synchrotron source at the Center for Advanced Microstructures and Devices (CAMD). Titanium valence states present in the spectra collected from Ti-doped NaAlH 4 after decomposition in air are Ti 3+ and Ti 4+ . The Ti 3+ is attributed to unreacted TiCl 3 . The Ti 4+ present in the sample is attributed to TiO 2 occurring after air oxidation. Coupled with studies of the kinetics of hydrogen desorption reactions, examination of dopant ion valence states after entry into the lattice may lead to better understanding of the interrelationship between lattice doping and desorption kinetics.

Study of Morphological Changes in MgH2 Destabilized LiBH4 Systems Using Computed X-ray Microtomography

The objective of this study was to apply three-dimensional x-ray microtomographic imaging to understanding morphologies in the diphasic destabilized hydride system: MgH2 and LiBH4. Each of the single phase hydrides as well as two-phase mixtures at LiBH4:MgH2 ratios of 1:3, 1:1, and 2:1 were prepared by high energy ball milling for 5 minutes (with and without 4 mol % TiCl3 catalyst additions). Samples were imaged using computed microtomography in order to (i) establish measurement conditions leading to maximum absorption contrast between the two phases and (ii) determine interfacial volume. The optimal energy for measurement was determined to be 15 keV (having 18% transmission for the MgH2 phase and above 90% transmission for the LiBH4 phase). This work also focused on the determination of interfacial volume. Results showed that interfacial volume for each of the single phase systems, LiBH4 and MgH2, did not change much with catalysis using 4 mol % TiCl3. However, for the mixed composite system, interphase boundary volume was always higher in the catalyzed system; increasing from 15% to 33% in the 1:3 system, from 11% to 20% in the 1:1 system, and 2% to 14% in the 2:1 system. The parameters studied are expected to govern mass transport (i.e., diffusion) and ultimately lead to microstructure-based improvements on H2 desorption and uptake rates.

Dispersion of Single Walled Carbon Nanotubes by Self Assembly of Polymers

Dispersion and stability of single walled nanotubes (SWNT) is one of the inhibiting factors affecting their tailorability for various electronic, chemical and mechanical applications . The realization of these applications depends on dispersing the SWNTs in aqueous media by inducing high forces of repulsion among the nanotubes. Steric repulsions are induced to the nanotubes by attaching polyelectrolytes, like poly styrene sulfonate (PSS) and poly allyl amine hydrochloride (PAH). In this work, Self Assembly technique is employed to attach polyelectrolytes, and thereby enhance the dispersion of SWNTs in aqueous media. The steric forces produced by the attached polyelectrolytes overcome the high van der waals force of attraction between the nanotubes and aid in the nanotubes dispersion. Characterization of the dispersions with UV-Vis Spectrophotometric method in kinetic mode revealed that nanotubes treated at pH 3 are seen to be more stable than the ones treated at pH 7. The effect of pH of the polyelectrolyte solutions in the assembly and its consequence on dispersion stability is also studied with zeta potential measurements. The morphology of the films produced by drying the nanotubes in vacuum on a silicon substrate is characterized by field emission scanning electron microscopy (FESEM).