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Intrinsic and extrinsic reduction of SnX tin chalcogenide (X=S, Se) lattice thermal conductivity through heterostructure layering

Rundle, Jordan 2022. Intrinsic and extrinsic reduction of SnX tin chalcogenide (X=S, Se) lattice thermal conductivity through heterostructure layering. PhD Thesis, Cardiff University.
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This work utilised Density Functional Theory and Wannier functions for the evaluation of the Boltzmann transport equations, and Tight Binding methods with the frozen phonon approach, to determine lattice thermal conductivity values of tin sulphide, tin selenide, and periodic heterostructure derivatives of these two structures. The properties were evaluated within the context of thermoelectric application, with the aim to lower lattice thermal conductivity via the creation of sandwiched SnSe/SnS tin chalcogenide periodic heterostructures. This approach was found to give significant reduction of lattice thermal conductivity values in all structures with layering above a 1 to 1 ratio (1 SnS: 1 SnSe), reaching ultralow lattice thermal conductivities for the 3SnS/3SnSe structure of 0.14, 0.60 and 0.51 W/m/K along the a, b and c axes respectively at 300 K, a significant reduction from the lattice thermal conductivities of Pnma-SnSe calculated at 0.34, 0.77 and 0.57 W/m/K along the a, b and c axes respectively 300 K. These lattice thermal conductivity reductions were shown to positively influence the full thermoelectric figure of merit over a range of relevant temperatures (300, 500 and 750 K), for several heterostructure motifs, with particular benefits seen in the n-doping region of the 3SnS/3SnSe, reaching a 300 K figure of merit theoretical maximum of 2.56, and in the p-doping region of the SnS/2SnSe structures, reaching a 300 K theoretical figure of merit maximum of 1.37. Both represent a pronounced improvement on the calculated maxima of Pnma-SnSe of 1.76 and 0.98 in the p- and n-doping regions respectively. The reasons beyond these significant improvements included phonon boundary scattering effects occurring at the interfaces between the SnS and SnSe regions and phonon softening modes, which reduce the group velocity of the phonons and disrupt the heat transport further. The combination of these intrinsic (phonon softening) and extrinsic (phonon scattering) factors comes into existence as the resulting heterostructures tends to become more similar, both in lattice and electronic transport, to the Cmcm high-temperature structure, which can be shown to exhibit lowered lattice thermal conductivities and greater electrical conductivity, low/large enough to compensate for a smaller Seebeck coefficient maximum.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 13 July 2022
Last Modified: 13 Jul 2022 16:02

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