54 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
X-ray computed micro tomography as complementary method for the characterization of activated porous ceramic preforms
X-ray computed micro tomography (CT) is an alternative technique to the classical methods such as mercury intrusion (MIP) and gas pycnometry (HP) to obtain the porosity, pore-size distribution, and density of porous materials. Besides the advantage of being a nondestructive method, it gives not only bulk properties, but also spatially resolved information. In the present work, uniaxially pressed porous alumina performs activated by titanium were analyzed with both the classical techniques and CT. The benefits and disadvantages of the applied measurement techniques were pointed out and discussed. With the generated data, development was proposed for an infiltration model under ideal conditions for the production of metal matrix composites (MMC) by pressureless melt infiltration of porous ceramic preforms. Therefore, the reliability of the results, received from different investigation techniques, was proved statistically and stereologicall
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
Yttrium and Hydrogen Superstructure and Correlation of Lattice Expansion and Proton Conductivity in the BaZr0.9Y0.1O2.95 Proton Conductor
Bragg reflections in Y-resonant x-ray diffractograms of BaZr0.9Y0.1O2.95 (BZY10) reveal that Y is organized in a superstructure. Comparison with neutron diffraction superstructure reflections in protonated/deuterated BZY10 suggests that both superstructures are linked, and that protons move in the landscape imposed by the Y. The thermal lattice expansion decreases abruptly for protonated BZY10 at Tâ„648±20 K, coinciding with the onset of lateral proton diffusion and suggesting a correlation of structural changes and proton conductivity. The chemical shift in the Y L1-shell x-ray absorption spectra reveals a reduction from Y3+ toward Y2+ upon protonation
Correlation of high temperature X-ray photoemission spectral features and conductivity of epitaxially strained (La0.8Sr0.2)0.95Ni0.2Fe0.8O3/SrTiO3(110)
Reversible and irreversible discontinuities at around 573 K and 823 K in the
electric conductivity of a strained 175 nm thin film of
(La0.8Sr0.2)0.95Ni0.2Fe0.8O3-{\delta} grown by pulsed laser deposition on
SrTiO3 (110) are reflected by valence band changes as monitored in
photoemission and oxygen K-edge x-ray absorption spectra. The irreversible jump
at 823 K is attributed to depletion of doped electron holes and reduction of
Fe4+ to Fe3+, as evidenced by oxygen and iron core level soft x-ray
spectroscopy, and possibly of a chemical origin, whereas the reversible jump at
573 K possibly originates from structural changes
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
Diffusing-wave spectroscopy of nonergodic media
We introduce an elegant method which allows the application of diffusing-wave
spectroscopy (DWS) to nonergodic, solid-like samples. The method is based on
the idea that light transmitted through a sandwich of two turbid cells can be
considered ergodic even though only the second cell is ergodic. If absorption
and/or leakage of light take place at the interface between the cells, we
establish a so-called "multiplication rule", which relates the intensity
autocorrelation function of light transmitted through the double-cell sandwich
to the autocorrelation functions of individual cells by a simple
multiplication. To test the proposed method, we perform a series of DWS
experiments using colloidal gels as model nonergodic media. Our experimental
data are consistent with the theoretical predictions, allowing quantitative
characterization of nonergodic media and demonstrating the validity of the
proposed technique.Comment: RevTeX, 12 pages, 6 figures. Accepted for publication in Phys. Rev.
X-ray computed micro tomography as complementary method for the characterization of activated porous ceramic preforms
The effect of compressive strain on the Raman modes of the dry and hydrated BaCe0.8Y0.2O3 proton conductor
The BaCe0.8Y0.2O3-{\delta} proton conductor under hydration and under
compressive strain has been analyzed with high pressure Raman spectroscopy and
high pressure x-ray diffraction. The pressure dependent variation of the Ag and
B2g bending modes from the O-Ce-O unit is suppressed when the proton conductor
is hydrated, affecting directly the proton transfer by locally changing the
electron density of the oxygen ions. Compressive strain causes a hardening of
the Ce-O stretching bond. The activation barrier for proton conductivity is
raised, in line with recent findings using high pressure and high temperature
impedance spectroscopy. The increasing Raman frequency of the B1g and B3g modes
thus implies that the phonons become hardened and increase the vibration energy
in the a-c crystal plane upon compressive strain, whereas phonons are relaxed
in the b-axis, and thus reveal softening of the Ag and B2g modes. Lattice
toughening in the a-c crystal plane raises therefore a higher activation
barrier for proton transfer and thus anisotropic conductivity. The experimental
findings of the interaction of protons with the ceramic host lattice under
external strain may provide a general guideline for yet to develop epitaxial
strained proton conducting thin film systems with high proton mobility and low
activation energy
Thermal decomposition of monosaccharides derivatives applied in ceramic gelcasting process investigated by the coupled DTA/TG/MS analysis
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