Mitigating the Effect of Nanoscale Porosity on Thermoelectric Power Factor of Si

Hosseini, S. Aria; Romano, Giuseppe; Greaney, P. Alex

doi:10.1021/acsaem.0c02640

Cited by 12 publications

(5 citation statements)

References 29 publications

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“…But the overall effect of nanostructuring could be more complicated, involving the effect on ph, and phenomena such as energy filtering. 11,[32][33][34][35] For the latter, energy filtering either for majority or minority carriers would reduce bipolar effects (effectively increase the bandgap) and reduce the effect we describe.…”

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confidence: 99%

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Graziosi

Neophytou

2022

Applied Physics Letters

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Low bandgap thermoelectric materials suffer from bipolar effects at high temperatures, with increased electronic thermal conductivity and reduced Seebeck coefficient, leading to reduced power factor and low ZT figure of merit. In this work we show that the presence of strong transport asymmetries between the conduction and valence bands can allow high phonon-limited electronic conductivity at finite Seebeck coefficient values, leading to largely enhanced power factors. The power factors that can be achieved can be significantly larger compared to their maximum unipolar counterparts, allowing for doubling of the ZT figure of merit. We identify this behavior in lowbandgap cases from the half-Heusler material family. Using both, advanced electronic Boltzmann transport calculations for realistic material bandstructures, as well as model parabolic electronic bands, we elaborate on the parameters that determine this effect. We then develop a series of descriptors which can guide machine learning studies in identifying such classes of materials with extraordinary power factors at nearly undoped conditions. For this we test more than 3000 analytical bandstructures and their features, and more than 120 possible descriptors, to identify the most promising ones that contain: i) only bandstructure features for easy identification from material databases, and ii) bandstructure and transport parameters that provide much higher correlations, but for which parameter availability can be somewhat more scarce.

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confidence: 99%

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Graziosi

Neophytou

2022

Applied Physics Letters

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“…The term

is the Kronecker delta function, and

is the first-order Bessel function of the first kind, and

is the pore’s length perpendicular to the transport direction. We have previously computed the scattering matrix operators for pores with rectangular and triangular cross-sections and these can be found in reference [ 66 ]. The number density of pores is related to porosity,

, and the pore size through the relationship

, where

is the volume of the pores.…”

Section: Charge Carriers Transport In Nanoporous Si 08 Ge 02mentioning

confidence: 99%

“…This is an insightful result, showing the systematic difference between discrete and extended pores on electron scattering processes. We have studied the effect of discreet (e.g., spherical) pores, on electronic coefficients in a recent paper [ 66 ]. There, we have shown that the low-energy scattering by pores induced a strong filtering effect that considerably enhances the Seebeck coefficient and thus mitigates the effect of nanoscale porosity on the thermoelectric power factor of dielectrics.…”

Section: Charge Carriers Transport In Nanoporous Si 08 Ge 02mentioning

confidence: 99%

Enhanced Thermoelectric Performance of Polycrystalline Si0.8Ge0.2 Alloys through the Addition of Nanoscale Porosity

2021

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Engineering materials to include nanoscale porosity or other nanoscale structures has become a well-established strategy for enhancing the thermoelectric performance of dielectrics. However, the approach is only considered beneficial for materials where the intrinsic phonon mean-free path is much longer than that of the charge carriers. As such, the approach would not be expected to provide significant performance gains in polycrystalline semiconducting alloys, such as SixGe1-x, where mass disorder and grains provide strong phonon scattering. In this manuscript, we demonstrate that the addition of nanoscale porosity to even ultrafine-grained Si0.8Ge0.2 may be worthwhile. The semiclassical Boltzmann transport equation was used to model electrical and phonon transport in polycrystalline Si0.8Ge0.2 containing prismatic pores perpendicular to the transport current. The models are free of tuning parameters and were validated against experimental data. The models reveal that a combination of pores and grain boundaries suppresses phonon conductivity to a magnitude comparable with the electronic thermal conductivity. In this regime, ZT can be further enhanced by reducing carrier concentration to the electrical and electronic thermal conductivity and simultaneously increasing thermopower. Although increases in ZT are modest, the optimal carrier concentration is significantly lowered, meaning semiconductors need not be so strongly supersaturated with dopants.

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“…Nanoengineered membranes including thin films, 2,3 nanowires, 4 nanomeshes, 5,6 nanocomposites, 7,8 and nanoporous structures [9][10][11][12] feature extremely low thermal conductivity, sometimes beyond the amorphous limit. 13 This is largely due to the scattering of phonons with large MFPs (generally with small frequencies), i.e., those that are comparable with the characteristic length of the material.…”

Section: Introductionmentioning

confidence: 99%

Mode- and Space- Resolved Thermal Transport of Alloy Nanostructures

Hosseini¹,

Khanniche²,

Snyder³

et al. 2022

Preprint

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Nanostructured semiconducting alloys obtain ultra-low thermal conductivity as a result of the scattering of phonons with a wide range of mean-free-paths (MFPs). In these materials, long-MFP phonons are scattered at the nanoscale boundaries whereas short-MFP high-frequency phonons are impeded by disordered point defects introduced by alloying. While this trend has been validated by simplified analytical and numerical methods, an ab-initio space-resolved approach remains elusive. To fill this gap, we calculate the thermal conductivity reduction in porous alloys by solving the mode-resolved Boltzmann transport equation for phonons using the finite-volume approach. We analyze different alloys, length-scales, concentrations, and temperatures, obtaining a very large reduction in the thermal conductivity over the entire configuration space. For example, a ∼97% reduction is found for Al0.8In0.2As with 25% porosity. Furthermore, we employ these simulations to validate our recently introduced "Ballistic Correction Model" (BCM), an approach that estimates the effective thermal conductivity using the characteristic MFP of the bulk alloy and the length-scale of the material. The BCM is then used to provide guiding principles in designing alloy-based nanostructures. Notably, it elucidates how porous alloys such as SixGe1−x obtain larger thermal conductivity reduction compared to porous Si or Ge, while also explaining why we should not expect similar behavior in alloys such as AlxIn1−xAs. By taking into account the synergy from scattering at different scales, we provide a route for the design of materials with ultra-low thermal conductivity.

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Mitigating the Effect of Nanoscale Porosity on Thermoelectric Power Factor of Si

Cited by 12 publications

References 29 publications

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Bipolar conduction asymmetries lead to ultra-high thermoelectric power factor

Enhanced Thermoelectric Performance of Polycrystalline Si0.8Ge0.2 Alloys through the Addition of Nanoscale Porosity

Mode- and Space- Resolved Thermal Transport of Alloy Nanostructures

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