High-performance SnTe-based thermoelectric materials via synergistically band and nanostructure engineering

Abstract

Thermoelectric materials and generators, enabling to convert thermal energy into electrical energy or vice versa, offer great potentials in solving the energy problem from an environmental-sustainable perspective. As one of emerging thermoelectric materials, tin telluride (SnTe) shows uniuqe characterisitcs, such as low-toxicity and eco-friendly behaviour. The recent trend shows that band engineering and nanostructuring can enable to achieve enhanced thermoelectric performance of SnTe in the temperature range from 300 to 800 K, which provides an alternative for toxic PbTe with the same operational temperature. This thesis firstly highlights the key strategies to enhance the thermoelectric performance of SnTe materials through band engineering, carrier concentration optimization, synergistic engineering and structure design. On the basis of the fundamental analysis, the underpinnings for the property improvement are elucidated and can boost the relevant research with a view to work on further performance enhancement of SnTe materials.While most of the reported work on SnTe uses conventional melting method, this thesis governs with the solvothermal synthesis method which has unique advantages over melting method. In solvothermal method, the reactant ions and/or molecules react in solution which can lead to different final structures of products even if the same reactants are used with the melting method. In addition to this, solvothermal method can yield well-controlled nanomaterials with low energy consumption. In theoretical perspective, it is important to see the electronic band structure of doped and undoped SnTe, hence, density functional theory calculations are preformed to see the effect of suitable dopants including In, In/Sr, In/Ag on the band structure and density of states (DOS) of SnTe. The success of the solvothermal method in this thesis with synergistic band engineering and structure engineering is summarized as followsTo improve the thermoelectric performance of pristine SnTe, single In dopant was introduced to modify the crystal structure and band structure of SnTe. In dopant creates InTe nanoprecipiate in the Sn1-xInxTe matrix and the structure of this nanoprecipitates has been clearly identified by extensive transmission electron microscopy analysis. It is found that the structure of InTe (a = 6.14 A) is face-centred cubic which is the similar crystal structure with pristine SnTe (a = 6.32 A). These nanoprecipitates together with the point defects and grain boundaries significantly reduce lattice thermal conductivity to ~0.45 W m-1 K-1. Density functional theory calculation shows that the distortion of DOS (resonance energy level) near the Fermi level leads to enhanced seebeck coefficient (S) from ~23 mV K-1 to ~88 mV K-1. Finally, a high power factor of ~21.8 mW cm-1 K-2 and a corresponding figure of merit, a figure of merit (ZT) of ~ 0.78 have been obtained in Sn0.99In0.01Te at 773 K.The co-dopants of In and Cd with extra Te were used to further improve the thermoelectric performance of pristine SnTe. In and Cd rich nanoprecipitates can be found in the SnTe matrix and can significantly ameliorate the thermal transport properties. Pisarenko plot shows higher S in the In/Cd co-doped SnTe compared with the pristine SnTe, revealing that the significant valence band convergence and the resonant energy effect co-exist in the Sn(CdIn)xTe1+2x. Consequently, a high ZT of ~1.12 is obtained at 773 K in the p-type SnIn0.03Cd0.03Te1.06.A systematic theoretical and extensive experimental analyses have been successfully perceived in the In/Sr co-doped SnTe. The synergistic band and structure engineering significantly improve the electrical and thermal transport properties of Sn1-3xInxSr2xTe. The contribution of different phonon scattering centers such as grain boundaries, point defects and nanoprecipitates on the reduction of the lattice thermal conductivity has been well understood by using phonon modelling calculations. Besides, strain field associated with the In/Sr rich nanoprecipates is also analyzed by geometrical phase analysis (GPA). As a result, a record high power factor of ~33.88 mWcm-1K-2 and a peak ZT of ~1.31 has been achieved at 823 K for the Sn0.925In0.025Sr0.05Te pellet.A solvothermal synthesis method was developed to increase the solubility of In/Ag co-dopant in SnTe. It is has been found that In/Ag co-doping with appropriate dopant ratio (In:Ag = 1:2) can significantly improve the valence band convergence and resonant energy effect by using extensive density functional theory calculations. High-density strain field, dislocations, point defects and grain boundaries are observed in the matrix, which significantly scatter heat carrying phonons and yield low lattice thermal conductivity in a whole temperature range. Consequently, a high peak ZT of ~1.38 at 823 K has been achieved in Sn0.85In0.05Ag0.10Te, outperforming most of SnTe-based materials.In summary, this thesis successfully demonstrates the effectiveness of the facile and reliable solvothermal method for synthesising high-performance SnTe based thermoelectric materials by using synergistic band engineering and structure engineering. The simulation investigations on electronic transport and the phonon modeling fundamentally illustrate the effect of the proposed concepts, which will direct the future development of other thermoelectric system.