20,194 research outputs found
Origin of Large Dielectric Constant with Large Remnant Polarization and Evidence of Magnetoelectric Coupling in Multiferroic La modified BiFeO3-PbTiO3 Solid Solution
The presence of superlattice reflections and detailed analyses of the powder
neutron and x-ray diffraction data reveal that La rich
(BF-LF)-(PT) (BF-LF-PT) has ferroelectric
rhombohedral crystal structure with space group \textit{} at ambient
conditions. The temperature dependence of lattice parameters, tilt angle,
calculated polarization , volume, and integrated intensity of
superlattice and magnetic reflections show an anomaly around 170 K. Impedance
spectroscopy, dielectric and ac conductivity measurements were performed in
temperature range to probe the origin of large remnant
polarization and frequency dependent broad transitions with large dielectric
constant near . Results of impedance spectroscopy measurements
clearly show contributions of both grain and grain boundaries throughout the
frequency range ( Hz Hz). It could be concluded
that the grain boundaries are more resistive and capacitive as compared to the
grains, resulting in inhomogeneities in the sample causing broad frequency
dependent dielectric anomalies. Enhancement in dielectric constant and remnant
polarization values are possibly due to space charge polarization caused by
piling of charges at the interface of grains and grain boundaries. The
imaginary parts of dielectric constant () Vs frequency
data were fitted using Maxwell-Wagner model at K) and model
fits very well with the data up to Hz. Magnetodielectric measurements
prove that the sample starts exhibiting magnetoelectric coupling at
K, which is also validated by neutron diffraction data.Comment: 20 pages, 10 figure
Mass transport and electrochemical properties of La2Mo2O9 as a fast ionic conductor
La2Mo2O9, as a new fast ionic conductor, has been investigated widely due to its high
ionic conductivity which is comparable to those of the commercialized materials. However,
little work has been reported on the oxygen transport and diffusion in this candidate
electrolyte material. The main purpose of this project was to investigate oxide ion diffusion
in La2Mo2O9 and also the factors which could affect oxygen transport properties.
Oxygen isotope exchange followed by Secondary Ion Mass Spectrometry (SIMS)
measurements were employed to obtain oxygen diffusion profiles. A correlation between
oxygen ion transport and the electrochemical properties such as ionic conductivity was
built upon the Nernst Einstein equation relating the diffusivity to electrical conductivity.
In-situ neutron diffraction and AC impedance measurements were designed and conducted
to investigate the correlation between crystal structure and oxygen transport in the bulk
materials. Other techniques, such as synthesis, microstructure studies, and thermal analysis
were also adopted to study the electrochemical properties of La2Mo2O9.
The results of the study on the effects of microstructure on oxygen diffusion in
La2Mo2O9 revealed that the grain boundary component played a significant role in
electrochemical performance, although the grain size seemed to have little influence on
oxygen transport. The oxygen isotope exchange in 18O2 was successfully carried out by
introducing a silver coating on the sample surface, which solved the main difficulty in
applying oxygen isotope exchange on pure ionic conductors. The ionic conductivity
obtained from the diffusion coefficients was consistent with the result from AC impedance spectroscopy. The number of mobile oxygen ions was estimated to be 5 per unit cell. There
was a difference of oxygen self diffusion coefficient when the isotope exchange was
conducted in 18O2 and H2
18O. The activation energy of oxygen diffusion in humidified
atmosphere was higher than that measured in dry atmosphere. It indicated that the
humidified atmosphere had affected oxygen transport in the material. The studies on
hydroxyl incorporation and transport explained the decreased oxygen diffusion coefficients
in wet atmosphere and also suggested proton conductivity in La2Mo2O9, which leads to
further investigation on applications of La2Mo2O9 as a proton conductor. In-situ neutron
diffraction and AC impedance measurement revealed a close relationship between crystal
structure and ionic conductivity. The successful application of this technique provides a
new method to simultaneously investigate crystal structure and electrical properties in
electro-ceramics in the future
Disentangling instrumental broadening
A new procedure aiming at disentangling the instrumental profile broadening
and the relevant X-ray powder diffraction (XRPD) profile shape is presented.
The technique consists of three steps: denoising by means of wavelet
transforms, background suppression by morphological functions and deblurring by
a Lucy--Richardson damped deconvolution algorithm. Real XRPD intensity profiles
of ceria samples are used to test the performances. Results show the robustness
of the method and its capability of efficiently disentangling the instrumental
broadening affecting the measurement of the intrinsic physical line profile.
These features make the whole procedure an interesting and user-friendly tool
for the pre-processing of XRPD data.Comment: 9 pages, 1 table, 1 figure; typos correcte
Mechanochemical synthesis and characterization of nanodimensional iron–cobalt spinel oxides
Iron–cobalt spinel oxide nanoparticles, CoxFe3−xO4 (x = 1, 2), of sizes below 10 nm have been prepared by combining chemical precipitation with high-energy ball milling. For comparison, their analogues obtained by thermal synthesis have also been studied. The phase composition and structural properties of the obtained materials have been investigated by means of X-ray diffraction, Mössbauer spectroscopy, infrared spectroscopy, temperature-programmed reduction and magnetization measurements. X-ray diffraction shows that after 1 h of mechanical treatment ferrites are formed. The measurement techniques employed indicate that longer milling induces an increase in crystal size while crystal defects decrease with treatment time. Magnetization and reduction properties are affected by the particles size, the iron/cobalt ratio and the synthesis conditions
Multiscale understanding of tricalcium silicate hydration reactions
Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium
hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly
understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is
intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline
phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow
tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H
gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective
clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore,
PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched
as recently predicted by first principles calculations. These outcomes, plus additional laboratory
characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed
densities and Ca/Si atomic ratios at the nano- and meso- scales.This work has been supported by Spanish MINECO through BIA2014-57658-C2-2-R, which is co-funded by
FEDER, BIA2014-57658-C2-1-R and I3 (IEDI-2016-0079) grants. We also thank CELLS-ALBA (Barcelona,
Spain) for providing synchrotron beam time at BL04-MSPD beamline
Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation and Experimentation
Among the many additive manufacturing (AM) processes for metallic materials,
selective laser melting (SLM) is arguably the most versatile in terms of its
potential to realize complex geometries along with tailored microstructure.
However, the complexity of the SLM process, and the need for predictive
relation of powder and process parameters to the part properties, demands
further development of computational and experimental methods. This review
addresses the fundamental physical phenomena of SLM, with a special emphasis on
the associated thermal behavior. Simulation and experimental methods are
discussed according to three primary categories. First, macroscopic approaches
aim to answer questions at the component level and consider for example the
determination of residual stresses or dimensional distortion effects prevalent
in SLM. Second, mesoscopic approaches focus on the detection of defects such as
excessive surface roughness, residual porosity or inclusions that occur at the
mesoscopic length scale of individual powder particles. Third, microscopic
approaches investigate the metallurgical microstructure evolution resulting
from the high temperature gradients and extreme heating and cooling rates
induced by the SLM process. Consideration of physical phenomena on all of these
three length scales is mandatory to establish the understanding needed to
realize high part quality in many applications, and to fully exploit the
potential of SLM and related metal AM processes
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