116 research outputs found
An electron hole doping and soft x-ray spectroscopy study on La1-xSrxFe0.75Ni0.25O3-{\delta}
The conductivity of the electron hole and polaron conductor
La1-xSrxFe0.75Ni0.25O3-{\delta}, a potential cathode material for intermediate
temperature solid oxide fuel cells, was studied for 0 <x < 1 and for
temperatures 300 K <T < 1250 K. In LaSrFe-oxide, an ABO3 type perovskite,
A-site substitu-tion of the trivalent La3+ by the divalent Sr2+ causes
oxidation of Fe3+ towards Fe4+, which forms conducting electron holes. Here we
have in addition a B-site substitution by Ni. The compound for x = 0.5 is
identified as the one with the highest conductivity ({\sigma} ~ 678 S/cm) and
lowest activation energy for polaron conductivity (Ep = 39 meV). The evolution
of the electronic structure was monitored by soft x-ray Fe and oxygen K-edge
spectroscopy. Homogeneous trend for the oxida-tion state of the Fe was
observed. The variation of the ambient temperature conductivity and activation
energy with relative Sr content (x) shows a correlation with the ratio of
(eg/eg+t2g) in Fe L3 edge up to x=0.5. The hole doping process is reflected by
an almost linear trend by the variation of the pre-peaks of the oxygen K-edge
soft x-ray absorption spectra
Energy separation in oscillatory Hamiltonian systems without any non-resonance condition
We consider multiscale Hamiltonian systems in which harmonic oscillators with
several high frequencies are coupled to a slow system. It is shown that the
oscillatory energy is nearly preserved over long times eps^{-N} for arbitrary
N>1, where eps^{-1} is the size of the smallest high frequency. The result is
uniform in the frequencies and does not require non-resonance conditions.Comment: 13 page
Correlation of conductivity and angle integrated valence band photoemission characteristics in single crystal iron perovskites for 300 K < T < 800 K: Comparison of surface and bulk sensitive methods
A single crystal monolith of La0.9Sr0.1FeO3 and thin pulsed laser deposited
film of La0.8Sr0.2Fe0.8Ni0.2O3 were subject to angle integrated valence band
photoemission spectroscopy in ultra high vacuum and conductivity experiments in
ambient air at temperatures from 300 K to 800 K. Except for several sputtering
and annealing cycles, the specimen were not prepared in-situ.. Peculiar changes
in the temperature dependent, bulk representative conductivity profile as a
result of reversible phase transitions, and irreversible chemical changes are
semi-quantitatively reflected by the intensity variation in the more surface
representative valence band spectra near the Fermi energy. X-ray photoelectron
diffraction images reflect the symmetry as expected from bulk iron perovskites.
The correlation of spectral details in the valence band photoemission spectra
(VB PES) and details of the conductivity during temperature variation suggest
that valuable information on electronic structure and transport properties of
complex materials may be obtained without in-situ preparation
High temperature oxygen NEXAFS valence band spectra and conductivity of LaFe3/4Ni1/4O3 from 300 K to 773 K
LaFe3/4Ni1/4O3 was subjected to oxygen near edge x-ray absorption fine
structure (NEXAFS) spectroscopy for 300 K < T < 773 K. The spectra show in the
pre-edge a small hole doped peak originating from Ni substitution. The relative
spectral weight of this transition to the weight of the hybridized O(2p) -
Fe(3d) transitions scales with T and has a maximum at around 600 K. The
characteristic energies of the thermal activated spectral intensity and
conductivity suggest that the concentration of charge transferred electrons
from O(2p) to Ni(3d) increases and that the pre-edges account in part for the
polaron activated transport
Effect of intergranular glass films on the electrical conductivity of 3Y-TZP
The electrical conductivity of 3Y-TZP ceramics containing SiO2 and Al2O3 has been investigated by complex impedance spectroscopy between 500 and 1270 K. At low temperatures, the total electrical conductivity is suppressed by the grain boundary glass films. The equilibrium thickness of intergranular films is 1-2 nm, as derived using the "brick-layerâ model and measured by HRTEM. A change in the slope of the conductivity Arrhenius plots occurs at the characteristic temperature Tb at which the macroscopic grain boundary resistivity has the same value as the resistivity of the grains. The temperature dependence of the conductivity is discussed in terms of a series combination of RC element
Entanglement of charge transfer, hole doping, exchange interaction and octahedron tilting angle and their influence on the conductivity of La1-xSrxFe0.75Ni0.25O3-{\delta}: A combination of x-ray spectroscopy and diffraction
Substitution of La by Sr in the 25% Ni doped charge transfer insulator LaFeO3
creates structural changes that inflect the electrical conductivity caused by
small polaron hopping via exchange interactions and charge transfer. The
substitution forms electron holes and a structural crossover from orthorhombic
to rhombohedral symmetry, and then to cubic symmetry. The structural crossover
is accompanied by a crossover from Fe3+-O2--Fe3+ superexchange interaction to
Fe3+-O2--Fe4+ double exchange interaction, as evidenced by a considerable
increase of conductivity. These interactions and charge transfer mechanism
depend on superexchange angle, which approaches 180{\deg} upon increasing Sr
concentration, leading an increased overlap between the O (2p) and Fe/Ni (3d)
orbitals
Redistribution of Aluminum Ions During Processing of Sialon Ceramics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65916/1/j.1151-2916.1986.tb04820.x.pd
Insolubility of Mg in Î-Si 3 N 4 in the System Al-Mg-Si-O-N
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65816/1/j.1151-2916.1978.tb09345.x.pd
On the Nonlinear Stability of Asymptotically Anti-de Sitter Solutions
Despite the recent evidence that anti-de Sitter spacetime is nonlinearly
unstable, we argue that many asymptotically anti-de Sitter solutions are
nonlinearly stable. This includes geons, boson stars, and black holes. As part
of our argument, we calculate the frequencies of long-lived gravitational
quasinormal modes of AdS black holes in various dimensions. We also discuss a
new class of asymptotically anti-de Sitter solutions describing noncoalescing
black hole binaries.Comment: 26 pages. 5 figure
Microstructure and Mechanical Properties of the in situ Î-Si 3 N 4 /ÎâČ-SiAlON Composite
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65911/1/j.1151-2916.1994.tb04604.x.pd
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