62 research outputs found
Texture and Porosity Effects on the Thermal Radiative Behavior of Alumina Ceramics
International audienceThermal and optical properties of ceramics are dependent on radiation scattering and cannot be determined by a knowledge of their chemical composition alone, as for single crystals. In this paper, extrinsic effects, such as roughness, porosity, and texture, on the spectral emissivity of alumina ceramics are investigated. Roughness effects have an influence mainly in the opaque zone; an important porosity dependence and the presence of a critical porosity threshold were observed in the semitransparent zone. Furthermore, it was shown that two ceramics with similar total porosities, but with different textures, possess radically different emissivities, showing that grain size, pore size, and spatial repartition of the grains are also crucial for an understanding of the thermal properties of the ceramics
The effect of sulfur on the glass transition temperature in anorthite-diopside eutectic glasses
International audienceThe effect of sulfur dissolved in anorthite-diopside eutectic (AD) glasses on the glasstransition temperature (Tg) has been investigated via Differential Scanning Calorimetricmeasurements (DSC) and Thermogravimetric Analysis (TGA) under moderately reducing tooxidizing conditions.In a series of AD glasses, we have measured the change in Tg as a function of S contentpresent as SO42- (HS- is also identified to a lesser extent) and H2O content. The AD glassesinvestigated have S contents ranging from 0 to 7519 ppm and H2O contents ranging from 0 to5.3 wt.%. In agreement with previous studies, increasing H2O content induces a strongexponential decrease in Tg: volatile free AD glass has a Tg at 758±13C and AD glass with5.18±0.48 wt.% H2O has a Tg at 450±11C. The change in Tg as a function of H2O is wellreproducedwith a third-order polynomial function and has been used to constrain Tg at anyH2O content. The effect of S on Tg is almost inexistent or towards a decrease in Tg withincreasing S content. For instance, at ~2.4 wt.% H2O, the addition of S induces a change inTg from 585±10°C with 0 ppm S to 523±3C with 2365±138 ppm S; a further increase in Sup to 7239±90 ppm S does not induce a dramatic change in Tg measured at 529±2C.The limited effect of S on the glass transition temperature contrasts with recent spectroscopicmeasurements suggesting that S dissolution as SO42- groups provokes an increase in thepolymerization degree. We propose an alternative view which reconciles the spectroscopicevidence with the Tg measurements. The dissolution of S as SO42- does not induce theformation of Si-O-Si molecular bonding through consumption of available non-bridgingoxygens (NBO) but instead we suggest that Si-O-S molecular bonds are formed which are not detectable by DSC measurements but mimic the increase in glass polymerization. Therefore, spectroscopic measurements must be used with caution in order to extract melt physicalproperties
Prediction of thermal radiative properties (300–1000 K) of La2NiO4+δ ceramics.
A multiscale numerical model is developed to predict the thermal radiative properties (TRP) of rough La2NiO4+δ coatings. The model integrates intrinsic and extrinsic contributions related to the chemical composition and the texture, respectively. High-temperature infrared reflectivity and thermogravimetric measurements on a La2NiO4+δ single crystal make it possible to understand the role of the excess oxygen in the intrinsic TRP. We show that dense ceramics with thicknesses higher than 4 μm are optically thick, and that one can adjust the surface roughness parameters to predict their TRP
Phonons and Hybrid Modes in the High and Low Temperature Far Infrared Dynamics of Hexagonal TmMnO3
We report on TmMnO3 far infrared emissivity and reflectivity spectra from
1910 K to 4 K. At the highest temperature the number of infrared bands is lower
than that predicted for centrosymmetric P63/mmc (D6h4) (Z=2) space group due
high temperature anharmonicity and possible defect induced bitetrahedra
misalignments. On cooling, at ~1600 K TmMnO3 goes from non-polar to an
antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at
the ~700 K. The 300 K reflectivity is fitted using 19 oscillators and this
number of phonons is maintained down to 4 K. A weak phonon anomaly in the band
profile at 217 cm-1 (4 K) suggests subtle Rare Earth magnetoelectric couplings
at ~TN and below. A low energy collective excitation is identified as a THz
instability associated with room temperature eg electrons in a d-orbital
fluctuating environment. It condenses into two modes that emerge pinned to the
E-type antiferromagmetic order hardening simultaneously down to 4 K. They obey
power laws with TN as the critical temperature and match known zone center
magnons. The one peaking at 26 cm-1, with critical exponent \b{eta}=0.42 as for
antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth.
The band at ~50 cm-1, with \b{eta}=0.25, splits at ~TN into two peaks. The
weaker band of the two is assimilated to the upper branch of gap opening in the
transverse acoustical (TA) phonon branch crossing the magnetic dispersion found
in YMnO3. (Petit et al, 2007 PRL 99, 266604). The stronger second at ~36 cm-1
corresponds to the lower branch of the TA gap. We assign both excitations as
zone center magnetoelectric hybrid quasiparticles concluding that in NdMnO3
perovskite the equivalent picture corresponds to an instability which may be
driven by an external field to transform NdMnO3 into a multiferroic compound by
perturbation enhancing the TA phonon-magnetic correlation.Comment: 39 pages, 9 Figure
Paramagnetic Collective Electronic Mode and Low Temperature Hybrid Modes in the Far Infrared Dynamics of Orthorhombic NdMnO3
We report on far- and mid-infrared reflectivity of NdMnO3 from 4 K to 300K.
Two main features are distinguished in the infrared spectra: active phonons in
agreement with the expected for orthorhombic D2h 16-Pbnm (Z=4) space group
remaining constant down to 4 K and a well-defined collective excitation in the
THz region due to eg electrons in a d-orbital fluctuating environment. We trace
its origin to the NdMnO3 high temperature orbital disordered intermediate phase
not being totally dynamically quenched at lower temperatures. This results in
minute orbital misalignments that translate in randomize non-static eg
electrons within orbitals yielding a room temperature collective excitation.
Below TN~78 K, electrons gradually localize inducing long-range magnetic order
as the THz band condenses into two modes that emerge pinned to the A-type
antiferromagmetic order. They harden simultaneously down to 4 K obeying power
laws with TN as the critical temperature and exponents {\beta}~0.25 and
{\beta}~0.53, as for a tri-critical point and Landau magnetic ordering,
respectively. At 4K they match known zone center spin wave modes. The power law
dependence is concomitant with a second order transition in which spin modes
modulate orbital instabilities in a magnetoelectric hybridized
orbital/charge/spin/lattice scenario. We also found that phonon profiles also
undergo strong changes at TN~78 K due to magnetoelasticity.Comment: 40 pages with 8 figure
High Temperature Far Infrared Dynamics of Orthorhombic NdMnO3: Emissivity and Reflectivity
We report on near normal far- and mid-infrared emission and reflectivity of
NdMnO3 perovskite from room temperature to sample decomposition above 1800 K.
At 300 K the number infrared active phonons is in close agreement with the 25
calculated for the orthorhombic D2h16-Pbnm (Z=4) space group. Their number
gradually decreases as we approach the temperature of orbital disorder at ~1023
K where the orthorhombic O' lower temperature cooperative phase coexists with
the cubic orthorhombic O. At above ~1200 K, the three infrared active phonons
coincide with the expected for cubic Pm-3m (Z=1) in the high temperature
insulating regime. Heating samples in dry air triggers double exchange
conductivity by Mn3+ and Mn4+ ions and a small polaron mid-infrared band. Fits
to the optical conductivity single out the octahedral antisymmetric and
symmetric vibrational modes as main phonons in the electron-phonon interactions
at 875 K. For 1745 K, it is enough to consider the symmetric stretching
internal mode. An overdamped defect induced Drude component is clearly outlined
at the highest temperatures. We conclude that Rare Earth manganites eg
electrons are prone to spin, charge, orbital, and lattice couplings in an
intrinsic orbital distorted perovskite lattice favoring embryonic low energy
collective excitations.Comment: 32 pages with 5 figure
Modelling of the radiative properties of an opaque porous ceramic layer
Solid Oxide Fuel Cells (SOFCs) operate at temperatures above 1,100 K where radiation effects can be significant. Therefore, an accurate thermal model of an SOFC requires the inclusion of the contribution of thermal radiation. This implies that the thermal radiative properties of the oxide ceramics used in the design of SOFCs must be known. However, little information can be found in the literature concerning their operating temperatures. On the other hand, several types of ceramics with different chemical compositions and microstructures for designing efficient cells are now being tested. This is a situation where the use of a numerical tool making possible the prediction of the thermal radiative properties of SOFC materials, whatever their chemical composition and microstructure are, may be a decisive help. Using this method, first attempts to predict the radiative properties of a lanthanum nickelate porous layer deposited onto an yttria stabilized zirconium substrate can be reported
Paramagnetic collective electronic mode and low temperature hybrid modes in the far infrared dynamics of orthorhombic NdMnO₃
We report on the far- and mid-infrared reflectivity of NdMnO₃ from 4 to 300 K. Two main features are distinguished in the infrared spectra: active phonons in agreement with expectations for the orthorhombic D¹⁶2h-Pbnm (Z = 4) space group remaining constant down to 4 K and a well defined collective excitation in the THz region due to eg electrons in a d-orbital fluctuating environment. We trace its origin to the NdMnO₃ high-temperature orbital disordered intermediate phase not being totally dynamically quenched at lower temperatures. This results in minute orbital misalignments that translate into randomized non-static eg electrons within orbitals yielding a room-temperature collective excitation. Below TN ∼ 78 K, electrons gradually localize, inducing long-range magnetic order as the THz band condenses into two modes that emerge pinned to the A-type antiferromagnetic order. They harden simultaneously down to 4 K, obeying power laws with TN as the critical temperature and exponents β ∼ 0.25 and β ∼ 0.53, as for a tri-critical point and Landau magnetic ordering, respectively. At 4 K they match known zone center spin wave modes. The power law dependence is concomitant with a second order transition in which spin modes modulate orbital instabilities in a magnetoelectric hybridized orbital-charge-spin-lattice scenario. We also found that phonon profiles also undergo strong changes at TN ∼ 78 K due to magnetoelasticity.Facultad de Ciencias ExactasCentro de Química Inorgánic
High temperature far-infrared dynamics of orthorhombic NdMnO₃: emissivity and reflectivity
We report on near normal far- and mid-infrared emission and reflectivity of NdMnO₃ perovskite from room temperature to sample decomposition above 1800 K. At 300 K the number infrared active phonons is in close agreement with the 25 calculated for the orthorhombic D¹⁶2h-Pbnm (Z=4) space group. Their number gradually decreases as we approach the temperature of orbital disorder at ~1023 K where the orthorhombic O' lower temperature cooperative phase coexists with the cubic orthorhombic O. At above ~1200 K, the three infrared active phonons coincide with the expected for cubic Pm-3m (Z=1) in the high temperature insulating regime.
Heating samples in dry air triggers double exchange conductivity by Mn³⁺ and Mn⁴⁺ ions and a small polaron mid-infrared band. Fits to the optical conductivity single out the octahedral antisymmetric and symmetric vibrational modes as main phonons in the electron-phonon interactions at 875 K. For 1745 K, it is enough to consider the symmetric stretching internal mode. An overdamped defect induced Drude component is clearly outlined at the highest temperatures. We conclude that Rare Earth manganites eg electrons are prone to spin, charge, orbital, and lattice couplings in an intrinsic orbital distorted perovskite lattice favoring embryonic low energy collective excitations.Facultad de Ciencias ExactasCentro de Química Inorgánic
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