Thermoelectric Power Factor in Nanostructured Materials With Randomized Nanoinclusions

Vargiamidis, Vassilios; Foster, Samuel; Neophytou, Neophytos

doi:10.1002/pssa.201700997

Cited by 7 publications

(7 citation statements)

References 36 publications

(47 reference statements)

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“…We simulate a dense, hexagonally oriented network, in order to maximize its effect on transport properties. Whether the network is regular or randomized, makes only little difference to the PF [196]. Potential barriers formed by NIs, similar to those formed in SLs or by grain boundaries, could be expected to also provide energy filtering and improve the Seebeck coefficient, and hopefully the PF [197,198].…”

Section: Nanoinclusions Voids and The Power Factormentioning

confidence: 99%

“…What is detrimental are variations in the barrier heights (that degrade the conductivity) and extremely thin, easy to tunnel barriers (which degrade the Seebeck coefficient) [69,180]. In the case of NIs, in a similar manner the variability in the geometry and positions of the NIs does not affect the power factor in a noticeable manner [196], but also for low V B the density and V B itself do not affect it either.…”

Section: Nanoinclusions Voids and The Power Factormentioning

confidence: 99%

Section: Nanoinclusions Voids and The Power Factormentioning

confidence: 99%

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Hierarchically nanostructured thermoelectric materials: challenges and opportunities for improved power factors

et al. 2020

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The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity, reaching values larger than two, consistently across materials and temperatures. Central to this ZT improvement is the drastic reduction in the material thermal conductivity due to the scattering of phonons on the numerous interfaces, boundaries, dislocations, point defects, phases, etc., which are purposely included. In these new generation of nanostructured materials, phonon scattering centers of different sizes and geometrical configurations (atomic, nano- and macro-scale) are formed, which are able to scatter phonons of mean-free-paths across the spectrum. Beyond thermal conductivity reductions, ideas are beginning to emerge on how to use similar hierarchical nanostructuring to achieve power factor improvements. Ways that relax the adverse interdependence of the electrical conductivity and Seebeck coefficient are targeted, which allows power factor improvements. For this, elegant designs are required, that utilize for instance non-uniformities in the underlying nanostructured geometry, non-uniformities in the dopant distribution, or potential barriers that form at boundaries between materials. A few recent reports, both theoretical and experimental, indicate that extremely high power factor values can be achieved, even for the same geometries that also provide ultra-low thermal conductivities. Despite the experimental complications that can arise in having the required control in nanostructure realization, in this colloquium, we aim to demonstrate, mostly theoretically, that it is a very promising path worth exploring. We review the most promising recent developments for nanostructures that target power factor improvements and present a series of design ‘ingredients’ necessary to reach high power factors. Finally, we emphasize the importance of theory and transport simulations for materialoptimization, and elaborate on the insight one can obtain from computational tools routinely used in the electronic device communities. Graphical abstract

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Section: Nanoinclusions Voids and The Power Factormentioning

confidence: 99%

Section: Nanoinclusions Voids and The Power Factormentioning

confidence: 99%

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Hierarchically nanostructured thermoelectric materials: challenges and opportunities for improved power factors

et al. 2020

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“…Finally, we note that recent theoretical studies about the effect of the effect of variations for electronic transport (where the mean-free-path is much shorted), is not as noticeable. 6,94 Thus, variability can be used as means to achieve lower thermal conductivity without affecting the electronic system, which could be advantageous for thermoelectric applications.…”

Section: Analytical Models -Extensions and Validationmentioning

confidence: 99%

Monte Carlo phonon transport simulations in hierarchically disordered silicon nanostructures

2018

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Hierarchical material nanostructuring is considered to be a very promising direction for high performance thermoelectric materials. In this work we investigate thermal transport in hierarchically nanostructured silicon. We consider the combined presence of nanocrystallinity and nanopores, arranged under both ordered and randomized positions and sizes, by solving the Boltzmann transport equation using the Monte Carlo method. We show that nanocrystalline boundaries degrade the thermal conductivity more drastically when the average grain size becomes smaller than the average phonon mean-free-path. The introduction of pores degrades the thermal conductivity even further. Its effect, however, is significantly more severe when the pore sizes and positions are randomized, as randomization results in regions of higher porosity along the phonon transport direction, which introduce significant thermal resistance. We show that randomization acts as a large increase in the overall effective porosity. Using our simulations, we show that existing compact nanocrystalline and nanoporous theoretical models describe thermal conductivity accurately under uniform nanostructured conditions, but overestimate it in randomized geometries. We propose extensions to these models that accurately predict the thermal conductivity of randomized nanoporous materials based solely on a few geometrical features. Finally, we show that the new compact models introduced can be used within Matthiessen's rule to combine scattering from different geometrical features within ~10% accuracy. . 3.7 Wm −1 K −1 for an average pore size of ~ 30 nm and grain sizes between 50 and 80 nm. 21 By reducing both pore and grain sizes, however, Basu et al. reported κ = 1.2 Wm −1 K −1 at 40% porosity in p-type silicon. 22 A recent work in SiGe nanomeshes, reported ultralow κ of 0.55 ± 0.10 Wm −1 K −1 for SiGe nanocrystalline nanoporous structures, a value well below the amorphous limit. 23 A significant amount of work can be found in the literature attempting to clarify these experimental observations. However, theoretical investigations of thermal conductivity in highly/hierarchically disordered nanostructures (which include not only crystalline boundaries, but also pores of random sizes placed at random positions) are very limited. Understanding the qualitative and quantitative details of such geometries on the thermal conductivity would allow the design of more efficient thermoelectrics and heat management materials in general. In this work, we solve the Boltzmann transport equation for phonons in disordered Si nanostructures using the Monte Carlo (MC) method. Monte Carlo, which can capture the details of geometry with relative accuracy, is widely employed to understand phonon transport in various nanostructures such as nanowires, 24,25,26 thin films, 27,28 nanoporous materials, 29,30,31,32,33 polycrystalline materials, 10,15,34,35,36 nanocomposites, 37,38 corrugated structures, 39,40,41,42 silicon-on-insulator devices, 43 etc. We consider geometries that include grai...

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“…More recent studies, reported in the p-type Pb0.98Na0.02Te-SrTe system an extremely low κ of 0.5 W K −1 m −1 and a ZT of 2.5 at 923K [13], and a similarly low κ = 0.55 W K −1 m −1 in a SiGe nanoporous system [7]. Moreover, our prior works also point out that hierarchical nanostructures can improve the thermoelectric power factor as well [8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Enhanced Phonon Boundary Scattering at High Temperatures in Hierarchically Disordered Nanostructures

Chakraborty

Oliveira

Neophytou

2019

Journal of Elec Materi

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Boundary scattering in hierarchically disordered nanomaterials is an effective way to reduce the thermal conductivity of thermoelectric materials and increase their performance. In this work we investigate thermal transport in silicon-based nanostructured materials in the presence of nanocrystallinity and nanopores at the range of 300 K -900 K using a Monte Carlo simulation approach. The thermal conductivity in the presence of nanocrystallinity follows the same reduction trend as in the pristine material. We show, however, that the relative reduction is stronger with temperature in the presence of nanocrystallinity, a consequence of the wavevector-dependent (q-dependent) nature of phonon scattering on the domain boundaries. In particular, as the temperature is raised the proportion of large wavevector phonons increases. Since these phonons are more susceptible to boundary scattering, we show that compared to the wave-vector independent thermal conductivity value this q-dependent surface scattering could account for even a ~40% reduction in the thermal conductivity of nanocrystalline Si. Introduction of nanopores with randomized positions enhances this effect, which suggests that hierarchical nanostructuring is actually more effective at high temperatures than previously thought.

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Thermoelectric Power Factor in Nanostructured Materials With Randomized Nanoinclusions

Cited by 7 publications

References 36 publications

Hierarchically nanostructured thermoelectric materials: challenges and opportunities for improved power factors

Hierarchically nanostructured thermoelectric materials: challenges and opportunities for improved power factors

Monte Carlo phonon transport simulations in hierarchically disordered silicon nanostructures

Enhanced Phonon Boundary Scattering at High Temperatures in Hierarchically Disordered Nanostructures

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