17 research outputs found
Lattice thermal conductivity of TiZrHfNiSn half-Heusler alloys calculated from first principles: Key role of nature of phonon modes
In spite of their relatively high lattice thermal conductivity
, the XNiSn (X=Ti, Zr or Hf) half-Heusler compounds are good
thermoelectric materials. Previous studies have shown that can
be reduced by sublattice-alloying on the X-site. To cast light on how the alloy
composition affects , we study this system using the phonon
Boltzmann-transport equation within the relaxation time approximation in
conjunction with density functional theory.The effect of alloying through
mass-disorder scattering is explored using the virtual crystal approximation to
screen the entire ternary TiZrHfNiSn phase diagram. The
lowest lattice thermal conductivity is found for the TiHfNiSn
compositions; in particular, there is a shallow minimum centered at
TiHfNiSn with taking values between 3.2 and 4.1 W/mK
when the Ti content varies between 20 and 80\%. Interestingly, the overall
behavior of mass-disorder scattering in this system can only be understood from
a combination of the nature of the phonon modes and the magnitude of the mass
variance. Mass-disorder scattering is not effective at scattering acoustic
phonons of low energy. By using a simple model of grain boundary scattering, we
find that nanostructuring these compounds can scatter such phonons effectively
and thus further reduce the lattice thermal conductivity; for instance,
TiHfNiSn with a grain size of nm experiences a 42\%
reduction of compared to that of the single crystal
First-principles determination of the phonon-point defect scattering and thermal transport due to fission products in ThO2
This work presents the first principles calculations of the lattice thermal
conductivity degradation due to point defects in thorium dioxide using an
alternative solution of the Pierels-Boltzmann transport equation. We have used
the non-perturbative Green's function methodology to compute the phonon point
defect scattering rates that consider the local distortion around the point
defect, including the mass difference changes, interatomic force constants and
structural relaxation near the point defects. The point defects considered in
the work include the vacancy of thorium and oxygen, substitution of helium,
krypton, zirconium, iodine, xenon, in the thorium site, and the three different
configuration of the Schottky defects. The results of the phonon-defect
scattering rate reveals that among the considered intrinsic defects, the
thorium vacancy and helium substitution in the thorium site scatter the phonon
most due to substantial changes in the force constant and structural
distortions. The scattering of phonons due to the substitutional defects
unveils that the zirconium atom scatters phonons the least, followed by xenon,
iodine, krypton, and helium. This is contrary to the intuition that the
scattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the mass
difference. This striking difference in the zirconium phonon scattering is due
to the local chemical environment changes. Zirconium is an electropositive
element with valency similar to thorium and, therefore, can bond with the
oxygen atoms, thus creating less force constant variance compared to iodine, an
electronegative element, noble gas helium, xenon, and krypton. These results
can serve as the benchmark for the analytical models and help the
engineering-scale modeling effort for nuclear design.Comment: 10 page
Thermal conductivity of crystalline AlN and the influence of atomic-scale defects
Aluminum nitride (AlN) plays a key role in modern power electronics and
deep-ultraviolet photonics, where an understanding of its thermal properties is
essential. Here we measure the thermal conductivity of crystalline AlN by the
3 method, finding it ranges from 674 56 W/m/K at 100 K to 186
7 W/m/K at 400 K, with a value of 237 6 W/m/K at room
temperature. We compare these data with analytical models and first principles
calculations, taking into account atomic-scale defects (O, Si, C impurities,
and Al vacancies). We find Al vacancies play the greatest role in reducing
thermal conductivity because of the largest mass-difference scattering.
Modeling also reveals that 10% of heat conduction is contributed by phonons
with long mean free paths, over ~7 m at room temperature, and 50% by
phonons with MFPs over ~0.3 m. Consequently, the effective thermal
conductivity of AlN is strongly reduced in sub-micron thin films or devices due
to phonon-boundary scattering
Modelling thermal transport in nanostructured materials
Zur Bestimmung der thermischen Eigenschaften von Si-Ge Legierungen haben wir zunächst ein kurzreichweitiges Kraftkonstantenmodell entwickelt. Anhand dieses Modells konnten wir zeigen, dass kurzreichweitige Potentiale zur Berechnung thermischer Eigenschaften von komplexen Nanostrukturen verwendet werden können. Desweiteren haben wir mit Hilfe von ab initio Rechnungen das Verhalten der thermischen Leitfähigkeit in Zn-Chalkogeniden untersucht. Diese zeigen ein interessantes Verhalten in Abhängigkeit von der Nanostruktur. In diesem Zusammenhang haben wir ein detailliertes Verständnis in Bezug auf die verschiedenen Phononenbeiträge in diesen Materialien entwickelt. Um die thermische Leitfähigkeit von großskaligen Si-Nanostrukturen zu berechnen, haben wir ein tight-binding Modell entwickelt, welches die thermische Leitfähigkeit von Si in der Diamantstruktur in guter Übereinstimmung mit Experimenten reproduziert. Außerdem zeigt es eine gute Übertragbarkeit auf andere Si-Strukturen.We have developed simple and short ranged models for simulation of thermal conductivity in nanomaterials for thermoelectric applications. A force constant model has been developed initially for calculating thermal properties in Si-Ge alloys. Through this model, we have demonstrated the possibility of short ranged models for correctly determining thermal properties of complex nanostructures. We have also done ab initio study of thermal conductivity in Zn-Chalcogenides which are known to show an interesting change in their thermal conductivity trend from bulk to nanoscale. We have developed a detailed understanding of this behaviour in terms of different phonon contributions in these materials. Furthermore, to calculate thermal conductivity at large length scales for Si nanostructures, we have developed a tight binding model for Si. The model predicts Si bulk thermal conductivity in good agreement with experiment and shows transferability to other structures of Si