To further reduce the lattice thermal conductivity of thermoelectric materials, the technique of embedding nano-inclusions into bulk matrix materials, in addition to point defect scattering via alloying, was widely applied. Differential Effective Medium (DEM) method was employed to calculate two-phase heterogeneous systems. However, in most effective medium treatment, the interface scattering of matrix phonons by embedded nanoparticle was underestimated by adopting particle's projected area as scattering cross-section. Herein, modified cross-section calculations, as well as grain sizes dispersions, are applied in DEM, with the calculations then validated by comparing with Monte-Carlo simulations and existing experimental data. Predictions of lattice thermal conductivity reduction on in-situ formed Full Heusler(FH)/Half Heusler(HH) nano/matrix system are discussed.
High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 18 medium- and high-entropy pyrochlores were synthesized to measure their thermal conductivity and Young's modulus. Up to 35% reductions in thermal conductivity were achieved with retained moduli, thereby attaining insulative yet stiff properties for potential thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter. Thus, this modified size diso…
In the work, a modified effective medium theory is constructed for calculating the effective properties of thermoelectric composites with different values of percolation thresholds. It is shown that even at concentrations beyond the critical region, the threshold value is essential for determining the effective properties. Two fundamentally different cases of a set of local properties of the composite are considered. In one of these cases, the conductivity and thermal conductivity of one of the phases is simultaneously greater than the conductivity and thermal conductivity of the other phase. …
We derive a new cross-property differential effective medium scheme for a composite material's thermal conductivity as a function of its electrical conductivity and vice versa. Our scheme assumes that one phase is embedded in the other as inclusions. The relations are independent of inclusion volume fraction, but depend on the aspect ratio of the inclusions. We show that the method successfully models published laboratory measurements on a copper-graphite composite, with the inferred aspect ratio matching the physical shape of the inclusions. This work complements earlier results on elastic-el…
Nanostructured materials enable high thermal transport tunability, holding promises for thermal management and heat harvesting applications. Predicting the effect that nanostructuring has on thermal conductivity requires models, such as the Boltzmann transport equation (BTE), that capture the non-diffusive transport of phonons. Although the BTE has been well validated against several key experiments, notably those on nanoporous materials, its applicability is computationally expensive. Several effective model theories have been put forward to estimate the effective thermal conductivity; howeve…
Thermal conductivity of nanocrystalline and ultra-nanocrystalline films is analyzed with effective medium theory (EMT) models. The existing EMT models use the spherical inclusion approximation. Although this approximation works quite well it is inconsistent, mostly with respect to the maximal packing of 74{\%}, which may be unrealistic for polycrystalline films. To check the consistency of these models we devise an EMT model with arbitrarily shaped inclusions. We pick the EMT model with cubic inclusions and we compare its results with the results of the EMT model with spherical inclusions. It …
There are several models for the effective thermal conductivity of two-phase composite materials in terms of the conductivity of the solid and the disperse material. In this paper, we generalise three models of Maxwell type (namely, the classical Maxwell model and two generalisations of it obtained from effective medium theory and differential effective medium theory) so that the resulting effective thermal conductivity accounts for radiative heat transfer within gas voids. In the high-temperature regime, radiative transfer within voids strongly influences the thermal conductivity of the bulk …
Electrical and thermal transport properties of disordered materials have long been of both theoretical interest and engineering importance. As a new class of materials with an intrinsic compositional disorder, high/medium-entropy alloys (HEAs/MEAs) are being immensely studied mainly for their excellent mechanical properties. By contrast, electrical and thermal transport properties of HEAs/MEAs are less well studied. Here we investigate these two properties of silicon (Si)-germanium (Ge)-tin (Sn) MEAs, where we keep the same content of Si and Ge while increasing the content of Sn from 0 to 1/3 …
It is discussed that the classical effective medium theory for the elastic properties of random heterogeneous materials is not congruous with the effective medium theory for the electrical conductivity. In particular, when describing the elastic and electro-conductive properties of a strongly inhomogeneous two-phase composite material, the steep rise of effective parameters occurs at different concentrations. To achieve the logical concordance between the cross-property relations, a modification of the effective medium theory of the elastic properties is introduced. It is shown that the qualit…
Quantifying the correlation between the complex structures of amorphous materials and their physical properties has been a long-standing problem in materials science. In amorphous Si, a representative covalent amorphous solid, the presence of a medium-range order (MRO) has been intensively discussed. However, the specific atomic arrangement corresponding to the MRO and its relationship with physical properties, such as thermal conductivity, remain elusive. Here, we solve this problem by combining topological data analysis, machine learning, and molecular dynamics simulations. By using persiste…
We present a review of the phonon thermal conductivity of segmented nanowires focusing on theoretical results for Si and Si/Ge structures with the constant and periodically modulated cross-sections. We describe the use of the face-centered cubic cell and Born-von Karman models of the lattice vibrations for calculating the phonon energy spectra in the segmented nanowires. Modification of the phonon spectrum in such nanostructures results in strong reduction of the phonon thermal conductivity and suppression of heat transfer due to a trapping of phonon modes in nanowire segments. Possible practi…
Thermal conductivity of amorphous carbon thin films
用$3ω$方法测量了非晶碳薄膜在80- 400 K温度范围内的热导率$Λ$。样品薄膜的范围从通过远程等离子体沉积制备的软a-C：H（室温下的$Λ= 0.20$ W m $^{-1}$ K $^{-1}$）到具有大部分从过滤弧源沉积的$sp^3$键合碳的非晶金刚石（$Λ= 2.2$ W m $^{-1}$ K $^{-1}$）。连续介质理论提供了电导率随质量密度变化的唯象描述。热导率与由测量的原子密度和纵向声速计算的最小热导率吻合得很好。
We explore the effects of geometry on the thermal conductivity (kappa) of silicon and germanium nanowires, with lengths between 10-120 nm and diameters up to 5-6 nm. To this end we perform molecular dynamics simulations with the LAMMPS software, using Tersoff interatomic potentials. We consider nanowires with polygonal cross section and we discuss the effect of the transverse geometry on the thermal conductivity. We also consider tubular (hollow) nanowires and core/shell combinations of Si/Ge and Ge/Si, and we compare the heat transport of the core/shell structure with that of the separated co…
