X-ray photoelectron spectroscopic studies on initial oxidation of iron and manganese mono-silicides

Initial oxidation of iron and manganese mono-silicides (FeSi and MnSi) surfaces was studied by X-ray photoelectron spectroscopy (XPS). Clean surfaces of these silicides were prepared by fracturing in an ultra high vacuum, and then the fractured surfaces were oxidized by exposing to high-purity oxygen at pressures up to 1.3 Pa. For the clean FeSi surface, positive chemical shifts of the Fe 2p_3/2 and Si 2p peaks from elemental Fe and Si were 0.5 eV and 0.1 eV, respectively. For the clean MnSi surface, a negative chemical shift of the Si 2p peak from elemental Si was 0.1 eV. Iron on the FeSi surface was oxidized at an oxygen pressure of 1.3 Pa, whereas the silicon was oxidized under the pressure of 1.3 × 10⁻⁶ Pa, indicating that oxidation of silicon occurred prior to that of iron. Manganese and silicon on the MnSi were simultaneously oxidized in the range from 1.3 × 10⁻⁶ Pa to 1.3 × 10⁻³ Pa; however, over the pressure of 1.3 Pa, the oxidation of manganese occurs prior to that of silicon. These oxidation behaviors at low oxygen pressures were similar to those of the FeSi and MnSi fractured in air.     

A Mössbauer-effect study of the isomer shifts of iron in nickel aluminides

The room-temperature isomer shifts of iron in nickel aluminides have been measured. A single Mössbauer-resonance line is observed for alloys of composition (NiAl)_100-cFe_c, 0 ? c ? 5 at. %, and Al₅₀Ni^50-cFec, 2 ? c ? 50 at. %. The isomer shift for dilute Fe concentration in stoichiometric NiAl is compared with that for FeAl at stoichiometric composition.     

Surface studies of 2,4-pentanedione on γ-Al2O3/NiAl (1 0 0) and NiAl (1 0 0)

The chemical compound 2,4-pentanedione (Hacac) has been shown to etch the oxidized metal surfaces metals such as copper and nickel, but not their unoxidized surfaces. Here it is shown that on the γ-Al₂O₃/NiAl (1 0 0) surface (oxidized NiAl (1 0 0)) etching of aluminum occurs at 170 K and 750 K. Reflection-absorption infrared spectroscopy (RAIRS) is used to show that Hacac binds to both the clean, metallic and oxidized surfaces, but decomposition and combustion products dominate on the metallic surface and no etching occurs. The binding process that involves a deprotonation reaction of the enol species was identified by redshift in the carbonyl peaks and the appearance of an Al-H peak observed in the IR spectrum. The implication of these results is that there is both an unusual low temperature and high temperature etching of the alumina by bound acac.     

Growth of Ni-Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni₃Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10¹⁵ Al/cm² and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10¹⁵ Al/cm². Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni₃Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni₃Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1₂ Ni₃Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni₃Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.     

Adsorption of methanol on Ni3Al(1 1 1) and NiAl(1 1 0): A high resolution PES study

The adsorption of methanol on Ni₃Al(1 1 1) and NiAl(1 1 0) has been studied using high resolution photoemission spectroscopy (HR-PES) and density functional theory (DFT). Both methanol and methoxy are formed on these surfaces after the initial methanol exposure at low temperatures. Heating to 200 K leads to further formation of methoxy. On NiAl(1 1 0) two different methoxy species are observed where the first is formed upon methanol adsorption, and the other results from methanol decomposition during heating. The DFT calculations show that methanol and methoxy interacts with the Al atoms on both surfaces. Methanol is found to bond through the oxygen atom to the Al on-top site on Ni₃Al(1 1 1) and NiAl(1 1 0) with the C–O axis tilted with respect to the surface normal. On Ni₃Al(1 1 1) methoxy is situated in a 2Ni+Al hollow site, whereas on NiAl(1 1 0) the Al–Al bridge site is preferred.     

Growth of thin, crystalline oxide, nitride and oxynitride films on metal and metal alloy surfaces

This paper reviews the current knowledge of the properties of ultrathin, well-ordered oxide, nitride and oxynitride films grown on metal and metal alloy surfaces. Different modes of preparation are discussed and the vibrational and structural properties are summarized. The focus will be put onto Al-oxides grown on surfaces of the intermetallic alloys NiAl, Ni₃Al and FeAl. The properties of Ga₂O₃ grown on surfaces of the intermetallic alloy CoGa are also described. In these cases Al- and Ga-atoms, respectively, segregate from the substrate and react with adsorbed oxygen. The grown Al- and Ga-oxide films, respectively, order at elevated temperatures. Systems are also discussed where Al-oxide is grown by oxidation of Al-atoms which are evaporated on surfaces of the transition metals Re and Ru. The growth of transition metal oxides CoO (1 1 1)/Co (0 0 0 1), CoO (1 0 0)/Co (1 1 2 0), NiO (1 0 0)/Ni (1 0 0), NiO/Ni (1 1 1) and Cr₂O₃/Cr (1 1 0) are also presented. Thin films of Al- and Ga-nitride, respectively, can be grown on the base of the intermetallic alloy NiAl and CoGa by low-temperature adsorption of ammonia. These nitride films order at elevated temperatures. Al- and Ga-oxynitride, respectively, can be grown on surfaces of NiAl and CoGa substrates by adsorption on nitric oxide. An ordering of these ultrathin oxynitride layers is observed at elevated temperatures.     

Reactivities of ultrathin alumina films exposed to intermediate pressures of H2O: Substrate-mediated mechanism for growth and loss of surface order

The response of ordered ultrathin Al₂O₃ films on NiAl(1 1 0) and Ni₃Al(1 1 0) substrates to sequential exposures at varying pressures of H₂O between 10⁻⁷ Torr and 10⁻³ Torr, ambient temperature, was characterized by LEED, AES and density functional theory (DFT) calculations. In all cases, an increase in average oxide thickness, as determined by AES, was observed, consistent with a field-induced oxide growth mechanism. Ordered oxide films of initial average thicknesses of 7 Å and 12 Å grown on NiAl(1 1 0) achieved a limiting thickness of 17(1) Å, while films of initial thickness of 7 Å and 11 Å grown on Ni₃Al(1 1 0) achieved a limiting thickness of 12(1) Å. The LEED patterns for the thinner (7 Å) films were not observed after exposure to 10⁻⁵ Torr (NiAl(1 1 0)), or 10⁻⁴ Torr (Ni₃Al(1 1 0)). In contrast, LEED patterns for the films of greater initial thickness persisted after exposures to 10⁻³ Torr UHV. DFT calculations indicate an Al vacancy formation energy that is significantly greater (by ∼0.5 eV) on the surface that has the thicker oxide film, directly opposite to what may be naively expected. A simple coordination argument supports these numerical results. Therefore, the greater limiting oxide thickness observed on NiAl(1 1 0) demonstrates that the rate determining step in the oxide growth process is not Al removal from the metal substrate and transport across the oxide/metal interface. Instead, the results indicate that the determining factor in the oxide growth mechanism is the kinetic barrier to Al diffusion from the substrate bulk to the oxide/metal interface. The persistence of the LEED patterns observed for the films of greater initial oxide thickness indicates that the surface disorder generally observed for alumina films grown on aluminide substrates and exposed to intermediate pressures of H₂O is due to the growth of a disordered alumina layer over an ordered substrate, rather than to direct H₂O interaction with terrace sites.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Size-dependent oxidation of Pdn (n ? 13) on alumina/NiAl(110): Correlation with Pd core level binding energies

Small Pd clusters Pd_n (n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O₂ at temperatures ranging from 100 to 500 K. After O₂ exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd₄, which did not oxidize under any conditions. The inertness of Pd₄/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O₂ exposures at 300 K or above, but He⁺ scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.     

An investigation of the oxidized Ni/InAs interface

Ni was resistively deposited onto bulk InAs and subsequently oxidized in an O₂ atmosphere. The anneal temperature and time were 450 °C and 2.5 h, respectively. X-ray diffraction of the oxidized Ni/InAs sample revealed the formation of In₃Ni₂ and In₂O₃ on the front suggesting inter diffusion of In, Ni and O. NiO was not detected by X-ray diffraction. In a preliminary study, using glass as a substrate, NiO readily formed when using these oxidation parameters. Conductivity measurements of the oxidized Ni/InAs surface revealed a conducting front and insulating rear surface while TEM of the Ni/InAs interface revealed an intermediate amorphous diffusion zone between the “oxidized” Ni layer and the bulk InAs. A closer investigation of the intermediate layer supports the X-ray diffraction results, suggesting compound formation due to diffusion of oxygen and nickel into the substrate, and out-diffusion of In and As from the bulk of the sample. AES was used to further elucidate these results.     

Adsorption of methanol and methoxy on NiAl(1 1 0) and Ni3Al(1 1 1): A DFT study

The adsorption of methanol and methoxy on NiAl(1 1 0) and Ni₃Al(1 1 1) has been investigated using density functional theory (DFT). Optimised adsorption geometries and core level shifts are presented. On both surfaces we find that methanol binds to the Al on-top site via its oxygen atom and with the C–O axis tilted away from the surface normal. Methoxy also shows a preference for Al-dominated sites. On NiAl(1 1 0), we predict that methoxy adsorbs with its oxygen atom in the Al–Al bridge site, while it is determined to be adsorbed with its oxygen atom in a 2Ni + Al hollow site on Ni₃Al(1 1 1), closer to Al than Ni. Surface and adsorbate induced binding energy shifts in the Al 2p states are calculated and found to be in good agreement with experimental high resolution photoelectron spectroscopy results.     

Comparison of O adsorption on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces through first-principles calculations

First-principles calculations were performed to study the properties of O adsorption on Ni₃Al (0 0 1), (0 1 1), and (1 1 1) surfaces using the Cambridge serial total package (CASTEP) code. Stable adsorption sites are identified. The atomic and electronic structures and adsorption energies are predicted. The adsorption sites for O on the Ni₃Al (0 0 1) surface are at the 2Ni–2Al fourfold hollow site, whereas O prefers to adsorb at the Ni–Al bridge site on (0 1 1) surface and 2Ni–Al threefold hollow site on (1 1 1) surface. It is found that O shows the strongest affinity for Al and the state of O is the most stabilized when O adsorbs on (0 0 1) surface, while the affinity of O for Al on (0 1 1) surface is weaker than (0 0 1) surface, and (1 1 1) surface is the weakest. The stronger O and Al affinity indicates more stable Al₂O₃ when oxidized. The experiment has shown that the oxidation resistance of single crystal superalloy in different orientations improves in the order of (1 1 1), (0 1 1), and (0 0 1) surface, suggesting that the oxidation in different crystallographic orientations may be related to the affinity of O for Al in the surface.     

Effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite

The effect of nickel substitution on defect chemistry, electrical properties, and dimensional stability of calcium-doped yttrium chromite was studied for use as an interconnect material in high temperature solid oxide fuel cells (SOFCs). The compositions of Y_0.8Ca_0.2Cr_1 − xNi_xO_3 ± δ (x = 0-0.15), prepared using the glycine nitrate process, showed single phase orthorhombic perovskite structures over a wide range of oxygen partial pressures (4.6 × 10^− 20 atm ≤ pO₂ ≤ 0.21 atm at 900 °C). X-ray diffraction (XRD) analysis indicated that most of the nickel ions replacing chromium ions are divalent and act as acceptor dopants, leading to a substantial increase in conductivity. In particular, the conductivity at 900 °C in air increased from 10 S/cm to 34 S/cm with 15% nickel substitution, and an increase in charge carrier density was confirmed by Seebeck measurements, which validated the predominant Ni²⁺ oxidation state. A point defect model was derived, and the relationship between electrical conductivity and oxygen partial pressure was successfully fitted into the proposed model. The defect modeling results indicated that nickel substitution improves the stability of calcium-doped yttrium chromite toward reduction and suppresses the oxygen vacancy formation, which results in significantly increased electrical conductivity in reducing environment. The electrical conductivity of Y_0.8Ca_0.2Cr_0.85Ni_0.15O_3 ± δ at 900 °C in reducing atmosphere (pO₂ = 10^− 17 atm) was 5.8 S/cm, which was more than an order of magnitude higher than that of Y_0.8Ca_0.2CrO_3 ± δ (0.2 S/cm). Improved stability in reducing atmosphere was further confirmed by dilatometry measurements showing reduced isothermal “chemical” expansion, and the isothermal expansion in reducing atmosphere (pO₂ = 10^− 17 atm) at 900 °C decreased from 0.07% for Y_0.8Ca_0.2CrO_3 ± δ to 0.03% for Y_0.8Ca_0.2Cr_0.85Ni_0.15O_3 ± δ. Based on these results, enhanced electrical performance and mechanical integrity is expected with nickel substitution on calcium-doped yttrium chromite in SOFC operating conditions.     

Study on contamination of projection optics surface for extreme ultraviolet lithography

Ru-capped Mo/Si multilayer mirrors were irradiated by EUV in a vacuum atmosphere with ethanol or decane gas, and their reflectivity changes by contamination were investigated by changing the amount of introduced gas. The reflectivity hardly decreased by EUV irradiation in the ethanol-introduced atmosphere. On the other hand, the reflectivity decreased by about 5% in the decane-introduced atmosphere at a decane pressure of P_Decane = 1.3 × 10⁻⁴ Pa, an EUV power of about 200 mW/mm², and an EUV dose of 150 J/mm². EUV irradiation to the Ru-capped multilayer mirrors was also performed in the presence of water vapor and decane. The surface oxidation by EUV irradiation with a water vapor pressure of PH₂O=1.3×10⁻⁵ Pa was controlled by the introduction of decane at a pressure of P_Decane = 7.0 × 10⁻⁷ to 1.3 × 10⁻⁶ Pa.     

Development of structural and optical properties of WOx films upon increasing oxygen partial pressure during reactive sputtering

WO_x films were prepared by reactive dc magnetron sputtering using tungsten target. Sputtering was carried out at a total pressure of 1.2 Pa using a mixture of argon plus oxygen in an effort to determine the influence of the oxygen partial pressure on structural and optical properties of the films. The deposition rate decreases significantly as the surface of the target is oxidized. X-Ray diffraction revealed the amorphous nature of all the films prepared at oxygen partial pressures higher than 1.71×10⁻³ Pa. For higher oxygen partial pressures, fully transparent films were deposited, which showed a slight increase in optical band gap with increasing oxygen partial pressure, while the refractive index was simultaneously decreased.     

Effects of substrate temperature and oxygen pressure on the magnetic properties and structures of CoFe2O4 thin films prepared by pulsed-laser deposition

CoFe₂O₄ thin films were grown on silicon substrates by pulsed-laser deposition techniques at various temperatures from 350 °C to 700 °C and different pressures from 0.1 Pa to 10 Pa. The CoFe₂O₄ films with highly (1 1 1)-preferred orientation and smooth surfaces were obtained. The high coercivities of the films were attributed to the residual stress in the films, and the saturation magnetizations were mainly dependent on the oxygen pressure. Higher oxygen pressure could decrease the oxygen deficiencies in the films. Sufficient oxygen ions in the films enhanced the exchange interactions between the magnetic ions, as a result, increasing the saturation magnetization.     

Electron microscopic study on interfacial characterization of electroless Ni-W-P plating on aluminium alloy

The interface between electroless plating Ni-W-P deposit and aluminium alloy (Al) matrix at different temperature heated for 1 h was studied using transmission electron microscope. The results show that the interface between as-deposited Ni-W-P deposit and Al matrix is clear. There are no crack and cavity. The bonding of Ni-W-P deposit and Al matrix is in good condition. The Ni-W-P plating is nanocrystalline phase (5-6 nm) in diameter. After being heated at 200 °C for 1 h, the interface of Ni-W-P deposit and Al matrix is clear, without the appearance of the diffusion layer. There exist a diffusion layer and educts of intermetallic compounds of nickle and aluminium such as Al₃Ni, Al₃Ni₂, NiAl, Ni₅Al₃ and so on between Ni-W-P deposit and Al matrix after being heated at 400 °C for 1 h.     

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni₃Al, NiAl, and NiAl₃ alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p_3/2 peak for NiAl and NiAl₃ from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel-aluminum alloys.     

Surface and redox chemistry of Sm0.95Ce0.05Fe1 − xNixO3 − δ perovskites

A new series of perovskite materials with formula Sm_0.95Ce_0.05Fe_1 − xNi_xO_3 − δ (0 ≤ x ≤ 0.10) has been prepared by sol-gel combustion via a citrate precursor route. X-ray diffraction data showed that materials prepared by this method had a single orthorhombic phase belonging to the Pnma (62) space group. The study of powders sintered in air and in reducing atmospheres reveals that these materials do not show phase separation in air (up to 1350 °C) nor under 5% v/v H₂/N₂ (up to 700 °C), but a phase separation of Sm₂O₃ does occur at and above 800 °C under 5% v/v H₂/N₂ without deterioration of the perovskite phase. The surfaces of all the powders (fresh, in-situ reduced and ex-situ reduced) were Sm rich, and multiple oxidation states for Fe were observed. XP analysis of in-situ reduced samples (800 °C and above) shows that metallic Fe forms in all nickel doped materials except x = 0.07. The surface oxygen vacancies and percentages of lattice and adsorbed oxygen for this series of Ni doped materials were determined and the oxygen recapturing ability is explained in terms of the multiple oxidation states of Fe.     

The effect of Pt on Ni3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni₃Al single crystals, with and without Pt addition, at 300-900 K under oxygen pressures of ca. 10⁻⁷ Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al_xO_y. At high-temperature (i.e., 700-800 K), NiO is replaced by a thick layer of Al_xO_y. By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al_xO_y; and (2) shift the establishment of the thick Al_xO_y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.