Oxidation Of NiAl Research Articles

Formation of multilayered scale during the oxidation of NiAl–Mo alloy

We have studied the oxidation behavior of a hypereutectic NiAl–Mo alloy. This alloy showed an initial rapid mass loss followed by a relatively steady state behavior. The oxide scale formed during the oxidation process was seen to have a multilayered structure comprising of NiO, NiAl2O4, NiMoO4 and Al2O3 with minor amounts of MoO2 in the sub-scale region. The oxidation behavior is influenced significantly by the formation and stability of the constituent oxides, especially NiMoO4. Hence the decomposition behavior of NiMoO4 in the 1100–1200°C was studied as well. The thermal decomposition of the NiMoO4 was slow at 1100°C, but accelerated at 1200°C, resulting in the formation of NiO, which remained in the oxide scale, and MoO3, which volatilized away.

X-ray Photoelectron Spectroscopy Study of the Passivation of NiAl(100) by Water Vapor

The oxidation of NiAl(100) surfaces by water vapor is studied using X-ray photoelectron spectroscopy (XPS) to elucidate the effect of temperature and vapor pressure on the surface passivation mechanism of the NiAl alloy. The water-vapor oxidation at ambient temperature (25 °C) results in self-limiting Al(OH)3/Al2O3 bilayer film growth to a less extent of the limiting thickness regimes, in which the growth of the inner Al2O3 layer occurs via dehydration of the outer Al(OH)3 layer. The growth of the passivating overlayer at the ambient temperature depletes Al and forms a Ni-rich layer at the oxide/alloy interface that impedes supply of Al atoms to the outer surface for Al(OH)3 formation via the hydration reaction, whereby resulting in a more Al-deficient structure of the outer Al(OH)3 layer upon increasing the vapor pressure. In contrast, the water-vapor oxidation at 300 °C results in Al2O3 single-layer film growth to a larger limiting thickness without involving the transient hydroxide phase of Al(OH)3. It is shown that increasing the oxidation temperatures results in the formation of a more compact Al2O3 film owning to the enhanced bulk diffusion rate that maintains an adequate supply of Al atoms to the oxide/alloy interface to sustain the oxide film growth to the full extent of the limiting thickness.

Growth of ultrathin amorphous alumina films during the oxidation of NiAl(100)

The effect of temperature on the oxidation of NiAl(100) is comparatively studied at 25°C and 300°C using X-ray Photoelectron Spectroscopy to elucidate the effect of oxide–alloy interfacial reaction on the growth of ultrathin alumina thin films. The oxidation at 25°C results in self-limiting aluminum oxide film growth to a less extent of the limiting thickness regimes with non-stoichiometric oxide films exhibiting a deficiency of Al cations, whereas for the oxidation at 300°C the oxide films grow to a larger limiting thickness with relatively enriched with Al at the limiting thickness. The temperature dependent limiting thickness and composition of the oxide films are ascribed to the transport velocity of Al from deeper layers to the oxide/alloy interface during the oxide growth. For the oxidation at 25°C the oxide film growth depletes Al and forms an underlying Ni-rich interfacial layer that blocks the supply of Al atoms to the oxide/substrate interface, whereas for the oxidation at 300°C the enhanced diffusion rate maintains adequate supply of Al atoms to the oxide/alloy interface to sustain the oxide film growth to the full extent of the limiting thickness.

The formation of double-row oxide stripes during the initial oxidation of NiAl(100)

The initial growth of ultrathin aluminum oxide film during the oxidation of NiAl(100) was studied with scanning tunneling microscopy. Our observations reveal that the oxide film grows initially as pairs of a double-row stripe structure with a lateral size equal to the unit cell of θ-Al2O3. These double-row stripes serve as the very basic stable building units of the ordered oxide phase for growing thicker bulk-oxide-like thin films. It is shown that the electronic properties of these ultrathin double-row stripes do not differ significantly from that of the clean NiAl surface; however, the thicker oxide stripes show a decreased conductivity.

Methane steam reforming: Kinetics and modeling over coating catalyst in micro-channel reactor

Kinetics of methane steam reforming for hydrogen production has been studied through experiment in a micro-channel reactor over coating catalyst. The catalyst coating prepared by cold spray on stainless steel substrate is based on a mixture of Ni–Al oxides which is normally employed in industry for methane primary steam reforming. Two kinetic laws namely parallel as well as inverse models have been derived at atmospheric pressure, and power law type kinetics have been established using non-linear least squares optimization. With the above kinetics, simulation study has been carried out to find out temperature distribution in the micro-channel over coating catalyst at two different types of boundary conditions. The results show a quite different “cold spot” character and reactants, products distribution character in the reaction channel due to its own distinct heat and mass transfer features. The kinetics and simulation study results can be applied in aid of micro-channel reactor design, and suggestion has been proposed for catalytic coating preparation and optimization.

Oxidation behavior and thermal stability of a NiAl-V eutectic alloy

Oxidation behavior and thermal stability of a NiAl–V alloy with eutectic composition processed by directional solidification technique has been investigated. The surface analysis at the elevated temperature indicated that the investigated microstructures are stable at the isothermal conditions and an inert atmosphere. The oxidation testing in the synthetic air showed that the temperature of 400 °C is critical. In addition, the oxidation of the NiAl–V eutectic alloy is characterized by: (a) alteration of composition immediately below the surface substrate/oxide; (b) formation of the oxide layer rich in V, adherent to the substrate; and (c) formation of external oxide layer that presents oxide mixture formed by vanadium, nickel, and aluminum. Microstructure of the substrate/oxide interface of NiAl–V alloy oxidized at 900 °C for 24 h.

2-2-2 Ni-Al酸化物系触媒のCO選択メタン化反応におけるバナジウム添加効果と耐久性(2-2 天然ガス科学,Session 2 天然ガス・メタンハイグレート等)

2-2-2 Ni-Al酸化物系触媒のCO選択メタン化反応におけるバナジウム添加効果と耐久性(2-2 天然ガス科学,Session 2 天然ガス・メタンハイグレート等)

Effect of water vapor on the phase transformation of alumina grown on NiAl at 950 °C

This particular study includes the analysis of the effect of H 2O on promoting the phase transformation in thermally grown aluminum oxides formed on NiAl at 950 °C. Oxidation of NiAl is carried out at 950 °C in O 2 and O 2 + 15 vol.% H 2O. It is observed that transient alumina initially formed on NiAl transforms to stable α-alumina in the presence of water vapor which promotes the subject transformation and eventually results in a compact scale, offering superior oxidation resistance. Present study includes the analysis of θ to α-alumina transformation under the effect of temperature and environment.

Pulsed Laser Deposition and In Situ Scanning Tunneling Microscopy of Pd clusters supported on alumina

ABSTRACTWith the aim of addressing the material gap issue between model and real systems in heterogeneous catalysis, we exploited Pulsed Laser Deposition (PLD) to produce Pd clusters supported on ultrathin alumina films (Pd/Al2O3/NiAl(001) and Pd/Al2O3-x/HOPG). The structural properties have been investigated by in situ Scanning Tunneling Microscopy (STM) in ultra high vacuum (UHV). At first, Pd clusters were deposited by evaporation and by PLD on Al2O3 surfaces grown by thermal oxidation of NiAl(001). The system shows thermal stability up to 650 K. By PLD we deposited Pd clusters with a good size control obtained by varying the background gas pressure and the target-to-substrate distance. We then realized aPd/Al2O3-x/HOPG system where both Pd clusters and the alumina film are produced by PLD showing that, by exploiting the same deposition technique, it is possible to synthesize both a model system addressable by in situ STM and a thick film (∼100 μm) closer to realistic systems.

High Catalytic Performance of Ruthenium‐Doped Mesoporous Nickel–Aluminum Oxides for Selective CO Methanation

Deep-cleaned hydrogen can be made available for fuel cells in one step. 1 vol % CO contained in hydrogen was removed to levels of less than 10 ppm over ruthenium-doped mesoporous Ni-Al oxides (see picture) by selective CO methanation, with a working temperature window greater than 50 °C that included the 200–250 °C range. The catalysts exhibited excellent long-term stability. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Growth of thin epitaxial alumina films onto Ni(111): an electron spectroscopy and diffraction study

AbstractAn epitaxial ultra‐thin alumina layer was prepared on Ni(111) by simultaneous Al deposition and oxidation at elevated temperatures. Different deposition procedures lead to creation of oxide layers of various thicknesses and quality. Low‐energy electron diffraction revealed epitaxial parameters of the films and the relationship between the nickel substrate and the oxide overlayer. Core‐level photoelectron spectroscopy using Al‐Kα and synchrotron‐generated photons show that the oxide‐metal interface is formed by oxygen atoms, which is contrary to the case of epitaxial alumina layers prepared by the oxidation of NiAl(110) and Ni3Al(111) surfaces. The results of a photoelectric work function measurement are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.

Ni–Al–Ti coatings obtained by microwave assisted SHS: Oxidation behaviour in the 750–900 °C range

Microwaves have been used to ignite the Self-propagating High-temperature Synthesis (SHS) of Ni and Al powder mixtures to produce a duplex intermetallic coating on Ti substrates. Due to the high exothermic nature of the reaction, the newly formed NiAl is in the liquid phase and can react with the underlying Ti to form a tough ternary intermediate layer, belonging to the Ti–Ni–Al system, in a one step process. Aim of this work is to assess the high-temperature performances of the Ti–Ni–Al layer, compared to NiAl coating and Ti. Experimental results demonstrate that the ternary layer presents oxidation resistance comparable to NiAl up to 750 °C. In this condition, the thick Ti–Ni–Al layer could replace the functionality of hard and brittle NiAl coatings. At 900 °C, instead, NiAl oxidation resistance results higher, and this can be ascribed to the relatively low Al content in the studied ternary compound, which hinders the formation of a continuous and protective scale.

High temperature water–gas shift reaction over hollow Ni–Fe–Al oxide nano-composite catalysts prepared by the solution-spray plasma technique

Abstract The effect of Fe content in Ni–Fe–Al oxide nano-composites prepared by the solution-spray plasma technique on their catalytic activity for the high temperature water–gas shift reaction was investigated. The composites showed a hollow sphere structure, with highly dispersed Fe–Ni particles supported on the outer surface of the spheres. When the water–gas shift reaction was performed over an Ni–Al oxide composite catalyst without Fe, undesired CO methanation took place predominantly compared to the water–gas shift reaction, and significant amounts of hydrogen were consumed. When appropriate amounts of Fe were added to the Ni–Al oxide composite catalyst during the plasma process, methanation was suppressed remarkably, without serious loss of activity for the water–gas shift reaction. The catalyst was characterized by STEM, XRD and H2 chemisorption measurements.

Effect of magnetic abrasive treatment on high-temperature oxidation of NiAl and NiAl–Re coatings

A comparative microstructural analysis of the scale formed on the NiAl–Re detonation coating shows that the mechanism of its high-temperature oxidation can be changed with preliminary magnetic abrasive treatment (MAT). The scale formed over the NiAl–Re coating in ordinary conditions includes several layers (NiO–NiAl2O4–Al2O3), while the scale formed on the MAT coating consists of an Al2O3 monolayer with spinel inclusions. The differences in the oxidation of the NiAl and NiAl–Re coatings after MAT are attributed to changes in their dislocation structure.

γ-Al2O3 Thin Films Catalyst Model Support Preparation on β-NiAl(110) Alloy

AbstractLow temperature oxidation of NiAl(110) alloy has been studied with surface characterization methods and cross-section TEM. Our work demonstrated thin and high quality single crystal γ-Al2O3(111) films have grown heteroepitaxially on NiAl(110) alloy. The orientation relationship of metal/oxide is NiAl(110) || γ-A12O3(111)and NiAl(211)∥γ-Al2O3(311) with very slight lattice mismatch. Al cation outward diffusion dominates during this low temperature oxidation confirmed by EELS mapping along interface.

Large oriented mesoporous self-supporting Ni–Al oxide films derived from layered double hydroxide precursors

Large (~1 cm2) transparent highly (111)-oriented mesoporous self-supporting Ni–Al oxide films of uniform small nanoparticles have been prepared using an Ni2Al(OH)6(NO3)·2.1H2O layered double hydroxide (LDH) as a single precursor. The monodisperse small LDH nanoparticles (about 20 nm in diameter) are first cast as an oriented assembly on a glass substrate to form large transparent self-supporting (00l)-oriented LDH films. Subsequent heating in air affords (111)-oriented mesoporous Ni–Al oxide films preserving the shape, dimensions and optical transparency of the films. The process involves a topotactic transformation from the LDH (00l) facet to the NiO and NiAl2O4 spinel (111) facets, demonstrated here for the first time, and does not require any template, structure-directing agent, or lattice-matched single crystal substrate. The nanostructures of the resulting mixed metal oxide films can be controlled by changing the calcination temperature: Al-doped NiO and composite NiO/NiAl2O4 films of uniform small nanoparticles have been obtained at 500 °C and 900 °C respectively. The pore size and pore size distribution increase monotonically with temperature due to the increased sintering of the nanoparticles at higher temperatures. The resulting large transparent Ni–Al oxide films have a narrow distribution of mesopores (<10 nm) and high thermal stability, suggesting their potential application as catalysts or catalyst supports, in sensors, and as ultrafiltration membranes in harsh environments.

NiAl Oxidation and Corrosion Resistant Coatings Obtained by Thermal Spraying

There are investigated the possibilities to avoid or at least to reduce the Al2O3 scales formation on NiAl powder particles at its plasma spray deposition on steel substrates. The optimum processing parameters and the necessity to surround the plasma jet by an inert gas have been established. In appropriate processing conditions, the obtained coating layer is formed by flattened particles, welded together and to the substrate, proving their melting during spraying. It is dense and adherent, consisting of NiAl with only small Al2O3 inclusions, proving the NiAl stability preserving without decomposition or a notable oxidation, as premises of its desired functionality achievement.

Growth of thin alumina films on a vicinal NiAl surface

Dramatic changes in the surface morphology have been observed during the oxidation of stepped NiAl(16, 14, 1) by LEED and STM. The initial sequence of identical (1 1 0) terraces is lifted in favor of large, triangular planes, whose mean size is determined by the mismatch-induced stress that accumulates in the thin alumina film. The asymmetry of the original step direction on NiAl(16, 14, 1) with respect to the orientation of the two alumina reflection domains favors the formation of one domain type, for which the stress relief via NiAl step edges is particularly efficient.

Erratum

“In situ x-ray study of the γ- to α-Al2O3 phase transformation during atmospheric pressure oxidation of NiAl(110)” [J. Mater. Res. 21, 3047 (2006)]

Nanoscopic nickel aluminate spinel (NiAl 2O 4) formation during NiAl(1 1 1) oxidation

Oxidation of a NiAl(1 1 1) single crystal surface was investigated using high resolution soft X-ray photoelectron spectroscopy (HRSXPS), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray mapping, and atomic force microscopy (AFM). After repeated oxygen exposure, annealing, and cleaning cycles under ultrahigh vacuum conditions, a new oxide phase in the form of tiny 3-dimensional surface structures was detected. These features are several micrometers long and ∼300 nm high and oriented along low index 〈 1 ¯ 0 1 〉 directions in the plane of the substrate; they have nickel aluminate spinel (NiAl 2O 4) stoichiometry. We propose that repeated cycles of oxygen dosing and annealing of the NiAl(1 1 1) surface leads to oxygen diffusion into the bulk and nucleation of spinel below the surface.