I07-Surface & interface diffraction
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Cem
Ornek
,
Fan
Zhang
,
Alfred
Larsson
,
Mubashir
Mansoor
,
Gary S.
Harlow
,
Robin
Kroll
,
Francesco
Carla
,
Hadeel
Hussain
,
Dirk L.
Engelberg
,
Bora
Derin
,
Jinshan
Pan
Diamond Proposal Number(s):
[23388]
Open Access
Abstract: The passive film stability on stainless steel can be affected by hydrogen absorption and lead to microstructure embrittlement. This work shows that the absorption of hydrogen results in surface degradation due to oxide reduction and ionic defect generation within the passive film, which decomposes and eventually vanishes. The passive film provides a barrier to entering hydrogen, but when hydrogen is formed, atomic hydrogen infuses into the lattices of the austenite and ferrite phases, causing strain evolution, as shown by synchrotron x-ray diffraction data. The vacancy concentration and hence the strains increase with increasing electrochemical cathodic polarization. Under cathodic polarization, the surface oxides are thermodynamically unstable, but the complete reduction is kinetically restrained. As a result, surface oxides remain present under excessive cathodic polarization, contesting the classical assumption that oxides are easily removed. Density-functional theory calculations have shown that the degradation of the passive film is a reduction sequence of iron and chromium oxide, which causes thinning and change of the semiconductor properties of the passive film from n-type to p-type. As a result, the surface loses its passivity after long cathodic polarization and becomes only a weak barrier to hydrogen absorption and hence hydrogen embrittlement.
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Aug 2023
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I07-Surface & interface diffraction
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Cem
Ornek
,
Mubashir
Mansoor
,
Alfred
Larsson
,
Fan
Zhang
,
Gary S.
Harlow
,
Robin
Kroll
,
Francesco
Carla
,
Hadeel
Hussain
,
Bora
Derin
,
Ulf
Kivisäkk
,
Dirk L.
Engelberg
,
Edvin
Lundgren
,
Jinshan
Pan
Diamond Proposal Number(s):
[23388]
Open Access
Abstract: Various mechanisms have been proposed for hydrogen embrittlement of duplex stainless steel, but the causation of hydrogen-induced material degradation has remained unclear. This work shows that phase instability (decomposition) of the austenite phase and ductile-to-brittle transition of the ferrite phase precedes hydrogen embrittlement. In-situ diffraction measurements revealed that Ni-rich sites of the austenite phase decompose into metastable hydrides. Hydride formation is possible by increasing the hydrogen chemical potential during electrochemical charging and low defect formation energy of hydrogen interstitials. Our findings demonstrate that hydrogen embrittlement can only be understood if measured in situ and in real-time during the embrittlement process.
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Jun 2023
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I07-Surface & interface diffraction
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Alexandra L.
Martin
,
Philip N.
Jemmett
,
Thomas
Howitt
,
Mary H.
Wood
,
Andrew W.
Burley
,
Liam R.
Cox
,
Timothy R.
Dafforn
,
Rebecca J. L.
Welbourn
,
Mario
Campana
,
Maximilian W. A.
Skoda
,
Joseph J.
Thompson
,
Hadeel
Hussain
,
Jonathan L.
Rawle
,
Francesco
Carla
,
Christopher L.
Nicklin
,
Thomas
Arnold
,
Sarah L.
Horswell
Diamond Proposal Number(s):
[14670, 16423, 19542]
Open Access
Abstract: The effect of lipid composition on models of the inner leaflet of mammalian cell membranes has been investigated. Grazing incidence X-ray diffraction and X-ray and neutron reflectivity have been used to characterize lipid packing and solvation, while electrochemical and infrared spectroscopic methods have been employed to probe phase behavior in an applied electric field. Introducing a small quantity of the anionic lipid dimyristoylphosphatidylserine (DMPS) into bilayers of zwitterionic dimyristoylphosphatidylethanolamine (DMPE) results in a significant change in the bilayer response to an applied field: the tilt of the hydrocarbon chains increases before returning to the original tilt angle on detachment of the bilayer. Equimolar mixtures, with slightly closer chain packing, exhibit a similar but weaker response. The latter also tend to incorporate more solvent during this electrochemical phase transition, at levels similar to those of pure DMPS. Reflectivity measurements reveal greater solvation of lipid layers for DMPS > 30 mol %, matching the greater propensity for DMPS-rich bilayers to incorporate water. Taken together, the data indicate that the range of 10–35 mol % DMPS provides optimum bilayer properties (in flexibility and function as a barrier), which may explain why the DMPS content of cell membranes tends to be found within this range.
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Feb 2023
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[21922]
Open Access
Abstract: A high-throughput method for the fabrication of ordered arrays of Au nanoparticles is presented. It is based on pulsed electrodeposition into porous anodic alumina templates. In contrast to many synthesis routes, it is cyanide-free, prior separation of the alumina template from the aluminium substrate is not required, and the use of contaminating surfactants/capping agents often found in colloidal synthesis is avoided. The aspect ratio of the nanoparticles can also be tuned by selecting an appropriate electrodeposition time. We show how to fabricate arrays of nanoparticles, both with branched bases and with hemispherical bases. Furthermore, we compare the different morphologies produced with electron microscopies and grazing-incidence synchrotron X-ray diffraction. We find the nanoparticles are polycrystalline in nature and are compressively strained perpendicular to the direction of growth, and expansively strained along the direction of growth. We discuss how this can produce dislocations and twinning defects that could be beneficial for catalysis.
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May 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[22710]
Open Access
Abstract: The surface region of austenitic stainless steel (SS) is investigated by synchrotron X-ray powder diffraction (XRPD) and X-ray absorption near edge structure (XANES) measurements, because its composition and structure is crucial for the corrosion resistance of SS. Grazing incidence XRPD of a polished AISI 304 bulk steel sample show that the near-surface structure is modified. The concentration of the ferrite phase of Fe, a typical minority phase in AISI 304, increases gradually from 10% to 30% when approaching the surface from 150 nm depth. XANES Fe K-edge investigations of ultra-thin, sputter deposited films also reveal much larger ferrite fractions than expected from the austenitic steel composition of the films. Reasons for the increased ferrite fraction in the surface region of bulk steel and thin films are discussed. However, right at the surface, the trend reverses. Analysis of XANES data for an ultra-thin, 4 nm SS film show that 80% of Fe is oxidized and 20% of metallic Fe is present only in austenite structure, suggesting that ferritic iron is preferentially subject to oxidation. The austenitic Fe is located at more than 2 - 3 nm below the surface where the Ni concentration is > 10%.
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Feb 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[18570]
Open Access
Abstract: Planar organic heterostructures are widely explored and employed in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. An important role for device performance plays the energy level alignment at the inorganic–organic and organic–organic interfaces. In this work, incremental ultraviolet photoelectron spectroscopy measurements and real-time X-ray scattering experiments are used to thoroughly investigate the thickness-dependent electronic and structural properties of a perfluoropentacene (PFP)-on-[6]phenacene heterostructure. For both materials an incremental increase of the material work function (positive interface dipole) is found. For [6]phenacene, this can be assigned to a thickness-dependent change of molecular arrangement evident from a change of the unit cell volume and a consequential alteration of the ionization energy. In the case of PFP the interface dipole stems from charge transfer from the substrate into unoccupied molecular orbitals resulting in an electrostatic potential on the surface. The magnitude of this potential can be correlated with an increased gap state density resulting from templated structural defects mediated by the bottom layer.
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Dec 2021
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I07-Surface & interface diffraction
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Open Access
Abstract: X-ray diffractometers primarily designed for surface X-ray diffraction are often used to measure the diffraction from powders, textured materials and fiber-texture samples in 2θ scans. Unlike in high-energy powder diffraction, only a fraction of the powder rings is typically measured, and the data consist of many detector images across the 2θ range. Such diffractometers typically scan in directions not possible on a conventional laboratory diffractometer, which gives enhanced control of the scattering vector relative to the sample orientation. There are, however, very few examples where the measured intensity is directly used, such as for profile/Rietveld refinement, as is common with other powder diffraction data. Although the underlying physics is known, converting the data is time consuming and the appropriate corrections are dispersed across several publications, often not with powder diffraction in mind. This paper presents the angle calculations and correction factors required to calculate meaningful intensities for 2θ scans with a (2 + 3)-type diffractometer and an area detector. Some of the limitations with respect to texture, refraction and instrumental resolution are also discussed, as is the kind of information that one can hope to obtain.
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Aug 2021
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B16-Test Beamline
B18-Core EXAFS
I08-Scanning X-ray Microscopy beamline (SXM)
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Debi
Garai
,
Vladyslav
Solokha
,
Axel
Wilson
,
Ilaria
Carlomagno
,
Ajay
Gupta
,
Mukul
Gupta
,
V. R.
Reddy
,
Carlo
Meneghini
,
Francesco
Carla
,
Christian
Morawe
,
Jorg
Zegenhagen
Diamond Proposal Number(s):
[17145]
Open Access
Abstract: This work reports about a novel approach for investigating surface processes during the early stages of galvanic corrosion of stainless steel in situ by employing ultra-thin films and synchrotron X-radiation. Characterized by X-ray techniques and voltammetry, such films, sputter deposited from austenitic steel, were found representing useful replicas of the target material. Typical for stainless steel, the surface consists of a passivation layer of Fe- and Cr-oxides, a couple of nm thick, that is depleted of Ni. Films of ≈ 4 nm thickness were studied in situ in an electrochemical cell under potential control (-0.6 to +0.8 V vs Ag/AgCl) during exposure to 0.1 M KCl. Material transport was recorded with better than 1/10 monolayer sensitivity by X-ray spectroscopy. Leaching of Fe was observed in the cathodic range and the therefor necessary reduction of Fe-oxide appears to be accelerated by atomic hydrogen. Except for minor leaching, reduction of Ni, while expected from Pourbaix diagram, was not observed until at ≈ +0.8 V Cr-oxide was removed from the film. After couple of minutes exposure at +0.8 V, the current in the electrochemical cell revealed a rapid pitting event that was simultaneously monitored by X-ray spectroscopy. Continuous loss of Cr and Ni was observed during the induction time leading to the pitting, suggesting a causal connection with the event. Finally, a spectroscopic image of a pit was recorded ex situ with 50 nm lateral and 1 nm depth resolution by soft X-ray scanning absorption microscopy at the Fe L2,3-edges by using a 80 nm film on a SiN membrane, which is further demonstrating the usefulness of thin films for corrosion studies.
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Dec 2020
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I07-Surface & interface diffraction
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Cem
Ornek
,
Alfred
Larsson
,
Gary S.
Harlow
,
Fan
Zhang
,
Robin
Kroll
,
Francesco
Carla
,
Hadeel
Hussain
,
Ulf
Kivisäkk
,
Dirk L.
Engelberg
,
Edvin
Lundgren
,
Jinshan
Pan
Diamond Proposal Number(s):
[23388]
Open Access
Abstract: Grazing-incidence x-ray diffraction was employed to measure, operando, during electrochemical hydrogen charging, the lattice strain development of the near-surface in super duplex stainless steel under applied tensile load. Hydrogen absorption led to the formation of tensile strains in both the austenite (γ) and ferrite (δ) phases perpendicular to the loading axis, whereas compressive strains were formed in the ferrite phase parallel to the loading direction, despite the acting tensile load. The earliest stages of degradation are discussed in light of understanding hydrogen embrittlement.
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Oct 2020
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I07-Surface & interface diffraction
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Cem
Ornek
,
Alfred
Larsson
,
Gary S.
Harlow
,
Fan
Zhang
,
Robin
Kroll
,
Francesco
Carla
,
Hadeel
Hussain
,
Ulf
Kivisäkk
,
Dirk L.
Engelberg
,
Edvin
Lundgren
,
Jinshan
Pan
Diamond Proposal Number(s):
[23388]
Open Access
Abstract: We report the evolution of metastable precursor structures during hydrogen infusion in the near-surface region of a super duplex stainless steel. Grazing-incidence x-ray diffraction was employed to monitor, operando, the lattice degradation of the austenite and ferrite phases. Electrochemical hydrogen charging resulted in the splitting of the diffraction peaks of the austenite phase, suggesting the evolution of a metastable precursor structure. This may be explained by the formation of quasi-hydrides, which convert back into the austenite parent structure during hydrogen effusion. The ferrite showed less lattice deformation than the austenite and no phase transformation.
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Sep 2020
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