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):
[24284]
Open Access
Abstract: A previous investigation of the structure of TCNQ adsorbed on Ag(111) using normal-incidence X-ray standing waves (NIXSW) and density functional theory (DFT) provided indirect evidence that Ag adatoms must be incorporated into the molecular overlayer. New surface X-ray diffraction (SXRD) results, presented here, provide direct evidence for the presence and location of these Ag adatoms and clearly distinguishes between two alternative models of the adatom registry favored by two different DFT studies.
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Feb 2023
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I07-Surface & interface diffraction
I09-Surface and Interface Structural Analysis
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Philip J.
Mousley
,
Luke A.
Rochford
,
Paul T. P.
Ryan
,
Philip
Blowey
,
James
Lawrence
,
David A.
Duncan
,
Hadeel
Hussain
,
Billal
Sohail
,
Tien-Lin
Lee
,
Gavin R.
Bell
,
Giovanni
Costantini
,
Reinhard J.
Maurer
,
Christopher
Nicklin
,
D. Phil
Woodruff
Diamond Proposal Number(s):
[14884, 4884]
Open Access
Abstract: While the phenomenon of metal substrate adatom incorporation into molecular overlayers is generally believed to occur in several systems, the experimental evidence for this relies on the interpretation of scanning tunneling microscopy (STM) images, which can be ambiguous and provides no quantitative structural information. We show that surface X-ray diffraction (SXRD) uniquely provides unambiguous identification of these metal adatoms. We present the results of a detailed structural study of the Au(111)-F4TCNQ system, combining surface characterization by STM, low-energy electron diffraction, and soft X-ray photoelectron spectroscopy with quantitative experimental structural information from normal incidence X-ray standing wave (NIXSW) and SXRD, together with dispersion-corrected density functional theory (DFT) calculations. Excellent agreement is found between the NIXSW data and the DFT calculations regarding the height and conformation of the adsorbed molecule, which has a twisted geometry rather than the previously supposed inverted bowl shape. SXRD measurements provide unequivocal evidence for the presence and location of Au adatoms, while the DFT calculations show this reconstruction to be strongly energetically favored.
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Apr 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
I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[24359, 23666]
Open Access
Abstract: Membranes with high selectivity offer an attractive route to molecular separations, where technologies such as distillation and chromatography are energy intensive. However, it remains challenging to fine tune the structure and porosity in membranes, particularly to separate molecules of similar size. Here, we report a process for producing composite membranes that comprise crystalline porous organic cage films fabricated by interfacial synthesis on a polyacrylonitrile support. These membranes exhibit ultrafast solvent permeance and high rejection of organic dyes with molecular weights over 600 g mol−1. The crystalline cage film is dynamic, and its pore aperture can be switched in methanol to generate larger pores that provide increased methanol permeance and higher molecular weight cut-offs (1,400 g mol−1). By varying the water/methanol ratio, the film can be switched between two phases that have different selectivities, such that a single, ‘smart’ crystalline membrane can perform graded molecular sieving. We exemplify this by separating three organic dyes in a single-stage, single-membrane process.
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Jan 2022
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[22176]
Abstract: Hypothesis: Despite the widespread industrial usage of erucamide as a slip additive to modify polymer surface properties, a controversy appears to have persisted regarding the nanostructure of erucamide surface layers, particularly the molecular orientation at the outermost layer. The erucamide nanostructure and molecular orientation, along with its surface coverage, hydrophobicity, and adhesive response, can be tuned by simply varying the erucamide concentration in the solution from which the spin coated layer is prepared. Experiments: Synchrotron X-ray reflectivity (XRR) allowed a comprehensive characterisation of the out-of-plane structural parameters (e.g. molecular packing and thickness) of the erucamide layers prepared via spin coating from nonaqueous solution on silica. Complementary Atomic Force Microscopy (AFM) imaging with high lateral resolution revealed localised in-plane structures. Contact angle measurements provided information on the wettability of erucamide-coated surfaces. Peak Force Quantitative Nanomechanical Mapping (QNM) allowed a correlation between the erucamide nanostructure with the surface nanomechanical properties (i.e. adhesive response). Findings: Our results reveal erucamide surface nanostructures on silica as patchy monolayers, isolated circular bilayers/rounded rectangle-like aggregates and overlapping plate-like multilayers as the erucamide concentration in the spin coating solution was varied. In all the cases, XRR and AFM results were consistent with the picture that the erucamide tails were oriented outwards. The QNM adhesion force mapping of all the observed morphologies also supported this molecular orientation at the outermost erucamide monolayer. The wettability study further confirmed this conclusion with the observed increase in the surface hydrophobicity and coverage upon increasing erucamide concentration, with the macroscopic water contact angle θ = 92.9° ± 2.9° at the highest erucamide concentration of 2 wt%.
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May 2021
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[24392]
Abstract: Understanding the nanostructure and nanomechanical properties of surface layers of erucamide, in particular the molecular orientation of the outermost layer, is important to its widespread use as a slip additive in polymer materials. Extending our recent observations of nanomorphologies of erucamide layers on a hydrophilic silica substrate, here we evaluate its nanostructure on a more hydrophobic polypropylene surface. Atomic force microscopy (AFM) imaging revealed the molecular packing, thickness, and surface coverage of the erucamide layers, while peak force quantitative nanomechanical mapping (QNM) showed that erucamide reduced the adhesive response on polypropylene. Synchrotron X-ray reflectivity (XRR) was used to probe the out-of-plane structure of the surface layers. Static contact angle measurements further corroborated on the resulting wettability, also demonstrating the efficacy of erucamide physisorption in facilitating control over polypropylene surface wetting. The results show the formation of erucamide monolayers, bilayers and multilayers, depending on the concentration in the spin-cast solution. Correlation of AFM, XRR and wettability results consistently points to the molecular orientation in the outermost layer, i.e. with the erucamide tails pointing outward for the surface nanostructures with different morphologies (i.e., bilayers and multilayers). Rare occurrence of monolayers with exposed hydrophilic head groups were observed only at the lowest erucamide concentration. Compared with our previous observations on the hydrophilic surface, the erucamide surface coverage was much higher on the more hydrophobic propylene surface at similar erucamide concentrations in the spin-cast solution. Furthermore, the structure, molecular orientation and nanomechanical properties of the spin-cast erucamide multilayers atop polypropylene were also similar to those on industrially relevant polypropylene fibers coated with erucamide via blooming. These findings shed light on the nanostructural features of the erucamide surface layer underpinning its nanomechanical properties, relevant to many applications in which erucamide is commonly used as a slip additive.
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May 2021
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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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