Clean Iron Research Articles

Decomposition of perfluoro-1-methoxy-2-ethoxy ethane on iron surfaces chemically modified with oxygen

Polymeric perfluoroalkylethers are being considered for use as lubricants in high temperature applications, but have been observed to catalytically decompose in the presence of metals. X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) were used to explore the decomposition of a model perfluorinated ether, perfluoro-1-methoxy-2-ethoxy ethane, CF 3OCF 2CF 2OCF 2CF 3, on oxidized polycrystalline iron surfaces and polycrystalline iron surfaces, modified with an overlayer of chemisorbed oxygen. Low temperature adsorption of the perfluorinated ether on both surfaces was molecular. Chemisorption of an oxygen overlayer lowered the reactivity of the surface to adsorption and decomposition of the perfluorinated ether, by blocking active sites on the metal surface. Incomplete coverage of the iron surface with chemisorbed oxygen results in a reaction, which resembles the defluorination reaction observed on the clean iron surface. Perfluoro-1-methoxy-2-ethoxy ethane reacts on the oxidized iron surface via a different pathway from that seen on the clean iron surface. A reaction mechanism is proposed which involves Lewis acid assisted cleavage of the carbon-oxygen bond, with preferential attack at the methoxy carbon.

Growth of AlN on clean and oxidised polycrystalline iron: an Auger study

The growth of aluminium nitride (AlN) thin films on clean and oxidised polycrystalline iron was studied by Auger electron spectroscopy. The films of AlN were deposited by reactive RF sputtering (argon + nitrogen atmosphere). On clean iron, a modelisation of the Auger peak-to-peak intensities evidences a Stranski-Krastanov growth mode (one layer followed by island formation) with an interfacial layer made of iron nitride covered by the AlN. Results on oxidised iron suggested the growth of a two-phase film with stoichiometry gradient of the (AlN) x ( Al 2 O 3) 1− x form, x increasing with the film thickness. Grains of AlN in the layer were shown to favour the diffusion of oxygen from the substrate to the film until thicknesses of about 80 monolayers are reached. The presence of grains of both phases supports the hypothesis of an island growth mode for this interface.

Investigation of the electronic structure reconstruction during various stages of iron oxidation by target current and Auger spectroscopy

MVV Auger spectra and target current (TC) spectra during various stages of iron oxidation were measured. In the TC-spectra of a clean iron the superposition of two maxima in the energy range of 0–5 eV was presented which can be attributed to transition from the Fe 3d-levels. During oxidation the new structure in the energy region of 5–10 eV was formed. It can be associated with 2p oxygen states. Valence band structure reconstruction obtained by back-convolution of MVV Auger spectra is in a good agreement with the target spectrum fine structure changes.

Initial metal fluoride formation at metal/fluorocarbon interfaces

The interaction of several perfluoroalkyl ethers [CF3OCF3, CF2OCF2OCF2 , and CF3OCF2OCF3] and an acyl fluoride [CF3OCF(CF3)CFO] with iron and aluminum surfaces has been studied with x-ray photoelectron spectroscopy. The perfluoroalkyl ethers were found to not interact with either the clean iron surface or the clean aluminum surface. In contrast, the acyl fluoride compound was observed to readily decompose when in contact with the clean aluminum surface. The same molecule formed a surface carboxylate structure when in contact the clean iron surface. Interactions were also observed on native oxides of aluminum and iron exposed to acyl fluoride. For the aluminum surface, the degradation of the acyl fluoride molecule is not limited to the carbonyl group as it is for the iron surface. Rather, the entire molecule dissociates on the surface.

Decomposition of fluorinated diethers on the clean iron surface

Polymeric perfluoroalkylethers are being considered for use as lubricants in high temperature applications, but have been observed to catalytically decompose in the presence of metals. X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) were used to explore the decomposition of three model fluorinated ethers on clean iron surfaces. Low temperature adsorption of the model fluorinated ethers was molecular. Defluorination of the three model compounds was observed at remarkably low temperatures, 155 K and below, with formation of iron fluoride. Preferential C-F bond scission occurred at methoxy, CF 3O, of perfluoro-1-methoxy-2-ethoxy ethane and perfluoro-1-methoxy-2-ethoxy propane and at CF 3 CF 2O of perfluo-o-1,3-diethoxy propane. The reactivity of the iron toward perfluoroalkylether decomposition when compared to other metals is due to the strength of the iron fluoride bond and the strong electron donating ability of the metallic iron.

Fe/Cr(111) interface formation investigated by synchrotron radiation-induced photoemission

The initial stages of interaction between iron and the Cr(111) clean surface have been studied at room temperature using synchrotron radiation-(SR)-induced photoemission in the 1–8 Å range of iron coverage. Analysis of the Fe3p level gave evidence for interaction between the two metals at the interface, that results in formation of a surface alloy. Annealing to 500 K of the Fe/Cr interface confirmed the alloying between the two metals and showed the segregation of Cr at the surface. In depth information has been obtained by using two incident photon energies at 5 Å Fe coverage and upon annealing the 8 Å Fe/Cr system. A comparison between the Fe3s multiplet splitting for iron in the Fe/Cr interface and for clean iron is also made.

Decomposition pathway for model fluorinated ethers on the clean iron surface

Polymeric perfluoroalkylethers (PFAE) are being considered for use as lubricants in high temperature applications, but in the presence of certain metals these molecules show enhanced decomposition. X-ray photoelectron spectroscopy (XPS) was used to explore the pathways for decomposition of three model fluorinated ethers on clean iron surfaces. The decomposition reaction, in vacuum, was observed to proceed at remarkably low temperatures, 155 K and below, via defluorination of the carbon–oxygen backbone, with formation of iron fluoride. The reactivity of iron toward perfluoroalkylether decomposition, when compared to other metals, is due to the strength of the iron fluoride bond. The remarkably low temperature threshold for the decomposition reflects a low activation barrier for C–F scission due to the strong electron donating ability of metallic iron.

Comment on "Thin oxide layers on clean iron surfaces: formation under vacuum and characterization by photoelectron spectroscopy and electrochemical reactions of probe molecules at the oxide/electrolyte interface"

Comment on "Thin oxide layers on clean iron surfaces: formation under vacuum and characterization by photoelectron spectroscopy and electrochemical reactions of probe molecules at the oxide/electrolyte interface"

A NEXAFS study on the adsorption of ammonia on clean and potassium-promoted iron

The adsorption of ammonia on thin films of iron with and without potassium has been investigated with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The NEXAFS results suggest that for low coverages of chemisorbed ammonia on iron there is a dilation of the NH bond length with respect to solid ammonia. At higher ammonia coverages there is less perturbation of NH 3, as inferred from NEXAFS and XPS experiments, suggesting that the adsorption energy of the ammonia is a strong function of surface coverage. The coadsorption of potassium metal is found to prevent the ammonia NH bond dilation observed over clean iron.

A NEXAFS study on the adsorption of ammonia on clean and potassium-promoted iron

The adsorption of ammonia on thin films of iron with and without potassium has been investigated with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The NEXAFS results suggest that for low coverages of chemisorbed ammonia on iron there is a dilation of the N-H bond length with respect to solid ammonia. At higher ammonia coverages there is less perturbation of NH 3, as inferred from NEXAFS and XPS experiments, suggesting that the adsorption energy of the ammonia is a strong function of surface coverage. The coadsorption of potassium metal is found to prevent the ammonia N-H bond dilation observed over clean iron.

A NEXAFS study on the adsorption of ammonia on clean and potassium-promoted iron

The adsorption of ammonia on thin films of iron with and without potassium has been investigated with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The NEXAFS results suggest that for low coverages of chemisorbed ammonia on iron there is a dilation of the NH bond length with respect to solid ammonia. At higher ammonia coverages there is less perturbation of NH 3, as inferred from NEXAFS and XPS experiments, suggesting that the adsorption energy of the ammonia is a strong function of surface coverage. The coadsorption of potassium metal is found to present the ammonia NH bond dilation observed over clean iron.

A NEXAFS study on the adsorption of ammonia on clean and potassium-promoted iron

The adsorption of ammonia on thin films of iron with and without potassium has been investigated with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The NEXAFS results suggest that for low coverages of chemisorbed ammonia on iron there is a dilation of the NH bond length with respect to solid ammonia. At higher ammonia coverages there is less perturbation of NH 3, as inferred from NEXAFS and XPS experiments, suggesting that the adsorption energy of the ammonia is a strong function of surface coverage. The coadsorption of potassium metal is found to prevent the ammonia NH bond dilation observed over clean iron.

Perfluoroalkylether reactions on iron and oxygen covered iron surfaces studied using x-ray photoelectron spectroscopy and secondary ion mass spectrometry

X-ray photoelectron spectroscopy and secondary ion mass spectrometry have been used to investigate the reaction of a model perfluoroalkylether (PFAE), CF3 OCF2 CF2 OCF2 CF3, with clean iron, oxygen modified iron and clean gold surfaces. These studies revealed an unexpected low temperature decomposition reaction on clean iron surfaces. The decomposition appeared to proceed through a defluorination of the carbon backbone, followed by a dissociation of the entire molecule. The clean gold and oxygen covered iron surfaces were essentially unreactive toward the PFAE.

Thin oxide layers on clean iron surfaces: formation under vacuum and characterization by photoelectron spectroscopy and electrochemical reactions of probe molecules at the oxide/electrolyte interface

Thin oxide layers on clean iron surfaces: formation under vacuum and characterization by photoelectron spectroscopy and electrochemical reactions of probe molecules at the oxide/electrolyte interface

Spin Polarized Photoemission Studies of Surfaces and Thin Films

AbstractSpin polarized photoemission is used to study the magnetic states associated with the clean iron (001) surface. These studies reveal evidence for a minority spin surface state in agreement with a first principles calculation. Studies of the same surface with silver and chromium epitaxial overlayers reveal evidence for interface states derived from this state found on the clean surface. In the case of the silver overlayer the binding energy of the new state is found to be dependent on the layer by layer thickness of the overlayer. With chromium overlayers the binding energy for the same interface state does not show the same thickness dependence. However a second interface state is observed immediately below the Fermi level. These changes in the interfacial electronic structure have implications for any modelling of magnetic coupling in multilayers dependent on the magnetic properties of the interface.

Adsorption of simple S-organic compounds on iron surfaces prepared in an UHV-system

In former studies [1 3], iron surfaces, chemically modified by n-decanthiol (n-DMc), have been investigated in order to get a better understanding of the corrosion protection of reactive metals by organic coatings. Surface analytical (XPS, AES) and electrochemical investigations have shown, that a stable bond and a well defined orientation of the organic molecules towards the surface are only observed, if the thiol is adsorbed onto oxide free surfaces. The preparation of this metallic clean iron surface was achieved by electrochemical methods. The Fe-electrode was polarized at a potential of 0 . 7 mVsnE in 1 mol/1 HC104 for several minutes. Then, the electrolyte was covered by a thin layer of n-DMc and the sample slowly pulled through the mercaptan-layer, the metal still being polarized to 0 . 7 mVsnE. But if the modified sample is then transferred through the laboratory atmosphere, the surface will be partly oxidized and covered with other contaminations. Therefore, the aim of this work is to achieve the adsorption of S-organic compounds onto clean iron-surfaces, which are prepared in an UHV-system using a specially constructed sample introduction system (Fig. 1) to the analysator chamber of the combined XPS/AES-spectrometer of the type LHS 10 (Leybold-Iteraeus). The airlock consists of three chambers: the first chamber is a initial vacuum chamber connected with the preparation chamber, which will later be used for electrochemical investigations. For the adsorption of the organic compounds the third chamber with a special substance inlet system was used. The chambers are separated by gate valves. The samples were transferred through the chambers on the sample disk, which is positioned at the end of a rod (IGT, Essen). The sample disk could be cooled down to -190°C (liquid nitrogen) and heated up to + 150 ° C during adsorption and during transport between the adsorption and analyser chamber.

Electrochemical and electron spectroscopic investigations of iron surfaces modified with thiols

AbstractIn order to develop new composite materials between reactive metals and polymers, it is proposed to place organic monomers at the metal/polymer phase boundary, which can be coupled by covalent/ionic bonds to reactive centres of the metal substrate and later by covalent bonds to the polymer itself. As a first step, the model system iron/n‐decanethiol (n‐decylmercaptan, C10H21SH) has been investigated by AES and XPS. It is shown that the mercaptan binds to metallic clean iron surfaces through the sulphur atom, probably by cleavage of the SH bond. The S 2p3/2 peak has a binding energy of 162.4 ± 0.2 eV. Anodic polarization and ageing in the laboratory atmosphere do not oxidize these chemisorbed molecules; the oxidation reactions of the substrate are suppressed too. After cyclovoltammetry (−1.0 VSHE⩽ U ⩽ + 0.6 VSHE), better oriented chemisorbed molecules are packed more densely on top of the surface.

Studies of ceramic-liquid metal reaction interfaces

An investigation has been made of the nature and extent of chemical reactions between various liquid metals and a range of engineering-grade ceramics typically used as cutting tool inserts. Such possible reactions are relevant to chemical wear effects during metal cutting but also relate to liquid metal containment by ceramics and ceramic-metal joining. The experimental procedure has involved immersing pre-polished ceramic sections in liquid metals for controlled times with subsequent sectioning and examination of the reaction interface. The ceramics studied were two alumina-based materials and five silicon nitrides and sialons. The metals were pure iron, pure nickel and four iron-nickel alloys (a mild steel, a stainless steel and two nickel-based superalloys) and span a range of Fe-Ni compositions. The reaction rates of the alumina materials were found to be much lower than those of the silicon nitride-based materials and reflect the chemical stability of the Al-O bond array. Zirconia-toughened alumina showed little evidence of reaction with clean iron alloys but substantial attack by oxygen-containing iron-based materials was found resulting in the formation of iron-aluminium spinel reaction products. Al2O3-TiC/N exhibited preferential metal attack of the carbonitride phase with dissolution and/or replacement of the TiC/N dispersion. Within the silicon nitride-based group, ferrous alloys were found to be more damaging than mainly nickel alloys and silicon nitrides were more readily attacked than sialons. The difference in behaviour between the sialons and silicon nitrides is attributed to alumina additions in the former group of materials increasing resistance to attack by molten metals. A detailed mechanism of attack for these mixed-phase ceramics is proposed whereby a silicon concentration gradient is established from the crystalline ceramic phases, through the glassy binding phase, to the metal. The result is dissolution of the crystalline phase and an increase in volume fraction of the glassy binder at the metal-ceramic interface with concomitant progressive disruption of the ceramic microstructure.

Comment on ‘adsorption of hydrogen on thin fcc-iron films grown on Cu(100) by C. Egawa, E.M. McCash and R.F. Willis’

In a recent Letter Egawa et al. [Surf. Sci. 215 (1989) L271] presented a detailed study of hydrogen adsorption on ultrathin iron films on the Cu(100) surface. They found the surface to reconstruct upon hydrogen adsorption and pointed out the analogy to a reconstruction reported in our previous Letter [Phys. Rev. Lett. 60 (1988) 2741]. In this Letter we present evidence that the cited reconstruction is a genuine feature of the clean iron films and is not caused by hydrogen adsorption.

Hydrogen transport and particle-induced hydrogen release in carbonization layers

The transport of hydrogen through amorphous hydrogenated carbon films (a-C : H) has been studied by investigating the molecular and plasma-driven permeation of hydrogen through an iron membrane which had been coated in-situ with a-C: H and a-C: D films of up to 100 nm thickness. A pronounced initial increase of the plasma-driven permeation flux compared with that of a clean iron membrane is observed during the early stage of the film deposition. This is attributed to a progressive decrease of the recombination rate σk r on the exposed surface of the membrane. At larger film thickness, d, the plasma-driven permeation rate decreases and is inversely proportional to d. The contribution of hydrogen isotopes incorporated during film deposition and that of the plasma species to the permeation is separated in measurements with a-C: H layers exposed to deuterium plasmas. The permeation from molecular driving pressures through the membrane is unaffected by the carbon film up to the largest thicknesses investigated (100 nm). The data are analyzed in the framework of a simple model assuming recombination of hydrogen atoms to molecules within their implantation range and rapid transport of H 2 molecules through the film. Tentative values for the transport coefficient have been derived.