Enhancement of Thermoelectric Properties of Polycrystalline SnSe via the Synergistic Multi-Doping Effect

다국어 초록 (Multilingual Abstract)

Thermoelectric (TE) materials can efficiently convert waste heat into electricity and vice versa. The efficiency of energy conversion in thermoelectric materials is determined by the thermoelectric figure of merit, ZT = S2σT/ κ (where S, σ, κ, and T represent the Seebeck coefficient, the electrical conductivity, the total thermal conductivity, and absolute temperature, respectively). To achieve a high figure of merit (ZT), thermoelectric materials must simultaneously exhibit high electrical conductivity (σ), a large Seebeck coefficient (S), and low thermal conductivity (κ). It is challenging to increase both electrical conductivity (σ) and the Seebeck coefficient (S) simultaneously, since they generally vary in opposite directions with changes in carrier concentration. Similarly, simultaneously enhancing electrical conductivity (σ) while suppressing thermal conductivity (κ) is challenging, since the electronic component of (κ) generally scales with (σ).
Tin selenide (SnSe) has emerged as one of the most promising lead-free TE materials because it is composed of earth-abundant, non-toxic elements and its intrinsically low lattice thermal conductivity. Single-crystalline SnSe exhibits an exceptional ZT of 2.6 at 923 K along the b-axis, resulting from its highly anisotropic layered orthorhombic structure (Pnma, No. 62) below 800 K and strongly anharmonic bonding. Nevertheless, despite the outstanding thermoelectric performance of SnSe single crystals, their intrinsic brittleness, the need for tightly controlled crystal growth conditions, and the high production cost have posed significant challenges for large-scale industrial application. To address these issues, research attention has increasingly turned toward the development of high-performance polycrystalline SnSe, which offers improved mechanical strength, easier fabrication, and greater potential for scalable production.
Therefore, in this study, we investigated the effect of multi-doping (Na, Cu) and (Na, Cu, and Sr) on SnSe-based materials that were synthesized through an annealing method followed by spark plasma sintering (SPS).
First, polycrystalline p-type Sn0.98Na0.02SeCux samples with copper (Cu) addition were prepared using an annealing process combined with spark plasma sintering (SPS). With Cu addition, both the carrier concentration and the carrier mobility exhibit slight enhancement due to Cu filling Sn vacancies and substituting Sn sites, which leads to an increase in electrical conductivity to get a high ZT value. The highest ZT value obtained in this work is around 0.54 at 723 K for the Sn0.98Na0.02SeCu0.01 sample measured along the direction perpendicular to the SPS compressing.

목차 (Table of Contents)

• Table of Contents i
• List of figures iii
• Chapter 1. General introduction 1
• 1.1. Introduction 1
• Chapter 2. General background 6
• 2.1. Thermoelectricity and thermoelectric effects 6
• 2.2. The dimensionless thermoelectric figure of merit ZT 9
• 2.3. Commercial and alternative thermoelectric material 14
• 2.4. Structural characteristics and properties of SnSe 18
• 2.4.1. α-SnSe and β-SnSe 19
• 2.4.2. Phonon scattering effect 20
• Chapter 3. Experimental 24
• 3.1. Synthesis of compound 24
• 3.1.1. Annealing process 25
• 3.1.2. Spark Plasma Sintering 26
• 3.2. Characterization 29
• 3.2.1. X-ray Diffraction Analysis 29
• 3.2.2. Scanning Electron Microscope (SEM) 31
• 3.2.3. Hall Effect Measurement 31
• 3.2.4. Electrical and Thermal Properties Measurement 33
• 3.2.5. Laser Flash Analysis 34
• Chapter 4. Investigation on the effect of Cu contents to the thermoelectric properties of polycrystalline p-type Sn0.98Na0.02SeCux 36
• 4.1. Introduction 36
• 4.2. Experimental section 41
• 4.3. Result and Discussion 43
• 4.3.1. Phase Analysis of Textured Bulk Sn0.98Na0.02SeCux 43
• 4.3.2. Microstructure of Textured Bulk Sn0.98Na0.02SeCux 45
• 4.3.3. Thermoelectric property of Sn0.98Na0.02SeCux 48
• 4.3.4. Thermal Conductivity and ZT of Sn0.98Na0.02SeCux 56
• 4.4. Conclusion 60
• Chapter 5. Investigation on the effect of contents Sr to the thermoelectric properties of polycrystalline p-type Sn0.98-xSrxNa0.02Cu0.01Se 61
• 5.1. Introduction 61
• 5.2. Experimental 64
• 5.3. Results and Discussion 66
• 5.3.1. Phase Analysis of Textured Bulk Sn0.98-xSrxNa0.02Cu0.01Se 66
• 5.3.2. Microstructure of Textured Bulk Sn0.98-xSrxNa0.02Cu0.01Se 67
• 5.3.3. Thermoelectric property of Sn0.98-xSrxNa0.02Cu0.01Se 69
• 5.3.4. Thermal Conductivity and ZT of Sn0.98-xSrxNa0.02Cu0.01Se 73
• 5.4. Conclusion 77
• References 78