2,202 research outputs found
Characterization of Iridium Coated Rhenium Used in High-Temperature, Radiation-Cooled Rocket Thrusters
Materials used for radiation-cooled rocket thrusters must be capable of surviving under extreme conditions of high-temperatures and oxidizing environments. While combustion efficiency is optimized at high temperatures, many refractory metals are unsuitable for thruster applications due to rapid material loss from the formation of volatile oxides. This process occurs during thruster operation by reaction of the combustion products with the material surface. Aerojet Technical Systems has developed a thruster cone chamber constructed of Re coated with Ir on the inside surface where exposure to the rocket exhaust occurs. Re maintains its structural integrity at high temperature and the Ir coating is applied as an oxidation barrier. Ir also forms volatile oxide species (IrO2 and IrO3) but at a considerably slower rate than Re. In order to understand the performance limits of Ir-coated Re thrusters, we are investigating the interdiffusion and oxidation kinetics of Ir/Re. The formation of iridium and rhenium oxides has been monitored in situ by Raman spectroscopy during high temperature exposure to oxygen. For pure Ir, the growth of oxide films as thin as approximately 200 A could be easily detected and the formation of IrO2 was observed at temperatures as low as 600 C. Ir/Re diffusion test specimens were prepared by magnetron sputtering of Ir on Re substrates. Concentration profiles were determined by sputter Auger depth profiles of the heat treated specimens. Significant interdiffusion was observed at temperatures as low as 1000 C. Measurements of the activation energy suggest that below 1350 C, the dominant diffusion path is along defects, most likely grain boundaries, rather than bulk diffusion through the grains. The phases that form during interdiffusion have been examined by x ray diffraction. Analysis of heated test specimens indicates that the Ir-Re reaction produces a solid solution phase of Ir dissolved in the HCP structure of Re
Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110)
We observe using low-energy electron microscopy the self-assembly of
monolayer-thick stripes of Au on W(110) near the transition temperature between
stripes and the non-patterned (homogeneous) phase. We demonstrate that the
amplitude of this Au stripe phase decreases with increasing temperature and
vanishes at the order-disorder transition (ODT). The wavelength varies much
more slowly with temperature and coverage than theories of stress-domain
patterns with sharp phase boundaries would predict, and maintains a finite
value of about 100 nm at the ODT. We argue that such nanometer-scale stripes
should often appear near the ODT.Comment: 5 page
Three-Fold Diffraction Symmetry in Epitaxial Graphene and the SiC Substrate
The crystallographic symmetries and spatial distribution of stacking domains
in graphene films on SiC have been studied by low energy electron diffraction
(LEED) and dark field imaging in a low energy electron microscope (LEEM). We
find that the graphene diffraction spots from 2 and 3 atomic layers of graphene
have 3-fold symmetry consistent with AB (Bernal) stacking of the layers. On the
contrary, graphene diffraction spots from the buffer layer and monolayer
graphene have apparent 6-fold symmetry, although the 3-fold nature of the
satellite spots indicates a more complex periodicity in the graphene sheets.Comment: An addendum has been added for the arXiv version only, including one
figure with five panels. Published paper can be found at
http://link.aps.org/doi/10.1103/PhysRevB.80.24140
Valence Band Circular Dichroism in non-magnetic Ag/Ru(0001) at normal emission
For the non-magnetic system of Ag films on Ru(0001), we have measured the
circular dichroism of photoelectrons emitted along the surface normal, the
geometry typically used in photoemission electron microscopy (PEEM).
Photoemission spectra were acquired from micrometer-sized regions having
uniformly thick Ag films on a single, atomically flat Ru terrace. For a single
Ag layer, we find a circular dichroism that exceeds 6% at the d-derived band
region around 4.5 eV binding energy. The dichroism decreases as the Ag film
thickness increases to three atomic layers. We discuss the origin of the
circular dichroism in terms of the symmetry lowering that can occur even in
normal emission.Comment: 9 pages, 4 figure
Imaging Spin Reorientation Transitions in Consecutive Atomic Co layers
By means of spin-polarized low-energy electron microscopy (SPLEEM) we show
that the magnetic easy-axis of one to three atomic-layer thick cobalt films on
ruthenium crystals changes its orientation twice during deposition:
one-monolayer and three-monolayer thick films are magnetized in-plane, while
two-monolayer films are magnetized out-of-plane, with a Curie temperature well
above room temperature. Fully-relativistic calculations based on the Screened
Korringa-Kohn-Rostoker (SKKR) method demonstrate that only for two-monolayer
cobalt films the interplay between strain, surface and interface effects leads
to perpendicular magnetization.Comment: 5 pages, 4 figures. Presented at the 2005 ECOSS conference in Berlin,
and at the 2005 Fall meeting of the MRS. Accepted for publication at Phys.
Rev. Lett., after minor change
Hydrogen-induced reversible spin-reorientation transition and magnetic stripe domain phase in bilayer Co on Ru(0001)
Imaging the change in the magnetization vector in real time by spin-polarized
low-energy electron microscopy, we observed a hydrogen-induced, reversible
spin-reorientation transition in a cobalt bilayer on Ru(0001). Initially,
hydrogen sorption reduces the size of out-of-plane magnetic domains and leads
to the formation of a magnetic stripe domain pattern, which can be understood
as a consequence of reducing the out-of-plane magnetic anisotropy. Further
hydrogen sorption induces a transition to an in-plane easy-axis. Desorbing the
hydrogen by heating the film to 400 K recovers the original out-of-plane
magnetization. By means of ab-initio calculations we determine that the origin
of the transition is the local effect of the hybridization of the hydrogen
orbital and the orbitals of the Co atoms bonded to the absorbed hydrogen.Comment: 5 figure
Factors influencing graphene growth on metal surfaces
Graphene forms from a relatively dense, tightly-bound C-adatom gas, when
elemental C is deposited on or segregates to the Ru(0001) surface. Nonlinearity
of the graphene growth rate with C adatom density suggests that growth proceeds
by addition of C atom clusters to the graphene edge. The generality of this
picture has now been studied by use of low-energy electron microscopy (LEEM) to
observe graphene formation when Ru(0001) and Ir(111) surfaces are exposed to
ethylene. The finding that graphene growth velocities and nucleation rates on
Ru have precisely the same dependence on adatom concentration as for elemental
C deposition implies that hydrocarbon decomposition only affects graphene
growth through the rate of adatom formation; for ethylene, that rate decreases
with increasing adatom concentration and graphene coverage. Initially, graphene
growth on Ir(111) is like that on Ru: the growth velocity is the same nonlinear
function of adatom concentration (albeit with much smaller equilibrium adatom
concentrations, as we explain with DFT calculations of adatom formation
energies). In the later stages of growth, graphene crystals that are rotated
relative to the initial nuclei nucleate and grow. The rotated nuclei grow much
faster. This difference suggests first, that the edge-orientation of the
graphene sheets relative to the substrate plays an important role in the growth
mechanism, and second, that attachment of the clusters to the graphene is the
slowest step in cluster addition, rather than formation of clusters on the
terraces
Viable thermionic emission from graphene-covered metals
Thermionic emission from monolayer graphene grown on representative
transition metals, Ir and Ru, is characterized by low-energy electron
microscopy (LEEM). Work functions were determined from the temperature
dependence of the emission current and from the electron energy spectrum of
emitted electrons. The high-temperature work function of the strongly
interacting system graphene/Ru(0001) is sufficiently low, 3.3 \pm 0.1 eV, to
have technological potential for large-area emitters that are spatially
uniform, efficient, and chemically inert. The thermionic work functions of the
less strongly interacting system graphene/Ir(111) are over 1 eV larger and vary
substantially (0.4 eV) between graphene orientations rotated by 30{\deg}.Comment: Published in Applied Physics Letter
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