75 research outputs found
Mid infrared spectra of four green and remoistened wood species
International audienc
Radiative Thermal Rectification between SiC and SiO2
By means of fluctuationnal electrodynamics, we calculate radiative heat flux
between two pla-nar materials respectively made of SiC and SiO2. More
specifically, we focus on a first (direct) situation where one of the two
materials (for example SiC) is at ambient temperature whereas the second
material is at a higher one, then we study a second (reverse) situation where
the material temperatures are inverted. When the two fluxes corresponding to
the two situations are different, the materials are said to exhibit a thermal
rectification, a property with potential applications in thermal regulation.
Rectification variations with temperature and separation distance are here
reported. Calculations are performed using material optical data experimentally
determined by Fourier transform emission spectrometry of heated materials
between ambient temperature (around 300 K) and 1480 K. It is shown that
rectification is much more important in the near-field domain, i.e. at
separation distances smaller than the thermal wavelength. In addition, we see
that the larger is the temperature difference, the larger is rectification.
Large rectification is finally interpreted due to a weakening of the SiC
surface polariton when temperature increases, a weakening which affects much
less SiO2 resonances
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
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
Mid-infrared optical properties of pyrolytic boron nitride in the 390 to 1050 C temperature range using spectral emissivity measurements
This paper shows a systematic experimental and theoretical study on the
temperature dependence of the infrared optical properties of pyrolytic boron
nitride (pBN), from 390 to 1050 CC for wavelengths between 4 and 16
{\mu}m. The temperature dependence of these properties has never been analyzed
before. The measured emissivity spectra were fitted to a dielectric function
model and an effective medium approximation. The phonon frequencies and
dielectric constants agreed well with room temperature experimental values from
the literature, as well as with ab initio and first principles calculations. In
addition, the phonon frequencies of the perpendicular mode and the dielectric
constants of the parallel mode showed an appreciable parabolic temperature
dependence, which justifies the interest of more theoretical efforts in order
to explain this behavior. Finally, the results of this work demonstrate that
thermal emission spectroscopy allows obtaining the values of the optical and
dielectric parameters of impure ceramic materials in a simple manner as a
function of temperature.Comment: 6 pages, 6 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
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 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
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
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