77 research outputs found
In situ electron microscopy studies of electric field assisted sintering of oxide ceramics
A wide range of studies shows a dramatic effect of applied electric fields or currents on the sintering behavior of oxide ceramic powders. However, the mechanisms accounting for the so-called flash sintering remain elusive despite the wide application potential. Using in-situ scanning and transmission electron microscopy, we aim to gain insight into the atomic origins of sintering behavior, as well as of the high conductivity states that occur in conjunction with flash events during field-assisted sintering.
We investigate the sintering dynamics of ZnO green bodies with a density between 50% and 70% and ZnO thin films with and without electric fields and under different oxidizing and reducing gas pressures. Specifically, we use a specially designed SEM heating stage to study the evolution of microstructure and morphology, including grain/void morphology, segregation, and precipitation, both with and without applied fields and with and without gas pressures up to 2 mbar. The in-situ TEM sintering studies, also under controlled electric field and gas pressure, allow us to detect chemical segregation and valence changes (using EDX and EELS) near the sintering boundaries. By gaining access to structural and chemical information down to the atomic scale, we hope to determine how the electric field causes flash sintering.
Please click Additional Files below to see the full abstract
Defect generation in Pd layers by ‘smart’ films with high H-affinity
In this paper, we demonstrate that the microstructure and the surface of a thin palladium (Pd) film can be intentionally altered by the presence of a subjacent niobium (Nb) film. Depending on the thickness of the Nb film and on the hydrogen gas pressure, defects in the Pd film can be healed or created. To demonstrate this effect, Pd/Nb/sapphire (Al₂O₃) stacks are studied during hydrogen gas exposure at room temperature by using scanning tunneling microscopy (STM), X-ray diffraction (XRD) and environmental transmission electron microscopy (ETEM). STM shows that hydrogen-induced topography changes in the Nb films depend on the film thickness which affects the height of the Nb surface corrugations, their lateral size and distribution. XRD measurements show that these changes in the Nb hydride film influence the microstructure of the overlaying Pd film. ETEM reveals that the modifications of the Pd film occur due to the precipitation and growth of the Nb hydride phase. The appearance of new defects, interface and surface roughening is observed in the Pd film above locally grown Nb hydride grains. These results can open a new route to design ‘smart’ catalysts or membranes, which may accommodate their microstructure depending on the gaseous environment
Ultrafast transport of laser-excited spin polarized carriers in Au/Fe/MgO(001)
A time domain approach to probe hot carrier-induced spin dynamics is
demonstrated. The experiments are performed in epitaxial Au/Fe/MgO(001), where
spin-polarized hot carriers are excited in the Fe layer by 35 fs laser pulses.
They propagate to the Au surface where the transient spin polarization is
detected by magneto-induced second harmonic generation. Different energies of
majority and minority hot carriers excited in the exchange-split Fe band
structure lead to their spindependent lifetimes in Au. Accordingly, two
spin-polarized current contributions which propagate superdiffusively at
different velocities result in a spin current pulse of about 100 fs duration.Comment: 5 pages, 4 figure
Angular dependence of Hall effect and magnetoresistance in SrRuO3−SrIrO3 heterostructures
Perovskite SrRuO is a prototypical itinerant ferromagnet which allows
interface engineering of its electronic and magnetic properties. We report
synthesis and investigation of atomically flat artificial multilayers of
SrRuO with the spin-orbit semimetal SrIrO in combination with
band-structure calculations with a Hubbard term and topological analysis.
They reveal an electronic reconstruction and emergence of flat Ru-4d
bands near the interface, ferromagnetic interlayer coupling and negative
Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall
effect and magnetoresistance measurements as a function of the field angle from
out of plane towards in-plane orientation (either parallel or perpendicular to
the current direction) by a two-channel model. The magnetic easy direction is
tilted by about from the sample normal for low magnetic fields,
rotating towards the out-of-plane direction by increasing fields. Fully
strained epitaxial growth enables a strong anisotropy of magnetoresistance. An
additional Hall effect contribution, not accounted for by the two-channel model
is compatible with stable skyrmions only up to a critical angle of roughly
from the sample normal. Within about from the thin film
plane an additional peak-like contribution to the Hall effect suggests the
formation of a non-trivial spin structure.Comment: to be published in Phys. Rev.
Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba<sub>2</sub>In<sub>2</sub>O<sub>5</sub> Thin Films
Solid
oxide oxygen ion and proton conductors are a highly important
class of materials for renewable energy conversion devices like solid
oxide fuel cells. Ba<sub>2</sub>In<sub>2</sub>O<sub>5</sub> (BIO)
exhibits both oxygen ion and proton conduction, in a dry and humid
environment, respectively. In a dry environment, the brownmillerite
crystal structure of BIO exhibits an ordered oxygen ion sublattice,
which has been speculated to result in anisotropic oxygen ion conduction.
The hydrated structure of BIO, however, resembles a perovskite and
the protons in it were predicted to be ordered in layers. To complement
the significant theoretical and experimental efforts recently reported
on the potentially anisotropic conductive properties in BIO, we measure
here both the proton and oxygen ion conductivity along different crystallographic
directions. Using epitaxial thin films with different crystallographic
orientations, the charge transport for both charge carriers is shown
to be anisotropic. The anisotropy of the oxygen ion conduction can
indeed be explained by the layered structure of the oxygen sublattice
of BIO. The anisotropic proton conduction, however, further supports
the suggested ordering of the protonic defects in the material. The
differences in proton conduction along different crystallographic
directions attributed to proton ordering in BIO are of a similar extent
as those observed along different crystallographic directions in materials
where proton ordering is not present but where protons find preferential
conduction pathways through chainlike or layered structures
- …