23 research outputs found
In-situ Observation of Martensite Decomposition in HAZ of Cr-Mo Steel Weldment
In-situ observation of martensite decomposition at Heat Affected Zone (HAZ) was investigated on a dissimilar joining between 2.25Cr-0.5Mo grade T22 as base material and ER90S-B9 as filler metal using GTAW process using LEEM at a synchrotron facility. A post weld heat treatment (PWHT) cycle was simulated on a welded specimen in high vacuum chamber by heating cartridge and electron bombardment. Both effects PWHT duration and weld areas were studied for comparisons. At the simulated PWHT between 690oC -700oC in CGHAZ, martensite started to decompose by the dissolution of carbide flakes. The prior-austenite grain boundaries were also shown during the process. The same phenomena were also observed in FGHAZ with different extent. In un-affected base material, ferrite and new pearlite grains presented and grew at the expense of old pearlite. Longer PWHT duration resulted in more ferrite formed in all weld areas. Raising PWHT temperature to 730oC could push the reaction above Eutectoid temperature as the new austenite formed at grain boundaries. The proposed mechanism for martensite decomposition would be in steps as dissolution of carbide followed by formation of ferrite and growth as PWHT proceeded
In-situ Observation of h-BN Formation on the Surface of Weld Dissimilar Joint Steels
In-situ observation of h-BN formation by surface precipitation on the surface of joined dissimilar steels is presented. Because the substrate consists of two different types of steels, different growth behaviors can be seen on different sides and also in the middle of the weld interface. This observation demonstrates that formations of 2D materials can occur on surfaces of steels under suitable conditions e.g. temperature, microstructures and concentrations of impurities. Characterizations by electron microscopy and synchrotron spectroscopy technics confirm that h-BN crystals that appear on the surface after annealing are of similar quality to those prepared by other methods such as chemical vapor deposition. Moreover, real-time observation during sample temperature swing above and below the phase transition temperature of Fe shows that h-BN islands reversibly form and dissociate on the surface. The results show that the formation of h-BN on steels is reversible and the analysis suggests that the process is likely affected by structural change of the steels near the phase transition temperature, which in-turn drives the diffusions of B and N atoms back and forth between surface and bulk
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Adsorption behavior of organic molecules: a study of benzotriazole on Cu(111) with spectroscopic and theoretical methods
The adsorption of organic molecules on solid substrates is important to applications in fields such as catalysis, photovoltaics, corrosion inhibition, adhesion, and sensors. The molecular level description of the surface-molecule interaction and of the adsorption structures in these complex systems is crucial to understand their properties and function. Here we present the investigation of one such system, benzotriazole (BTAH) on single crystal Cu(111) in vacuum conditions. BTAH is the most widely used corrosion
inhibitor for copper and thus a molecule of great industrial relevance. We show that the co-application of a wide range of spectroscopic techniques with theoretical methods provides unique insight in the description of the atomistic details of the adsorbed structures. Specifically, spectroscopic photoemission, absorption and standing wave experiments combined with ab initio computational modeling allowed us to identify that benzotriazole forms overlayers of intact BTAH when deposited at low temperature and it dissociates into BTA and H at room temperature and above. The dissociated molecule then forms complex structures of mixed chains and dimers of BTA bound to copper adatoms. Our work also reveals that copper adatoms at low concentrations, such as the theoretically predicted superstructures cannot be be resolved by means of current X-ray photoelectron spectroscopy (XPS) as the modelled Cu 2p spectra are practically indistinguishable from those for a Cu surface without adatoms. Overall this study significantly deepens understanding of BTAH on Cu - a system studied for more than 50 years - and it highlights the benefits of combining spectroscopic and computational methods in order to obtain a complete picture of a complex adsorption system
Low-Temperature Growth of Graphene on a Semiconductor
The industrial realization of graphene has so far been limited by challenges
related to the quality, reproducibility, and high process temperatures required
to manufacture graphene on suitable substrates. We demonstrate that epitaxial
graphene can be grown on transition metal treated 6H-SiC(0001) surfaces, with
an onset of graphitization starting around . From the
chemical reaction between SiC and thin films of Fe or Ru,
carbon is liberated from the SiC crystal and converted to
carbon at the surface. The quality of the graphene is demonstrated using
angle-resolved photoemission spectroscopy and low-energy electron diffraction.
Furthermore, the orientation and placement of the graphene layers relative to
the SiC substrate is verified using angle-resolved absorption spectroscopy and
energy-dependent photoelectron spectroscopy, respectively. With subsequent
thermal treatments to higher temperatures, a steerable diffusion of the metal
layers into the bulk SiC is achieved. The result is graphene supported on
magnetic silicide or optionally, directly on semiconductor, at temperatures
ideal for further large-scale processing into graphene based device structures.Comment: 10 pages, 4 figures, 51 reference
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The partial oxidation of methane over Pd/Al2O3 catalyst nanoparticles studied in-situ by near ambient-pressure x-ray photoelectron spectroscopy
Near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is used to study the chemical state of methane oxidation catalysts in-situ. Al2O3{supported Pd catalysts are prepared with different particle sizes ranging from 4 nm to 10 nm. These catalysts were exposed to conditions similar to those used in the partial oxidation of methane (POM) to syn-gas and simultaneously monitored by NAP-XPS and mass spectrometry. NAP-XPS data show changes in the oxidation state of the palladium as the temperature in-
creases, from metallic Pd0 to PdO, and back to Pd0.
Mass spectrometry shows an increase in CO production whilst the Pd is in the oxide phase, and the metal is reduced back under presence of newly formed H2. A particle size effect is observed, such that CH4 conversion starts at lower temperatures with larger sized
particles from 6 nm to 10 nm. We find that all nanoparticles begin CH4 conversion at lower temperatures than polycrystalline Pd foil
Directions and Breakup of Self-Running In Droplets on Low-Index InP Surfaces
The
nucleation and dynamics of multiple generations of In droplets
formed from Langmuir evaporation of InP (001), (111)A, and (111)B
surfaces are reported. In situ mirror electron microscopy reveals
that the majority of first-generation, or mother, droplets break up
immediately before they run from the nucleation sites, leaving behind
daughter droplets and etch trails where more droplets emerge. These
subsequent droplets grow with time and run once a critical size is
reached. The breakup and running characteristics are explained in
terms of crystallography, viscosity, chemical potential, and temperature
and will likely affect the growth processes and designs of various
droplet-catalyzed nanostructures and devices
Self-Running Ga Droplets on GaAs (111)A and (111)B Surfaces
Thermal decomposition of GaAs (111)A
and (111)B surfaces in ultrahigh
vacuum results in self-running Ga droplets. Although Ga droplets on
the (111)B surface run in one main direction, those on the (111)A
surface run in multiple directions, frequently taking sharp turns
and swerving around pyramidal etch pits, leaving behind mixed smooth-triangular
trails as a result of simultaneous in-plane driving and out-of-plane
crystallographic etching. The droplet motion is partially guided by
dislocation strain fields. The results hint at the possibilities of
using subsurface dislocation network and prepatterned, etched surfaces
to control metallic droplet motion on single-crystal semiconductor
surfaces
Electronic and Thermoelectric Properties of Graphene on 4H-SiC (0001) Nanofacets Functionalized with F4-TCNQ
International audienceThe functionalization of graphene is a well-established route for modulating its optoelectronic properties for a wide range of applications. Here, we studied, using photoemission spectroscopies and synchrotron radiation, the band structure upon evaporation of a p-type dopant tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) molecules and determined the work function (WF) shift over a large area of epitaxial graphene grown on a 4H-SiC (0001) silicon carbide substrate. This system exhibits peculiar nanostructures composed of mono and multilayers, notably at the step edges where the electronic properties differ from the terraces. We observed, owing to the high spatial resolution of photoemission electron microscopy (PEEM), that after the adsorption of F4-TCNQ, multilayer graphene on step edges was subjected to less charge transfer compared to the monolayer graphene on terraces, making their final WF smaller. We calculated the thermoelectric properties of this functionalized graphene system by using density functional theory and Boltzmann transport formalism within the range of the Fermi level (EF), and the carrier concentration, which was experimentally determined. We show that the Seebeck coefficient (S) on the nanofacets is 25% larger than on the monolayer terraces, and the maximum power factor (PF) is on the order of 10−2 W/K2m. This order of magnitude is comparable to the PF of commercial thermoelectric materials such as bulk bismuth telluride