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Phonon transport simulations in hierarchical and highly disordered nanostructures
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Chakraborty, Dhritiman (2020) Phonon transport simulations in hierarchical and highly disordered nanostructures. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3493113~S15
Abstract
Nanostructuring is considered a very promising direction for high performance thermoelectric materials, which can convert waste heat into useful energy. These materials can reduce dependence on fossil fuels and enhance thermal energy harvesting, with huge environmental and societal benefits. In this work we investigate thermal transport in nanostructures and study methods to reduce the thermal conductivity (which enhances thermoelectric efficiency). Using silicon as an example, we consider the combined presence of nanocrystallinity and nanopores, arranged under both ordered and disordered (randomized) positions and sizes by using a phonon transport simulator constructed as a part of this work. We show that nanocrystalline boundaries degrade the thermal conductivity more drastically when the average grain size becomes smaller than the material average phonon mean-free-path. Introduction of pores in a hierarchical fashion 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 this randomization, or disorder, 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 disordered geometries. We propose extensions to these models that accurately predict the thermal conductivity of disordered nanoporous materials based solely on a few geometrical features. Additionally, 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. Looking at high temperature regimes, we show that the relative reduction in thermal conductivity is stronger at high temperatures in the presence of nanocrystallinity, a consequence of the wavevector-dependent nature of phonon scattering on the nanocrystalline grain domain boundaries.
We next consider asymmetric nanoporous structures, and investigate the combined effects of porosity, inter-pore distance, and pore position on thermal rectification in nanoporous silicon. We define thermal rectification in terms of system mean-free-paths rather than non-linearity in temperature – as conventionally done. We show that systems: i) with denser, compressed pore arrangements (i.e. with smaller inter-pore distances), ii) with pores positioned closer to the device edge/contact, and iii) with pores in a triangular arrangement, can achieve rectification of over 55%. Introducing hierarchically smaller pores into existing porous geometries increases rectification even further. Importantly, for the structures we simulate, we show that sharp rectifying junctions, separating regions of long from short phonon mean-free-paths are more beneficial than spreading the asymmetry throughout the material along the heat direction in a graded fashion.
Lastly, comparing a full wave-based quantum mechanical Non-Equilibrium Green's Function (NEGF) method, and a particle-based classical ray-tracing approach, we investigate the qualitative differences in the wave and particle-based phonon transport at the vicinity of nanoscale features, indicating when simplified particle based approaches fail, and when not. Insight extracted from this work can be used to provide better and more complete understanding of phonon transport in nanomaterials.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TA Engineering (General). Civil engineering (General) | ||||
Library of Congress Subject Headings (LCSH): | Nanostructured materials -- Thermal properties, Nanostructured materials -- Effect of temperature on, Nanosilicon -- Thermal properties, Nanosilicon -- Effect of temperature on | ||||
Official Date: | May 2020 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Neophytou, Neophytos | ||||
Format of File: | |||||
Extent: | xiii, 180 leaves : illustrations (chiefly colour) | ||||
Language: | eng |
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