Off-campus UMass Amherst users: To download campus access dissertations, please use the following link to log into our proxy server with your UMass Amherst user name and password.
Non-UMass Amherst users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Dissertations that have an embargo placed on them will not be available to anyone until the embargo expires.
ORCID
https://orcid.org/0000-0002-7279-1951
Access Type
Open Access Thesis
Document Type
thesis
Degree Program
Electrical & Computer Engineering
Degree Type
Master of Science in Electrical and Computer Engineering (M.S.E.C.E.)
Year Degree Awarded
2020
Month Degree Awarded
May
Abstract
The current high demand for sustainable and renewable energy sources to solve world energy crisis has enormously increased interest in looking at alternative sources of energy. All the machines used in manufacturing process, electricity generation, residential applications, transportation etc., rejects energy in the form of heat into environment. Thermoelectric materials can convert thermal-to-electrical and electrical-to-thermal energy and can be utilized in waste-heat harvesting, more efficient cooling to reduce energy usage and CO2 emissions. Significant research efforts have been devoted over the past decade to thermoelectric materials, with particular emphasis being placed on combining materials selection with nanostructuring. The overarching goal was to reduce thermal conductivity through selective phonon scattering and thus boost the thermoelectric figure-of-merit (ZT). SiGe alloys, as well as superlattices and nanocomposites made from them, showed significant improvements upon nanostructuring and ZT exceeding one at high temperatures. Other group IV alloys were not studied in the context of thermoelectrics. However, SiSn alloys are widely studied for their optoelectronic properties because they were predicted to become direct-gap materials when Sn composition increased beyond about 50%. To address this gap, we study the thermoelectric properties of SiSn alloys. Furthermore, we develop an iterative full-band solver for the electron Boltzmann transport equation and use it to compute the electron and hole mobility and Seebeck coeffcient in SiSn alloys. The electronic structure of SiSn alloys was computed in the virtual crystal approximation from non-local empirical pseudopotentials, while the application of strain allowed us to extract the electron-phonon coupling deformation potentials for each alloy composition. We benchmark our code against available mobility data for Si and SiGe alloys and find that it accurately reproduces the measured values. Full phonon dispersion was computed from the adiabatic bond charge model, which was shown to accurately reproduce measured dispersion, and used in our phonon BTE solver to compute lattice thermal conductivities. Scattering rates include anharmonic phonon-phonon, impurity, isotope, alloy, and boundary mechanisms. The lowest thermal conductivity was obtained in SiSn alloys, which have been experimentally demonstrated with up to 18% Sn composition. This carries through when combined with calculations of electronic power factor, where mobilities and Seebeck coeffcients of SiSn alloys are comparable to those of SiGe. Furthermore, ZT is optimized through doping for every composition. The ZT improves dramatically at higher temperatures, reaching ZT of 1.9, 2.36 is obtained for Sn composition of 10% and 50% in a n-doped bulk SiSn alloys at a temperature of 1480 K. However, such high Sn composition of 50% is unlikely to be synthesized due to low solid solubility of Sn in Si. Lastly, we study the impact of nanostructuring in thin films on the ZT. We also establish the limits on how much the ZT can be improved through nanostructuring by studying thin films of SiSn alloys across temperature from room temperature up to 1500 K. We conclude that in bulk SiSn alloys, even at modest Sn concentration of 10%, ZT can reach 1.9, while in 20 nm thin films of n-type SiSn alloys, it can reach the long-sought target of ZT>3 and ZT of 2.16 is obtained in p-type nanostructured SiSn alloys.
DOI
https://doi.org/10.7275/17649805
First Advisor
Zlatan Aksamija
Recommended Citation
Dusetty, Venkatakrishna, "Numerical Simulation of Thermoelectric Transport in Bulk and Nanostructured SiSn Alloys" (2020). Masters Theses. 935.
https://doi.org/10.7275/17649805
https://scholarworks.umass.edu/masters_theses_2/935